ML072430005
| ML072430005 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 09/21/2007 |
| From: | Hiltz T NRC/NRR/ADRO/DORL/LPLIV |
| To: | Ford B Entergy Operations |
| vaidya B, NRR/DLPM, 415-3308 | |
| References | |
| TAC MD3044 | |
| Download: ML072430005 (19) | |
Text
September 21, 2007 Mr. Brian S. Ford Senior Manager Nuclear Safety & Licensing Entergy Operations, Inc.
1340 Echelon Parkway Jackson, MS 39286-1995
SUBJECT:
GRAND GULF NUCLEAR STATION UNIT 1 - REQUEST FOR ALTERNATIVE GG-ISI-002 - IMPLEMENT RISK-INFORMED INSERVICE INSPECTION PROGRAM BASED ON AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE, CODE CASE N-716 (TAC NO. MD3044)
Dear Mr. Ford:
By letter dated September 22, 2006, as supplemented by letters dated May 23, July 2, and August 27, 2007, Entergy Operations, Inc. (Entergy or the licensee), pursuant to paragraph 50.55a(a)(3)(i) of Title 10 of the Code of Federal Regulations (10 CFR), submitted Request for Alternative GG-ISI-002. The request for relief would implement a risk-informed/safety based (RIS_B) inservice inspection (ISI) program for piping. The proposed program is based, in part, on the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, Code Case N-716.
The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the subject request GG-ISI-002, dated September 22, 2006, as supplemented, for Grand Gulf Nuclear Station, Unit 1, and concludes that you have adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(a)(3)(i) and the proposed alternative provides an acceptable level of quality and safety. Therefore, the NRC staff authorizes the proposed alternative in accordance with 10 CFR 50.55a(a)(3)(i) for the remainder of the licensees second 10-year ISI interval, which was extended by the NRC letter dated February 13, 2007, until fall 2008, and for its third 10-year ISI interval ending in 2017. The NRC staffs approval of the licensees RIS_B program does not constitute approval of Code Case N-716. The NRC staff's related Safety Evaluation is enclosed.
Sincerely,
/RA/
Thomas G. Hiltz, Chief, Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-416
Enclosure:
Safety Evaluation
cc w/encl: See next page
September 21, 2007 Mr. Brian S. Ford Senior Manager Nuclear Safety & Licensing Entergy Operations, Inc.
1340 Echelon Parkway Jackson, MS 39286-1995
SUBJECT:
GRAND GULF NUCLEAR STATION UNIT 1 - REQUEST FOR ALTERNATIVE GG-ISI-002 - IMPLEMENT RISK-INFORMED INSERVICE INSPECTION PROGRAM BASED ON AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE, CODE CASE N-716 (TAC NO. MD3044)
Dear Mr. Ford:
By letter dated September 22, 2006, as supplemented by letters dated May 23, July 2, and August 27, 2007, Entergy Operations, Inc. (Entergy or the licensee), pursuant to paragraph 50.55a(a)(3)(i) of Title 10 of the Code of Federal Regulations (10 CFR), submitted Request for Alternative GG-ISI-002. The request for relief would implement a risk-informed/safety based (RIS_B) inservice inspection (ISI) program for piping. The proposed program is based, in part, on the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, Code Case N-716.
The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the subject request GG-ISI-002, dated September 22, 2006, as supplemented, for Grand Gulf Nuclear Station, Unit 1, and concludes that you have adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(a)(3)(i) and the proposed alternative provides an acceptable level of quality and safety. Therefore, the NRC staff authorizes the proposed alternative in accordance with 10 CFR 50.55a(a)(3)(i) for the remainder of the licensees second 10-year ISI interval, which was extended by the NRC letter dated February 13, 2007, until fall 2008, and for its third 10-year ISI interval ending in 2017. The NRC staffs approval of the licensees RIS_B program does not constitute approval of Code Case N-716. The NRC staff's related Safety Evaluation is enclosed.
Sincerely,
/RA/
Thomas G. Hiltz, Chief, Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-416
Enclosure:
Safety Evaluation cc w/encl: See next page DISTRIBUTION:
PUBLIC LPLIV r/f RidsOgcRp RidsAcrsAcnwMailCenter RidsRgn4MailCenter RidsNrrDraApla(MRubin)
RidsNrrDorlLpl4 RidsNrrDciCpnb(TChan)
AKeim, NRR/CPNB RidsNrrPMBVaidya RidsNrrLAJBurkhardt SDinsmore, NRR/APLA ACCESSION NO: ML072430005
(*) No substantial change from SE Input, (**) see previous concurrence OFFICE NRR/LPL4/PM NRR/LPL4/LA NRR/CPNB/BC(*)
NRR/APLA/BC(*)
OGC(**)
NRR/LPL4/BC NAME BVaidya JBurkhardt TChan MRubin SBrock THiltz DATE 9/21/07 9/21/07 09/05/07 09/05/07 9/20/07 9/21/07 OFFICIAL RECORD COPY
September 2007 Grand Gulf Nuclear Station cc:
Senior Vice President Entergy Nuclear Operations P.O. Box 31995 Jackson, MS 39286-1995 Senior Vice President and COO Entergy Operations, Inc.
P.O. Box 31995 Jackson, MS 39286-1995 Vice President, Operations Support Entergy Services, Inc.
P.O. Box 31995 Jackson, MS 39286-1995 General Manager, Plant Operations Entergy Operations, Inc.
P.O. Box 756 Port Gibson, MS 39150 Director, Nuclear Safety & Licensing Entergy Services, Inc.
440 Hamilton Avenue White Plains, NY 10601 Director, Nuclear Safety Assurance Entergy Operations, Inc.
P.O. Box 756 Port Gibson, MS 39150 Senior Manager Nuclear Safety & Licensing Entergy Services, Inc.
