ML17261A029

9/19/2017 - NEI Presentation for Common Cause Failure Public Meeting

ML17261A029
Person / Time
Site:	Nuclear Energy Institute
Issue date:	09/19/2017
From:	Krueger G Nuclear Energy Institute
To:	Office of Nuclear Reactor Regulation
Schwab A
References
Download: ML17261A029 (13)

Text

COMMON CAUSE FAILURE METHODS IMPACTING SDP EVALUATIONS Greg Krueger Senior Technical Advisor Jim Slider Senior Project Manager September 19, 2017

SITUATION

• PROBLEM: NRCs current quantitative-focused approach to the treatment of Common Cause Failure (CCF) in event/condition assessment is not aligned with PRA best practices for risk-informing key technical decisions

• CONSEQUENCES: Misalignment leads to resource-intensive debate with licensees when NRC treatment of CCF drives an SDP result or disincentivizes behaviors helpful to safety

• OPPORTUNITY: Gaining alignment on how to risk-inform CCF insights in an event/condition assessment would boost confidence in, and clarity of, SDP outcomes 2

TODAYS TOPICS

1. CCF considerations for event/condition evaluations

2. Consideration of a quantitative sensitivity method to support a structured evaluation of CCF

3. Potential development/enhancement of guidance associated with qualitative CCF considerations in event/condition assessment 3

MEETING GOALS

• Discuss elements of a risk informed framework to support focused application of CCF for Significance Determination Process (SDP) evaluations

• Illustrate the potential to streamline the use of industry and NRC resources applied during SDPs

• Discuss approaches to assess the impact of levels of defense against CCF

• Discuss development and use of additional qualitative CCF guidance for SDP purposes 4

CCF CONSIDERATIONS FOR EVENT/CONDITION EVALUATION

• CCF can be a significant contributor for event/condition applications typically associated with SDP evaluations

• The application of common cause in SDP evaluations involving multi-train systems should reflect a plant-specific assessment of CCF defenses

• Limitations of the NRCs current CCF database can drive an overestimation of the CCF impact in an event/condition evaluation 5

QUANTITATIVE CCF RESULTS Current RASP CCF Treatment if Draft NUREG - the alpha-factor estimates are no additional information is not plant-specific, and estimates do not reflect available on extent of 3.00E-06 plant-to-plant variability condition/cause is known 2.50E-06 Delta CDF Nominal CCF 2.00E-06 Treatment if robust CCF defenses in 1.50E-06 place and extent of condition/cause is 1.00E-06 known 5.00E-07 0.00E+00 1 5 10 20 50 100 233 6 Factor Increase in CCF Green/White Threshold

BENEFITS OF CONSIDERING CCF SENSITIVITIES

• A sensitivity approach highlights how much a quantitative CCF contributor drives the conclusion of significance Illustrating the sensitivities in a graphical manner highlights the magnitude the CCF data may have on the decision-making process

• The impact of known causal factors or specific conditions can be qualitatively applied when a range of results are presented

• The sensitivity approach improves understanding of the relationship between the CCF data used and the event/condition evaluation 7

POTENTIAL IMPACT ON RESULTS WHEN CCF IS APPLIED

• Overestimating CCF risk significance can cause undue effort for both the utility and NRC that may not be commensurate with safety significance of the issue

• Applying full conditional causal factors which do not reflect the proximate cause and plant-specific differences may overestimate the risk associated with an event/condition 8

EXAMPLES OF QUALITATIVE ELEMENTS FOR IMPROVED RISK-INFORMED DECISION-MAKING ON CCF

• Consideration of just-in-time training and technical human performance briefs

• Oversight and observations by others

• Use of subject matter experts

• Original equipment manufacturer consultation

• Quality Assurance modification inspections

• Procedure impacts

• Evidence of individual errors 9

EXAMPLES OF QUALITATIVE ELEMENTS FOR IMPROVED RISK-INFORMED DECISION-MAKING ON CCF (CONT.)

• Passive failure modes

• State of knowledge of actual similar CCF events

• Discussion of time of the subsequent failure relative to the original failure Time needed for failure to manifest itself 10

SUMMARY

• Consideration of CCF for event/condition assessment in a risk-informed context is needed to streamline NRC and industry resources applied during SDP evaluations

• Application of quantitative CCF sensitivities can be accomplished using existing models and methods and provide insight of CCF impact on SDP decision-making

• A defined set of qualitative factors allows for a graded approach that more accurately represents the condition or event being evaluated. They will also provide a roadmap of pre-emptive industry defenses to limit the potential for common cause 11

POSSIBLE NEXT STEPS

1. Work to develop a set of qualitative factors that could be used as prompts during the SDP decision-making process

2. Develop simple criteria that could be used to guide the quantitative sensitivity process 12

NEI Contacts:

Greg Krueger, gak@nei.org Jim Slider, jes@nei.org