ML083590038
| ML083590038 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 01/22/2009 |
| From: | Raghavan L Watts Bar Special Projects Branch |
| To: | Campbell W Tennessee Valley Authority |
| Lamb John G./NRR/DORL, 415-3100 | |
| References | |
| TAC MD9596 | |
| Download: ML083590038 (17) | |
Text
January 22, 2009 Mr. William R. Campbell, Jr.
Chief Nuclear Officer and Executive Vice President Tennessee Valley Authority 6A Lookout Place 1101 Market Street Chattanooga, TN 37402-2801
SUBJECT:
WATTS BAR NUCLEAR PLANT, UNIT 1, RELIEF REQUEST C AMERICAN SOCIETY OF MECHANICAL ENGINEERS, SECTION XI, ALTERNATE INSERVICE INSPECTION PROGRAM C RISK-INFORMED INSERVICE INSPECTION PROGRAM (TAC NO. MD9596)
Dear Mr. Campbell:
Tennessee Valley Authority=s (TVA=s) letter of December 4, 2007, as supplemented on December 3, December 10, and December 11, 2008, requested approval of a risk-informed inservice inspection (RI-ISI) program for American Society of Mechanical Engineers (ASME)
Class 1 and Class 2 piping welds as an alternative to the current ISI program at Watts Bar Nuclear Plant, Unit 1. TVA developed the proposed RI-ISI program in accordance with Electric Power Research Institute Topical Report 112657 Revision B-A, Revised Risk-Informed Inservice Inspection Evaluation Procedure. The U.S. Nuclear Regulatory Commission (NRC) staff concludes that the proposed RI-ISI program is an acceptable alternative to the requirements of the ASME Boiler and Pressure Vessel Code,Section XI, for inservice inspection of Code Class 1 piping, Categories B-F and B-J welds, and Class 2 piping, Categories C-F-1 and C-F-2 welds. Therefore, TVA=s Request for Relief 1-RI-ISI-01 is authorized pursuant to Title 10, Code of Federal Regulations, Section 50.55a(a)(3)(i) for the second 10-year ISI interval on the basis that the alternative provides an acceptable level of quality and safety.
A copy of the NRC staff safety evaluation is enclosed.
Sincerely,
/RA/ PMilano for L. Raghavan, Chief Watts Bar Special Projects Branch Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-390
Enclosure:
Safety Evaluation cc w/enclosure: Distribution via Listserv
- via memorandum OFFICE LP-WBB/PM LP-WB/LA APLA/ABC CPNB/BC OGC - NLO LP-WB/BC NAME JLamb BClayton AHowe*
TChan*
EWilliamson PMilano for LRaghavan DATE 12 / 31 /08 12 / 31/08 01 / 21 /09 01 / 16 /09 12 / 31 /08 01/ 22 /09
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RISK-INFORMED INSERVICE INSPECTION PROGRAM TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT 1 DOCKET NO. 50-390
1.0 INTRODUCTION
By letter dated December 4, 2007 (Agencywide Documents Access and Management System Accession No. ML073390086), as supplemented December 3 (ML083390185), December 10 (ML083460213), and December 11, 2008 (ML083570574), Tennessee Valley Authority (TVA, the licensee) proposed a relief request (RR) for the Watts Bar Nuclear Plant (WBN), Unit 1, Risk-Informed (RI) Inservice Inspection (ISI) Program. The scope of the RI-ISI program is limited to the American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code (ASME Code),Section XI, Class 1 and 2 piping (Categories B-F, B-J, C-F-1, and C-F-2 welds) only. The proposed RR would modify the methodology for the RI-ISI program and would be used for the second ISI interval, which began May 27, 2007. TVA proposes to use the alternative methodology described in Electric Power Research Institute (EPRI) Technical Report (TR) TR-112657, Revision B-A, Revised Risk-Informed Inservice Inspection Evaluation Procedure, and the associated U.S. Nuclear Regulatory Commission (NRC) staff safety evaluation (SE).
For WBN Unit 1, the applicable edition of the Code for the second 10-year ISI interval is the 2001 Edition of ASME Boiler and Pressure Vessel Code Section XI through the 2003 Addenda.
