APPLICATION OF IMPORTANCE MEASURES TO NUCLEAR REACTOR SYSTEMS AND THE DEVELOPMENT OF SYSTEMS PERFORMANCE STANDARDS BASED ON PROBABILISTIC RISK ASSESSMENTS

Probabilistic Risk Assessments (PRA) of nuclear reactors now represent a substantial data base which describes relative plant accident potentials, plant failure modes, and plant system failure probabilities. The PRA analyses can be used to make design related decisions. In general, however, nuclear plant design relies on traditional engineering approaches along with quality control and regulatory review. This dissertation proposes a method whereby typical PRA methods can be used to develop design related standards for plant core melt and subsystem performance.;The approach taken used system and plant failure data from six Pressurized Water Reactor (PWR) PRAs. Importance measures were applied to gage the impact of various systems on plant core melt. These measures were also applied to modified plant configurations which met a dominant sequence core melt contribution limit of 1.0 x 10('-5) core melts per plant per reactor year. In addition, plant failure modes were put into functional groups, and sequences were placed into accident classes. Together, these approaches defined the failure characteristics of the six plants and sought to discern the existence of performance standards. Continuing, a broad range of sensitivity studies were performed including studies of the impact of eliminating dependent events. Unavailability and reliability decrease factors were introduced as a means of manipulating system unavailabilities between the perfectly good state and the perfectly bad state. As a refinement of this technique, a method of limiting the sensitivity of the plant (measured as an increase in core melt frequency) to hypothetical increases in event unavailabilities was proposed. Bounding estimates of the effect of these "standard pairs" on plant core melt frequency were derived.;Overall, it was found that plant failure modes were extremely diverse, reflecting the unique characteristics of all six of the PWRs. Implementation of standard pairs by changing system unavailabilities as necessary showed that similar standards could be met from plant to plant when the total plant core melt frequencies were reduced to similar levels. Thus, plant standards which limit overall core melt frequency and plant standards which limit plant sensitivity to system unavailability increases can be shown to produce similar core melt results and system design requirements.