ML22088A006

TLR-RES/DE/REB-2021-14-R1 - Probabilistic Leak-Break Evaluations of Pressurized-Water Reactor Piping Systems Using the Extremely Low Probability of Rupture Code

ML22088A006
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Issue date:	04/01/2022
From:	Matthew Homiack NRC/RES/DE
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Homiack M
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Technical Letter Report TLRRES/DE/REB202114R1 Probabilistic Leak-Before-Break Evaluations of Pressurized-Water Reactor Piping Systems using the Extremely Low Probability of Rupture Code Date:

April 2022 Prepared in response to Task 4 of User Need Request NRR2014004, by:

C. J. Sallaberry R. Kurth Engineering Mechanics Corporation of Engineering Mechanics Corporation of Columbus Columbus E. Kurth-Twombly F. W. Brust Engineering Mechanics Corporation of Engineering Mechanics Corporation of Columbus Columbus NRC Project Managers:

Matthew Homiack Shah Malik Materials Engineer Sr. Materials Engineer Component Integrity Branch Component Integrity Branch Division of Engineering Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

DISCLAIMER This report was prepared as an account of work sponsored by an agency of the U.S. Government.

Neither the U.S. Government nor any agency thereof, nor any employee, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party's use, or the results of such use, of any information, apparatus, product, or process disclosed in this publication, or represents that its use by such third party complies with applicable law.

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This report does not contain or imply legally binding requirements. Nor does this report establish or modify any regulatory guidance or positions of the U.S. Nuclear Regulatory Commission and is not binding on the Commission.

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EXECUTIVE

SUMMARY

This study used the Extremely Low Probability of Rupture (xLPR) probabilistic fracture mechanics code to demonstrate that pressurized-water reactor (PWR) piping systems previously approved for leak-before-break (LBB) continue to exhibit an extremely low probability of rupture consistent with the requirements of Title 10 of the Code of Federal Regulations, Part 50, Appendix A, General Design Criterion (GDC) 4, when subject to the effects of primary water stress-corrosion cracking (PWSCC). The U.S. Nuclear Regulatory Commission (NRC)

Office of Nuclear Regulatory Research conducted this study at the request of the Office of Nuclear Reactor Regulation to complete the evaluation of such systems after initially demonstrating that a subset indeed continue to demonstrate an extremely low probability of rupture.

This study included an expanded scope of piping systems beyond the typical Westinghouse four-loop PWR designs considered in an initial study. All piping systems which have received prior LBB approvals from the NRC staff and which contain Alloy 82/182 dissimilar metal welds (DMWs) that are susceptible to PWSCC were binned for this study as follows:

Westinghouse 4-loop reactor vessel inlet and outlet nozzle DMWs Westinghouse pressurizer surge line nozzle DMWs Combustion Engineering and Babcock and Wilcox reactor coolant pump nozzle DMWs Westinghouse steam generator nozzle DMWs Combustion Engineering hot leg branch line nozzle DMWs Combustion Engineering cold leg branch line nozzle DMWs Westinghouse two- and three-loop reactor vessel inlet and outlet nozzle DMWs For each bin, a representative weld was analyzed using actual plant data when available and engineering judgement when not. Probability distributions were used to represent the material variability, inherent uncertainties associated with the weld residual stress (WRS) profiles, and other uncertainties. Deterministic inputs for the analyses of each bin were selected such that they would bound all inservice DMWs represented by the bin. Based on previous analytical experience, the highest normal operating loads, temperatures, and pressures were selected, along with the largest outer diameters and thinnest pipe wall thicknesses.

Several cases were used to analyze the piping in each bin. A base case included the effects of PWSCC initiation and growth for both circumferential and axial cracks with leak rate detection, inservice inspection, and safe shutdown earthquake events. These cases were used to estimate the base probabilities of rupture with a 1 gallon per minute leak rate detection capability. Since these probabilities were typically zero even with a large sample size, additional quantities of interest (QoIs), such as the time-dependent probabilities of first crack, first leak, and rupture both with and without a 10-year inspection frequency were also estimated.

The base case was supplemented with a sensitivity study case where each realization begins with one axial and one circumferential crack at top dead center of the weld. As outlined in the prior study, the LBB ratio and LBB time lapse QoIs are not impacted by the crack initiation iv

models; therefore, estimates for these two QoIs were more accurately calculated using this approach. In addition, prior studies have highlighted the importance of WRS and the associated uncertainties. Thus, an additional sensitivity study case that considered a more severe WRS profile was also included for each bin. Other sensitivity studies were included to analyze the impacts of fatigue and mechanical mitigation, as appropriate.

The xLPR code analyzes the risks associated with a single weld; however, GDC 4 requires an aggregation of results at the system-level. Therefore, a piping system-level analysis was necessary to combine the individual bin results and estimate the total probability of rupture for the various PWR piping systems of interest. Consistent with the prior study, the probability of rupture with a 1 gallon per minute leak rate detection capability served as the QoI used to assess whether such piping systems demonstrate an extremely low probability of rupture consistent with the requirements of GDC 4. The estimated probabilities using this QoI were zero with exceptions studied and explained, so aggregation of the results at the system-level was also zero. The system-level results are thus below the acceptance criterion of 1 x 10-6 ruptures per reactor-year and, therefore, all the piping systems considered continue to meet the requirements of GDC 4.

To illustrate the contributions of the various welds at the system-level, estimates were prepared for the probabilities of first crack, first leak, and rupture with and without a 10-year inspection frequency for three representative piping systems that bound the various configurations in the operating PWRs. The representative piping systems were previously approved for LBB in Westinghouse four-loop PWRs, Westinghouse two- and three-loop PWRs, and CE and B&W PWRs. The aggregation method considered all the welds to be independent consistent with the prior study. The largest contributing components were shown to vary depending on the QoI under consideration.

Successful application of the xLPR code in this study to demonstrate that the rupture probabilities of PWR piping systems that contain DMWs and were previously approved for LBB remain extremely low when subject to PWSCC reinforces the role of probabilistic fracture mechanics in making the demonstrations required by GDC 4 as originally envisioned by the Commission. Accordingly, the Office of Nuclear Regulatory Research recommends no changes to the GDC 4 regulations. Additionally, in the absence of a strong industry interest in future LBB applications, the Office of Nuclear Regulatory Research recommends no changes to NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, Section 3.6.3, Leak-Before-Break Evaluation Procedures, Revision 1, issued March 2007, to support probabilistic LBB applications. Should a demand for probabilistic LBB guidance arise in the future, an expansion of the deterministic review procedures may be pursued based on the results of this study.

Revision 1 of the report clarifies in Sections 2.2.3 and 2.2.4 that the LBB time lapse and LBB ratio QoIs calculate rupture based on the combined normal operating and non-probabilistically treated seismic loads. The threshold values for these QoIs where ruptures are precluded under certain conditions have also been included. In addition, Revision 1 includes updated analyses for the Westinghouse steam generator nozzle DMWs. Specifically, the DMW configurations v

with no mechanical mitigation and with overlays for PWSCC mitigation were re-analyzed using updated WRS and other inputs that more accurately represent conditions in the PWR fleet.

Sections 3.5.4 and 3.5.5 document the updated inputs and results. Because the updated results had potential ruptures before leak rate detection under combined normal operating and non-probabilistically treated seismic loads, supplemental analyses were conducted to estimate the probabilities of rupture with leak rate detection and probabilistically treated seismic events.

The results, which are summarized in newly added Sections 3.5.4.2.6 and 3.5.5.2.6, indicate that seismic events are not a significant contributor to the probabilities of rupture in these cases.

The reports prior conclusions and recommendations were not impacted by any of the updated analysis results.

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ACKNOWLEDGEMENTS This study was facilitated in part through a collaborative effort between the NRC Office of Nuclear Regulatory Research and the Electric Power Research Institute (EPRI) under an addendum to their Memorandum of Understanding on cooperative nuclear safety research. The authors benefited from the experience, knowledge, and work of many individuals supporting both organizations. The authors would particularly like to thank Mr. Matthew Homiack of the NRC staff and Dr. Craig Harrington of EPRI for their guidance in leading these efforts. The authors also thank Mr. Markus Burkardt of Dominion Engineering, Inc. as he was a major force in gathering input data and reviewing various documents. All the fruitful discussions and exchanges with him over the course of the project were much appreciated. The authors also thank Nathan Glunt of EPRI for his valuable assistance in developing specific inputs using his experience and knowledge. The authors would also like to thank Mr. Bruce Bishop from Phoenix Engineering Associates, Inc., who was a constant and reliable source of historical information on both deterministic and probabilistic approaches and associated rationales. He reviewed most of the analyses performed and his insights led to many improvements. Finally, the authors wish to thank John Broussard of Dominion Engineering, Inc. for his help in identifying key information for some of the welds of interest so that the associated weld residual stress profiles could be developed.

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TABLE OF CONTENTS EXECUTIVE

SUMMARY

............................................................................................................. iv ACKNOWLEDGEMENTS .......................................................................................................... vii TABLE OF CONTENTS ............................................................................................................ viii LIST OF TABLES ........................................................................................................................ xi LIST OF FIGURES ..................................................................................................................... xii ACRONYMS .............................................................................................................................. xvi 1 INTRODUCTION ................................................................................................................... 1 1.1 Summary of Prior Probabilistic LBB Study ..................................................................... 1 1.2 Objectives of the Present Study ..................................................................................... 2 2 ANALYSIS APPROACH ....................................................................................................... 4 2.1 Piping Systems of Interest .............................................................................................. 4 2.2 Quantities of Interest .................................................................................................... 13 2.2.1 Probability of Rupture with Detection .................................................................... 14 2.2.2 Leak Rate Jump .................................................................................................... 14 2.2.3 LBB Time Lapse .................................................................................................... 14 2.2.4 LBB Ratio .............................................................................................................. 15 2.3 Statistical Approach ...................................................................................................... 15 2.3.1 Sample Size .......................................................................................................... 15 2.3.2 Sampling Loop and Random Seed ....................................................................... 17 2.4 Computational Platforms and Simulation Execution Strategy ...................................... 18 2.5 Project Team ................................................................................................................ 18 2.6 Necessary Code Corrections and Modifications ........................................................... 19 3 ANALYSES ......................................................................................................................... 20 3.1 Scope ........................................................................................................................... 20 3.1.1 Bases Cases with PWSCC Initiation and Growth ................................................. 20 3.1.2 Initial Flaw Sensitivity Study Cases ....................................................................... 20 3.1.3 More Severe WRS Sensitivity Study Cases .......................................................... 20 3.1.4 Mechanical Mitigation Sensitivity Study Cases ..................................................... 20 viii

3.1.5 Fatigue Sensitivity Study Cases ............................................................................ 21 3.2 Bin 1: Westinghouse Four-Loop RVON and RVIN DMWs ........................................... 25 3.2.1 Base Case ............................................................................................................. 25 3.2.2 Initial Flaws ............................................................................................................ 32 3.3 Bin 2: Westinghouse Pressurizer Surge Line Nozzle DMWs ...................................... 36 3.3.1 Base Case ............................................................................................................. 36 3.3.2 Initial Flaws ............................................................................................................ 42 3.3.3 More Severe WRS ................................................................................................ 48 3.3.4 Overlay Mitigation .................................................................................................. 53 3.3.5 Fatigue .................................................................................................................. 60 3.3.6 MSIP Mitigation................................................................................................... 64 3.4 Bin 3: CE and B&W RCP Nozzle DMWs ..................................................................... 69 3.4.1 Base Case ............................................................................................................. 69 3.4.2 Initial Flaws ............................................................................................................ 71 3.4.3 More Severe WRS ................................................................................................ 75 3.5 Bin 4: Westinghouse Steam Generator Nozzle DMWs ............................................... 76 3.5.1 Base Case ............................................................................................................. 77 3.5.2 Initial Flaws ............................................................................................................ 80 3.5.3 More Severe WRS ................................................................................................ 84 3.5.4 Overlay Mitigation .................................................................................................. 89 3.5.5 No Mechanical Mitigation ...................................................................................... 96 3.6 Bin 5a: CE Hot Leg Branch Line Nozzle DMWs ........................................................ 103 3.6.1 Base Case ........................................................................................................... 104 3.6.2 Initial Flaws .......................................................................................................... 107 3.6.3 More Severe WRS .............................................................................................. 111 3.7 Bin 5b: CE Cold Leg Branch Line Nozzle DMWs ...................................................... 113 3.7.1 Base Case ........................................................................................................... 113 3.7.2 Initial Flaws .......................................................................................................... 116 3.8 Bin 6: Westinghouse Two- and Three-Loop RVON and RVIN DMWs ...................... 120 3.8.1 Base Case ........................................................................................................... 120 3.8.2 Initial Flaws .......................................................................................................... 125 4 PIPING SYSTEM FAILURE PROBABILITY ..................................................................... 129 4.1 Methodology ............................................................................................................... 129 ix

4.2 Piping System Failure Frequency Results .................................................................. 130 4.2.1 Piping Systems Investigated ............................................................................... 130 4.2.2 Westinghouse Four-Loop PWRs ......................................................................... 132 4.2.3 Westinghouse Two-loop and Three-Loop PWRs ................................................ 137 4.2.4 CE and B&W PWRs ............................................................................................ 141 5 ANALYSIS ASSUMPTIONS ............................................................................................. 146 5.1 Conservatisms ............................................................................................................ 146 5.2 Unknowns ................................................................................................................... 146 6 ASSESSMENT OF NRC REGULATORY FRAMEWORK FOR LEAK-BEFORE-BREAK ..................................................................................................................................... 147 6.1 Background on LBB for Nuclear Piping ...................................................................... 147 6.2 General Design Criterion 4 ......................................................................................... 150 6.3 Standard Review Plan Section 3.6.3 .......................................................................... 150 7

SUMMARY

AND CONCLUSIONS .................................................................................... 154 8 REFERENCES .................................................................................................................. 157 APPENDIX A

SUMMARY

OF RESULTS ................................................................................. A-1 APPENDIX B ANALYSIS INPUTS ........................................................................................... B-1 APPENDIX C WELDING RESIDUAL STRESS PROFILE DEVELOPMENT .......................... C-1 x

LIST OF TABLES Table 2-1 Scope of piping systems analyzed .............................................................................. 9 Table 2-2 Computational platforms ........................................................................................... 18 Table 3-1 Summary of analysis cases ...................................................................................... 22 Table 4-1 Numbers of components bounding different plant designs ..................................... 131 xi

LIST OF FIGURES Figure 2-1 Typical Westinghouse four-loop PWR nuclear steam supply system piping arrangement .......................................................................................................... 6 Figure 2-2 Typical CE PWR nuclear steam supply system piping arrangement .................... 7 Figure 2-3 Typical B&W PWR nuclear steam supply system piping arrangement ................. 8 Figure 3-1 Cases 1.1.6a and 1.1.6c WRS profiles................................................................ 26 Figure 3-2 Case 1.1.6a LBB time lapse results .................................................................... 27 Figure 3-3 Case 1.1.6a LBB ratio results .............................................................................. 28 Figure 3-4 Case 1.1.6a time-dependent probabilities of first crack....................................... 29 Figure 3-5 Case 1.1.6a time-dependent probabilities of first leak with a 10-year inspection frequency ............................................................................................................. 29 Figure 3-6 Comparison of Case 1.1.6c time-dependent probabilities of first leak, first circumferential leak, and first leak with ISI with Case 1.1.6a............................... 30 Figure 3-7 Case 1.1.6a time-dependent probabilities of rupture with a 10-year inspection frequency ............................................................................................................. 31 Figure 3-8 Comparison of Case 1.1.6c time-dependent probabilities of rupture and rupture with ISI with Case 1.1.6a ..................................................................................... 32 Figure 3-9 Case 1.1.6b LBB time lapse results .................................................................... 33 Figure 3-10 Case 1.1.6b LBB ratio results .............................................................................. 34 Figure 3-11 Case 1.1.6b time-dependent probabilities of first leak......................................... 35 Figure 3-12 Case 1.1.6b time-dependent probabilities of rupture........................................... 35 Figure 3-13 Case 2.1.0 WRS profiles ..................................................................................... 37 Figure 3-14 Case 2.1.0 LBB time lapse results ...................................................................... 38 Figure 3-15 Case 2.1.0 LBB ratio results ................................................................................ 39 Figure 3-16 Case 2.1.0 time-dependent probabilities of first crack......................................... 40 Figure 3-17 Case 2.1.0 time-dependent probabilities of first leak........................................... 41 Figure 3-18 Case 2.1.0 time-dependent probabilities of rupture............................................. 42 Figure 3-19 Case 2.1.1 LBB time lapse results ...................................................................... 43 Figure 3-20 Case 2.1.1 LBB ratio results ................................................................................ 44 Figure 3-21 Case 2.1.1 time-dependent probabilities of first leak........................................... 45 Figure 3-22 Case 2.1.1 time-dependent probabilities of rupture............................................. 46 Figure 3-23 Comparison of Cases 2.1.1 and 1.1.6b first circumferential leak rates ............... 47 Figure 3-24 Comparison of Cases 2.1.1 and 1.1.6b leak rates one month before rupture ..... 48 Figure 3-25 Case 2.1.2 WRS Profiles ..................................................................................... 49 Figure 3-26 Case 2.1.2 LBB lapse time results ...................................................................... 50 Figure 3-27 Case 2.1.2 LBB ratio results ................................................................................ 51 Figure 3-28 Case 2.1.2 time-dependent probabilities of first crack......................................... 52 Figure 3-29 Case 2.1.2 time-dependent probabilities of first leak........................................... 52 Figure 3-30 Case 2.1.2 time-dependent probabilities of rupture............................................. 53 Figure 3-31 Case 2.1.3 WRS profiles ..................................................................................... 54 Figure 3-32 Case 2.1.3 time-dependent probabilities of rupture with leak rate detection and ISI ........................................................................................................................ 55 xii

Figure 3-33 Case 2.1.3 LBB time lapse results ...................................................................... 56 Figure 3-34 Case 2.1.3 LBB ratio results ................................................................................ 57 Figure 3-35 Case 2.1.3 time-dependent probabilities of first crack......................................... 58 Figure 3-36 Case 2.1.3 time-dependent probabilities of first leak........................................... 59 Figure 3-37 Case 2.1.3 time-dependent probabilities of rupture............................................. 59 Figure 3-38 Case 2.1.4 LBB lapse time results ...................................................................... 61 Figure 3-39 Case 2.1.4 LBB ratio results ................................................................................ 62 Figure 3-40 Case 2.1.4 time-dependent probabilities of first crack......................................... 63 Figure 3-41 Case 2.1.4 time-dependent probabilities of first leak........................................... 63 Figure 3-42 Case 2.1.4 time-dependent probabilities of rupture............................................. 64 Figure 3-43 Case 2.1.5 WRS profiles ..................................................................................... 65 Figure 3-44 Case 2.1.5 LBB time lapse results ...................................................................... 66 Figure 3-45 Case 2.1.5 LBB ratio results ................................................................................ 67 Figure 3-46 Case 2.1.5 time-dependent probabilities of first crack......................................... 68 Figure 3-47 Case 2.1.5 time-dependent probabilities of first leak........................................... 68 Figure 3-48 Case 2.1.5 time-dependent probabilities of rupture............................................. 69 Figure 3-49 Case 3.1.0 WRS profiles ..................................................................................... 70 Figure 3-50 Case 3.1.0 time-dependent probabilities of first crack......................................... 71 Figure 3-51 Case 3.1.1 LBB time lapse results ...................................................................... 72 Figure 3-52 Case 3.1.1 LBB ratio results ................................................................................ 73 Figure 3-53 Case 3.1.1 time-dependent probabilities of first leak........................................... 74 Figure 3-54 Case 3.1.1 time-dependent probabilities of rupture............................................. 74 Figure 3-55 Case 3.1.2 WRS profiles ..................................................................................... 75 Figure 3-56 Case 3.1.2 time-dependent probabilities of first crack......................................... 76 Figure 3-57 Case 4.1.0 WRS profiles ..................................................................................... 78 Figure 3-58 Case 4.1.0 time-dependent probabilities of first crack......................................... 79 Figure 3-59 Case 4.1.0 time-dependent probabilities of first leak........................................... 79 Figure 3-60 Case 4.1.1 probability of leak rate jump .............................................................. 80 Figure 3-61 Case 4.1.1 time-dependent probabilities of first leak........................................... 81 Figure 3-62 Case 4.1.1 time-dependent probabilities of rupture............................................. 82 Figure 3-63 Case 4.1.1 through-wall crack representation for one realization ....................... 83 Figure 3-64 Case 4.1.2 WRS profiles ..................................................................................... 85 Figure 3-65 Case 4.1.2 time-dependent probabilities of rupture with leak rate detection ....... 86 Figure 3-66 Case 4.1.2 time-dependent probabilities of first crack......................................... 87 Figure 3-67 Case 4.1.2 time-dependent probabilities of first leak........................................... 88 Figure 3-68 Case 4.1.2 time-dependent probabilities of rupture............................................. 89 Figure 3-69 Case 4.1.3 unmitigated and overlay mitigation WRS profiles.............................. 90 Figure 3-70 Case 4.1.3 time-dependent probability of leak rate jump .................................... 91 Figure 3-71 Case 4.1.3 LBB time lapse results ...................................................................... 92 Figure 3-72 Case 4.1.3 LBB ratio results ................................................................................ 93 Figure 3-73 Case 4.1.3 time-dependent probabilities of first crack......................................... 94 Figure 3-74 Case 4.1.3 time-dependent probabilities of first leak........................................... 95 Figure 3-75 Case 4.1.3 time-dependent probability of rupture ............................................... 96 xiii

Figure 3-76 Case 4.1.3 time-dependent probability of rupture with leak rate detection under probabilistically treated seismic loads ................................................................. 97 Figure 3-77 Case 4.1.4 time-dependent probability of leak rate jump .................................... 98 Figure 3-78 Case 4.1.4 LBB time lapse results ...................................................................... 99 Figure 3-79 Case 4.1.4 LBB ratio results .............................................................................. 100 Figure 3-80 Case 4.1.4 time-dependent probabilities of first crack....................................... 101 Figure 3-81 Case 4.1.4 time-dependent probabilities of first leak......................................... 102 Figure 3-82 Case 4.1.4 time-dependent probabilities of rupture........................................... 103 Figure 3-83 Case 4.1.4 time-dependent probability of rupture with leak rate detection under probabilistically treated seismic loads ............................................................... 104 Figure 3-84 Case 5.1.0 WRS profiles ................................................................................... 105 Figure 3-85 Case 5.1.0 time-dependent probabilities of first crack....................................... 106 Figure 3-86 Case 5.1.0 time-dependent probabilities of first leak......................................... 107 Figure 3-87 Case 5.1.1 LBB lapse time results .................................................................... 108 Figure 3-88 Case 5.1.1 LBB ratio results .............................................................................. 109 Figure 3-89 Case 5.1.1 time-dependent probabilities of first leak......................................... 110 Figure 3-90 Case 5.1.1 time-dependent probabilities of rupture........................................... 110 Figure 3-91 Case 5.1.2 WRS profiles ................................................................................... 111 Figure 3-92 Case 5.1.2 time-dependent probabilities of first crack....................................... 112 Figure 3-93 Case 5.1.2 time-dependent probabilities of first leak......................................... 113 Figure 3-94 Case 5.2.0 WRS profiles ................................................................................... 114 Figure 3-95 Case 5.2.0 time-dependent probabilities of first crack....................................... 115 Figure 3-96 Case 5.2.0 time-dependent probabilities of first leak......................................... 116 Figure 3-97 Case 5.2.1 LBB lapse time results .................................................................... 117 Figure 3-98 Case 5.2.1 LBB ratio results .............................................................................. 118 Figure 3-99 Case 5.2.1 time-dependent probabilities of first leak......................................... 119 Figure 3-100 Case 5.2.1 time-dependent probabilities of rupture........................................... 119 Figure 3-101 Case 1.3.0 WRS profiles ................................................................................... 120 Figure 3-102 Case 1.3.0 LBB time lapse results .................................................................... 122 Figure 3-103 Case 1.3.0 LBB ratio results .............................................................................. 123 Figure 3-104 Case 1.3.0 time-dependent probabilities of first crack....................................... 124 Figure 3-105 Case 1.3.0 time-dependent probabilities of first leak......................................... 124 Figure 3-106 Case 1.3.0 time-dependent probabilities of rupture........................................... 125 Figure 3-107 Case 1.3.1 LBB time lapse results .................................................................... 126 Figure 3-108 Case 1.3.1 LBB ratio results .............................................................................. 127 Figure 3-109 Case 1.3.1 time-dependent probabilities of first leak......................................... 128 Figure 3-110 Case 1.3.1 time-dependent probabilities of rupture........................................... 128 Figure 4-1 Bounding Westinghouse four-loop time-dependent probabilities ...................... 133 Figure 4-2 Westinghouse four-loop system probability of first crack and component contributions ...................................................................................................... 133 Figure 4-3 Westinghouse four-loop system probability of first circumferential crack and component contributions ................................................................................... 134 Figure 4-4 Westinghouse four-loop system probability of first leak and component contributions ...................................................................................................... 134 xiv

Figure 4-5 Westinghouse four-loop system probability of first leak with a 10-year inspection frequency and component contributions ........................................................... 135 Figure 4-6 Westinghouse four-loop system probability of first circumferential leak and component contributions ................................................................................... 135 Figure 4-7 Westinghouse four-loop system probability of rupture and component contributions ...................................................................................................... 136 Figure 4-8 Westinghouse four-loop system probability of rupture with 10-year inspection frequency and component contributions ........................................................... 136 Figure 4-9 Westinghouse two- and three-loop time-dependent probabilities...................... 137 Figure 4-10 Westinghouse two- and three-loop system probability of first crack and component contributions ................................................................................... 138 Figure 4-11 Westinghouse two- and three-loop system probability of first circumferential crack and component contributions ............................................................................ 138 Figure 4-12 Westinghouse two- and three-loop system probability of first leak and component contributions ...................................................................................................... 139 Figure 4-13 Westinghouse two- and three-loop system probability of first leak with a 10-year inspection frequency and component contributions .......................................... 139 Figure 4-14 Westinghouse two- and three-loop system probability of first circumferential leak and component contributions ............................................................................ 140 Figure 4-15 Westinghouse two- and three-loop system probability of rupture and component contributions ...................................................................................................... 140 Figure 4-16 Westinghouse two- and three-loop system probability of rupture with a 10-year inspection frequency and component contributions .......................................... 141 Figure 4-17 CE and B&W time-dependent probabilities ....................................................... 142 Figure 4-18 CE and B&W system probability of first crack and component contributions .... 142 Figure 4-19 CE and B&W system probability of first circumferential crack and component contributions ...................................................................................................... 143 Figure 4-20 CE and B&W system probability of first leak and component contributions ...... 143 Figure 4-21 CE and B&W system probability of first leak with a 10-year inspection frequency and component contributions ............................................................................ 144 Figure 4-22 CE and B&W system probability of first circumferential leak and component contributions ...................................................................................................... 144 Figure 4-23 CE and B&W system probability of rupture and component contributions ........ 145 Figure 4-24 CE and B&W system probability of rupture with a 10-year inspection frequency and component contributions ............................................................................ 145 xv

ACRONYMS cc cubic centimeters CDF cumulative distribution function CE Combustion Engineering CFR Code of Federal Regulations COV coefficient of variation DM1 Direct Model 1 DMW dissimilar metal weld DW deadweight EFPY effective full-power years EPRI Electric Power Research Institute FOI factor of improvement GDC general design criterion gpm gallons per minute ID inner diameter ISI in-service inspection kg kilogram LBB leak-before-break LHS Latin hypercube sampling mon month MPa megapascal MSIP Mechanical Stress Improvement Process NLKH non-linear kinematic hardening NRC Nuclear Regulatory Commission PDI Performance Demonstration Initiative PFM probabilistic fracture mechanics POD probability of detection PWHT post-weld heat treatment PWR pressurized-water reactor PWSCC primary water stress-corrosion cracking RCP reactor coolant pump RVIN reactor vessel inlet nozzle RVON reactor vessel outlet nozzle SRP standard review plan SS stainless steel SSE safe shutdown earthquake TWC through-wall crack WRS weld residual stress xLPR Extremely Low Probability of Rupture xvi

1 INTRODUCTION 1.1 Summary of Prior Probabilistic LBB Study NUREG-2247, Extremely Low Probability of Rupture Version 2 Probabilistic Fracture Mechanics Code, issued August 2021 [1], describes the Extremely Low Probability of Rupture (xLPR) code. In a prior study, as documented in U.S. Nuclear Regulatory Commission (NRC)

Technical Letter Report, TLR-RES/DE/REB-2021-09, Probabilistic Leak-Before-Break Evaluation of Westinghouse Four-Loop Pressurized-Water Reactor Primary Coolant Loop Piping using the Extremely Low Probability of Rupture Code, issued August 13, 2021 [2], the xLPR code was used to demonstrate that a selected pressurized-water reactor (PWR) piping system exhibits an extremely low probability of rupture consistent with the requirements of Title 10 of the Code of Federal Regulations (10 CFR), Part 50, Appendix A, General Design Criterion (GDC) 4 [3], when subject to the effects of primary water stress-corrosion cracking (PWSCC). The piping system selected for that study was the primary or main loop piping in a Westinghouse four-loop PWR design. This configuration was selected because it is the predominant piping system for which the NRC staff has granted prior leak-before-break (LBB) approvals. This design also has multiple dissimilar metal welds (DMWs), which supported the second objective of the study to combine the estimates from multiple welds to generate an annual, piping system-level failure frequency. Finally, most of the input data needed to analyze the piping system were already available and conveniently assembled in the required xLPR input set format.

The primary objectives of the prior study were twofold:

1. use the xLPR code to generate numerical failure frequency estimates with uncertainties for welds in a representative PWR piping system considering the effects of PWSCC, fatigue, leak rate detection, in-service inspection (ISI), mitigation, and seismic events

2. combine the estimates from multiple welds to generate an annual, piping system-level failure frequency to determine whether the requirements of GDC 4 were met Several quantities of interest (QoIs) were defined and calculated in the prior study to support the desired safety conclusions. These metrics were the probabilities of rupture with leak rate detection, leak rate jump, LBB time lapse, and LBB ratio. The analyses for the reactor vessel outlet nozzle (RVON) and reactor vessel inlet nozzle (RVIN) DMWs included base cases and sensitivity study cases to investigate the effects of specific analysis parameters and assumptions.

Some important observations were made based on all the cases that were analyzed in the prior study. First, the probability of a rupture occurring before a leak is detected (i.e., a break-before-leak scenario) is extremely low and should not be an issue for either the RVON or RVIN welds considering the inputs used in the simulations. Also, the WRS profile and its uncertainty was found to be one of the most influential inputs. In addition, axial cracks only impacted the probability of first leak, but the predicted leak rates were so low that the associated cracks 1

would only be detected through ISI. Furthermore, the likelihood of having both an axial crack and a circumferential crack was so low that it did not affect the results. Therefore, it would be appropriate to exclude axial cracks under similar analysis conditions.

A system-level analysis was performed to aggregate the results from the multiple RVON and RVIN welds in the main loop piping. Only the results for the four RVON welds were combined, however, as the RVIN weld results were too low to have an impact. The approach considered each weld to be independent, which is reasonably conservative. Since the individual weld probabilities were low, the aggregated probabilities were only increased by roughly a factor four, which corresponds with the number of RVON welds in the system. Thus, the system-level results did not affect the conclusions drawn on an individual weld basis and remain below 1 x 10-6 ruptures per reactor-year consistent with the basis for the GDC 4 rulemaking. In conclusion, the prior study demonstrated that, for a typical primary loop piping system in a Westinghouse four-loop PWR, the probability of rupture with consideration of the active degradation mechanism PWSCC is extremely low consistent with the requirements of GDC 4.

1.2 Objectives of the Present Study The present study builds on the results from the prior study by using the xLPR code to analyze the remaining PWR piping systems that contain DMWs and were previously approved for LBB to determine whether the rupture probabilities remain extremely low when subject to PWSCC as required by GDC 4. The primary objectives of the present study are the same as in the prior study. An additional objective of the present study is to assess NRCs regulatory framework with respect to LBB to determine if any changes are necessary.

As in the prior study, the following set of QoIs were considered:

Rupture with Detection - This QoI directly estimates the occurrence of ruptures with consideration of a 1 gallon per minute (gpm) leak rate detection capability and ISI, if necessary. It is represented as a cumulative probability over the simulated period of plant operation.

Leak Rate Jump - This QoI estimates the probability of a sudden jump in leakage from below a lower leak rate threshold value to above an upper leak rate threshold value from one simulation time step to the subsequent time step. The probabilistic result is expressed as a time-dependent probability over the simulated period of plant operation.

It is based directly on the recommendations in the technical basis document on acceptance criteria [4].

LBB Time Lapse - This QoI estimates the time between a detectable leak rate and a rupture. The probabilistic result is a cumulative distribution of the LBB time lapse over the simulated period of plant operation conditional on having cracks that both leak and rupture the pipe. It provides useful insights by capturing the time-dependent behavior of the system, which cannot be captured in a deterministic LBB analysis.

LBB Ratio - This QoI estimates the ratio between the critical crack length at rupture and the length of a crack that results in detectable leakage. It is the probabilistic analog to 2

the deterministic LBB acceptance criterion from NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition (SRP), Section 3.6.3, Leak-Before-Break Evaluation Procedures, Revision 1, issued March 2007 [5]. The probabilistic result is a cumulative distribution of the LBB ratio over the simulated period of plant operation conditional on having cracks that both leak and rupture the pipe.

These QoIs are only impacted by circumferential cracks because: (a) the axial crack leak rates are too small to impact the leak rate jump event when multiple axial cracks are present, and (b) the remaining QoIs depend on ruptures, which are caused only by circumferential cracks. For some of the cases considered, the QoIs may be zero or not applicable (e.g., without leakage, the LBB time lapse and LBB ratio cannot be calculated). Thus, as in the prior study, the time-dependent probabilities of first crack, first leak, and rupture without leak rate detection and ISI were also analyzed for each case. These QoIs are the standard indicator outputs from the xLPR code.

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2 ANALYSIS APPROACH 2.1 Piping Systems of Interest The piping systems of interest in this study are ones that that contain Alloy 82/182 DMWs and were previously approved for LBB by the NRC staff. The scope includes the main coolant loop piping in most Westinghouse, Combustion Engineering (CE), and Babcock and Wilcox (B&W)

PWRs. The scope also includes the pressurizer surge line piping in some Westinghouse and CE PWRs. Additionally, at one CE PWR, the scope also includes the high-pressure injection and shutdown coolant system branch line piping. For reference, Figure 2-1, Figure 2-2, and Figure 2-3 illustrate the typical reactor coolant system piping arrangements for Westinghouse, CE, and B&W PWRs, respectively.

Table 2-1 identifies the applicable operating PWRs, piping systems, and locations of the DMWs in each system. Although licensed to operate when this study was conducted, Diablo Canyon Nuclear Power Plant, Units 1 and 2, and Indian Point Nuclear Generating, Units 2 and 3 were not explicitly included because their owners had announced plans to cease operations. Some of the DMWs have been mechanically mitigated against PWSCC, and the table also identifies the type of mechanical mitigation, if applicable.

For the purposes of this study, the piping systems were organized into bins to optimize the number of analyses that were performed. The bins were determined primarily based on the plant designs, piping systems, and locations of the PWSCC-susceptible welds. The six bins were as follows:

1. Westinghouse four-loop reactor vessel inlet and outlet nozzle DMWs

2. Westinghouse pressurizer surge line piping to pressurizer nozzle DMWs

3. CE and B&W reactor coolant pump (RCP) inlet and outlet nozzle DMWs

4. Westinghouse steam generator nozzle DMWs

5. CE hot leg branch connection DMWs and CE high-pressure injection system DMWs

6. Westinghouse two- and three-loop reactor vessel inlet and outlet nozzle DMWs Some of the unique aspects of each bin are as follows:

Although Westinghouse four-loop reactor vessel outlet nozzle (RVON) DMWs were included in the prior study, new reference cases were developed for the present study in Bin 1. The new reference cases are largely based on Case 1.1.6 from the prior study, but with the inclusion of axial cracks and a bounding value for the hydrogen concentration in the reactor coolant for consistency with cases defined for the other bins.

These settings were determined to be the most appropriate based on the sensitivity study results from the prior study.

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No welding residual stress (WRS) profiles had previously been developed for Bin 2. In addition, the operating temperatures are higher, and the diameters and thicknesses are smaller as compared to the Westinghouse RVON DMWs. Further, all the welds have been subject to mechanical mitigation, thus requiring more cases to cover the different sensitivities.

Bin 3 has a relatively low WRS value at the inside diameter, so even with uncertainty in the WRS profile, the values are comparable to the normal operating stresses and thus the probabilities of crack initiation and rupture were expected to be quite low.

The welds in Bin 4 all have a double-vee groove configuration, which has a significantly different WRS profile that needed to be developed.

Bin 5 has smaller diameters and higher operating temperatures as compared to the Westinghouse RVON DMWs. It also includes auxiliary lines not previously studied. The second set of Bin 5 cases operate at cold leg temperatures; therefore, a substantially lower rupture frequency as compared to components that operate at hot leg temperatures was expected based on the results from the prior study.

The welds in Bin 6 grouped RVONs in Westinghouse 2-loop and 3-loop PWRs because Westinghouse 4-loop PWRs were the focus of the prior study. These systems are like the previously studied Westinghouse four-loop systems, so no significant differences were expected.

As PWSCC is the primary degradation mechanism of interest, only the Alloy 82/182 DMWs were analyzed.

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Figure 2-1 Typical Westinghouse four-loop PWR nuclear steam supply system piping arrangement 6

Figure 2-2 Typical CE PWR nuclear steam supply system piping arrangement 7

Figure 2-3 Typical B&W PWR nuclear steam supply system piping arrangement 8

Table 2-1 Scope of piping systems analyzed Plant Design Piping System(s) Location of Welds Mechanical Approved for LBB Susceptible to Mitigation PWSCC Arkansas Reactor Coolant RCP Inlet and Outlet Nuclear One, B&W None Piping [6] Nozzles Unit 1 Arkansas Reactor Coolant RCP Inlet and Outlet Nuclear One, CE None Piping [7] Nozzles Unit 2 None for the RVON and RVIN Hot and Cold Legs RVONs, RVINs, and DMWs; Overlay Beaver Valley, Westinghouse

[8], Pressurizer Surge Pressurizer Surge for the Unit 2 3-loop Line [9] Line Nozzle Pressurizer Surge Line Nozzle DMW Braidwood, Westinghouse Hot and Cold Legs RVONs and RVINs MSIP Units 1 and 2 4-loop [10]

Byron, Westinghouse Hot and Cold Legs RVONs and RVINs MSIP Units 1 and 2 4-loop [10]

Callaway, Westinghouse Hot and Cold Legs RVONs and RVINs None*

Unit 1 4-loop [11]

Calvert Cliffs, Reactor Coolant RCP Inlet and Outlet CE None Units 1 and 2 Piping [7] Nozzles Catawba, Westinghouse Hot and Cold Legs RVONs and RVINs None Unit 2 4-loop [12]

None for the RVON and RVIN Comanche Hot and Cold Legs RVONs, RVINs, and DMWs; Overlay Westinghouse Peak, [13], Pressurizer Pressurizer Surge for the 4-loop Units 1 and 2 Surge Line [14], [15] Line Nozzle Pressurizer Surge Line Nozzle DMWs The Callaway, Unit 1 RVON and RVIN DMWs have been peened, but the impacts of peening were not considered in this study.

