Inspection Credit for PWSCC Mitigation via Peening Surface ... · tubes have been shot peened over the past 30 years, extending their useful life Successful application of shot peening

© 2016 Electric Power Research Institute, Inc. All rights reserved.

Glenn A. White, Kyle P. Schmitt, Kevin J. Fuhr, Markus Burkardt, and Jeffrey A. Gorman

Dominion Engineering, Inc.

Paul CrookerElectric Power Research Institute

William SimsEntergy Nuclear Inc.

EPRI International Light Water Reactor Materials Reliability Conference and Exhibition 2016

Chicago, IL, August 1-4, 2016

Inspection Credit for PWSCC

Mitigation via Peening Surface

Stress Improvement

2© 2016 Electric Power Research Institute, Inc. All rights reserved.

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


IntroductionBackground 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)


IntroductionExperience with Peening in the Nuclear Power Industry

Peening has been previously applied at numerous components in Japanese PWRs and BWRs

Japanese PWRs (starting in 2001)

– At least 23 out of 24 PWR units have applied water jet peening (WJP) or laser peening (LP) to bottom-mounted nozzles (BMNs) and/or reactor vessel inlet/outlet nozzle dissimilar metal welds (DMWs)

– WJP and LP have also been applied to reactor vessel safety injection nozzles

– Ultrasonic Shot Peening (USP) has been applied to

Steam generator inlet or outlet nozzles at more than 10 PWRs

Pre-service peening of 9 replacement reactor vessel heads with Alloy 690 nozzles

Japanese BWRs (starting in 1999)

– WJP and LP have extensively been applied to core shrouds and bottom head penetrations (i.e., CRD stud tubes)

– Applying to new ABWR units during the fabrication and construction phases

The abrasive water jet (AWJ) conditioning process has been widely used in the U.S. for more than a decade in PWR nozzle repair applications

Several hundred thousand steam generator tubes have been shot peened over the past 30 years, extending their useful life

Successful application of shot peening on Alloy 600 pressurizer heater sheaths installed in 1990 at one PWR

First WJP or LP application in the U.S. successfully completed in spring 2016

Peening applications at several additional PWR units are scheduled through 2017


Process for Acceptance of Inspection ReliefASME 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


Process for Acceptance of Inspection ReliefRegulatory 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 regulation-

mandated 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)


Process for Acceptance of Inspection ReliefDevelopment 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


EPRI Materials Reliability Program Technical Documentation

EPRI MRP has prepared multiple documents in support of PWSCC mitigation by surface stress improvement (peening)

MRP-267R1Technical 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-335R3Topical 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-336R1Specification 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

http://www.epri.com/

http://www.epri.com/


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


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


MPR-335R3 Inspection Requirementsfor Peened DMWs and RPVHPNs (Tables 4-1 and 4-3)


Matrix of Deterministic Crack Growth CasesApproach

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


Matrix of Deterministic Crack Growth CasesInspection Schedule

Hot Head Cold Head

Pre-Peening

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

Inspection

Inspection Time (yr)

min(RIY=2.25, 8yr)

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

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


Matrix of Deterministic Crack Growth CasesExample DMW Cases

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

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 Unmitigated DMW Cases (Table 5-8 through 5-10)


Matrix of Deterministic Crack Growth Cases Overall Results

DMW –

Peened

DMW –

No Peening

RPVHPN –

Peened

RPVHPN –

No Peening

Never Leaks, Never

Detected10 of 72 0 of 72 28 of 72 10 of 72

Detected in Follow-

Up Exam31 of 72 N/A 30 of 72 N/A

Detected in ISI Exam20 of 72 48 of 72 12 of 72 52 of 72

Leaks Before

Extension of Intervals8 of 72

24 of 72

0 of 72

10 of 72Leaks After Extension

of Intervals3 of 72 2 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 95th percentile crack growth rate behavior

There is a very low probability of cases like this occurring in practice


Matrix of Deterministic Crack Growth CasesConclusions

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 slow-growing 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


Probabilistic AnalysesApproach

The DMW and RPVHPN probabilistic models applied inMRP-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


Probabilistic AnalysesConclusions

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


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.


Together…Shaping the Future of Electricity

Documents

Inspection Credit for PWSCC Mitigation via Peening Surface ... · tubes have been shot peened over the past 30 years, extending their useful life Successful application of shot peening