ML020180003
| ML020180003 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 02/05/2002 |
| From: | Anthony Mendiola NRC/NRR/DLPM/LPD3 |
| To: | Kingsley O Exelon Generation Co |
| Chawla, ML, NRR/DLPM/LPD III-2, 415-8371 | |
| References | |
| TAC MB0722, TAC MB2767 TR-112655, Rev B-A | |
| Download: ML020180003 (17) | |
Text
February 5, 2002 Mr. Oliver D. Kingsley, President Exelon Nuclear Exelon Generation Company, LLC 4300 Winfield Road Warrenville, IL 60555
SUBJECT:
QUAD CITIES NUCLEAR POWER STATION, UNITS 1 AND 2 - RELIEF REQUEST CR-33, RISK INFORMED INSERVICE INSPECTION PROGRAM -
ALTERNATIVE TO THE ASME BOILER AND PRESSURE VESSEL CODE SECTION XI REQUIREMENTS FOR CLASS 1 AND 2 PIPING WELDS FOR QUAD CITIES NUCLEAR POWER STATION, UNITS 1 AND 2 (TAC NOS.
MB0721 AND MB0722)
Dear Mr. Kingsley:
By letter dated November 30, 2000, Commonwealth Edison Company (ComEd) requested approval of an alternative risk-informed inservice inspection (RI-ISI) program for the Quad Cities Nuclear Power Station, Units 1 and 2, (Quad Cities) for ASME Class 1 and 2 piping welds. The letter included an enclosure describing the proposed program. Additional clarifying information was provided in Exelons letters dated August 10, 2001, October 31, 2001, and November 20, 2001.
The Quad Cities RI-ISI program was developed in accordance with Electric Power Research Institute (EPRI) Topical Report TR-112657, Revision B-A, using the Nuclear Energy Institute template methodology. The U.S. Nuclear Regulatory Commission (NRC) staff has completed the review of the subject relief request. The NRC staffs safety evaluation (SE) is enclosed.
The staff concludes that Exelons proposed RI-ISI program is an acceptable alternative to the requirements of the American Society of Mechanical Engineers Code Section XI for inservice inspection, and therefore, the licensees request for relief is authorized during the third 10-year ISI interval, which ends on February 17, 2003, for Unit 1, and on March 9, 2003, for Unit 2, pursuant to 10 CFR 50.55a(a)(3)(i), on the basis that the alternative provides an acceptable level of quality and safety. The staff also concludes that the licensees proposed alternative to volumetric examinations for those high safety significant socket welds identified in the RI-ISI program provides reasonable assurance of structural integrity. Complying with the Electric Power Research Institute (EPRI) procedure TR-112657 requirement for volumetric examinations would result in unusual difficulty without a compensating increase in the level of safety and quality. Therefore, the licensees request for relief is authorized during the third 10-year ISI interval, which ends on February 17, 2003, for Unit 1, and on March 9, 2003, for Unit 2, pursuant to 10 CFR 50.55a(a)(3)(ii), on the basis that the alternative provides reasonable assurance of structural integrity.
O. Kingsley If you have any questions about this review, please contact Mahesh Chawla at (301) 415-1321 or by e-mail at mlc@nrc.gov.
Sincerely,
/RA/
Anthony J. Mendiola, Chief, Section 2 Project Directorate III Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket Nos. 50-254 and 50-265
Enclosure:
Safety Evaluation cc: See next page
O. Kingsley If you have any questions about this review, please contact Mahesh Chawla at (301) 415-1321 or by e-mail at mlc@nrc.gov.
Sincerely,
/RA/
Anthony J. Mendiola, Chief, Section 2 Project Directorate III Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket Nos. 50-254 and 50-265
Enclosure:
Safety Evaluation cc: See next page DISTRIBUTION:
PUBLIC M. Chawla F. Lyon PD3-2 r/f C. Rosenberg T. Chan OGC M. Ring, RIII S. Ali A. Mendiola ACRS G. Hill (4)
H. Nieh S. Dinsmore ADAMS Accession Number: ML020180003
- SE input dated 12/12/01 OFFICE PM:PD3-2 PM:PD3-2 LA:PD3-2 BC:EMEB OGC Nlo SC:PD3-2 NAME MChawla FLyon CRosenberg TChan*
RHoefling AMendiola DATE 01/ 22/02 01/22/02 01/18/02 12/12/01 01/29/02 01/ /02 OFFICIAL RECORD COPY 2/5/02
O. Kingsley Quad Cities Nuclear Power Station Exelon Generation Company, LLC Units 1 and 2 cc:
Exelon Generation Company, LLC Site Vice President - Quad Cities 22710 206th Avenue N.
Cordova, Illinois 61242-9740 Exelon Generation Company, LLC Station Manager - Quad Cities 22710 206th Avenue N.
