Basis for the Treatment of Potential Common-Cause Failure in the Significance Determination Process (NUREG-2225)

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Publication Information

Manuscript Completed: September 2018
Date Published: September 2018

Prepared by:

A.Mosleh¹
K.Coyne
C.Hunter
S.Shen¹

¹University of California, Los Angeles
Samueli School of Engineering
7400 Boelter Hall
Los Angeles, CA 90095

Christopher Hunter, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

Event and condition assessment is an application of probabilistic risk assessment in which observed equipment failures, degradations, and outages are mapped into a risk model to obtain a numerical estimate of risk significance. These retrospective assessments are used in regulatory applications, such as the Significance Determination Process (SDP), Accident Sequence Precursor Program, and Incident Investigation Program. In the SDP, which is the focus for this report, the identified performance deficiency is mapped in the risk model by adjusting the failure probability of the affected (i.e., nonconforming) component. In addition, the probability of common-cause failure (CCF) is adjusted to reflect the performance deficiency that could have affected all redundant components in the affected system (i.e., within the same common-cause component group in the risk model). Experience has shown that increased potential for CCF is often a substantial contributor to the risk significance of a performance deficiency.

This NUREG documents the basis for the treatment of CCF potential at the level of the observed performance deficiency within the scope of the SDP, describes important technical terms associated with CCF modeling and how these terms relate to event and conditional assessments versus base probabilistic risk assessment modeling, and provides examples of CCF potential for actual conditions. It also describes the basic assumptions and key principles for treating CCF of redundant components in SDP risk assessments when one or more of the redundant components are failed or functionally degraded due to a deficiency in licensee performance. This report does not introduce new methods for CCF and is intended to summarize the U.S. Nuclear Regulatory Commission's philosophical basis for treating CCF within retrospective risk assessments performed as part of the SDP.