ML23146A097
| ML23146A097 | |
| Person / Time | |
|---|---|
| Issue date: | 06/14/2023 |
| From: | Ali Rezai, David Rudland NRC/NRR/DNRL/NPHP, NRC/NRR/DNRL |
| To: | |
| References | |
| Download: ML23146A097 (1) | |
Text
Industry / NRC Materials Programs Technical Information Exchange French SCC Issue - Update and NRC Activities David Rudland Senior Technical Advisor NRC Headquarters June 14-15, 2023
Outline
- Timeline from ASN/EDF
- Cracking details
- Safety implications
- Staff plans
- Since we went over this issue last year, we will not go through the history in detail 06/14/2023 Industry / NRC Annual Technical Exchange 2
ASN/EDF Timeline 06/14/2023 Industry / NRC Annual Technical Exchange 3
October 2021 November 2021 December 2021 January 2022 Civaux reactor 1 - 2nd ten-year outage:
Detection of circumferential flaws in close proximity to a weld in the safety injection system piping Hypothesis: thermal fatigue Anticipated outage for inspection of Civaux reactor 2:
flaws are detected (safety injection system)
Penly reactor 1-3rd ten-year outage: flaws are detected (safety injection system)
Civaux reactor 1: Laboratory analyses concluded that these cracks were due to IGSCC Anticipated outage for inspection of Chooz reactors B1 and B2 Penly reactor 1: Laboratory analyses concluded that the cracks were due to IGSCC Extension of inspection to other welds of safety injection and RHR systems of Penly reactor 1 and 1450 MWe units (N4 type)
ASN/EDF Timeline 06/14/2023 Industry / NRC Annual Technical Exchange 4
Spring 2022 Summer 2022 Implementation of preliminary inspection program on priority reactors :
Reactors identified from reanalysis of previous inspection reports (6 reactors)
Reference reactor (one of each design)
Those with upcoming10-year outages Implementation of Reinforced reactor oversight to secure the ability to quickly detect any leak Following the preliminary inspection program Availability of new NDT method able to accurately detect and size the defects Over 110 welds destructive tests performed Beginning of the repairs program Early 2023 Beginning of the implementation of the new inspection program, concerning all reactors in operation, on the areas that might be susceptible to IGSCC (end of the program by 2025)
Large flaw found at Penly 2 Autumn 2022 End of inspections on << prioritized reactors >> and repairs Discussion of the new inspection program between ASN and EDF
Original cracking
- Civaux 1 cracking The pipe is 300mm in diameter with a wall thickness of 30mm Safety injection piping near cold leg Stress corrosion cracking (SCC): flaw propagation is intergranular Location heat affected zone (HAZ) on both sides of the weld Crack length is 360° (full circumference) and the maximum depth is 5.6mm Relatively high hardness (250<HV<290)
Relatively big first layer of the weld (manual TIG weld )
Residual stress supports the arrest of through cracking Suspected to be due to large thermal stratification loads 06/14/2023 Industry / NRC Annual Technical Exchange 5
Inside
New Cracking
- Penly 2 Cracking
- Found after expansion of the inspection plan
- Safety injection piping near hot leg
- Suspected SCC
- 85% deep, 6-inch-long circumferential crack in HAZ
- Little to no thermal stratification
- Weld was repaired twice - suspected driver
- Due to unexpected size and location of cracking, understanding the safety implication to the US fleet is important 06/14/2023 Industry / NRC Annual Technical Exchange 6
Hot Leg Crack
Where are these flaws?
06/14/2023 Industry / NRC Annual Technical Exchange 7
Cold Leg Hot Leg Crack Crack Typical Westinghouse four loop ECCS Safety Injection System
Safety Implications
- Could this cracking occur in US fleet?
- If it can, what are the safety implications?
- Cracks are in unisolable section of pipe
- Break can cause LOCA
- Only one ECCS train is affected?
- Can earthquake or startup of ECCS cause failure?
- New cracking was very long and deep
- Staff looked at risk-triplet through their BeRiskSMART process (NUREG/KM-0016)
- What can go wrong/what can go right
- What are the consequences of each
- What is the expected likelihood of each
- Qualitative assessment 06/14/2023 Industry / NRC Annual Technical Exchange 8
Safety Implications, Staff Plans
- Conclusion from BeRiskSMART
- Uncertainties are large with unknown impact of break on plant safety
- Are our plants susceptible? Water chemistry, etc.
- Do we have unknown (undocumented/unrecorded) weld repairs in these locations?
- Even though these systems are susceptible to thermal fatigue, are we inspecting these locations, i.e. are these welds in the risk-informed program?
- What is the impact to the plant if we lose this system?
- Staff concluded that a more quantitative analysis is needed
- Initiated a LIC-504, Integrated Risk-Informed Decisionmaking Process for Emergent Issues.
- Using a Simplified approach with the goal of completion by July 2023.
06/14/2023 Industry / NRC Annual Technical Exchange 9
Safety Implications, Staff Plans
- LIC-504 Plan
- Investigation into the probability of rupture of an unisolable pipe in the safety injection system
- Perform xLPR analyses or use past runs to make a conservative prediction
- Sensitivity studies as necessary
- Investigate the conditional core damage probability resulting from loss of the portion of the safety injection system
- Exercise Standardized Plant Analysis Risk (SPAR) models for a variety of Westinghouse 4-loop and 3-loop plants
- Determine risk impact and make recommendation
- Using simplified approach to be completed in 3 months
- Other Actions
- Continue to follow industry actions and report development 06/14/2023 Industry / NRC Annual Technical Exchange 10