2.0 BACKGROUND
The WBN Unit 1 RI-ISI program for the first 10-year ISI interval was submitted to the NRC by letters dated May 21, 2001 (ML011500231) and September 28, 2001 (ML012740488). This initial RI-ISI program was developed in accordance with the methodology contained in Westinghouse Owners Group Topical Report WCAP-14572, Revision 1-NP-A, Westinghouse Owners Group Application of Risk-Informed Methods to Piping Inservice Inspection Topical Report, dated February, 1999 (ML042610469). The NRC staff authorized WBN Unit 1, to implement a RI-ISI program during the first 10-year ISI interval by letter dated January 24, 2002 (ML020240481).
3.0 REGULATORY EVALUATION
Paragraph 50.55a(g) of Title 10 of the Code of Federal Regulations (10 CFR) specifies that ISI of nuclear power plant components shall be performed in accordance with the requirements of the ASME Code except where specific written relief has been granted by the Commission
pursuant to 10 CFR 50.55a(g)(6)(i). It states in 10 CFR 50.55a(a)(3) that alternatives to the requirements of paragraph (g) may be used, when authorized by the NRC, if (i) the proposed alternatives would provide an acceptable level of quality and safety, or (ii) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Pursuant to 10 CFR 50.55a(g)(4), ASME Code Class 1, 2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, to the extent practical within the limitations of design, geometry, and materials of construction of components. The regulations require that inservice examination of components and system pressure tests conducted during the first 10-year interval and subsequent intervals comply with the requirements of the latest edition and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR50.55a(b) 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein. The applicable code of record for the second 10-year ISI interval for WBN Unit 1, is the ASME Code,Section XI, 2001 Edition through the 2003 Addenda.
The NRC staff has reviewed and evaluated the licensees proposed RI-ISI program, including those portions related to the applicable methodology and processes, based on guidance and acceptance criteria provided in the EPRI TR-112657, Revision B-A; Regulatory Guide (RG) 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis (ML003740133); RG 1.178, An Approach for Plant-Specific Risk-Informed Decision Making: Inservice Inspection of Piping (ML003740181);
and Standard Review Plan (SRP), Chapter 3.9.8, Standard Review Plan for the Review of Risk-Informed Inservice Inspection of Piping (ML032510135).
4.0 TECHNICAL EVALUATION
The licensee has proposed to use an RI-ISI program as an alternative to the ASME Code,Section XI requirements. The licensee states that this proposed program was developed using the RI-ISI methodology described in the EPRI-TR. The NRC staffs SE of October 28, 1999 (ML013470102), approving the methodology described in the EPRI-TR, concluded that the methodology conforms to guidance provided in RGs 1.174 and 1.178, and that no significant risk increase should be expected from the changes to the ISI program resulting from applying the methodology.
4.1 Proposed Changes to the ISI Program The licensee has proposed to use a RI-ISI program for ASME Class 1 and 2 piping (Examination Categories B-F, B-J, C-F-1, and C-F-2 welds) as an alternative to the ASME Code,Section XI requirements. The Code requires, in part, that for each successive 10-year ISI interval, 100 percent of Category B-F welds and 25 percent of Category B-J welds for the Code Class 1 nonexempt piping be selected for volumetric and/or surface examination, based on existing stress analyses and cumulative usage factors. For Category C-F welds, 7.5 percent of nonexempt welds are selected for volumetric and/or surface examination.
The scope of the licensees proposed RI-ISI program is limited to ASME Class 1 and Class 2 piping welds for the following Examination Categories: B-F for pressure retaining dissimilar metal welds in vessel nozzles, B-J for pressure retaining welds in piping, C-F-1 for pressure retaining welds in austenitic stainless steel or high alloy piping, and C-F-2 for pressure retaining welds in
carbon or low alloy steel piping. The RI-ISI program is proposed as an alternative to the existing ISI requirements of the ASME Code,Section XI. A general description of the proposed changes to the ISI program is provided in Sections 3 and 5 of the letter dated December 4, 2007.
