Joint EPRI MRP/PWR Owners Group Baffle-Former-Bolt Focus Group · 2017-05-23 · – Ginna: 2-loop, Downflow, Type 347SS (total of 728 BFBs) 9% UT Indications (Of these, 14 were sent

© 2017 Electric Power Research Institute, Inc. All rights reserved.

Industry-NRC Materials Exchange MeetingRockville, MDMay 23, 2017

Joint EPRI MRP/PWR Owners Group Baffle-Former-Bolt Focus Group

Update

2© 2017 Electric Power Research Institute, Inc. All rights reserved.

Westinghouse Baffle-Former Assembly Details

Source: ML15331A179

Baffle

Baffle Plate Edge Bolt (Baffle-to-Baffle Bolts)

Baffle-Former Bolt (Long & Short)

Corner Edge Bracket Baffle to

Former Bolt

Core Barrel to Former Bolt

Core BarrelFormer

Core barrel, baffle and former plates– Type 304 austenitic stainless steel material

Baffle-Former Bolts (BFBs)– Attach the baffle plates to the former plates in

the reactor lower internals assembly– Type 347 or Type 316 cold worked austenitic

stainless steel material– Bolt head designs and shank lengths vary

from plant-to-plant, which can challenge the NDE technique

Source: ML15331A179


Westinghouse Coolant Flow Configurations

Source: ML073190376

Large Differential Pressure (∆P) Across the Baffle Plate -> greater BFB bolt loads

May cause baffle jetting

Small Differential Pressure (∆P) across the baffle plate -> lower BFB bolt loads

Alleviates baffle jetting

*Figure shows modifications made for Upflow Conversion


Operating Experience Overview

Note: UT deployed as it became available and qualified for the various sites

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

DC Cook2 finds degraded bolts by visual inspection

First UT baffle-former bolts (BFB) inspections in French

PWR CP0 units and first cracks found

Indian Point 2, Salem 1,

DCCook2 find degraded bolts

(visual+UT)

Ginna performs first MRP-227

inspectionsFirst degraded baffle-former bolts

found in U.S.

WCAP-13266: BFB Program for the

Westinghouse Owners Group - Plant

Categorization

NRC Information Notice 98-11

on BFBs MRP publishes assessment of

French BFB OE (MRP-03)

NRC reviews & approves MRP-227

Westinghouse Technical Bulletin TB-12-5, related to the DC Cook OE

MRP publishes Reactor Internals

Inspection Guidelines (MRP-227)

Operating Experience

Guidance

NSAL-16-1AREVA CSB-16-02

Interim Guidance

Per MRP-227-A, BFB UT exam is performed for WEC plants initially at 25-35EFPY and repeated every 10-yearsNote: UT deployed as it becomes available and qualified for the various sites


Joint Owners Baffle Bolt Program (15-22 EFPY) Sponsored Inspections of four plants (1998-2000)

– Ginna: 2-loop, Downflow, Type 347SS (total of 728 BFBs) 9% UT Indications (Of these, 14 were sent for metallurgical examination. Results showed no

indications of cracking, so this 9% likely contains a number of false calls) Partial replacement program (56)

– Point Beach Unit 2: 2-loop, Upflow (converted), Type 347SS (total of 728 BFBs) 8% UT Indications Partial replacement program (175)

– Farley Unit 1: 3-loop, Upflow (converted), Type 316SS (total of 1088 BFBs) No UT Indications Proactive replacement of minimum pattern (276)

– Farley Unit 2: 3-loop, Converted Upflow (downflow at time of inspection), Type 316SS (total of 1088 BFBs) No UT Indications Proactive replacement of minimum pattern (203)

Based on work at the time, inspection results and metallurgical exams of bolts removed during this program led to conclusion that BFB degradation was not a concern for the original plant operating period and that this could be addressed by an aging management program for license renewal (MRP-227-A).


