ML20176A286
| ML20176A286 | |
| Person / Time | |
|---|---|
| Issue date: | 06/25/2020 |
| From: | David Rudland NRC/NRR/DNRL |
| To: | |
| David Rudland-NRR/DNRL 301-415-1896 | |
| References | |
| Download: ML20176A286 (13) | |
Text
10CFR50.55a Condition for Pressure Testing of Mechanical Joints Category 2 Public Meeting June 25, 2020
Purpose, Outcome, Process
Purpose:
Follow up meeting on 10 CFR 50.55a(b)(2)(xxvi)-
Pressure Testing Mechanical Joints
- Outcome: Present path forward on condition
- Process: See meeting agenda 06/25/2020 2
NRC staff slides - ML20176A286
Meeting Agenda Time Topic Speaker 1:00pm-1:05pm Introduction NRC 1:05pm-1:35pm Summary of safety analyses and path forward NRC 1:35pm-2:45pm Discussion NRC/NEI/ASME 2:45pm-3:00pm Public comment All 3:00pm Adjourn NRC 06/25/2020 3
Assessment of Condition 10 CFR 50.55a(b)(2)(xxvi)
- Process
- Qualitative safety assessment of pressure tests after repair/replacement of mechanical joints
- Assessment of exemptions
- Develop path forward 06/25/2020 4
Qualitative Safety Assessment
- The staff used the principles of risk-informed decision making to evaluate the safety significance of the pressure test after repair/replacement activities
- Staff utilized
- Personal experience,
- Operational experience,
- Past research and bolting issues (GSI-29),
- Other factors providing assurance of the leak tightness and integrity of bolted connections 06/25/2020 5
Deterministic Structural Analysis
- During the resolution of GSI-29, EPRI conducted deterministic analyses (NP-5769) to look at the stability of mechanical joints
- Structural and leakage analyses were conducted on a variety of safety-related Class 1 mechanical connections.
- Between 1 and 3 failed bolts are required for the leakage to surpass 1GPM (2-4 bolts for 10GPM)
- At this point, the margin to failure of adjacent bolts range between 2.2 and 3.3 06/25/2020 6
Risk Triplet
- Utilized the risk triplet for a variety of scenarios
- What can happen?
- How likely is it to happen?
- What are the consequences?
- Focused on the damage to the joint, e.g., leakage, rupture
- Also considered other risks to plant
- Staff developed likelihood and consequence rankings (high, medium, low) for each item in the scenarios 06/25/2020 7
Risk Scenarios Considered Mechanical joint failure at operating pressure, bolt failure, flange failure, etc.
Leaking steam, leading to steam cutting, erosion/corrosion or SCC, leading to joint failure or external damage Borated water leakage leading to external damage Non-Borated water leakage leading to external damage Water leakage leading to inventory loss Missed leak that is found in service forces outage - unscheduled outage for Class 1 may increase plant risk Class 2 lines not normally at operating conditions might have leaks that cannot be found by system pressure tests Leakage (water or steam) causes damage to electrical system Leakage causes the removing of a train to repair the system Steam leakage causes a personnel safety issue or ALARA issue 06/25/2020 8
Risk determined to be low
Principles of Risk Informed Decision Making
- Small change in risk
- Sufficient performance monitoring
- Leak tests as part of the QA program
- Twice daily walkdowns in all accessible areas by Operations staff including inspecting for leaks as part of rounds.
- Containment monitoring for identified and unidentified leakage.
- Pressure test of reactor coolant loop after refueling outage (next slide)
- Consistent with defense-in-depth philosophy
- Many systems, ECCS, etc. are in place to maintain core cooling if a primary system has a flange failure
- Many Code systems have redundant trains
- Sufficient safety margins
- Analyses suggest joint will leak before it fails illustrating significant safety margins exists with leaking joints 06/25/2020 9
Pressure Test Each Refuel Outage 06/25/2020 10 (a) The pressure-retaining boundary during the system leakage test shall correspond to the reactor coolant boundary, with all valves in the position required for normal reactor operation startup. The visual examination shall, however, extend to and include the second closed valve at the boundary extremity.
Other Considerations
- Operational Experience (OE)
- Worldwide passive system failure database given a total of 11,578 events, 7% of the events (leakage) were revealed by hydrotesting. No catastrophic failure events after leakage were noted.
- Staff found some OE on leakage with limited periphery damage and no ruptures 06/25/2020 11
Assessment Conclusions
- When coupled with the sufficient performance monitoring, maintaining defense-in-depth and sufficient safety margin, the risk analyses suggest that the absence of the pressure test after repair/replacement activities imposes a minimal safety concern
- The staff recognizes the importance of the items currently conducted to provide assurance of the leak tightness and integrity of bolted connections 06/25/2020 12
Path Forward
- The staff finds acceptable the industry position that the items in IWA-4540(b) are exempt from pressure tests
- For items not included in IWA-4540(b), the staff are open to alternatives to 10CFR50.55a related to this topic
- The staff will revisit this condition in the next ASME Code edition update rulemaking 06/25/2020 13