Insights from the Analyses of Other External Hazards for Nuclear Power Plants

Because the probable maximum events selected in FSAR for nuclear plants may not be the maximum possible events, they could possibly be exceeded by more severe events in the future. As such, there is a need to re-evaluate the other external hazards, especially those associated with the natural phenomena. To ensure that the maximum possible intensities of the natural phenomenon hazards are identified and analyzed, one has to be able to identify the physical limits of the parameters that define the intensities of the hazards. However, in some cases, it is truly difficult to identify the absolute, physical limits of parameters associated with selected natural hazards. One way to address the issue of exceeding the probable maximum event is to evaluate the quantitative risk in terms of core damage and large early release frequencies resulting from the specific hazard of concern. This will require the estimation of the hazard frequency. While it may be possible to assess the occurrence frequencies of selected natural phenomena of limited intensity, the uncertainty in the assessed frequencies of events with magnitude beyond the range of historical occurrences may be uncomfortably high. Furthermore, some of the external hazards may not lend themselves to an easy assessment of their occurrence frequencies. As such, deterministic criteria will still need to be used for the risk evaluation of selected hazard events. This paper groups the entire set of other external hazards into a number of categories and discusses the characteristics, PRA evaluation methods, and other aspects of each of these groups.