Westinghouse NSSS MRP-227-A BFB Inspections2010 through 2016 (excluding IP2, SAL1, DCCook2)

Ginna: 2-Loop, Downflow, 347SS – 2nd Inspection (2011)(partial inspection of 123

original bolts and 56 replacement bolts): one additional UT Indication (Partial Replacement of an additional 25 bolts)

Point Beach Unit 1: 2-Loop, Upflow(converted), 347SS – 1st Inspection (2013): No UT Indications

Point Beach Unit 2: 2-Loop, Upflow(converted), 347SS – 2nd Inspection (2014): 2% Additional UT

Indications Prairie Island 1: 2-Loop, Downflow, 347SS

– 1st Inspection (2014): 6% UT Indications Prairie Island 2: 2-Loop, Downflow, 347SS

– 1st Inspection (2013): 10% UT Indications

2-Loop plants currently have 2 to 10% UT indications

Surry Unit 1: 3-Loop, Downflow, 347SS– 1st Inspection (2010): <1% UT Indications

Surry Unit 2: 3-Loop, Downflow, 347SS– 1st Inspection (2011): <1% UT Indications

Robinson: 3-Loop, Downflow, 347SS– 1st Inspection (2013): <1% UT Indications

Turkey Point 3: 3-Loop,Downflow, 347SS– 1st Inspection (2015 - partial inspection of 305

bolts out of 1088): No UT Indications North Anna 1, 3-loop, Upflow (converted),

316SS– 1st Inspection (2016): <1% UT indications

3-Loop plants currently have <1% UT indications


B&W NSSS and International Plant Results Crystal River Unit 3, Type 304SS (2005)

– No relevant UT indications - UT performed due to visual indication from baffle-to-baffle bolts (aka ‘WEC edge-bolts’)

Oconee Unit 1, Type 304SS (2012)– No relevant UT indications - Four BFBs uninspectable due

to large welds on locking bars

Oconee Unit 2, Type 304SS (2013)– No relevant UT indications - One BFB uninspectable due to

UT probe not seating properly

Oconee Unit 3, Type 304SS (2014)– One BFB identified with crack-like indications - One BFB

uninspectable due to UT probe not seating properly

ANO Unit 1, Type 304SS (2016)– No relevant UT indications

Doel 1: 2-Loop Downflow, Type 316SS– 1st Inspection: No relevant UT indications (1991)– 2nd Inspection (2005) and 3rd Inspection (2015): 2% UT

Indications (replaced 9 bolts in 2015) Doel 2: 2-Loop Downflow, Type 316SS

– 1st Inspection (2006) and 2nd Inspection (2015): <1% UT Indications (replaced 7 bolts in 2015)

Krsko: 2-Loop, Downflow (prior to inspection), Type 316SS– 1st Inspection: <1% UT Indications (2013)

Tihange 1: 3-Loop, Upflow (converted), Type 316SS– 960 of 1088 bolts inspected in each of the following inspections– 1st Inspection: 4% UT Indications (1995)– 2nd Inspection: 3% UT Indications (2002)– Most recent Inspection: No relevant UT Indications (5 bolts either

not inspectable or not interpretable) (2014) Ringhals 2: 2-Loop Upflow (converted), Type 316SS

– 1st, 2nd, 3rd exams: <1% UT Ind., (2000, 2008, 2015) Ringhals 3: 3-Loop Downflow, Type 316SS

– 1st and 2nd Inspections: <1% UT Indications/uninspectable (2000 and 2007)

– 3rd Inspection: <1% UT Indications/uninspectable (2016) Ringhals 4: 3-Loop Upflow with cooling holes, Type 316SS

– 1st inspection: <1% UT Ind. (2010, next exam in 2019)


Observations from Broader OE Excluding the OE at DC Cook Unit 2, Indian Point Unit 2+3, and Salem Unit 1

(discussed later in the presentation), the following observations can be made based on inspection OE gathered to date from international and domestic plants:– Bolts with UT indications tend to be randomly distributed– Findings are consistent with expectations of IASCC failures and fluence effects near belt-line of

core– Quantity and distribution of bolts with indications are bounded by historical generic safety

assessment generated in mid-1990s (documented in PWROG report WCAP-15328) Future UT inspection findings may be driven by plant-specific parameters


DC Cook Unit 2 (10/2010 / 22 EFPY) (4-Loop Downflow) Fuel failure in peripheral assembly from fretting wear at broken bolt head Bolt heads and lock bars found on lower core plate after fuel removal Visual inspections revealed 18 degraded bolts on wide baffle plate

