ML12118A484
| ML12118A484 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/26/2012 |
| From: | Price J Virginia Electric & Power Co (VEPCO) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| N1-I4-CMP-001 | |
| Download: ML12118A484 (16) | |
Text
VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 April 26, 2012 U.S. Nuclear Regulatory Commission Serial No.12-308 Attention: Document Control Desk NLOS/ETS RO One White Flint North Docket No. 50-338 11555 Rockville Pike License No. NPF-4 Rockville, MD 20852-2738 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)
NORTH ANNA POWER STATION UNIT 1 - RELIEF REQUEST NI-14-CMP-001
SUMMARY
RESULTS OF THE STRESS ANALYSES FOR THE STEAM GENERATOR HOT LEG INLET NOZZLES FULL STRUCTURAL WELD OVERLAY FINAL CONFIGURATIONS By letter dated March 30, 2011 (Serial No.11-120), Dominion requested NRC approval for a proposed alternative to certain ASME Code Section Xl - 2004 Edition requirements associated with the Steam Generator hot leg nozzle repairs. The proposed alternative permits the application of full structural weld overlays (FSWOLs) to mitigate the potential for primary water stress corrosion cracking (PWSCC) susceptibility at North Anna Unit 1. By letter dated March 13, 2012, the NRC approved Relief Request Nl-14-CMP-001 that permits the use of FSWOLs as an alternative.
Dominion has installed FSWOLs on the Steam Generator hot leg nozzle dissimilar metal welds (DMWs) during the North Anna Unit 1 spring 2012 refueling outage. As a condition of the alternative, Dominion committed to provide a summary of the stress analysis performed for each of the FSWOL final configurations. The attachment to this letter provides a summary of the stress analysis performed for the three hot leg Steam Generator nozzles.
If you have any questions regarding this submittal, please contact Mr. Thomas Shaub at (804) 273-2763.
Sincerely,
=sident - Nuclear Engineering
Attachment:
Stress Analysis Summary, Steam Generator Hot Leg Nozzles Commitments made in this letter: None
-Actl IUUL
Serial No.12-308 Docket No. 50-338 Page 2 of 2 cc: U.S. Nuclear Regulatory Commission Region II Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, Georgia 30303-1257 Mr. J. E. Reasor, Jr. (without attachment)
Old Dominion Electric Cooperative Innsbrook Corporate .Center 4201 Dominion Blvd.
Suite 300 Glen Allen, Virginia 23060 NRC Senior Resident Inspector (without attachment)
North Anna Power Station Dr. V. Sreenivas (without attachment)
NRC Project Manager U. S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop 08 G-9A Rockville, Maryland 20852 Ms. K. R. Cotton (without attachment)
NRC Project Manager U. S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop 08 G-9A Rockville, Maryland 20852 Mr. M. M. Grace (without attachment)
Authorized Nuclear Insurance Inspector North Anna Power Station
12-308 Docket No. 50-338 ATTACHMENT Stress Analysis Summary Steam Generator Hot Leg Nozzles North Anna Power Station Unit 1 Virginia Electric and Power Company (Dominion)12-308 Docket No. 50-338 Attachment Page 1 of 13 STRESS ANALYSIS
SUMMARY
STEAM GENERATOR HOT LEG NOZZLES NORTH ANNA POWER STATION UNIT I
1.0 INTRODUCTION
Virginia Electric and Power Company (Dominion) applied full structural weld overlays (FSWOLs) on dissimilar metal welds (DMWs) between the low alloy steel nozzle and stainless steel safe end of the three Steam Generator (SG) hot leg inlet nozzles at North Anna Power Station Unit 1.
The purpose of these overlays is to eliminate dependence upon the primary water stress corrosion cracking (PWSCC) susceptible Alloy 82/182 welds as pressure boundary welds and to mitigate any potential future PWSCC in these welds. The overlays were installed using a PWSCC resistant weld filler material; Alloy 52M [1].
This report, which satisfies Dominion Commitment No. 1 of the Relief Request [2],
summarizes the results of the nozzle specific residual stress analyses and the fracture
-mechanics evaluations, and also documents that all ASME Code,Section III stress and fatigue criteria are met. This information is to be submitted prior to entry into Mode 4 following completion of the overlays.