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Plant Design Piping System(s) Location of Welds Mechanical Approved for LBB Susceptible to Mitigation PWSCC MSIP for the Unit 1 RVON and RVIN DMWs; None for the Unit 2 RVON Hot and Cold Legs RVONs, RVINs, and D.C. Cook, Westinghouse and RVIN

[16], Pressurizer Pressurizer Surge Units 1 and 2 4-loop DMWs; Overlay Surge Line [17] Line Nozzle for the Units 1 and 2 Pressurizer Surge Line Nozzle DMWs Davis-Besse, Reactor Coolant RCP Inlet and Outlet B&W Overlay Unit 1 Piping [6] Nozzles None for the RVON and RVIN Hot and Cold Legs RVONs, RVINs, and DMWs; Overlay Farley, Westinghouse

[18], Pressurizer Pressurizer Surge for the Units 1 and 2 3-loop Surge Line [19] Line Nozzle Pressurizer Surge Line Nozzle DMWs McGuire, Westinghouse Hot and Cold Legs RVONs and RVINs None Unit 1 4-loop [20]

None for the RCP Inlet and Outlet Nozzle Reactor Coolant RCP Inlet and Outlet DMWs; Overlay Piping [7], Nozzles, Hot Leg for the Hot Leg Pressurizer Surge Surge Line Nozzle, Surge Line Millstone, CE Line [21], Shutdown Hot Leg Shutdown Nozzle, Hot Leg Unit 2 Cooling Line [22], Cooling Nozzle, Cold Shutdown Safety Injection Line Leg High Pressure Cooling Nozzle,

[23] Injection Nozzle and Cold Leg High Pressure Injection Nozzle DMWs Millstone, Westinghouse Hot and Cold Legs RVONs and RVINs None Unit 3 4-loop [24]

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Plant Design Piping System(s) Location of Welds Mechanical Approved for LBB Susceptible to Mitigation PWSCC Overlay for Unit 1 Steam Generator Inlet Nozzle DMWs; None for Unit 1 Steam Generator North Anna, Westinghouse Hot and Cold Legs Stem Generator Inlet and Outlet Units 1 and 2 3-loop [25] Outlet Nozzle Nozzles DMWs; Inlay for Unit 2 Steam Generator Inlet and Outlet Nozzle DMWs Oconee, Reactor Coolant RCP Inlet and Outlet B&W None Units 1, 2, and 3 Piping [6] Nozzles Reactor Coolant RCP Inlet and Outlet Palisades CE None Piping [7] Nozzles Inlay for Steam Hot and Cold Legs Steam Generator Point Beach, Westinghouse Generator Inlet

[26], Pressurizer Inlet and Outlet Units 1 and 2 2-loop and Outlet Surge Line [27] Nozzles Nozzle DMWs Hot and Cold Legs Prairie Island Westinghouse Pressurizer Surge

[28], Pressurizer Overlay Units 1 and 2 2-loop Line Nozzle Surge Line [29]

Robinson, Westinghouse Hot and Cold Legs RVONs and RVINs None Unit 2 3-loop [30]

MSIP for Unit 1 RVON and RVIN DMWs; MSIP Salem, Westinghouse Hot and Cold Legs RVONs and RVINs for Unit 2 RVON Units 1 and 2 4-loop [31]

DMWs; None for Unit 2 RVIN DMWs Seabrook, Westinghouse Hot and Cold Legs RVONs and RVINs MSIP Unit 1 4-loop [32]

Hot and Cold Legs Sequoyah, Westinghouse Pressurizer Surge

[33], Pressurizer Overlay Units 1 and 2 4-loop Line Nozzle Surge Line [34]

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Plant Design Piping System(s) Location of Welds Mechanical Approved for LBB Susceptible to Mitigation PWSCC MSIP for Shearon Harris, Westinghouse Hot and Cold Legs RVON DMWs; RVONs and RVINs Unit 1 3-loop [35] None for RVIN DMWs MSIP for Unit 1 RVON and RVIN DMWs; None for Unit 2 RVON Hot and Cold Legs RVONs, RVINs, and South Texas, Westinghouse and RVIN

[36], Pressurizer Pressurizer Surge Units 1 and 2 4-loop DMWs; Overlay Surge Line [37], [38] Line Nozzle for Units 1 and 2 Pressurizer Surge Line Nozzle DMWs St. Lucie, Reactor Coolant RCP Inlet and Outlet CE None Units 1 and 2 Piping [7] Nozzles None for the Hot Leg A RVON DMW, which was Replaced with Alloy 52; RVONs, RVINs, and MSIP for the V.C. Summer, Westinghouse Hot and Cold Legs Steam Generator Hot Leg B and Unit 1 3-loop [39], [40] Inlet and Outlet C RVON Nozzles DMWs; None for RVIN DMWs; Inlay for Steam Generator Inlet and Outlet Nozzle DMWs MSIP for Units 1 and 2 RVON DMWs; None for Units 1 and 2 Hot and Cold Legs RVONs, RVINs, and Vogtle, Westinghouse RVIN DMWs;

[41], Pressurizer Pressurizer Surge Units 1 and 2 4-loop Overlay for Units Surge Line [42] Line Nozzle 1 and 2 Pressurizer Surge Line Nozzle DMWs 12

Plant Design Piping System(s) Location of Welds Mechanical Approved for LBB Susceptible to Mitigation PWSCC None for the RCP Inlet and Outlet Nozzle RCP Inlet and Outlet Reactor Coolant DMWs; Overlay Nozzles, Pressurizer Waterford, Piping [7], for the CE Surge Line Nozzle, Unit 3 Pressurizer Surge Pressurizer and Hot Leg Surge Line [43] Surge Line Line Nozzle Nozzle and Hot Leg Surge Line Nozzle DMWs Hot and Cold Legs RVONs, RVINs, and Watts Bar, Westinghouse

[44], Pressurizer Pressurizer Surge MSIP Units 1 and 2 4-loop Surge Line [45] Line Nozzle Wolf Creek, Westinghouse Hot and Cold Legs RVONs and RVINs None*

Unit 1 4-loop [11]

2.2 Quantities of Interest The QoIs considered in this study were selected on the basis that they could provide information to a decisionmaker to determine whether a piping system has an extremely low probability of rupture consistent with the requirements of GDC 4. The evaluation period for all the QoIs was 80 effective full-power years (EFPY) to bound plant operation as would be authorized by an original 40-year operating license and up to two renewed operating licenses. The 80-EFPY evaluation period assumes a plant capacity factor of 100 percent throughout the entire period of licensed operation. Thus, as presented in this report, 80 EFPY is equivalent to 80 calendar years of operation. This assumption is conservative as it subjects the piping components to the most amount of degradation in a simulation.

Sections 2.2.1 through 2.2.4 describe the primary QoIs in detail (i.e., probability of rupture with detection, leak rate jump, LBB time lapse, and LBB ratio). Two leak rate detection capabilities were considered in this study for the LBB time lapse and LBB ratio calculations: 1 and 10 gpm.

The latter was chosen because many licensees had demonstrated a 1 gpm leak rate detection capability for approval of their original, deterministic LBB analyses. However, per SRP Section 3.6.3, a safety factor of 10 was applied, which results the 10 gpm value. This safety factor was meant to address many of the uncertainties that the xLPR code addresses directly; therefore, the former 1 gpm leak rate detection capability was also examined. The other QoIs considered included the probabilities of first crack, first leak, and rupture without detection.

The Wolf Creek, Unit 1 RVON and RVIN DMWs have been peened, but the impacts of peening were not considered in this study.

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These QoIs were considered for informational purposes to support the general LBB concept, and because they can be more useful when analyzing results from sensitivity studies.

2.2.1 Probability of Rupture with Detection The probability of rupture is one of the standard xLPR indicator results. With consideration of the offsetting effects of leak rate detection, it was used in this study as the decisionmaking QoI for probabilistic LBB assessment because it directly addresses the language in GDC 4 (i.e.,

demonstrate that piping rupture is extremely low). The probability of rupture with leak rate detection is the average of an indicator function that takes on: (a) a value of zero for each realization without rupture and for each realization that had a leak rate greater than the leak rate detection threshold before rupture, and (b) a value of one for each realization that ruptured before the leak was detected. A 1 gpm leak rate detection capability was considered, and when the probability of rupture with leak rate detection was nonzero, the impact of a 10-year inspection frequency was also considered. The probabilistic result is expressed as a time-dependent probability of occurrence over the 80-EFPY evaluation period.

2.2.2 Leak Rate Jump The leak rate jump and associated thresholds are based directly on the recommendations in the technical basis document on acceptance criteria [4]. It was considered as an informative QoI in this study because it estimates the probability of a sudden jump in leakage from below a lower threshold value (defined as 10 gpm) to above an upper threshold value (defined as 50 gpm) from one simulation time step to the subsequent time step (defined as 1 month). Several conservatisms are present in this approach as detailed in [4]. This QoI can be used to support probabilistic LBB assessment because a quickly increasing leak rate would indicate fast and potentially unstable crack growth that could lead to rupture or a loss of coolant accident.

Ruptures are also counted in the leak rate jump results if they occur before the lower leak rate jump threshold is reached. An advantage of this QoI is that it accounts for the temporal aspects of the problem. As such, it provides greater insights as compared to deterministic LBB analyses prepared following the guidance in SRP Section 3.6.3, which does not account for such aspects.

Additionally, as explained in [4], this QoI is tied to the NRCs risk-informed decisionmaking framework. The main factors that affect the leak rate jump are the crack growth rate and crack opening displacement. The probabilistic result is expressed as a time-dependent probability of occurrence over the 80-EFPY evaluation period.

2.2.3 LBB Time Lapse The LBB time lapse estimates the time between a detectable leak rate under normal operating loads and a rupture under combined normal operating and non-probabilistically treated seismic loads. It was considered as an informative QoI in this study because, when a through-wall crack (TWC) is experiencing subcritical crack growth, the time between when the TWC is detected by a plants leakage detection system and when the TWC becomes unstable and leads to rupture provides insights into the time-dependent behavior of the system. The main factors that affect the LBB time lapse are the crack growth rate and uncertainties in the leak rate models. The probabilistic result is a distribution of the LBB time lapse conditional on having 14

cracks that leak and a seismic event that ruptures the pipe. It is expressed as a cumulative distribution function (CDF) based on the 80-EFPY evaluation period. Because it assumes that the seismic loads are applied in every time step, an LBB time lapse greater than zero indicates that leak rate detection precludes seismic rupture. Additionally, since the combined normal operating and seismic loads are greater than the normal operating loads by themselves, rupture under normal operating loads is also precluded when the LBB time lapse is greater than zero.

2.2.4 LBB Ratio The LBB ratio estimates the ratio between (a) the critical crack length at rupture under combined normal operating and non-probabilistically treated seismic loads, and (b) the length of a crack that results in detectable leakage under the normal operating loads. It was considered as an informative QoI in this study because it is the probabilistic analog to the current deterministic LBB acceptance criterion. In accordance with SRP Section 3.6.3, the deterministic LBB analysis should demonstrate that there is a margin of at least two between the leakage crack size and the critical crack size. The leakage crack size represents the size of the TWC under normal operating loads that will produce a leak rate 10 times greater than the reactor coolant system leak rate detection capability. The critical crack size represents the size of the crack at the onset of instability under normal operating plus safe shutdown earthquake (SSE) loads. Both 1 and 10 gpm leak rate detection capabilities were chosen to define the LBB ratio in this study, however. The main factors that affect the LBB ratio are the crack size, crack opening displacement, leak rate, and crack stability models. The probabilistic result is a distribution of the LBB ratio conditional on having cracks that leak and a seismic event that ruptures the pipe.

It is expressed as a CDF based on the 80-EFPY evaluation period. Because it assumes that the seismic loads are applied in every time step, an LBB ratio greater than one indicates that leak rate detection precludes seismic rupture. Additionally, since the combined normal operating and seismic loads are greater than the normal operating loads by themselves, rupture under normal operating loads is also precluded when the LBB ratio is greater than one.

2.3 Statistical Approach 2.3.1 Sample Size An annual failure frequency of less than 1 x 10-6 was used as the acceptance criterion in this study. Such a threshold is consistent with the basis for the GDC 4 rulemaking. Considering that the evaluation period is 80 EFPY consistent with the prior study, it was estimated that a sample size of 100,000 is necessary to guarantee that any undesirable event will not be missed in the analysis.

The probability of having at least one unwanted outcome, (e.g., pipe rupture), with an annual frequency, , occurring over a period of years is shown as follows:

, 1 1 Equation 1 15

Using Equation 1, the probability of an event with an annual frequency of 1 x 10-6 to occur over 80 EPFY is roughly , 1 8 10 . Further, a sample size of for an event, , with a probability of occurrence, , leads to expected events. The probability of not generating a single event (i.e., missing the likelihood of an event whose probability of occurrence is ) can be estimated as follows:

0 1 1 0 Equation 2 Then, let represent an event with an unwanted outcome of annual frequency 10 occurring over a period of 80 years. The expected number of events and probabilities of not generating any events for sample sizes of 10,000; 50,000; 70,000; and 100,000 are respectively as follows:

10 0.8; 0.45 44.93%

50 4; 0.018 1.83%

70 5.6; 0.0037 0.37%

100 8; 0.0003 0.03%

Considering these estimates, a sample size of 10,000 is not enough to confidently capture events with an annual frequency of 1 x 10-6, and a sample size of at least 50,000 would be necessary to have a 98 percent confidence level (i.e., only a 2 percent error).

Alternatively, if some margin is desired over the annual frequency and, for instance, the sample size is expected to capture events with an annual frequency of 5 x 10-7, then let represent an event with an unwanted outcome of annual frequency 10 occurring over a period of 80 years. The expected number of events and probabilities of not generating any events considering the same sample sizes as before are respectively as follows:

10 0.4 ; 0.67 67.03%

50 2; 0.14 13.53%

70 2.8 ; 0.06 6.08%

100 4; 0.02 1.83%

Under these conditions, a sample size of 100,000 would be necessary to have a 98 percent confidence level. A traditional threshold in statistics is to use a 95 percent confidence level (i.e.,

only a 5 percent error). Accordingly, a sample size of 100,000 was determined to be the most appropriate for the desired level of accuracy in this study.

Furthermore, due to the age of the piping systems under consideration and the fact that most welds have not experienced any detectable PWSCC, it is possible that estimation of the annual 16

frequency will not be based on the entire 80-EFPY evaluation period, but rather on the final years of plant operation as a predictive frequency. Since mechanistically there are periods of time between crack initiation, leakage, and rupture, such a predictive frequency is expected to be higher than a frequency based on the entire 80-EFPY evaluation period.

With a shorter simulation time, a larger sample size is required to confirm that an event with an unwanted outcome and an annual frequency of 1 x 10-6 does not occur. Suppose, for instance, that represents an event with an unwanted outcome of annual frequency 10 occurring over a period of 30 years. The expected number of events and probabilities of not generating any events considering the same sample sizes as before are respectively as follows:

10 0.3 ; 0.74 74.08%

50 1.5 ; 0.22 22.31%

70 2.1 ; 0.13 12.25%

100 3.0 ; 0.05 4.98%

Under these conditions, any sample size below 100,000 would lead to a confidence level lower than 95 percent.

In conclusion, a sample size of 100,000 is appropriate to confidently demonstrate that events with unwanted outcomes have annual frequencies of occurrence lower than 1 x 10-6. This sample size was used for all the simulations in this study that used Direct Model 1 for crack initiation. The adequacy of this PWSCC initiation model was demonstrated through the sensitivity studies performed in the prior study. A sample of size 5,000 was used for all simulations that used the initial flaw density option (i.e., pre-existing cracks) conditional on the assumption that the probability of having a circumferential crack occurring over the 80-EFPY evaluation period will be at most 0.05 (i.e., 5,000 occurrences of crack out of 100,000 samples).

2.3.2 Sampling Loop and Random Seed As demonstrated in the prior study [2], using a single loop consisting entirely of either the aleatory (inner) or epistemic (outer) loop produced statistically equivalent results. As a result, 7 separate simulations with 15,000 realizations on the epistemic (outer) loop were generally used to produce a 105,000-realization composite simulation. Latin hypercube sampling (LHS) was used to take advantage of its denser stratification of each uncertain input distribution. A similar approach was used for the present study, except that the number of realizations was reduced to 10,000 and the number of replicate simulations was increased to 10. Such an approach produced 100,000-realization composite simulations when Direct Model 1 was used. A single, 5,000-realization simulation was used again for simulations that used the initial flaw density option.

The GoldSim random number generator has been tested extensively, and these tests confirmed that the choice of random seeds does not affect the statistical results, even when 17

consecutive random seeds are selected. While only the epistemic (outer) loop was used, and the quasi-totality* of the uncertain parameters was set to epistemic, both the epistemic and aleatory random seeds were changed for each replicate simulation. The random seeds used in each simulation are recorded in Appendix B.

2.4 Computational Platforms and Simulation Execution Strategy All the analyses were executed on the computational platforms described in Table 2-2. Nearly all the simulations used GoldSim Pro with its parallel processing capabilities to decrease code run times.

Table 2-2 Computational platforms Platform 1 Platform 2 Platform 3 Random-Access 32 GB 32 GB 16GB Memory Central Intel Core' i9- Intel Core' i9- Intel Core' i7-Processing Unit 9920X @ 3.5GHz 10920X @ 3.5GHz 4790X @ 3.6GHz Operating Microsoft Windows Microsoft Windows Microsoft Windows System 10 Pro 10 Pro 10 Pro Disk Drive Solid State Drive Solid State Drive Hard Disk Drive GoldSim License GoldSim Pro GoldSim Pro GoldSim Pro GoldSim Version 11.1.7 11.1.7 11.1.7 2.5 Project Team This study was facilitated in part through a collaborative effort between the NRCs Office of Nuclear Regulatory Research and EPRI under an addendum to their general memorandum of understanding on cooperative nuclear safety research [46]. Separate NRC and EPRI analysis teams were formed, which included NRC staff, EPRI staff, and their contractors. Case definition and data collection activities necessary to gather sources for inputs were largely a cooperative effort. The teams were then left to independently complete the agreed upon scope of analyses.

All results were shared between the teams, but their ultimate presentation and the conclusions drawn were strictly an independent exercise. This report presents the NRC teams results and conclusions.

The xLPR code only supports an aleatory uncertainty type for the fatigue crack initiation model parameter Co (Global ID 2528).

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2.6 Necessary Code Corrections and Modifications Several xLPR code problems were corrected and improvements were implemented for the prior study. In summary, these changes were as follows:

Corrected a problem that led to double counting the pressure in the circumferential crack opening displacement calculations.

Corrected a problem that led to double counting the pressure in the circumferential surface crack and circumferential TWC stability calculations.

Extended the range of validity of the axial crack opening displacement module to provide more accurate calculations for low values of , which represents the ratio between the half-crack length and the square root of the median radius multiplied by the thickness (i.e., / ).

Corrected a problem that affected the results conditional on ISI when a rupture occurred in the same time step as an inspection.

Corrected a problem that affected the ISI results when the option to consider only the deepest surface crack was selected (Global ID 0820).

All these changes have been included in the version of the xLPR code used for the runs in the present study. The major version including all these changes was xLPR v2.0d, and the minor version was xLPR v2.0d_002. The minor version removes unused output results to reduce the final file size and includes additional outputs relevant to the present study. These version designations are specific to this study.

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3 ANALYSES 3.1 Scope The scope of analyses performed is summarized in the case matrix shown in Table 3-1. The bins and cases were selected and defined by the NRC and EPRI project teams. Appendix A summarizes the results for each case. The analysis inputs are listed in Appendix B.

Sections 3.1.1 through 3.1.5 describe the cases selected for each bin. Sections 3.2 through 3.8 provide the results for each bin.

3.1.1 Bases Cases with PWSCC Initiation and Growth A base case analysis was performed for all bins to assess the likelihood of failure due to PWSCC initiation and growth for both circumferential and axial cracks. The effects of leak detection, ISI, and SSE events were also assessed in these analyses.

3.1.2 Initial Flaw Sensitivity Study Cases A sensitivity study was performed for all bins where, instead of Direct Model 1 for crack initiation, pre-existing flaws were assumed in both the axial and circumferential orientations and subject to PWSCC growth. These cases were included so that the impact of having a crack in the weld could be more accurately assessed from a larger number of realizations with cracks.

Note that all these probabilities are conditional on having a crack at the beginning of the simulation, and they should not be interpreted without correction from a risk standpoint. The effects of leak rate detection, ISI, and seismic events were also assessed in these analyses.

The LBB time lapse and LBB ratio QoIs are conditional on having a crack occurring and are not affected by crack initiation. As a result, it was expected that the resulting CDFs would be similar between the base case and the initial flaw sensitivity study case, with the latter providing a finer representation because of more realizations with ruptures.

3.1.3 More Severe WRS Sensitivity Study Cases A sensitivity study was performed considering a more severe, yet plausible, WRS profile for Bins 2, 3, 4 and 5. The definition of severity was based on factors that would more likely lead to rupture using engineering judgement and operating experience. In all cases, to ensure plausibility, the more severe WRS profile was based on either another modelers results for the same weld geometry or from another location in the weld (i.e., in the butter rather than the weld centerline). The WRS profiles used in for each bin are documented and summarized in Appendix C.

3.1.4 Mechanical Mitigation Sensitivity Study Cases Sensitivity studies were performed applying mechanical mitigation for Bins 2 and 4. These analyses simulated the effects of overlays and the mechanical stress improvement process (MSIP). Data on overlays applied to existing operating plants were assessed to determine an 20

average time of application and overlay thickness. MSIP mitigation used the rules from the xLPR WRS Subgroup report [47] to develop a post-mitigation WRS profile, and the time of application was determined from the operating plant records. Additionally, a sensitivity study was performed for Bin 4 (i.e., for Westinghouse steam generator nozzle DMWs), where an inlay was applied in the first month of operation to represent DMWs in this bin that were placed into service with the inlay already applied. Finite element analysis (FEA) was performed to generate the WRS profile for this case as detailed in Appendix C.

3.1.5 Fatigue Sensitivity Study Cases A sensitivity study was performed for Bin 2 to consider the impacts of fatigue crack initiation and growth. The pressurizer surge line experiences thermal stratification transients and insurge-outsurge transients that are not present in the primary coolant loop piping. Therefore, this analysis assessed the impact of these transients on the resulting probabilities. Fatigue sensitivity study cases were not included for the other bins based on the findings from the prior study.

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Table 3-1 Summary of analysis cases Weld Bin Case Report Crack Crack Initiation Crack Objective No. Section Orientations Method Growth Mechanism 1.1.6a Assess the base likelihood of failure caused by Circumferential PWSCC and 3.2.1 PWSCC PWSCC initiation and growth without mechanical Bin 1: and Axial (Direct Model 1) 1.1.6c mitigation Westinghouse 4-loop RVON Initial Flaw Assess the base likelihood of failure with pre-and RVIN Circumferential Density existing flaws and subsequent PWSCC growth of DMWs 1.1.6b 3.2.2 PWSCC and Axial (1 Axial and 1 circumferential and axial cracks without mechanical Circ. Crack) mitigation Assess the base likelihood of failure caused by Circumferential PWSCC 2.1.0 3.3.1 PWSCC PWSCC initiation and growth without mechanical and Axial (Direct Model 1) mitigation Initial Flaw Assess the base likelihood of failure with pre-Circumferential Density existing flaws and subsequent PWSCC growth of 2.1.1 3.3.2 PWSCC and Axial (1 Axial and 1 circumferential and axial cracks without mechanical Circ. Crack) mitigation Bin 2: Circumferential PWSCC Sensitivity study of Case 2.1.0 considering a more Westinghouse 2.1.2 3.3.3 PWSCC and Axial (Direct Model 1) severe WRS profile Pressurizer Surge Line Circumferential PWSCC Sensitivity study of Case 2.1.0 considering overlay 2.1.3 3.3.4 PWSCC Nozzle DMWs and Axial (Direct Model 1) mitigation PWSCC Assess the base likelihood of failure caused by Circumferential PWSCC &

2.1.4 3.3.5 (Direct Model 1) & fatigue initiation and growth without mechanical and Axial Fatigue Fatigue mitigation Circumferential PWSCC Sensitivity study of Case 2.1.0 considering MSIP 2.1.5 3.3.6 PWSCC and Axial (Direct Model 1) mitigation 22

Weld Bin Case Report Crack Crack Initiation Crack Objective No. Section Orientations Method Growth Mechanism Assess the base likelihood of failure caused by Circumferential PWSCC 3.1.0 3.4.1 PWSCC PWSCC initiation and growth without mechanical and Axial (Direct Model 1) mitigation Bin 3: Initial Flaw Assess the base likelihood of failure with pre-CE and B&W Circumferential Density existing flaws and subsequent PWSCC growth of 3.1.1 3.4.2 PWSCC RCP Nozzle and Axial (1 Axial and 1 circumferential and axial cracks without mechanical DMWs Circ. Crack) mitigation Circumferential PWSCC Sensitivity study of Case 3.1.0 considering a more 3.1.2 3.4.3 PWSCC and Axial (Direct Model 1) severe WRS profile Circumferential PWSCC Assess the base likelihood of failure caused by 4.1.0 3.5.1 PWSCC and Axial (Direct Model 1) PWSCC initiation and growth with inlay mitigation Initial Flaw Assess the base likelihood of failure with pre-Circumferential Density existing flaws and subsequent PWSCC growth of 4.1.1 3.5.2 PWSCC Bin 4: and Axial (1 Axial and 1 circumferential and axial cracks with inlay Westinghouse Circ. Crack) mitigation Steam Circumferential PWSCC Sensitivity study of Case 4.1.0 considering a more Generator 4.1.2 3.5.3 PWSCC and Axial (Direct Model 1) severe WRS profile Nozzle DMWs Circumferential PWSCC Sensitivity study of Case 4.1.0 considering overlay 4.1.3 3.5.4 PWSCC and Axial (Direct Model 1) instead of inlay mitigation Circumferential PWSCC Sensitivity study of Case 4.1.0 without mechanical 4.1.4 3.5.5 PWSCC and Axial (Direct Model 1) mitigation 23

Weld Bin Case Report Crack Crack Initiation Crack Objective No. Section Orientations Method Growth Mechanism Assess the base likelihood of failure caused by Circumferential PWSCC 5.1.0 3.6.1 PWSCC PWSCC initiation and growth without mechanical and Axial (Direct Model 1) mitigation Bin 5a:

Initial Flaw Assess the base likelihood of failure with pre-CE Hot Leg Circumferential Density existing flaws and subsequent PWSCC growth of Branch Line 5.1.1 3.6.2 PWSCC and Axial (1 Axial and 1 circumferential and axial cracks without mechanical Nozzle DMWs Circ. Crack) mitigation Circumferential PWSCC Sensitivity study of Case 5.1.0 considering a more 5.1.2 3.6.3 PWSCC and Axial (Direct Model 1) severe WRS profile Assess the base likelihood of failure caused by Circumferential PWSCC 5.2.0 3.7.1 PWSCC PWSCC initiation and growth without mechanical Bin 5b: and Axial (Direct Model 1) mitigation CE Cold Leg Branch Line Initial Flaw Assess the base likelihood of failure with pre-Nozzle DMWs Circumferential Density existing flaws and subsequent PWSCC growth of 5.2.1 3.7.2 PWSCC and Axial (1 Axial and 1 circumferential and axial cracks without mechanical Circ. Crack) mitigation Assess the base likelihood of failure caused by Circumferential PWSCC Bin 6: 1.3.0 3.8.1 PWSCC PWSCC initiation and growth without mechanical and Axial (Direct Model 1)

Westinghouse mitigation 2- and 3-loop Initial Flaw Assess the base likelihood of failure with pre-RVON and Circumferential Density existing flaws and subsequent PWSCC growth of RVIN DMWs 1.3.1 3.8.2 PWSCC and Axial (1 Axial and 1 circumferential and axial cracks without mechanical Circ. Crack) mitigation 24

3.2 Bin 1: Westinghouse Four-Loop RVON and RVIN DMWs Westinghouse four-loop RVON and RVIN DMWs were analyzed in the prior study [2].

Cases 1.1.6a, 1.1.6b, and 1.1.6c were created to represent these welds in the present study.

They are equivalent to the base case (i.e., Case 1.1.0) in the prior study with the inclusion of axial cracks and with the hydrogen concentration in the reactor coolant reduced from 37 cubic centimeters per kilogram (cc/kg) to 25 cc/kg. Using the reduced hydrogen concentration is a bounding approach as it increases crack growth rates and represents the lower bound of the PWR operating conditions as described in EPRI Technical Report 1022852, Materials Reliability Program: Probabilistic Assessment of Chemical Mitigation of Primary Water Stress Corrosion Cracking in Nickel-Base Alloys (MRP-307), issued June 29, 2011 [48].

The Westinghouse four-loop RVON base cases for this study (i.e., Cases 1.1.6a and 1.1.6c),

and the corresponding initial flaw sensitivity study case (i.e., Case 1.1.6b), are described in Sections 3.2.1 and 3.2.2, respectively. Cases 1.1.6a and 1.1.6c are equivalent, except that the former uses a 10-year inspection frequency, and the latter uses a 5-year inspection frequency.

The different inspection frequencies were considered to assess the impact of the length of time between inspections.

3.2.1 Base Case 3.2.1.1 Case Description The objective of Case 1.1.6a was to generate a new reference case for Westinghouse four-loop RVON DMWs to establish a point of comparison for all the cases in the present study that use Direct Model 1 for crack initiation. Case 1.1.6a is based on Cases 1.1.6 and 1.1.14 from the prior study, which were sensitivity studies on the effects of axial cracks and dissolved hydrogen concentration, respectively. Figure 3-1 shows the WRS profiles used to analyze Case 1.1.6a.

They are the same WRS profiles that were used in Case 1.1.0 from the prior study and are described in Section C6.1. Section B1 describes the specific inputs and other simulation details used to analyze the case.

Case 1.1.6c is like Case 1.1.6a only with a different inspection frequency to assess the impact of the length of time between inspections. The inspection frequency in Case 1.1.6a was set to 1 every 10 years; whereas, in Case 1.1.6c it was set to 1 every 5 years consistent with American Society of Mechanical Engineers (ASME) Code Case N-770-5, Alternative Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated With UNS N06082 or UNS W86182 Weld Filler Material With or Without Application of Listed Mitigation Activities, approved November 7, 2016 [49]. This code case is mandated by the NRC in 10 CFR 50.55a(g)(6)(ii)(F). The outputs affected by this change are the time-dependent probabilities of leakage with ISI and rupture with ISI.

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Figure 3-1 Cases 1.1.6a and 1.1.6c WRS profiles 3.2.1.2 Results and Analysis 3.2.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.2.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.2.1.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

36.6 +/- 1.2 months (minimum observed: 14 months) 25.1 +/- 0.9 months (minimum observed: 9 months)

All LBB time lapses beyond 12 EFPY were conservatively excluded as they were found to strongly influence the mean. Such a long period of time is also not of practical interest. For example, if the LBB time lapse is greater than 12 EFPY, it is typically due to a slow-growing crack, an arrested crack, or a crack that has been mechanically mitigated, if applied.

Figure 3-2 shows the LBB time lapse CDFs for Case 1.1.6a compared with Case 1.1.0 from the prior study. The leftward shift for the Case 1.1.6a results is due to the bounding hydrogen concentration used in the analysis.

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Figure 3-2 Case 1.1.6a LBB time lapse results 3.2.1.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

9.64 +/- 0.07 (minimum observed: 1.66) 4.58 +/- 0.02 (minimum observed: 1.56)

Figure 3-3 shows the LBB ratio CDFs for Case 1.1.6a compared with Case 1.1.0 from the prior study. As can be observed, the LBB ratio distributions are not affected by the hydrogen concentration, and thus the Case 1.1.6a results lie on top of the results for Case 1.1.0.

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Figure 3-3 Case 1.1.6a LBB ratio results 3.2.1.2.5 Standard Indicators Figure 3-4 shows the probabilities of first crack and first circumferential crack for Case 1.1.6a.

The probabilities are consistent with the values reported for Case 1.1.6 in the prior study [2].

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Figure 3-4 Case 1.1.6a time-dependent probabilities of first crack Figure 3-5 shows the probabilities of first leak, first circumferential crack leak, and first leak with ISI for Case 1.1.6a with a 10-year inspection frequency. The values are slightly higher than the ones reported in the prior study because of the bounding hydrogen concentration, which leads to increased crack growth rates.

Figure 3-5 Case 1.1.6a time-dependent probabilities of first leak with a 10-year inspection frequency 29

Figure 3-6 shows the probabilities of first leak, first circumferential crack leak, and first leak with ISI for Cases 1.1.6a and 1.1.6c, which considered 10- and 5-year inspection frequencies, respectively. As expected, only the probability of first leak with ISI is affected. The more frequent inspections in Case 1.1.6c reduce the probability of first leak to 2.92 x 10-5 over 80 EFPY, which corresponds to an annual frequency of 3.65 x 10-7.

Figure 3-6 Comparison of Case 1.1.6c time-dependent probabilities of first leak, first circumferential leak, and first leak with ISI with Case 1.1.6a Figure 3-7 shows the probabilities of rupture, rupture with SSE, and rupture with ISI for Case 1.1.6a, which considered a 10-year inspection frequency. There were no occurrences of rupture over 80 EFPY when leak detection is considered.

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Figure 3-7 Case 1.1.6a time-dependent probabilities of rupture with a 10-year inspection frequency Figure 3-8 shows the probabilities of rupture and rupture with ISI for Cases 1.1.6a and 1.1.6c, which considered 10- and 5-year inspection frequencies, respectively. As expected, only the probability of rupture with ISI is affected. The more frequent inspections in Case 1.1.6c reduce the probability of rupture with ISI to 1.71 x 10-6 over 80 EFPY, which corresponds to an annual frequency of 2.14 x 10-8.

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Figure 3-8 Comparison of Case 1.1.6c time-dependent probabilities of rupture and rupture with ISI with Case 1.1.6a 3.2.2 Initial Flaws 3.2.2.1 Case Description The objective of Case 1.1.6b was to generate a new reference case for Westinghouse four-loop RVON DMWs to establish a point of comparison for all the cases in the present study that consider pre-existing flaws. This case uses the same inputs as Case 1.1.6a except that, instead of Direct Model 1 for crack initiation, it uses pre-existing axial and circumferential flaws.

Section B2 describes the specific inputs and other simulation details used to analyze the case.

3.2.2.2 Results and Analysis 3.2.2.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.2.2.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.2.2.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

38.66 +/- 0.26 months (minimum observed: 10 months) 26.43 +/- 0.18 months (minimum observed: 7 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-9 shows the LBB time lapse CDFs for Case 1.1.6b as compared to Case 1.1.6a. The results lie on top of each other, which indicates that starting with an existing crack does not affect this QoI.

Figure 3-9 Case 1.1.6b LBB time lapse results 33

3.2.2.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

9.86 +/- 0.01 (minimum observed: 7.71) 4.64 +/- 0.00* (minimum observed: 3.99)

Figure 3-10 shows the LBB ratio CDF plots for Cases 1.1.6a and 1.1.6b. There is good agreement between the results of the two cases.

Figure 3-10 Case 1.1.6b LBB ratio results 3.2.2.2.5 Standard Indicators Figure 3-11 shows the probabilities of first leak for Case 1.1.6b. The results for the circumferential cracks are consistent with the observations from the Case 1.1.14 sensitivity study on the hydrogen concentration from the prior study. The 10-year inspection frequency reduces the probability by about one order of magnitude after 80 EFPY.

The standard errors are reported with 2-digit accuracy. A standard error of 0.00 is lower than 0.01.

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Figure 3-11 Case 1.1.6b time-dependent probabilities of first leak Figure 3-12 shows the probabilities of rupture for Case 1.1.6b. There were no occurrences of rupture over 80 EFPY when leak detection is considered.

Figure 3-12 Case 1.1.6b time-dependent probabilities of rupture 35

3.3 Bin 2: Westinghouse Pressurizer Surge Line Nozzle DMWs The following cases were used to analyze the Westinghouse pressurizer surge line nozzle DMWs represented by Bin 2:

Case 2.1.0: base case Case 2.1.1: initial flaws Case 2.1.2: more severe WRS Case 2.1.3: overlay mitigation Case 2.1.4: fatigue Case 2.1.5: MSIP mitigation The cases and associated analyses are described in Sections 3.3.1 through 3.3.6, respectively.

3.3.1 Base Case 3.3.1.1 Case Description The objective of Case 2.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used bounding values for the geometry and loading, both normal operating and SSE stresses, based on the information from the applicable references in Table 2-1. The ISI parameters are from the xLPR ISI module validation report [50]. Figure 3-13 shows the mean WRS profiles used to analyze the case. They were developed using FEA for a representative pressurizer surge line nozzle with a fill-in type weld geometry. Figure C-1 illustrates the geometry in detail, and further details on the development of the WRS profiles are in Section C2.1. Section B4 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-13 Case 2.1.0 WRS profiles 3.3.1.2 Results and Analysis 3.3.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.3.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.3.1.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

6.50 +/- 0.67 months (minimum observed: 4 months) 1.08 +/- 0.08 months (minimum observed: 1 month)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-14 shows the LBB time lapse CDFs for Case 2.1.0. The values are a lot lower as compared with Case 1.1.6a, which indicate shorter times from detectable leakage to rupture in the case of the pressurizer surge line nozzle DMWs. This result is because of the smaller diameter of the pressurizer surge line, which leads to larger crack sizes in proportion to the circumference to reach 1 or 10 gpm leak rates. The proportionally larger crack sizes are closer 37

both in time and size to the critical crack size, as shown in the supplemental analysis in Section 3.3.2.3. The result can also be attributed to the higher operating temperature of the pressurizer surge line.

Figure 3-14 Case 2.1.0 LBB time lapse results 3.3.1.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

4.42 +/- 0.12 (minimum observed: 3.73) 1.99 +/- 0.07 (minimum observed: 1.62)

Figure 3-15 shows the LBB ratio CDFs for Case 2.1.0. As for the LBB time lapse CDFs, the values are lower than Case 1.1.6a due to the smaller diameter of the pressurizer surge line.

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Figure 3-15 Case 2.1.0 LBB ratio results 3.3.1.2.5 Standard Indicators Figure 3-16 shows the probabilities of first crack for Case 2.1.0 as compared with Case 1.1.6a.