P.O. Box 31995 Jackson, MS 39286-1995 Manager, Licensing Entergy Operations, Inc.
P.O. Box 756 Port Gibson, MS 39150 State Health Officer State Health Board P.O. Box 1700 Jackson, MS 39215 Attorney General Asst. Attorney General State of Mississippi P.O. Box 22947 Jackson, MS 39225-2947 Office of the Governor State of Mississippi Jackson, MS 39201 Attorney General Department of Justice State of Louisiana P.O. Box 94005 Baton Rouge, LA 70804-9005 President Claiborne County Board of Supervisors P.O. Box 339 Port Gibson, MS 39150 Richard Penrod, Senior Environmental Scientist/State Liaison Officer Office of Environmental Services Northwestern State University Russell Hall, Room 201 Natchitoches, LA 71497 Chief, Energy and Transportation Branch Environmental Compliance and Enforcement Division Mississippi Department of Environmental Quality P.O. Box 10385 Jackson, MS 39289-0385 Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-8064 Senior Resident Inspector U.S. Nuclear Regulatory Commission P.O. Box 399 Port Gibson, MS 39150
September 2007 Grand Gulf Nuclear Station cc:
Mr. William R. Brian Site Vice President Grand Gulf Nuclear Station Entergy Operations, Inc.
P. O. Box 756 Port Gibson, MS 39150
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR ALTERNATIVE GG-ISI-002 TO IMPLEMENT RISK-INFORMED INSERVICE INSPECTION (ISI) PROGRAM BASED ON THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS CODE, ASME CODE CASE N-716 ENTERGY OPERATIONS, INC.
GRAND GULF NUCLEAR STATION DOCKET NO. 50-416
1.0 INTRODUCTION
By letter dated September 22, 2006 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML062720254) (Reference 1), as supplemented by letters dated May 23 (Reference 2), July 2 (Reference 3), and August 27, 2007 (Reference 4) (ADAMS Accession No. ML071500047, ML071910233, and ML072410257), respectively (hereafter, collectively referred to as submittal), Entergy Operations, Inc. (Entergy or the licensee),
pursuant to paragraph 50.55a(a)(3)(i) of Title 10 of the Code of Federal Regulations (10 CFR),
submitted request for alternative GG-ISI-002. The request for alternative GG-ISI-002 proposes a risk-informed/safety based inservice inspection (RIS_B) program as an alternative to a portion of its current inservice inspection (ISI) program for Grand Gulf Nuclear Station, Unit 1 (GGNS),
on the basis that the alternative provides an acceptable level of quality and safety. The licensee requested implementation of this alternative for the remainder of the licensees second 10-year ISI interval, which was extended by the NRC letter dated February 13, 2007, until fall 2008, and for its third 10-year ISI interval which ends in 2017. Entergy proposed the use of the RIS_B process for the inservice inspection of American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) Class 1 and Class 2 piping, Examination Categories B-F, B-J, C-F-1, and C-F-2 piping welds. Entergy requests to implement an RIS_B program based, in part, on ASME Code Case N-716, Alternative Piping Classification and Examination Requirements,Section XI Division 1 (CC N-716), which the licensee incorporated as Enclosure 2 in its submittal. The provisions of CC N-716 define additional requirements for Class 3 piping or non-Class piping.
CC N-716 is founded in large part on the risk-informed inservice inspection (RI-ISI) process as described in the Electric Power Research Institute Topical Report TR-112657, Revision B-A (EPRI-TR), which was previously reviewed and approved by the U.S. Nuclear Regulatory Commission (NRC). The licensee proposed the RIS_B program as an alternative to the ENCLOSURE requirements in the ASME Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, pursuant to 10 CFR Section 50.55a(a)(3)(i). The licensee requested implementation of this alternative for the third period of the second 10-year ISI interval at GGNS and for the third 10-year ISI interval.
CC N-716 has not been endorsed for generic use by the NRC. Entergys relief request refers to the methodology described in CC N-716 instead of describing the details of the methodology in the relief request. Entergy has, however, modified the methodology described in CC N-716 while developing its proposed RIS_B program. When the methodology used by the licensee is accurately described in CC N-716, this safety evaluation report (SER) refers to the details found in CC N-716. When the methodology used by the licensee deviates from or expands upon the methodology described in CC N-716, this SER refers to details in the submittal. Therefore, CC N-716 is incorporated in this safety evaluation (SE) only as a source for some of the detailed methodology descriptions as needed and the staff is not endorsing the use of ASME Case N-716, Alternative Piping Classification and Examination Requirements,Section XI Division 1, in this SER.
2.0 REGULATORY EVALUATION
Pursuant to 10 CFR 50.55a(g), ASME Code Class 1, 2, and 3 components (including supports) shall meet the requirements set forth in the Code to the extent practical within the limitations of design, geometry, and materials of construction of the components.
Paragraph 50.55a(g) of 10 CFR also states that ISI of the ASME Code, Class 1, 2, and 3 components is to be performed in accordance with Section XI of the ASME Code and applicable addenda, except where specific written relief has been granted by the NRC. The objective of the ISI program, as described in Section XI of the ASME Code and applicable addenda, is to identify conditions (i.e., flaw indications) that are precursors to leaks and ruptures in the pressure boundary of these components that may impact plant safety.
The regulations also require, during the first 10-year ISI interval and during subsequent intervals, the licensees ISI program comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by reference into 10 CFR 50.55a(b) 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein. GGNS is in the third period of the second 10-year ISI interval. The applicable edition of Section XI of the ASME Code for GGNS for the second 10-year ISI interval is the 1992 Edition and for the third 10-year ISI interval is the 2001 Edition through the 2003 Addenda for piping welds.