The requirements for the design of weld overlay repairs are defined in the Relief Request [2], which is based on ASME Code Case N-740-2 [3]. Weld overlay repairs are considered to be acceptable long-term repairs for PWSCC susceptible weldments if they meet a conservative set of design assumptions which qualify them as "full structural" weld overlays. The design basis flaw assumption for FSWOLs is a circumferentially oriented flaw that extends 3600 around the component; that is, completely through the original component wall thickness. A combination of internal pressure, deadweight, seismic and other dynamic stresses is applied to the overlaid nozzles containing this assumed design basis flaw, and they must meet the requirements of ASME Code,Section XI, IWB-3641 [4].
ASME Code,Section III stress and fatigue usage evaluations are also performed that supplement the existing safe end and nozzle stress reports to demonstrate that the overlaid components continue to meet ASME Code,Section III requirements. The original construction Code for the SG hot leg inlet nozzles was ASME Code,Section III, 1968 Edition with Addenda through Winter 1968. However, as allowed by the ASME Code Section XI, Code Editions and Addenda later than the original construction Code may be used. ASME Code,Section III, 2004 [5] was used for these analyses.
In addition to providing structural reinforcement to the PWSCC susceptible locations with a resistant material, weld overlays have also been shown to produce beneficial compressive residual stresses that mitigate PWSCC in the underlying DMWs. The weld overlay approach has been used to repair stress corrosion cracking in U.S. nuclear
12-308 Docket No. 50-338 Attachment Page 2 of 13 plants on hundreds of welds, and there have been no reports of subsequent crack extension after application of weld overlays. Thus, the compressive stresses caused by the weld overlay have been effective in mitigating new crack initiation and/or growth of existing cracks.
Finally, evaluations will be performed, based on as-built measurements taken after the overlays were applied, to demonstrate that the overlays meet their design basis requirements, and that they will not have an adverse effect on the balance of the piping systems. These include comparison of overlay dimensions to design dimensions, evaluations of shrinkage and added weight effects on the piping systems.
2.0 ANALYSIS
SUMMARY
AND RESULTS 2.1 Weld Overlay Structural Sizing Calculations Detailed sizing calculations for weld overlay thickness were performed using the ASME Code, Section Xl, IWB-3640 [4] evaluation methodology. Normal Operating (Level A),
Upset (Level B), Emergency (Level C), and Faulted (Level D) load combinations were considered in this evaluation, and the design was based on the more limiting results.
Additionally, per the seismic margin management plan [10], increased seismic loads were also considered in the evaluation. The resulting minimum required overlay thicknesses are summarized in Table 2-1.
As stated in Section 1.0, FSWOLs were installed using Alloy 52M filler metal. However, Alloy 52M weld metal has demonstrated sensitivity to certain impurities, such as sulfur, when deposited onto austenitic stainless steel base materials. Therefore, a buffer (transitional) layer of austenitic stainless steel filler metal was applied across the austenitic stainless steel base material. The austenitic stainless steel buffer layer is not included in the structural weld overlay thickness defined above.
The weld overlay length must consider: (1) length required for structural reinforcement, (2) length required for access for preservice and inservice examinations of the overlaid weld, and (3) residual stress improvement. In accordance with the Relief Request [2]
and ASME Code Case N-740-2 [3], the minimum weld overlay length required for structural reinforcement was established by evaluating the axial shear stress due to transfer of primary axial loads from the safe end into the overlay and back into the nozzle, on either side of the weld being overlaid. Axial weld overlay lengths were established such that this stress is less than the ASME Section III limit for pure shear stress. The resulting minimum length requirements are summarized in Table 2-1.
The overlay length and profile must also be such that the required post-WOL examination volume can be inspected using Performance Demonstration Initiative (PDI) qualified nondestructive examination (NDE) techniques. This requirement can cause required overlay lengths to be longer than the minimums for structural reinforcement. A schematic of the weld overlay design for the North Anna Unit 1 SG hot leg inlet nozzles is illustrated in Figure 2-1. The design was reviewed by qualified NDE personnel to
12-308 Docket No. 50-338 Attachment Page 3 of 13 ensure that it meets inspectability requirements, and the overlay was designed to satisfy full structural requirements for the DMWs. The design thickness and length specified on the design drawing bounds the calculated minimum values, and may be greater to facilitate the desired geometry for examination.
