The Use of PSAs in Piping Components Maintenance Prioritization. F. Ardorino and L. Magne, Electricité de France - Research and Development Division - Safety and Reliability Branch, 1 avenue du Général de Gaulle - 92141 Clamart – France, e-mail: Florence.Ardorino@edfgdf.fr, Fax: (33-1) 47 65 51 73
A process is being developed at EDF to extend the current Reliability Centered Maintenance program to piping structural components. This process, called "OMF-structures" leads to optimize maintenance through a prioritization of resources for segments that are important in term of safety, availability and maintenance costs. In this process, PSAs are used to evaluate consequences on safety of the segments' failure modes, and to identify segments where potential failure modes would be critical to safety.
The global process includes probabilistic and deterministic aspects. The probabilistic criteria come from the use of PSA models: level one PSAs "adapted" for taking into account the passive components failure consequences. The deterministic aspects are considered through the potential initiation of an emergency procedure, unavailability governed by Operating Technical Specifications, and regulatory definition of important to safety equipment.
Such an approach has already been applied for active components (like valves or pumps ...). Two risk indicators provided by PSAs were used for characterizing the importance of components failures: a measure of contribution to core melt risk, and a measure of risk achievement due to a failure. But with the extension to passive components, the process for selecting PSA safety critical components had to be adapted due to the lack of knowledge about piping segments
- The lack of piping segments failure rates keeps from using PSA indicators alone for evaluating the core melt risk importance of piping segments. In the global process, PSAs are used in conjunction with results coming from reliability models that can be implemented to assess components failure rates, when a potential degradation mechanism has been identified.
- The lack of knowledge about the structural failure consequences in the existing PSA models makes it difficult to evaluate risk indicators as it was done for active components. Contributions of piping segments failures to core melt risk can be "hidden" in the model, for example when accident initiators are potentially impacted. But instead of refining PSA models by adding more complete description of the consequences of piping segments rupture, we chose to refine the process for evaluating importance indicators, to take into account the special case of structural elements.
We explain in this paper the way existing PSAs can be used for evaluating the core melt risk importance of passive components, and more specially which are the risk indicators used, and how they can be evaluated to take into account the consequences of piping segments failure modes.
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