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Nathan PalmEPRI

Annual NRC-Materials MeetingNRC HQ, Rockville, MD

June 3, 2015

Core Shroud Boat Sample Evaluation

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Contents

BackgroundObjectivesPlanned TestingCompleted ActivitiesVisual Examination ResultsSectioning PlansAssessment of Test ResultsCurrent StatusCore Shroud Focus Group / Future BWRVIP Actions

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Background

Atypical cracking identified in a BWR core shroud during 2008 visual exam– Oriented perpendicular to the weld – Extension beyond heat affected zone into base metal and also across

weld in some cases.BWRVIP focus group convened to investigate cracking

– Identified the atypical cracking as potentially being irradiation assisted stress corrosion cracking (IASCC)

– Recommended further inspection using UT and removal of a boat sample

Further investigation performed in Spring 2014– UT examinations performed– Boat sample removed containing one atypical crack– Detailed presentation of inspection results provided at March 2015

Regulatory Information Conference

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Atypical Core Shroud Cracking - Example

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Boat Sample

Sample taken at a location ~9.5” counter-clockwise from V4 and ~1.5” above H4Captured flaw - 0.82” max

depthWeld, heat-affected zone,

and base metal capturedUtility removed boat sampleBWRVIP is funding testing

and evaluation of the boat sampleBoat sample evaluation

being performed by team ofSIA, B&W, and PNNL

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Objectives

Overall Objective: Determine whether the cracking is due to IASCC or to some other mechanismSpecific Objectives

– Determine the chemical composition of the base metal, weld metal and compare to specifications

– Determine the microstructure and grain boundary chemistry in the regions surrounding the cracks and compare to non-cracked regions

– Assess Pt deposition within crack– Determine the variation in hardness as a function of depth in order to

assess the possible role of cold work– Measure the fracture toughness and tensile properties in the base

metal– Estimate the neutron fluence for comparison to code predictions

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Planned Testing

Visual Inspections and PhotographyOptical MetallographyAutomated MicrohardnessSEM/EDS ExaminationsTensile TestingFracture Toughness TestingDosimetry AnalysisChemical Analysis by ICP/MSFEG-SEM and EBSD CharacterizationATEM Characterization of Base and Weld MaterialsATEM Characterization of Crack Tips

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Completed Activities

Visual Inspection and PhotographySectioning of Sample

– Initial division of sample into 4 smaller pieces (A, B, C, & D)– Lower dose of smaller pieces allowed for more detailed inspection– Final sectioning plans developed and machining performed

Tensile Testing– Average Yield Strength ~82 ksi– Average Ultimate Strength ~87 ksi– Results of tensile testing used to determine size of fracture toughness

specimenDesign of Fracture Toughness Specimens

– 1/4T CT specimen– Maximum possible fracture toughness of 165 ksiin – exceeds values

specified in BWRVIP guidance

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Visual Examination Results

Shroud ID Surface – 100x

EDM Surface – 20x

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Sectioning Plan – Pieces B and D – Tensile and Fracture Toughness Specimens

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Sectioning Plan – Piece C

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Sectioning Plan – Piece C (continued)

Left of Crack Right of CrackCenter of Crack

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Assessment of Testing Results Factors Indicating a Preferred Cracking Mechanism

RIS = Radiation Induced SegregationGB = Grain Boundary

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Assessment of Testing ResultsDecision Tree for Determining Cracking Mechanism

Mechanical TestsCold Working vs

Radiation Hardening: Loss of Ductility and

Toughness, Localized Deformation

Hardness Mapping (correlate to YS changes)

Fracture Toughness & Tensile Tests

Optical & SEMStructural & Chemical

Signatures Along Crack Path

Crack path through weld, HAZ and base metal,

check for sensitization, fracture surface examinations for

signatures of mechanism

Structural changes, EBSD for strain gradients, chemical analysis of

fracture surfaces, crack tips, base metal, Weld,

HAZ

TEM & ATEMNanoscale Signatures

of Radiation Damage/Hardening, RIS, Sensitization,

Impurities

TEM analysis for dislocation loops, network

dislocation, twinning, martensite, dislocation channeling, 2nd phases,

spinodal in weld

STEM for GB RIS, oxide structure and chemistry, impurities in base metal,

weld, HAZ, surfaces, crack walls and tips

(APT support as needed)

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Assessment of Testing ResultsChoice of Cracking Mechanisms based on Observations

• No localized deformation, low radiation hardening, base metal yield strength comparatively low, transition from transgranular initiation to intergranular propagation

• Residual weld stresses combined with residual cold work• Externally applied stresses from structure?• Sensitized only, no RIS of significance, possible environmental impurities

contributing to cracking• Combination of sensitization and cold work?• Hardness maps may reveal residual stresses

IGSCCdriven

• High level of radiation hardening, loss of uniform ductility (loops, precipitation)• Mechanical testing may reveal localized deformation mode due to dislocation

channeling• Significant RIS driven Cr depletion in absence of sensitization, or in

conjunction with sensitization• Localized deformation around crack tips, revealed by mechanical testing,

SEM, EBSD & TEM, low toughness, denuded zones around grain boundaries• Transition from transgranular initiation to intergranular propagation

IASCC driven

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Current Status

Recently Completed Activities– Optical Metallography– Automated Microhardness– SEM/EDS Examinations– Shipment of first set of specimens from B&W to PNNL

Near Term Activities– Dosimetry Analysis– Chemical Analysis– FEG-SEM and EBSD Characterization– ATEM Characterization– Fracture Toughness Testing

Future Activities– Draft BWRVIP Report for Committee Review – August 2015– Final BWRVIP Report – November 2015

Results not yet available

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Core Shroud Focus Group / Future BWRVIP Actions

Focus group convened to review incidences of atypical cracking and determine if actions are needed– Considering interim inspection requirements to assess extent of

condition in fleet – Requirements subject to BWRVIP approval– Evaluation guidance and acceptance criteria for “off-axis” flaws are

under development

Core Shroud Focus Group will be responsible for reviewing results and conclusions of boat sample evaluationFuture revision to BWRVIP inspection criteria is dependent

upon:– Results and conclusions of boat sample testing– Results of interim inspections (pending approval for implementation)– Lessons learned from development of off-axis flaw evaluation

guidance and acceptance criteria

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