Development of Chloride- Induced Stress Corrosion Cracking (CISCC) Aging Management Guidelines

Transcription

1 Development of Chloride- Induced Stress Corrosion Cracking (CISCC) Aging Management Guidelines Shannon Chu Senior Technical Leader International Light Water Reactor Materials Reliability Conference and Exhibition August 2-4,

2 CISCC Chloride-Induced Stress Corrosion Cracking (CISCC) of stainless steel reactor components has occurred when three elements are all present: Tensile stress Susceptible Material Corrosive Environment Surface contamination by atmospheric chlorides Sufficient humidity (deliquescence) EPRI has a multi-year project to Evaluate susceptibility to CISCC for welded stainless steel used fuel canisters Develop related aging management guidelines Evaluate the consequences of through-wall CISCC in used fuel canisters Tensile Stress SCC Corrosive Environment Susceptible Material 2

3 Tensile Stress - Welding Residual Stress EPRI and NRC modeled WRS, reasonable agreement in results Sufficient stress in canister welds to support CISCC, through wall tensile stress is present 3

4 Susceptible Material SS 304 and 316 CISCC of Stainless Steels Operating Experience (Not Complete List) 1999 St. Lucie Type 304 stainless steel ECCS suction piping 2001 Koberg (South Africa) 304L stainless steel tanks 2005 Turkey Point 304 stainless steel spent fuel pool cooling line 2009 SONGS Type 304 stainless steel piping (3 separate locations) (NRC Information Notice ) Type 316L Specimen Treated with Salt Fog and Held At 43 C [109 F] for 32 Weeks (NUREG/CR-7030) Type 304 Specimens Deposited With 1 g/m 2 Salt Held at 52 C [136 F] for 8 Months (NUREG/CR-7170) 4

5 Corrosive Environment (?) Chloride sources include breaking ocean waves, cooling tower drift, road salt; no established lower bound threshold chloride areal surface concentration for CISCC Canister surface temperatures that are about 30 C [ 86 F] above ambient or lower may lead to aqueous conditions due to deliquescence at high local humidities CISCC propagation rates increase with higher temperature Fastest propagation would tend to occur on surfaces that are just cool enough to sustain deliquescent brine Stress Corrosion Cracking Deliquescence of Chloride 5

6 Chloride-Induced Stress Corrosion Cracking R&D and beyond CISCC Test Programs and Model Development Voluntary Surface Sampling and Environmental Monitoring Collaborative Research and Development Efforts Field Data Compilation and Assessment Canister Breach Risk and Consequence Assessment ASME Task Group on Inservice Inspection of Spent Fuel Storage & Transportation Containments Canister Mitigation and Repair Technology Nondestructive Examination Techniques & Delivery Systems Visual Examinations of Canister Surfaces NEI Guidance for Operations-Based Aging Management for Dry Cask Storage, Rev. 1 Flaw Growth & Flaw Tolerance Assessment EPRI Reports CISCC Susceptibility Assessment Criteria Aging Management Guidelines Confinement Breach Consequence Analysis 6

7 CISCC Initiation & Growth Testing, Modeling Testing results generally not considered representative of field conditions Cold work, stress/strain higher than expected in order to accelerate testing Humidity/Cl deposition (field concentrations unknown, complex behavior of brines on surfaces) Some continuing CISCC initiation and growth testing, very limited scope Developing consensus on testing approach remains challenging Scale and scope of need much greater than available resources Sandia National Laboratories, Evaluation of the Frequencies for Canister Inspections for SCC (February 2, 2016) Identifies significant limitations of existing flaw growth models and associated experimental crack growth data Outlines significant differences among currently proposed flaw growth models, including EPRI s flaw growth model 7

8 Voluntary Surface Sampling and Environmental Monitoring Past efforts collected surface samples from canisters at 3 sites, many challenges to evaluating the data, did find very small amounts of chloride present Two volunteer sites are continuing with projects to collect samples from surrogate stainless steel surfaces, one site also using wet candle and filter methods to monitor environmental chlorides Chloride aerosol concentration is measured by dozens of EPA sites across the U.S. 8

