Core Shroud Boat Sample Evaluation

Transcription

1 Core Shroud Boat Sample Evaluation Nathan Palm EPRI Annual NRC-Materials Meeting NRC HQ, Rockville, MD June 3, 2015

2 Contents Background Objectives Planned Testing Completed Activities Visual Examination Results Sectioning Plans Assessment of Test Results Current Status Core Shroud Focus Group / Future BWRVIP Actions 2

3 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 3

4 Atypical Core Shroud Cracking - Example 4

5 Boat Sample Sample taken at a location ~9.5 counter-clockwise from V4 and ~1.5 above H4 Captured flaw max depth Weld, heat-affected zone, and base metal captured Utility removed boat sample BWRVIP is funding testing and evaluation of the boat sample Boat sample evaluation being performed by team of SIA, B&W, and PNNL 5

6 Objectives Overall Objective: Determine whether the cracking is due to IASCC or to some other mechanism Specific 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 6

7 Planned Testing Visual Inspections and Photography Optical Metallography Automated Microhardness SEM/EDS Examinations Tensile Testing Fracture Toughness Testing Dosimetry Analysis Chemical Analysis by ICP/MS FEG-SEM and EBSD Characterization ATEM Characterization of Base and Weld Materials ATEM Characterization of Crack Tips 7

8 Completed Activities Visual Inspection and Photography Sectioning 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 specimen Design of Fracture Toughness Specimens 1/4T CT specimen Maximum possible fracture toughness of 165 ksi in exceeds values specified in BWRVIP guidance 8

9 Visual Examination Results Shroud ID Surface 100x EDM Surface 20x 9

10 Sectioning Plan Pieces B and D Tensile and Fracture Toughness Specimens 10

11 Sectioning Plan Piece C 11

12 Sectioning Plan Piece C (continued) Left of Crack Center of Crack Right of Crack 12

13 Assessment of Testing Results Factors Indicating a Preferred Cracking Mechanism RIS = Radiation Induced Segregation GB = Grain Boundary 13

14 Assessment of Testing Results Decision Tree for Determining Cracking Mechanism Mechanical Tests Cold Working vs Radiation Hardening: Loss of Ductility and Toughness, Localized Deformation Hardness Mapping (correlate to YS changes) Fracture Toughness & Tensile Tests Optical & SEM Structural & 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 & ATEM Nanoscale Signatures of Radiation Damage/Hardening, RIS, Sensitization, Impurities TEM analysis for dislocation loops, network dislocation, twinning, martensite, dislocation channeling, 2 nd 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) 14

15 Assessment of Testing Results Choice 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 IGSCC driven 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 15

16 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 Results not yet available Draft BWRVIP Report for Committee Review August 2015 Final BWRVIP Report November

17 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 evaluation Future 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 17

18 Together Shaping the Future of Electricity 18