Cordova, Illinois 61242-9740 Exelon Generation Company, LLC Regulatory Assurance Manager - Quad Cities 22710 206th Avenue N.
Cordova, Illinois 61242-9740 U.S. Nuclear Regulatory Commission Quad Cities Resident Inspectors Office 22712 206th Avenue N.
Cordova, Illinois 61242 William D. Leech Manager - Nuclear MidAmerican Energy Company P.O. Box 657 Des Moines, Iowa 50303 Vice President - Law and Regulatory Affairs MidAmerican Energy Company One River Center Place 106 E. Second Street P.O. Box 4350 Davenport, Iowa 52808 Chairman Rock Island County Board of Supervisors 1504 3rd Avenue Rock Island County Office Bldg.
Rock Island, Illinois 61201 Regional Administrator U.S. NRC, Region III 801 Warrenville Road Lisle, Illinois 60532-4351 Illinois Department of Nuclear Safety Office of Nuclear Facility Safety 1035 Outer Park Drive Springfield, Illinois 62704 Document Control Desk-Licensing Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. John Skolds Chief Operating Officer Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. John Cotton Senior Vice President, Operation Support Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. William Bohlke Senior Vice President, Nuclear Services Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. Robert J. Hovey Vice President Mid-West Regional Operating Group Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. Christopher Crane Senior Vice President Mid-West Regional Operating Group Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555
O. Kingsley Quad Cities Nuclear Power Station Exelon Generation Company, LLC Units 1 and 2 Mr. Jeffrey Benjamin Vice President - Licensing and Regulatory Affairs Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. K. A. Ainger Director - Licensing Mid-West Regional Operating Group Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555 Mr. Robert Helfrich Senior Counsel, Nuclear Mid-West Regional Operating Group Exelon Generation Company, LLC 4300 Winfield Road Warrenville, Illinois 60555
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO REQUEST FOR RELIEF FOR RISK INFORMED INSERVICE INSPECTION PROGRAM PLAN - RELIEF REQUEST CR-33 EXELON GENERATION COMPANY, LLC AND MIDAMERICAN ENERGY COMPANY QUAD CITIES NUCLEAR POWER STATION, UNITS 1 AND 2 DOCKET NUMBERS 50-254 AND 50-265
1.0 INTRODUCTION
Current inservice inspection (ISI) requirements for the Quad Cities Nuclear Power Station (QCNPS) are contained in the 1989 Edition of Section XI, Division 1 of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, entitled Rules for Inservice Inspection of Nuclear Power Plant Components (hereinafter referred to as the ASME Code). In a submittal dated November 30, 2000 (Reference 1), Commonwealth Edison Company (the former licensee), proposed a new risk-informed inservice inspection (RI-ISI) program as an alternative to a portion of its current ISI program. Additional clarifying information was provided in letters from Exelon Generation Company (the current licensee) dated August 10, 2001 (Reference 2), October 31, 2001 (Reference 3), and November 20, 2001 (Reference 4).
Subsequent to the date of the original relief request, ComEd was merged into Exelon Generation Company (Exelon). By letter dated February 7, 2001, Exelon informed the Nuclear Regulatory Commission (NRC) that it assumed responsibility for all pending NRC actions that were requested by ComEd.
The RI-ISI program is limited to ASME Class 1 and Class 2 piping welds. The program was developed in accordance with the Electric Power Research Institute (EPRI) methodology contained in the NRC approved EPRI Topical Report EPRI TR-112657, Revision B-A (TR-112657), (Reference 5).
In the proposed RI-ISI program, piping failure potential estimates were determined using TR-112657 guidance, which utilizes industry piping failure history, plant-specific piping failure history, and other relevant information. Using the failure potential and supporting insights on piping failure consequences from the licensees probabilistic risk assessment (PRA), a safety ranking of piping segments was established for determination of new inspection locations. The proposed program maintains the fundamental requirements of the ASME Code, such as the examination technology, examination frequency, and acceptance criteria. However, the proposed program reduces the required examination locations significantly while demonstrating that an acceptable level of quality and safety is maintained. Thus, the proposed alternative approach is based on the conclusion that it provides an acceptable level of quality and safety and, therefore, is in conformance with Title 10, Code of Federal Regulations (10 CFR),
Part 50.55a(a)(3)(i).
2.0
SUMMARY
OF PROPOSED APPROACH The ASME Code,Section XI, requires that for each successive 10-year ISI interval, 100 percent of Category B-F welds, and 25 percent of Category B-J welds for ASME Code Class 1 piping greater than one inch in nominal diameter be selected for volumetric and/or surface examination based on existing stress analyses and cumulative usage factors. For Category C-F piping welds, 7.5 percent of non-exempt welds shall be selected for volumetric and/or surface examination. The licensee submitted the application as an RI-ISI "template" application.
