ML21162A383
| ML21162A383 | |
| Person / Time | |
|---|---|
| Issue date: | 05/11/2021 |
| From: | Ching Ng NRC/NRR/DRA/APOB |
| To: | |
| Fields L | |
| Shared Package | |
| ML21162A391 | List: |
| References | |
| Download: ML21162A383 (10) | |
Text
Common Cause Failure - Causal Alpha Factor Method Activities for Significance Determination Process Ching Ng Reliability & Risk Analyst PRA Oversight Branch May 11, 2021
Purpose
- Share the progress on Common-Cause Failure modeling and parameters estimation
- Solicit further feedback from industry and other stakeholders 2
Topics
- Background
- Stakeholders Activities
- Causal Alpha Factor Method
- Next Steps 3
Background
- The ASME/ANS PRA standard (2013) defines CCF as a failure of two or more components during a short period of time as a result of a single shared cause
- In this context, the term short period of time is meant to be synonymous with the mission time, if the mission time is sufficiently short (e.g., 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />)
- The concept of a shared cause resulting in malfunction is the key aspect of a CCF event 4
SDP Modeling
- NRC uses the Alpha Factor methodology:
- Documented in NUREG-2225, Basis for the Treatment of Potential Common-Cause Failure in the Significance Determination Process, Sept 2018, ML18274A198
- Failures directly related to the Performance Deficiency in the event or condition are modeled as TRUE in the PRA model.
- Equipment and operator action successes - are not set to FALSE (probability of zero). Basic events are left at nominal failure probabilities 5
Stakeholders Activities
- Past interactions with industry indicate the importance of CCF potential in events and condition assessments
- Exelon letter to Bill Dean on January 2017 and NRC response (ML17066A245)
- 2017-2018 - Public workshop, Public teleconference, and an Industry White Paper
- Dec 2018 - Public Meeting on CCF (ML19011A139)
- Mar 2019 - Staffs letter to NRR/DRA Director on path forward
- Based on these interactions and specific statements made by industry stakeholders, NRR tasked RES to investigate an important subset of these issues
- Causal Alpha Factor Method (CAFM) Development
- Existing CCF Parameters and Modeling 6
- Next - Office of Nuclear Regulatory Research 7
- Next - Office of Nuclear Reactor Regulation 8
Next Steps
- Continue to evaluate insights gained
- Consider changes to NRC guidance (e.g. RASP Handbook) 9
Questions?