BWR Core Shroud Boat Sample Metallurgical Testing Summary

Transcription

1 BWR Core Shroud Boat Sample Metallurgical Testing Summary Daniel Sommerville, Structural Integrity Associates, Inc. Heather Jackson Structural Integrity Associates, Inc. Nathan Palm Electric Power Research Institute James Hyres Babcock & Wilcox 2016 International LWR Material Reliability Conference Chicago, IL Danny Edwards Pacific Northwest National Laboratory

2 Outline Background and Introduction Test Objectives Summary of Key Results Implications of Boat Sample Results SLIDE 2

3 Background Visual examination of a domestic core shroud in 2008 identified indications having attributes not typical of IGSCC Indications oriented perpendicular to the weld (off-axis) Extending well outside the weld HAZ into base metal (several inches in some cases) Some indications appeared to extend into or through welds A review of the available data was performed (BWRVIP-253) and included the following recommendations: Perform volumetric examinations Remove a boat sample to better understand the material condition of the shroud Expand inspections to other plants in the fleet SLIDE 3

4 Background (continued) Examinations performed by utility in Spring 2014 utilizing specialized UT Identified several deep / through-wall indications Identified numerous smaller indications SLIDE 4

5 Background (concluded) In spring 2014 outage, following specialized UT, utility owner removed a boat sample Boat sample taken from shroud ID Significant dose rate: 310 R/hr at 30 cm Captured off-axis crack, including crack tip Includes weld metal, HAZ and base metal Significant investment by both utility and the BWRVIP SLIDE 5

6 Boat Sample Analysis and Evaluation Team Electric Power Research Institute Structural Integrity Associates, Inc. BWX Technologies, Inc. Pacific Northwest National Laboratory Southern Nuclear Operating Company Westmoreland Mechanical Testing and Research GE-Hitachi Nuclear Energy JLN Consulting (Larry Nelson) Tri-State Motor Transit Co. (Transportation of cask) SLIDE 6

7 Test Program Objectives Primary Objective: 1. Investigate influence of irradiation on atypical cracking observed in core shroud Secondary Objectives: 2. Measure tensile and fracture properties for the base metal to assist in determining structural margin 3. Estimate neutron fluence for comparison to code predictions 4. Assess Pt deposition within crack and on ID surface to provide input to mitigation efforts 5. Provided specimens to PNNL for He measurements in support of EPRI WRTC irradiated materials welding program 6. Provided archival specimens to ORNL to support PSCR long-term R&D projects SLIDE 7

8 Test Summary The following testing/analyses performed to support the test program objectives: Crack mechanism Visual inspection and photography (1) Optical metallography (1) SEM/EDS on open crack surfaces (1) FEG-SEM and EBSD of cross-sections through cracks (1) ATEM along crack path and at crack tips (1) Chemical composition Bulk chemical analysis by ICP-MS (1) Neutron fluence Dosimetry analysis to benchmark fluence evaluations (3) Microstructure and grain boundary chemistry ATEM to characterize radiation defects (1) ATEM to characterize extent of RIS (1) Mechanical properties Tensile testing (1,2) Fracture toughness testing (2) Microhardness profiles (1) Pt deposition FEG-SEM of shroud ID surface and open cracks (4) SLIDE 8

9 Boat Sample Sectioning Plan Mechanical test specimens obtained from Pieces B and D Piece B Piece D SLIDE 9

10 Boat Sample Sectioning Plan, cont. Specimens for metallurgical, chemical, and dosimetry analyses were obtained from Piece C Piece C Left of crack Right of crack SLIDE 10

11 KEY TEST RESULTS SLIDE 11

12 Cracking in HAZ and Base Metal is Essentially Entirely Intergranular Shroud ID Shroud ID Secondary cracks HAZ Primary crack Base metal open crack surface SLIDE 12

13 Cracking in Weld is Interdendritic SLIDE 13

14 Boat Sample Exhibited Elevated Strength and Hardness due to Both Irradiation Hardening and Fabrication-Induced Cold Work Boat Sample Mechanical Properties Yield strength at 288 C (550 F) 553 to 577 MPa 80 to 84 ksi Ultimate tensile strength at 288 C (550 F) 587 to 608 MPa 85 to 88 ksi Vickers microhardness at room temperature 3.8 to 22 mm (0.15 to 0.85 inch) below ID 0 to 3.8 mm (0 to 0.15 inch) below ID 25 m (0.001 inch) below ID 300 HV 340 to 350 HV 390 to 506 HV Strength properties are consistent with expected values for this fluence level Significant hardness observed through the entire sample thickness; above threshold values known to promote SCC (i.e., 270 HV) Boat Sample Fluence (E > 1 MeV) At ID surface 2.24 to 2.37 x n/cm to 3.4 dpa 0.5 inch below surface 1.85 to 1.94 x n/cm to 2.8 dpa Fluence benchmark against dosimetry measurements shows +37% bias error (atypical since most RAMA benchmarks show <10% bias error) SLIDE 14

15 Hardness was Significantly Higher at ID Surface due to Grinding and Cold Work ID surface Cold-worked layer Grinding on ID surface ID Surface ~23 mm (~0.9 inch) below ID Hardness HV Base metal 0.15 in 0.08 in 0.01 in Hardness HV Full-thickness traverse in base metal Distance from ID (in) Distance from ID (in) SLIDE 15