Table 2-1: Weld Overlay Structural Thickness and Length Requirements Location SG Hot Leg Inlet Nozzle Minimum Nozzle Side 1.37 Thickness (in.) Safe End Side 1.37 Minimum Nozzle Side 2.58 Length*
(in.) Safe End Side 3.81
- Length shown is the minimum required for structural acceptance and does not include additional length necessary to meet inspectability requirements.
2.2 ASME Code,Section III Stress Analyses Stress intensities for the weld overlaid SG hot leg inlet nozzles were determined from finite element analyses for the various specified load combinations and transients using the ANSYS software package [6]. Linearized stresses were evaluated at various stress locations using 3-dimensional solid models. A typical finite element model showing stress path locations is provided in Figure 2-2. The stress intensities at these locations were evaluated in accordance with ASME Code,Section III, Sub-articles NB-3200 and NB-3600 [5], and compared to applicable Code limits. A summary of the stress and fatigue usage comparisons for the most limiting locations is provided in Table 2-2. The stresses and fatigue usage in the weld overlaid nozzles are within the applicable Code limits.
Table 2-2: Limiting Stress Results for Weld Overlaid Nozzles Nozzle Load Combination Type Calculated Allowable SG Hot Level A/B Primary + Secondary (P + Q) (ksi)* 44.63 51.69 Leg Inlet Nozzle Fatigue Cumulative Usage Factor 0.56 1.000
- Primary stress acceptance criteria are met via the sizing calculations discussed in Section 2.1.12-308 Docket No. 50-338 Attachment Page 4 of 13 2.3 Residual Stress Analysis Weld residual stresses for the North Anna SG hot leg inlet nozzle weld overlays were determined by detailed elastic-plastic finite element analyses. The analysis approach has been previously documented to provide predictions of weld residual stresses that are in reasonable agreement with experimental measurements [7]. A two-dimensional, axisymmetric finite element model was developed for the SG hot leg inlet nozzle configurations. The modeling of weld nuggets used in the analysis to lump the combined effects of several weld beads is, illustrated in Figure 2-3. The model simulated an inside surface (ID) repair at the DMW location with a depth-of approximately 50% of the original wall thickness. This assumption is considered to conservatively bound any weld repairs that may have been performed during plant construction from the standpoint of producing tensile residual stresses on the ID of the weld.
An analysis was performed to simulate the welding process of the ID weld repair, the safe end-to-pipe weld, the overlay welding process, and finally, a slow heatup to operating temperature. The analysis consists of a thermal pass to determine the temperature response of the model to each individual lumped weld nugget as it is added in sequence, followed by a non-linear elastic-plastic stress pass to calculate the residual stress due to the temperature cycling from the application of each lumped weld pass.
Since residual stress is a function of the welding history, the stress pass for each nugget is applied to the residual stress field induced from all previously applied weld nuggets.
After completion of the weld overlay simulation, the model was allowed to cool to a uniform steady state temperature of 70 0 F, and then heated up to a uniform steady state temperature of 621.9 0 F and a pressure of 2;235 psig to obtain the residual stresses at normal operating conditions.
The resulting residual stresses were evaluated on two paths through the DMW. These
.path definitions are shown in Figure 2-4. The resulting through wall residual stresses along these paths are shown in Figure 2-5.
2.4 Section XI Crack Growth Analyses The residual stress calculations were utilized, along with stresses due to applied loadings and thermal transients, to demonstrate that assumed cracks that could be missed by inspections or the as-found cracks in the weld will not exceed the overlay design basis during the ASME Section XI inservice inspection interval due to fatigue or PWSCC. In the fatigue crack growth analysis, the 40-year design quantity of cycles for each applied transient was applied. Since the examination volume for the PDI qualified post-overlay UT inspections includes the weld overlay thickness plus the outer 25% of the pre-WOL wall thickness, an ID connected flaw that is 75% of the pre-WOL weld thickness is assumed as the largest flaw that could escape detection by this examination. Thus, crack growth is computed assuming an initial flaw depth of 75% of
12-308 Docket No. 50-338 Attachment Page 5 of 13 the pre-WOL weld thickness for the "A" and "C" SG hot leg inlet nozzles. For the "B",SG hot leg inlet nozzle, the as-found flaws were used in the analysis. As the flaws were 100% through-wall, ID connected flaws that were 100% of the pre-WOL machined weld thickness were used. The amount of time it takes for this assumed flaw to reach the overlay design basis thickness is then calculated. The results are shown in Table 2-3 for the nozzles.