Pursuant to 10 CFR 50.55a(g), a certain percentage of ASME Code Category B-F, B-J, C-F-1, and C-F-2 pressure retaining piping welds must receive ISI during each 10-year ISI interval.
ASME Code requires 100 percent of all B-F welds, 25 percent of all B-J welds greater than 1-inch nominal pipe size be selected for volumetric or surface examination, or both, on the basis of existing stress analyses. For Categories C-F-1 and C-F-2 piping welds, 7.5 percent of non exempt welds are selected for volumetric or surface examination, or both. As discussed in Reference 6, the selection of welds in the ASME program is based on design stress reports.
These analyses are typically very conservative and may not provide an accurate representation of failure potential. Industry experience has shown that pipe failures, while rarely occurring, are due primarily to corrosion or fatigue type failure mechanisms and typically occur in locations not addressed in the plant's ASME ISI program. While the use of conservative assumptions is appropriate in the design of piping systems to achieve high levels of reliability, insights supported by operating experience indicate that it provides misleading expectations regarding the effectiveness of piping inspection programs.
According to 10 CFR 50.55a(a)(3), the NRC may authorize alternatives to the requirements of 10 CFR 50.55a(g), if an applicant demonstrates that the proposed alternatives would provide an acceptable level of quality and safety, or that compliance with the specified requirement would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. The licensee has proposed to use an RIS_B program for ASME Code Class 1 and Class 2 piping (Examination Categories B-F, B-J, C-F-1, and C-F-2 piping welds), as an alternative to the ASME Code,Section XI requirements. The provisions of CC N-716 define additional requirements for Class 3 piping or non-Class piping. The licensee asserts that this proposed program provides an acceptable level of quality and safety and therefore satisfies the requirements of 10 CFR 50.55a(a)(3)(i).
The licensee states that CC N-716 is founded in large part on the RI-ISI process as described in the EPRI-TR, which was previously reviewed and approved by the NRC. The licensee further states that the risk-informed application based upon CC N-716 meets the intent and principles of Regulatory Guide (RG) 1.174, An Approach for Using Probabilistic Risk Assessment [PRA]
In Risk-Informed Decisions On Plant-Specific Changes to the Licensing Basis, and RG 1.178, An Approach For Plant-Specific Risk-Informed Decisionmaking - Inservice Inspection of Piping.
The objectives of the licensee's proposed RIS_B program are to identify high safety significant piping at the plant, determine the locations to be inspected within the identified piping, and to identify appropriate inspection methods. The licensee's proposed program will inspect ten percent of high safety significant welds using an inspection method capable of addressing the degradation mechanism(s) of concern and most likely to occur at each location to be inspected.
In general, the licensee simplified the EPRI-TR method because it does not evaluate system parts that have been generically identified as high safety-significant (HSS) parts, and uses screening PRA analyses to evaluate in detail only system parts that cannot be screened out as low safety-significant (LSS). The staff has reviewed and evaluated the licensees proposed RIS_B program based on guidance and acceptance criteria provided in the following documents:
RG 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis RG 1.178, An Approach for Plant-Specific Risk-Informed Decisionmaking for Inservice Inspection of Piping NRC NUREG-0800, Chapter 3.9.8 Standard Review Plan For the Review of Risk-Informed Inservice Inspection of Piping EPRI TR-112657, Revision B-A, Revised Risk-Informed Inservice Inspection Evaluation Procedure Since the issuance of the EPRI-TRs SER, several instances of primary water stress-corrosion cracking (PWSCC) of Alloy 82/182 dissimilar metal welds have occurred at pressurized-water reactors (PWRs). This has prompted the NRC to send a letter (Reference 11) to the Chairman of the ASME Subcommittee on Nuclear Inservice Inspection, stating that the operating experience with leakage and flaws caused by PWSCC at PWRs supports a position that current ASME Code inspection requirements are not sufficient for managing PWSCC-susceptible butt welds in the reactor coolant pressure boundary of PWRs. This letter represents a departure from the staffs conclusions about PWSCC in the EPRI-TRs SER. This issue of PWSCC of dissimilar metal welds at PWRs is not relevant at GGNS since GGNS is a boiling-water reactor.
The staff is including this information to demonstrate that as issues arise, modifications to RI-ISI programs and RIS_B programs may be warranted, as required in the NRC approval of EPRI-TR.
3.0 TECHNICAL EVALUATION
RG 1.174 provides guidance on the use of PRA findings and risk insights in support of licensee request for changes to a plants licensing basis. RG 1.178 describes an RI-ISI program as one that incorporates risk insights that can focus inspections on more important locations while at the same time maintaining or improving public health and safety. EPRI-TR provides a detailed methodology that the staff has previously concluded will result in an acceptable RI-ISI. The RIS_B program proposed by GGNS also incorporates risk insights to focus inspection on more important locations although the methodology differs in several respects from the EPRI-TR methodology. This SE describes and evaluates the differences between the endorsed EPRI-TR methodology and the proposed RIS_B methodology to reach a conclusion about the acceptability of the proposed method.
An acceptable RI-ISI program replaces the number and locations of non-destructive examination (NDE) inspections based on ASME Code,Section XI, requirements by the number and locations of these inspections based on the RI-ISI guidelines. The proposed RIS_B program permits alternatives to the requirements of IWB-2420, IWB3-2430, and IWB-2500 (Examination Categories B-F and B-J) and IWC-2420, IWC-2430, and IWC-2500 (Examination Categories C-F-I and C-F-2), or as additional requirements for Subsection IWD, and may be used for ISI and preservice inspection of Class 1, 2, 3, or non-Class piping. All piping components, regardless of risk classification, will continue to receive ASME Code-required pressure and leak testing, as part of the current ASME Code, Section Xl,program. Visual examinations (VT-2) are scheduled in accordance with the GGNS pressure and leak test program, which remains unaffected by the proposed RlS_B program.