The probability of first crack is higher by a factor of 2 due to axial cracks; however, the probability of circumferential crack is lower by one order of magnitude.

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Figure 3-16 Case 2.1.0 time-dependent probabilities of first crack Figure 3-17 shows the probabilities of first leak for Case 2.1.0 as compared with Case 1.1.6a.

As with the probability of first crack, the probability of leakage is higher due to the inclusion of axial cracks. The impact of a 10-year inspection frequency is reduced due to a sharp rise in probability during the first 10 EFPY. With an increased inspection frequency for the surge line as is currently required by ASME Code Case N-770-5, which is mandated by 10 CFR 50.55a(g)(6)(ii)(F), the impact of ISI would be more pronounced. However, since the main concern is rupture and not leakage, it is not necessary to revisit the 10-year inspection frequency used for the analysis. The probability of leakage due to only circumferential cracks is lower by more than one order of magnitude, which is consistent with the probability of first leak results. This reduction is attributed, in part, to the difference between the mean axial and hoop WRS at the inside diameter of the weld.

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Figure 3-17 Case 2.1.0 time-dependent probabilities of first leak Figure 3-18 shows the probabilities of rupture for Case 2.1.0 as compared to Case 1.1.6a.

These probabilities are more than one order of magnitude lower for Case 2.1.0. It should be noted that the inspection frequency was set to one inspection every 10 years to give a consistent comparison for all the welds in this study, and currently the pressurizer surge line welds are required by 10 CFR 50.55a(g)(6)(ii)(F) to be inspected more frequently (i.e., every other refueling outage, which is approximately every 3 to 4 years). The probability of rupture when ISI is considered is lower than 1 x 10-6 ruptures per year with the 10-year inspection frequency, and it is expected to be even lower with the current NRC inspection requirements.

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Figure 3-18 Case 2.1.0 time-dependent probabilities of rupture 3.3.2 Initial Flaws 3.3.2.1 Case Description The objective of Case 2.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation. This case uses the same inputs as Case 2.1.0 except that, instead of Direct Model 1 for crack initiation, it uses pre-existing axial and circumferential flaws. The WRS profiles were the same as in the Case 2.1.0 analysis. Section B5 describes the specific inputs and other simulation details used to analyze the case.

3.3.2.2 Results and Analysis 3.3.2.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.3.2.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.3.2.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

6.59 +/- 0.07 months (minimum observed: 2 months) 1.28 +/- 0.02 months (minimum observed: 0 month)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-19 shows the LBB time lapse CDFs from Case 2.1.1 as compared to Case 2.1.0.

Consistent with the expectations described in Section 3.1.2, the CDFs are similar. The reason for the low LBB time lapses is described in Section 3.3.2.3.

Figure 3-19 Case 2.1.1 LBB time lapse results 3.3.2.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

4.49 +/- 0.02 (minimum observed: 2.48) 1.99 +/- 0.01 (minimum observed: 1.26) 43

Figure 3-20 shows the LBB ratio CDF plots for Case 2.1.1. As expected, there is good agreement with the results from Case 2.1.0.

Figure 3-20 Case 2.1.1 LBB ratio results 3.3.2.2.5 Standard Indicators Figure 3-21 shows the probabilities of first leak for Case 2.1.1 as compared with Case 1.1.6b.

The rapid crack growth over the first 10 EPFY greatly reduces the impact of inspections when conducted every 10 years. More frequent inspections, as are currently required by 10 CFR 50.55a(g)(6)(ii)(F), would show a greater incidence on the results.

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Figure 3-21 Case 2.1.1 time-dependent probabilities of first leak Figure 3-22 shows the probabilities of rupture for Case 2.1.1 as compared with Case 1.1.6b. In smaller diameter piping, circumferential cracks take less time to reach the critical crack size, which leads to higher probabilities of rupture at earlier times as compared to larger diameter piping, such as the Westinghouse RVON DMW, when every realization starts with an existing circumferential crack. As a result, the probability of rupture with ISI is also higher in Case 2.1.1.

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Figure 3-22 Case 2.1.1 time-dependent probabilities of rupture 3.3.2.3 Supplemental Analyses To illustrate the impact of the pipe diameter on the leak rate, distributions of the circumferential crack leak rates at (a) the time of first circumferential leak occurrence, and (b) the time step before rupture, were estimated and compared to similar distributions from Case 1.1.6b. The Case 2.1.1 results were used instead of the results from Case 2.1.0 because more instances of leakage were generated, and both Figure 3-19 and Figure 3-20 confirm that the two cases lead to similar results. Figure 3-23 shows the distribution of leak rate at the time of first circumferential leak for both Cases 2.1.1 and 1.1.6b. The leak rates for the larger diameter pipe are, on average, about 3 times greater.

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Figure 3-23 Comparison of Cases 2.1.1 and 1.1.6b first circumferential leak rates Figure 3-24 shows the distribution of leak rate one month before rupture for both Cases 2.1.1 and 1.1.6b. A logarithmic scale is used on the horizontal axis denoting that the difference between the two cases is about two orders of magnitude. This analysis shows that smaller diameter pipes are more sensitive to the detectable leak rate.

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Figure 3-24 Comparison of Cases 2.1.1 and 1.1.6b leak rates one month before rupture 3.3.3 More Severe WRS 3.3.3.1 Case Description Case 2.1.2 was a sensitivity study of Case 2.1.0 considering a more severe WRS profile. This case used the same inputs as Case 2.1.0 but with a change to the mean hoop and axial WRS profiles. The standard deviations used to represent uncertainties in the WRS profiles were the same as in Case 2.1.0. Figure 3-25 shows the WRS profiles used to analyze the case. These profiles were developed using the same FEA that was used to develop the WRS profiles for Case 2.1.0; however, for Case 2.1.2, the WRS profiles were extracted from the weld butter rather than from the weld centerline. The WRS profile is considered more severe because the higher inside diameter stress favors PWSCC initiation, which has been shown through prior sensitivity analyses to have a large influence on the probability of rupture as documented in TLR-RES/DE/CIB-2021-11, Sensitivity Studies and Analyses Involving the Extremely Low Probability of Rupture Code, issued May 14, 2021 [51]. Additional details on development of the WRS profiles are in Section C2.2. Section B6 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-25 Case 2.1.2 WRS Profiles 3.3.3.2 Results and Analysis 3.3.3.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.3.3.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.3.3.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

4.58 +/- 0.22 months (minimum observed: 1 month) 1.30 +/- 0.08 months (minimum observed: 0 month)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-26 shows the LBB time lapse CDF plots for Case 2.1.2. As compared to the Case 2.1.1 results, the LBB time lapse is reduced when the more severe WRS profile is used, especially for a 1 gpm leak rate detection capability.

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Figure 3-26 Case 2.1.2 LBB lapse time results 3.3.3.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

3.70 +/- 0.07 (minimum observed: 1.95) 2.01 +/- 0.03 (minimum observed: 1.43)

Figure 3-27 shows the LBB ratio CDF plots for Case 2.1.2. As compared to the Case 2.1.1 results, the LBB ratios for a 1 gpm leak rate detection capability are lower with the more severe WRS profile.

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Figure 3-27 Case 2.1.2 LBB ratio results 3.3.3.2.5 Standard Indicators Figure 3-28 shows the probabilities of first crack for Case 2.1.2 as compared with Case 2.1.0.

The more severe WRS profile in Case 2.1.2 leads to increases in both the probabilities of first crack and first circumferential crack.

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Figure 3-28 Case 2.1.2 time-dependent probabilities of first crack Figure 3-29 shows the probabilities of first leak for Case 2.1.2 as compared with Case 2.1.0. As with the probabilities of first crack, the probabilities of first leak are greater by roughly one order of magnitude.

Figure 3-29 Case 2.1.2 time-dependent probabilities of first leak 52

Figure 3-30 shows the probabilities of rupture for Case 2.1.2 as compared with Case 2.1.0. The probabilities for Case 2.1.2 are more than one order of magnitude lower.

Figure 3-30 Case 2.1.2 time-dependent probabilities of rupture 3.3.4 Overlay Mitigation 3.3.4.1 Case Description Case 2.1.3 was a sensitivity study of Case 2.1.0 considering overlay mitigation. Most of the pressurizer surge line nozzle DMWs represented by Bin 2 have overlays. In the analysis, the overlay was applied at 25 EFPY based on the average application time for the DMWs represented by Bin 2. The overlay thickness was set to 12.5 centimeters (cm), which was the minimum thickness of all welds represented by the bin. Figure 3-31 shows the WRS profiles used to analyze the case. They were developed by applying the mechanical mitigation rules from the xLPR WRS Subgroup report [47] to the WRS profiles used for Case 2.1.0. Section B7 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-31 Case 2.1.3 WRS profiles 3.3.4.2 Results and Analysis 3.3.4.2.1 Probability of Rupture with Detection Two out of 100,000 realizations had ruptures with a 1 gpm leak rate detection capability in Case 2.1.3. While it may seem counterintuitive, the application of the overlay is the cause of these ruptures. As modeled in the xLPR code, any existing crack in the original weld material will continue to grow through the weld thickness, and it will not be stopped at the interface between the original weld and the overlay. Thus, the cracks can grow through the PWSCC-susceptible Alloy 82/182 weld to the more PWSCC-resistant Alloy 52/152 overlay. At this point, crack growth in the depth direction slows in the overlay because of the PWSCC growth factor of improvement (FOI), which was set to 324 (i.e., PWSCC growth in the Alloy 52/152 overlay was modeled to occur 324 times more slowly than PWSCC growth in the Alloy 82/182 original weld metal). The FOI of 324 represents the 75th percentile FOI as recommended in EPRI Technical Report 3002010756, Materials Reliability Program: Recommended Factors of Improvement for Evaluating Primary Water Stress Corrosion Cracking (PWSCC) Growth Rates of Thick-Wall Alloy 690 Materials and Alloy 52, 152, and Variants Welds (MRP 386), issued December 22, 2017 [52]. However, the faster PWSCC growth continues in the Alloy 82/182 weld around the circumference, and eventually the cracks become unstable when they grow large enough. One of the two cases was because of a surface crack rupture. In the other case, rupture occurred as soon as the crack grew through-wall.

The associated annual frequency of rupture is 2.5 x 10-7, which is below the 1 x 10-6 acceptance threshold considered in this study. When the effects of a 10-year inspection frequency are considered, the annual frequency is reduced to 1.25 x 10-9 (i.e., essentially zero) after 80 EFPY.

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The probability of surface crack rupture when a 10-year inspection frequency is considered is lower than 1.0 x 10-10 (i.e., essentially zero). More frequent inspections per ASME Code Case N-770-5, which is currently mandated by 10 CFR 50.55a(g)(6)(ii)(F), would further reduce the probability of surface crack rupture.

Figure 3-32 Case 2.1.3 time-dependent probabilities of rupture with leak rate detection and ISI 3.3.4.2.2 Leak Rate Jump Two realizations out of 100,000 had a leak rate jump, giving a probability of leak rate jump of 2 x 10-5. These correspond with the two realizations that had ruptures with leak rate detection as discussed in Section 3.3.4.2.1.

3.3.4.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

10.8 +/- 5.7 months (minimum observed: 0 month) 8.6 +/- 4.9 months (minimum observed: 0 month)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-33 shows the LBB time lapse CDF plots for Case 2.1.3. The low number of ruptures does not allow for an accurate representation of the CDF. However, the tendencies are to have 55

(a) a shorter LBB time lapse when the rupture occurs before 25 EFPY when the overlay is applied, and (b) a longer LBB time lapse for the few ruptures that occur after 25 EFPY.

Figure 3-33 Case 2.1.3 LBB time lapse results 3.3.4.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

2.06 +/- 0.35 (minimum observed: 0.85) 1.39 +/- 0.11 (minimum observed: 0.85)

Figure 3-34 shows the LBB ratio CDF plots for Case 2.1.3. The low number of ruptures does not provide an accurate representation of the CDF. The distributions are lower as compared to Case 2.1.0 because most of the higher LBB ratios in Case 2.1.0 happen at later times.

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Figure 3-34 Case 2.1.3 LBB ratio results 3.3.4.2.5 Standard Indicators Figure 3-35 shows the probabilities of first crack for Case 2.1.3 as compared to Case 2.1.0.

The overlay applied at 25 EFPY is too late to have much effect on the overall probability of first crack, whereas it leads to an increase in the probability of first circumferential crack.

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Figure 3-35 Case 2.1.3 time-dependent probabilities of first crack Figure 3-36 shows the probabilities of first leak for Case 2.1.3 as compared with Case 2.1.0.

The impact of the overlay on the probability of first leak is strong, which leads to a strong delay for the increase in crack size. The decrease in probability is because of the repair of any TWCs when the overlay is applied. The increase in circumferential crack leakage is consistent with the increase in circumferential crack initiation. However, the overlay delays the occurrence of leakage because of slower crack growth in the depth direction when cracks reach the overlay.

As a result, even though there is a factor of 40 difference in circumferential crack initiation, there is only a factor of 3 difference in circumferential crack leak.

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Figure 3-36 Case 2.1.3 time-dependent probabilities of first leak Figure 3-37 shows the probabilities of rupture for Case 2.1.3 as compared to Case 2.1.0. The probabilities increase slightly for Case 2.1.3, but they remain low.

Figure 3-37 Case 2.1.3 time-dependent probabilities of rupture 59

3.3.5 Fatigue 3.3.5.1 Case Description The objective of Case 2.1.4 was to assess the base likelihood of failure caused by fatigue initiation and growth without mechanical mitigation. The fatigue crack initiation and growth parameters used for the analysis were from the xLPR Inputs Group report [53]. The transient definitions were developed based on information from the following reports:

Structural Integrity Associates, Inc., SIR-98-096, Pressurizer Surge Line Leak-Before-Break Evaluation Millstone Nuclear Power Station, Unit 2, issued October 1998 [21]

Structural Integrity Associates, Inc., Report No. 1301103.401, Flaw Tolerance Evaluation of St. Lucie Surge Line Welds Using ASME Code Section XI, Appendix L, issued May 2015 [54]

Structural Integrity Associates, Inc., Report No. 1100756.401, Revision 1, Flaw Tolerance Evaluation of Turkey Point Surge Line Welds Using ASME Code Section XI, Appendix L, issued May 2012 [55]

Based on conversations with EPRI and Westinghouse Electric Company personnel, it was concluded that these transients would be sufficient to gain an understanding of the effects of fatigue on Westinghouse pressurizer surge line nozzle DMWs. The WRS profiles were the same as in the Case 2.1.0 analysis. Section B8 describes the specific inputs and other simulation details used to analyze the case.

3.3.5.2 Results and Analysis 3.3.5.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.3.5.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.3.5.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

2.45 +/- 0.28 months (minimum observed: 1 month) 0.73 +/- 0.14 months (minimum observed: 0 month)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-38 shows the LBB time lapse CDF plots for Case 2.1.4. As observed for the fatigue and PWSCC sensitivity study case in the prior study (i.e., Case 1.1.15), fatigue accelerates 60

crack growth especially for deep surface cracks or TWCs. Thus, the time from detectable leak to rupture is reduced.

Figure 3-38 Case 2.1.4 LBB lapse time results 3.3.5.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

4.85 +/- 0.67 (minimum observed: 3.28) 2.49 +/- 0.48 (minimum observed: 1.33)

Figure 3-39 shows the LBB ratio CDF plots for Case 2.1.4. Fatigue does not affect the crack size required for a given leak rate, nor does it impact the critical crack size for rupture; therefore, the LBB ratio is not affected. The change in LBB ratio is simply due to the larger size of the crack when it ruptures due to faster crack growth. This leads to inaccuracy in the calculation of the critical crack size, which affects the LBB ratio calculation.

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Figure 3-39 Case 2.1.4 LBB ratio results 3.3.5.2.5 Standard Indicators Figure 3-40 shows the probabilities of first crack from Case 2.1.4. The results are essentially identical to Case 2.1.0, which indicates that fatigue does not cause additional crack initiations.

All the crack initiations are because of PWSCC.

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Figure 3-40 Case 2.1.4 time-dependent probabilities of first crack Figure 3-41 shows the probabilities of first leak for Case 2.1.4. These results are virtually identical to Case 2.1.0, which indicates that fatigue does not impact the probabilities of first leak.

Figure 3-41 Case 2.1.4 time-dependent probabilities of first leak Figure 3-42 shows the probabilities of rupture for Case 2.1.4. The addition of fatigue should only increase the probability of rupture. One realization was counted as an SSE rupture when 63

fatigue was added. A close examination of the results revealed that the normalized outer half-length had a value of 2 or more, as if the outer crack length was twice the circumference, which is not physically possible. It appears from these results that the fatigue mechanisms cause crack growth that is not expected by the xLPR code. This issue was referred to the maintenance process for further investigation and correction.

1.E01 1.E02 1.E03 1.E04 Probability 1.E05 1.E06 P(rupture) 2.1.4 1.E07 P(rupture) 2.1.0 P(rupture w/ SSE) 2.1.4 1.E08 P(rupture w/ SSE) 2.1.0 1.E09 P(rupture w/ ISI) 2.1.4 P(rupture w/ ISI) 2.1.0 1.E10 0 10 20 30 40 50 60 70 80 Time (EFPY)

Figure 3-42 Case 2.1.4 time-dependent probabilities of rupture 3.3.6 MSIP Mitigation 3.3.6.1 Case Description Case 2.1.5 was a sensitivity study of Case 2.1.0 considering MSIP mitigation. MSIP was applied at 12 EFPY, which was the latest time of MSIP application for the welds represented by the bin. Figure 3-43 shows the WRS profiles used to analyze the case. They were developed using rules from the xLPR WRS Subgroup report [47] and applied to the WRS profile used for the Case 2.1.0 analysis. Section B9 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-43 Case 2.1.5 WRS profiles 3.3.6.2 Results and Analysis 3.3.6.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case 3.3.6.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.3.6.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection.

capabilities were respectively as follows:

4.5 +/- 0.5 months (minimum observed: 4 months) 1.0 +/- 0.0 month (minimum observed: 1 month)

Since only 2 out of 100,000 realizations led to rupture in this case, these numbers are included for completeness but should not be used to draw any conclusions.

Figure 3-44 shows the LBB time lapse CDF plots for Case 2.1.5. Due to the limited number of ruptures before MSIP application, the CDFs are linear and rough; however, they follow the same trends as in Case 2.1.1.

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Figure 3-44 Case 2.1.5 LBB time lapse results 3.3.6.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

4.49 +/- 0.23 (minimum observed: 4.25) 2.12 +/- 0.17 (minimum observed: 1.95)

Like the LBB time lapse results, the mean LBB ratios and CDFs are based on only 2 realizations and are included for completeness only; they should not be used to draw any conclusions.

Figure 3-45 shows the LBB ratio CDF plots for Case 2.1.5. As for the LBB lapse time results, the CDFs follow the same trends as Case 2.1.1.

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Figure 3-45 Case 2.1.5 LBB ratio results 3.3.6.2.5 Standard Indicators Figure 3-46 shows the probabilities of first crack for Case 2.1.5 as compared with Case 2.1.0.

The MSIP application at 12 EFPY stops any additional occurrences of both axial and circumferential cracks.

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Figure 3-46 Case 2.1.5 time-dependent probabilities of first crack Figure 3-47 shows the probabilities of first leak for Case 2.1.5 as compared with Case 2.1.0.

Like the probabilities of crack initiation, the probabilities of first leak do not increase after the MSIP is applied at 12 EFPY.

Figure 3-47 Case 2.1.5 time-dependent probabilities of first leak 68

Figure 3-48 shows the probabilities of rupture from Case 2.1.5 as compared with Case 2.1.0.

Again, these probabilities stop increasing when the MSIP is applied at 12 EFPY.

Figure 3-48 Case 2.1.5 time-dependent probabilities of rupture 3.4 Bin 3: CE and B&W RCP Nozzle DMWs The following cases were used to analyze the RCP nozzle DMWs represented by Bin 3:

Case 3.1.0: base case Case 3.1.1: initial flaws Case 3.1.2: more severe WRS The cases and associated analyses are described in Sections 3.4.1 through 3.4.3, respectively.

3.4.1 Base Case 3.4.1.1 Case Description The objective of Case 3.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. The analysis of this case used bounding values for the geometry and loading, both normal operating and SSE stresses, based on the licensing submittals referenced in Table 2-1 for the bin. The ISI parameters used were the same as those from the xLPR Inputs Group report [53]. Figure 3-49 shows the WRS profiles used to analyze the case. These profiles were based on the xLPR WRS Subgroup report [47]. More information on these WRS 69

profiles is in Section C3.1. Section B10 describes the specific inputs and other simulation details used to analyze the case.

Figure 3-49 Case 3.1.0 WRS profiles 3.4.1.2 Results and Analysis 3.4.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.4.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.4.1.2.3 LBB Time Lapse There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

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3.4.1.2.4 LBB Ratio There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.4.1.2.5 Standard Indicators Figure 3-50 shows the probabilities of first crack for Case 3.1.0 as compared with Case 1.1.6a.

Only axial cracks occur in Case 3.1.0, and the associated probability is around 3 x 10-4 at 80 EFPY. The probabilities of leakage and rupture were all zero with the sample size considered and are thus not plotted.

Figure 3-50 Case 3.1.0 time-dependent probabilities of first crack 3.4.2 Initial Flaws 3.4.2.1 Case Description The objective of Case 3.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used the same inputs as Case 3.1.0 except that, instead of Direct Model 1 for crack initiation, it used pre-existing axial and circumferential flaws. Section B11 describes the specific inputs and other simulation details used to analyze the case.

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3.4.2.2 Results and Analysis 3.4.2.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.4.2.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.4.2.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

78.1 +/- 4.8 months (minimum observed: 25 months) 53.5 +/- 3.3 month (minimum observed: 16 months)

Figure 3-51 shows the LBB time lapse CDF plots for Case 3.1.1. Since no ruptures occurred in Case 3.1.0, the results were compared with Case 1.1.6b. The LBB time lapses are noticeably longer for the RCP nozzle DMWs.

Figure 3-51 Case 3.1.1 LBB time lapse results 72

3.4.2.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

10.1 +/- 0.12 (minimum observed: 8.64) 4.63 +/- 0.04 (minimum observed: 4.17)

Figure 3-52 shows the LBB ratio CDF plots for Case 3.1.1 as compared with Case 1.1.6b. The results are similar, which is expected considering that the weld sizes in the two cases are similar.

Figure 3-52 Case 3.1.1 LBB ratio results 3.4.2.2.5 Standard Indicators Figure 3-53 shows the probabilities of first leak for Case 3.1.1. The probability of first leak is around 2.7 x 10-2 at 80 EFPY, and the probability of first circumferential crack leak is around 7.4 x 10-3. The probability of first leak decreases to around 2.1 x 10-4 when a 10-year inspection frequency is considered.

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Figure 3-53 Case 3.1.1 time-dependent probabilities of first leak Figure 3-54 shows the probabilities of rupture for Case 3.1.1. The probability of rupture is about 5.8 x 10-3 at 80 EFPY, which is slightly lower than the probability of first circumferential crack leak. The probability of rupture decreases to 2.8 x 10-7 with a 10-year inspection frequency.

Figure 3-54 Case 3.1.1 time-dependent probabilities of rupture 74

3.4.3 More Severe WRS 3.4.3.1 Case Description Case 3.1.2 was a sensitivity study of Case 3.1.0 considering a more severe WRS profile. This case used the same inputs as Case 3.1.0 but with a change to the mean hoop and axial WRS profiles. The standard deviations used to represent uncertainties in the WRS profiles were the same as in Case 3.1.0. Figure 3-55 shows the WRS profiles used to analyze the case. They were developed from FEA results corresponding with the greatest inside diameter stresses, which occur in the weld butter. Such a profile is considered more severe because the higher inside diameter stress favors PWSCC initiation, which has been shown through prior sensitivity analyses to have a large influence on the probability of rupture as documented in TLR-RES/DE/CIB-2021-11 [51]. Additional details on development of the WRS profiles are in Section C3.2. Section B12 describes the specific inputs and other simulation details used to analyze the case.

Figure 3-55 Case 3.1.2 WRS profiles 3.4.3.2 Results and Analysis 3.4.3.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.4.3.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.4.3.2.3 LBB Time Lapse There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.4.3.2.4 LBB Ratio There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.4.3.2.5 Standard Indicators Figure 3-56 shows the probabilities of first crack for Case 3.1.2 as compared with Case 3.1.0.

The more severe axial WRS profile was selected to increase the probability of circumferential crack initiation. However, the increased stress was not enough result in any circumferential cracks. Furthermore, the mean hoop WRS at the inside diameter was lower than in Case 3.1.0, which led to a decreased probability of axial crack occurrence.

Figure 3-56 Case 3.1.2 time-dependent probabilities of first crack 3.5 Bin 4: Westinghouse Steam Generator Nozzle DMWs The following cases were used to analyze the Westinghouse steam generator nozzle DMWs represented by Bin 4:

Case 4.1.0: base case with inlay mitigation Case 4.1.1: initial flaws 76

Case 4.1.2: more severe WRS Case 4.1.3: overlay mitigation Case 4.1.4: no mechanical mitigation The cases and associated analyses are described in Sections 3.5.1 through 3.5.4.2.6, respectively.

3.5.1 Base Case 3.5.1.1 Case Description The objective of Case 4.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth with inlay mitigation. The effects of leak detection, ISI, and SSE were also assessed. Some steam generators have been replaced with a double-vee groove weld geometry with an Alloy 52 inlay. The steam generators at Point Beach Nuclear Plant, Unit 2; North Anna Power Station, Units 1 and 2; and Virgil C. Summer Nuclear Station, Unit 1 have this configuration. This case used bounding values for the geometry and loading, both normal operating and SSE, based on the licensing submittals referenced in Table 2-1 for the bin. The ISI parameters used were from the xLPR Inputs Group report [53]. The normal operating temperature was set to 328°C, which represents the conditions in the hot leg piping.

The DMW in this case has an inlay applied at 1 month to represent welds that were put in service with an inlay already applied. Since the xLPR code does not allow any mitigation to be applied as an initial condition, the application timing was set to 1 month, which is the first possible time step. The inlay material is Alloy 52, and the thickness is 3.3 millimeters (mm) based on the North Anna Power Station, Unit 2 geometry as reported in the April 22, 2013, letter from E. S. Grecheck, Vice President - Nuclear Engineering and Development, Virginia Electric and Power Company, to the NRC Document Control Desk [56]. Figure 3-57 shows the WRS profiles used to analyze the case. They were developed to represent the double-vee groove geometry with an Alloy 52 inlay applied. Additional details on the WRS profile development are in Section A1.1. Section B13 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-57 Case 4.1.0 WRS profiles 3.5.1.2 Results and Analysis 3.5.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.5.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.5.1.2.3 LBB Time Lapse There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.5.1.2.4 LBB Ratio There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.5.1.2.5 Standard Indicators Figure 3-58 shows the probabilities of first crack for Case 4.1.0. The probabilities are higher as compared to Case 1.1.6a. This result is because of the FOI of 24 on crack initiation used for the Alloy 52 inlay, which did not offset the increased hoop and axial WRS at the inside diameter.

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Figure 3-58 Case 4.1.0 time-dependent probabilities of first crack Figure 3-59 shows the probabilities of first leak for Case 4.1.0. While the probability of first leak is slightly higher as compared to Case 1.1.6a, it is only because of the axial crack leaks. There were no circumferential crack leaks.

Figure 3-59 Case 4.1.0 time-dependent probabilities of first leak 79

3.5.2 Initial Flaws 3.5.2.1 Case Description The objective of Case 4.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks with inlay mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used the same inputs as Case 4.1.0 except that, instead of Direct Model 1 for crack initiation, it used pre-existing axial and circumferential flaws. The WRS profiles used were the same as in the Case 4.1.0 analysis.

Section B14 describes the specific inputs and other simulation details used to analyze the case.

3.5.2.2 Results and Analysis 3.5.2.2.1 Probability of Rupture with Detection Figure 3-60 shows the probability of rupture with a 1 gpm leak rate detection capability compared with the probability of leak rate jump. Case 4.1.1 generated a large probability of rupture with leak rate detection of 1.2 x 10-2 at 80 EFPY.

3.5.2.2.2 Leak Rate Jump As shown in Figure 3-60, Case 4.1.1 generated leak rate jump events with a probability of 1.6 x 10-2 at 80 EFPY. A large portion of these realizations had ruptures with leak rate detection.

Figure 3-60 Case 4.1.1 probability of leak rate jump 80

3.5.2.2.3 LBB Time Lapse The nature of the ruptures in Case 4.1.1 makes the LBB time lapse CDFs irrelevant.

3.5.2.2.4 LBB Ratio The nature of the ruptures in Case 4.1.1 makes the LBB ratio CDFs irrelevant.

3.5.2.2.5 Standard Indicators Figure 3-61 shows the probabilities of first leak for Case 4.1.1. The probability of first leak is lower as compared to Case 1.1.6b. Also, a comparison of the differences between the probabilities of first leak with the probabilities of first leak with ISI for each case show that a 10-year inspection frequency has less impact in Case 4.1.1. This result indicates that many of the cracks remain at shallow depths for long times before they grow through-wall and produce leakage. The probability of first leak is 2.5 x 10-1 at 80 EFPY. This probability is reduced by a factor of 2 (i.e., to 1.4 x 10-1) when a 10-year inspection frequency is considered. This relatively low reduction is because of the inlay. Most of the cracks remain in the inlay, whose thickness is less than 10 percent of the weld thickness, and the ISI model parameters were set such that cracks with depths less than 10 percent of the weld thickness are not detected. For the few cracks that grew beyond the thickness of the inlay, they tended to then grow quickly to penetrate through-wall (e.g., leak in less than 10 EFPY), and only the ones close enough to the next inspection were detected. The probability of first circumferential crack leak is 1.88 x 10-2 at 80 EFPY.

Figure 3-61 Case 4.1.1 time-dependent probabilities of first leak 81

Figure 3-62 shows the probabilities of rupture for the different simulations. The probability of rupture is close to the probability of first circumferential leak. It is equal to 1.6 x 10-2 at 80 EFPY, which is the same as the probability of leak rate jump.

Figure 3-62 Case 4.1.1 time-dependent probabilities of rupture 3.5.2.3 Supplemental Analyses While the results for Case 4.1.1 are conditional on having a circumferential crack, the high probability of rupture with leak rate detection (i.e., in the 1 x 10-2 range) was further investigated.

The first area that was evaluated was the distribution used for the initial crack depth. It was a lognormal distribution with a geometric mean of 1.5 mm and a geometric standard deviation of 1.419. The probability of leak rate jump was equal to 1.6 x 10-2, which roughly corresponds to the 98th percentile of the CDF. The same quantile on crack depth represents a depth of 3.2 mm, which is close to the inlay depth of 3.3 mm. Thus, about 1.6 percent of the realizations begin with a crack that is deeper or close to the depth of the inlay. In all these realizations, the cracks can grow faster in depth in the Alloy 82/182 weld material, while growth in length is reduced in the Alloy 52 inlay material. When these cracks grow through-wall, they have a trapezoidal shape with a smaller opening on the inside diameter and a larger opening on the outside diameter. Figure 3-63 illustrates one such crack.

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Figure 3-63 Case 4.1.1 through-wall crack representation for one realization These cracks have low leak rates because of their small inside diameter crack opening areas.

In flow Regime 1, the leak rate module only uses the inside diameter crack length to calculate the leak rate. Leak rates for the realizations that led to rupture ranged from 0.1 to 3.3 gpm, with 75 percent falling below the 1 gpm leak rate detection capability at the time of the rupture.

Then, when the outside diameter crack length is equal to the circumference, the crack is unstable and leads to rupture.

The results generated by Case 4.1.1 are thus not the result of a problem in the xLPR code.

However, the results are not necessarily valid because of some assumptions made in the models and inputs. Some of these assumptions are the following:

The input distribution for the initial crack depth was assumed to be the same as the distribution used for Alloy 82/182 materials. This assumption may not be appropriate considering the PWSCC-resistant properties of the Alloy 52 inlay. Its possible that a more realistic initial crack depth would lead to the disappearance of, or strong reduction in, the rupture events.

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The trapezoidal shape of the TWCs penetrating the inlay does not match well with FEA models, which show bubble-shaped cracks resulting from such conditions. It is unknown how much the shape of the crack would affect the rupture results.

The assumption of using only the inside diameter crack length to estimate the leak rate may not be appropriate in this case.

A series of deterministic analyses were performed by Rudland and others to evaluate the inlay process as a mitigation strategy for PWSCC as reported in [57] and [58]. WRS profiles were developed for inlays in several weld geometries and scenarios. PWSCC was then modeled using advanced FEA for several scenarios. Bubble-shaped crack growth was predicted in all cases where the crack penetrated the Alloy 52/152 inlay and entered the original Alloy 82/182 weld material, because the crack growth rate in the inlay was two orders of magnitude slower than in the original weld. As a simplification, the xLPR code uses a trapezoidal crack shape to approximate crack growth predicted with advanced FEA. Thus, while the xLPR code can reasonably approximate the general extent of such crack growth (e.g., length and depth), it cannot directly model the bubble shape.

Further investigation into these aspects is beyond the scope of this report. Nonetheless, the potential causes are outlined here, which may be pursued to enhance the current input recommendations and models to increase confidence in the results.

3.5.3 More Severe WRS 3.5.3.1 Case Description Case 4.1.2 was a sensitivity study of Case 4.1.0 considering a more severe WRS profile. This case uses the same inputs as Case 4.1.0 but with a change to the mean hoop and axial WRS profiles. The standard deviations used to represent uncertainties in the WRS profiles were the same as in Case 4.1.0. Figure 3-64 shows the WRS profiles used to analyze the case. These profiles were developed using the same FEA as was used to develop the WRS profiles for Case 4.1.0; however, for Case 4.1.2, the WRS profiles were extracted from the location with the highest stresses on the inside diameter, rather than at the weld centerline. The WRS profile is considered more severe because the higher inside diameter stress favors PWSCC initiation, which has been shown through prior sensitivity analyses to have a large influence on the probability of rupture as documented in TLR-RES/DE/CIB-2021-11 [51]. Additional details on development of the WRS profiles are in Section C4.3. Section B15 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-64 Case 4.1.2 WRS profiles 3.5.3.2 Results and Analysis 3.5.3.2.1 Probability of Rupture with Detection Figure 3-65 shows the probability of rupture results with a 1 gpm leak rate detection capability.

The results are like the probability of rupture without leak rate detection. When a 10-year inspection frequency is considered, the probability decreases to 2.4 x 10-6 at 80 EFPY, which leads to an annual frequency of 3 x 10-8. The causes of these non-zero probabilities are like those presented for Case 4.1.1.

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Figure 3-65 Case 4.1.2 time-dependent probabilities of rupture with leak rate detection 3.5.3.2.2 Leak Rate Jump The probability of leak rate jump is equal to 2 x 10-4 at 80 EFPY, which represents an annual frequency of 2.5 x 10-6. The causes for these non-zero probabilities are like those presented in the previous section for Case 4.1.1.

3.5.3.2.3 LBB Time Lapse The nature of the ruptures in Case 4.1.2 makes the LBB time lapse CDFs irrelevant.

3.5.3.2.4 LBB Ratio The nature of the ruptures in Case 4.1.2 makes the LBB time lapse CDFs irrelevant.

3.5.3.2.5 Standard Indicators Figure 3-66 shows the probabilities of first crack for Case 4.1.2. Both the probabilities of first crack and first circumferential crack increase as compared to Case 4.1.0 due to the more severe WRS profile.

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Figure 3-66 Case 4.1.2 time-dependent probabilities of first crack Figure 3-67 shows the probabilities of first leak for Case 4.1.2. The new hoop WRS profile dips around 40 percent through the wall thickness, which leads to a reduction in the probability of first leak as compared to Case 4.1.0. All the leaks are from circumferential cracks because the probability of first circumferential crack leak lies on top of the probability of first leak. The probability of first leak with ISI is also reduced, because it applies only to the circumferential cracks.

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Figure 3-67 Case 4.1.2 time-dependent probabilities of first leak Figure 3-68 shows the probabilities of rupture for Case 4.1.2. No ruptures were observed in Case 4.1.0, so no results from that case have been included for comparison. The probability of rupture is around 2 x 10-4 at 80 EFPY, and most of the ruptures lead to leak rate jumps. The probability of rupture when ISI is considered is around 5.8 x 10-7 at 80 EFPY, which leads to an annual frequency of 7.3 x 10-9.

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Figure 3-68 Case 4.1.2 time-dependent probabilities of rupture 3.5.4 Overlay Mitigation 3.5.4.1 Case description Case 4.1.3 was a sensitivity study of Case 4.1.0 considering overlay instead of inlay mitigation.

The steam generator welds represented by this sensitivity study case had overlays applied after 17 years of service, which was bounded in the analysis by applying the overlay at 20 EFPY.

The WRS profiles from Case 4.1.0 were used to represent the unmitigated WRS profiles for Case 4.1.3. Although the Case 4.1.0 WRS profiles were generated for a double-vee groove weld with an Alloy 52/152 inlay, they are also applicable to the Case 4.1.3 Alloy 82/182 double-vee groove weld because the welding sequences are the same and the temperature-dependent stress strain curves are essentially identical between Alloy 52/152 and Alloy 82/182. The overlay WRS profiles for Case 4.1.3 were generated by using the same standard deviations as the unmitigated WRS profiles and applying the overlay rules in [47] to the mean values. The resulting mean overlay WRS profiles are plotted in Figure 3-69 along with the mean unmitigated WRS profiles for comparison. The overlay thickness was set to 0.04075 m, which is equal to one third of the original weld thickness. This represents the minimum overlay thickness for a full structural weld overlay as specified in EPRI Technical Report 1016602, Materials Reliability Program: Technical Basis for Preemptive Weld Overlays for Alloy 82/182 Butt Welds in PWRs (MRP-169) Revision 1, issued June 11, 2008 [59]. Section B16 describes the specific inputs and other simulation details used to analyze the case.

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Figure 3-69 Case 4.1.3 unmitigated and overlay mitigation WRS profiles 3.5.4.2 Results and Analysis 3.5.4.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.5.4.2.2 Leak Rate Jump As shown in Figure 3-70, Case 4.1.3 generated leak rate jump events with a probability of 1.6 x 10-3 at 80 EFPY. Most of these events (i.e., 92 percent) occurred before the overlay was applied at 20 EFPY. The remaining events corresponded with realizations that had deep surface cracks when the overlay was applied. However, there were no ruptures with a 1 gpm leak rate detection capability as stated in Section 3.5.4.2.1. In this case, the highly tensile mean WRS value at the inside diameter and the highly compressive mean WRS value around 40 percent through the weld thickness led to larger inner crack lengths when the cracks penetrated through the weld thickness.