The submittal follows the NRC staff approved RI-ISI process and methodology delineated in the EPRI-TR. By assessing piping failure potential and piping failure consequences, and performing probabilistic risk assessment (PRA) and safety significance ranking of piping segments, the number of inspection locations is significantly reduced. However, the program retains the fundamental requirements of the Code, such as inspection methods, acceptance guidelines, pressure testing, corrective measures, documentation requirements and quality control requirements. Thus, ISI program requirements of other nonrelated portions of the ASME Code,Section XI are unaffected.
The licensee stated that the augmented ISI program for flow accelerated corrosion (FAC) implemented in response to NRC Generic Letter 89-08, Erosion/Corrosion - Induced Pipe Wall Thinning, is not changed by the RI-ISI program. Additionally, augmented inspection programs have been implemented for certain piping welds with multiple weld repairs and for Alloy 600 welds in accordance with NRC Order EA-03-009 and Business Practice BP-257, Integrated Material Issues Management Plan. The licensee states that while some of these welds were selected for examination under the RI-ISI program, the examinations under these augmented programs will continue.
4.2 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 EPRI-TR states that industry and plant-specific piping failure information, if any, is to be utilized to identify piping degradation mechanisms and failure modes (i.e., failure potential), and consequence evaluations are to be performed using probabilistic risk assessments to establish piping segment safety ranking for determining new inspection locations.
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 EPRI TR-112657. The degradation mechanisms identified in the submittal include thermal fatigue including thermal stratification, cycling and striping (TASCS) and thermal transients, intergranular stress corrosion cracking, and FAC. The licensee stated in Section 2.2 that the augmented inspection program for FAC is relied upon to manage this mechanism and is not changed by the RI-ISI program. The NRC staff finds that this process and these degradation mechanisms are consistent with the methodology in the ERPI-TR and are, therefore, acceptable.
The licensee reported a deviation from the methodology in the EPRI-TR related to the failure potential assessment. Additional considerations for determining the potential for thermal fatigue as a result of the effects of TASCS provided the licensee an allowance for the consideration for cycle severity. The additional considerations include: TASCS resulting from turbulent penetration at branch piping connections, low flow TASCS, valve leakage TASCS and convection heating TASCS. The methodology for assessing the susceptibility of piping to TASCS used by the licensee is consistent with the methodology provided by EPRI in letters dated February 28, 2001, and March 28, 2001 (ML010650169 and ML011070238). The NRC staff finds these considerations to be appropriate for determining the potential for TASCS and, thus, acceptable for use.
The licensee states that the consequences of pressure boundary failures were evaluated and ranked based on their impact on core damage and containment performance (isolation, bypass, and large early release). Also, the licensee indicated that impact on the above measures due to both direct and indirect effects was considered using the guidance provided in the EPRI-TR. The staff finds that the consequence measures used and the effects of the failure that the licensee used are consistent with the methodology in the ERPI-TR and are, therefore, acceptable.
The licensee reported a deviation from the approved consequence evaluation described in the EPRI-TR. Instead of defining piping segments as continuous runs of piping that had the same consequences and degradation mechanisms, the licensee defined segments as continuous runs of piping that had the same consequence. The license introduced the concept of risk groups to bring the degradation mechanism appropriately into the evaluation. RG 1.178 states that the definition of piping segments is acceptable if a sound engineering and accounting record is maintained and can be applied to an engineering analysis in a consistent and sound process. In a letter dated December 11, 2008, the licensee clarified that risk groups are groups of all inspection elements (welds) within a single system that have the same risk category, and are all potentially subject to the same degradation mechanism. Figure 3-2 in the EPRI TR-112657, combines the consequence category and degradation mechanism to place all welds into seven risk categories. The minimum percentage of welds that should be inspected is defined for the risk categories. The change in risk calculations are based on changes to inspection locations within the risk categories. The licensee's further subdivision of a risk category into groups with the same degradation mechanism should not affect the population of welds in the final risk categories. The licensee's methodology is an accounting method that will place the same population of welds in each of the risk categories as the approved method and is, therefore, acceptable.