The EPRI-TR process includes the following steps which, when successfully applied, satisfy the guidance provided in RGs 1.174 and 1.178:
Scope Definition Consequence Evaluation Degradation Mechanism Evaluation Piping Segment Definition Risk Categorization Inspection/NDE Selection Risk Impact Assessment Implementation Monitoring and Feedback These processes result in a program consistent with the concept that, by focusing inspections on the most safety-significant welds, the number of inspections can be reduced while at the same time maintaining adequate protection of public health and safety. As described below, the acceptability of the licensees proposed RIS_B program is evaluated by comparing the processes it has applied to develop its program with these steps.
3.1 Scope Definition The scope of evaluation to support RIS_B program development, and of the proposed changes, in CC N-716 and the licensees submittal includes ASME Code Class 1, 2, 3 and non-Class piping welds. Standard Review Plan (SRP) 3.9.8, Risk-Informed Inspection of Piping, and RG 1.178 address scope issues. The primary acceptance guideline in the SRP is that the selected scope needs to support the demonstration that any proposed increase in core damage frequency (CDF) and risk are small. The scope of GGNSs evaluation included all piping where ASME inspections could be discontinued providing assurance that the change in risk estimate would, as a minimum, capture the risk increase associated with implementing the RIS_B program in lieu of the ASME program. The change in risk estimate is used by the licensee to demonstrate that risk increases are small. RG 1.178 clarifies that a full-scope risk-informed evaluation is acceptable, where the definition of full-scope is consistent with the scope as defined in CC N-716. Therefore, the staff concludes that the full-scope extent of the piping included in the RIS_B program changes satisfies the SRP and RG guidelines and is acceptable.
3.2 Consequence Evaluation The methodology described in RG 1.178 and the EPRI-TR divide all piping within the scope of the proposed EPRI-TR RI-ISI program into piping segments. The consequence of each segment failure must be estimated as a conditional core damage probability (CCDP) and conditional large early release probability (CLERP), or by using a set of tables in the EPRI-TR that yield equivalent results. The consequences are used to determine the safety significance of the segments. In contrast, CC N-716 does not require that the consequence of each segments failure be estimated to determine the safety significance of piping segments.
Instead, CC N-716 identifies portions of systems that should be generically classified as high safety significant (HSS) at all plants. A consequence analysis is not required for system parts generically classified as HSS because there is no higher safety-significance category to which the system part can be assigned and degradation mechanisms, not consequence, are used to select inspection locations in the HSS weld population. The licensees PRA is subsequently used to search for any additional, plant-specific HSS segments that are not included in the generic HSS population.
Sections 2(a)(1) through 2(a)(4) in CC N-716 provide guidance that identifies the portions of systems that should be generically classified as HSS based on a review of almost 50 RI-ISI programs. These RI-ISI programs were all developed by considering both direct and indirect effects of piping pressure boundary failures and the different failure modes of piping. This is consistent with the guidelines for evaluating pipe failures with PRA described in RG 1.178, the EPRI-TR and SRP 3.9.8 and, therefore, the generic results are derived from acceptable analyses.
Section 2(a)(5) in CC N-716 provides guidance that defines additional, plant-specific HSS segments that should be identified using a plant-specific PRA of pressure boundary failures.
The licensee stated that it used its PRA of pressure boundary failures (flooding analysis) to search for additional plant-specific HSS segments. GGNS further described its flooding analysis in its submittal. Consistent with CC N-716, GGNS's flooding analysis did not require an estimate of the CCDP and CLERP of each length of pipe. Instead, the licensee describes a systematic process that defines flood zones, identifies their contents (e.g., important equipment), identifies potential flood sources, identifies flood propagation pathways, and then uses qualitative and quantitative screening to identify significant flood events. The CDF and large early release frequency (LERF) contributions from the significant flood events are quantified. The licensee stated that the analysis considered both the direct and indirect effects of pressure boundary failure and the different failure modes of the piping. Evaluation of flooding scenarios considered both the frequency and the size of the flood initiators in each flood zone. This is consistent with the guidelines for evaluating pipe failures with PRA described in RG 1.178, the EPRI-TR, and SRP 3.9.8.
Each of the consequence evaluations (the generic and the plant-specific flooding analysis) considers both direct and indirect effects of piping pressure boundary failures and the different sized piping failures to systematically use risk insights and PRA results to characterize the consequences of piping failure. This is consistent with the guidelines for evaluating pipe failures with PRA described in RG 1.178 and SRP and, therefore, the staff considers the consequence analysis performed by the licensee for this application to be acceptable.
3.3 Degradation Mechanism Evaluation The EPRI-TR requires a determination of the susceptibility to all degradation mechanisms of every weld within the scope of the proposed program. The degradation mechanisms which should be identified are described in the EPRI-TR. This information is used to support the safety significance determination for all segments, to target inspections toward the locations with damage mechanisms in the segments that require inspections, and to provide estimates of weld failure frequencies to support the change in risk calculation. Once a segment is placed in the LSS category, the degradation mechanisms at the welds in that segment are not further used in the development of an EPRI-TR RI-ISI program because no inspections are required in LSS segments and the discontinued inspections in LSS segments are not included in the change in risk estimate.
CC N-716 identifies a generic population of HSS welds, followed by a search for plant-specific HSS welds. The Code Case requires a determination of the susceptibility to all degradation mechanisms of all welds assigned generically assigned HSS. The degradation mechanisms to be considered in CC N-716 are consistent with those identified in EPRI-TR which the staff has previously concluded is a sufficiently comprehensive list of the applicable mechanisms.