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Figure 3-70 Case 4.1.3 time-dependent probability of leak rate jump 3.5.4.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

5.31 +/- 0.25 months (minimum observed: 0 months) 5.21 +/- 0.24 months (minimum observed: 0 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3. The minimum observations were estimated at 0 months. Some of the 0-month LBB time lapse estimates were because of the 1-month time step used for the simulation, and a smaller time step would have given a more accurate estimate in these realizations. One realization also had a break before detectable leak, which could have produced a negative LBB time lapse. However, the LBB time lapse in this instance was also estimated as 0 months because, when a pipe ruptures, it also leaks.

Figure 3-71 shows the LBB time lapse CDFs for Case 4.1.3 as compared with Case 1.1.6a. As this figure shows, the LBB time lapses in Case 4.1.3 are much lower than in Case 1.1.6a, which indicates shorter times from detectable leakage to rupture for steam generator nozzle DMWs.

This result is partially influenced by the overlay application at 20 EFPY, as supported by the observation that no ruptures occurred thereafter. Thus, the CDF for Case 4.1.3 only reflects ruptures that occurred during the first 20 EFPY, and the overlay prevented cracks with slower growth rates from rupturing the pipe.

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In some realizations, the LBB time lapses were 0 months, and these events all occurred before the overlay was applied. Because the LBB time lapse QoI was calculated based on the combined normal operating and non-probabilistically treated seismic loads as explained in Section 2.2.3, values less than or equal to zero mean that there is some seismic contribution to the failure frequency. This frequency is estimated in Section 3.5.4.2.6. Under the normal operating loads by themselves, these cracks did not rupture the pipe for an additional 2 to 15 months.

Figure 3-71 Case 4.1.3 LBB time lapse results 3.5.4.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

3.07 +/- 0.05 (minimum observed: 0.85) 2.82 +/- 0.04 (minimum observed: 0.85)

Like the LBB time lapse, the critical crack size calculations for the LBB ratio are based on the combined normal operating and non-probabilistically seismic loads as explained in Section 2.2.4. This explains why the minimum LBB ratios are lower than 1 despite there being no ruptures with leak rate detection. Figure 3-72 shows the LBB ratio CDF plots for Case 4.1.3 as compared with Case 1.1.6a.

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Figure 3-72 Case 4.1.3 LBB ratio results 3.5.4.2.5 Standard Indicators Figure 3-73 shows the probabilities of first crack from Case 4.1.3 as compared to Case 4.1.0.

Due to the relatively high mean hoop WRS value at the inside diameter, the likelihood of having a crack is high in both cases. However, in Case 4.1.3 the absence of the Alloy 52/152 inlay leads to an even higher probability of having a crack. Once the overlay is applied at 20 EFPY, the probabilities for Case 4.1.3 remain essentially flat.

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Figure 3-73 Case 4.1.3 time-dependent probabilities of first crack Figure 3-74 shows the probabilities of first leak from Case 4.1.3 as compared to Case 4.1.0.

The probabilities of first leak are higher in Case 4.1.3 for the first 20 EFPY because of the mean hoop WRS profile. When the overlay is applied, any TWCs become surface cracks that must then grow through the thickness of the overlay to become TWCs once again. The probability of such an event is very low (i.e., 4.4 x 10-3) for the remaining 60 EFPY and halved (i.e.,

1.99 x 10-3) when a 10-year inspection frequency is considered. These results demonstrate a large improvement from using an overlay for PWSCC mitigation purposes.

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Figure 3-74 Case 4.1.3 time-dependent probabilities of first leak Figure 3-75 shows the probabilities of rupture from Case 4.1.3 as compared to Case 4.1.0. In Case 4.1.0 there were no ruptures. In Case 4.1.3, the probability of rupture is very low (i.e.,

3.3 x 10-3) and reflects ruptures that only occurred during the first 20 EFPY before the overlay was applied. The probability of rupture is reduced by a factor of 5 (i.e., to 6.8 x 10-4) when a 10-year inspection frequency is considered. Along with the results for first leak, these results demonstrate a large improvement from using an overlay for PWSCC mitigation purposes.

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Figure 3-75 Case 4.1.3 time-dependent probability of rupture 3.5.4.2.6 Supplemental Analyses In Case 4.1.3, 15 of the 100,000 realizations had potential ruptures under combined normal operating and non-probabilistically treated seismic loads before leak rate detection. Figure 3-76 shows an estimate of the probability of rupture with leak rate detection and probabilistically applied seismic loads. The results include a modification based on a 1 x 10-3 annual frequency of a seismic event leading to an applied bending stress of 161.9 MPa. The 1 x 10-3 annual frequency is the maximum from Section E.3.1 of [60]. This analysis considers probabilistically treated seismic events during the time between (a) the first potential rupture under combined normal operating and non-probabilistically treated seismic loads, and (b) the first rupture under the normal operating loads by themselves. Based on this analysis, the probability of rupture with leak rate detection when seismic events are probabilistically considered is around 6 x 10-8 at 20 EFPY, which gives an annual rupture frequency of around 3 x 10-9. The probability is reported at 20 EFPY in this case because all 15 of the events occurred before the overlay mitigation was applied at 20 EFPY.

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Figure 3-76 Case 4.1.3 time-dependent probability of rupture with leak rate detection under probabilistically treated seismic loads 3.5.5 No Mechanical Mitigation 3.5.5.1 Case description Case 4.1.4 was a sensitivity study of Case 4.1.0 without mechanical mitigation. The WRS profiles from Case 4.1.0 were used to represent the WRS profiles for Case 4.1.4. Although the Case 4.1.0 WRS profiles were generated for a double-vee groove weld with an Alloy 52/152 inlay, they are also applicable to the Case 4.1.4 Alloy 82/182 double-vee groove weld because the welding sequences are the same and the temperature-dependent stress strain curves are essentially identical between Alloy 52/152 and Alloy 82/182. The normal operating temperature used to analyze this case reflects cold-leg operating conditions because those are the conditions for the limited number of inservice welds represented by this case. The WRS profiles are displayed in Figure 3-57. Section B17 describes the specific inputs and other simulation details used to analyze the case.

3.5.5.2 Results and Analysis 3.5.5.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

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3.5.5.2.2 Leak Rate Jump As shown in Figure 3-77, Case 4.1.4 generated leak rate jump events with a probability of 3.2 x 10-4 at 80 EFPY, which corresponds to an annual frequency of 4.0 x 10-6. However, there were no ruptures with a 1 gpm leak rate detection capability as stated in Section 3.5.5.2.1. The highly tensile mean axial WRS value at the insider diameter led to similar causes of leak rate jump as presented for Case 4.1.3 in Section 3.5.4.2.2.

Figure 3-77 Case 4.1.4 time-dependent probability of leak rate jump 3.5.5.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

23.8 +/- 3.2 months (minimum observed: 0 months) 23.7 +/- 3.2 months (minimum observed: 0 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3. The minimum observations were estimated at 0 months because the time step used for the simulation was 1 month. A smaller time step would have yielded more accurate estimates of the LBB time lapses in these realizations.

Figure 3-78 shows the LBB time lapse CDFs for Case 4.1.4 as compared with Case 4.1.3. The LBB time lapses for Case 4.1.4 are higher as compared to Case 4.1.3, which indicates longer times from detectable leakage to rupture. This result is expected because the lower operating temperature used in the Case 4.1.4 analysis produced slower crack growth rates. Like 98

Case 4.1.3, in a few realizations, the LBB time lapses were zero in Case 4.1.4. Because the LBB time lapse QoI was calculated based on the combined normal operating and non-probabilistically treated seismic loads as explained in Section 2.2.3, values less than or equal to zero mean that there is some seismic contribution to the failure frequency. This frequency is estimated in Section 3.5.5.2.6. Under just the normal operating loads, these cracks did not rupture the pipe for an additional 16 to 24 months.

Figure 3-78 Case 4.1.4 LBB time lapse results 3.5.5.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

3.08 +/- 0.12 (minimum observed: 1.45) 2.88 +/- 0.09 (minimum observed: 1.45)

Like the LBB time lapse, the critical crack size calculations for the LBB ratio are based on the combined normal operating and non-probabilistically seismic loads as explained in Section 2.2.4. This explains the LBB ratio values lower than 2. Figure 3-79 shows the LBB ratio CDF plots for Case 4.1.4 as compared with Case 4.1.3. The overall distributions are similar, which is expected because the two cases use the same WRS profiles for the first 20 EFPY, and all the ruptures occur within the first 20 EFPY in Case 4.1.3.

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In contrast with Case 4.1.3, the minimum LBB ratios observed were greater than 1 in Case 4.1.4. Despite the minimum LBB time lapses being estimated as 0 months, the minimum LBB ratios were greater than 1 because of the selected time step of 1 month. In addition, the reason for these discrepancies is that the crack sizes leading to either a 1 or 10 gpm leak rate as used in the LBB ratio calculations were estimated using a linear interpolation. The leakage crack sizes were still lower than the critical crack sizes. Thus, in each of these realizations, the detectable leak rate occurred in the beginning of the timestep when the crack was still stable, and the crack continued to grow and lead to rupture under the combined normal operating and non-probabilistically treated seismic loads at the end of the same timestep.

Figure 3-79 Case 4.1.4 LBB ratio results 3.5.5.2.5 Standard Indicators Figure 3-80 shows the probabilities of first crack for Case 4.1.4 as compared with Case 4.1.0.

Despite the lower operating temperature in Case 4.1.4, the likelihood of having a crack is higher than Case 4.1.0 because there is no Alloy 52/152 inlay.

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Figure 3-80 Case 4.1.4 time-dependent probabilities of first crack Figure 3-81 shows the probabilities of first leak for Case 4.1.4 as compared to Case 4.1.0. As observed for Case 4.1.3, the probabilities of first leak in Case 4.1.4 are higher than in Case 4.1.0 because of the more tensile mean hoop WRS profile. However, due to the lower operating temperature, which reduces the crack growth rates, the influence of a 10-year inspection frequency is stronger for Case 4.1.4 and leads to a lower probability of first leak at 80 EFPY.

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Figure 3-81 Case 4.1.4 time-dependent probabilities of first leak Figure 3-82 shows the probabilities of rupture from Case 4.1.4 as compared to Case 4.1.0. In Case 4.1.0 there were no ruptures. In Case 4.1.4, the probability of rupture at 80 EFPY is low at 5.4 x 10-4, which represents an annual rupture frequency of 6.75 x 10-6. The probability becomes negligible (i.e., 6.5 x 10-10) when a 10-year inspection frequency is considered.

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Figure 3-82 Case 4.1.4 time-dependent probabilities of rupture 3.5.5.2.6 Supplemental Analyses In Case 4.1.4, 4 of the 100,000 realizations had potential ruptures before leak rate detection under combined normal operating and non-probabilistically treated seismic loads. Figure 3-83 shows an estimate of the probability of rupture with leak rate detection when seismic loads are considered probabilistically. Like for Case 4.1.3, the results include a modification based on a 1 x 10-3 annual frequency of a seismic event leading to an applied bending stress of 161.9 MPa.

The 1 x 10-3 annual frequency is the maximum from Section E.3.1 of [60]. This analysis considers probabilistically treated seismic events during the time between (a) the first potential rupture under combined normal operating and non-probabilistically treated seismic loads, and (b) the first rupture under the normal operating loads by themselves. Based on this analysis, the probability of rupture with leak rate detection when seismic events are probabilistically considered is around 6.3 x 10-8 at 80 EPFY, which gives an annual rupture frequency of around 8 x 10-10.

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Figure 3-83 Case 4.1.4 time-dependent probability of rupture with leak rate detection under probabilistically treated seismic loads 3.6 Bin 5a: CE Hot Leg Branch Line Nozzle DMWs The following cases were used to analyze the CE hot leg branch line nozzle DMWs represented by Bin 5a:

Case 5.1.0: base case Case 5.1.1: initial flaws Case 5.1.2: more severe WRS The cases and associated analyses are described in Sections 3.6.1 through 3.6.3, respectively.

3.6.1 Base Case 3.6.1.1 Case Description The objective of Case 5.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used bounding values for the geometry and loading, both normal operating and SSE, based on the licensing submittals referenced in Table 2-1 for the bin. This piping system contains aged cast austenitic stainless steels (SS), which may lead to lower fracture toughness. The ISI parameters used were the same as those used for the pressurizer surge line nozzle DMW analyses. The applicability of the pressurizer surge line nozzle ISI parameters to the CE hot leg branch line nozzle DMWs is documented in the 104

applicability assessment guidance for POD curves [61]. Figure 3-84 shows the WRS profiles used to analyze the case. They were generated from FEA results, and additional details on their development are in Section C5.2. Section B18 describes the specific inputs and other simulation details used to analyze the case.

Figure 3-84 Case 5.1.0 WRS profiles 3.6.1.2 Results and Analysis 3.6.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case 3.6.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.6.1.2.3 LBB Time Lapse There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.6.1.2.4 LBB Ratio There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

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3.6.1.2.5 Standard Indicators Figure 3-85 shows the probabilities of first crack for Case 5.1.0 as compared with Case 1.1.6a.

Due to the high mean hoop WRS value at the inside diameter, the likelihood of having an axial crack is higher in Case 5.1.0. However, there were no circumferential crack initiations because of the highly compressive mean axial WRS value at the inside diameter.

Figure 3-85 Case 5.1.0 time-dependent probabilities of first crack Figure 3-86 shows the probabilities of first leak for Case 5.1.0 as compared with Case 1.1.6a.

Like the probability of first crack, the probability of first leak results are higher in Case 5.1.0 because of the higher likelihood of axial cracks.

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Figure 3-86 Case 5.1.0 time-dependent probabilities of first leak 3.6.2 Initial Flaws 3.6.2.1 Case Description The objective of Case 5.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used the same inputs as Case 5.1.0 except that, instead of Direct Model 1 for crack initiation, it used pre-existing axial and circumferential flaws. The WRS profiles used were the same as used in the Case 5.1.0 analysis. Section B19 describes the specific inputs and other simulation details used to analyze the case.

3.6.2.2 Results and Analysis 3.6.2.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.6.2.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.6.2.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

54.3 +/- 2.4 months (minimum observed: 23 months) 15.5 +/- 0.6 months (minimum observed: 6 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-87 shows the LBB time lapse CDF plots for Case 5.1.1 as compared with Case 1.1.6b.

The lowest values are associated with the 10 gpm leak rate detection capability. Considering the smaller diameter pipe size in Case 5.1.1, a 10 gpm leak rate requires a larger crack angle and is thus closer to the critical crack size at rupture.

Figure 3-87 Case 5.1.1 LBB lapse time results 108

3.6.2.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

3.42 +/- 0.02 (minimum observed: 2.93) 1.81 +/- 0.01 (minimum observed: 1.66)

Figure 3-88 shows the LBB ratio CDF plots for Case 5.1.1. The results are lower as compared to Case 1.1.6b because of the smaller diameter of the piping. Figure 3-20 shows a similar comparison for the pressurizer surge line nozzle DMW, which also has a smaller diameter in comparison to the Westinghouse RVON and RVIN DMWs.

Figure 3-88 Case 5.1.1 LBB ratio results 3.6.2.2.5 Standard Indicators Figure 3-89 shows the probabilities of first leak for Case 5.1.1. Compared to the Case 1.1.6b results, the probability of first leak is higher at 80 EFPY, and the impact of ISI is reduced because, at 10 EFPY, most of the realizations have already begun to leak. Of note, almost all the first leaks are caused by axial cracks. The probability of first circumferential crack leak is in the range of 2 x 10-2 at 80 EFPY.

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Figure 3-89 Case 5.1.1 time-dependent probabilities of first leak Due to the low probability of first circumferential crack leak, the probability of rupture in Case 5.1.1 is also low and reaches about 1.76 x 10-2 at 80 EFPY as seen in Figure 3-90. When a 10-year inspection frequency is considered, the probability reduces to the 1 x 10-8 range.

Figure 3-90 Case 5.1.1 time-dependent probabilities of rupture 110

3.6.3 More Severe WRS 3.6.3.1 Case Description Case 5.1.2 was a sensitivity study of Case 5.1.0 considering a more severe WRS profile. This case used the same inputs as Case 5.1.0 but with a change to the mean hoop and axial WRS profiles. The standard deviations used to represent uncertainties in the WRS profiles were the same as in Case 5.1.0. Figure 3-91 shows the WRS profiles used to analyze the case. They were developed using FEA of a generic CE branch line geometry with the distance between the DMW and the SS weld changed to the maximum length of all welds represented by the bin.

The WRS profile is considered more severe because, in general, a greater distance between the DMW and SS weld will result in more tensile stresses. Additional details on development of the WRS profiles are in Section C5.3. Section B20 describes the specific inputs and other simulation details used to analyze the case.

Figure 3-91 Case 5.1.2 WRS profiles 3.6.3.2 Results and Analysis 3.6.3.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.6.3.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.6.3.2.3 LBB Time Lapse There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.6.3.2.4 LBB Ratio There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.6.3.2.5 Standard Indicators Figure 3-92 shows the probabilities of first crack for Case 5.1.2 as compared to Case 5.1.0.

The use of a more severe WRS profile in Case 5.1.2 only slightly increased the probability of first crack. Although there were no circumferential crack initiations in Case 5.1.0, the probability of circumferential crack occurrence in Case 5.1.2 was 3.6 x 10-5 at 80 EFPY.

Figure 3-92 Case 5.1.2 time-dependent probabilities of first crack Figure 3-93 shows the probability of first leak for Case 5.1.2. Like the probability of first crack, the more severe WRS profile led to an increase in the probability of first leak. However, the probability of first circumferential crack leak remained at zero. There were no occurrences of rupture over the 80-EFPY evaluation period.

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Figure 3-93 Case 5.1.2 time-dependent probabilities of first leak 3.7 Bin 5b: CE Cold Leg Branch Line Nozzle DMWs The following cases were used to analyze the CE cold leg branch line nozzle DMWs represented by Bin 5b:

Case 5.2.0: base case Case 5.2.1: initial flaws The cases and associated analyses are described in Sections 3.7.1 and 3.7.2, respectively.

3.7.1 Base Case 3.7.1.1 Case Description The objective of Case 5.2.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. The analysis of this case used bounding values for the geometry and loading, both normal operating and SSE stresses, based on the licensing submittals referenced in Table 2-1 for this bin. This piping system contains aged cast austenitic SS, which may lead to lower fracture toughness. The ISI parameters used were the same as those used for the pressurizer surge line nozzle DMW analyses. The applicability of the pressurizer surge line ISI parameters to the CE cold leg branch line nozzle DMWs is documented in the applicability assessment guidance for POD curves [61]. Figure 3-94 shows the WRS profiles used to analyze the case. They were developed using a general CE branch line geometry; therefore, 113

they are the same as the WRS profiles used in the analysis of Case 5.1.0. Section B21 describes the specific inputs and other simulation details used to analyze the case.

Figure 3-94 Case 5.2.0 WRS profiles 3.7.1.2 Results and Analysis 3.7.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.7.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.7.1.2.3 LBB Time Lapse There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.7.1.2.4 LBB Ratio There were no circumferential crack leaks or ruptures for this case; therefore, this QoI cannot be reported.

3.7.1.2.5 Standard Indicators The Case 5.2.0 results were compared with Case 5.1.0 because the CE hot and cold leg branch line nozzle DMWs have similar geometries and the same WRS profiles. Figure 3-95 shows the 114

probabilities of first crack. The cold leg branch line results for Case 5.2.0 are lower than the hot leg branch line results from Case 5.1.0, and there were no occurrences of circumferential cracks.

Figure 3-95 Case 5.2.0 time-dependent probabilities of first crack Figure 3-96 shows the probabilities of first leak for Case 5.2.0 as compared with Case 5.1.0.

Like the probability of first crack results, the results from Case 5.2.0 are lower than the results from Case 5.1.0.

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Figure 3-96 Case 5.2.0 time-dependent probabilities of first leak 3.7.2 Initial Flaws 3.7.2.1 Case Description The objective of Case 5.2.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used the same inputs as Case 5.2.0 except that, instead of Direct Model 1 for crack initiation, it used pre-existing axial and circumferential flaws. The WRS profiles were the same as used in the Case 5.2.0 analysis. Section B22 describes the specific inputs and other simulation details used to analyze the case.

3.7.2.2 Results and Analysis 3.7.2.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.7.2.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.7.2.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

84.3 +/- 5.2 months (minimum observed: 30 months) 31.8 +/- 2.5 months (minimum observed: 11 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-97 shows the LBB time lapse CDF plots for Case 5.2.1. When compared to Case 5.1.1, the LBB time lapses are longer for the cold leg branch line, which indicate that the hot leg branch line results can be considered as an upper bound for the welds represented by Bins 5a and 5b.

Figure 3-97 Case 5.2.1 LBB lapse time results 117

3.7.2.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

4.29 +/- 0.03 (minimum observed: 3.88) 1.97 +/- 0.01 (minimum observed: 1.85)

Figure 3-98 shows the LBB ratio CDF plots for Case 5.2.1 as compared with Case 5.1.1. Like the LBB time lapse results, the cold leg branch line LBB ratios are greater than the hot leg branch line ratios, which further indicate that the hot leg branch line results can be considered as an upper bound for Bins 5a and 5b.

Figure 3-98 Case 5.2.1 LBB ratio results 3.7.2.2.5 Standard Indicators Figure 3-99 shows the probabilities of first leak for Case 5.2.1. As compared to Case 5.1.1, the results are lower.

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Figure 3-99 Case 5.2.1 time-dependent probabilities of first leak Figure 3-100 shows the probabilities of rupture for Case 5.2.1. Like the probabilities of first leak, the cold leg branch line results are lower, which again indicates that the hot leg branch line results can be considered as an upper bound.

Figure 3-100 Case 5.2.1 time-dependent probabilities of rupture 119

3.8 Bin 6: Westinghouse Two- and Three-Loop RVON and RVIN DMWs The following cases were used to analyze the Westinghouse two- and three-loop RVON DMWs represented by Bin 6:

Case 1.3.0: base case Case 1.3.1: initial flaws The cases and associated analyses are described in Sections 3.8.1 and 3.8.2, respectively.

3.8.1 Base Case 3.8.1.1 Case Description The objective of Case 1.3.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. Since the present study is an extension of the prior study, this case was included in Bin 1, which was established in the prior study for Westinghouse four-loop RVON and RVIN DMWs. This case used bounding values for the geometry and loading, both normal operating and SSE, based on the licensing submittals referenced in Table 2-1 for the bin. The ISI parameters used were the same as in Case 1.1.0 from the prior study [2]. Figure 3-101 shows the WRS profiles used to analyze the case. They are the same as the WRS profiles used for Case 1.1.0 analysis. Additional information on this WRS profile can be found in Section C2.1. Section B23 describes the specific inputs and other simulation details used to analyze the case.

Figure 3-101 Case 1.3.0 WRS profiles 120

3.8.1.2 Results and Analysis 3.8.1.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.8.1.2.2 Leak Rate Jump There were no leak rate jump events for this case.

3.8.1.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

34.0 +/- 1.6 months (minimum observed: 13 months) 21.8 +/- 1.2 months (minimum observed: 8 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-102 shows the LBB time lapse CDF plots for Case 1.3.0. The results are equivalent to the Case 1.1.6a results. The small differences can be attributed to the statistical accuracy.

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Figure 3-102 Case 1.3.0 LBB time lapse results 3.8.1.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

9.9 +/- 0.1 (minimum observed: 7) 4.44 +/- 0.04 (minimum observed: 3.51)

Figure 3-103 shows the LBB ratio CDF plots for Case 1.3.0. Like the LBB time lapses, the results are statistically equivalent to the Case 1.1.6a results.

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Figure 3-103 Case 1.3.0 LBB ratio results 3.8.1.2.5 Standard Indicators Figure 3-104 shows the probabilities of first crack for Case 1.3.0. As compared to Case 1.1.6a, the probability of first crack is higher, while the probability of first circumferential crack is lower.

Considering that circumferential crack ruptures are the primary concern, the Case 1.3.0 results are bounded by Case 1.1.6a.

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Figure 3-104 Case 1.3.0 time-dependent probabilities of first crack Figure 3-105 shows the probabilities of first leak for Case 1.3.0. Like the probability of first crack, the Case 1.3.0 results are bounded by Case 1.1.6a because the probability of first circumferential leak in the former is lower.

Figure 3-105 Case 1.3.0 time-dependent probabilities of first leak 124

Figure 3-106 shows the probabilities of rupture for Case 1.3.0 as compared to Case 1.1.6a. As expected from the first crack and first leak results, the Case 1.3.0 results are lower.

Figure 3-106 Case 1.3.0 time-dependent probabilities of rupture 3.8.2 Initial Flaws 3.8.2.1 Case Description The objective of Case 1.3.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation. The effects of leak detection, ISI, and SSE were also assessed. This case used the same inputs as Case 1.3.0 except that, instead of Direct Model 1 for crack initiation, it used pre-existing axial and circumferential flaws. The WRS profiles were the same as used in the Case 1.3.0 analysis. Section B24 describes the specific inputs and other simulation details used to analyze the case.

3.8.2.2 Results and Analysis 3.8.2.2.1 Probability of Rupture with Detection There were no ruptures with a 1 gpm leak rate detection capability for this case.

3.8.2.2.2 Leak Rate Jump There were no leak rate jump events for this case.

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3.8.2.2.3 LBB Time Lapse The mean LBB time lapses and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

33.4 +/- 0.2 months (minimum observed: 6 months) 20.9 +/- 0.14 months (minimum observed: 6 months)

Note that all results beyond 12 EFPY have been excluded for the reasons explained in Section 3.2.1.2.3.

Figure 3-107 shows the LBB time lapse CDF plots for Case 1.3.1. The results are slightly lower as compared to Case 1.1.6b. However, the results are statistically close, and the difference is only a few months.

Figure 3-107 Case 1.3.1 LBB time lapse results 126

3.8.2.2.4 LBB Ratio The mean LBB ratios and standard errors with 1 and 10 gpm leak rate detection capabilities were respectively as follows:

10.00 +/- 0.01 (minimum observed: 6.33) 4.52 +/- 0.01 (minimum observed: 3.42)

Figure 3-108 shows the LBB ratio CDF plots for Case 1.3.1. The results are statistically equivalent with Case 1.1.6b.

Figure 3-108 Case 1.3.1 LBB ratio results 3.8.2.2.5 Standard Indicators Figure 3-109 shows the probabilities of first leak for Case 1.3.1. The probability of first leak is higher as compared to Case 1.1.6b. However, the increase is associated with axial cracks only, and the likelihood of circumferential crack leakage is slightly lower as highlighted by the probability of rupture shown in Figure 3-110.

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Figure 3-109 Case 1.3.1 time-dependent probabilities of first leak Figure 3-110 shows the probabilities of rupture for Case 1.3.1. The probabilities of rupture with ISI are lower as compared to Case 1.1.6b.

Figure 3-110 Case 1.3.1 time-dependent probabilities of rupture 128

4 PIPING SYSTEM FAILURE PROBABILITY The xLPR code can only analyze one weld in a simulation. However, the piping systems of interest in this study contain multiple welds. Thus, a system-level analysis was necessary to combine the individual weld results and estimate a total probability of failure.

4.1 Methodology In Section 4.2 of NUREG/CR-2189, Probability of Pipe Fracture in the Primary Coolant Loop of a PWR Plant, Volume 5, Probabilistic Fracture Mechanics Analysis, Load Combination Program, Project I Final Report, issued August 1981 [62], two methods are presented for combining the failure probabilities of multiple welds to estimate a single, system-level failure probability. The first method considers all welds to be independent. This method provides an upper bound on the probability of failure. The second method considers only a single weld associated with the worst-case conditions as the weld that will fail first. This method provides a lower bound on the probability of failure and considers the properties and conditions to be perfectly correlated among all the welds. In practice, the true probability should lie between these two bounds; however, depending on the analysis considered, one of the bounds may be more representative of the true value.

As discussed in Section 4 of the prior study [2], the first approach is based on independence among the results from each weld and is considered the most representative method for evaluating event probabilities for the following reasons:

PWSCC is the dominant degradation mechanism for both crack initiation and crack growth. This mechanism does not affect all the piping system components similarly, nor at the same time during the simulated plant operating period. This contrasts with fatigue transients, which are modeled as occurring at the same time and with correlated intensities.

The uncertain physical parameters that influence crack initiation and growth, such as the WRS, are not expected to be correlated among the welds included in these analyses.

The operating conditions (e.g., temperature and pressure) could affect all the welds, but these parameters are constant in each realization. Therefore, by default, they are applied equally to each weld with respect to the expected value.

The probability of an event affecting multiple welds of the same type (e.g., multiple RVON welds within the same plant) is estimated using a classical statistical approach.

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If a single weld has, at a given time, , a probability, , of the event occurring, then the probability of having welds experiencing the event out of a pool of welds (with 0 ) is defined as follows:

. 1 Equation 3 where is the notation for the combination of elements from a pool of elements.

This probability can be used to estimate the likelihood of each potential scenario, from no welds failing up to all welds failing. If only the probability of at least one event occurring is of interest, then Equation 3 can be simplified as follows:

1 1 0 1 1 Equation 4 This concept can be extended to multiple weld types. For example, let , , , be the weld types considered, with the respective number of each type of weld being , , , ,

and the probabilities of having the event occurring at time be , , , . Then, the probability of having welds of type , welds of type , and so on up to welds of type

, would be as follows:

, ,, . 1 Equation 5 with the equivalent of Equation 4 becoming:

1 1 0 1 1 Equation 6 As noted in the prior study, an underlying assumption of this method is that the event considered would have the same impact if it happens for any of the welds under consideration in the piping system. If for one loop or one weld, the event is of higher or lower consequence, then a quantitative impact factor needs to be included. In the present analysis, all the events considered have the same impact on the piping system regardless of the weld type.

4.2 Piping System Failure Frequency Results 4.2.1 Piping Systems Investigated Two of the time-dependent QoIs (i.e., probability of leak rate jump and probability of rupture with a 1 gpm leak rate detection capability) were estimated to be essentially 0 in all the individual weld analyses performed as part of this study. Thus, any aggregation of these results at the system level would also be zero. In consequence, these QoIs cannot be used to illustrate the method for developing a system-level failure probability. Instead, the methodology was applied 130

to the probabilities of first crack, first leak, and rupture without leak rate detection both with and without a 10-year inspection frequency.

Three groupings were considered in the system analysis to provide an upper bound for the various piping system configurations in the PWR fleet. These combinations contain the maximum number of weld types for a given group of plants, but they do not necessarily represent actual piping system configurations. The three groupings are summarized in Table 4-1.

Table 4-1 Numbers of components bounding different plant designs Grouping RVON Pressurizer Steam RCP Inlet Hot Leg Cold Leg DMWs Surge Line Generator Nozzle Branch Branch Nozzle Inlet Nozzle DMWs Line Nozzle Line Nozzle DMWs DMWs DMWs DMWs Westinghouse 5 1 Not Not Not Not 4-Loop PWRs (Case 1.1.6a) (Case 2.1.0) Applicable Applicable Applicable Applicable Westinghouse 4 Not 6 Not Not Not 2-and 3-Loop (Case 1.3.0) Applicable (Case 4.1.0) Applicable Applicable Applicable PWRs CE and B&W Not 1 Not 8 2 4 PWRs Applicable (Case 2.1.0) Applicable (Case 3.1.0) (Case 5.1.0) (Case 5.2.0)

The first grouping bounds all possible configurations in Westinghouse 4-loop PWRs. It consists of 9 welds:

4 RVON DMWs 4 RVIN DMWs 1 pressurizer surge line nozzle DMW The RVIN DMWs were analyzed in the prior study [2], and the QoIs generated were at least one order of magnitude below those for the RVON DMW. Since no RVIN DMW cases were run in the present study, the 4 RVIN DMWs were conservatively represented by 1 RVON DMW. The plant-level aggregation used the individual weld results from Case 1.1.6a for the RVON DMWs and from Case 2.1.0 for the pressurizer surge line nozzle DMW. Section 4.2.2 presents the aggregated results for this grouping.

The second grouping bounds all possible configurations in Westinghouse two- and three-loop PWRs. It consists of 12 welds:

3 RVON DMWs 3 RVIN DMWs 3 steam generator inlet nozzle DMWs 131

3 steam generator outlet nozzle DMWs As for the previous grouping, the 3 RVIN DMWs were conservatively represented by 1 RVON DMW. Only the steam generator inlet nozzle DMW was analyzed in this study. Because it is subject to higher operating temperatures, its results were also used to represent the steam generator outlet nozzle DMWs. The plant-level aggregation used the individual weld results from Case 1.3.0 for the RVON DMWs and from Case 4.1.0 for the steam generator inlet nozzle DMWs. Section 4.2.3 presents the aggregated results for this grouping.

The third grouping bounds all possible configurations in CE and B&W plants. It consists of 15 welds:

1 pressurizer surge line nozzle DMW 4 RCP inlet nozzle DMWs 4 RCP outlet nozzle DMWs 4 high-pressure injection nozzle DMWs 1 shutdown cooling nozzle DMW 1 pressurizer surge line to hot leg nozzle DMW The high-pressure injection nozzle DMWs are connected to the cold leg, while the shutdown cooling and pressurizer surge line to hot leg nozzle DMWs are both connected to the hot leg.

Because the RCP inlet nozzle DMWs are subject to higher operating temperatures, they were also used to represent the RCP outlet nozzle DMWs. The plant-level aggregation used the individual weld results from Case 2.1.0 for the pressurizer surge line nozzle DMW, Case 3.1.0 for the RCP nozzle DMWs, Case 5.1.0 for the hot leg branch line nozzle DMWs, and Case 5.2.0 for the cold leg branch line nozzle DMWs. Section 4.2.4 presents the aggregated results for this grouping.

4.2.2 Westinghouse Four-Loop PWRs Figure 4-1 shows the time-dependent probability plots estimated using Equation 6 to bound Westinghouse four-loop PWRs. At 80 EFPY, the plant-level results are as follows:

6.9 x 10-2 probability of first crack 1.6 x 10-2 probability of first circumferential crack 5.6 x 10-2 probability of first leak 9.7 x 10-3 probability of first circumferential leak 2.2 x 10-2 probability of first leak with a 10-year inspection frequency 9.4 x 10-3 probability of rupture without ISI or leak rate detection 1.3 x 10-3 probability of rupture with a 10-year inspection frequency 132

Figure 4-1 Bounding Westinghouse four-loop time-dependent probabilities Each of these probabilities can be split into the contributions from each group of components.

This approach approximates the contributions, which is more accurate for low probabilities.

Figure 4-2 presents such a decomposition for the probability of first crack. As can be seen in this figure, the pressurizer surge line nozzle DMW and the 5 RVON DMWs have about the same contribution at 80 EFPY for this QoI.

Figure 4-2 Westinghouse four-loop system probability of first crack and component contributions Figure 4-3 presents a similar figure for the probability of first circumferential crack. It shows that the contribution from the RVON DMWs is dominant, and the contribution from the pressurizer surge line nozzle DMW has been reduced by more than 2 orders of magnitude.

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Figure 4-3 Westinghouse four-loop system probability of first circumferential crack and component contributions Figure 4-4 shows the contributions to the time-dependent probability of first leak. Because of the smaller diameter of the piping, the pressurizer surge line nozzle DMW quickly leads to leakage as compared to the other welds considered, and thus it is the major contributor in the early years of plant operation. At 80 EFPY, the contribution of the 5 RVON DMWs are similar to the contribution from the 1 pressurizer surge line nozzle DMW.

Figure 4-4 Westinghouse four-loop system probability of first leak and component contributions As shown in Figure 4-5, the contribution from the pressurizer surge line nozzle DMW drives the probability of first leak with a 10-year inspection frequency. Figure 4-6 shows a similar trend for 134

the probability of first circumferential leak. Because there were fewer occurrences of circumferential cracks for the pressurizer surge line nozzle DMW, this probability is equivalent to the probability of the five RVON DMWs combined. These results illustrate that only one set of components could be considered to estimate the probability at the system level.

Figure 4-5 Westinghouse four-loop system probability of first leak with a 10-year inspection frequency and component contributions Figure 4-6 Westinghouse four-loop system probability of first circumferential leak and component contributions Figure 4-7 and Figure 4-8 show the probabilities of rupture and rupture with a 10-year inspection frequency, respectively. Since only circumferential cracks lead to ruptures, these figures are 135

consistent with the probability of first circumferential leak where the system-level probability is driven by the five RVON DMWs.

Figure 4-7 Westinghouse four-loop system probability of rupture and component contributions Figure 4-8 Westinghouse four-loop system probability of rupture with 10-year inspection frequency and component contributions 136

4.2.3 Westinghouse Two-loop and Three-Loop PWRs Figure 4-9 shows the time-dependent probability plots estimated using Equation 6 to bound Westinghouse two- and three-loop PWRs. At 80 EFPY, the system-level results are as follows:

3.1 x 10-1 probability of first crack 3.9 x 10-2 probability of first circumferential crack 9.9 x 10-2 probability of first leak 3.5 x 10-3 probability of first circumferential leak 4.8 x 10-2 probability of first leak with a 10-year inspection frequency 3.3 x 10-3 probability of rupture without ISI or leak rate detection 3.2 x 10-4 probability of rupture with a 10-year inspection frequency Figure 4-9 Westinghouse two- and three-loop time-dependent probabilities Figure 4-10 presents the contributions of each weld type to the probability of first crack. As can be seen in this figure, the steam generator nozzle DMWs drive the probability. Figure 4-11 presents a similar figure for the probability of first circumferential crack. The contribution from the steam generator nozzle DMWs remains dominant.

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Figure 4-10 Westinghouse two- and three-loop system probability of first crack and component contributions Figure 4-11 Westinghouse two- and three-loop system probability of first circumferential crack and component contributions Figure 4-12 shows the contributions to the time-dependent probability of first leak. It shows that the steam generator nozzle DMWs and RVON DMWs have similar contributions that switch in importance over time. The steam generator nozzle DMWs become the biggest contributor over time when a 10-year inspection frequency is considered for the probability of first leak, as displayed in Figure 4-13.

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Figure 4-12 Westinghouse two- and three-loop system probability of first leak and component contributions Figure 4-13 Westinghouse two- and three-loop system probability of first leak with a 10-year inspection frequency and component contributions Figure 4-14 shows the probability of first leak for circumferential cracks only. Because of the slow circumferential crack growth in the steam generators nozzle DMWs, this probability is equivalent to the probability from the four RVON DMWs.

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Figure 4-14 Westinghouse two- and three-loop system probability of first circumferential leak and component contributions Figure 4-15 and Figure 4-16 show the probabilities of rupture and rupture with a 10-year inspection frequency, respectively. Since only circumferential cracks led to rupture, these figures are consistent with the probabilities of first circumferential crack leak with the system-level probability being driven by the four RVON DMWs.

