Event frequency risk screening

Risk screening (Section 4.3) Information used in consequence modeling I Generic frequency data on events of concern from similar plants I Risk tolerance criteria or methodologies ... 

Risk screening relies upon generic frequency data to provide conservative approximations of individual and aggregate risks. Table 4.9 offers an example of generic frequencies for major explosions for certain types of refinery process units. A methodology is also offered to predict expected event frequencies where generic data are not available. 

Analyses have shown that external events are major contributors to core damage frequencies. But experience witfj screening analyses of essentially all external events to which the plant could conceivably be exposed shows that only a very few of these are significant contributors to risk at US plants. Whether these events can be screened out will depend on the site characteristics and the plant design on a case by case basis. 

QRA is generally resource intensive. FEIR, is much less although it uses the framework of QRA for screening and ranking systems by relative risk using order-of-magnitude estimates of the frequencies and consequences of events. It incorporates plant experience and industry data to estimate the potential for future losses. 

Qualitative frequency evaluation can be used to screen out events that are extremely unlikely to occur (i.e., have such a remote chance of occurring that further evaluation is unnecessary). This method is particularly appropriate for use in conjunction with qualitative consequence evaluation as a means of ranking risks. 

---