Template applications are short overview submittals intended to expedite preparation and review of RI-ISI submittals that comply with a pre approved methodology. The licensee proposed to implement the staff-approved RI-ISI methodology delineated in TR-112657.
In accordance with Table 6.2 of the EPRI TR-112657, some augmented inspection programs may be integrated into the RI-ISI program. In accordance with the guidance in TR-112657, thermal fatigue is subsumed into the RI-ISI program since the issues raised by NRC Bulletin 88-08 are all addressed by the thermal fatigue assessment as part of the RI-ISI program.
Augmented programs for intergrannular stress-corrosion cracking (IGSCC) Category B-G (Generic Letter 88-01), service water integrity (Generic Letter 89-13), flow accelerated corrosion (FAC, Generic Letter 89-09), and high energy line break (USNRC Branch Technical Position MEB 3-1) are not subsumed into the RI-ISI program and remain unaffected.
The licensee indicated that it has not yet completed its evaluation of existing relief requests to determine which can be withdrawn due to changes that occur from implementing the RI-ISI program. The licensee further stated that until this evaluation is completed, all existing relief requests remain in place.
The licensee requested approval of this alternative for the remainder of the third inspection period of the third ten-year interval for Unit 1 and the entire third inspection period of the third ten-year interval for Unit 2. According to the information provided in References 1 and 2, QCNPS Unit 1 is currently in the third 10-year interval that started on February 18, 1993, and ends on February 17, 2003. The current period (i.e., the third period of the interval) started on October 1, 2000, and ends on February 17, 2003. Unit 2 is currently in the third 10-year interval that started on March 10, 1993, and ends on March 9, 2003. The current period (i.e., the third period of the interval) started on March 10, 2000, and ends on March 9, 2003.
The implementation of an RI-ISI program for piping should be initiated at the start of a plants 10-year inservice inspection interval consistent with the requirements of the ASME Code and Addenda committed to by the licensee in accordance with 10 CFR 50.55a. However, the implementation may begin at any point in an existing interval as long as the examinations are scheduled and distributed consistently with the ASME Code requirements (e.g., the minimum examinations completed at the end of the three inspection intervals under ASME Code Program B should be 16 percent, 50 percent, and 100 percent, respectively, and the maximum examinations credited at the end of the respective periods should be 34 percent, 67 percent, and 100 percent).
It is also the staffs view that the inspections for the RI-ISI program and for the balance of the ISI program should be on the same interval start and end dates. This can be accomplished by either implementing the RI-ISI program at the beginning of the interval or merging the RI-ISI program into the ISI program for the balance of the inspections if the RI-ISI program is to begin during an existing ISI interval. One reason for this view is that it eliminates the problem of having different Codes of record for the RI-ISI program and for the balance of the ISI program.
A potential problem with using two different interval start dates and hence two different Codes of record would be having two sets of repair/replacement rules depending upon which program identified the need for repair (e.g., a weld inspection versus a pressure test). In Reference 2, the licensee stated that the ASME Code minimum and maximum inspection percentages for the first and second inspection periods have already been satisfied. The licensee also stated that the RI-ISI program implementation in the third period will result in 100 percent of the RI-ISI inspections being completed during the current third interval for both units.
The staff finds that the QCNPS, Units 1 and 2, RI-ISI programs meet the ASME Code and 10 CFR 50.55a requirements for minimum and maximum inspections during inspection periods and intervals, and for program submittal to the NRC.
3.0 EVALUATION The licensees submittal was reviewed with respect to the methodology and criteria contained in TR-112657. Further guidance in defining acceptable methods for implementing a RI-ISI program is also provided in Regulatory Guide (RG) 1.174, RG 1.178, and Standard Review Plan (SRP) Chapter 3.9.8 (References 6, 7, and 8).
3.1 Proposed Changes to the ISI Program Pursuant to 10 CFR 50.55a(a)(3)(i), the licensee has proposed to implement an RI-ISI program in accordance with the methodology contained in TR-112657 as an alternative to the ASME Code examination requirements for ASME Class 1 and 2 piping for QCNPS, Units 1 and 2. A general description of the proposed changes to the ISI program was provided in Section 3 of the licensees submittal.
3.2 Engineering Analysis In accordance with the guidance provided in RGs 1.174 and 1.178, an engineering analysis of the proposed changes is required using a combination of traditional engineering analysis and supporting insights from the PRA. The licensee elaborated as to how the engineering analyses conducted for the QCNPS RI-ISI program ensures that the proposed changes are consistent with the principles of defense-in-depth. This is accomplished by evaluating a locations susceptibility to a particular degradation mechanism and then performing an independent assessment of the consequence of a failure at that location. No changes to the evaluation of design basis accidents in the final safety analysis report are being made by the RI-ISI process.