Figure 4-15 Westinghouse two- and three-loop system probability of rupture and component contributions 140

Figure 4-16 Westinghouse two- and three-loop system probability of rupture with a 10-year inspection frequency and component contributions 4.2.4 CE and B&W PWRs Figure 4-17 shows the time-dependent probability plots estimated using Equation 6 to bound the CE and B&W PWRs. At 80 EFPY, the system-level results are as follows:

2.4 x 10-1 probability of first crack 1.3 x 10-4 probability of first circumferential crack 2.0 x 10-1 probability of first leak 1.1 x 10-4 probability of first circumferential leak 1.2 x 10-1 probability of first leak with a 10-year inspection frequency 1.1 x 10-4 probability of rupture without ISI or leak rate detection 4.0 x 10-5 probability of rupture with a 10-year inspection frequency 141

Figure 4-17 CE and B&W time-dependent probabilities Figure 4-18 presents the decomposition for the probability of first crack. As can be seen in this figure, the hot leg branch line with 2 DMWs and the cold leg branch line with 4 DMWs are the biggest contributors, followed by the pressurizer surge line nozzle DMW and the RCP nozzle DMWs. Figure 4-19 presents a similar figure for the probability of first circumferential crack.

Here the only contributor is the pressurizer surge line nozzle DMW because the other welds did not have any circumferential cracks over 80 EFPY for the sample sizes considered. As a result, the probabilities of first circumferential leak and rupture will also be dependent only on the pressurizer surge line nozzle DMW contribution.

Figure 4-18 CE and B&W system probability of first crack and component contributions 142

Figure 4-19 CE and B&W system probability of first circumferential crack and component contributions Figure 4-20 shows the contribution to the time-dependent probability of first leak. The hot leg and cold leg branch line nozzle DMWs are the biggest contributors to the probability of first crack followed by the pressurizer surge line nozzle DMW. The RCP nozzle DMWs did not generate any leakage, so they are not a contributor. The hot leg branch line nozzle DMW becomes the dominant contributor when a 10-year inspection frequency is considered for the probability of first leak, as shown in Figure 4-21. Figure 4-22 shows the probability of first leak for circumferential cracks only. As expected, it is equivalent to the probability of first leak for the pressurizer surge line nozzle DMW.

Figure 4-20 CE and B&W system probability of first leak and component contributions 143

Figure 4-21 CE and B&W system probability of first leak with a 10-year inspection frequency and component contributions Figure 4-22 CE and B&W system probability of first circumferential leak and component contributions Figure 4-23 and Figure 4-24 show probabilities of rupture and rupture with a 10-year inspection frequency, respectively. Since only circumferential cracks led to rupture, these figures are consistent with the probability of first circumferential leak, and the system-level probabilities are equivalent to their pressurizer surge line nozzle DMW equivalents.

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Figure 4-23 CE and B&W system probability of rupture and component contributions Figure 4-24 CE and B&W system probability of rupture with a 10-year inspection frequency and component contributions 145

5 ANALYSIS ASSUMPTIONS 5.1 Conservatisms Biases in the overall analysis approach used for the present study make the results upper bound estimates. The models retain the biases presented in the report on sources and treatment of uncertainties in the xLPR code [63]. The following input assumptions contribute additional bias:

The highest normal operating loads, pressures, and temperatures were used to represent the welds in each bin. The normal operating loads also reflect design-basis values.

The smallest thicknesses and largest outside diameters were used to represent the welds in each bin. This approach leads to quicker times to through-wall cracks and higher applied stresses.

A lower-bound hydrogen concertation was used in all cases. This approach leads to faster crack growth rates.

For uniformity, a 10-year inspection frequency was used in all cases; however, many of the DMWs are currently required by 10 CFR 50.55a(g)(6)(ii)(F) to be inspected more frequently.

An SSE frequency of occurrence of 1 x 10-3 events per year was assumed in conjunction with design-basis SSE stresses.

In addition, as described in Section 4, when combining the individual weld results into a system level analysis, assuming that the welds are independent provides an upper bound on the system-level probabilities.

5.2 Unknowns The distance between the DMWs and the safe-end-to-pipe, or SS closure, welds represents an unknown. This distance influences the WRS profile, and a typical value was used for the analyses in this study. Sufficient information was not available to generate a distribution of these values for each bin.

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6 ASSESSMENT OF NRC REGULATORY FRAMEWORK FOR LEAK-BEFORE-BREAK This section evaluates NRCs current regulatory framework for LBB considering the results from this study. The evaluation focuses on the following elements:

requirements in GDC 4 guidance in SRP Section 3.6.3 Section 6.1 provides historical background on LBB in nuclear power plant piping systems.

Potential changes to the above elements of NRCs regulatory framework are discussed in Sections 6.2 and 6.3, respectively.

6.1 Background on LBB for Nuclear Piping In 1971 [64], the Atomic Energy Commission, the predecessor agency to the NRC, promulgated 10 CFR Part 50, Appendix A, General Design Criteria for Nuclear Power Plants. GDC 4 of this appendix required that structures, systems, and components important to safety be protected against the dynamic effects of postulated large piping ruptures. GDC 4 was then conservatively applied to require all nuclear power reactors to employ massive pipe whip restraints and jet impingement shields to mitigate the dynamic effects of a postulated guillotine rupture in the largest piping in the reactor coolant system.

In 1975, the NRC staff was informed of newly defined asymmetric blowdown loads that result by postulating rapid-opening, double-ended ruptures at the most adverse location in the PWR primary piping system. The topic was designated as Unresolved Safety Issue A-2. In response to a conclusion based on analyses in 1980 that some plants might require extensive modifications to address this safety issue, Westinghouse Electric Corporation undertook a deterministic fracture mechanics evaluation to demonstrate that the assumed double-ended rupture is not a credible design-basis event for PWR piping base and weld metals.

Westinghouse Electric Corporation reports WCAP-9570, Mechanistic Fracture Evaluation of Reactor Coolant Pipe Containing a Postulated Circumferential Through-Wall Crack, Revision 2, issued June 1981 [65], and WCAP-9788, Tensile and Toughness Properties of Primary Piping Weld Metal for Use in Mechanistic Fracture Evaluation, issued June 1981 [66], document the evaluations for piping base and weld metals, respectively.

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Around the same time, the NRC staff published the results of a probabilistic PWR piping fracture study in NUREG/CR-2189, Volume 5 [62]. The Westinghouse Electric Corporation and NRC staff-sponsored studies used different methodologies; however, both studies supported the conclusion that double-ended ruptures in PWR primary system piping are extremely low probability events. The results of these studies were submitted to the NRCs Advisory Committee on Reactor Safeguards. In a June 14, 1983, letter to the NRCs Executive Director for Operations, the Advisory Committee on Reactor Safeguards stated that:

Fracture mechanics analysis clearly indicates that in PWR primary piping a substantial range of stable crack sizes exists between those which give detectable leaks and the much larger size that results in sudden failure However any relaxation of requirements to cope with double-ended guillotine break should be preceded by rigorous reexamination of the integrity of heavy component supports under all design conditions.

As a result of these developments, the NRC proposed to modify the requirements of GDC 4 in 1985 [67] and 1986 [68]. The resulting amendments to GDC 4 in 1986 [69] and 1987 [70] allow for analyses to serve as the basis for excluding the consideration of dynamic effects associated with certain piping system ruptures. These analyses constitute what is commonly referred to as the LBB concept. The deterministic and probabilistic analyses showed that, for the primary loop piping in PWRs, double-ended guillotine or longitudinal ruptures are extremely unlikely. The analyses depend on advanced fracture mechanics techniques and include investigations of potential indirect failure mechanisms which could lead to piping rupture. The objective of the LBB approach is to demonstrate by analysis that the detection of small flaws, either by ISI or by leakage monitoring systems, is assured long before the flaws could grow to critical or unstable sizes and lead to large breaks, such as the double-ended guillotine pipe rupture. Acceptable analytical procedures are outlined in NUREG-1061, Report of the U.S. Nuclear Regulatory Commission Piping Review Committee, Volume 3, Evaluation of Potential Pipe Breaks, issued November 1984 [71].

The general design criteria in 10 CFR Part 50, Appendix A [3], require that the emergency core cooling systems of nuclear power plants be capable of tolerating a double-ended guillotine break. Specifically, GDC 4 states that:

Structures, systems, and components important to safety shall be designed to accommodate the effects of and to be compatible with the environmental conditions associated with normal operation, maintenance, testing, and postulated accidents, including loss-of-coolant accidents. These structures, systems, and components shall be appropriately protected against dynamic effects, including the effects of missiles, pipe whipping, and discharging fluids, that may result from equipment failures and from events and conditions outside the nuclear power unit. However, dynamic effects associated with postulated pipe ruptures in nuclear power units may be excluded from the design basis when analyses reviewed and approved by the Commission demonstrate that 148

the probability of fluid system piping rupture is extremely low under conditions consistent with the design basis for the piping.

With loss of coolant accidents defined as:

those postulated accidents that result from the loss of reactor coolant at a rate in excess of the capability of the reactor coolant makeup system from breaks in the reactor coolant pressure boundary, up to and including a break equivalent in size to the double-ended rupture of the largest pipe of the reactor coolant system.

LBB technology was applied to commercial nuclear power plant piping beginning in the 1980s in the U.S. GDC 4 requires mitigation against the potential dynamic events from a postulated dynamic piping break (i.e., use of pipe whip restraints and jet impingement shields), unless it can be shown there is an extremely low probability of rupture. The NRC staff developed a deterministic LBB procedure documented in NUREG-1061, Volume 3 [71], which is based on a stringent set of screening criteria and a deterministic fracture mechanics flaw tolerance evaluation. In 1987, the NRC solicited public comments on SRP Section 3.6.3 [72], which incorporated the screening criteria and deterministic fracture mechanics review procedures. Per this guidance, for a piping system to be eligible for LBB analysis, it should, among other factors, have the following:

no active degradation mechanisms that can be potential sources of pipe rupture (e.g.

erosion, corrosion, and fatigue) no materials that are susceptible to brittle cleavage-type failure (i.e., fracture) remote causes of rupture from water hammer and other potential indirect sources The failure mode of concern for PWR primary loop reactor coolant system piping is circumferential cracking in butt-welds. Axial cracking has not been an issue in seamless or seam-welded piping that is stress-relieved or solution-annealed.

Following the procedures in SRP Section 3.6.3, a crack size in the reactor coolant system piping is calculated for a leak rate equal to the plant leakage detection system threshold (e.g., 1 gpm) multiplied by a safety factor of 10 to account for uncertainties. Once the leakage crack size is determined at normal operating conditions, the critical crack size at normal plus SSE loading conditions is determined by limit load analysis with Z-factors or elastic-plastic fracture mechanics analysis, if the material toughness is low enough to require it. This includes a margin of 2 on the crack length or 1.4 on the stresses. Therefore, an extremely low probability of rupture is demonstrated deterministically by having a piping system with no active degradation mechanisms that can be potential sources of pipe rupture and an analytical flaw tolerance evaluation.

Around 2000, a new degradation mechanism was identified in PWR piping systems that were previously approved for LBB. The new mechanism was termed PWSCC. It is characterized by a long crack initiation time and a relatively fast crack growth rate. It occurs in the DMWs (i.e.,

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Alloy 82/182 filler weld metals) between stainless and ferritic steel piping components. The susceptible welds were not stress-relieved, and the normal operating stresses were low, so subcritical crack growth was primarily driven by the WRS. A revision to SRP Section 3.6.3 issued in 2007 [5] clarified that the NRC staff considers PWSCC to be an active degradation mechanism in Alloy 600/82/182 materials in PWRs. After the occurrence of additional PWSCC events, more detailed deterministic and probabilistic analyses were conducted, such as reported in EPRI Technical Report 1020752, Materials Reliability Program: Primary Water Stress Corrosion Cracking of Alloy 600Proceedings of the 2007 International Conference and Exhibition (MRP-221), issued in 2010 [73], and EPRI Technical Report 1011808 Materials Reliability Program: Leak-Before-Break Evaluation for PWR Alloy 82/182 Welds (MRP-140),

issued 2005 [74].

From a deterministic viewpoint, there are two primary scenarios: (1) when the combined WRS and normal operating stresses produce a crack that grows quickly through the pipe wall thickness, and (2) when a surface crack grows a long distance around the inside circumference before becoming a TWC. The first scenario represents LBB behavior; however, the second has the opposite effect (i.e., represents undesirable break-before-leak behavior) unless more complicated analytical evaluations are performed which consider the effects of ISI.

6.2 General Design Criterion 4 The xLPR code was developed, in part, to calculate whether DMWs in PWR piping systems exhibit an extremely low probability of rupture consistent with the requirements of GDC 4 when subject to the effects of PWSCC. The Office of Nuclear Regulatory Research conducted the present study at the request of the Office of Nuclear Reactor Regulation because many PWR licensees eliminated the use of certain equipment that protected against the failure of these piping systems. The approvals for these systems were based on the NRC staffs approval of the licensees deterministic LBB analyses. Since PWSCC had not been addressed in the original licensee analyses, a regulatory question remained as to whether the piping systems with PWSCC in PWRs continue to demonstrate an extremely low probability of rupture consistent with the requirements of GDC 4.

Indeed, PFM analyses were used to support the GDC 4 rulemakings that allow LBB analyses, and the Commission has always envisioned the use of PFM analyses to make the demonstrations required by this regulation. A deterministic approach, however, was favored historically. Successful application of the xLPR code in this study demonstrates that the probabilities of PWR piping system ruptures remain extremely low when subject to PWSCC, which serves to reinforce the role of PFM in making the demonstrations required by GDC 4.

Accordingly, the Office of Nuclear Regulatory Research recommends no changes to the GDC 4 regulations.

6.3 Standard Review Plan Section 3.6.3 SRP Section 3.6.3 allows for an NRC staff-approved LBB analysis to permit a licensee to remove protective hardware such as pipe whip restraints and jet impingement barriers; redesign 150

pipe-connected components, their supports, and their internals; and make other related changes. Compliance with GDC 4 requires that components important to safety be designed to accommodate the effects of, and be compatible with, environmental conditions associated with normal operation, maintenance, testing, and postulated accidents, including loss of coolant accidents. LBB analyses should demonstrate that the probability of pipe rupture is extremely low under conditions consistent with the design basis for the piping to be consistent with GDC 4.

Previously a deterministic evaluation, as described in Section 6.1, of the piping system that demonstrates sufficient margins against failure, including verified design and fabrication and an adequate ISI program, was assumed to satisfy the extremely low probability criterion.

The current SRP Section 3.6.3 review procedures state that the NRC staff should verify the applicants or licensees LBB analysis with consideration of the following factors:

1. The reviewer should verify that the licensees or applicants LBB evaluation uses design basis loads and is based on the as-built piping configuration, as opposed to the design configuration.

2. The reviewer should evaluate the potential for degradation by erosion, erosion-corrosion, and erosion-cavitation due to unfavorable flow conditions and water chemistry.

3. The review should evaluate the material susceptibility to corrosion, the potential for high residual stresses, and environmental conditions that could lead to degradation by stress-corrosion cracking.

4. The reviewer should evaluate the adequacy of the leakage detection systems associated with the reactor coolant system. Determination of leakage from a piping system under pressure involves uncertainties and, therefore, margins are needed.

5. The reviewer should verify that the potential for water hammer in the candidate piping systems is very low.

6. The reviewer should verify that the candidate piping is not susceptible to creep and creep-fatigue.

7. The reviewer should evaluate the corrosion resistance of piping, which can be demonstrated by the frequency and degree of corrosion in the specific piping systems.

8. The reviewer should assess the potential for indirect sources of pipe ruptures to ensure that indirect failure mechanisms defined in the plant safety analysis report are negligible causes of pipe rupture.

9. The reviewer should determine that the piping material will not become susceptible to brittle, cleavage-type failures over the full range of system operating temperatures.

10. The reviewer should determine that the candidate piping does not have a history of fatigue cracking or failure.

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11. The reviewer should review the acceptability of the deterministic LBB evaluation procedure.

12. The reviewer should review the considerations for review of design certification and combined license applications.

The analyses in this and prior studies have demonstrated that previously approved LBB piping systems continue to demonstrate an extremely low probability of rupture consistent with the requirements of GDC 4 with the presence of active degradation mechanisms. As guidance, SRP Section 3.6.3 does not preclude licensees from other such probabilistic LBB demonstrations, which the NRC staff can review directly against the GDC 4 requirements on a case-by-case basis. Accordingly, in the absence of a strong industry interest in future LBB applications, the Office of Nuclear Regulatory Research recommends no immediate changes to SRP Section 3.6.3 to support probabilistic LBB applications. SRP Section 3.6.3 may be retained as-is to support the NRC staffs review of deterministic LBB analyses as needed.

Should a strong demand for probabilistic LBB guidance arise in the future, based on the results of this and the prior study, the Office of Nuclear Regulatory Research recommends a broad expansion of the deterministic review procedures as follows:

Review procedure item 1 addresses the use of design-basis loads. For probabilistic LBB analyses, it is recommended that the reviewer instead verify that the input distributions represent the loads or stresses as applicable to the analysis.

Review procedure items 2, 3, 5, 6, 7, and 10 address the potential for various damage and degradation mechanisms. For probabilistic LBB analyses, it is recommended that the reviewer should instead verify that the applicable mechanisms are explicitly modeled in the analysis with verified and validated models and inputs. The non-applicability or low potential of damage or degradation mechanisms not explicitly modeled in the analysis may be demonstrated following the existing deterministic review procedures.

Review procedure item 4 addresses margins on the leakage detection system. For probabilistic LBB analyses, it is recommended that such margins are not necessary, provided uncertainties in the leak rate calculations have been explicitly modeled in the analysis.

Review procedure item 8 addresses indirect sources of pipe ruptures to ensure that indirect failure mechanisms defined in the plant safety analysis report are negligible causes of pipe rupture. For probabilistic LBB analyses, it is recommended that the reviewer instead verify that the applicable indirect failure mechanisms are explicitly modeled in the analysis with verified and validated models and inputs. The non-applicability or low potential of indirect failure mechanisms not explicitly modeled in the analysis may be demonstrated following the existing deterministic review procedures.

Review procedure item 9 addresses material susceptibility to brittle cleavage-type failures over the full range of system operating temperatures. For probabilistic LBB analyses, it is recommended that the reviewer should instead verify that the fracture 152

behavior of the materials is explicitly modeled in the analysis with verified and validated models and inputs.

Review procedure item 11 addresses steps for an acceptable deterministic LBB evaluation procedure. For probabilistic LBB analyses, it is recommended that the reviewer verify the applicable inputs, models, computational sequences, and outputs.

The pertinent QoI for the analysis is the probability of rupture, which should be extremely low consistent the requirements of GDC 4. The probability of rupture results may reflect any explicitly modeled detection capabilities as necessary (e.g., leak rate detection, ISI, or both as may be necessary).

Review procedure item 12 addresses considerations for the review of design certification and combined license applications. For probabilistic LBB analyses, it is recommended that this item continue to apply.

Not addressed in the current deterministic review procedures are review procedures for PFM analyses. For probabilistic LBB analyses, it is recommended that the reviewer verify that the analysis follows applicable guidance for preparing PFM submittals. The Office of Nuclear Regulatory Research is currently in the process of preparing such guidance.

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7

SUMMARY

AND CONCLUSIONS Following the analyses performed for the prior study, the present generalization study extended the welds under consideration beyond primary piping systems in Westinghouse four-loop PWRs. All piping systems which have received prior LBB approvals from the NRC staff and which contain Alloy 82/182 DMWs that are susceptible to PWSCC were binned for this study as follows:

Bin 1: Westinghouse four-loop RVON and RVIN DMWs Bin 2: Westinghouse pressurizer surge line nozzle DMWs Bin 3: CE and B&W RCP nozzle DMWs Bin 4: Westinghouse steam generator nozzle DMWs Bin 5a: CE hot leg branch line nozzle DMWs Bin 5b: CE cold leg branch line nozzle DMWs Bin 6: Westinghouse two- and three-loop RVON and RVIN DMWs For each bin, a representative weld was analyzed using actual plant data when available and engineering judgement when not. Probability distributions were used to represent the material variability and inherent uncertainties associated with the WRS profiles, among other uncertainties. Otherwise, deterministic inputs for the analysis of each bin were selected such that they would bias the results towards higher probabilities of rupture, thereby bounding all welds represented by the bin. Based on experience, the highest normal operating loads, temperatures, and pressures were selected along with the highest outside diameters and thinnest wall thicknesses. The objective was to define a bounding, although realistic, weld for each bin. In some instances, this basic approach was revised to keep the representative weld within reasonable conditions (e.g., the highest load was not selected for one bin because it was associated with the only weld with MSIP mitigation, and such mitigation was not considered for the base case).

A base case was analyzed for each bin. The base case included the effects of PWSCC initiation and growth for both circumferential and axial cracks with leak rate detection, ISI, and SSE events. These cases were used to estimate the base probability of rupture with a 1 gpm leak rate detection capability. Since these values were zero for all the base cases, even with a large sample size, other QoIs such as the time-dependent probabilities of first crack, first leak, and rupture both with and without ISI were also estimated. The base case was supplemented with a sensitivity study where each realization begins with one axial and one circumferential crack at the top dead center of the weld. As outlined in the prior study, two of the QoIs (i.e., the LBB ratio and LBB time lapse) are not impacted by crack initiation, and thus they can be more accurately estimated with this approach. Prior studies have outlined the importance of WRS and its associated uncertainties. Thus, an additional sensitivity study case considering a more severe WRS profile was also included for each bin. Other sensitivity studies were included to analyze the impacts of fatigue and mechanical mitigation, as appropriate.

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Bin 1 covered Westinghouse four-loop RVON and RVIN DMWs. Although these welds were the focus of the prior study, they were reanalyzed as part of the present study to provide a consistent basis for comparison for all the cases. As in the prior study, the probability of rupture with a 1 gpm leak rate detection capability was estimated to be zero in all cases.

Bin 2 covered Westinghouse pressurizer surge line nozzle DMWs. The probability of rupture with a 1 gpm leak rate detection capability was estimated to be zero in all cases, except in the sensitivity study case that included a weld overlay for mitigation purposes. Although counterintuitive, because of the overlay, some circumferential cracks grew slowly in depth through the more PWSCC-resistant Alloy 52/152 overlay while growing more quickly in length in the more PWSCC-susceptible Alloy 82/182 original weld metal. However, the occurrence of such events was below 1 x 10-6 ruptures per year. When the effects of a 10-year inspection frequency are also considered, the frequency drops to 1 x 10-9 ruptures per year. It should be noted that this inspection frequency was selected for consistency across all the bins to enable comparisons of the results; however, the surge line is currently required by 10 CFR 50.55a(g)(6)(ii)(F) to be inspected more frequently (i.e., every other refueling outage, which is approximately every 3 to 4 years). Modeling such a frequency would only further reduce the annual frequency of rupture. Due to their smaller diameters and different leak rates, the pressurizer surge line nozzle DMWs required larger relative crack sizes to generate 1 or 10 gpm leak rates. These relatively larger crack sizes resulted in lower LBB ratios and LBB time lapses as compared to the Bin 1 results.

Bin 3 covered CE and B&W RCP nozzle DMWs. The probability of rupture with a 1 gpm leak rate detection capability was estimated to be zero in all cases. None of the cases generated a rupture. As a result, the LBB ratio and LBB time lapse QoIs could not be estimated. Based on the prior study, these results were expected given the lower operating temperature of the cold leg where these welds are located. Of note, the base case had a higher probability of first crack as compared to the sensitivity study case with a more severe WRS profile. All the cracks in the base case were axial. The more severe WRS profile was selected to increase the likelihood of circumferential crack initiation; however, it was not enough to initiate any circumferential cracks, and the companion hoop WRS profile was also lower leading to a lower probability of axial cracks.

Bin 4 covered Westinghouse steam generator nozzle DMWs. The base case was defined differently from the other base cases because the inlay was modeled from the beginning of the simulation on account it being applied before the components were placed in service. The probability of rupture with a 1 gpm leak rate detection capability was estimated to be zero in all cases except Case 4.1.2, which was a sensitivity study considering a more severe WRS profile.

This case resulted in a probability of rupture with a 1 gpm leak rate detection capability of 1 x 10-4, which equates to 1.4 x 10-6 ruptures per year. However, the frequency was reduced by 2 orders of magnitude to 7.3 x 10-9 ruptures per year when a 10-year inspection frequency is considered.

Bins 5a and 5b covered CE hot and cold leg branch line nozzle DMWs, respectively. The probability of rupture with a 1 gpm leak rate detection capability was zero in all cases. The cold 155

leg branch line nozzle DMW probabilities of first crack and first leak at 80 EFPY were lower than their hot leg equivalents by roughly a factor of 3. As compared to the Bin 1 results, the smaller diameter piping in Bins 5a and 5b resulted in lower LBB ratios. The LBB time lapses, however, were on average in the same range or higher due to the slower crack growth.

Bin 6 covered Westinghouse two- and three-loop RVON and RVIN DMWs. The probability of rupture with a 1 gpm leak rate detection capability was zero in all cases. The results demonstrate that the Westinghouse four-loop RVON and RVIN DMW analysis results also bound the two- and three-loop designs.

The xLPR code analyzes the risks associated with a single weld; however, GDC 4 requires an aggregation of the results at the system-level. Thus, a piping system-level analysis was necessary to combine the individual bin results and estimate the total probability of rupture for the various PWR piping systems of interest in this study. The probability of rupture with a 1 gpm leak rate detection capability and ISI, as necessary, served as the QoI used to assess whether such piping systems demonstrate an extremely low probability of rupture consistent with the requirements of GDC 4. Since these estimated probabilities were zero in all the base cases, aggregation of the results at the system-level was also zero. Some of the estimates for the sensitivity study cases were non-zero, and these cases were studied and explained. The system-level results for the probability of rupture with detection are thus below the acceptance criterion of 1 x 10-6 ruptures per reactor-year and, therefore, the piping systems continue to meet the requirements of GDC 4.

To illustrate the contributions of the various welds at the system-level, the probabilities of first crack, first leak, and rupture with and without a 10-year inspection frequency were estimated for three groupings of components that bound the various configurations in operating PWRs. The groupings were for Westinghouse four-loop PWR piping systems, Westinghouse two- and three-loop PWR piping systems, and CE and B&W PWR piping systems with prior LBB approvals.

The aggregation method considered all the welds to be independent consistent with the prior study. The largest contributing welds types were shown to vary depending on the QoI under consideration.

Successful application of the xLPR code in this study to demonstrate that the probabilities of PWR piping system ruptures remain extremely low when subject to PWSCC serves to reinforce the role of PFM for making the demonstrations required by GDC 4. Accordingly, the Office of Nuclear Regulatory Research recommends no changes to the GDC 4 regulations as result.

Additionally, in the absence of a strong industry interest in future LBB applications, the Office of Nuclear Regulatory Research recommends no changes to SRP Section 3.6.3 to support probabilistic LBB applications. Should a strong demand for probabilistic LBB guidance arise in the future, a broad expansion of the deterministic review procedures may be considered based on the results of this study.

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[82] M. Uddin and others, Flaw Evaluation Procedure for Cast Austenitic Stainless Steel Materials Using A Newly Developed Statistical Thermal Aging Model, PVP2019-93711, in 164

Proceedings of the ASME 2019 Pressure Vessels & Piping Conference, San Antonion, TX, 2019.

[83] F. W. Brust and others, Part Repair and Start/Stop Effects and Three Dimensional Residual Stress in Dissimilar Metal Welds, PVP2015-45785, in Proceedings of the ASME 2015 Pressure Vessels and Piping Conference, Boston, MA, 2015.

[84] F. W. Brust and others, Chapter 7, Residual Stress and Distortion, in Welding Handbook, 10th Edition, Volume 1, Miami, FL, American Welding Society, 2019.

[85] C. Harrington, MRP Project Manager, EPRI, letter to NRC Document Control Desk, MRP 2021-014, Transmittal of xLPR Welding Residual Stress Essential Parameters and Profile Selection, September 28, 2021, ADAMS Accession Number ML21271A591.

[86] NRC, NUREG-2162, Weld Residual Stress Finite Element Analysis Validation: Part 1 -

Data Development Effort, March 2014, ADAMS Accession Number ML14087A118.

[87] NRC, "NUREG-2228, Weld Residual Stress Finite Element Analysis Validation, Part II -

Proposed Validation Procedure, Final Report, July 2020, ADAMS Accession Number ML20212L592.

[88] H. J. Rathburn and others, NRC Welding Residual Stress Validation Program International Round Robin Program and Findings, PVP2011-57642, in Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference, Baltimore, MD, 2011.

[89] F. W. Brust and others, Weld Residual Stress Analysis and Axial PWSCC Predictions in a Double Vee Groove Large Diameter Nozzle, PVP2013-98065, in Proceedings of the ASME 2013 Pressure Vessels and Piping Conference, Paris, France, 2013.

[90] F. W. Brust and others, Natural PWSCC Crack Growth in Dissimilar Metal Welds with Inlay, PVP2010-26108, in Proceedings of ASME 2010 Pressure Vessels and Piping Division/K-PVP Conference, Bellevue, WA, 2010.

165

APPENDIX A

SUMMARY

OF RESULTS The following table summarizes the results of the probabilistic LBB evaluation of representative PWR piping welds using the xLPR code. The numerical results are presented as mean estimates for each case with the standard error to provide an indication of the level of uncertainty in the estimates. The results for the probability of rupture with a 1 gpm leak rate detection capability were zero for all the bases cases and are thus not reported in the table.

Case No. xLPR Sample Mean Mean Mean Probability Probability Probability Beta Size Occurrence LBB Time Lapse (1) LBB Ratio of 1st of 1st Leak of Rupture Version of Leak Crack at at at 1 gpm 10 gpm 1 gpm 10 gpm Rate Jump 80 EFPY 80 EFPY 80 EFPY (2)

Leak Rate Leak Rate Leak Rate Leak Rate at 80 EFPY Detection Detection Detection Detection Capability Capability Capability Capability (Months) (Months) xLPR 7.4 x 10-3 5.14 x 10-3 1.85 x 10-3 1.1.6a v2.0d 100,000 0 36.5 +/- 1.2 25.1 +/- 0.90 9.6 +/- 0.07 4.6 +/- 0.02 +/- 2.7 x 10-4 +/- 2.3 x 10-4 +/- 1.4 x 10-4 xLPR 9.9 x 10-1 9.6 x 10-1 1.1.6b v2.0d 5,000 0 38.7 +/- 0.3 26.4 +/- 0.2 9.9 +/- 0.01 4.6 +/- 0.00 1+/-0 +/- 1.1 x 10 -3

+/- 3.0 x 10-3 xLPR 7.4 x 10-3 5.14 x 10-3 1.85 x 10-3 1.1.6c v2.0d 100,000 0 36.5 +/- 1.2 25.1 +/- 0.90 9.6 +/- 0.07 4.6 +/- 0.02 +/- 2.7 x 10-4 +/- 2.3 x 10-4 +/- 1.4 x 10-4 xLPR 1.24 x 10-2 1.04 x 10-2 8.36 x 10-4 1.3.0 v2.0d 100,000 0 33.9 +/- 1.6 21.8 +/- 1.8 9.9 +/- 0.1 4.4 +/- 0.04 +/- 3.5 x 10-4 +/- 3.2 x 10-4 +/- 9.1 x 10-5 xLPR 9.73 x 10-1 7.5 x 10-1 1.3.1 v2.0d 5,000 0 33.4 +/- 0.2 20.9 +/- 0.1 10 +/- 0.02 4.5 +/- 0.01 1+/-0 +/- 2.3 x 10 -3

+/- 6.1 x 10-3 xLPR 3.40 x 10-2 3.12 x 10-2 1.09 x 10-4 2.1.0 v2.0d 100,000 0 6.5 +/- 0.7 1.1 +/- 0.1 4.4 +/- 0.1 2.0 +/- 0.07 +/- 5.7 x 10-4 +/- 5.5 x 10-4 +/- 3.3 x 10-5 A-1

Case No. xLPR Sample Mean Mean Mean Probability Probability Probability Beta Size Occurrence LBB Time Lapse (1) LBB Ratio of 1st of 1st Leak of Rupture Version of Leak Crack at at at 1 gpm 10 gpm 1 gpm 10 gpm Rate Jump 80 EFPY 80 EFPY 80 EFPY (2)

Leak Rate Leak Rate Leak Rate Leak Rate at 80 EFPY Detection Detection Detection Detection Capability Capability Capability Capability (Months) (Months) xLPR 1.00 x 10-0 (3) 8.69 x 10-1 2.1.1 v2.0d 5,000 0 6.6 +/- 0.1 1.28 +/- 0.02 4.5 +/- 0.02 2.0 +/- 0.01 1+/-0 +/- 2. x 10 -4

+/- 4.8 x 10-3 xLPR 3.58 x 10-1 3.57 x 10-1 1.03 x 10-3 2.1.2 v2.0d 100,000 0 4.6 +/- 0.2 1.3 +/- 0.08 3.7 +/- 0.07 2.0 +/- 0.03 +/- 1.5 x 10-3 +/- 1.5 x 10-3 +/- 1.0 x 10-4 xLPR 2 x 10-5 1.90 x 10-2 6.4 x 10-5 8.2 x 10-5 2.1.3 v2.0d 100,000 +/- 1.4 x 10-5 11.7 +/- 5.6 10.8 +/- 5.7 2.1 +/- 0.4 1.4 +/- 0.1 +/- 4.3 x 10-4 +/- 2.5 x 10-5 +/- 2.9 x 10-5 (4) xLPR 3.40 x 10-2 3.12 x 10-2 1. x 10-4 2.1.4 v2.0d 100,000 0 2.5 +/- 0.3 0.73 +/- 0.14 4.8 +/- 0.7 2.5 +/- 0.5 +/- 5.7 x 10-4 +/- 5.5 x 10-4 +/- 3.2 x 10-5 xLPR 1.20 x 10-2 8.4 x 10-3 1.8 x 10-5 2.1.5 v2.0d 100,000 0 4.5 +/- 0.5 1 +/- 0.00 4.5 +/- 0.2 2.1 +/- 0.2 +/- 3.5 x 10-4 +/- 2.9 x 10-4 +/- 1.4 x 10-5 xLPR 3.3 x 10-4 3.1.0 (5) (5) (5) (5) v2.0d 100,000 0 NA NA NA NA +/- 5.7 x 10-5 0+/-0 0+/-0 xLPR 2.7 x 10-2 5.8 x 10-3 3.1.1 v2.0d 5,000 0 78 +/- 7 53 +/- 4 10.1 +/- 0.1 4.63 +/- 0.04 1+/-0 +/- 2.3 x 10-3 +/- 1.1 x 10-3 xLPR 3.6 x 10-5 3.1.2 v2.0d 100,000 0 NA(5) NA(5) NA(5) NA(5) +/- 1.9 x 10-5 0+/-0 0+/-0 xLPR 5.21 x 10-2 1.04 x 10-2 4.1.0 v2.0d 100,000 0 NA(5) NA(5) NA(5) NA(5) +/- 7.0 x 10-4 +/- 3.2 x 10-4 0+/-0 xLPR 1.6 x 10-2+/- 2.5 x 10-1 +/- 1.6 x 10-2 +/-

4.1.1 v2.0d 5,000 4.0 x 10-4 NA (5)

NA (5)

NA (5)

NA (5) 1+/-0 6.1 x 10-3 1.8 x 10-3 A-2

Case No. xLPR Sample Mean Mean Mean Probability Probability Probability Beta Size Occurrence LBB Time Lapse (1) LBB Ratio of 1st of 1st Leak of Rupture Version of Leak Crack at at at 1 gpm 10 gpm 1 gpm 10 gpm Rate Jump 80 EFPY 80 EFPY 80 EFPY (2)

Leak Rate Leak Rate Leak Rate Leak Rate at 80 EFPY Detection Detection Detection Detection Capability Capability Capability Capability (Months) (Months) xLPR 2.5 x 10-4+/- 1.3 x 10-1 3.1 x 10-3 2.1 x 10-4 4.1.2 v2.0d 100,000 5.0 x 10-5 NA (5)

NA (5)

NA (5)

NA (5)

+/- 1.1 x 10-3 +/- 5.6 x 10-5 +/- 4.6 x 10-5 xLPR 1.6 x 10-3+/- 1.38 x 10-1+/- 4.4 x 10-3+/- 3.3 x 10-3+/-

4.1.3 v2.0d 100,000 1.3 x 10-4 5.31 +/- 0.25 5.21 +/- 0.24 3.07 +/- 0.05 2.82 +/- 0.04 1.1 x 10-3 2.1 x 10-4 1.8 x 10-4 xLPR 1.6 x 10-3+/- 9.8 x 10-2+/- 6.0 x 10-2+/- 5.4 x 10-4+/-

4.1.4 v2.0d 100,000 1.3 x 10-4 23.8 +/- 3.2 23.7 +/- 3.2 3.08 +/- 0.12 288 +/- 0.09 9.4 x 10-4 7.5 x 10-4 7.5 x 10-5 xLPR 3.2 x 10-4+/- 6.62 x 10-2 6.42 x 10-2 5.1.0 v2.0d 100,000 5.7 x 10-5 NA(5) NA(5) NA(5) NA(5) +/- 7.9 x 10-4 +/- 7.8 x 10-4 0+/-0 xLPR 9.95 x 10-1 1.8 x 10-2 5.1.1 v2.0d 5,000 0 54.3 +/- 2.4 15.6 +/- 0.6 3.4 +/- 0.02 1.8 +/- 0.01 1+/-0 +/- 1.0 x 10-3 +/- 2.0 x 10-3 xLPR 1.06 x 10-1 1.03 x 10-1 5.1.2 v2.0d 100,000 0 NA(5) NA(5) NA(5) NA(5) +/- 9.7 x 10-4 +/- 9.6 x 10-4 0+/-0 xLPR 2.38 x 10-2 1.57 x 10-2 5.2.0 v2.0d 100,000 0 NA(5) NA(5) NA(5) NA(5) +/- 4.8 x 10-4 +/- 3.9 x 10-4 0+/-0 xLPR 3.4 x 10-1 1.00 x 10-3 5.2.1 v2.0d 5,000 0 84 +/- 5 32 +/- 2.5 4.29 +/- 0.03 1.97 +/- 0.01 1+/-0 +/- 6.7 x 10-3 +/- 4.5 x 10-4 A-3

Notes:

(1)

All results beyond 12 EFPY excluded as they strongly influence the mean for the reasons explained in Section 3.2.1.2.3.

(2)

Excludes the effects of leak rate detection and ISI.

(3)

The probability is 0.9998, which was rounded up to 1.00 x 10-0 at two significant digits (4)

The probability of rupture is higher than the probability of first leak in Case 2.1.3 due to surface crack rupture.

(5)

Indicates that the QoI could not be calculated (i.e., there were no ruptures in the simulation).

A-4

APPENDIX B ANALYSIS INPUTS B1 Case 1.1.6a The objective of Case 1.1.6a was to assess the base likelihood of failure caused by PWSCC initiation and growth with without mechanical mitigation for Westinghouse 4-loop RVON and RVIN DMWs.