As described above, CC N-716 augments the generic HSS welds with a search for plant-specific HSS welds based on the flooding analysis. The flooding analysis first identifies areas that may be sensitive to floods (i.e., potential HSS areas) and then evaluates the failure potential of piping segments in areas that are sensitive to flooding. In the submittal, the licensee stated that the flood initiating frequencies were obtained from Reference 12. The failure frequencies in Reference 12 are greater than or equal to 4.38E-7/foot-year. In EPRI-TR, the staff has accepted an estimated failure frequency of 2E-7/weld-year for welds exposed to any degradation mechanism (aside from water hammer and flow-accelerated corrosion (FAC))
to support the development of RI-ISI programs. The failure frequency estimates used by the licensee are conservative because welds normally are located more than several feet apart.
The use of conservative values would identify all HSS segments that would be identified using realistic values based on identified degradation mechanisms. Therefore, the staff concludes that the analysis preformed by the licensee is acceptable despite not searching for degradation mechanisms in potentially HSS piping segments because the licensee uses conservative failure frequencies instead.
The licensee stated that a review was conducted to verify that LSS piping was not susceptible to FAC or water hammer, the two degradation mechanisms that would assign a high-failure frequency to a weld. In lieu of conducting a degradation mechanism evaluation for all the LSS piping, all locations in LSS piping were conservatively assigned to the medium failure potential for the purpose of assigning a failure frequency to be used to calculate the change in risk. This results in conservative failure frequency estimates because these failure frequencies would always be equal to or greater than those used in the analysis if the susceptibility of all LSS welds to all degradation mechanisms was determined.
The licensee identified degradation mechanisms at all generic HSS welds and in LSS segments with potential high-failure frequency (i.e., susceptible to FAC or water hammer). The licensee used conservative failure frequency estimates from Reference 12 while searching for plant-specific HSS welds. The lack of some information on degradation mechanisms in the LSS segments results in a conservative estimate of the risk increase from discontinued inspections, the only analysis step where degradation mechanisms in LSS segments are used.
Therefore, the staff concurs that limiting the search for degradation mechanisms compared to the search required by the EPRI-TR is acceptable because the licensees method fulfills the requirements for identifying locations that should be inspected, identifying additional plant-specific HSS segments, and developing a bounding estimate for the change in risk.
3.4 Piping Segment Definition Previous guidance on RI-ISI including RG 1.178, SRP 3.9.8, and both approved industry methodologies centered on defining and using piping segments. RG 1.178 states, for example, that the analysis and definition of a [piping] segment must be consistent and technically sound.
The primary purpose of segments is to group welds so that consequence analyses can be done for the smaller number of segments instead of for each weld. Sections 2(a)(1) to 2(a)(4) in CC N-716 identify system parts (segments and groups of segments) that are generically assigned HSS without requiring a plant-specific consequence determination and any subdivision of these system parts is unnecessary. Section 2(a)(5) in CC N-716 uses a PRA to identify plant-specific piping that might be assigned HSS. The process described by the licensee to search for plant-specific HSS piping first identifies zones that may be sensitive to flooding, and then evaluates the failure potential of piping in these zones. Lengths of piping whose failure impacts the same plant equipment within each zone are equivalent to piping segments. Therefore piping segments are either not needed to reduce the number of consequence analyses required (for the generic HSS piping) or, when needed during the plant-specific analysis, the length of pipe included in the analysis is consistent with the definition of a segment in RG 1.178 and SRP 3.9.8.
An additional purpose of piping segments in the EPRI-TR is as an accounting/tracking tool. In the EPRI methodology, all parts of all systems within the selected scope of the RI-ISI program are placed in segments and the safety significance of each segment is developed. For each safety-significant classification, a fixed percentage of welds within all the segments of that class are selected. Additional selection guidelines ensure that this fixed percentage of inspections is distributed throughout the segments to ensure that all damage mechanisms are targeted and all piping systems continue to be inspected. CC N-716 generically defines a large population of welds as HSS. An additional population of welds may be added based on the risk-informed search for plant-specific HSS segments. When complete, the N-716 process yields a well-defined population of HSS welds from which inspections must be selected. This accomplishes the same objective as accounting for each weld throughout the analysis by using segments. CC N-716, as applied by the licensee, provides additional guidelines to ensure that this fixed percentage is appropriately distributed throughout the population of welds subject to inspection, all damage mechanisms are targeted, and all piping systems continue to be inspected.
The staff concludes that the segment identification in RG 1.178 as used as an accounting tool is not needed within the generic population of HSS welds. The risk-informed search for HSS segments based on a flooding PRA divides up piping systems into segments based on consequences which is consistent with the segment definition in RG 1.178. Therefore, the proposed method accomplishes the same objective as the approved methods without requiring that segments be identified and defined for all piping within the scope of the RIS_B program.
3.5 Risk Categorization Sections 2(a)(1) through 2(a)(4) in CC N-716 identify the portions of systems that should be generically classified as HSS and Section 2(a)(5) requires a search for plant-specific HSS segments. Application of the guideline in Section 2(a)(5) in CC N-716 identifies plant-specific piping segments that are not assigned to the generic HSS category but that are risk-significant at a particular plant. CC N-716 requires that any segment with a total estimated CDF greater than 1E-6/year be assigned the HSS category. The licensee augmented this CC N-716 metric on CDF with the requirement to assign the HSS category to any segment with a total estimated LERF greater than 1E-7/year. The licensee stated that these guideline values are suitably small and consistent with the decision guidelines for acceptable changes in CDF and LERF found in RG 1.174. The licensee further argues that allocating resources (i.e., NDE) to components below this guideline will provide negligible risk benefit while expending unnecessary worker dose and radwaste.