4.3 Probabilistic Risk Assessment TVAs letter dated December 4, 2007, stated that the WBN Unit 1 probabilistic safety assessment (PSA) model, Revision 4, was used to evaluate the consequences of pipe failures for the purpose of the RI-ISI program. A Westinghouse Owners Group PRA Peer Review Team reviewed Revision 3 to the WBN Unit 1 PSA. Revision 4 to the PSA model was performed to incorporate changes to the model needed in support of the Mitigating System Performance Index program. These changes included resolving the category A & B Facts & Observations (F&Os) from the peer review that impacted internal events; incorporating failure and unavailability data collected in support of the Maintenance Rule program; incorporating comments on the systems analysis made by plant personnel; and changes to permit the calculation of Fussel-Vesley importance values for certain maintenance alignments and components. The licensee reviewed the remaining open findings in the PSA open item data base and determined they did not have an impact on the RI-ISI analysis.
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 core damage frequency (CDF) and large early release frequency (LERF). Inaccuracies in the models or assumptions that are large enough to invalidate the broad categorizations developed to support the RI-ISI should have been identified in the licensees review. Minor errors or inappropriate assumptions will only affect the consequence categorization of a few segments and will not invalidate the general results or conclusions. Therefore, the NRC staff finds that the quality of the PRA is sufficient to support the submittal.
As required by Section 3.7 of the EPRI-TR, WBN Unit 1 evaluated the change in risk expected from replacing the ASME ISI program with the RI-ISI program. The calculations estimated the change in risk due to removing locations and adding locations to the inspection program. The expected change in risk was quantitatively evaluated using the Simplified Risk Quantification Method described in Section 3.7.2 of the EPRI-TR. For high consequence category segments, the licensee used the conditional core damage probability (CCDP) and conditional large early release probability (CLERP) based on the highest estimated CCDP and CLERP. For medium consequence category segments, bounding estimates of CCDP and CLERP were used. The licensee estimated the change in risk using bounding pipe failure rates from the EPRI methodology.
The licensee performed their bounding analysis with and without taking credit for enhanced inspection effectiveness due to an increased probability of detection. The quantitative results are reported in the letter dated December 4, 2007, with a minor correction reported in the letter dated December 11, 2008. The results indicate that the aggregate change in all CDF and LERF estimates is expected to be a decrease in risk. All system level increases in CDF and LERF are less than the acceptance guidelines in EPRI-TR.
The NRC staff finds the licensees process to evaluate and bound the potential change in risk 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 effects of enhanced inspection. System level and aggregate estimates of the changes in CDF and LERF are less than the corresponding guideline values in the EPRI-TR.
The staff finds that re-distributing 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 use the approved methodology and on the reported results, the staff finds that any change in risk associated with the implementation of the RI-ISI program is small and meets the guidelines of RG 1.174, and is, therefore, acceptable.
4.4 Integrated Decisionmaking As described in the licensees submittal, 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 using the results of the risk category rankings and other operational considerations are described in Section 3.5 of the submittal. Table 5-1 of the submittal provides the number of locations and inspections by risk category for the various WBN Unit 1 systems. Table 5-2 provides a table comparing the number of inspections required under the existing ASME Code,Section XI ISI program with the alternative RI-ISI program for WBN Unit 1. The degradation mechanisms and risk impact analysis results for each system for WBN Unit 1 are summarized in Table 3.6. The licensee used the methodology described in the EPRI-TR to guide the selection of examination elements within high and medium risk ranked piping segments. Based on the NRC staffs review of the above tables (containing the results of element selection), the staff concludes that the element selection results are consistent with the process described in the EPRI-TR guidelines.
Therefore, the NRC staff concludes, based on its review, the licensees selection of element locations, which includes consideration of degradation mechanisms in addition to those covered by augmented inspection programs, is acceptable.
4.5 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, the NRC must determine that the licensee has provided an adequate level of quality and safety.
The licensee stated 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, such as inspection methods, acceptance guidelines, pressure testing, corrective measures, documentation requirements, and quality control requirements would be retained.