16 Boat Sample Material Retained Significant Ductility and Fracture Toughness Uniform elongation at 288 C (550 F) 10 to 12 % Total elongation at 288 C (550 F) 16 to 18 % Reduction in area at 288 C (550 F) 18 to 24 % J Q at 288 C (550 F) 3.18 mm (0.125 inch) below ID 15.9 mm (0.625 inch) below ID K JQ at 288 C (550 F) 3.18 mm (0.125 inch) below ID 15.9 mm (0.625 inch) below ID 146 kj/m kj/m in-lb/in in-lb/in MPa m 218 MPa m Fracture toughness falls within significant scatter observed in BWRVIP-100, Rev. 1 Fracture toughness significantly higher than the lower bound published in BWRVIP-100, Rev. 1 1 x n/cm 2 < f < 3 x n/cm 2 K Ic = 112 ksi in 166 ksi in 198 ksi in Deformation of fracture toughness specimens SLIDE 16

17 Grain Boundary Cr Depletion due to Radiation Induced Segregation (RIS) Observed Cr Cr Fe Si Ni Mn P Ni Energy Dispersive Spectroscopy (EDS) compositional profiles across a high-angle grain boundary in the base metal (also seen in HAZ) show RIS within ~5 nm of grain boundary: Depletion of Cr (as low as 12-14% vs. 18% bulk concentration) and Fe Enrichment of Ni Very slight enrichment of Si and P and depletion of Mn Results are consistent with other IASCC investigations (i.e. BWR Top Guide) No evidence of weld-induced sensitization SLIDE 17

18 Analytical Transmission Electron Microscopy Examination of Crack Tips Boat Sample Crack Primary crack tip Secondary crack tip in HAZ Appearance of gap in oxide layer of primary crack tip may indicate reactor coolant has access to crack tip Primary Crack Tip Location Secondary Crack Tip Location In previous examinations of BWR shroud and top guide samples, oxide-filled crack tips and Ni enrichment ahead of crack tips were features associated with inactive cracks SLIDE 18

19 Analytical Transmission Electron Microscopy Examination of Crack Tips Secondary crack tip adjacent to primary crack tip STE M Secondary crack tip in HAZ Fe Ni Oxide in crack Nickel enrichment ahead of crack tip apparent for all cracks perhaps not as highly enriched for primary crack tip Crack tip Cr S Ni enrichment ahead of crack tip O Oxide in crack C Mn Si Ni enrichment ahead of crack tip Primary crack tip In previous examinations of BWR shroud and top guide samples, oxidefilled crack tips and Ni enrichment ahead of crack tips were features associated with inactive cracks Oxide in crack Ni enrichment ahead of crack tip SLIDE 19

20 Key Results Crack morphology essentially completely intergranular (or interdendritic within the weld fusion zone), with minor branching in all areas examined. Chemical composition of the base metal found to meet the requirements of SA-240 Type 304. Peak fluence of the boat sample at the shroud ID surface estimated to be 2.37 x n/cm 2 (3.4 dpa), attenuating to ~1.9 x n/cm 2 (2.7 dpa) at a depth of 12.7 mm (0.5 inch) below the ID surface. Fluence estimates reflect a bias of 37% against dosimetry measurements, suggesting that the fluence of the boat sample may be overpredicted by up to 37%. RIS was observed at grain boundaries in the base metal and HAZ. Cr was depleted as low as 12 wt%, versus the bulk concentration of 18 wt%. Ni was enriched to 14 wt% versus 9 wt% in the bulk. SLIDE 20

21 Key Results Yield and ultimate tensile strength are consistent with expected properties based on fluence Fracture toughness values, K JQ, ranged from 182 to 218 MPa m (166 to 198 ksi in) At high end of expected range of values Hardness depth profiles show elevated hardness through thickness of boat sample (~300 HV in the bulk), with significantly elevated hardness (>390 HV up to 506 HV) close to ID surface No evidence of weld-induced sensitization Potentially aggressive species observed in crack environment: S, Cu, and Pb SLIDE 21

22 IMPLICATIONS OF RESULTS SLIDE 22

23 Implications of Boat Sample Results Changes in boat sample material properties typical for this fluence range, based on previous material testing for the fleet Irradiation has increased material susceptibility to SCC: RIS and radiation hardened microstructure are consistent with a fluence of ~2 x n/cm 2 (~3 dpa) No evidence of weld-induced sensitization Through-sample hardness sufficiently high to make material susceptible to SCC Non-irradiation factors were also present that may contribute to IGSCC: Significant fabrication-induced cold work Potentially aggressive species observed in crack environment: S, Cu, Pb Examination of boat sample crack tips suggests: Secondary crack tip is likely dormant Primary crack tip may exhibit reactor coolant present at crack tip which could indicate active crack SLIDE 23

24 BWRVIP Perspective The results do not change the perspective that off-axis cracking does not represent a challenge to long-term shroud integrity Mechanical properties consistent with existing irradiated materials database and fracture toughness well above the conservative lower bound values used as the basis for core shroud structural evaluations No clear evidence of recent / active crack growth and no data indicating that the off-axis cracks in the Hatch core shroud will grow significantly in the future SLIDE 24

25 Acronyms APT Atom probe tomography ATEM Analytical transmission electron microscopy EBSD Electron backscatter diffraction EDS Energy dispersive X-ray spectroscopy FEG Field emission gun GB Grain boundary HAZ Weld heat-affected zone HV Vickers microhardness IASCC Irradiation-assisted stress corrosion cracking ICP-MS Inductively coupled plasma mass spectrometry IGSCC Intergranular stress corrosion cracking RIS Radiation induced segregation SEM Scanning electron microscopy SLIDE 25

26 Questions SLIDE 26