The licensee, in the letter dated July 2, 2007 (Reference 3), clarified that these ancillary metrics were added as a defense-in-depth measure to provide a method of ensuring that plant-specific locations that are important to safety are identified. Any piping that has inspections added or removed per CC N-716 is required to be included in the change in risk assessment and an acceptable change in risk estimate is used to demonstrate compliance with RG 1.174 acceptance criteria. The ancillary metrics and guidelines on CDF and LERF are only used to add HSS segments and not, for example, to remove system parts generically assigned to the HSS in Sections 2(a)(1) through 2(a)(4).
The staff concurs that a plant-specific analysis to identify plant-specific locations that are important to safety is a necessary element of RI-ISI program development. The results of the plant-specific risk categorization analysis provides confidence that the goal of inspecting the more risk-significant locations is met while permitting the use of generic HSS system parts to simplify and standardize the evaluation. Any evaluation that categorizes the safety significance of structures, systems, and components requires metrics and guideline values, such as the Fussel-Vessley and risk achievement worth guidelines endorsed in RG 1.201, Guidelines for Categorizing Structures, Systems, and Components in Nuclear Power Plants According to Their Safety Significance. Such metrics are subordinate to the change in risk metrics in RG 1.174 which are used to determine whether the increase in risk associated with a proposed change is small and consistent with the intent of the Commissions Safety Goal Policy Statement.
Satisfying the guidelines in Sections 2(a)(5) requires confidence that the flooding PRA is capable of successfully identifying all, or most, of the significant flooding contributors to risk that are not included in the generic results. The licensee described the facts and observations from the peer review of the GGNS flooding analysis and described how the models had been changed to address any issues that could have affected the RIS_B program. The licensee reviewed the results of its flooding analysis to identify any segments that may have a CDF and LERF greater than 1E-6/year or 1E-7/year, respectively. The licensee reported significant flooding CDF scenarios as low as 2E-8/year indicating that the screening process is capable of identifying the much higher frequencies (1E-6/year) used to assess if the guideline values are exceeded.
The staff concurs that the CDF and LERF metrics proposed by the licensee are acceptable because they address the risk elements that form the basis for risk-informed applications (i.e.,
core damage and large early release). The staff accepts the proposed guideline values because these ancillary guidelines are applied in addition to the change in risk acceptance guidelines in RG 1.174 and only add plant-specific HSS segments to the RIS_B program, i.e.,
they may not be used to reassign any generic HSS segment into the LSS category.
3.6 Inspection/NDE Selection The submittal discusses the impact of the proposed RIS_B application on the various augmented inspection programs.
CC N-716 contains no provisions for reducing the number of inspections in the inspection program for high energy line breaks (HELB) outside containment, implemented in response to Updated Final Safety Analysis Report (UFSAR) Section 6.6.8, "Augmented Inservice Inspection to Protect against Postulated Piping Failures." However, CC N-716 does include a provision to increase the number of HELB inspections if the HELB program is inspecting less than 10% of the welds in this region. Changes to the HELB program may be made as authorized by EPRI-TR-1006937, Extension of the EPRI Risk Informed ISI Methodology to the Break Exclusion Region Programs, or by another process found acceptable by the staff.
CC N-716 contains no provisions for changing the flow accelerated corrosion (FAC) augmented program developed in response to NRC Generic Letter 89-08, Erosion/Corrosion-Induced Pipe Wall Thinning.
CC N-716 subsumes all inspection for Category A welds in the augmented inspection program for intergrannular stress-corrosion cracking (IGSCC) implemented in response to NRC Generic Letter 88-01, NRC Position on IGSCC in BWR Austentic Stainless Steel Piping. The existing augmented program for Categories B through C welds remains unaffected by the RIS_B program.
MRP-139 as an augmented inspection program, for the inspection and management of PWSCC susceptible dissimilar metal welds and will supplement the RI-ISI program selection process.
Section 4 in CC N-716 requires that 10 percent of HSS welds shall be selected for examination.
The licensee, in the letter dated May 23, 2007 (Reference 2), clarified that the population of welds for which 10 percent must be inspected includes all butt welds and socket welds categorized as HSS. The licensee further clarified that all inspections shall be volumetric inspections.
Sections 4(b)(1) through 4(b)(3) in CC N-716 describe how the inspection locations will be selected. The selection process includes guidance that ensures that inspection locations are distributed physically throughout the HSS piping systems and that all degradation mechanisms will be represented in the selected locations. The guidance provides some flexibility in the distribution of locations to satisfy all the guidelines but the number of inspections must be increased beyond 10 percent if necessary to met the quantitative risk acceptance guidelines in Section 5(b).
In contrast to the EPRI-TR, which only changed the types of ISI inspections and the locations of inspections, CC N-716 also discontinues preservice inspection requirements for LSS welds.
These preservice examinations are performed to obtain a baseline inspection using the examination method that will be used for subsequent ISI examinations. Similar to the ASME Code which requires preservice examination of all Class 1 welds, CC N-716 requires preservice examination of all HSS welds. Preservice examinations are performed on ASME Class 2 welds that are initially selected for ISI, which is 7.5 percent of Class 2 piping welds. Any Class 2, Class 3 or non-Code welds that are selected for inspection in the RIS_B program will be HSS welds. Therefore, preservice examinations will continue to be performed on all welds selected for examination using the CC N-716 to obtain a baseline inspection using the examination method that will be used for subsequent ISI examinations.
In addition to the preservice exams as required, repair/replacement activities involving welding or brazing areas and welded joints made for installation of items shall be examined in accordance with the Construction Code identified in the licensees Repair/Replacement Plan.
