Performance of a German Seismic PRA for Nuclear Power Plants by Using Safety Margins

摘要

The German regulatory guide requires the performance of a Probabilistic Risk Assessment (PRA) including External Events. In 2005, a new Methodology Guideline (Methodenband) [1] based on the current state of science and technology was released to provide the analyst with a set of suitable tools and methods for the analysis of all PRA events. In the case of earthquake, a multilevel procedure is suggested. The analysis level which has to be applied depends on the seismic risk at the site of the plant. For sites in areas with low seismic activity no analysis or only a reduced analysis is proposed. This paper describes the evaluation of safety margins of buildings, structures, components and systems for plants at sites with high seismic risk, corresponding to the German Methodology Guideline, as a basis for the determination of seismic failure probabilities (fragilities). The seismic PRA results in an estimation of core damage frequency caused by earthquakes. Additionally, the described approach can also be adapted for the usage in a reduced analysis for sites with lower earthquake risks. Westinghouse has wide experience in performing seismic PRA for both, BWR as well as PWR plants. Westinghouse uses the documented set of seismic design analyses dating from the construction phase of the plant and from later updates, as a basis for the seismic PRA, which means that usually no costly new structural mechanics analyses have to be performed.