Therefore, sufficient safety margins will be maintained.
The licensees RI-ISI program at QCNPS Station is applicable to ASME Class 1 Categories B-F and B-J and ASME Class 2 Categories C-F-1 and C-F-2 piping welds. The licensee stated in its submittal that other non-related portions of the ASME Code will be unaffected by this program. Piping systems defined by the scope of the RI-ISI program were divided into piping segments. Pipe segments are defined as lengths of pipe whose failure leads to similar consequences and are exposed to the same degradation mechanisms. That is, some lengths of pipe whose failure would lead to the same consequences may be split into two or more segments when two or more regions are exposed to different degradation mechanisms.
In response to NRC Question 6 (Reference 2), the licensee stated that QCNPS will perform visual VT-2 examinations during each refueling outage as an alternative to the volumetric examinations specified in Reference 5, for those high safety significant (HSS) socket welds identified in the RI-ISI program. This request is reasonable because the volumetric examination is inconclusive and impractical due to the geometric limitations imposed by a socket weld.
However, the staff notes that Table IWB-2500-1 of the Code requires surface examination, not volumetric examination, at the socket welds, and surface examination (i.e., liquid penetration examination) is an effective method for discovery of potential surface flaws on the outside surface, and specifically, flaws induced by low-cycle, high-bending stress thermal fatigue or by external chloride stress corrosion cracking (ECSCC). The licensee indicated that the QCNPS socket weld piping is not located in areas that are subject to an environment promoting ECSCC, and an outside surface-initiated flaw has a very low probability of occurrence due to the inclusion of thermal cyclic loads in the piping design. Therefore, these conditions do not exist in socket welds at QCNPS. As for a potential outside surface flaw caused by vibration-induced fatigue, such a flaw is likely to take a long period for initiation. After the initiation phase, the flaw will likely propagate rapidly and cause the pipe to leak. Hence, the staff concludes that performance of a VT-2 visual examination is sufficiently effective and therefore acceptable.
Pursuant to 10 CFR 50.55a(a)(3)(ii), the staff concurs that performing volumetric or surface examinations of these socket welds would result in unusual difficulty without a compensating increase in the level of quality and safety.
The submittal states that failure potential categories were generated utilizing industry failure history, plant-specific failure history, and other relevant information using the guidance provided in TR-112657. The degradation mechanisms identified in the submittal include thermal fatigue, IGSCC, erosion-cavitation (E-C), and flow-accelerated corrosion (FAC).
Augmented programs for IGSCC Category B-G (Generic Letter 88-01), service water integrity (Generic Letter 89-13), flow accelerated corrosion (Generic Letter 89-08), and high energy line break (USNRC Branch Technical Position MEB 3-1) are not subsumed into the RI-ISI program and remain unaffected. Elements in the scope of QCNPS that were also covered by these augmented programs were included in the consequence assessment, degradation assessment, and risk categorization evaluations to determine whether the affected piping was subject to damage mechanisms other than those addressed by the augmented program. If another damage mechanism was identified, the element was retained within the scope of consideration for element selection as part of the RI-ISI program. When inspections are required under the RI-ISI and augmented programs, all inspection requirements for both RI-ISI and augmented programs are met. If no other damage mechanism was identified, the element was excluded from the RI-ISI element selection population (i.e., not included in the population of elements from which 25 percent or 10 percent must be selected for inspection) and retained in the appropriate augmented inspection program. The QCNPS approach deviates from the approved methodology because the methodology in TR-112657 includes all elements in the RI-ISI element selection population but allows crediting up to 50 percent of the augmented inspections as RI-ISI element inspections. The deviation as described in References 1 and 2 is acceptable since inspections required only in the augmented programs are not credited as RI-ISI inspections, elements in the augmented programs will continue to be inspected for the appropriate degradation mechanisms, and the RI-ISI program will address other damage mechanisms.
The licensee stated that the consequences of pressure boundary failure were evaluated and ranked based on their impact on core damage probability and large early release probability.
Both direct and indirect effects of pipe ruptures were evaluated and included in the consequence characterization. The licensee used its PRA models to directly support its estimation of the consequences of pressure boundary failure for each piping element in the evaluation. Since the licensee reported no deviations from the segment definition and consequence characterization methodology approved by the staff in TR-112657, the staff concludes that the licensees analyses are acceptable.