The random seeds used for the Case 1.1.6a analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 1.1.6 input set from the prior study [2] as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 1.1.06a - Based on Description case description B-1

Global Name Value / Distribution Parameters Units Basis ID 3002 Unmitigated 25 cc/kg Bounds H2 level Constant the operating experience of PWRs as reported in

[48]

B-2

B2 Case 1.1.6b The objective of Case 1.1.6b was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation for Westinghouse 4-loop RVON and RVIN DMWs.

The random seeds used for the Case 1.1.6b analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 1.1.6a input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 1.1.06b - Based on case description Description Crack 0 Based on case description Initiation 0501 -

Type Choice 1 Considers the impact of one Constant circumferential crack and one axial crack because the Number of 1209 - likelihood of multiple cracks Flaws (Circ) is low enough to not affect the results as demonstrated in [2]

Initial Flaw Lognormal Based on PWSCC initial 1210 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1211 -

Full-Length (Circ)

Initial Flaw Lognormal Based on PWSCC initial 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes

(*) max=0.0663)

B-3

Global Name Value / Distribution Parameters Units Basis ID Multiplier 1 Based on PWSCC initial 1213 Starting Constant - flaw sizes Depth (Circ) 1 Considers the impact of one Constant circumferential crack and Number of one axial crack because the 1214 Flaws - likelihood of multiple cracks (Axial) is low enough to not affect the results as demonstrated in [2]

Initial Flaw Lognormal Based on PWSCC initial 1215 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1216 -

Full-Length (Axial)

Initial Flaw Lognormal Based on PWSCC initial 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1218 -

Depth (Axial)

B-4

B3 Case 1.1.6c The objective of Case 1.1.6c was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation for Westinghouse 4-loop RVON and RVIN DMWs when a 5-years in-service inspection schedule is considered.

The random seeds used for the Case 1.1.6c analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 1.1.6 input set from the prior study [2] as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 1.1.06c - Based on case Description description 0811 Inspection 1 Inspection schedule Schedule set by frequency Input Type 0812 Pre-Mitigation 0.2 yr-1 Annual frequency set Inspection to one inspection every Freq. 5 years 0813 Post- 0.2 yr-1 Not necessary, but set Mitigation to the same value as Inspection Global ID 0812 for Freq. completeness B-5

Global Name Value / Distribution Parameters Units Basis ID 3002 Unmitigated 25 cc/kg Bounds the operating H2 level Constant experience of PWRs as reported in [48]

B-6

B4 Case 2.1.0 The objective of Case 2.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation for Westinghouse pressurizer surge line nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 2.1.0 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 3, Scenario 3, input set from the xLPR Inputs Group report [53] as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 2.1.00 - Based on case Description description 0001 Plant 960 mon Based on case Operation description time 0402 Period End 961 mon Based on case Time (Op description Period #1) 0403 Input Type 2 - Based on case Choice (Op description Period #1)

B-7

Global Name Value / Distribution Parameters Units Basis ID 0405 Period End 962 mon Based on case Time (Op description Period #2) 0811 Inspection 1 - Inspection Schedule schedule set by Input Type frequency 0812 Pre- 0.1 1/yr Annual frequency Mitigation set to one Inspection inspection every Freq 10 years 0820 Number of 1 - Based on case cracks description detected 0904 Max time 1 mon Based on case between 2 description check -

single TWC -

CC All Data Source Epistemic - Outer loop uncertain preserves LHS variables, structure except Global ID 2528 1001 Effective Full 80 yr Based on case Power Years Constant description (EFPY) 1101 Pipe Outer 0.3556 m Typical value for Diameter Constant Westinghouse four-loop pressurizer surge line 1102 Pipe Wall 0.028575 m Minimum pipe wall Thickness Constant thickness for Westinghouse four-loop pressurizer surge line B-8

Global Name Value / Distribution Parameters Units Basis ID 1103 Weld Width 0.02648 m Outside diameter Constant weld width from Figure 1 of [75]

1104 Weld 0.028575 m Set to same value Material Constant as Global ID 1102 Thickness 3002 Unmitigated 25 cc/kg Bounds the H2 Level Constant operating experience of PWRs as reported in [48]

3102 Operating 345 °C Typical operating Temperature Constant temperature for Westinghouse four-loop pressurizer surge line as reported in

[76]

4001 Earthquake 1E-3 1/yr From Section Probability Constant E.3.1 of [60], the maximum earthquake probability is 1E-3 4002 Earthquake 169.64 MPa Maximum SSE Total Constant load from Figure Membrane 2-6 of [60] is combined membrane and bending 4003 Earthquake 0 MPa Maximum SSE Inertial Constant load from Figure Bending 2-6 of [60] is combined membrane and bending 4004 Earthquake 0 MPa Maximum SSE Anchor Constant load from Figure Bending 2-6 of [60] is combined membrane and bending B-9

Global Name Value / Distribution Parameters Units Basis ID 4005 Sigma_SSa 0 MPa Based on case Constant description 4006 Sigma_SSh 0 MPa Based on case Constant description 4101 Fx (Dead 0 kN Set to 0 because Weight) Constant all loads input as stresses instead of forces and moments 4102 Mx (Dead 0 kN-m Set to 0 because Weight) Constant all loads input as stresses instead of forces and moments 4103 My (Dead 0 kN-m Set to 0 because Weight) Constant all loads input as stresses instead of forces and moments 4104 Mz (Dead 0 kN-m Set to 0 because Weight) Constant all loads input as stresses instead of forces and moments 4105 Fx (Thermal 0 kN Set to 0 because Expansion) Constant all loads input as stresses instead of forces and moments 4106 Mx (Thermal 0 kN-m Set to 0 because Expansion) Constant all loads input as stresses instead of forces and moments 4107 My (Thermal 0 kN-m Set to 0 because Expansion) Constant all loads input as stresses instead of forces and moments B-10

Global Name Value / Distribution Parameters Units Basis ID 4108 Mz (Thermal 0 kN-m Set to 0 because Expansion) Constant all loads input as stresses instead of forces and moments 4121 Membrane 0 MPa Stress from [60] is Stress (DW) Constant combined DW and thermal, so this input is set to 0, and Global ID 4123 contains the DW contribution 4122 Maximum 0 MPa Stress from [60] is Bending Constant combined DW and Stress (DW) thermal, so this input is set to 0, and Global ID 4124 contains the DW contribution 4123 Membrane 5.06 MPa Stress from [60] is Stress Constant combined DW and (Thermal) thermal; used the limiting thermal maximum across all plants represented in Figure 2-7, Plant C 4124 Bending 100.32 MPa Stress from [60] is Stress Constant combined DW and (Thermal) thermal; used the limiting thermal maximum across all plants represented in Figure 2-8, Plant I B-11

Global Name Value / Distribution Parameters Units Basis ID

- Hoop WRS Mean Std. Dev. MPa Mean profile and Pre- -32.21 76.7 standard deviation Mitigation 16.12 76.7 are based on 10 51.74 76.7 FEA results as reported in 61.55 76.7 Figures 14 and 15 76.59 76.7 of [75]

90.81 76.7 110.18 76.7 87.32 76.7 78.81 76.7 68.77 76.7 44.21 76.7

-10.78 76.7

-61.59 76.7

-85.78 76.7

-91.10 76.7

-59.64 76.7

-14.89 76.7 30.41 76.7 79.04 76.7 112.83 76.7 147.84 76.7 172.87 76.7 192.41 76.7 171.25 76.7 145.10 76.7 128.88 76.7 B-12

Global Name Value / Distribution Parameters Units Basis ID

- Axial WRS Mean Std. Dev. MPa Mean profile and Pre- -176.05 57.9 standard Mitigation -155.45 57.9 deviation are

-135.68 57.9 based on 10 FEA results as

-122.95 57.9 reported in

-108.40 57.9 Figures 14 and

-107.12 57.9 15 of [75]

-102.24 57.9

-98.37 57.9

-105.16 57.9

-121.23 57.9

-152.48 57.9

-188.90 57.9

-208.65 57.9

-205.37 57.9

-173.00 57.9

-127.62 57.9

-61.80 57.9 13.09 57.9 87.12 57.9 160.00 57.9 228.12 57.9 296.98 57.9 352.88 57.9 351.06 57.9 339.51 57.9 335.30 57.9 5004 Lower bound 0 - Even though it is POD, POD0 not used, the default 0.999 value would lead to 99.9 percent probability of detection for a crack of zero depth B-13

Global Name Value / Distribution Parameters Units Basis ID 5101-5110 Pre- beta_0 (circ): Normal (2.71, 0.21) - Based on [50]

Mitigation beta_1 (circ): Normal (0.31, 0.45)

Inspection beta_0 (axial): Normal (-0.8, 0.38)

Properties beta_1 (axial): Normal (8.3, 1.45) a (circ): Normal (0.034, 0.006) b (circ): Normal (0.955, 0.013) a (axial): Normal (0.041, 0.011) b (axial): Normal (0.88, 0.029)

Sigma_depth (circ): 0.072 Sigma_depth (axial): 0.078 Correlation Intercept, B0 -0.86 - Based on [50]

5101-5102 (circ)

Intercept, B1 (circ)

Correlation Intercept, B0 -0.93 - Based on [50]

5103-5104 (axial)

Intercept, B1 (axial)

Correlation a (circ) -0.867 - Based on [50]

5105-5106 b (circ)

Correlation a (axial) -0.87 - Based on [50]

5107-5108 b (axial)

B-14

B5 Case 2.1.1 The objective of Case 2.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation for Westinghouse pressurizer surge line nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 2.1.1 analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 2.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 2.1.01 - Based on case Description description Crack 0 Based on case 0501 Initiation - description Type Choice 1 Considers the impact of Constant one circumferential crack and one axial Number of crack because the 1209 -

Flaws (Circ) likelihood of multiple cracks is low enough to not affect the results as demonstrated in [2]

Initial Flaw Lognormal Based on PWSCC 1210 Full-Length (1, 4.3E-3, 2.226) m initial flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC Starting Full- Constant initial flaw sizes 1211 -

Length (Circ)

B-15

Global Name Value / Distribution Parameters Units Basis ID Initial Flaw Lognormal Based on PWSCC 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m initial flaw sizes

(*) max=0.0663)

Multiplier 1 Based on PWSCC 1213 Starting Constant - initial flaw sizes Depth (Circ) 1 Considers the impact of Constant one circumferential crack and one axial Number of crack because the 1214 Flaws -

likelihood of multiple (Axial) cracks is low enough to not affect the results as demonstrated in [2]

Initial Flaw Lognormal Based on PWSCC 1215 Full-Length (1, 4.3E-3, 2.226) m initial flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC Starting Full- Constant initial flaw sizes 1216 -

Length (Axial)

Initial Flaw Lognormal Based on PWSCC 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m initial flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC Starting Constant initial flaw sizes 1218 -

Depth (Axial)

B-16

B6 Case 2.1.2 Case 2.1.2 was a sensitivity study of Case 2.1.0 considering a more severe WRS profile for Westinghouse pressurizer surge line nozzle DMWs.

The random seeds used for the Case 2.1.2 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 2.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Units Basis ID Parameters N/A Case 2.1.02 - Based on case description Description B-17

Global Name Value / Distribution Units Basis ID Parameters

- Hoop WRS Mean Std. Dev. MPa More severe hoop WRS Pre-Mitigation 208.41 76.7 profile estimated from FEA 292.19 76.7 results for the weld butter as 338.16 76.7 discussed in Section C2.2 358.36 76.7 330.63 76.7 369.53 76.7 394.38 76.7 377.63 76.7 306.73 76.7 138.90 76.7 11.55 76.7

-82.04 76.7

-96.97 76.7

-75.07 76.7

-37.10 76.7 3.23 76.7 72.29 76.7 140.58 76.7 214.52 76.7 308.49 76.7 346.74 76.7 428.52 76.7 417.46 76.7 446.92 76.7 409.06 76.7 382.11 76.7 B-18

Global Name Value / Distribution Units Basis ID Parameters N/A Axial WRS Mean Std. Dev. MPa More severe axial WRS Pre-Mitigation -118.00 57.9 profile estimated from FEA

-60.80 57.9 results for the weld butter as

-25.21 57.9 discussed in Section C2.2

-21.12 57.9

-20.85 57.9

-18.21 57.9

-17.66 57.9

-107.85 57.9

-226.94 57.9

-323.93 57.9

-369.72 57.9

-331.22 57.9

-247.03 57.9

-249.79 57.9

-217.62 57.9

-160.65 57.9

-66.23 57.9

-42.58 57.9 33.29 57.9 112.75 57.9 196.05 57.9 303.46 57.9 371.40 57.9 479.99 57.9 510.97 57.9 529.34 57.9 B-19

B7 Case 2.1.3 Case 2.1.3 was a sensitivity study of Case 2.1.0 considering overlay mitigation for Westinghouse pressurizer surge line nozzle DMWs.

The random seeds used for the Case 2.1.3 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using Case 3, Scenario 9, from the xLPR Inputs Group report

[53] as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case Description 2.1.03 - Based on case description 0301 Mitigation Type 1 - Based on case Choice description 0305 Stress Mitigation 2 - Based on case Choice description B-20

Global Name Value / Distribution Parameters Units Basis ID 0306 Stress Mitigation 300 mon Average Time overlay mitigation application for the set of pressurizer surge line nozzles represented by bin 0803 Post-Overlay 1 - Consistent Trunc Meas Error with pre-mitigation approach 0804 Post-Overlay Eval 0 - Default value Length Effects 0805 Full Structural 0 - Minimum WOL overlay thickness selected is consistent with an optimized weld overlay 0813 Post-Mitigation 0.1 1/yr Annual Inspection Freq. frequency set to one inspection every 10 years 0815 Post-Overlay 0 - Default value Ligament Flag 1105 Weld Overlay 0.0125 m Smallest Thickness Constant overlay thickness for Waterford, Unit 3 from

[77], which bounds the welds represented by bin B-21

Global Name Value / Distribution Parameters Units Basis ID 2701 Yield Strength, Lognormal MPa Alloy 52/152 Sigy (317, 54.99, min=209, max=466) material property from xLPR Inputs Group report

[53]

2702 Ultimate Strength, Lognormal MPa Alloy 52/152 Sigu (542, 26.81, min=483, max=608) material property from xLPR Inputs Group report

[53]

2705 Elastic Modulus, E Normal MPa Alloy 52/152 (196800, 29520, min=167280, material max=226320) property from xLPR Inputs Group report

[53]

2706 Material Init J- Normal N/mm Alloy 52/152 Resistance, Jic (524.3, 182, min=225.1, material max=947.4) property from xLPR Inputs Group report

[53]

2707 Material Init J- Normal N/mm Alloy 52/152 Resist Coef, C (586, 76.2, min=460.9, max=763.6) material property from xLPR Inputs Group report

[53]

2708 Material Init J- Normal - Alloy 52/152 Resist Exponent, (0.661, 0.074, min=0.2, max=1) material m property from xLPR Inputs Group report

[53]

B-22

Global Name Value / Distribution Parameters Units Basis ID 2743 Multiplier proport. Lognormal - Based on Const. A (DM1) (0.0417, 17.99) minimum FOI of 24 from Pacific Northwest National Laboratories test data on Alloy 52/152 crack initiation and using a similar method as in [78]. The Alloy 82/182 distribution median from the xLPR Inputs Group report [53] was divided by this FOI.

Note that this input was is not used in the simulation because cracks initiate on the inside diameter, not on the outside diameter where the overlay is applied. It was included for completeness.

B-23

Global Name Value / Distribution Parameters Units Basis ID 2788 Power Law 2.01E-12 (m/s)(MPa- Set equal to Constant, Alpha Constant m1/2)^(- the Alloy beta) 82/182 power law constant since the FOI for Alloy 52/152 is applied in Global ID 2796 2789 Power Law 1.6 - Alloy 52/152 Exponent, Beta Constant material property from xLPR Inputs Group report

[53]

2790 SIF Threshold, Kth 0 MPa-m1/2 Alloy 52/152 Constant material property from xLPR Inputs Group report

[53]

2791 Activation Energy, Normal kJ/mol Alloy 52/152 Qg (130, 20) material property from xLPR Inputs Group report

[53]

2792 Comp-to-Comp Lognormal - Alloy 52/152 Variab Factor, (1, 1.803, min=0.313, max=2.64) material fcomp property from xLPR Inputs Group report

[53]

2793 Within-Comp Lognormal - Alloy 52/152 Variab Factor, (1, 1.617, min=0.309, max=3.24) material fflaw property from xLPR Inputs Group report

[53]

B-24

Global Name Value / Distribution Parameters Units Basis ID 2794 Peak-to-Valley 1 - Alloy 52/152 ECP Ratio - 1, P-1 Constant material property from xLPR Inputs Group report

[53]

2795 Charact Width of 1 mV Alloy 52/152 Peak vs ECP, c Constant material property from xLPR Inputs Group report

[53]

2796 Factor of 324 - Represents Improvement, IF Constant 75th percentile FOI from [52]

2797 Reference 325 Cdeg Alloy 52/152 Temperature Constant material property from xLPR Inputs Group report

[53]

Correlation Yield Strength, 0.709 - Alloy 52/152 2701-2702 Sigy material Ultimate Strength, property from Sigu xLPR Inputs Group report

[53]

4351 Hoop WRS Post- Epistemic - Uncertainty Mitigation applied to the post-mitigation WRS profile 4353 Axial WRS Post- Epistemic - Uncertainty Mitigation applied to the post-mitigation WRS profile B-25

Global Name Value / Distribution Parameters Units Basis ID N/A Hoop WRS Post- Mean Std. Dev. MPa Overlay mitigation -232.21 76.7 mitigation rules

-175.88 76.7 from [47]

-132.26 76.7 applied to the unmitigated

-114.45 76.7 mean WRS

-91.41 76.7 profile.

-69.19 76.7 Standard

-41.82 76.7 deviation set

-56.68 76.7 equal to the

-57.19 76.7 unmitigated

-59.23 76.7 WRS profile

-75.79 76.7 standard deviation.

-122.78 76.7

-165.59 76.7

-181.78 76.7

-179.1 76.7

-139.64 76.7

-86.89 76.7

-33.59 76.7 23.04 76.7 64.83 76.7 107.84 76.7 140.867 76.7 168.41 76.7 155.25 76.7 137.1 76.7 128.88 76.7 B-26

Global Name Value / Distribution Parameters Units Basis ID N/A Axial WRS Post- Mean Std. Dev. MPa Overlay mitigation -71.85 57.9 mitigation rules

-110.84 57.9 from [47]

-152.67 57.9 applied to the unmitigated

-189.00 57.9 mean WRS

-238.47 57.9 profile.

-274.20 57.9 Standard

-207.02 57.9 deviation set

-154.39 57.9 equal to the

-122.41 57.9 unmitigated

-82.01 57.9 WRS profile

-15.66 57.9 standard deviation.

24.11 57.9 32.11 57.9 22.69 57.9 69.42 57.9 143.87 57.9 202.51 57.9 203.98 57.9 215.93 57.9 213.70 57.9 164.56 57.9 119.35 57.9 78.72 57.9 32.64 57.9

-25.57 57.9

-64.50 57.9 5201 Depth repair 0 - Set to pre-threshold x_TH Constant mitigation (during) value because no applicable values for overlays 5202 Depth repair 0 - Set to pre-threshold x_TH Constant mitigation (post) value because no applicable values for overlays B-27

Global Name Value / Distribution Parameters Units Basis ID 5301-5312 Post-Overlay beta_0 (circ): Normal (2.71, 0.21) - Set equal to Inspection beta_1 (circ): Normal (0.31, 0.45) the pre-Properties beta_0 (axial): Normal (-0.8, 0.38) mitigation beta_1 (axial): Normal (8.3, 1.45) inspection property a (circ): Normal (0.034, 0.006) values b (circ): Normal (0.955, 0.013) because no a (axial): Normal (0.041, 0.011) applicable b (axial): Normal (0.88, 0.029) values for Sigma_depth (circ): 0.072 overlays Sigma_depth (axial): 0.078 x_small: 0.1 x_LB: 0 Correlation Intercept, B0 (circ) -0.86 - Set equal to 5301-5302 Slope, B1 (circ) the pre-mitigation inspection property values because no applicable values for overlays Correlation Intercept, B0 -0.93 - Set equal to 5301-5302 (axial) the pre-Slope, B1 (axial) mitigation inspection property values because no applicable values for overlays Correlation a (circ) -0.867 - Set equal to 5301-5302 b (circ) the pre-mitigation inspection property values because no applicable values for overlays B-28

Global Name Value / Distribution Parameters Units Basis ID Correlation a (axial) -0.87 - Set equal to 5301-5302 b (axial) the pre-mitigation inspection property values because no applicable values for overlays B-29

B8 Case 2.1.4 The objective of Case 2.1.4 was to assess the base likelihood of failure caused by fatigue initiation and growth without mechanical mitigation for Westinghouse pressurizer surge line nozzle DMWs.

The random seeds used for the Case 2.1.4 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using Case 2.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID

- Case 2.1.04 Based on case Description description 0411 Transient Type 2 - Option to Selection consider thermal (Load 1) stratification with heatup transient 0411.2 Transient Type 2 - Option to Selection consider thermal (Load 2) stratification with cooldown transient B-30

Global Name Value / Distribution Parameters Units Basis ID 0411.3 Transient Type 1 - Option to Selection consider plant (Load 3) loading transient without thermal stratification 0411.4 Transient Type 1 - Option to Selection consider plant (Load 4) unloading transient without thermal stratification 0411.5 Transient Type 1 - Option to Selection consider step (Load 5) load increase transient without thermal stratification 0411.6 Transient Type 1 - Option to Selection consider step (Load 6) load decrease transient without thermal stratification 0411.7 Transient Type 1 - Option to Selection consider loss of (Load 7) load transient without thermal stratification 0411.8 Transient Type 1 - Option to Selection consider partial (Load 8) loss of flow transient without thermal stratification 0411.9 Transient Type 1 - Option to Selection consider reactor (Load 9) trip transient without thermal stratification B-31

Global Name Value / Distribution Parameters Units Basis ID 0411.10 Transient Type 3 - Option to Selection consider (Load 10) operating basis earthquake transient 0501 Crack Initiation 3 - Option to Type Choice consider fatigue initiation consistent with case description 0601 Crack Growth 1 - Option to Type Choice consider fatigue growth consistent with case description 1201 Fatigue Initial Lognormal mm Based on Flaw Full- (0, 8.61, 4.849) Case 3, Length (*) Scenario 10, input set from

[50]

1202 Multiplier 1 - Based on Fatigue Initial Constant Case 3, Full-Length Scenario 10, input set from

[50]

1203 Fatigue Initial Lognormal mm Based on Flaw Depth (*) (0, 3, 0.05) Case 3, Scenario 10, input set from

[50]

1204 Multiplier 1 - Based on Fatigue Initial Constant Case 3, Depth Scenario 10, input set from

[50]

3001 Flow Rate 0.18 m/s From [55]

Constant B-32

Global Name Value / Distribution Parameters Units Basis ID 3103 Dissolved 40 ppm Based on Oxygen Constant Case 3, Scenario 10, input set from

[50]

9001 Fatigue Growth Lognormal - Based on CKTH (1, 1, 1.149, 0, 4.559) Case 3, Scenario 10, input set from

[50]

Correlation Strain 1 - Value imposed 2525-2528 Threshold, by the xLPR STH code Co Transient Points, Times, 1, 0, -287.78, -1.34E7 -, s, Cdeg, Plant heatup Definitions Delta Ts, and 2, 20880, -32.24, 0 Pa transient from Tab Delta Ps for Table 4-2 of [54]

Transient 1 Transient Points, Times, 1, 0, 0, 0 -, s, Cdeg, Plant cooldown Definitions Delta Ts, and 2, 20880, -323.89, -1.54E7 Pa transient from Tab Delta Ps for Table 4-2 of [54]

Transient 2 Transient Points, Times, 1, 0, 0, 0 -, s, Cdeg, Plant loading Definitions Delta Ts, and 2, 50, 0, 0 Pa transient from Tab Delta Ps for Table 4-2 of [54]

3, 50.1, 0, 4.14E5 Transient 3 4, 1120, 0, 4.14E5 5, 1120.1, 0, 4.14E5 Transient Points, Times, 1, 0, 0, -6.89E4 -, s, Cdeg, Plant unloading Definitions Delta Ts, and 2, 0.1, 0, -6.89E4 Pa transient from Tab Delta Ps for Table 4-2 of [54]

3, 30, 0, 0 Transient 4 4, 30.1, 0, 0 5, 200, 0, -1.86E6 6, 1100, 0, -3.45E5 7, 1100.1, 0, -3.45E5 8, 1200, 0, 4.83E5 B-33

Global Name Value / Distribution Parameters Units Basis ID Transient Points, Times, 1, 0, 0, 0 -, s, Cdeg, Plant step load Definitions Delta Ts, and 2, 10, 0, 0 Pa increase Tab Delta Ps for transient from 3, 10.1, 0, 0 Transient 5 Table 4-2 of [54]

4, 140, 0, 6.21E5 5, 140.1, 0, 6.21E5 6, 350, 0, 0 7, 350.1, 0, 0 Transient Points, Times, 1, 0, 0, 6.89E4 -, s, Cdeg, Plant step load Definitions Delta Ts, and 2, 0.1, 0, 1.93E5 Pa decrease Tab Delta Ps for transient from 3, 10, 0, 2E5 Transient 6 Table 4-2 of [54]

4, 10.1, 0, 2E5 5, 150, 0, 2.07E5 6, 150.1, 0, -4.83E5 7, 350, 0, -4.83E5 8, 350.1, 0, -4.83E5 Transient Points, Times, 1, 0, 0, 0 -, s, Cdeg, Plant loss of load Definitions Delta Ts, and 2, 2, 0, 0 Pa transient from Tab Delta Ps for Table 4-2 of [54]

3, 2.1, 0, 0 Transient 7 4, 13, 0, 0, 1.03E6 5, 13.1, 0, 1.03E6 6, 60, -20.56, -3.38E6 7, 120, -20.56, -3.65E6 8, 120.1, -20.56, -3.65E6 Transient Points, Times, 1, 0, 0, 0 -, s, Cdeg, Plant partial loss Definitions Delta Ts, and 2, 2, 0, 0 Pa of flow transient Tab Delta Ps for from Table 4-2 of 3, 2.1, 0, 0 Transient 8 [54]

4, 13, 0, 0, 1.03E6 5, 13.1, 0, 1.03E6 6, 60, -20.56, -3.38E6 7, 120, -20.56, -3.65E6 8, 120.1, -20.56, -3.65E6 B-34

Global Name Value / Distribution Parameters Units Basis ID Transient Points, Times, 1, 0, 0, 0 -, s, Cdeg, Plant reactor trip Definitions Delta Ts, and 2, 2, 0, 0 Pa transient from Tab Delta Ps for Table 4-2 of [54]

3, 2.1, 0, 0 Transient 9 4, 13, 0, 0, 1.03E6 5, 13.1, 0, 1.03E6 6, 60, -20.56, -3.38E6 7, 120, -20.56, -3.65E6 8, 120.1, -20.56, -3.65E6 Type 2 +/- Membrane 2.08, MPa, Heatup transient Transient Stress, 73.73, MPa, loading from Inputs on +/- Bending Table 4 of [55],

0, mon, TIFFANY Stress, and frequency Inputs 960, mon, from Table 4-2 of Start Month, Worksheet, 0.5, -, [54]

End Month, Transient 1 8.33, 1/yr, Front-Back Loading, 1 -

Frequency,

1. of Cycles per Event Type 2 +/- Membrane 2.08, Mpa, Cooldown Transient Stress, +/- 73.73, MPa, transient loading Inputs on Bending from Table 4 of 0, mon, TIFFANY Stress, Start [55], and Inputs Month, 960, mon, frequency from Worksheet, End Month, 0.5, -, Table 4-2 of [54]

Transient 2 8.33, 1/yr, Front-Back Loading, 1 -

Frequency,

1. of Cycles per Event Type 1 Start Month, 0, mon, Plant loading Transient End Month, 960, mon, transient Inputs on frequency from Front-Back 0.5, -,

TIFFANY Table 4-2 of [54]

Loading, 250, 1/yr, Inputs Worksheet, Frequency, 1 -

Transient 3 # of Cycles per Event B-35

Global Name Value / Distribution Parameters Units Basis ID Type 1 Start Month, 0, mon, Plant unloading Transient End Month, 960, mon, transient Inputs on frequency from Front-Back 0.5, -,

TIFFANY Table 4-2 of [54]

Loading, 250, 1/yr, Inputs Worksheet, Frequency, 1 -

Transient 4 # of Cycles per Event Type 1 Start Month, 0, mon, Step load Transient End Month, 960, mon, increase Inputs on transient Front-Back 0.5, -,

TIFFANY frequency from Loading, 33.33, 1/yr, Inputs Table 4-2 of [54]

Worksheet, Frequency, 1 -

Transient 5 # of Cycles per Event Type 1 Start Month, 0, mon, Step load Transient End Month, 960, mon, decrease Inputs on transient Front-Back 0.5, -,

TIFFANY frequency from Loading, 33.33, 1/yr, Inputs Table 4-2 of [54]

Worksheet, Frequency, 1 -

Transient 6 # of Cycles per Event Type 1 Start Month, 0, mon, Loss of load Transient End Month, 960, mon, transient Inputs on frequency from Front-Back 0.5, -,

TIFFANY Table 4-2 of [54]

Loading, 1.33, 1/yr, Inputs Worksheet, Frequency, 1 -

Transient 7 # of Cycles per Event Type 1 Start Month, 0, mon, Partial loss of Transient End Month, 960, mon, flow transient Inputs on frequency from Front-Back 0.5, -,

TIFFANY Table 4-2 of [54]

Loading, 1.33, 1/yr, Inputs Worksheet, Frequency, 1 -

Transient 8 # of Cycles per Event B-36

Global Name Value / Distribution Parameters Units Basis ID Type 1 Start Month, 0, mon, Reactor trip Transient End Month, 960, mon, transient Inputs on frequency from Front-Back 0.5, -,

TIFFANY Table 4-2 of [54]

Loading, 10.33, 1/yr, Inputs Worksheet, Frequency, 1 -

Transient 9 # of Cycles per Event Type 3 +/- Membrane 4.52, Mpa, Loading from Transient Stress, 60.61, MPa, Table 4 of [55];

Inputs on +/- Bending frequency, 0, mon, TIFFANY Stress, # cycles, and rise Inputs 960, mon, time based on Start Month, Worksheet, 0.5, -, Case 3, Transient End Month, Scenario 10, 0.1, 1/yr, 10 Front-Back input set from 10, -,

Loading, [50]

1 s Frequency,

1. of Cycles per Event, Rise Time Uncertainty Transients 1 Epistemic - Based on on through 10 Lognormal Case 3, TIFFANY (1, 0.5, 1.4142, 0.25, 1) Scenario 10, Inputs input set from Worksheet [50]

B-37

B9 Case 2.1.5 Case 2.1.5 was a sensitivity study of Case 2.1.0 considering MSIP mitigation for Westinghouse pressurizer surge line nozzle DMWs.

The random seeds used for the Case 2.1.5 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using Case 2.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID

- Case 2.1.05 Based on case Description description 0301 Mitigation Type 1 - Based on case Choice description 0305 Stress 1 - Based on case Mitigation description Choice B-38

Global Name Value / Distribution Parameters Units Basis ID 0306 Stress 144 mon Equivalent to Mitigation Time 12 years, which is the latest time of MSIP application for the welds represented by bin 0806 Post-MSIP 1 - Based on case Trunc Meas description Error 0807 Post-MSIP 0 - Default value Eval Length Effects 0813 Post-Mitigation 0.1 1/yr Annual frequency Inspection set to one Freq inspection every 10 years 0816 Post-MSIP 0 - Default value Ligament Flag 4351 Hoop WRS Epistemic MPa Uncertainty post-mitigation applied to the post-mitigation WRS profile 4353 Axial WRS Epistemic MPa Uncertainty post-mitigation applied to the post-mitigation WRS profile 5201 Depth repair 0 - Set equal to the threshold, Constant pre-mitigation x_TH (during) inspection property values because no applicable values for MSIP B-39

Global Name Value / Distribution Parameters Units Basis ID 5202 Depth repair 0 - Set equal to the threshold, Constant pre-mitigation x_TH (post) inspection property values because no applicable values for MSIP 5401-5412 Post-MSIP beta_0 (circ): Normal (2.71, 0.21) - Set equal to the Inspection beta_1 (circ): Normal (0.31, 0.45) pre-mitigation Properties beta_0 (axial): Normal (-0.8, 0.38) inspection beta_1 (axial): Normal (8.3, 1.45) property values because no a (circ): Normal (0.034, 0.006) applicable values b (circ): Normal (0.955, 0.013) for MSIP a (axial): Normal (0.041, 0.011) b (axial): Normal (0.88, 0.029)

Sigma_depth (circ): 0.072 Sigma_depth (axial): 0.078 Correlation Intercept, B0 -0.86 - Set equal to the 5401-5402 (circ) pre-mitigation Slope, B1 (circ) inspection property values because no applicable values for MSIP Correlation Intercept, B0 -0.93 - Set equal to the 5403-5404 (axial) pre-mitigation Slope, B1 inspection (axial) property values because no applicable values for MSIP Correlation a (circ) -0.867 - Set equal to the 5405-5406 b (circ) pre-mitigation inspection property values because no applicable values for MSIP B-40

Global Name Value / Distribution Parameters Units Basis ID Correlation a (axial) -0.87 - Set equal to the 5407-5408 b (axial) pre-mitigation inspection property values because no applicable values for MSIP N/A Post-Mitigation Mean Std. Dev. MPa MSIP mitigation Hoop WRS -284.704 76.7 rules from [47]

-231.14 76.7 applied to the

-190.27 76.7 unmitigated mean WRS

-175.22 76.7 profile. Standard

-154.94 76.7 deviation set

-135.48 76.7 equal to the

-110.86 76.7 unmitigated WRS

-128.48 76.7 profile standard

-131.74 76.7 deviation.

-136.54 76.7

-155.86 76.7

-205.61 76.7

-251.17 76.7

-270.11 76.7

-270.19 76.7

-233.49 76.7

-183.50 76.7

-132.96 76.7

-79.09 76.7

-40.05 76.7 0.20 76.7 30.48 76.7 55.26 76.7 39.35 76.7 18.44 76.7 7.46 76.7 B-41

Global Name Value / Distribution Parameters Units Basis ID N/A Post-Mitigation Mean Std. Dev. MPa MSIP mitigation Axial WRS -294.48 57.9 rules from [47]

-269.15 57.9 applied to the

-244.64 57.9 unmitigated mean WRS

-227.18 57.9 profile. Standard

-207.89 57.9 deviation set

-201.87 57.9 equal to the

-182.16 57.9 unmitigated WRS

-163.81 57.9 profile standard

-156.48 57.9 deviation.

-158.81 57.9

-176.67 57.9

-200.07 57.9

-207.16 57.9

-191.58 57.9

-147.28 57.9

-90.34 57.9

-13.31 57.9 72.42 57.9 156.94 57.9 239.93 57.9 313.26 57.9 385.70 57.9 445.19 57.9 446.96 57.9 439.00 57.9 438.37 57.9 B-42

B10 Case 3.1.0 The objective of Case 3.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation for CE and B&W RCP nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 3.1.0 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 2, Scenario 3, input set from the xLPR Inputs Group report [53] as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 3.1.00 - Based on case description Description 0001 Plant 960 mon Based on case description Operation Time 0402 Period End 961 mon Based on case description Time (Op Period #1) 0403 Input Type 2 - Based on case description Choice (Op Period #1)

B-43

Global Name Value / Distribution Parameters Units Basis ID 0405 Period End 962 mon Based on case description Time (Op Period #2) 0808- Inspection N/A mon Inspection defined as an 0808.10 Month annual frequency 0811 Inspection 1 - Inspection defined as an schedule annual frequency input type 0812 Pre- 0.1 1/yr Annual frequency set to one mitigation inspection every 10 years inspection freq 0820 Number of 1 - Based on case description cracks detected 0904 Max time 1 mon Based on case description between 2 check - single TWC - CC All Data Source Epistemic - Outer loop preserves LHS uncertain structure variables, except Global ID 2528 1001 Effective Full 80 yr Based on case description Power Years Constant (EFPY) 1101 Pipe Outer 0.8509 m B&W geometry from Diameter Constant Appendix E of [50];

Section 3.1, Table 4-1, of

[79]; and Table 2.2 of [7]

1102 Pipe Wall 0.06985 m B&W geometry from Thickness Constant Appendix E of [50];

Section 3.1, Table 4-1, of

[79]; and Table 2.2 of [7]

1104 Weld Material Thickness 1103 Weld Width 0.01905 m B&W geometry from Constant Appendix E of [50]

B-44

Global Name Value / Distribution Parameters Units Basis ID 3002 Unmitigated 25 cc/kg Bounds the operating H2 Level experience of PWRs as reported in [48]

3101 Operating 15.51 MPa Operating pressure for CE Pressure Constant plants from [7], which is higher than 14.82 MPa operating pressure for B&W plants from [6]

3102 Operating 293 °C Maximum temperature Temperature Constant reported for B&W plants in Appendix G of [50]

4001 Earthquake 0.001 1/yr Same value as used for Probability Constant analyses of other cases in this study (e.g., Case 2.1.0) 4002 Earthquake 0.13 MPa Appendix F of [50]

Total Constant Membrane 4003 Earthquake 0 MPa All bending stresses Inertial Constant captured in Global ID 4004 Bending 4004 Earthquake 44.35 MPa Appendix F of [50]

Anchor Constant Bending 4005 Sigma_SSa 0 MPa Based on case description Constant 4006 Sigma_SSh 0 MPa Based on case description Constant 4101 Fx (Dead 0 kN Set to 0 because all loads Weight) Constant input as stresses instead of forces and moments 4102 Mx (Dead 0 kN-m Set to 0 because all loads Weight) Constant input as stresses instead of forces and moments 4103 My (Dead 0 kN-m Set to 0 because all loads Weight) Constant input as stresses instead of forces and moments B-45

Global Name Value / Distribution Parameters Units Basis ID 4104 Mz (Dead 0 kN-m Set to 0 because all loads Weight) Constant input as stresses instead of forces and moments 4105 Fx (Thermal 0 kN Set to 0 because all loads Expansion) Constant input as stresses instead of forces and moments 4106 Mx (Thermal 0 kN-m Set to 0 because all loads Expansion) Constant input as stresses instead of forces and moments 4107 My (Thermal 0 kN-m Set to 0 because all loads Expansion) Constant input as stresses instead of forces and moments 4108 Mz (Thermal 0 kN-m Set to 0 because all loads Expansion) Constant input as stresses instead of forces and moments 4121 Membrane 0.07 MPa Same as reference case Stress (DW) Constant 4122 Maximum 0.35 MPa Same as reference case Bending Constant Stress (DW) 4123 Membrane 4.72 MPa Same as reference case Stress Constant (Thermal) 4124 Bending 120.5 MPa Same as reference case Stress Constant (Thermal)

N/A Hoop WRS Unchanged from reference case MPa No-repair hoop WRS profile Pre-Mitigation N/A Axial WRS Unchanged from reference case MPa No-repair axial WRS profile Pre-Mitigation 5004 Lower bound 0 - Even though it is not used, POD (POD0) Constant the default 0.999 value would lead to 99.9 percent probability of detection for a crack of zero depth B-46

Global Name Value / Distribution Parameters Units Basis ID 5001- Inspection Unchanged from reference case - RVON inspection 5510 Properties parameters are used following the recommendations in [61]

9003 TW Crack Unchanged from reference case mm Assumed same as RVON Distance based on similar geometries Rule Modifier B-47

B11 Case 3.1.1 The objective of Case 3.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation for CE and B&W RCP nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 3.1.1 analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 3.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 3.1.01 - Based on case description Description Crack 0 Based on case description Initiation 0501 -

Type Choice 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1209 -

Flaws (Circ) multiple cracks is low enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1210 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1211 -

Full-Length (Circ)

B-48

Global Name Value / Distribution Parameters Units Basis ID Initial Flaw Lognormal Based on PWSCC initial 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes

(*) max=0.0663)

Multiplier 1 Based on PWSCC initial 1213 Starting Constant - flaw sizes Depth (Circ) 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1214 Flaws -

multiple cracks is low (Axial) enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1215 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1216 -

Full-Length (Axial)

Initial Flaw Lognormal Based on PWSCC initial 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1218 -

Depth (Axial)

B-49

B12 Case 3.1.2 Case 3.1.2 was a sensitivity study of Case 3.1.0 considering a more severe WRS profile for CE and B&W RCP nozzle DMWs.