– Additional bolts removed from plate with visual indications to define extent of localized degradation (approx. 40 bolts in single patch)

– Additional test bolts removed from symmetrical locations to evaluate potential for degradation on other plates (all of these test bolts were found to be intact)

No UT inspections performed in 2010 (at that time UT was not qualified or optimized for the Cook 2 bolt design) Degraded and test bolts replaced (total of 52 bolts and 2 empty holes)

– Performed testing on some removed bolts in hot cell in 2011Westinghouse issued Technical Bulletin TB-12-5, dated 3/7/2012

– Failures may have resulted from crack propagation due to combination of IASCC and fatigue

100% Visual VT-3 inspection conducted in 2012 with no indications


Indian Point Unit 2 (3/2016 / 31 EFPY) (4-Loop Downflow) Degraded bolts/lock bars noted visually prior to planned MRP-227 100% UT exams Markings on periphery of neighboring fuel assembly identified (no fuel failure) Inspections identified 227 BFB with visual or UT indications (includes 14 uninspectable)

UT indications were clustered– Spanned various quadrants, mostly in upper region of core baffles– Multiple groups of 10+ adjacent failures / At least one cluster of 50+ adjacent failures

Observed failures exceed WCAP-17096-NP-A engineering acceptance criteria Site-specific response was needed

– Performed Acceptable Bolting Pattern Analysis (ABPA) and Replacement Bolting Pattern Analysis– Performed baffle-former bolt removal and replacement (278), including ‘anti-cluster’ bolts– Quarantined and shipped select bolts for testing in hot cell


Salem Unit 1 (4/2016 / 28 EFPY) (4-Loop Downflow) Conducted visual exams every other refueling outage in response to DC Cook Unit 2 OE from

2010 and the resulting TB-12-5; MRP-227 UT exams were not originally planned until 2017 Degraded bolts/lock bars noted in visual exams followed by doing 100% UT exams Loose/protruding bolt heads resulted in fuel fretting and one fuel clad failure Inspections identified 182 BFB with visual degradation or UT indications (includes 18 uninspectable) UT indications were clustered

– More concentrated (than Indian Point 2) to a few adjacent octants– Multiple groups of 10+ adjacent failures / At least one cluster of 50+ adjacent failures

Observed failure pattern exceeds WCAP-17096-NP-A engineering acceptance criteria Site-specific response was needed

– Performed Acceptable Bolting Pattern Analysis (ABPA) and Replacement Bolting Pattern Analysis– Performed baffle-former bolt removal and replacement (189)– Quarantined and shipped select bolts for testing in hot cell


DC Cook Unit 2 (11/2016 / 28 EFPY) (4-Loop Downflow) Two (2) on-line fuel leaks identified during the last fuel cycle associated with two (2) empty bolt-

holes from bolts that were not replaced in 2010 (suspected damaged by jetting thru vacant hole) Inspections identified 179 BFB with visual degradation or UT indications (includes 9 that were not

inspectable, and three (3) with visibly degraded lock-bar welds)– Includes six (6) replacement BFBs from 2010 event that exhibit UT indications (first-of-a-kind finding)

– Five (5) Baffle-Edge-Bolts on one seam appear visually failed (first-of-a-kind finding)

UT indications were clustered– Spanned various quadrants– Multiple groups of 10+ adjacent failures / At least one cluster of 50+ adjacent failures

Observed failure pattern exceeds WCAP-17096-NP-A engineering acceptance criteria Site-specific response was needed

– Performed Acceptable Bolting Pattern Analysis (ABPA) and Replacement Bolting Pattern Analysis– Performed baffle-former bolt removal and replacement (201)– Quarantined and shipped several bolts for testing in hot cell


Indian Point Unit 3 (3/2017 / 28.6 EFPY) (4-Loop Downflow) Inspections identified 259 BFB with UT indications (includes 3 that were not inspectable)

– No visual evidence of missing or protruding BFB heads or lock-bars– No visual evidence of degradation of edge bolts

UT indications were clustered– Spanned various quadrants– Multiple groups of 10+ adjacent failures / At least one cluster of 30+ adjacent failures