3.3 Probabilistic Risk Assessment To support this RI-ISI submittal, the licensee used the Quad Cities Nuclear Stations 1999 Updated PRA Model, Rev. 0, Calculation #QDC-0200-M-0803 for core damage frequency (CDF) and the Quad Cities Nuclear Stations 1999 Large Early Release Frequency (LERF)
Model, Rev. 0, Calculation #QDC-1600-N-0981 for LERF. The licensee reported a CDF of 4.6E-6/yr and a LERF of 3.1E-6/yr for each unit. The licensee submitted its individual plant examination (IPE) on December 13, 1993, and a modified version of the IPE on December 17, 1996. The staff evaluation report on the IPE submitted in December 1996, was issued July 9, 1997, and concluded that the IPE satisfied the intent of Generic Letter 88-20 but noted that the common cause factors used in the IPE were lower than the generic factors. In Reference 2, the licensee stated that it has incorporated the common cause factors from NUREG/CR-5497, Common-Cause Failure Parameter Estimations (Reference 9) into the PRA analysis used to support the RI-ISI submittal.
The licensees Dresden Nuclear Power Stations PRA underwent the Boiling Water Reactors peer review certification in January 1998. The licensee stated that Dresden and Quad Cities are plants with similar designs and PRAs. The upgrade of the Quad Cities PRA was done in parallel with the update of the Dresden PRA to reflect enhancements identified by the peer review process. The licensee also reported that it has implemented a PRA Maintenance and Update Procedure that formalizes the PRA update process.
The approved TR-112657 requires that functions relied upon to mitigate external events and to mitigate transients during operation modes outside the scope of the PRA also be systematically included in the categorization. The licensee reported no deviations from the approved methodology in this area and therefore the staff finds its evaluation acceptable.
The staff did not review the PRA analysis to assess the accuracy of the quantitative estimates.
Quantitative results of the PRA are used, in combination with a quantitative characterization of the pipe segment failure likelihood, to support the assignment of segments into broad safety significance categories reflecting the relative importance of pipe segment failures on CDF and LERF and to provide an illustrative estimate of the change in risk. Inaccuracies in the models or assumptions large enough to invalidate the analyses developed to support RI-ISI should have been identified in the licensees or the staffs reviews. Minor errors or inappropriate assumptions will only affect the consequence categorization of a few segments and will not invalidate the general results or conclusions. Furthermore, the continuous use and documented maintenance of the PRA provide further opportunities to identify inaccuracies, if any, in the PRA models and assumptions. The staff finds that the quality of the Quad Cities PRA is sufficient to support this submittal.
As required by Section 3.7 of TR-112657, the licensee evaluated the change in risk expected from replacing the current ISI program with the RI-ISI program. The analysis estimates the net change in risk due to the positive and negative influence of adding and removing locations from the inspection program. As discussed in Section 3.2 of this safety evaluation (SE), the licensee deviated from the EPRI methodology by excluding some elements from the population of elements from which RI-ISI locations for inspection were selected. In Reference 2, the licensee stated that the change in risk estimates included the increase in risk caused by discontinued Section XI inspections in the population of elements excluded from RI-ISI element selection.
Therefore, excluding some elements from the population of elements for possible inspection does not affect the change in risk calculations. The failure frequencies used in the calculation are the frequencies excluding the degradation mechanism addressed by the augmented program. This is consistent with the staffs position that the augmented programs adequately control the degradation mechanism and is acceptable.
The licensee used the failure frequencies developed in EPRI Topical Report TR-111880 (Reference 10) to support the estimate for the change in risk. The non-proprietary version of TR-111880 (Reference 11) illustrates the characteristics and format of the information used, but does not include the calculated parameters. Reference 10 provides failure frequency estimates according to system type and exposure to a degradation mechanism. The method used to develop the frequencies in Reference 10 was reviewed and approved during the approval of TR-112657 although the process to select and to adapt the frequencies from the report for use in the change in risk calculations was not specified. The change in risk is calculated utilizing the Markov model described in EPRI Topical Report TR-111061 (Reference 12) and described in detail in Reference 2.
The method uses the Markov model to estimate the inspection efficiency factor (IEF). The IEF calculation incorporates the time between ISI inspections and the time between opportunities to detect a leak together with the probability of detection (POD) to estimate the reduction in pipe failure frequency arising from including the element in an ISI program. The method, and the input parameters, are the same as used by the licensee, and approved by the staff in the Dresden RI-ISI submittal (Reference 13). The staff finds the calculations acceptable to use in support of this RI-ISI submittal.
The licensee estimated the change in CDF and LERF for QCNPS 1 to be 6.23E-9/yr and 1.85E-9/yr, respectively (Reference 3). For QCNPS 2 the estimated changes in CDF and LERF are 5.60E-9/yr and 1.84E-9/yr, respectively (Reference 3). The licensee also reported the system level changes for all the systems included in the scope of the submittal. All the estimated changes in risk are below the EPRI guideline criteria for acceptable estimated changes in CDF and LERF. These estimates use the IEF and include the effect of change in POD. Sensitivity studies provided in Reference 3 indicate that bounding estimates are also below the EPRI guideline criteria.
