Inspection Credit for PWSCC Mitigation via Peening Surface Stress Improvement Glenn A. White, Kyle P. Schmitt, Kevin J. Fuhr, Markus Burkardt, and Jeffrey A. Gorman Dominion Engineering, Inc. Paul Crooker Electric Power Research Institute William Sims Entergy Nuclear Inc. EPRI International Light Water Reactor Materials Reliability Conference and Exhibition 2016 Chicago, IL, August 1-4, 2016
Outline Introduction Process for Acceptance of Inspection Relief MRP-335R3 Technical Basis for Inspection Relief Inspection Objectives Performance Criteria Inspection Requirements Deterministic Approach Probabilistic Approach Conclusions 2
Introduction Background on Peening Surface Stress Improvement Methods for PWRs Peening surface stress improvement (SSI) mitigates PWSCC by inducing compressive residual stress at the surface exposed to reactor coolant Initiation of PWSCC flaws requires tensile stress at the surface above a threshold Any existing flaws that are fully within the surface compressive normal plus operating stress zone cannot grow via PWSCC Peening provides an option to mitigate reactor vessel closure head penetration nozzles instead of replacing the entire head Peening provides an option to mitigate components that are not easily replaced or mitigated from outer surface using weld overlay or mechanical stress improvement (e.g., some reactor vessel inlet/outlet nozzles) 3
Introduction Experience with Peening in the Nuclear Power Industry Peening has been previously applied at Japanese BWRs (starting in 1999) numerous components in Japanese PWRs and WJP and LP have extensively been applied BWRs to core shrouds and bottom head Japanese PWRs (starting in 2001) penetrations (i.e., CRD stud tubes) At least 23 out of 24 PWR units have applied Applying to new ABWR units during the water jet peening (WJP) or laser peening fabrication and construction phases (LP) to bottom-mounted nozzles (BMNs) The abrasive water jet (AWJ) conditioning and/or reactor vessel inlet/outlet nozzle process has been widely used in the U.S. for dissimilar metal welds (DMWs) more than a decade in PWR nozzle repair WJP and LP have also been applied to applications reactor vessel safety injection nozzles Several hundred thousand steam generator Ultrasonic Shot Peening (USP) has been tubes have been shot peened over the past 30 applied to years, extending their useful life Steam generator inlet or outlet nozzles at Successful application of shot peening on Alloy more than 10 PWRs 600 pressurizer heater sheaths installed in 1990 Pre-service peening of 9 replacement at one PWR reactor vessel heads with Alloy 690 First WJP or LP application in the U.S. nozzles successfully completed in spring 2016 Peening applications at several additional PWR units are scheduled through 2017 4
Process for Acceptance of Inspection Relief ASME and Regulatory Requirements in U.S. Inspection requirements for Alloy 600/82/182 PWR primary system pressure boundary components are specified in ASME Boiler & Pressure Vessel Code Cases that are made mandatory with conditions by U.S. NRC regulations (10 CFR 50.55a): N-770-1: Alloy 82/182 piping dissimilar metal butt welds (DMWs) N-729-1: Reactor pressure vessel head penetration nozzles (RPVHPNs) N-722-1: Reactor vessel bottom-mounted instrumentation nozzles (BMNs) Inspection requirements identify Nondestructive exam (NDE) inspection methods Inspection frequency Inspection coverage Flaw acceptance standards 5
Process for Acceptance of Inspection Relief Regulatory Process in U.S. Asset Management (10 CFR 50.59 Process) Licensees may make changes to the facility and procedures and conduct tests and experiments without prior NRC approval If change requires no license or technical specifications modifications If change does not meet one or more of the eight criteria specified in 10 CFR 50.59(c)(2) No license amendment request required This process does not grant inspection relief NRC has confirmed that ASME Code requirements do not prohibit peening for purpose of surface stress improvement Alternative Inspection Intervals (10 CFR 50.55a Process) For NRC approved and regulationmandated programs (e.g. in-service inspection programs) separate from the license 10 CFR 50.55a specifies the processes for requesting alternatives to, or relief from, the in-service inspection and testing requirements of the ASME Code Licensees may submit relief requests to the NRC NRC reviews and approves relief requests by Safety Evaluations (SE) 6
Process for Acceptance of Inspection Relief Development of ASME Code Requirements and U.S. NRC Review of Topical Report Parallel paths were taken for acceptance of inspection relief 1. ASME Section XI Code Cases for peening inspection credit 2. NRC Safety Evaluation (SE) based on NRC review of peening topical report Revised ASME Section XI Code Cases include performance criteria and inspection intervals for components mitigated by peening: DMWs per Code Case N-770-4, approved by ASME May 7, 2014 RPVHPNs per Code Case N-729-5, approved by ASME October 7, 2015 These revised code cases have not yet been approved by U.S. NRC and incorporated by reference in 10 CFR 50.55a See ASME Paper PVP2016-64008 (D. Weakland, et al.) The industry s topical report (MRP-335R3) was submitted to NRC for review Specifies program of pre-peening, follow-up, and long-term inservice inspection (ISI) exams Submitted to NRC February 19, 2016 Upon NRC approval, the topical report (with SE) will be cited in relief requests submitted by individual licensees as the basis for inspection credit for peening processes demonstrated to meet a set of performance criteria The topical report (with SE) does not automatically grant inspection relief for individual plants 7