The random seeds used for the Case 3.1.2 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 3.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID

- Case 3.1.02 Based on case Description description B-50

Global Name Value / Distribution Parameters Units Basis ID

- Hoop WRS Mean Std. Dev. MPa More severe hoop WRS Pre- -139.48 50.4 profile estimated from Mitigation -93.80 50.4 FEA results for the weld 72.51 50.4 butter as discussed in Section C3.2 170.81 50.4 141.69 50.4 112.08 50.4

-32.80 50.4

-154.57 50.4

-166.86 50.4

-153.04 50.4

-132.55 50.4

-87.83 50.4

-49.64 50.4

-65.32 50.4 45.86 50.4 38.11 50.4 128.27 50.4 205.25 50.4 194.06 50.4 250.03 50.4 257.42 50.4 254.42 50.4 225.10 50.4 196.58 50.4 167.43 50.4 144.17 50.4 B-51

Global Name Value / Distribution Parameters Units Basis ID N/A Axial WRS Mean Std. Dev. MPa More severe axial WRS Pre- -145.06 28.3 profile estimated from Mitigation -123.38 28.3 FEA results for the weld

-29.33 28.3 butter as discussed in Section C3.2 1.87 28.3

-20.60 28.3

-106.75 28.3

-256.09 28.3

-337.07 28.3

-313.15 28.3

-276.01 28.3

-251.71 28.3

-192.52 28.3

-156.08 28.3

-122.04 28.3

-92.61 28.3

-58.29 28.3 13.87 28.3 90.03 28.3 156.95 28.3 214.69 28.3 277.74 28.3 338.45 28.3 355.12 28.3 332.82 28.3 314.71 28.3 298.81 28.3 B-52

B13 Case 4.1.0 The objective of Case 4.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth with inlay mitigation for Westinghouse steam generator nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 4.1.0 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 3, Scenario 9, input set from the xLPR Inputs Group Report [53] as a template with the following modifications:

Global ID Name Value / Distribution Parameters Units Basis

- Case 4.1.00 - Based on case Description description 0001 Plant 960 mon Based on case Operation description Time 0306 Stress 1 mon Based on case Mitigation description Time 0402 Period End 961 mon Based on case Time (Op description Period #1) 0403 Input Type 2 - Based on case Choice (Op description Period #1)

B-53

Global ID Name Value / Distribution Parameters Units Basis 0405 Period End 962 mon Based on case Time (Op description Period #2) 0808- Inspection N/A mon Inspection defined as 0808.10 Month (Pre- an annual frequency Mitigation) 0809- Inspection N/A mon Inspection defined as 0809.10 Month an annual frequency (Post-Mitigation) 0811 Inspection 1 - Inspection defined as Schedule an annual frequency Input Type 0812 Pre- 0.1 1/yr Annual frequency set Mitigation to one inspection Inspection every 10 years Freq 0813 Post- 0.1 1/yr Annual frequency set Mitigation to one inspection Inspection every 10 years Freq 0820 Number of 1 - Based on case cracks description detected 0904 Max time 1 mon Based on case between 2 description check - single TWC - CC All Data Source Epistemic - Outer loop preserves uncertain LHS structure variables, except Global ID 2528 1001 Effective Full 80 yr Based on case Power Years Constant description (EFPY) 1101 Pipe Outer 1.03266 m Outside diameter for Diameter Constant North Anna, Units 1 and 2 steam generator welds from

[80], which bounds B-54

Global ID Name Value / Distribution Parameters Units Basis welds represented by bin 1102 Pipe Wall 0.12225 m Thickness for North Thickness Constant Anna, Units 1 and 2 steam generator welds from [80], which bounds welds represented by bin 1103 Weld Width 0.04064 m Weld width for North Constant Anna, Units 1 and 2 steam generator welds from [80], which bounds welds represented by bin 1104 Weld Material 0.12225 m Set to same value as Thickness Constant Global ID 1102 1106 Inlay 0.0033 m Figure 7 from [56]

Thickness Constant 3002 Unmitigated 25 cc/kg Bounds the operating H2 Level Constant experience of PWRs as reported in [48]

3101 Operating 15.51 MPa Maximum operating Pressure Constant pressure for V.C.

Summer, Unit 1 from

[80], which bounds welds represented by bin 3102 Operating 328 °C Maximum operating Temperature Constant temperature for North-Anna, Units 1 and 2 from [80], which bounds welds represented by bin 3103 Dissolved 40 ppm Based on Case 3, Oxygen Scenario 10, input set from [50]

4001 Earthquake 0.001 1/yr Same value as used Probability for analyses of other cases in this study (e.g., Case 2.1.0)

B-55

Global ID Name Value / Distribution Parameters Units Basis 4002 Earthquake 0 MPa All SSE stresses Total captured in Global Membrane ID 4004 4003 Earthquake 0 MPa All SSE stresses Inertial Constant captured in Global Bending ID 4004 4004 Earthquake 161.9 MPa Maximum SSE stress Anchor Constant for North Anna, Unit 1 Bending from Tables 3-1 through 3-4 in [80],

which bounds welds represented by bin 4005 Sigma_SSa 0 MPa Based on case Constant description 4006 Sigma_SSh 0 MPa Based on case Constant description 4101 Fx (Dead 0 kN Set to 0 because all Weight) Constant loads input as stresses instead of forces and moments 4102 Mx (Dead 0 kN-m Set to 0 because all Weight) Constant loads input as stresses instead of forces and moments 4103 My (Dead 0 kN-m Set to 0 because all Weight) Constant loads input as stresses instead of forces and moments 4104 Mz (Dead 0 kN-m Set to 0 because all Weight) Constant loads input as stresses instead of forces and moments 4105 Fx (Thermal 0 kN Set to 0 because all Expansion) Constant loads input as stresses instead of forces and moments 4106 Mx (Thermal 0 kN-m Set to 0 because all Expansion) Constant loads input as stresses instead of forces and moments B-56

Global ID Name Value / Distribution Parameters Units Basis 4107 My (Thermal 0 kN-m Set to 0 because all Expansion) Constant loads input as stresses instead of forces and moments 4108 Mz (Thermal 0 kN-m Set to 0 because all Expansion) Constant loads input as stresses instead of forces and moments 4121 Membrane 0 MPa Reference [80] makes Stress (DW) Constant no distinction between membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the DW contribution 4122 Maximum 0 MPa Reference [80] makes Bending Constant no distinction between Stress (DW) membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the DW contribution 4123 Membrane 0 MPa Reference [80] makes Stress Constant no distinction between (Thermal) membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the thermal membrane stress contribution 4124 Bending 92 MPa Maximum stress for Stress Constant North Anna, Unit 2 (Thermal) from Tables 3-1 through 3-4 in [80],

which bounds welds represented by bin N/A Hoop WRS Same as hoop WRS post-mitigation MPa Set for consistency Pre- throughout the mitigation simulation B-57

Global ID Name Value / Distribution Parameters Units Basis N/A Axial WRS Same as axial WRS post-mitigation MPa Set for consistency Pre- throughout the mitigation simulation N/A Hoop WRS Mean Std. Dev. MPa Mean hoop WRS Post- 188.73 67 profile developed as mitigation 289.19 67 discussed in 269.03 67 Section 1.1.1.1.1A1.1.

The standard 296.16 67 deviation is based on 295.05 67 the maximum 308.65 67 standard deviation 328.51 67 from [47].

309.94 67 165.39 67

-43.41 67

-68.83 67 67.31 67 182.84 67 169.10 67 139.86 67 181.21 67 228.36 67 261.16 67 304.80 67 305.55 67 302.81 67 283.76 67 272.20 67 281.17 67 241.45 67 261.87 67 N/A Axial WRS Mean Std. Dev. MPa Axial hoop WRS Post- 52.15 67 profile developed as mitigation 120.65 67 discussed in 96.01 67 Section 1.1.1.1.1A1.1.

The standard 100.73 67 deviation is based on 65.89 67 the maximum 60.41 67 standard deviation 75.84 67 from [47].

33.72 67

-137.24 67

-317.64 67 B-58

Global ID Name Value / Distribution Parameters Units Basis

-323.08 67

-216.92 67

-142.67 67

-112.86 67

-104.57 67

-67.69 67

-11.59 67

-22.16 67 23.99 67 48.60 67 74.25 67 111.38 67 132.88 67 197.13 67 170.55 67 134.40 67 5004 Lower bound 0 - Even though it is not POD, POD0 Constant used, the default 0.999 value would lead to 99.9 percent probability of detection for a crack of zero depth 2743 Multiplier Lognormal - Based on minimum Proport. (0.0417, 17.99) FOI of 24 from Pacific Const. A Northwest National (DM1) Laboratories test data on Alloy 52/152 crack initiation and using a similar method as in

[78]. The Alloy 82/182 distribution median from the xLPR Inputs Group report [53] was divided by this FOI.

2788 Power Law 2.01E-12 (m/s)(MP Set equal to the Constant, Constant a- Alloy 82/182 power Alpha m1/2)^(- law constant because beta) the FOI is applied in Global ID 2796 B-59

Global ID Name Value / Distribution Parameters Units Basis 2796 Factor of 324 - Represents 75th Improvement Constant percentile FOI from IF [52]

B-60

B14 Case 4.1.1 The objective of Case 4.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks with inlay mitigation for Westinghouse steam generator nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 4.1.1 analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 4.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 4.1.01 - Based on case description Description Crack 0 Based on case description Initiation 0501 -

Type Choice 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1209 -

Flaws (Circ) multiple cracks is low enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1210 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1211 -

Full-Length (Circ)

B-61

Global Name Value / Distribution Parameters Units Basis ID Initial Flaw Lognormal Based on PWSCC initial 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes

(*) max=0.0663)

Multiplier 1 Based on PWSCC initial 1213 Starting Constant - flaw sizes Depth (Circ) 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1214 Flaws -

multiple cracks is low (Axial) enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1215 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1216 -

Full-Length (Axial)

Initial Flaw Lognormal Based on PWSCC initial 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1218 -

Depth (Axial)

B-62

B15 Case 4.1.2 Case 4.1.2 was a sensitivity study of Case 4.1.0 considering a more severe WRS profile for Westinghouse steam generator nozzle DMWs.

The random seeds used for the Case 4.1.2 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 4.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 4.1.02 - Based on Description case description B-63

Global Name Value / Distribution Parameters Units Basis ID N/A Hoop WRS Mean Std. Dev. MPa More severe Post- 250.13 67 hoop WRS Mitigation 134.75 67 profile 55.27 67 estimated from FEA 14.51 67 results for the

-2.01 67 weld butter

-21.50 67 as discussed

-81.53 67 in

-141.06 67 Section C4.3

-177.72 67

-195.79 67

-183.49 67

-165.58 67

-139.10 67

-108.05 67

-90.13 67

-72.45 67

-65.47 67

-62.11 67

-47.94 67

-36.58 67

-22.76 67

-4.85 67 24.51 67 92.47 67 202.39 67 250.13 67 N/A Hoop WRS Same as hoop WRS post-mitigation MPa Set for Pre- consistency Mitigation throughout the simulation B-64

Global Name Value / Distribution Parameters Units Basis ID N/A Axial WRS Mean Std. Dev. MPa More severe Post- 247.82 67 axial WRS Mitigation 191.42 67 profile 98.57 67 estimated from FEA 46.10 67 results for the 30.31 67 weld butter 10.36 67 as discussed

-56.05 67 in

-127.90 67 Section C4.3

-183.04 67

-213.67 67

-219.63 67

-201.55 67

-150.39 67

-99.58 67

-60.25 67

-33.63 67

-19.39 67

-10.82 67 8.82 67 30.82 67 56.92 67 90.43 67 122.54 67 154.68 67 221.83 67 254.66 67 N/A Axial WRS Same as axial WRS post-mitigation MPa Set for Pre- consistency Mitigation throughout the simulation B-65

B16 Case 4.1.3 Case 4.1.3 was a sensitivity study of Case 4.1.0 considering overlay instead of inlay mitigation.

The random seeds used for the Case 4.1.3 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 4.1.0 input set as a template with the following modifications:

Global ID Name Value / Distribution Parameters Units Basis N/A Case 4.1.03 - Based on case Description description 0305 Stress 2 - Setting for weld Mitigation overlay Choice 0306 Stress 240 mon The steam mitigation generators time represented by this case had overlays applied after 17 years of service, which was bounded in the analysis by applying the overlay at 20 EFPY (i.e., 240 months)

B-66

Global ID Name Value / Distribution Parameters Units Basis 0803 Post-Overlay 1 Consistent with pre-Trunc Meas mitigation approach Error default value 0804 Post-Overlay 0 Default value Eval Length Effects 0805 Full 1 Overlay represented Structural is a full structural WOL weld overlay 0815 Post-Overlay 0 Default value Ligament Flag 1105 Weld overlay 0.04075 m Weld thickness is thickness Constant 0.12225 m. Overlay thickness set to one third that value, which is the minimum acceptable thickness for a full structural weld overlay as stated in

[59]

5201 Depth repair 0 - Set to pre-mitigation threshold Constant value because no x_TH applicable values for (during) overlays 5202 Depth repair 0 - Set to pre-mitigation threshold Constant value because no x_TH (post) applicable values for overlays 5301-5312 Post-Overlay beta_0 (circ): Normal (5.41, 3.64) - Set equal to the pre-Inspection beta_1 (circ): Normal (0.86, 6.02, min=0, mitigation inspection Properties max=14.86) property values beta_0 (axial): Normal (3.07, 2.07) because no beta_1 (axial): Normal (0.64, 4.46, min=0, applicable values for max=11.02) overlays a (circ): Normal (0.018, 0.017) b (circ): Normal (0.971, 0.029) a (axial): Normal (0.018, 0.017) b (axial): Normal (0.971, 0.029)

Sigma_depth (circ): 0.04 Sigma_depth (axial): 0.04 B-67

Global ID Name Value / Distribution Parameters Units Basis x_small: 0.1 x_LB: 0 Correlation Intercept, B0 -0.92 - Set equal to the pre-5301-5302 (circ) mitigation inspection Slope, B1 property values (circ) because no applicable values for overlays Correlation Intercept, B0 -0.92 - Set equal to the pre-5303-5304 (axial) mitigation inspection Slope, B1 property values (axial) because no applicable values for overlays Correlation a (circ) -0.94 - Set equal to the pre-5305-5306 b (circ) mitigation inspection property values because no applicable values for overlays Correlation a (axial) -0.94 - Set equal to the pre-5307-5308 b (axial) mitigation inspection property values because no applicable values for overlays B-68

Global ID Name Value / Distribution Parameters Units Basis N/A Hoop WRS Mean Std. Dev. MPa Same as hoop WRS Pre- 188.73 67 profile used for Case Mitigation 289.19 67 4.1.0 due to similar 269.03 67 double-vee groove welding sequences 296.16 67 and temperature-295.05 67 dependent weld 308.65 67 material stress-strain 328.51 67 curves 309.94 67 165.39 67

-43.41 67

-68.83 67 67.31 67 182.84 67 169.10 67 139.86 67 181.21 67 228.36 67 261.16 67 304.80 67 305.55 67 302.81 67 283.76 67 272.20 67 281.17 67 241.45 67 261.87 67 B-69

Global ID Name Value / Distribution Parameters Units Basis N/A Axial WRS Mean stdev MPa Same as axial WRS Pre- 52.15 67 profile used for Case Mitigation 120.65 67 4.1.0 due to similar 96.01 67 double-vee groove welding sequences 100.73 67 and temperature-65.89 67 dependent weld 60.41 67 material stress-strain 75.84 67 curves 33.72 67

-137.24 67

-317.64 67

-323.08 67

-216.92 67

-142.67 67

-112.86 67

-104.57 67

-67.69 67

-11.59 67

-22.16 67 23.99 67 48.60 67 74.25 67 111.38 67 132.88 67 197.13 67 170.55 67 134.40 67 B-70

Global ID Name Value / Distribution Parameters Units Basis N/A Hoop WRS Mean Std. Dev. MPa Overlay mitigation Post- -11.27 67 rules from xLPR Mitigation 97.19 67 WRS Subgroup 85.03 67 report [47] applied to pre-mitigation 120.16 67 mean hoop WRS 127.05 67 profile 148.65 67 176.51 67 165.94 67 29.39 67

-171.41 67

-188.83 67

-44.69 67 78.84 67 73.10 67 51.86 67 101.21 67 156.36 67 197.16 67 248.80 67 257.55 67 262.81 67 251.76 67 248.20 67 265.17 67 233.45 67 261.87 67 B-71

Global ID Name Value / Distribution Parameters Units Basis N/A Axial WRS Mean Std. Dev. MPa Overlay mitigation Post- -43.72 67 rules from xLPR Mitigation 24.78 67 WRS Subgroup 0.14 67 report [47] applied to pre-mitigation 4.86 67 mean axial WRS

-29.98 67 profile

-35.46 67 4.89 67

-14.69 67

-165.47 67

-328.06 67

-318.05 67

-198.83 67

-113.87 67

-75.72 67

-61.47 67

-20.98 67 36.35 67 25.77 67 71.92 67 96.54 67 122.18 67 159.32 67 180.81 67 245.07 67 218.48 67 182.33 67 B-72

B17 Case 4.1.4 Case 4.1.4 was a sensitivity study of Case 4.1.0 without mechanical mitigation.

The random seeds used for the Case 4.1.4 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 4.1.0 input set as a template with the following modifications:

Global ID Name Value / Distribution Parameters Units Basis N/A Case 4.1.04 - Case number for Description reference 0301 Mitigation 0 - Setting for no type choice mitigation 3101 Operating 15.59 MPa Same operating Pressure Constant pressure as Case 4.1.0 3102 Operating 296 °C Cold leg temperature Temperature Constant corresponding with location of the steam generator outlet nozzles B-73

Global ID Name Value / Distribution Parameters Units Basis 4001 Earthquake 0.001 - Same value as used probability Constant for analyses of other cases in this study (e.g., Case 2.1.0) 4002 Earthquake 0 MPa All SSE membrane Total Constant stresses captured in Membrane Global ID 4004 4003 Earthquake 0 MPa All SSE bending Inertial Constant stresses captured in Bending Global ID 4004 4004 Earthquake 161.9 MPa Maximum SSE stress Anchor Constant for North Anna, Unit 1 Bending from Tables 3-1 through 3-4 in [80],

which bounds welds represented by bin 4121 Membrane 0 MPa Reference [80] makes stress (DW) Constant no distinction between membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the DW contribution 4122 Maximum 0 MPa Reference [80] makes Bending Constant no distinction between Stress (DW) membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the DW contribution 4123 Membrane 0 MPa Reference [80] makes Stress Constant no distinction between (Thermal) membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the thermal membrane stress contribution B-74

Global ID Name Value / Distribution Parameters Units Basis 4124 Bending 92 MPa Reference [80] makes Stress Constant no distinction between (Thermal) membrane and bending stresses, so this input is set to 0, and Global ID 4124 contains the DW contribution B-75

Global ID Name Value / Distribution Parameters Units Basis N/A Hoop WRS Mean Std. Dev. MPa Same as hoop WRS Pre-Mitigation 188.73 67 profile used for Case 289.19 67 4.1.0 due to similar 269.03 67 double-vee groove welding sequences 296.16 67 and temperature-295.05 67 dependent weld 308.65 67 material stress-strain 328.51 67 curves 309.94 67 165.39 67

-43.41 67

-68.83 67 67.31 67 182.84 67 169.10 67 139.86 67 181.21 67 228.36 67 261.16 67 304.80 67 305.55 67 302.81 67 283.76 67 272.20 67 281.17 67 241.45 67 261.87 67 B-76

Global ID Name Value / Distribution Parameters Units Basis N/A Axial WRS Mean stdev MPa Same as axial WRS Pre-Mitigation 52.15 67 profile used for Case 120.65 67 4.1.0 due to similar 96.01 67 double-vee groove welding sequences 100.73 67 and temperature-65.89 67 dependent weld 60.41 67 material stress-strain 75.84 67 curves 33.72 67

-137.24 67

-317.64 67

-323.08 67

-216.92 67

-142.67 67

-112.86 67

-104.57 67

-67.69 67

-11.59 67

-22.16 67 23.99 67 48.60 67 74.25 67 111.38 67 132.88 67 197.13 67 170.55 67 134.40 67 N/A Hoop WRS Same as hoop WRS pre-mitigation MPa Not used in the Post- simulation but filled for Mitigation completeness N/A Axial WRS Same as axial WRS pre-mitigation MPa Not used in the Post- simulation but filled for Mitigation completeness B-77

B18 Case 5.1.0 The objective of Case 5.1.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation for CE hot leg branch line nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 5.1.0 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using Case 2.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID

- Case 5.1.00 - Based on case Description description

- Weld Type CE HL Branch DMW - Based on case Choice description 0808- Inspection N/A mon Inspection 0808.10 Month defined as an annual frequency 0811 Inspection 1 - Inspection Schedule defined as an Input Type annual frequency B-78

Global Name Value / Distribution Parameters Units Basis ID 0812 Pre- 0.1 1/yr Annual frequency Mitigation set to one Inspection inspection every Freq 10 years All Data Source Epistemic - Outer loop uncertain preserves LHS variables, structure except Global ID 2528 1101 Pipe Outer 0.324 m Typical shutdown Diameter Constant cooling system pipe outside diameter from

[22]

1102 Pipe Wall 0.036 m Typical shutdown Thickness Constant cooling system pipe wall thickness from

[22]

1103 Weld Width 0.036 m Outside diameter Constant weld width from generic CE branch line weld configuration 1104 Weld 0.036 m Set to same Material Constant value as Global Thickness ID 1102 2101 Yield Lognormal MPa Mean from Table Strength, (179.5, 26.87, min=128, max=269) 4-2 of [22] and Sigy distribution developed using the coefficient of variation (COV) from Case 2.1.0, Global ID 2101 B-79

Global Name Value / Distribution Parameters Units Basis ID 2102 Ultimate Lognormal MPa Mean from Table Strength, (461.2, 60.72, min=359, max=700) 4-2 of [22] and Sigu distribution developed using the COV from Case 2.1.0, Global ID 2102 2105 Elastic Normal MPa Mean from Table Modulus, E (179270, 26800, min=148716, 4-2 of [22] and max=201204) distribution developed using the COV from Case 2.1.0, Global ID 2105 2106 Material Init Normal N/mm Mean from Table J- (105.076, 58.6, min=10, max=254.1) 4-2 of [22] and Resistance, distribution Jic developed using the COV from Case 2.1.0, Global ID 2106 2107 Material Init Normal N/mm Mean from Table J-Resist (448.85, 138, min=91.6, max=615.9) 4-2 of [22] and Coef, C distribution developed using the COV from Case 2.1.0, Global ID 2107 2108 Material Init Normal - Mean from Table J-Resist (0.274, 0.0317, min=0.1, max=1) 4-2 of [22] and Exponent, m distribution developed using the COV from Case 2.1.0, Global ID 2108 2301 Yield Lognormal MPa Distribution Strength, (201, 22.42, min=154, max=253) developed based Sigy on data from [81]

2302 Ultimate Lognormal MPa Distribution Strength, (360, 54.01, min=235, max=485) developed based Sigu on data from [81]

B-80

Global Name Value / Distribution Parameters Units Basis ID 2305 Elastic Normal MPa Mean value from Modulus, E (179959.5, 27000, min=150212, [23]

max=203228) 2306 Material Init Normal N/mm Distribution J- (106.6, 65, min=7, max=211) developed based Resistance, on [23] and [82]

Jic 2307 Material Init Normal N/mm Distribution J-Resist (216, 135, min=44, max=467) developed based Coef, C on [23] and [82]

2308 Material Init Normal - Distribution J-Resist (0.44, 0.09, min=0.21, max=0.56) developed based Exponent, m on [23] and [82]

3002 Unmitigated 25 cc/kg Bounds the H2 Level Constant operating experience of PWRs as reported in [48]

3102 Operating 318 °C Typical operating Temperature Constant temperature for shutdown cooling system as reported in [22]

and pressurizer surge line as reported in [21]

4001 Earthquake 0.001 1/yr From Section Probability Constant E.3.1 of [60], the maximum earthquake probability is 1E-3 4002 Earthquake 16.43 MPa Maximum SSE Total Constant stress from Table Membrane 4-6 [22] is combined membrane and bending stress 4003 Earthquake 0 MPa SSE bending Inertial Constant stresses captured Bending in Global ID 4002 B-81

Global Name Value / Distribution Parameters Units Basis ID 4004 Earthquake 0 MPa SSE bending Anchor Constant stresses captured Bending in Global ID 4002 4121 Membrane 0 MPa Stress from [22]

Stress (DW) Constant is combined DW and thermal, so this input is set to 0, and Global ID 4124 contains the DW contribution 4122 Maximum 0 MPa Stress from [22]

Bending Constant is combined DW Stress (DW) and thermal, so this input is set to 0, and Global ID 4124 contains the DW contribution 4123 Membrane 0 MPa Stress from [22]

Stress Constant is combined DW (Thermal) and thermal, so this input is set to 0, and Global ID 4124 contains the thermal contribution 4124 Bending 21.51 MPa Stress from Stress Constant Table 4-5 of [22]

(Thermal) is combined DW and thermal; used the limiting thermal maximum B-82

Global Name Value / Distribution Parameters Units Basis ID N/A Hoop WRS Mean Std. Dev. MPa Mean hoop WRS Pre- 91.43 76.7 profile and Mitigation 136.17 76.7 standard 173.20 76.7 deviation based on analysis for 223.04 76.7 generic CE 321.41 76.7 branch line 353.55 76.7 geometry as 323.67 76.7 documented in 279.46 76.7 Section C5.2 229.43 76.7 159.52 76.7 114.54 76.7 72.28 76.7 71.63 76.7 77.13 76.7 136.84 76.7 110.97 76.7 163.95 76.7 229.93 76.7 209.21 76.7 218.51 76.7 250.47 76.7 261.70 76.7 243.47 76.7 234.81 76.7 248.73 76.7 252.10 76.7 B-83

Global Name Value / Distribution Parameters Units Basis ID N/A Axial WRS Mean Std. Dev. MPa Mean axial WRS Pre- -152.59 57.9 profile and Mitigation -100.70 57.9 standard

-45.09 57.9 deviation based on analysis for 1.38 57.9 generic CE 75.69 57.9 branch line 97.53 57.9 geometry as 64.70 57.9 documented in 22.01 57.9 Section C5.2

-76.55 57.9

-129.83 57.9

-145.23 57.9

-215.12 57.9

-214.80 57.9

-204.85 57.9

-179.97 57.9

-136.36 57.9

-69.80 57.9 12.75 57.9 47.64 57.9 108.06 57.9 156.35 57.9 187.73 57.9 185.37 57.9 194.95 57.9 210.02 57.9 205.36 57.9 5004 Lower bound 0 - Even though it is POD, POD0 Constant not used, the default 0.999 value would lead to 99.9 percent probability of detection for a crack of zero depth B-84

Global Name Value / Distribution Parameters Units Basis ID 5101-5110 Pre- beta_0 (circ): Normal (2.71, 0.21) - Uses values for Mitigation beta_1 (circ): Normal (0.31, 0.45) the pressurizer Inspection beta_0 (axial): Normal (-0.8, 0.38) surge line per the Properties beta_1 (axial): Normal (8.3, 1.45) recommendations in [61]

a (circ): Normal (0.034, 0.006) b (circ): Normal (0.955, 0.013) a (axial): Normal (0.041, 0.011) b (axial): Normal (0.88, 0.029)

Sigma_depth (circ): 0.072 Sigma_depth (axial): 0.078 Correlation Intercept, B0 -0.86 - Uses values for 5101-5102 (circ) the pressurizer Intercept, B1 surge line per the (circ) recommendations in [61]

Correlation Intercept, B0 -0.93 - Uses values for 5103-5104 (axial) the pressurizer Intercept, B1 surge line per the (axial) recommendations in [61]

Correlation a (circ) -0.867 - Uses values for 5105-5106 b (circ) the pressurizer surge line per the recommendations in [61]

Correlation a (axial) -0.87 - Uses values for 5107-5108 b (axial) the pressurizer surge line per the recommendations in [61]

9003 TW Crack Uniform mm The Distance (0, 508) circumference of Rule Modifier the shutdown cooling system piping is similar enough to the pressurizer surge line to use the same upper bound from Case 2.1.0 B-85

B19 Case 5.1.1 The objective of Case 5.1.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation for CE hot leg branch line nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 5.1.1 analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 5.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 5.1.01 - Based on case description Description Crack 0 Based on case description Initiation 0501 -

Type Choice 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1209 -

Flaws (Circ) multiple cracks is low enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1210 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1211 -

Full-Length (Circ)

B-86

Global Name Value / Distribution Parameters Units Basis ID Initial Flaw Lognormal Based on PWSCC initial 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes

(*) max=0.0663)

Multiplier 1 Based on PWSCC initial 1213 Starting Constant - flaw sizes Depth (Circ) 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1214 Flaws -

multiple cracks is low (Axial) enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1215 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1216 -

Full-Length (Axial)

Initial Flaw Lognormal Based on PWSCC initial 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1218 -

Depth (Axial)

B-87

B20 Case 5.1.2 Case 5.1.2 was a sensitivity study of Case 5.1.0 considering a more severe WRS profile for CE hot leg branch line nozzle DMWs.

The random seeds used for the Case 5.1.2 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 5.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 5.1.02 - Based on Description case description B-88

Global Name Value / Distribution Parameters Units Basis ID N/A Hoop WRS Mean Std. Dev. MPa More severe Pre- 135.82 76.7 hoop WRS Mitigation profile 177.92 76.7 estimated 209.02 76.7 from FEA 251.85 76.7 results for the weld 340.89 76.7 butter as 369.79 76.7 discussed in Section C5.3 339.69 76.7 295.63 76.7 248.33 76.7 178.62 76.7 131.49 76.7 91.90 76.7 89.13 76.7 85.42 76.7 144.50 76.7 119.70 76.7 165.50 76.7 228.94 76.7 208.52 76.7 221.96 76.7 257.46 76.7 268.67 76.7 250.29 76.7 242.98 76.7 257.83 76.7 260.38 76.7 B-89

Global Name Value / Distribution Parameters Units Basis ID N/A Axial WRS Mean Std. Dev. MPa More severe Pre- -105.58 57.9 axial WRS Mitigation -47.70 57.9 profile 8.75 57.9 estimated from FEA 50.80 57.9 results for 117.52 57.9 the weld 132.72 57.9 butter as 94.79 57.9 discussed in 47.29 57.9 Section C5.3

-55.16 57.9

-112.78 57.9

-131.67 57.9

-202.11 57.9

-203.49 57.9

-201.05 57.9

-180.28 57.9

-141.19 57.9

-82.63 57.9

-9.57 57.9 19.61 57.9 72.45 57.9 118.34 57.9 147.84 57.9 144.86 57.9 152.16 57.9 163.89 57.9 160.22 57.9 B-90

B21 Case 5.2.0 The objective of Case 5.2.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation for CE cold leg branch line nozzle DMWs.

The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 5.2.0 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using Case 5.1.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 5.2.00 - Based on case Description description N/A Weld Type CE CL Branch DMW - Based on case Choice description 0808- Inspection N/A mon Inspection 0808.10 Month defined as an annual frequency 0811 Inspection 1 - Inspection Schedule defined as an Input Type annual frequency B-91

Global Name Value / Distribution Parameters Units Basis ID 0812 Pre- 0.1 1/yr Annual frequency Mitigation set to one Inspection inspection every Freq 10 years All Data Source Epistemic - Outer loop uncertain preserves LHS variables, structure except Global ID 2528 1101 Pipe Outer 0.32385 m Typical safety Diameter Constant injection system pipe outside diameter from

[23]

1102 Pipe Wall 0.0361947 m Typical safety Thickness Constant injection system pipe wall thickness from

[23]

1103 Weld Width 0.0355 m Outside diameter Constant weld width from generic CE branch line weld configuration 1104 Weld 0.0361947 m Set to same Material Constant value as Global Thickness ID 1102 2101 Yield Lognormal MPa Mean from Table Strength, (179.5, 26.87, min=128, max=269) 4-2 of [22] and Sigy distribution developed using the COV from Case 2.1.0, Global ID 2101 B-92

Global Name Value / Distribution Parameters Units Basis ID 2102 Ultimate Lognormal MPa Mean from Table Strength, (461.2, 60.72, min=359, max=700) 4-2 of [22] and Sigu distribution developed using the COV from Case 2.1.0, Global ID 2102 2105 Elastic Normal MPa Mean from Table Modulus, E (179270, 26800, min=148716, 4-2 of [22] and max=201204) distribution developed using the COV from Case 2.1.0, Global ID 2105 2106 Material Init Normal N/mm Mean from Table J- (105.076, 58.6, min=10, max=254.1) 4-2 of [22] and Resistance, distribution Jic developed using the COV from Case 2.1.0, Global ID 2106 2107 Material Init Normal N/mm Mean from Table J-Resist (448.85, 138, min=91.6, max=615.9) 4-2 of [22] and Coef, C distribution developed using the COV from Case 2.1.0, Global ID 2107 2108 Material Init Normal - Mean from Table J-Resist (0.274, 0.0317, min=0.1, max=1) 4-2 of [22] and Exponent, m distribution developed using the COV from Case 2.1.0, Global ID 2108 2301 Yield Lognormal MPa Distribution Strength, (201, 22.42, min=154, max=253) developed based Sigy on data from [81]

2302 Ultimate Lognormal MPa Distribution Strength, (360, 54.01, min=235, max=485) developed based Sigu on data from [81]

B-93

Global Name Value / Distribution Parameters Units Basis ID 2305 Elastic Normal MPa Mean from [23]

Modulus, E (179959.5, 27000, min=150212, max=203228) 2306 Material Init Normal N/mm Distribution J- (106.6, 65, min=7, max=211) developed based Resistance, on [22] and [82]

Jic 2307 Material Init Normal N/mm Distribution J-Resist (216, 135, min=44, max=467) developed based Coef, C on [22] and [82]

2308 Material Init Normal - Distribution J-Resist (0.44, 0.09, min=0.21, max=0.56) developed based Exponent, m on [22] and [82]

3002 Unmitigated 25 cc/kg Bounds the H2 Level Constant operating experience of PWRs as reported in [48]

3102 Operating 288 °C Typical operating Temperature Constant temperature for the safety injection system from [23]

4001 Earthquake 0.001 1/yr From Section Probability Constant E.3.1 of [60], the maximum earthquake probability is 1E-3 4002 Earthquake 29.71 MPa Maximum SSE Total Constant stress from Table Membrane 4-6 [23] is combined membrane and bending stress 4003 Earthquake 0 MPa SSE bending Inertial Constant stresses captured Bending in Global ID 4002 4004 Earthquake 0 MPa SSE bending Anchor Constant stresses captured Bending in Global ID 4002 B-94

Global Name Value / Distribution Parameters Units Basis ID 4121 Membrane 0 MPa Stress from [23]

Stress (DW) Constant is combined DW and thermal, so this input is set to 0, and Global ID 4124 contains the DW contribution 4122 Maximum 0 MPa Stress from [23]

Bending Constant is combined DW Stress (DW) and thermal, so this input is set to 0, and Global ID 4124 contains the DW contribution 4123 Membrane 0 MPa Stress from [23]

Stress Constant is combined DW (Thermal) and thermal, so this input is set to 0, and Global ID 4124 contains the thermal contribution 4124 Bending 74.38 MPa Stress from [23]

Stress Constant is combined DW (Thermal) and thermal; used the limiting thermal maximum B-95

Global Name Value / Distribution Parameters Units Basis ID N/A Hoop WRS Mean Std. Dev. MPa Mean hoop WRS Pre- 91.43 76.7 profile and Mitigation 136.17 76.7 standard 173.20 76.7 deviation based on analysis for 223.04 76.7 generic CE 321.41 76.7 branch line 353.55 76.7 geometry as 323.67 76.7 documented in 279.46 76.7 Section C5.2 229.43 76.7 159.52 76.7 114.54 76.7 72.28 76.7 71.63 76.7 77.13 76.7 136.84 76.7 110.97 76.7 163.95 76.7 229.93 76.7 209.21 76.7 218.51 76.7 250.47 76.7 261.70 76.7 243.47 76.7 234.81 76.7 248.73 76.7 252.10 76.7 B-96

Global Name Value / Distribution Parameters Units Basis ID N/A Axial WRS Mean Std. Dev. MPa Mean axial WRS Pre- -152.59 57.9 profile and Mitigation -100.70 57.9 standard

-45.09 57.9 deviation based on analysis for 1.38 57.9 generic CE 75.69 57.9 branch line 97.53 57.9 geometry as 64.70 57.9 documented in 22.01 57.9 Section C5.2

-76.55 57.9

-129.83 57.9

-145.23 57.9

-215.12 57.9

-214.80 57.9

-204.85 57.9

-179.97 57.9

-136.36 57.9

-69.80 57.9 12.75 57.9 47.64 57.9 108.06 57.9 156.35 57.9 187.73 57.9 185.37 57.9 194.95 57.9 210.02 57.9 205.36 57.9 5101-5110 Pre- beta_0 (circ): Normal (2.71, 0.21) - Uses values for Mitigation beta_1 (circ): Normal (0.31, 0.45) the pressurizer Inspection beta_0 (axial): Normal (-0.8, 0.38) surge line per the Properties beta_1 (axial): Normal (8.3, 1.45) recommendations in [61]

a (circ): Normal (0.034, 0.006) b (circ): Normal (0.955, 0.013) a (axial): Normal (0.041, 0.011) b (axial): Normal (0.88, 0.029)

