A critique of methods for dependent failure analysis

Dependent failures make a significant contribution to the failure probability of safety systems. It is important, therefore, that they be taken into account when performing Probabilistic Safety Assessments of nuclear reactors. The way in which the CEGB currently does this is to adopt a ''cut-off'' as a lower limit for the failure probability of a system. The value of this cut-off is arrived at judgmentally. This approach and various other existing methods are reviewed and the CEGB's requirements for an ideal method for treating dependent failures are listed. It is explained how these methods fail to meet these requirements, and outlines a new method, the Distributed Failure Probability (DFP) approach, which was developed within the CEGB to meet its needs. This DFP approach provides a soundly based and scrutable technique by which dependent failures can be quantitatively assessed, making the best use of both judgment and data. (author)