EPRI Materials Reliability Program Technical Documentation EPRI MRP has prepared multiple documents in support of PWSCC mitigation by surface stress improvement (peening) MRP-267R1 Technical Basis for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement Provides background on peening methods and the technical basis for effectiveness of peening as a mitigation method Includes extensive data generated by peening vendors as well as confirmatory testing sponsored by EPRI Freely downloadable at www.epri.com, Product ID # 1025839 MRP-335R3 Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement Supports acceptance of peening as a mitigation method, including appropriate extension of the required inspection intervals following mitigation if performance criteria are met Freely downloadable at www.epri.com, Product ID # 3002007392 MRP-336R1 Specification Guideline for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement Provides guidance to utilities regarding items that should be addressed in detail by the utility and vendor 8
Inspection Objectives for Peened Components Inspection requirements for unmitigated Alloy 600/82/182 components were developed to maintain an acceptably low effect on nuclear safety of PWSCC: MRP-110, MRP-117, MRP-105, MRP-113, MRP-139R1, and MRP-206 These inspection requirements also result in low probability of through-wall cracking and leakage, ensuring defense in depth The goal of MRP-335R3 was to develop inspection requirements and performance criteria for peened components that maintain this acceptably low effect on nuclear safety As shown by probabilistic analyses, the requirements of MRP-335R3 actually result in an increased nuclear safety margin, plus a large reduction in the probability of leakage occurring The leakage prevention benefit of peening performed in accordance with MRP-335R3 is further demonstrated through a matrix of deterministic crack growth cases Peening mitigation implemented in accordance with the requirements of MRP-335R3 provides a substantial risk benefit for a risk that is already low 9
MRP-335R3 Performance Criteria MRP-335R3 (Section 4.2 DMWs, Section 4.3 RPVHPNs) defines the following peening performance criteria: Peening coverage Surface stress magnitude including operating stress Compressive residual stress depth Sustainability Inspectability Lack of adverse effects NDE Qualification Peening vendors are required to establish essential variables and associated ranges of acceptable application-specific values Part of the controlled special process procedures submitted for licensee pre-implementation approval Vendor qualification tests demonstrate that the MRP performance criteria are met Acceptable ranges of essential variables will ensure that specified performance criteria are met Essential variables are documented in process qualification and post-implementation reports 10
MPR-335R3 Inspection Requirements for Peened DMWs and RPVHPNs (Tables 4-1 and 4-3) 11
Matrix of Deterministic Crack Growth Cases Approach A matrix of deterministic crack growth cases that model growth to through-wall penetration and leakage is included in MRP-335R3 Cases model growth for range of initial flaw depths up to the NDE detectability limit at time of peening Flaws at least as deep as the NDE detectability limit are detectable during the follow-up and ISI exams Vary initiating location, orientation, operating temperature, crack growth rate material variability factor, initial crack aspect ratio, weld residual stress profile, and effective bending moment (DMWs) Matrix consists of 72 cases for mitigated components and 72 cases for unmitigated components, each for DMWs and RPVHPNs 12
Matrix of Deterministic Crack Growth Cases Inspection Schedule The inspection schedules shown below are applied to determine during which inspection a crack is predicted to be detected, or if the crack is predicted to lead to a leak Inspection Time (yr) Inspection Hot-Leg DMW Cold-Leg DMW Pre-Peening every 5 every 7 1st Follow Up 5 10 2nd Follow Up 10 N/A 1st ISI 20 20 2nd ISI 30 30 3rd ISI 40 40 4th ISI 50 50 5th ISI 60 60 6th ISI 70 70 Never Leaks 80 80 Inspection Time (yr) Inspection Hot Head Cold Head Pre-Peening min(riy=2.25, 8yr) 1st Follow Up 2 3 2nd Follow Up 4 N/A 1st ISI 14 13 2nd ISI 24 23 3rd ISI 34 33 4th ISI 44 43 5th ISI 54 53 6th ISI 64 63 7th ISI 74 73 Never Leaks 80 80 13