For crack growth due to PWSCC, the total sustained stress intensity factor during normal steady state plant operating conditions was determined as a function of assumed crack depth, considering internal pressure stresses, residual stresses, steady state thermal stresses, and stresses due to sustained piping loads (including deadweight). Zero PWSCC growth is predicted for assumed crack depths at which the combined stress intensity factor due to sustained steady state operating conditions is less than zero.
Table 2-3: Crack Growth Results Time for Postulated Flaw to Reach Overlay Flaw Design Basis Thickness "A" and "C" SG Hot Leg Inlet Nozzle Circumferential (DMW)l 16 years Axial (DMW) 1 18 years "B" SG Hot Leg Inlet Nozzle Circumferential (DMW) 1 10 years Axial (DMW)l 13 years Notes:
- 1. DMW = Dissimilar metal weld.12-308 Docket No. 50-338 Attachment Page 6 of 13 2.5 Leak-Before-Break Evaluation A leak-before-break (LBB) evaluation that includes the weld overlay was performed.
The evaluation has demonstrated that with the application of the weld overlay, the LBB margins required in SRP 3.6.3 [8] and NUREG-1061, Vol. 3 [9] are maintained. A range of weld overlay thicknesses was also evaluated, showing that the actual thickness attained during overlay application does not change the LBB behavior significantly. The results of the LBB evaluation, specifically the leakage values for the critical flaw sizes, are shown in Table 2-4. All the leakage values exceed the minimum allowable leak detection rate.
Table 2-4: Leak-Before-Break Leakage Results Minimum Path 1 Path 2 Path 3 Allowed Leak Detection Rate Min. WOL, gpm 79.37 18.52 124.21 10.0 Max. WOL, gpm 81.21 1.8.69 123.91 10.0 2.6 Evaluation of As-Built Conditions The Relief Request [2] and Code Case N-740-2 [3] require evaluation of the as-built weld overlays to determine the effects of any changes in applied loads, as a result of weld shrinkage from the entire overlay, on other items in the piping system. These evaluations will be performed and documented separately from this report and will include the effects of the disposition of any non-conformances that occurred during weld overlay installation. In anticipation of the required as-built evaluations, calculations were performed based on design dimensions to confirm that the overlays would not adversely affect critical piping components. Specifically, the effect of the added weight of the overlays on the adjacent piping systems, based on maximum design dimensions, was evaluated and found to be insignificant.12-308 Docket No. 50-338 Attachment Page 7 of 13 Weld C Figure 2-1: Illustration of Weld Overlay Design
12-308 Docket No. 50-338 Attachment Page 8 of 13 00 Section 900 Section 1800 Section Figure 2-2: Finite Element Model for Section III Stress Evaluation showing Stress Paths12-308 Docket No. 50-338 Attachment Page 9 of 13 F
Figure 2-3: Finite Element Model for Residual Stress Analysis showing Nuggets used for Welding Simulations
Docket No.12-308 50-338 Attachment Page 10 of 13 Figure 2-4:
Finite Element Model for Residual Stress Analysis showing Stress Paths12-308 Docket No. 50-338 Attachment Page 11 of 13 Path 1 Through-Wall Stress Post WOL 80 70 60 50 40 30 20 10 0
'ft 0 0
-10
-20
-30
________-0--Axial 70'F
-40 0
___ _,_-,49--HoopM7 F
-50
-.&r-- Axial 621.9 F/2235 psig
-60
-v-- Hoop 62 .9°F/2235 psig
-70 r r -
0 1 2 3 4 5 6 Distance from ID Surface (in)
Path 2Through-Wall Stress Post WOL 80 70 60 50 40 30 20 10
'ft
'ft 0
0
-10
-20
-30
-40
-50
-60
-70 0 1 2 3 4 5 6 Distance from ID Surface (in)
Figure 2-5: Residual Stress Results along Stress Paths through the DMW
12-308 Docket No. 50-338 Attachment Page 12 of 13
3.0 CONCLUSION
S The design of the North Anna Power Station Unit 1 weld overlay was performed in accordance with the requirements of the Relief Request [2], which is based on ASME Code Case N-740-2 [3]. The weld overlay design is demonstrated to provide long-term mitigation of PWSCC in these welds based on the following:
" In accordance with the Relief Request [2], structural design of the overlays was performed to meet the requirements of ASME Code, Section Xl, IWB-3640 [4]
based on an assumed circumferential flaw 100% through and 3600 around the original welds (which bounds a 100% through-wall axial flaw). Therefore, the resulting FSWOLs restore the original safety margins of the nozzles with no credit taken for the underlying, PWSCC-susceptible material.
