ML18348A631
| ML18348A631 | |
| Person / Time | |
|---|---|
| Site: | Nuclear Energy Institute |
| Issue date: | 12/12/2018 |
| From: | Nuclear Energy Institute |
| To: | Cj Fong NRC/NRR/DRA/APOB |
| Ng C | |
| References | |
| Download: ML18348A631 (13) | |
Text
©2018 Nuclear Energy Institute Crediting observed defenses against common cause using a qualitative approach Common Cause Failure Impact within the SDP December 12, 2018
©2018 Nuclear Energy Institute 2 Present issues and provide recommendations Add perspective for risk-informed decision-making Improve framework for SERP with focus on Ground Rule 3 Promote simple and more effective SDP evaulations Objectives
©2018 Nuclear Energy Institute 3 January 2018 NEI White Paper Addressing Common Cause Failure (CCF) Impact within the Significance Determination Process (SDP)
Proposal to enhance how CCF is treated in RASP Handbook NUREG-2225 Basis for the Treatment of Potential Common-Cause Failure in the Significance Determination Process issued Documents basis for treatment of CCF potential at performance deficiency level in quantitative SDP risk assessments Owners Group, industry and public comments NRC Development of Supplemental Guidance for CCF
Background
©2018 Nuclear Energy Institute 4 Expand the framework for crediting observed defenses against common cause qualitatively outside of the quantitative risk analysis Recommendation 1: Add an additional paragraph to Section 5.3 in RASP Handbook to use sensitivity results between baseline or nominal CCF calculation and conditional CCF calculation Recommendation 2: Add text to Ground Rule 3 in the RASP Handbook to reflect qualitative factors using the above approach Help both SERP deliberations as well as utilities better view the application of common cause with respect to the defenses employed Industry White paper
©2018 Nuclear Energy Institute 5 SDP is a risk-informed process and, therefore, factors not explicitly considered in the quantitative risk assessment can be qualitatively considered during the enforcement process Many CCF defenses (e.g., staggered maintenance) were in place during observed events and are already represented in parameter estimates; therefore, qualitative credit should only be considered for unique defenses not already factored in the CCF data The staffs treatment of CCF (e.g., maintaining observed successes at their nominal likelihood) is consistent with failure memory approach which is state-of-practice for event and condition assessment (ECA)
Significance Determination Process
Reference:
NRC Response to Public Comments on draft NUREG-2225
©2018 Nuclear Energy Institute 6 SERP can best provide a regulatory perspective regarding the link between a performance deficiency and the associated risk evaluation Impetus to challenge the analysis decreases when we continue to focus on the quantitative aspects of the safety significance evaluation Its difficult to conceptualize the partitioning of common cause failure coupling factors to reflect application of data There is not solid enough data to factor out apparently missing barriers to CCF (i.e. the mean increases)
Focus: RASP Handbook Ground Rule 3
©2018 Nuclear Energy Institute 7 Guidance for the Senior Reactor Analysts (SRAs) and management members of the Significance and Enforcement Review Panel (SERP)
Adjustments need to take into account new factors not previously accounted for in the development of alpha-factor data Questions: can causal factors be deduced from the data? Are additional methods already accounted for in the existing data?
Any new approach needs to take into account factors that not only lower but also increase CCF contribution (intersystem dependencies)
Simple and clear to use with defensible and consistent results Proposed NRC Supplemental Guidance for Common Cause Failure
©2018 Nuclear Energy Institute 8 INL data classification process categorizes the strength of a CCF event using impact vectors but the SDP has CCF either on or off NRC treats every failure as having a ~5% chance of being a CCF many failures have a much lower chance of being an actual CCF Developing new alpha factors adds more complexity to process Licensees have taken measures to reduce CCF (maintenance crew changes, staggered testing, etc.) but its unclear if the benefit has been reflected in the current alpha-factor data gathered by INL Observations
©2018 Nuclear Energy Institute 9 Credit superior defenses address when less than expected Design control, testing and preventive maintenance practices, procedure review, training, quality control, diversity and barriers Addressing maintenance and operational coupling factors eliminates relationship and reduces number of individual failures Use proximate cause codes in NUREG/CR-6268 data analysis to identify strategies that help illustrate the relationship between deliberate utility actions and a qualitative adjustment factor Promote the use of a defense strategy against proximate causes
©2018 Nuclear Energy Institute 10 Qualitatively adjust the applicability of the cause and the coupling factor of the event due to the strength of common cause defense Use to inform the quantitative result during SERP deliberation Acknowledges a range of actions or organizational factors Focus on the deficiency in question Strength of Defenses Qualitative Factors Complete Nominal or baseline CCF Superior Nominal CCF - 0.5 Conditional CCF Moderate 0.5 Conditional CCF - Conditional CCF Minimal Conditional CCF (NRCs quantitative estimate)
©2018 Nuclear Energy Institute 11 Industry Performance Trend 0
100 200 300 400 500 600 700 800 900 2015 2016 2017 2018 Trend in Findings (from 11/28 Counterparts Presentation)
Total findings Green findings
©2018 Nuclear Energy Institute 12 Data Issues
Data used in SDP examples does not reflect current fleet performance
Weaker coupling factors lead to conservative conditional probabilities
Data (alpha2 @ 4%) based on two events and a prior distribution:
May 1997 failure of AFW pump breaker to close during test; pump manually started from control room
November 2006 switches for both MDAFW pumps; couldve been manually started from control room
Neither of the above examples is considered a hardware CCF and the prior contains events that may not be countable CCFs
CCF database contains easily recoverable failures, contemporate failures, modeled support system failures and other events that should not be used for alpha factors which will be used by the SDP
©2018 Nuclear Energy Institute 13 Use of qualitative factors within existing framework has the potential to:
Improve characterization of performance deficiencies Better focus on higher organizational deficiencies Optimize resource expenditures by focusing on performance rather than quantitative refinements for more robust decision making Add perspective to the evaluation of a performance deficiency Increase the recognition and use of common cause defenses by utilities, thereby proactively promoting good practices Closing Remarks