The licensee stated in Section 4 of the submittal 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. The submittal also states that, as a minimum, risk ranking of piping segments will be reviewed and adjusted on an ASME ISI period basis and that significant changes may require more frequent adjustment as directed by NRC bulletin or generic letter requirements, or by industry and plant-specific feedback.
In Section 3.5.1 of the licensees submittal, TVA presented the criteria for engineering evaluation and additional examinations if unacceptable flaws or relevant conditions are found during examinations. The submittal stated that the evaluation will include whether other elements in the segment or segments are subject to the same root cause conditions. Additional examinations will be performed on those elements with the same root cause conditions or degradation mechanisms. The additional examinations will include high-risk significant elements and medium-risk significant elements, if needed, up to a number equivalent to the number of elements required to be inspected on the segment or segments during the current outage. The submittal further states that if unacceptable flaws or relevant conditions are again found similar to the initial problem, the remaining elements identified as susceptible will be examined. In the letter dated December 11, 2008, the licensee clarified that the additional examination sample would be examined during the current outage and, if unacceptable flaws or relevant conditions are identified in the first additional examination sample, the remaining elements identified as susceptible would also be examined during the current outage. The NRC staff finds this approach to be acceptable since the additional examinations, if required, will be performed during the outage that the unacceptable flaws or relevant indications are identified.
The proposed periodic reporting requirements meet existing ASME Code requirements and applicable regulations and, therefore, are considered acceptable. The NRC staff finds that the proposed process for RI-ISI program updates meets the guidelines of RG 1.174, which provide that risk-informed applications should include performance monitoring and feedback provisions; therefore, the licensees proposed process for program updates is acceptable.
5.0 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. The licensee has proposed as alternative developed using the risk-informed process described in the NRC-approved EPRI-TR. As discussed in Section 4.0 above, the NRC staff concludes that the licensees proposed RI-ISI program, as described in its submittal, will provide an acceptable level of quality and safety pursuant to 10 CFR 50.55a with regard to the number of inspections, locations of inspections, and methods of inspections.
The NRC staff finds that the results of the different elements of the engineering analysis are considered in an integrated decisionmaking 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 1.178 guidelines. The WBN Unit 1 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 VT-2 examination of piping structural elements will continue to be performed on all Class 1 and 2 locations in accordance with the ASME Code,Section XI program. The RI-ISI program applies the same performance measurement strategies as existing ASME Code requirements and, in addition, increases the inspection volumes at weld locations that are exposed to thermal fatigue.
The WBN Unit 1 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 because the methodology provides reasonable confidence that any reduction in existing inspections will not lead to degraded piping performance when compared to existing performance levels. Safety margins are maintained because no changes to the evaluation of design basis accidents in the final safety analysis report are being made by the RI-ISI process.
WBN reported two deviations from methodology approved in the EPRI-TR. In the first deviation, the licensee assessed the susceptibility of piping to TASCS in a manner that is consistent with the methodology provided by EPRI in letters dated February 28 and March 28, 2001, instead of in the EPRI-TR. As described above, the NRC staff concluded that these considerations are appropriate for determining the potential for TASCS and, thus, acceptable for use. In the second deviation, the licensee defined piping segments as continuous runs of piping that had the same consequences instead of defining piping segments as in the EPRI-TR as continuous runs of piping that had the same consequences and degradation mechanisms. As described above, the NRC staff finds that the licensee's methodology is an appropriate accounting method that will place the same population of welds in each of the risk categories as the approved method in the EPRI-TR and is, therefore, acceptable.
The NRC staffs review of the licensees proposed RI-ISI program concludes that the program is an acceptable alternative to the current ISI program, which is based on ASME Code,Section XI, requirements for Class 1 and Class 2 welds. Therefore, the licensees RR 1-RI-ISI-03 is authorized pursuant to 10 CFR 50.55a(a)(3)(i) on the basis that the request provides an acceptable level of quality and safety.
This safety evaluation authorizes application of the proposed RI-ISI program during the second 10-year ISI interval for WBN Unit 1.
Principal Contributors: Stephen C. Dinsmore Carol A. Nove Date: January 22, 2009