Seismic Probabilistic Risk Assessment (SPRA) Methods
There are three distinct analyses that together comprise seismic PRA, the seismic hazard analysis, the fragility analysis, and the systems-accident analysis. These are brought together in the quantification analysis to produce the bottom-line analysis of core-damage frequency or frequency of large releases. As stated in the ASME-ANS Standard :
- (1) Seismic hazard analysis is used to develop frequencies of occurrence of different levels of ground motion (e.g., peak ground acceleration [PGA]) at the site;
- (2) Seismic-fragility evaluation is used to estimate the conditional probability of failure of important structures and equipment whose failure may lead to unacceptable damage to the plant (e.g., core damage); a plant walkdown is an important activity in conducting this task;
- (3) Systems/accident sequence analysis is used to model the various combinations of structural and equipment failures that could initiate and propagate a seismic core damage sequence;
- (4) Risk quantification assembles the results of the seismic hazard, fragility, and systems analyses to estimate the frequencies of core damage and plant damage states. A so-called “level-2 ” or “level-3 ” seismic PRA assesses the impact of seismic events on the containment and consequence analyses, and then integrates these results with the core damage analysis to obtain estimates of seismic risk in terms of effects on public health (e.g., early deaths and latent cancer fatalities).
This process is shown schematically in Fig. 8.11 .
Next, in the following separate sections, current status of each of these parts of SPRA is discussed.
FIG. 8.11 SCHEMATIC OVERVIEW OF A SEISMIC PRA