The licensees use of CC N-716 does not affect the examinations required to verify the integrity of welds associated with repair/replacement activities. Therefore, the staff finds that there is no impact on the change in risk calculations associates with repair activities under the RIS_B program because the examinations required to verify the integrity of repaired or replaced welds are not affected and will continue to be performed.
3.7 Risk Impact Assessment The licensee uses a change in risk estimation process approved by the staff in the EPRI-TR.
The change in risk assessment in the EPRI-TR permits using each segments CCDP and CLERP or, alternatively, placing each segment into high, medium, or low-consequence bins and using a single bounding CCDP and CLERP for all segments in each bin. CC N-716 also includes both alternatives, and the bounding values to be used in the bounding analysis are the same as those approved for use in the EPRI-TR. GGNS uses the alternative of placing each segment into consequence bins and using the associated bounding values for all segments in each bin during the change in risk assessment.
In the submittal, the licensee identified the different types of pipe failures that cause major plant transients such as those causing loss-of-coolant accidents (LOCAs), isolable LOCAs, and potential LOCAs. Conservative CCDP and CLERP estimates were developed from the PRA based on these initiating events. When the scenario was not appropriately modeled in the PRA, the licensee developed scenarios based on the PRA results and associated plant-specific equipment failures. The staff concurs that the scenarios described are reasonable because they identify the appropriate equipment failure modes that cause a sequence to progress, and the licensee uses generally acceptable values for those failure modes. Based on these estimates, the segments were assigned into the appropriate consequence bin.
The licensee relied on its flooding analysis to identify the appropriate consequence bin for segments in standby and balance-of-plant systems. The licensee stated that its flooding analysis did not identify any high-consequence segments (lower bound CCDP and CLERP of 1E-4 and 1E-5, respectively). These high CCDPs and CLERPs are associated with the flood-induced failure of components so critical to accident mitigation that the random failure of only one or two additional components would cause core damage or early release. The staff concurs that such critical equipment is normally readily identifiable and, therefore, the staff finds that the location-dependent steps in the screening process (i.e., identifying the important equipment in a flood zone and evaluating the impact of potential flood sources on that equipment) is capable of identifying high-consequence zones and associated piping segments.
Instead of performing additional screening analysis to identify medium (as opposed to low) risk contributors, the licensee placed all piping in standby and balance-of-plant systems into the medium-consequence bin.
1CC N-716 requires no estimated risk increase for discontinuing surface examinations at locations that are not susceptible to outside diameter attack [e.g., external chloride stress-corrosion cracking]. The staff determined during the review and approval of the EPRI-TR that the surface exams do not appreciably contribute to safety and need not be included in the change in risk quantification and, therefore, exclusion of surface exam from the change in risk evaluations is acceptable.
Section 5 in CC N-716 requires that any piping that has NDE inspections1 added or removed per CC N-716 is required to be included in the change in risk assessment. Acceptance criteria provided in Section 5(d) in CC N-716 include limits of 1E-7/year and 1E-8/year for increase of CDF and LERF for each system, and limits of 1E-6/year and 1E-7/year for the total increase in CDF and LERF associated with replacing the ASME Code,Section XI, program with the RIS_B program. These guidelines and guideline values are consistent with those approved by the staff in the EPRI-TR and therefore acceptable.
The change in risk evaluation approved in the EPRI-TR method is a final screening to ensure that a licensee replacing the Section XI program with the risk-informed alternative evaluates the potential change in risk resulting from that change and implements it only upon determining with reasonable confidence that any increase in risk is small and acceptable. The licensees method is consistent with the approved EPRI-TR method with the exception that the change in risk calculation in CC N-716 includes the risk increase from discontinued inspection in LSS.
Based on the detailed analysis of every segment required by the EPRI-TR, the staff concluded that there is a high confidence that the total increase in risk from all discontinued inspections in LSS segments would be negligible. The staff concludes that the licensees method described in the submittal is acceptable because the deviation from the approved EPRI-TR method expands the scope of the calculated change in risk providing confidence that the less detailed analyses of LSS segments required by CC N-716 does not result in an unanticipated and potentially unacceptable risk increase.
The licensee provided the results of the change in risk calculations in the submittal and noted that all the estimates satisfy both the system level and the total guidelines and, therefore, the staff finds that any increase in risk is small and acceptable.
3.8 Implementation Monitoring and Feedback The objective of this element of RGs 1.174 and 1.178 is to assess performance of the affected piping systems under the proposed RI-ISI program by implementing monitoring strategies that conform the assumptions and analysis used in developing the RIS_B program. The licensee states that it will implement program updates as described in Section 7 of CC N-716. Section 7 states that the examination selections shall be reevaluated on the basis of inspection periods that coincide with the inspection program requirements for Inspection Program A or B of IWA-2431 or IWA-2432, i.e., the currently acceptable interval before each licensee is required to update their ASME program to the latest ASME requirements.
The proposed periodic reevaluation interval meet existing ASME Code requirements and, therefore, should be considered acceptable. The revaluation shall determine if any changes to the inspection selections need to be made because of plant design changes, changes in postulated conditions, inspection results, piping failures, and updates to the PRA. This list of possible changes includes all changes at the facility or in the PRA that could affect the evaluation used to develop the RIS_B program and is, therefore, acceptable.
These periodic reevaluations satisfy the guidelines in RG 1.178 that an RI-ISI program should be self correcting as experience dictates because the proposed update interval is consistent with the acceptable ASME code update interval, and all the factors that could change the program are included in the reevaluation.