The staff finds that the licensees process to evaluate the potential change in risk is reasonable because it accounts for the change in the number and location of elements inspected, recognizes the difference in degradation mechanism related to failure likelihood, and considers the synergistic effects of multiple degradation mechanisms within the same piping segment.
The staff finds that redistributing the welds to be inspected with consideration of the safety-significance of the segments provides assurance that segments whose failures have a significant impact on plant risk receive an acceptable, and often improved, level of inspection.
Based on the reported quantitative results, the staff concludes that any increase in risk associated with the implementation of the RI-ISI program is small and is consistent with the intent of the Commissions Policy Statement and with RG 1.178 and that, therefore, implementation of the RI-ISI program as described in the licensees application is acceptable.
3.4 Integrated Decision-Making As described in the August 10, 2001, and October 31, 2001, Exelon Generation Company submittals, an integrated approach is utilized in defining the proposed RI-ISI program by considering in concert the traditional engineering analysis, risk evaluation, and the implementation and performance monitoring of piping under the program. This is consistent with the guidelines of RG 1.178.
The selection of pipe segments to be inspected is described in Section 3.5 of the submittal using the results of the risk category rankings and other operational considerations. The submittal states that in accordance with the EPRI TR, 25 percent of high HSS and 10 percent of medium safety significant (MSS) elements are selected for inspection. As discussed in the submittal and earlier in this SE, these percentages are drawn from the population of welds included in the RI-ISI element selection population. The inspections are generally selected on a system-by-system basis. The licensee stated that an attempt is made to ensure that all damage mechanisms and all combinations of damage mechanisms are represented in the elements selected for inspection.
Table 2 of the submittal provides the failure potential assessment summary for Units 1 and 2.
Tables 3 and 4 of the submittal identify on a per system basis for Units 1 and 2, respectively, the number of elements (welds) by risk category. Tables 5 and 6 provide a summary comparing the number of inspections required under the existing ASME Code ISI program with the alternative RI-ISI program for each applicable system.
The licensee used the methodology described in TR-112657 to guide the selection of examination elements within high and medium ranked piping segments. The EPRI report describes targeted examination volumes (typically associated with welds) and methods of examination based on the type(s) of degradation expected. The staff has reviewed these guidelines and has determined that, if implemented as described, the RI-ISI examinations should result in improved detection of service-related discontinuities over that currently required by the ASME Code.
The staff finds the location selection process to be acceptable since it is consistent with the process approved in TR-112657, takes into account defense-in-depth, and includes coverage of welds subjected to degradation mechanisms in addition to those covered by augmented inspection programs. As described in section 3.2 of this SE, excluding elements exposed only to a damage mechanism addressed by an augmented program from the RI-ISI element selection population is an acceptable deviation from the EPRI methodology.
The objective of ISI required by the ASME Code is to identify conditions (i.e., flaw indications) that are precursors to leaks and ruptures in the pressure boundary that may impact plant safety. Therefore, the RI-ISI program must meet this objective to be found acceptable for use.
Further, since the risk-informed program is based on inspection for cause, element selection should target specific degradation mechanisms. Chapter 4 of TR-112657 provides guidelines for the areas and/or volumes to be inspected as well as the examination method, acceptance standard, and evaluation standard for each degradation mechanism. Based on the review of the cited portion of the EPRI report, the staff concludes that the examination methods are appropriate since they are selected based on specific degradation mechanisms, pipe sizes, and materials of concern.
3.5 Implementation and Monitoring Implementation and performance monitoring strategies require careful consideration by the licensee and are addressed in Element 3 of RG 1.178 and SRP 3.9.8. The objective of Element 3 is to assess the performance of the affected piping systems under the proposed RI-ISI program by implementing monitoring strategies that confirm the assumptions and analyses used in the development of the RI-ISI program. To approve an alternative pursuant to 10 CFR 50.55a(a)(3)(i), implementation of the RI-ISI program, including inspection scope, examination methods, and methods of evaluation of examination results, must provide an acceptable level of quality and safety.
The licensee stated in its submittal that upon approval of the RI-ISI program, procedures that comply with the EPRI TR-112657 guidelines will be prepared to implement and monitor the RI-ISI program. The licensee confirmed that the applicable portions of the ASME Code not affected by the change, such as inspection methods, acceptance guidelines, pressure testing, corrective measures, documentation requirements, and quality control requirements would be retained.
The licensee stated in Reference 2 and further clarified in Reference 4 that the RI-ISI program is a living program and its implementation will require feedback of new relevant information to ensure the appropriate identification of high safety significant piping locations. Such relevant information would include major updates to the QCNPS Units 1 and 2 PRA models which could impact both the risk characterization and risk impact assessments, any new trends in service experience with piping systems at QCNPS and across the industry, and new information on element accessibility that will be obtained as the risk informed inspections are implemented.