" The weld metal used for the overlay is Alloy 52M, which has been shown to be resistant to PWSCC [1], thus providing a PWSCC resistant barrier. Therefore, no PWSCC crack growth is expected into the overlay.
" Application of the weld overlays was shown to not impact the conclusions of the existing nozzle Stress Report. Following application of the overlay, all ASME Code,Section III stress and fatigue criteria are met.
" A nozzle specific residual stress analysis was performed, afterfirst simulating a severe ID weld repair in the nozzle-to-safe-end weld, prior to applying the weld overlay. The post weld overlay residual stresses were shown to result in beneficial compressive stresses or reduced tensile stresses on the inside surface of the components, and into the thickness of the original DMW, assuring that future PWSCC initiation or crack growth into the overlay is highly unlikely.
" A fracture mechanics analysis was performed to determine the amount of future crack growth that would be predicted in the nozzles, assuming that cracks exist that are equal to or greater than the thresholds of the NDE techniques used to examine the nozzles. Both fatigue and PWSCC crack growth were considered and found to be acceptable.
" Leak-before-break evaluations were performed to demonstrate that the weld overlay, when implemented, would meet regulatory requirements for critical flaw and leak detection.
Based on the above observations and the fact that nozzle-to-safe end weld overlays have been applied to other plants since 1986 with no subsequent problems identified, it is concluded that the North Anna Power Station Unit I SG hot leg inlet nozzle DMWs have received long term mitigation against PWSCC.
- 12-308 Docket No. 50-338 Attachment Page 13 of 13
4.0 REFERENCES
- 1. Materials Reliability Program (MRP): Resistance to Primary Water Stress Corrosion Cracking of Alloys 690, 52, and 152 in Pressurized Water Reactors (MRP-1 11),
EPRI, Palo Alto, CA: 2004. 1009801.
- 2. Virginia Electric and Power Company, North Anna Power Station Unit 1 Relief Request No. NI-14-CMP-001, "Use of Weld Overlays as an Alternative Repair Technique for Steam Generator Hot Leg Nozzles," Docket No. 50-338, dated March 30, 2011.
- 3. ASME Boiler and Pressure Vessel Code, Code Case N-740-2, "Full Structural Dissimilar Metal Weld Overlay for Repair or Mitigation of Class 1, 2, and 3 Items,Section XI, Division 1."
- 4. ASME Boiler and Pressure Vessel Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, 2004 Edition.
- 5. ASME Boiler and Pressure Vessel Code,Section III, Rules for Construction of Nuclear Facility Components, 2004.
- 6. ANSYS Mechanical APDL and PrepPost, Release 12.1 x64, ANSYS, Inc.,
November 2009.
- 7. Materials Reliability Program: Technical Basis for Preemptive Weld Overlays for Alloy 82/182 Butt Welds in PWRs (MRP-1 69), Revision 1, EPRI, Palo Alto, CA:
2008. 1016602.
- 8. NUREG-0800, "U.S. Nuclear Regulatory Commission Standard Review Plan, Office of Nuclear Reactor Regulation, Section 3.6.3, Leak-Before-Break Evaluation Procedure," Revision 1, March 2007.
- 9. NUREG-1 061, Volume 3, "Report of the U.S. Nuclear Regulatory Commission Piping Review Committee," prepared by the Piping Review Committee, NRC, April 1985.
- 10. North Anna Standard No. STD-GN-0038, Rev. 11, "Seismic Margin Management Plan (SMMP) for North Anna Units 1 and 2 Structures, Systems and Components to Address the August 23, 2011 M5.8 Mineral VA Earthquake."