3.9 Examination Methods In accordance with CC N-716 LSS welds will be exempt from the volumetric, surface, VT-1, and VT-3 visual examination requirements of Section XI. Ten percent of the HSS welds will be selected for examination as addressed in Section 3.6 of this safety evaluation. Section 4 of CC N-716 directs users to Table 1 for the examination requirements of the welds selected for examination. The examination method is based on the postulated degradation for the selected weld. Table 1 of CC N-716 is consistent with the traditional RI-ISI approach for examination methods as approved in EPRI-TR-112657. The examination methods are based on an inspection for cause philosophy so that when there is a potential for a certain degradation mechanism, the examination method selected would be one that would be able to detect that type of degradation.
4.0 CONCLUSION
S Pursuant to 10 CFR 50.55a(a)(3)(i), alternatives to the requirements of 10 CFR 50.55a(g) may be used, when authorized by the NRC, if the licensee demonstrates that the proposed alternatives will provide an acceptable level of quality and safety. In this case, the licensee, has proposed to use an alternative to the risk-informed process described in NRC-approved EPRI-TR-112657.
The implementation strategy is consistent with the RG 1.178 guidelines because the number and location of inspections is a product of a systematic application of the risk-informed process.
Other aspects of the licensees ISI program, such as system pressure tests and visual examination of piping structural elements will continue to be performed on all Class 1, 2, and 3 systems in accordance with ASME Code,Section XI. This provides a measure of continued monitoring of areas that are being eliminated from the NDE portion of the ISI program. As required by the EPRI-TR methodology, the existing ASME Code performance measurement strategies will remain in place. In addition, the CC N-716 methodology provides for increased inspection volumes for those locations that are included in the NDE portion of the program.
RG 1.174 establishes requirements for risk-informed decisions involving a change to a plants licensing basis. RG 1.178 establishes requirements for risk-informed decisions involving alternatives to the ISI program requirements of 10 CFR 50.55a(g), and its directive to follow the requirements of the ASME Code,Section XI. The EPRI-TR RI-ISI methodology contains details for developing an acceptable an RI-ISI program. The CC N-716, modified as described by the licensee in its submittal, describes a methodology similar to the EPRI-TR methodology but with several differences as described in this SE. The staff has evaluated each of the differences and determined that the licensees proposed methodology, when applied as described, meets the intent of all the steps endorsed in EPRI-TR, is consistent with the guidance provided in RG 1.178, and therefore satisfies the guidelines established in RG 1.174.
The staff concludes that the proposed RIS_B program will provide an acceptable level of quality and safety pursuant to 10 CFR 50.55a(a)(3)(i) for the proposed alternative to the piping ISI requirements with regard to (1) the number of locations, (2) the locations of inspections, and (3) the methods of inspection. Therefore, the proposed RI-ISI program is authorized for the remainder of the licensees second 10-year ISI interval and for its third 10-year ISI interval pursuant to 10 CFR 50.55a(a)(3)(i) on the basis that this alternative will provide an acceptable level of quality and safety.
5.0 REFERENCES
1.
Letter from F. G. Burford (Entergy Operations, Inc.) to U.S. Nuclear Regulatory Commission, containing Request for Alternative GG-ISI-002, Request to Use ASME Code Case N-716, September 22, 2006 (ADAMS Accession No. ML062720254).
2.
Letter from Steven A. Saunders (Entergy Operations, Inc.) to U.S. Nuclear Regulatory Commission, containing Request for Alternative GG-ISI-002, Responses to Requests for Additional Information, May 23, 2007 (ADAMS Accession No. ML071500047).
3.
Letter from John F. McCann (Entergy Operations, Inc.) to U.S. Nuclear Regulatory Commission, containing Request for Alternative GG-ISI-002, Response to Request for Additional Information, July 2, 2007 (ADAMS Accession No. ML071910233).
- 4.
Letter from John F. McCann (Entergy Operations, Inc.) to U.S. Nuclear Regulatory Commission, Request for Alternative GG-ISI-002, Response to Request for Additional Information, August 27, 2007 (ADAMS Accession No. ML072410257).
5.
ASME Code Case N-716, Alternative Piping Classification and Examination Requirements,Section XI Division 1, © ASME, New York, New York, April 19, 2006.
6.
EPRI TR-112657, Revision B-A, Revised Risk-Informed Inservice Inspection Evaluation Procedure, December 1999 (ADAMS Accession No. ML013470102).
7.
RG 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, November 2002 (ADAMS Accession No. ML023240437).
8.
RG 1.178, An Approach for Plant-Specific Risk-Informed Decisionmaking for Inservice Inspection of Piping, September 2003 (ADAMS Accession No. ML032510128).
9.
NRC NUREG-0800, Chapter 3.9.8, Standard Review Plan For the Review of Risk-Informed Inservice Inspection of Piping, September 2003 (ADAMS Accession No. ML032510135).
10.
EPRI-TR-1006937, Extension of the EPRI Risk-Informed ISI Methodology to the Break Exclusion Region Programs, June 27, 2002 (ADAMS Accession No. ML021790518).
11.
Letter from J.E. Dyer, U.S. Nuclear Regulatory Commission to G.C. Park, ASME Subcommittee on Nuclear Inservice Inspection, Related to Codification of Inspection Requirements of Reactor Coolant System Dissimilar Metal Welds at Pressurized-Water Reactors, December 20, 2005 (ADAMS Accession No. ML053480359).
12.
EGG-SSRE-8875, Generic Component Failure Data Base for Light Water and Liquid Sodium Reactor PRAs, EG&G Idaho Inc., Idaho Falls, Idaho, February 1990.
13.
NRC Generic Letter 89-08, Erosion/Corrosion-Induced Pipe Wall Thinning, May 2, 1989 (ADAMS Accession No. ML031200731).
14.
NRC Generic Letter 88-01, NRC Position on IGSCC in BWR Austentic Stainless Steel Piping, January 25, 1988 (ADAMS Accession No. ML031130463).
Principal Contributors: A. Keim S. Dinsmore Date: September 21, 2007