Observed indication pattern exceeds WCAP-17096-NP-A engineering acceptance criteria Site-specific response was needed

– Performed Acceptable Bolting Pattern Analysis (ABPA) and Replacement Bolting Pattern Analysis– Performed baffle-former bolt removal and replacement (270)– Quarantined several bolts for potential future testing in hot cell if deemed necessary


Salem Unit 2 (4/2017 / 25.4 EFPY) (4-Loop Downflow) Inspections identified 9 BFB with UT indications, all of the BFBs were UT inspectable

No evidence of clustered indications

Observed indication pattern meets existing WCAP-17096-NP-A engineering acceptance criteria

Site-specific response considered pro-active BFB replacements for additional margin– Performed Acceptable Bolting Pattern Analysis (ABPA) and Replacement Bolting Pattern Analysis– Performed baffle-former bolt removal and replacement (129 BFBs)


Conclusions from Recent OE These five US plants share a common plant design configuration (4-loop

downflow), bolt design (int.hex+lock bar), and material (347SS) – Tier 1a Assessing impact of new findings from fall 2016 DC Cook 2 exams:

– Replacement 316 CW BFBs (6) from 2010 event with UT indications, were found broken upon removal

– Visually degraded edge bolts (5) on one panel, in the center of a large area/cluster of BFB failures (left in-place; two adjacent fuel assemblies were hardened with solid stainless pins)

Bolts with visual and/or UT indications tend to be ‘clustered’ together Distributions seem to indicate the presence of a mechanism causing adjacent

bolts to become more susceptible to failure (i.e., ‘unzippering’) The latest inspection results from Spring 2017 are still being evaluated


NDE (UT and visual) exams are finding defective bolts

Bolt removal findings validate significant quantities of broken bolts and loose bolts (based on torque wrench)

BFB Exam/Removal Experiences from Recent Outages


Industry BFB FG Immediate Actions Completed

Supported presentation to NSIAC on 5/23/2016– Westinghouse Technical Bulletin TB-12-5 remains validProvided Industry Alert Letter from the PMMP Chairman to PWR site

VPs on 6/1/2016 (MRP-2016-014)– NEI 03-08 Interim Guidance will require the 4-loop plants identified in the Westinghouse TB-

12-5 bulletin to perform UT inspections of all the BFBs or replace an acceptable pattern of bolts at their next outage.

– Consideration should also be given to proceeding with procurement of replacement bolts prior to issuance of interim guidance due to potentially long manufacturing lead times.

Westinghouse NSAL 16-1 issued 07/05/16 and revised 08/01/16AREVA Customer Service Bulletin issued 07/14/16 Industry technical exchange meeting with NRC staff 7/19/2016Supported ACRS metallurgy subcommittee meeting 11/16/2016


Intermediate Term Industry BFB FG Actions Completed

Issued NEI 03-08 “Needed” Interim Guidance regarding BFB exams as identified in Westinghouse NSAL 16-1– Tier 1 plants provided interim guidance in MRP-2016-021, dated 7/25/2016 – Remaining plants provided interim guidance in MRP-2017-009, dated 3/15/2017– Industry team reviewed interim guidance with NRC in public meeting on 4/12/2017– Additional technical discussions with NRC staff are tentatively planned for summer 2017

Assessed 2017 and 2018 outage seasons for developing contingency plans for tooling and BFB material needs– Spring 2017: 3 UT inspections (Tier 1a plants) and 1 VT-3 inspection (Tier 1b plant)– Fall 2017: 2 planned UT inspections (Tier 1a plants), one which is a re-examination – CY2018: 5 planned UT inspections (1 Tier 1a plant, 2 Tier 1b plants and 2 Tier 2b plant)

Performing Hot Cell Post Irradiation Examinations of BFBs– Microscopic examinations and mechanical tests nearing completion


Objectives of Hot Cell Testing

What is the material condition of the bolt?• Chemical composition• Radionuclide analysis

What was the condition of the bolt at the onset of failure?• Tensile testing of machined specimens• Whole bolt pull testing

Which mechanisms contributed to the failure of the bolts?• Optical and scanning electron

microscopy

What is the correlation between material condition and failure process?• Cross-sectional metallography

