engleski

Comparing Two Different Approaches to Modeling of the Common Cause Failures in Fault Trees

The potential for common cause failures in systems that perform critical functions has been recognized as very important contributor to risk associated with operation of nuclear power plants. Consequentially, modelling of common cause failures (CCF) in fault trees has become one among the essential elements in any probabilistic safety assessment (PSA). Detailed and realistic representation of CCF potential in fault tree structure is sometimes very challenging task. This is especially so in the cases where a common cause group involves more than two components. During the last ten years the difficulties associated with this kind of modelling have been overcome to some degree by development of integral PSA tools with high capabilities. Some of them allow for the definition of CCF groups and their automated expanding in the process of Boolean resolution and generation of minimal cutsets. On the other hand, in PSA models developed and run by more traditional tools, CCF-potential had to be modelled in the fault trees explicitly. With explicit CCF modelling, fault trees can grow very large, especially in the cases when they involve CCF groups with three or more members, which can become an issue for the management of fault trees and basic events with traditional non-integral PSA models. For these reasons various simplifications had to be made. Speaking in terms of an overall PSA model, there are also some other issues that need to be considered, such as maintainability and accessibility of the model. In this paper a comparison is made between the two approaches to CCF modelling. It is based on a fault tree from a full-scope Level 1 PSA model for internal events, which had originally been developed by a traditional PSA tool and later, transferred to a new-generation PSA tool with automated CCF modelling capabilities. The aspects and issues described above are discussed in the paper.

Probabilistic safety assessment; falut tree; common cause failure

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