Matrix of Deterministic Crack Growth Cases Example DMW Cases Example Unmitigated DMW Cases (Table 5-8 through 5-10) Crack Weld MRP-115 Initial Growth Growth Time Aspect Total Total Orient. Detect. Residual A182 Initial Initial Aspect Time to from Detect Ratio at Length at Length at Case Axial/ Limit Stress CGR Temp. Depth Depth Ratio Bending Detect Limit Limit to Leak Detected/ Detect LimitDetect LimitDetect Limit Number Circ (%TW) Profile %ile ( F) (in.) (mm) (2c 0 /a 0 ) Moment (yr) (yr) Leaks (2c /a ) (in.) (mm) 49 - NP Axial 1.4% Low 5% 593 0.039 1.00 6.0 N/A 0.0 26.1 Detected 6.00 0.236 6.00 50 - NP Axial 1.4% Median 50% 605.5 0.039 1.00 8.0 N/A 0.0 4.5 Leaks 8.00 0.315 8.00 51 - NP Axial 1.4% High 95% 618 0.039 1.00 10.0 N/A 0.0 0.9 Leaks 10.00 0.394 10.00 52 - NP Circ 1.4% Low 5% 593 0.039 1.00 6.0 Base Case 0.0 28.8 Detected 6.00 0.236 6.00 53 - NP Circ 1.4% Median 50% 605.5 0.039 1.00 8.0 Base Case 0.0 6.4 Detected 8.00 0.315 8.00 54 - NP Circ 1.4% High 95% 618 0.039 1.00 10.0 Base Case 0.0 1.5 Leaks 10.00 0.394 10.00 Example Mitigated DMW Cases (Table 5-5 through 5-7) Crack Weld MRP-115 Initial Growth Growth Time Aspect Total Total Orient. Detect. Residual A182 Initial Initial Aspect Time to from Detect Ratio at Length at Length at Case Axial/ Limit Stress CGR Temp. Depth Depth Ratio Bending Detect Limit Limit to Leak Detection Detect Limit Detect Limit Detect Limit Number Circ (%TW) Profile %ile ( F) (in.) (mm) (2c 0 /a 0 ) Moment (yr) (yr) Time (2c /a ) (in.) (mm) 2 Axial 1.4% Median 50% 605.5 0.010 0.25 8.0 N/A 15.8 7.7 1st ISI 2.4 0.093 2.37 12 Circ 1.4% High 95% 618 0.010 0.25 10.0 High 3.1 1.7 Leaks before extension of interval 2.5 0.100 2.54 13 Axial 1.4% Low 5% 538 0.010 0.25 6.0 N/A 274.5 174.9 Never Leaks 1.9 0.076 1.94 29 Circ 1.4% Median 50% 605.5 0.020 0.50 8.0 Base Case 4.1 11.1 1st Follow Up 4.1 0.160 4.06 63 Axial 1.4% High 95% 559 0.039 1.00 10.0 N/A 0.0 6.4 Leaks 10.0 0.394 10.00 14
Matrix of Deterministic Crack Growth Cases Overall Results Never Leaks, Never Detected Detected in Follow- Up Exam Detected in ISI Exam Leaks Before Extension of Intervals Leaks After Extension of Intervals DMW Peened DMW No Peening RPVHPN Peened RPVHPN No Peening 10 of 72 0 of 72 28 of 72 10 of 72 31 of 72 N/A 30 of 72 N/A 20 of 72 48 of 72 12 of 72 52 of 72 8 of 72 0 of 72 24 of 72 3 of 72 2 of 72 10 of 72 All but one of the peened cases that result in leakage assume the combination of a high tensile weld residual stress profile, the highest operating temperature for their category, and 95 th percentile crack growth rate behavior There is a very low probability of cases like this occurring in practice 15
Matrix of Deterministic Crack Growth Cases Conclusions Frequency of cases with leakage is much reduced compared to unpeened components inspected per current requirements Large fraction of cases with peening show no leakage subsequent to extension of inspection intervals The long-term ISI exams address the residual risk of slowgrowing pre-existing flaws The MRP inspection requirements are effective to prevent leakage Deterministic matrix results are consistent with and complement the probabilistic results As for unmitigated heads, any J-groove weld cracking is addressed by the visual exams for leakage 16
Probabilistic Analyses Approach The DMW and RPVHPN probabilistic models applied in MRP-335R3 are similar to models that have been applied for more than 12 years to assess PWSCC: Inspection requirements to address PWSCC of Alloy 600 and Alloy 690 RPVHPNs (MRP-105, MRP-375, and MRP-395) Inspection requirements to address PWSCC of Alloy 600 BMNs (MRP-206) Assessment of depth-sizing uncertainty of flaws in large-diameter piping welds (MRP-373) Probabilistic analyses assess the benefit of peening on the probability of pressure boundary leakage or rupture assuming reduced frequency of inspection Component loading including effect of peening on residual stress field PWSCC crack initiation and growth Probability of PWSCC detection Various inspection options 17
Probabilistic Analyses Conclusions Follow-up and ISI exams address the possibility of growth of pre-existing PWSCC flaws that were not detected in the pre-peening exam Flaws tend to become more easily detectable as they grow in size The probabilistic analysis results are compared to acceptance criteria: Alloy 82/182 piping butt welds: Peening mitigation with the recommended inspection intervals results in a large reduction in the probability of leakage compared to no mitigation and standard intervals RPVHPNs: Peening mitigation with the recommended inspection intervals results in: an acceptably low nozzle ejection frequency a nozzle ejection frequency that is below that calculated for no mitigation and standard intervals a large reduction in the probability of leakage compared to no mitigation and standard intervals 18
Conclusions Advanced laser peening and water jet peening technologies have been successfully adapted for use in the nuclear power industry Peening has been demonstrated as a long-term mitigation method for PWSCC, without adverse effects Technical basis documents have been developed to support appropriate inspection relief for peened components This technical basis is now under regulatory review in the U.S. Peening for asset management has successfully been performed in the U.S. 19
Together Shaping the Future of Electricity 20