Reference 4 states that as a minimum, risk ranking of piping segments will be reviewed and adjusted on an ASME-period basis and that significant changes may require more frequent adjustment as directed by NRC bulletin or generic letter requirements, or by industry or plant-specific feedback. Reference 4 also states that the RI-ISI program will be updated and submitted to the NRC at the end of the 10-year ISI interval and may be submitted to the NRC prior to the end of the 10-year ISI interval if there is a deviation from the RI-ISI methodology described in the initial 10-year interval ISI submittal to the NRC or if industry experience determines that there is a need for significant revision to the program as described in the initial 10-year interval ISI submittal to the NRC for that interval.
The licensees submittal (Reference 1) presented the criteria for engineering evaluations and additional examinations if unacceptable flaws or relevant conditions are found during examinations. The submittal stated that in lieu of the evaluation and sample expansion requirements of Section 3.6.6.2 in Reference 3, QCNPS will utilize the requirements of Subarticle-2430 in Code Case N-578-1. Reference 2 clarified that the additional examinations shall include piping structural elements with the same postulated failure mode and the same or higher failure potential. Reference 2 stated that the number of additional elements shall be the number of piping structural elements with the same postulated failure mode originally scheduled for that fuel cycle. Reference 2 further stated that the scope of the additional examinations may be limited to those HSS piping structural elements (Risk Group Categories 1 through 5) within systems, whose material and service conditions are determined by an evaluation to have the same postulated failure mode as the piping structural element that contained the original flaw or relevant condition. Further clarifications of additional examinations are also provided in Reference 2. The staff finds the criteria for additional examinations to be acceptable based on the clarifications in Reference 2 in response to the staff Question 5.
The proposed periodic reporting requirements meet existing ASME Code requirements and applicable regulations and are, therefore, acceptable to the staff. The proposed process for RI-ISI program updates meets the guidelines of RG 1.174 that risk-informed applications must include performance monitoring and feedback provisions. Therefore, the staff concludes that the process for program updates is acceptable.
4.0 CONCLUSION
S In accordance with 10 CFR 50.55a(a)(3)(i), proposed alternatives to regulatory requirements may be used when authorized by the NRC when the applicant demonstrates that the alternative provides an acceptable level of quality and safety. In this case, the licensee's proposed alternative is to use the risk-informed process described in the NRC approved EPRI TR-112657. The staff concludes that the licensees proposed RI-ISI program, which is consistent with the methodology described in EPRI TR-112657, 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 the number of inspections, locations of inspections, and methods of inspection.
The staff finds that the results of the different elements of the engineering analysis are considered in an integrated decision-making process. The impact of the proposed change in the ISI program is founded on the adequacy of the engineering analysis and acceptable change in plant risk in accordance with RG 1.174 and RG 1.178 guidelines.
QCNPS methodology also considers implementation and performance monitoring strategies.
Inspection strategies ensure that failure mechanisms of concern have been addressed and there is adequate assurance of detecting damage before structural integrity is affected. The risk significance of piping segments is taken into account in defining the inspection scope for the RI-ISI program.
System pressure tests and visual examination of piping structural elements will continue to be performed on all ASME Class 1, 2, and 3 systems in accordance with the ASME Code program.
The RI-ISI program applies the same performance measurement strategies as existing ASME Code requirements and, in addition, increases the inspection volumes at some weld locations.
QCNPS methodology provides for conducting an engineering analysis of the proposed changes using a combination of engineering analysis with supporting insights from a PRA.
Defense-in-depth and quality are not degraded in that the methodology provides reasonable confidence that any reduction in existing inspections will not lead to degraded piping performance when compared to existing performance levels. Inspections are focused on locations with active degradation mechanisms as well as selected locations that monitor the performance of piping systems.
The licensee has stated that the ASME Code minimum and maximum inspection requirements for Program B will be met and that the RI-ISI inspections and the balance of the inspections will be on the same interval start and end dates. The licensee stated that QCNPS, Units 1 and 2, would continue to submit their 10-year interval ISI programs including the RI-ISI programs every 10 years. The licensee also stated that the RI-ISI program will be maintained as a living program and reviewed and adjusted on an ASME-period basis. The licensee stated that changes that could impact the RI-ISI program include major changes to QCNPS PRA or changes to weld selection. The staff finds that the QCNPS, Units 1 and 2, RI-ISI programs meet the ASME Code requirements for minimum and maximum inspections during inspection periods and intervals. The staff also finds that the QCNPS, Units 1 and 2, RI-ISI programs meet the 10 CFR 50.55a requirements for program submittal to the NRC.