9 Field Data Compilation and Assessment CISCC aging management approach will require consideration of new data and results as they become available Operating Experience and Inspection Results ISFSI Aging Management INPO Database (AMID) Environmental Monitoring, Laboratory Testing Results and Other Research EPRI Extended Storage Collaboration Program (ESCP) 9

10 Canister Breach Risk and Consequence Assessment NRC Public Meeting: Discussion of risk-informing framework for spent fuel storage certification and licensing activities held April 28, 2016 Expect continued collaboration of NRC, EPRI, NEI, and cask vendors via Regulatory Issue Resolution Protocol (RIRP) EPRI has current project to evaluate available risk information (PRAs, dose assessments, failure mode analyses) and define scope of additional work needed for consequence analysis that is specific to through-wall CISCC of one or more dry storage canisters 10

11 ASME Boiler Pressure Vessel Section XI Task Group on In-service Inspection of Spent Fuel Storage and Transportation Containments Task group was formed at the request of the Nuclear Regulatory Commission Developing the final recommendations for canister inspection scope, methods, frequency, and acceptance criteria EPRI participating and intending our aging management guidance to be a significant resource Targeting 2019 for a final code case with recommendations 11

12 Canister Mitigation and Repair Technology Evaluating technologies applied to operating components for relevance to canisters Interested in uniquely applicable technologies (cleaning) CRIEPI has specifically considered application of peening for dry storage canisters Challenges Competitive/proprietary issues among cask vendors Surface of canisters already in service is exposed to environment but is not readily physically accessible 12

13 NDE and Delivery System Development Dry Canister Storage System Inspection and Robotic Delivery System Development A prototype robotic delivery system has been developed that is capable of entering vertical dry storage cask designs. Future design iterations are planned to enter horizontal cask designs. The robotic system has been successfully deployed into two empty cask systems with an NDE payload. NDE inspection development has shown significant potential to identify defects in canisters. EPRI NDE and delivery system demonstrations in 2016 McGuire May 16 Maine Yankee July 11 EPRI mockups available for testing NDE technologies Vendor development of visual and UT inspection capabilities is also proceeding, coordinating efforts via ESCP subcommittee 13

14 Format, Content, and Implementation Guidance for Dry Cask Storage Operations-Based Aging Management (NEI Rev. 1) Introduces tollgates and learning aging management Tollgates obligate the licensee to perform periodic assessments of the aggregate state of knowledge of aging-related operational experience, research, monitoring, and inspections to ascertain the ability of dry cask storage structures, systems and components (SSCs) to continue performing their intended safety functions throughout the period of extended operation NRC endorsement requested, review process and additional information exchange is on-going 14

15 15 EPRI Susceptibility Assessment Criteria Criteria define site conditions and canister parameters associated with earlier potential for CISCC initiation and growth based on literature review, failure modes and effects analysis, and flaw growth and tolerance assessment Criteria allow ranking of canisters to set priorities for inspection and other aging management efforts in order to direct resources to sites where CISCC is more likely to occur, limited guidance on use of ranking criteria to identify canisters that are considered bounding ISFSI Susceptibility Ranking Proximity to chloride source and local absolute humidity are key variables Canister Susceptibility Ranking Intended to identify canister(s) to be inspected at a given site and to guide scope expansion if needed Geometry (horizontal or vertical) affects locations of maximum chloride deposition and locations of minimum temperature, canisters with different geometries are ranked separately Canister material, storage duration, and current canister power are key variables

16 EPRI CISCC Aging Management Guidance Consider risk-informed and learning approaches Use Susceptibility Assessment Criteria in setting recommended inspection scope and expansion criteria Provide criteria for timing of initial inspection (20 years + delay) Provide inspection acceptance criteria Include details of confinement integrity assessment as an appendix This assessment compares the relative probability of through-wall CISCC for various inspection regimes and parameters to a no-inspection case Discuss mitigation and repair in an appendix Significant reference/resource for ASME Task Group on ISI of Canisters Continued effort to support ASME Task Group and provide implementation training after publication 16

17 Together Shaping the Future of Electricity 17