Sigma_depth (circ): 0.072 Sigma_depth (axial): 0.078 B-97

Global Name Value / Distribution Parameters Units Basis ID Correlation Intercept, B0 -0.86 - Uses values for 5101-5102 (circ) the pressurizer Intercept, B1 surge line per the (circ) recommendations in [61]

Correlation Intercept, B0 -0.93 - Uses values for 5103-5104 (axial) the pressurizer Intercept, B1 surge line per the (axial) recommendations in [61]

Correlation a (circ) -0.867 - Uses values for 5105-5106 b (circ) the pressurizer surge line per the recommendations in [61]

Correlation a (axial) -0.87 - Uses values for 5107-5108 b (axial) the pressurizer surge line per the recommendations in [61]

9003 TW Crack Uniform mm The Distance (0, 508) circumference of Rule Modifier the shutdown cooling system piping is similar enough to the pressurizer surge line to use the same upper bound from Case 2.1.0 B-98

B22 Case 5.2.1 The objective of Case 5.2.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation for CE cold leg branch line nozzle DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 5.2.1 analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 5.2.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 5.2.01 - Based on case description Description Crack 0 Based on case description Initiation 0501 -

Type Choice 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1209 -

Flaws (Circ) multiple cracks is low enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1210 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1211 -

Full-Length (Circ)

B-99

Global Name Value / Distribution Parameters Units Basis ID Initial Flaw Lognormal Based on PWSCC initial 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes

(*) max=0.0663)

Multiplier 1 Based on PWSCC initial 1213 Starting Constant - flaw sizes Depth (Circ) 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1214 Flaws -

multiple cracks is low (Axial) enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1215 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1216 -

Full-Length (Axial)

Initial Flaw Lognormal Based on PWSCC initial 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1218 -

Depth (Axial)

B-100

B23 Case 1.3.0 The objective of Case 1.3.0 was to assess the base likelihood of failure caused by PWSCC initiation and growth without mechanical mitigation for Westinghouse two- and three-loop RVON and RVIN DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 1.3.0 analyses were as follow:

Simulation Replicate Epistemic Aleatory Description Simulation Random Random No. Seed Seed 100,000-realization 1 1515 13118 composite simulation 2 1974 713705 using the epistemic (outer) loop 3 2002 1503 4 2004 909 5 2010 907 6 3131 131521 7 4512 1685 8 5121 919 9 41520 2025 10 1415 23118 The other inputs were developed using the Case 1.1.0 inputs set from the prior study [2] as a template with the following modifications:

Global ID Name Value / Distribution Parameters Units Basis N/A Case 1.3.00 - Based on Description case description 003 Crack 3 - Option to Orientation include both circumferential and axial cracks 0808- Inspection N/A - Inspection 0808.10 Month defined as an annual frequency B-101

Global ID Name Value / Distribution Parameters Units Basis 0811 Inspection 1 - Inspection Schedule defined as an Input Type annual frequency 0812 Pre- 0.1 1/yr Annual Mitigation frequency set Inspection to one Freq inspection every 10 years 0820 Number of 1 - All cracks cracks detected detected independently per case description 0904 Max time 1 mon For post-between 2 processing check - purposes per single TWC case

- CC description All Data Source Epistemic - Outer loop uncertain preserves variables, LHS structure except Global ID 2528 1101 Pipe Outer 0.863 m Largest Diameter Constant diameter for the plants represented by bin 1102 Pipe Wall 0.056 m Smallest Thickness Constant thickness for the plants represented by bin 1104 Weld 0.056 m Set to same Material Constant value as Thickness Global ID 1102 B-102

Global ID Name Value / Distribution Parameters Units Basis 3002 Unmitigated 25 cc/kg Bounds the H2 Level Constant operating experience of PWRs as reported in

[48]

3101 Operating 15.51 MPa Highest Pressure Constant operating pressure for the plants represented by bin 3102 Operating 326 °C Highest Temperature Constant operating temperature for the plants represented by bin 4001 Earthquake 0.001 1/yr Same value Probability Constant as used for analyses of other cases in this study (e.g.,

Case 2.1.0) 4002 Earthquake 7.6 MPa Largest SSE Total Constant membrane Membrane stress for the plants represented by bin 4003 Earthquake 29.8 MPa Largest SSE Inertial Constant bending stress Bending for the plants represented by bin 4004 Earthquake 0 MPa All SSE Anchor Constant bending Bending stresses captured in Global ID 4003 B-103

Global ID Name Value / Distribution Parameters Units Basis 4101 Fx (Dead 0 kN Set to 0 Weight) Constant because all loads input as stresses instead of forces and moments 4102 Mx (Dead 0 kN-m Set to 0 Weight) Constant because all loads input as stresses instead of forces and moments 4103 My (Dead 0 kN-m Set to 0 Weight) Constant because all loads input as stresses instead of forces and moments 4104 Mz (Dead 0 kN-m Set to 0 Weight) Constant because all loads input as stresses instead of forces and moments 4105 Fx (Thermal 0 kN Set to 0 Expansion) Constant because all loads input as stresses instead of forces and moments 4106 Mx (Thermal 0 kN-m Set to 0 Expansion) Constant because all loads input as stresses instead of forces and moments B-104

Global ID Name Value / Distribution Parameters Units Basis 4107 My (Thermal 0 kN-m Set to 0 Expansion) Constant because all loads input as stresses instead of forces and moments 4108 Mz (Thermal 0 kN-m Set to 0 Expansion) Constant because all loads input as stresses instead of forces and moments 4121 Membrane 0 MPa All membrane Stress (DW) Constant stresses captured in Global ID 4123 4122 Maximum 0 MPa All bending Bending Constant stresses Stress (DW) captured in Global ID 4124 4123 Membrane 1.69 MPa Largest Stress Constant membrane (Thermal) stress for the plants represented by bin 4124 Bending 100.92 MPa Largest Stress Constant bending stress (Thermal) for the plants represented by bin N/A Hoop WRS Unchanged from reference case MPa No-repair Pre- hoop WRS mitigation profile N/A Axial WRS Unchanged from reference case MPa No-repair axial Pre- WRS profile mitigation B-105

Global ID Name Value / Distribution Parameters Units Basis 5004 Lower 0 - Even though it bound POD, Constant is not used, POD0 the default 0.999 value would lead to 99.9 percent probability of detection for a crack of zero depth B-106

B24 Case 1.3.1 The objective of Case 1.3.1 was to assess the base likelihood of failure with pre-existing flaws and subsequent PWSCC growth of circumferential and axial cracks without mechanical mitigation for Westinghouse 2- and 3-loop RVON and RVIN DMWs. The effects of leak detection, ISI, and SSE were also assessed.

The random seeds used for the Case 1.3.1 analyses were as follow:

Simulation Epistemic Aleatory Description Random Random Seed Seed 5000-realization simulation using the 6128 369 epistemic (outer) loop The other inputs were developed using the Case 1.3.0 input set as a template with the following modifications:

Global Name Value / Distribution Parameters Units Basis ID N/A Case 1.3.01 - Based on case description Description Crack 0 Based on case description Initiation 0501 -

Type Choice 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1209 -

Flaws (Circ) multiple cracks is low enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1210 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Circ) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1211 -

Full-Length (Circ)

B-107

Global Name Value / Distribution Parameters Units Basis ID Initial Flaw Lognormal Based on PWSCC initial 1212 Depth (Circ) (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes

(*) max=0.0663)

Multiplier 1 Based on PWSCC initial 1213 Starting Constant - flaw sizes Depth (Circ) 1 Considers the impact of Constant one circumferential crack and one axial crack Number of because the likelihood of 1214 Flaws -

multiple cracks is low (Axial) enough to not affect the results as demonstrated in

[2]

Initial Flaw Lognormal Based on PWSCC initial 1215 Full-Length (1, 4.3E-3, 2.226) m flaw sizes (Axial) (*)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1216 -

Full-Length (Axial)

Initial Flaw Lognormal Based on PWSCC initial 1217 Depth (1, 1.5E-3, 1.419, min=5E-4, m flaw sizes (Axial) (*) max=0.0663)

Multiplier 1 Based on PWSCC initial Starting Constant flaw sizes 1218 -

Depth (Axial)

B-108

APPENDIX C WELDING RESIDUAL STRESS PROFILE DEVELOPMENT C1 Introduction to Weld Residual Stress Profile Development Welding is the preferred method for connecting many components in nuclear power plants.

Welds are used for vessel fabrication, piping and nozzle connections, reactor and piping supports, vessel head and bottom penetration connections, along with many other component fabrications. The welding process consists of applying a heat source and often weld filler metal along the weld path. Shrinkage of the weld beads during cooling leads to the development of WRS in components. The WRS profiles may have stress components greater than the yield stress because the stress state is multiaxial and, at locations where the mean stress is high, the component stresses can be quite high. Material hardening also plays a role. Moreover, in many applications, especially nuclear components, weld repairs are often necessary to remove defects. The WRS profiles caused by the repair welds are often more severe (i.e., produce higher tensile WRS that promotes crack growth) as compared with the original WRS state.

Also, WRS profiles are often self-equilibrating. For instance, the axial WRS profiles produced from the nozzle-to-piping dissimilar metal butt-welds considered in this study are typically close to self-equilibrating while the hoop WRS profiles are not. However, it is noted that repair welds have repair lengths only partway around the circumference of the weld. Therefore, the WRS profile near the start and stop locations of the repair are often quite different from those at the midpoint as reported by Brust and others [83].

A physical perspective for the development of WRS profiles is provided in Chapter 7, Residual Stress and Distortion, of the 2019 Welding Handbook [84], which describes weld bead shrinkage and geometry effects of residual stress development, among other factors. For complex geometries, the development of the WRS profile can be more involved and requires a nonlinear finite element solution of the welding process where the deposition of each pass is modeled. The history behind the development of computational weld models is summarized in many of the references cited in the Welding Handbook.

C1.1 Existing Library of WRS Profiles A series of WRS profiles were developed for use in xLPR code simulations for several different nozzle geometries as documented in the xLPR WRS Subgroup report [47]. Hoop and axial WRS profiles were developed for three typical PWR nozzle geometries: (i) hot and cold leg nozzles, (ii) steam generator inlet and outlet nozzles, and (iii) RCP inlet and outlet nozzles.

These nozzles are representative of many of the nozzles of interest for LBB assessment using the xLPR code. The factors that affect the WRS include the number of weld and butter passes, nozzle geometry (e.g., thickness, diameter, and taper), heat input, weld groove geometry, distance between the DMW and the SS closure weld, and weld repair depth, among others. For each nozzle geometry, the xLPR WRS Subgroup developed no-repair WRS profiles and WRS C-1

profiles considering repair depths of 15 and 50 percent. A separate procedure summarized in the WRS essential parameters and profile selection document [85] was developed for use by the NRC and EPRI project teams for the purposes of this study to determine which, if any, of the existing library of WRS solutions were applicable to the geometries of interest in the present study. If it was determined following this procedure that a particular geometry was not sufficiently represented by an existing solution, then a new solution was developed.

The WRS profiles in the xLPR WRS Subgroup report [47] were developed so that uncertainties could be calculated for use in the xLPR code simulations. These uncertainties were developed by having four experienced modelers develop solutions for each of the geometries considered.

Each modeler developed solutions using both isotropic and nonlinear kinematic hardening (NLKH) laws because these laws strongly influence the magnitude of the predicted results. The average of the isotropic and NLKH solutions for each modeler was used because it provides the best estimate of the WRS profiles as described in [47]. The modeling efforts provided a series of WRS profiles for each geometry and repair depth and the WRS uncertainty was assessed as discussed in [47] and then used as an input to the xLPR code.

The standard deviation was estimated at each point through the weld thickness in [47]. An average standard deviation over all 26 points was then recommended to represent uncertainties. This averaging approach produced a more stable estimate of the standard deviation because it was based on 504 data points (i.e., 4 WRS profiles times 26 through-thickness data points per profile) versus only 4 data points at each through-thickness location.

This approach was validated as discussed in [47], and the same approach was applied for the WRS profiles developed for this study.

C1.2 Mechanical Mitigation The two types of mechanical mitigation against PWSCC most often used in PWR nuclear power plants that affect the WRS profile are weld overlay and MSIP. Both overlays and MSIP reduce the inside diameter WRS profile, often making the magnitude compressive. A third type of mitigation is an inlay. It consists of depositing a layer of PWSCC-resistant Alloy 52 weld metal on the inside diameter. It was beyond the scope of the xLPR WRS Subgroup to develop mechanically mitigated WRS profiles directly from FEA solutions, so instead, they were estimated by applying a series of rules for each mechanical mitigation type that can be applied to the base, unmitigated WRS profile of interest. The rationale behind the development of these rules is discussed in [47].

C1.3 Overview of the WRS Profiles Used for the Generalization Study The WRS profiles for each nozzle DMW considered in the present study are summarized in the Sections C2 through C6. For each DMW, it was first determined if any of the WRS profiles from the existing library of solutions in [47] would apply, and justifications for applicability of these solutions are provided. Summaries are provided for cases where new WRS profiles were developed. In the WRS essential parameters and profile selection document [85], rules were developed to identify the closest match between a selected weld type and the library of existing C-2

WRS solutions in [47]. These rules were used as guidance to select the appropriate weld geometry for the FEA solutions presented in Sections C2 through C6.

Section C2 describes the Westinghouse pressurizer surge line nozzle DMW solutions for the base case and the more severe WRS profile that were developed for this study. Note that the base case uses a no-repair WRS profile. The more severe WRS profile was defined as one that results in a higher tensile stress at the weld inside diameter to favor more crack initiations. For the base case, the WRS profile was developed from the weld centerline as described in Section C2.1. For the more severe WRS case, it was developed from a path defined through the butter region as described in Section C2.2.

Section C3 describes the WRS profiles used for the CE and B&W RCP nozzle DMW analyses.

For the base case, the WRS profile from the existing library of solutions described in [47] was selected for the reasons provided in Section C2.1. A no-repair WRS profile was used.

Section C2.2 describes the more severe WRS profile developed from a path defined through the butter region.

Section C4 describes the WRS profiles used for the Westinghouse steam generator nozzle DMW analyses. Reference [47] provides the WRS profiles for a conventional, single-vee groove geometry. Section C4.1 describes the geometry and welding sequence for the double-vee groove replacement stream generator case. Section 1.1.1.1.1A1.1 describes the base case WRS profiles. Section C4.3 describes the more severe WRS profiles, which were again developed from a path defined through the butter region. As discussed in [47], the inlay WRS profiles were estimated by developing rules based on inlay solutions in the literature, because it was beyond the scope of the xLPR WRS Subgroup to develop mechanical mitigation WRS profiles directly from FEA solutions.

Section C5 summarizes the WRS profiles developed for the CE hot and cold leg branch line nozzle DMWs. Section C5.1 describes the geometries and welding sequences. Section C5.2 describes the base case WRS profiles, which are no-repair. The more severe WRS profile is summarized in Section C5.3. This profile was developed for a closure weld that was farther from the DMW as compared to the base case.

Finally, Section C6 describes the WRS profiles used for the Westinghouse two- and three-loop RVON and RVIN DMW analyses. These WRS profiles come from the existing library of solutions in [47]. The baseline WRS profiles are summarized in Section C6.1. They are for the no-repair case. The more severe WRS profiles are summarized in Section C6.2. These profiles were developed from a different PWR RVON weld geometry.

C2 Westinghouse Pressurizer Surge Line Nozzle DMWs The pressurizer surge line connection to the pressurizer is a geometry that does not fit into the categories of WRS profiles summarized in [47]. The nozzle geometry is unique because it has a fill-in weld, which is shown in Figure C-1. During the fabrication process, the fill-in weld is applied prior to the SS weld. A Westinghouse fill-in type pressurizer surge nozzle weld was chosen to develop the WRS profile because it is the most prevalent type. Of note, the fill-in C-3

weld tends to produce higher tensile WRS fields. The NRC/EPRI Phase 2b round robin mockup problem was used to obtain the WRS profiles for Case 2.1.0 assuming no repairs. The mockup description, model geometry, dimensions, material properties, and WRS solution results for ten independent modelers are summarized in NUREG-2162, Weld Residual Stress Finite Element Analysis Validation: Part 1 - Data Development Effort, issued March 2014 [86], NUREG-2228, Weld Residual Stress Finite Element Analysis Validation, Part II - Proposed Validation Procedure, Final Report, issued July 2020 [87], the Phase 2b finite element round robin results technical letter report, issued December 2015 [75], and by Rathburn and others [88]. The geometry and weld definition are illustrated in Figure C-1.

The welding procedure for this geometry consists of the following steps:

1. add butter

2. apply post-weld heat treatment (PWHT)

3. machine the butter in preparation for the DMW

4. add DMW beads

5. add fill-in weld on the inside diameter

6. add SS closure safe-end-to-pipe weld There was no repair weld considered for this case. Each of the 10 participants provided their WRS profiles along the weld centerline path, which is illustrated in Figure C-1, as the average of the isotropic and NLKH results. These data were then compiled so that the uncertainty in the participants results could be used to define the uncertainty in the WRS profile.

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Figure C-1 FEA model for pressurizer surge line nozzle with fill-in weld used for WRS profile development C2.1 Baseline and Mitigated WRS Profiles The axial and hoop WRS profiles plotted through the center of the pressurizer surge line nozzle weld are illustrated in Figure C-2 and Figure C-3, respectively. There is some variation among the WRS modelers, but in general the trends are quite similar. There were also WRS measurements made on this mock-up as discussed in the Phase 2b finite element round robin results technical letter report [75], and those measurements also exhibit some scatter. In general, the measurements reasonably validated the modelers analytical predications.

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Figure C-2 Pressurizer surge line nozzle axial WRS profiles through the weld centerline from 10 modelers C-6

Figure C-3 Pressurizer surge line nozzle hoop WRS profiles through the weld centerline from 10 modelers The average axial and hoop WRS profiles for the pressurizer surge line weld analyses are shown in Figure C-4 and Figure C-5, respectively. The WRS uncertainty parameters were determined using the same procedure as outlined in the xLPR WRS Subgroup report [47].

Following this procedure, a weighted mean and weighted standard deviation were calculated to reduce the impact of an outlier profile. Following this approach, the outlier solutions from Figure C-2 and Figure C-3 were given less weight in determining the mean and standard deviation values.

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Figure C-4 Pressurizer surge line nozzle average axial WRS profiles through the weld centerline C-8

Figure C-5 Pressurizer surge line nozzle average hoop WRS profiles through the weld centerline From the case descriptions, mechanical mitigation of the pressurizer surge line nozzle DMWs was also considered. For Case 2.1.3, the WRS profiles were obtained using the rules in [47] for overlay mitigation as shown in Figure 3-31. The main effect of the overlay is to reduce the inside diameter residual hoop stress, thereby reducing the probability of axial PWSCC initiation.

The overlay slightly increases the inside diameter residual axial stress. For Case 2.1.5, the rules in [47] for MSIP mitigation were applied to the mean pre-mitigation WRS profiles. The results are shown in Figure 3-43. The effect of MSIP is to reduce the inside diameter residual stress, thereby reducing the probability of both axial and circumferential PWSCC initiation.

C2.2 More Severe WRS Profile Case 2.1.2 was analyzed using a more severe WRS profile. For this case, an off-center location was selected. From the WRS contour plots shown in Figure C-6, a location in the Alloy 182 butter was chosen. The WRS at the inside diameter in the butter region is less compressive than at the weld centerline (i.e., -210 MPa versus -280 MPa). This figure shows the WRS generated using the isotropic hardening law; however, the same effect is seen with NLKH. Because the participants in the round robin study only extracted WRS results at the weld centerline, only the Emc2 solution was used to develop the more severe WRS profile.

Figure C-7 shows line plots for the Emc2 results in the butter. They provide a higher axial WRS C-9

at the inside diameter as compared to the Emc2 weld centerline results. The average of the 10 participants results for the weld centerline are shown for reference in the figure. The differences between the Emc2 weld centerline results and the 10 participants weld centerline results were used to adjust the Emc2 butter results so that it would be more representative of a solution as if it were based on data from all 10 participants.

Figure C-6 Pressurizer surge line nozzle axial and hoop WRS contour plots generated from FEA with isotropic hardening law C-10

Figure C-7 Pressurizer surge line nozzle axial WRS profiles through the weld centerline and butter using the average of the isotropic and NLKH solutions C3 CE and B&W RCP Nozzle DMWs C3.1 Baseline WRS Profiles B&W RCP WRS profiles are available in the xLPR WRS Subgroup report [47]. The only dissimilar metal butt-welds in the reactor coolant system main loop piping in a B&W plant are at the piping connections to the RCP at the lower cold leg and the upper cold leg locations. Of note, a B&W plant has two steam generators with two RCPs per generator; therefore, a total of eight DMWs are present within the system. The nozzle weld geometry and materials are shown in Figure C-8, and a more complete description of the FEA model and results is provided in [47].

For the RCP, the no-repair WRS profile was used because it has the highest mean stress on the inside diameter. For completeness, the mean profile is shown in in Figure C-9 along with results from the four modelers.

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Figure C-8 FEA model used for RCP nozzle WRS profile development C-12

Figure C-9 RCP nozzle axial and hoop WRS profiles through the weld centerline C3.2 More Severe WRS Profile For the RCP, the more severe WRS profile was developed by finding a region in the DMW where the inside diameter residual stresses were the highest. Figure C- 10 shows contour plots from the Emc2 FEA solution (the only available) using the isotropic hardening law. The upper C-13

illustration shows the WRS around the weld centerline, and the lower illustration shows the WRS around the butter where the higher stresses exist at the inside diameter.

Figure C- 10 RCP nozzle axial WRS contour plots generated from FEA with isotropic hardening law Figure C-11 shows the weld centerline axial WRS profile plots for all four analysts along with the mean. These WRS profiles are discussed in detail in [47]. Also shown is the WRS profile in the butter from the Emc2 analysis. The differences between the Emc2 centerline profile and the 4 C-14

participants centerline profiles were used to adjust the Emc2 butter results so that they would be more representative of a solution as if it were based on data from all 4 modelers. Figure C-12 shows a comparison of the axial WRS profiles though the weld centerline and butter, the latter of which represents the more severe case.

Figure C-11 RCP nozzle axial WRS profiles through the weld centerline and butter from four modelers C-15

Figure C-12 RCP nozzle axial WRS profiles through the weld centerline and butter from Emc2 C4 Westinghouse Steam Generator Nozzle DMWs Westinghouse steam generator nozzle WRS profiles are available in the xLPR WRS Subgroup report [47] for no-repair and 15 percent and 50 percent repair depths. These solutions were also presented in detail by Brust and others [89], where PWSCC growth was also modeled as driven by the WRS profiles. These solutions are for the more typical single-vee groove geometry. However, some steam generators have been replaced with a double-vee groove geometry with an Alloy 52 inlay. These steam generators include the ones at Peach Bottom Atomic Power Station, Unit 2; North Anna Power Station, Units 1 and 2; and Virgil C. Summer Nuclear Station, Unit 1.

C4.1 Replacement Steam Generator Nozzle Geometry and Welding Sequence The geometry, dimensions, and materials for the replacement steam generator double-vee groove weld are shown in Figure C-13. The nozzle is tapered with a 122.33 mm thickness along the weld centerline. The safe end is 177 mm from the DMW. At such a distance, the closure weld is unlikely to reduce the WRS profiles at the DMW inside diameter. Based on experience modeling other nozzles, 100 mm or less is typically required to see such an effect.

The dimensions and other details regarding the weld were obtained from the April 22, 2013, letter from E. S. Grecheck, Vice President - Nuclear Engineering and Development, Virginia Electric and Power Company, to the NRC Document Control Desk [56].

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Figure C-14 Replacement steam generator nozzle geometry, dimensions, and materials The FEA model is presented in Figure C-15. The welding sequence and modeling procedure is summarized in the following steps. As applicable, references to region numbers in these steps refer to the numbered areas in the figure.

1. Model butter application, PWHT, and machining of the butter (neglected here)

2. Model the Alloy 152 tie in Region 1 (5.6 mm thick)

3. Model the inner Region 2 vee partial groove welds

4. Model back gouge below Region 3 and add back gouge passes

5. Model the outer vee Region 3 weld passes

6. Model the inner vee Region 4 weld passes

7. Model Alloy 152 inlay in Region 5 (3.4 mm thick)

8. Model the SS closure weld

9. Model the application of the hydrotest and removal

10. Apply operating temperature of 300°C C-17

The DMW has a double-vee groove. The butter was first applied to the nozzle, subjected to PWHT, and then machined to facilitate the double-vee groove weld. The butter and PWHT process was not included in this WRS model because, from experience, when the PWHT is applied after the butter, the effect of neglecting the PWHT is small on the final WRS field. Next, the Alloy 152 inlay tie-in (shown as Region 1 in the figure) was deposited, which was followed by the inner vee weld (Region 2). Then the back-gouge removal was modeled, and the Region 3 Alloy 182 weld was deposited followed by completion of the inside diameter weld (Region 4). Finally, the Alloy 152 inlay (shown as Region 5 in the figure) was modeled and then the SS closure weld. As is the case with all the xLPR WRS Subgroup solutions [47], afterwards a hydrostatic test was modeled and then three cycles of service load (pressure and end cap stresses only), which can help to shake down (i.e., reduce) the WRS profile to some extent.

Finally, all results are presented at the operating temperature of 300°C.

An inlay performed in the field consists of removing Alloy 82/182 material under the DMW and then adding Alloy 52/152, PWSCC resistant material as discussed in detail in the xLPR WRS Subgroup report [47]. The inlay feature shown in Figure C-15 was actually applied during the steam generator replacement process and was added to enhance the DMWs resistance to PWSCC. The temperature-dependent material stress-strain curve for Alloy 82/182 and Alloy 52/152 are nearly identical. Therefore, the WRS profiles shown in Figure C-16 and Figure C-17 are the same regardless of whether Alloy 82/182 or Alloy 52/152 is deposited. Therefore, analyses can be made using this WRS field for either material.

Figure C-15 FEA model used for replacement steam generator nozzle WRS profile development C-18

C4.2 Baseline and Mitigated WRS Profiles As described above, the replacement steam generator weld is a unique geometry because it includes an inlay of PWSCC-resistant Alloy 152 material, but the inlay was deposited during the original fabrication sequence rather than during an outage.

Figure C-16 and Figure C-17 provide the axial and hoop WRS profiles, respectively, for isotropic and NLKH laws, and the average stresses through the weld centerline. The average WRS profile is used for Cases 4.1.0, 4.1.1, and 4.1.2. The uncertainty in the WRS profile was assumed to be similar as in [47]. Because the Alloy 152 inlay was deposited at the end of the weld sequence, and the closure weld had little effect, the inside diameter axial stresses are tensile.

Overlay mitigation was also considered for the steam generator nozzle DMWs in Case 4.1.3.

The WRS profiles for this case were obtained using the rules from [47] and are shown in Figure 3-69.

Figure C-16 Replacement steam generator nozzle axial WRS profiles C-19

Figure C-17 Replacement steam generator nozzle hoop WRS profiles C4.3 More Severe WRS Profile The more severe WRS profile was determined from a location within the steam generator weld or butter where the axial stresses at the inside diameter were the highest because these conditions promote PWSCC initiation. Figure C-18 and Figure C-19 provide contour plots of the axial and hoop WRS, respectively. A path in the butter region was chosen to obtain the more severe WRS profile. Figure C-20 and Figure C-21 show the average axial and hoop WRS profiles, respectively. The weld centerline stresses are shown for comparison. Figure C-20 shows that the axial WRS profile in the butter is higher at the inside diameter as compared to the weld centerline; however, the hoop stresses are lower in the butter as shown in Figure C-21.

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Figure C-18 Replacement steam generator nozzle baseline axial WRS contour plot generated from FEA C-21

Figure C-19 Replacement steam generator nozzle hoop WRS contour plot generated from FEA C-22

Figure C-20 Comparison of replacement steam generator nozzle baseline and more severe axial WRS profiles C-23

Figure C-21 Comparison of replacement steam generator nozzle and more severe hoop WRS profiles C5 CE Hot and Cold Leg Branch Line Nozzle DMWs The CE plants have DMWs for some of the hot leg and cold leg branch line connections. This includes a DMW from the surge line to the hot leg and to the pressurizer, which is different from Westinghouse plants where the DMW is only to the pressurizer. The geometry and welding process for these lines are proprietary, so they were estimated for this study based on field measurements and other documents provided by EPRI. The generic geometry is applicable for the hot leg to surge weld and other lines, including shutdown cooling and high-pressure injection. It is noted that this geometry does not fall within the guidelines of the xLPR WRS Subgroup report [47] or the WRS essential parameters and profile selection document [85], so a new WRS profile was developed for this study.

C5.1 Geometry and Welding Sequence A representative geometry for the CE branch lines is shown in Figure C-22. The distance between the DMW and SS weld is 95.25 mm (3.75 inches), which is the average distance for all the nozzles. The maximum distance was 107.95 mm (4.25 inches), and this distance was used to obtain the more severe WRS profile as discussed in Section C5.3.

C-24

Figure C-22 Representative CE branch line nozzle geometry and dimensions The FEA model and materials for the CE branch line are shown in Figure C-23 and Figure C-24.

The weld has a single-vee groove geometry. The weld sequence is shown in Figure C-25. The DMW was modeled with 36 weld bead passes and the SS weld with 24 passes. The different colors in Figure C-23 and Figure C-25 illustrate the weld passes. As in the approach used to develop the xLPR WRS Subgroup solutions [47], a hydrostatic test was modeled, and then three cycles of service load (pressure and end cap stress only), which can help to shake down the WRS profile to some extent. Finally, all results are presented at an operating temperature of 300°C.

Figure C-23 FEA model used for CE branch line nozzle WRS profile development C-25

Figure C-24 Details of FEA model used for CE branch line nozzle WRS profile development C-26

Figure C-25 Fabrication sequence used for CE branch line nozzle WRS profile development C5.2 Baseline WRS Profiles Contour plots of the axial and hoop WRS for the CE branch line weld are shown in Figure C-26.

The corresponding axial and hoop WRS profiles are shown in Figure C-27 and Figure C-28.

The average of the isotropic and NLKH law results was used. Since only one analysis was performed, the uncertainty was defined by assuming similar uncertainty to that shown in [47] for the hot leg.

C-27

Figure C-26 CE branch line nozzle axial (left) and hoop (right) WRS contour plots generated from FEA at 300°C Figure C-27 CE branch line nozzle axial WRS profiles through the weld centerline C-28

Figure C-28 CE branch line nozzle hoop WRS profiles through the weld centerline C5.3 More Severe WRS Profile There is 95.25 mm between the DMW and the SS weld in the typical CE branch line as shown in Figure C-29(a). This distance is the average of all the CE branch lines based on information provided by EPRI. The greatest distance was 107.95 mm as shown in Figure C-29(b). In general, if this distance is greater, the WRS profile will be more tensile at the inside diameter, because the SS closure weld applies a ring shrinkage load to the pipe. This load affects the WRS field in the DMW, and the farther this weld is from the DMW, the less the WRS field is reduced. As such, this greater distance was used to develop the more severe WRS profile.

Figure C-30 and Figure C-31 show the axial and hoop WRS profiles through the weld centerline for different DMW to SS closure weld distances. The axial WRS profile is higher by about 50 MPa at the inside diameter, and the tensile stress has a higher maximum of about 40 MPa at a depth of about 20 percent of the wall thickness. In addition, the compressive stress in the middle of the nozzle is about the same. The more severe WRS profile will thus lead to more crack initiations and a greater chance for axial crack leakage. From Figure C-31, there is also an increase in the hoop WRS profile for the greater DMW to SS closure weld distance.

C-29

(a)

(b)

Figure C-29 CE branch line nozzle DMW to SS closure weld distances of (a) 95.25 mm and (b) 107.95 mm as shown on the FEA models C-30

Figure C-30 CE branch line nozzle axial WRS profiles for different DMW to SS closure weld distances C-31

Figure C-31 CE branch line nozzle hoop WRS profiles for different DMW to SS closure weld distances Finally, the butter location provides the higher axial WRS profile as compared to the weld centerline location for the larger DMW to SS closure weld distance. This location was thus chosen to extract the more severe WRS profile for the CE branch line weld. Comparisons of the weld centerline and butter WRS profiles are shown in Figure C-32 and Figure C-33 for axial and hoop stresses, respectively. As shown in these figures, the butter region for the DMW to SS closure weld distance of 107.95 mm represents the more severe WRS profile for the CE branch line nozzles.

C-32

Figure C-32 Comparison of CE branch line nozzle axial WRS profiles through weld centerline and butter Figure C-33 Comparison of CE branch line nozzle hoop WRS profiles through the weld centerline and butter C5.4 DMW to SS Weld Distance Study One of the main drivers of the WRS in DMWs is the distance between the DMW and the SS closure weld. If the closure weld is within about 100 mm from the DMW, the shrinkage of the C-33

SS weld acts like a shrink fit clamp ring that modifies the WRS profile in both the DMW and the SS weld. There are several factors that make the DMW WRS profile more compressive. These factors include the distance between the two welds, the thickness of both the DMW and the SS safe end, and the SS weld groove size, which control the amount of weld shrinkage and hence the clamping force caused by the SS weld.

One of the challenges in developing WRS profiles for use in xLPR code simulations is that there are several variables that affect the final DMW WRS profile, especially the distance between the DMW and SS closure weld. There has been no systematic study of this effect to develop rules for modifying the existing library of WRS profiles. The effect of mechanical mitigation had been implemented in [47] through rules that can be applied to modify the WRS profile for mechanical mitigation (i.e., overlay, MSIP, and inlay). A companion study was thus undertaken to examine the effect of the distance between the DMW and SS closure weld to develop rules to extend the existing library of WRS profiles. This study only considered the geometry of the CE surge line nozzle; however, similar studies of other, thicker nozzle geometries could be similarly performed.

Figure C-34 FEA mesh for DMW to SS closure weld distance Figure C-34 illustrates the concept. The top mesh was used to develop the CE branch line WRS profiles. The distance between the DMW and the SS closure weld is 95.25 mm, which represents the average of all the different CE branch lines. To obtain the more severe WRS profile, a second analysis was performed by setting this distance to 107.95 mm, which was the largest distance among all the CE branch line welds. As seen in the bottom illustration of Figure C-34, this latter case can be modeled by simply adding 12.7 mm (0.5 inches) to the mesh length. Meshes with distances of 120.35 and 133.35 mm were generated along with a case where there was no SS closure weld, which represented an infinite distance. By adding the C-34

additional elements in the region between the DMW and SS closure weld and beginning with node and element numbering range larger than the original mesh (95.25 mm distance), there was no need to modify the weld pass numbering or any other mesh parameters necessary to perform the analysis. This made rerunning of the analyses quick and straightforward. The case with no SS closure weld represents the limit of the solution.

Figure C-35 shows the effect of the DMW to SS closure weld distance on the axial WRS profile.

The stresses increase at the inside diameter as the distances between the DMW and SS closure weld increase until there is no closure weld. The inside diameter stresses increase as the outside diameter stresses decrease because of the axisymmetric nature of the solution.

Figure C-36 shows a similar trend for the hoop stresses. These results could be used to develop rules to account for this important variable for use in xLPR code simulations. These results are for a thinner nozzle geometry, but by generating results for a few thicker nozzle geometries (e.g., RVON and steam generator), the effect of this parameter could be quantified to expand the existing set of WRS solutions. The rules would be developed as a function of a normalized distance through the weld thickness.

Figure C-35 Axial WRS profiles for CE branch line considering different DMW to SS closure weld distances C-35

Figure C-36 Hoop WRS profiles for CE branch line considering different DMW to SS closure weld distances C6 Westinghouse Two- and Three-Loop RVON and RVIN DMWs The RVON and RVIN DMWs for Westinghouse four-loop plants were analyzed in the prior study. The Westinghouse two- and three-loop DMWs are examined here. The nozzle geometries, materials, and welding sequences are similar among the two-, three- and four-loop designs. Therefore, the WRS profile used for the four-loop designs was also used for the two-and three-loop analyses.

C6.1 Baseline WRS Profiles The no-repair WRS profile was used because the inside diameter stresses are higher as compared to the 15 and 50 percent repair cases. The closure weld is close to the DMW, so the WRS profile is compressive. A more complete description of the RVON geometry, fabrication process, and WRS results is available in [47]. For completeness, the axial and hoop WRS profiles are shown in Figure C-37.

C-36

Figure C-37 Axial and hoop WRS profiles for Westinghouse three- and four-loop RVONs C6.2 More Severe WRS Profile The more severe WRS profiles for the Westinghouse RVON cases were developed from a nozzle with a different geometry [47]. The geometries considered for this case are described in the evaluation of the inlay process as a mitigation strategy for PWSCC [57] along with the C-37

welding processes and fabrication sequence. Since that evaluation was performed in 2010, the model was used to redevelop the WRS profiles using the xLPR WRS Subgroup approach [47].

The WRS profiles were generated from an average of the isotropic and NLKH law results. The model is shown in Figure C-38. The geometry is different from that used in [47] with the distance from the DMW to SS closure weld set to 93.1 mm. The mesh near the inside diameter is quite fine, because this region is where the small temper bead inlay passes were deposited for the evaluation in [57]. However, for the present analysis no inlay was modeled and only deposition of the Alloy 82/182 weld material was included. The fabrication modeling process consisted of the following steps:

1. add butter passes and corresponding PWHT

2. deposit the DMW passes

3. add the SS closure weld

4. hydrotest modeling

5. apply three loading cycles including temperature application, pressure, and removal

6. heat to operating temperature of 300°C It is noted that further details of the WRS profile calculations and PWSCC growth predictions in the inlay can be found in [57] and the publications by Brust and others [90] and Rudland and others [58]. The natural PWSCC growth predictions lead to a bubble-shape after the crack grows through the Alloy 52 material and enters the Alloy 182 material. Such a shape causes the leak rate to be constrained by the small crack size in the inlay, and thus the crack may not be easily detected in the simulation, even if a large crack exists outside the inlay.

C-38

Figure C-38 RVON nozzle model for more severe WRS profile A contour plot of the axial WRS profiles, showing the butter and weld outlines, is shown in Figure C-39 for the isotropic hardening law case. The axial and hoop WRS profiles at the weld centerline are shown in Figure C-40. The average WRS profiles were used for the RVON analyses. Finally, the axial WRS profile and more severe WRS profile are shown in Figure C-41. These profiles were used for the Case 1.3.0 and 1.3.1 analyses.

C-39

Figure C-39 More severe axial WRS contour plot for the RVON weld C-40

Figure C-40 More severe axial and hoop WRS profiles for the RVON weld C-41

Figure C-41 Comparison of more severe and axial WRS profiles for the RVON weld C-42