Event tree analysis scope

Figure 21.2 illustrates how the starting point, the directions and the scope of each method fit into the accident-analysis framework of Chapter 6. Two of the methods. Fault tree analysis and Comparison analysis are deductive in that they start with the unwanted event. They proceed by analysing the underlying incidents and deviations (Fault tree analysis) or contributing factors (Comparison analysis). Several of the methods are mainly inductive in that they start with a deviation and proceed by studying the effects of this deviation. This applies to HAZOP, Failure mode and effect analysis. Event tree analysis and CRIOP, although they also have a component of causal analysis. Coarse analysis and Job-safety analysis start with the hazard and use a combination of inductive and deductive analyses. 

Failure rates for both equipment and peoples responses are assigned and the frequency and severity of a TOP Event can be calculated. Should the risk be found to be unacceptable, additional process safety hardware or additional procedures can be recommended. Then, calculations can be made to determine the benefits of the additional hardware or procedures. The Fault Tree Analysis method of evaluation is very sophisticated and a detailed explanation is beyond the scope of this book. 

The first step in performing a fault tree analysis is to collect the appropriate project description documents, existing hazard analyses, and guidance documents and carefully review them to determine the limits, scope, and ground rules for the FTA.This review includes defining the system to be analyzed, the depth or indenture levels to be included in the effort, and, of course, the nature of the undesired event or failure to be studied. 

The reviews of the event trees indicate that the trees accurately model the responses to the postulated initiating events, given the simplifying assumptions used to reduce the complexities of the analysis. The principal scoping assumption, i.e., that the reactor is operating,at full power, is typical of most reactor PRAs. Some concern was expressed that shutdown operations and operation at less than full power were not modelled. Commercial reactor experience has shown that analyses of these operational phases can provide important insights. These concerns need not be resolved before restart, because it is expected they will have only small contributions to core damage frequency and plant risk. 

Cost identification often involves the development of a probability or decision tree of the therapeutic pathway that describes all relevant downstream events related to use of that therapy and its comparator(s). Once the relevant resources are identified and measured (e.g. number of physician visits, treatment of side effects, number and duration of hospital visits, etc.), local costs/prices can be applied to those resources to determine the overall cost of that intervention. The scope of the resources (and costs) included in an analysis is determined by the perspective (or intended audience) of the study. 

Over the next several years, the PRA was detailed to the point it included detailed fault trees of the mechanical, electrical, and instrumentation and control systems and the scope was expanded to include shutdown, fire, flood events, and large release frequency and off-site dose quantifications. Core damage frequency PRA was supported by extensive plant thermal-hydraulic analysis to justify success criteria. Extensive testing and thermal-hydrauUc analysis, to support containment integrity during core melt sequences, underpirmed the large release PRA. 

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