At the Palisades Nuclear Power Plant, a review of systems, structures and components is being performed for a variety of risk-informed applications. Among these applications is the In-Service Testing program developed under Section XI of the ASME Boiler and Pressure Vessel Code. In reviewing the scope of the IST program, a relatively new risk ranking technique is being utilized called Top Event Prevention (TEP). TEP differs from more traditional methods of risk ranking in that it uses defense-in-depth methods to select what SSCs are important to safety. The results of the TEP analysis are also somewhat unique as compared to more traditional approaches in that combinations of equipment important to safety are identified as opposed to determining importance one component at a time. In addition, entire paths or trains of equipment are identified as being required to manage safety. Furthermore, the components identified in the selected trains are minimal. That is, TEP identifies the minimum set of equipment modeled in the PRA that is necessary to manage safety. Because TEP provides a structured, deterministic approach to selection of components that are important to managing safety, consideration is being given to classifying pumps and valves into one of three safety categories; high safety significant components, low safety significant components and a third non safety significant category. The first two categories of safety significance are similar to those already defined in Regulatory Guide 1.175 [1], and will identify where current testing methods remain appropriate while suggesting those components for which testing intervals can be extended. The third or non-safety significant category has been established to identify those pumps and valves to which the PRA and safety are insensitive. Preliminary results from the TEP analysis of the Palisades IST scope suggest that of the more than 500 pumps and valves modeled in the PRA, just under one third are not important from a defense-in-depth perspective. Probabilistic testing of these approximately 140 components shows that risk as modeled by the PRA is not dependent on the reliability of these components. This suggests that the PRA is relatively insensitive to the reliability of these components collectively and that they can be considered as candidates for significant relaxation of testing requirements, possibly including exclusion from the formal IST program.

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