Systems engineering aspects of seismic margin studies

Increasing knowledge in the geoscience field has led to the understanding that, although highly unlikely, it is possible for a nuclear power plant to be subjected to earthquake ground motion greater than that for which the plant was designed. While it is recognized that there are conservatisms inherent in current design practices, interest has developed in evaluating the seismic risk of operating plants. Several plant-specific seismic probabilistic risk assessments (SPRA) have been completed to address questions related to the seismic risk of a plant: What ground motion levels and plant component failures dominate seismic risk? What is the median (or mean) seismic capacity of the plant? How significant is the seismic risk in comparison to the nonseismic risk? The results from such SPRAs are quite informative, but such studies may entail a considerable amount of expensive analysis of large portions of the plant. As an alternative to an SPRA, it may be more practical to select an earthquake level above the design basis for which plant survivability is to be demonstrated. In such a seismic margin evaluation, however, the objectives are more limited than those of a seismic risk evaluation. Namely, the principal question to be addressed in a seismic margin evaluation is: “At what gound motion levels does one have a high confidence that the probability of seismically induced core damage is sufficiently low?”