Metallurgical testing was conducted on bolts from Indian Point 2 and Salem 1 to answer the following questions:


Summary of BFB Hot Cell Testing ResultsMechanical / Chemical Properties

Chemical compositions of the bolts are consistent with the CMTRs for the 347SS heats Measured values of dose (dpa) are

consistent with the calculated values and are in the range of 12 to 25 dpa(underside of the bolt heads) Yield stress, ultimate tensile stress,

and ductility for whole BFBs and also tensile specimens machined from bolts are consistent with MRP-211 data for irradiated Type 347 SS

Fractography Characteristics

Initiation is always by intergranular cracking - typical of IASCC Fracture surfaces can contain regions

of IG-SCC, TG-SCC, Fatigue, and Ductile Overload – Extent of IG cracking found to vary from 4% to

66%– Extent of cracking mechanisms varies bolt-to-bolt– No obvious correlation of extent of each cracking

mode with bolt location or expected dose. IASCC observed on anti-cluster bolts

that were removed and found to be in 2 pieces


BFB Metallurgical Testing Findings - Examples Exams of 347SS BFB fracture surfaces have found IASCC and fatigue:

(bolt identified as rejectable by UT exam)


BFB Metallurgical Testing Findings - Examples Exams of 347SS BFB fracture surfaces have found IASCC and fatigue:

(bolt identified as rejectable by UT exam)


BFB Hot Cell Testing – Crack Initiation Type 347SS BFB fracture surface

following pull testing (red, intact bolt) IASCC region is ~10% sectional area

which is consistent with the as-found UT signal indications from multiple channels


BFB Hot Cell Testing – Crack Propagation

IASCC• IASCC was seen on the

fracture surface of bolts that were removed in two pieces

• IASCC was also seen on the fracture surface of an IP “anti-cluster” green bolt

• In some cases, IASCC is only a few grains deep

Transgranular Cracking• TG and mixed-mode

cracking is apparent on many fracture surfaces

Fatigue• In some regions, fatigue

striations can be seen; although, the striations may be attributed to transgranular stress corrosion cracking

Ductile Overload• The fracture of the

remaining ligament shows retained ductility on most bolts consistent with tensile/pull test results


BFB Hot Cell Testing – Cross-section Metallography

Flowlines found in head-to-shank radius, along with aligned precipitationNo other microstructural features identified that might potentially lead

to invalid UT results (e.g., large inclusion or precipitates)


BFB Hot Cell Testing Summary

Hot cell results are consistent with prior findings from 1990s– Hot cell work has not identified a different crack initiation or crack

growth mechanism than predicted or previously seen– Defective or incorrect materials have not been observed for these

BFBs– Intact bolts with UT indications have maintained their load-carrying

capabilities– Metallography results have not identified significant microstructural

features that contributed to UT results


Inspection Tasks - Steps to Address NDE QuestionsModeling of BFB UT Techniques during Summer 2017– Enhancements to UT techniques have been made to improve reliability– Currently investigating geometric effects and beam mode conversion at

head-to-shank radius of internal hex bolts– Considering investigating impacts of variations in UT probe contact

surfaces from plant-to-plant– Considering investigating possibility of UT beam redirection due to flow

lines in microstructureConsider enhancements to BFB UT exam protocol for 2018

– Revising procedure qualification to require 90% probability of detection (POD)

– Enhance UT inspection personnel qualification for detecting flaws of various sizes and orientations


Industry Focus Group Assessment of NDE Technique

The internal-hex head geometries are known to be the most challenging to examine due to the presence of lock-bars and weldsUT methods were qualified and demonstrated to find a range

of cracks in qualification bolts under conditions simulating field exams– Uses underwater remote control delivery systems in irradiation field– Applicable to multiple head geometries but requires different probe

headsNDE experts are working to enhance UT methods


Together…Shaping the Future of Electricity

Documents

Joint EPRI MRP/PWR Owners Group Baffle-Former-Bolt Focus Group · 2017-05-23 · – Ginna: 2-loop, Downflow, Type 347SS (total of 728 BFBs) 9% UT Indications (Of these, 14 were sent