The licensee stated that QCNPS will perform visual VT-2 examinations during each refueling outage as an alternative to the volumetric examinations specified in Reference 5, for HSS socket welds identified in the RI-ISI program. This request is reasonable because the volumetric examination is inconclusive and impractical due to the geometric limitations imposed by a socket weld. The licensee also indicated that the QCNPS socket weld piping is not located in areas that are subject to an environment promoting ECSCC, and an outside surface-initiated flaw has a very low probability of occurrence due to the inclusion of thermal cyclic loads in the piping design. Therefore, these conditions do not exist in socket welds at QCNPS. Hence, the staff concludes that performance of a VT-2 visual examination is sufficiently effective and therefore acceptable. Pursuant to 10 CFR 50.55a(a)(3)(ii), the staff authorizes the licensees request for relief on the basis that performing volumetric or surface examinations of these socket welds would result in unusual difficulty without a compensating increase in the level of quality and safety.
Based on the discussion above the staff authorizes application of the proposed RI-ISI program during the third ten-year ISI interval for both QCNPS, Unit 1 and Unit 2.
5.0 REFERENCES
1.
Letter, Joel Dimmette Jr. (Quad Cities Nuclear Power Station), to U.S. Nuclear Regulatory Commission, Quad Cities Nuclear Power Station, Units 1 and 2, Facility Operating License Nos. DPR-29 and DPR-30, NRC Docket Nos. 50-254 and 50-265, Risk Informed Inservice Inspection Program Alternative to the ASME Boiler and Pressure Vessel Code Section XI Requirements for Class 1 and 2 Piping Welds, dated November 30, 2000.
2.
Letter, Timothy Tulon (Quad Cities Generating Station), to U.S. Nuclear Regulatory Commission, Quad Cities Nuclear Power Station, Units 1 and 2, Facility Operating License Nos. DPR-29 and DPR-30, NRC Docket Nos. 50-254 and 50-265, Response to request for Additional Information Concerning Risk Informed Inservice Inspection Program Alternative to the ASME Boiler and Pressure Vessel Code Section XI Requirements for Class 1 and 2 Piping Welds, dated August 10, 2001.
3.
Letter, P.R. Simpson (Exelon Generation), to U.S. Nuclear Regulatory Commission, Quad Cities Nuclear Power Station, Units 1 and 2, Facility Operating License Nos.
DPR-29 and DPR-30, NRC Docket Nos. 50-254 and 50-265, Clarification of Responses to Request for Additional Information Concerning Risk Informed Inservice Inspection Program Alternative to the ASME Boiler and Pressure Vessel Code Section XI Requirements for Class 1 and 2 Piping Welds, dated October 31, 2001.
4.
Letter, K.R. Jury (Exelon Generation), to U.S. Nuclear Regulatory Commission, Quad Cities Nuclear Power Station, Units 1 and 2, Facility Operating License Nos. DPR-29 and DPR-30, NRC Docket Nos. 50-254 and 50-265, Responses to Request for Additional Information Regarding Risk Informed Inservice Inspection Program, dated November 20, 2001.
5.
Electric Power Research Institute, Revised Risk-Informed Inservice Inspection Evaluation Procedure, EPRI TR-112657, Revision B-A, January 2000.
6.
U.S. Nuclear Regulatory Commission, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Regulatory Guide 1.174, July 1998.
7.
U.S. Nuclear Regulatory Commission, An Approach for Plant-Specific Risk-Informed Decision Making: Inservice Inspection of Piping, Regulatory Guide 1.178, September 1998.
8.
U.S. Nuclear Regulatory Commission, Standard Review Plan for Trial Use for the Review of Risk-Informed Inservice Inspection of Piping, NUREG-0800, SRP Chapter 3.9.8, September 1998.
9.
U.S. Nuclear Regulatory Commission, Common-Cause Failure Parameter Estimations, NUREG/CR-5497, October 1998.
10.
Electric Power Research Institute, Piping System Failure Rates and Rupture Frequencies for Use in Risk-Informed In-Service Inspection Applications, EPRI TR-111880, September 1999 (proprietary).
11.
Electric Power Research Institute, Piping System Failure Rates and Rupture Frequencies for Use in Risk-Informed In-Service Inspection Applications, EPRI TR-111880-NP, November 2000.
12.
Electric Power Research Institute, Piping System Reliability and Failure Rate Estimation Modes for Use in Risk-Informed In-Service Inspection Applications, EPRI TR-110161, December 1998 (proprietary).
13.
Letter, A. Mendiola (U. S. Nuclear Regulatory Commission) to O. D. Kingsley (Exelon Nuclear), Dresden Nuclear Power Station, Units 2 and 3, American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) - Relief for Risk-Informed Inservice Inspection of Piping, dated September 5, 2001.
Principal Contributors:
Syed Ali Stephen Dinsmore