Event selection probability

Since dependency analysis is not needed, we can go on to the BUILD program. Go to FTAPSUIT and select 5 "Run Build." It asks you for the input file name including extender. Type "pv.pch," It asks you for name and extender of the input file for IMPORTANCE. Type, for examle, "pv.ii . It next asks for the input option. Type "5" for ba.sic event failure probabilities. This means that any failure rates must be multiplied by their mission times as shown in Table 7.4-1. (FTAPlus was written only for option 5 which uses probabilities and error factors. Other options will require hand editing of the pvn.ii file. The switch 1 is for failure rate and repair time, switch 2 is failure rate, 0 repair time, switch 3 is proportional hazard rate and 0 repair time, and switch 4 is mean time to failure and repair time.)... 

The probability of selecting a male at random can be calculated by adding the probabilities for the events "male < 45 years," "male 46-64 years," and "male > 65 years," because these are all mutually exclusive events. The probability can be calculated as follows ... 

Fault Tree. When direct data allowing to calculate the probability of a failure mode are not available or this failure form is complex, it is proposed the elaboration of a fault tree. It is a method of multidisciplinar analysis that begins with the selection of a failure mode or event that is tried to avoid. The event is developed into its immediate causes, and the sequence of events continues until basic causes are identified. The fault tree is constructed showing the logical event relationships that are necessary to result in the top event. The fault tree reaches terminal events whose probability must be calculated or estimated. These events can be basic events, which do not require to be explained by means of other previous events, or events which are not developed because it is not considered necessary or for lack of information. 

For hazard category 1 and 2 research reactors, the design basis wind can be evaluated on the basis of the selected probability of exceedance for the external event hazard according to the performance goal assigned to their safety class 1 and 2 structures, systems and components. For more sophisticated investigations and analysis, further guidance is provided in Ref. [22],... 

In so doing, we obtain the condition of maximum probability (or, more properly, minimum probable prediction error) for the entire distribution of events, that is, the most probable distribution. The minimization condition [condition (3-4)] requires that the sum of squares of the differences between p and all of the values xi be simultaneously as small as possible. We cannot change the xi, which are experimental measurements, so the problem becomes one of selecting the value of p that best satisfies condition (3-4). It is reasonable to suppose that p, subject to the minimization condition, will be the arithmetic mean, x = )/ > provided that... 

Layer of protection analysis (LOPA) is a simplified form of event tree analysis. Instead of analyzing all accident scenarios, LOPA selects a few specific scenarios as representative, or boundary, cases. LOPA uses order-of-magnitLide estimates, rather than specific data, for the frequency of initiating events and for the probability the various layers of protection will fail on demand. In many cases, the simplified results of a LOPA provide sufficient input for deciding whether additional protection is necessary to reduce the likelihood of a given accident type. LOPAs typically require only a small fraction of the effort required for detailed event tree or fault tree analysis. 

Selection 6 goes to FTAPlus, from which menu 5 is selected, to provide the inl.,.. ation needed for IMPORT. This program presents event names as given in FTAP and POST and requests failure rates or probabilities (e-format), uncertainty (f-format), and an event description. Each time this set is given, it presents the next event name until all events are specified. Then it returns to the main FTAPlus menu to be exited by selecting 6 to go to the FTAPSUIT menu. 

For acute releases, the fault tree analysis is a convenient tool for organizing the quantitative data needed for model selection and implementation. The fault tree represents a heirarchy of events that precede the release of concern. This heirarchy grows like the branches of a tree as we track back through one cause built upon another (hence the name, "fault tree"). Each level of the tree identifies each antecedent event, and the branches are characterized by probabilities attached to each causal link in the sequence. The model appiications are needed to describe the environmental consequences of each type of impulsive release of pollutants. Thus, combining the probability of each event with its quantitative consequences supplied by the model, one is led to the expected value of ambient concentrations in the environment. This distribution, in turn, can be used to generate a profile of exposure and risk. 

Figure 23-1 shows the hazards identification and risk assessment procedure. The procedure begins with a complete description of the process. This includes detailed PFD and P I diagrams, complete specifications on all equipment, maintenance records, operating procedures, and so forth. A hazard identification procedure is then selected (see Haz-ard Analysis subsection) to identify the hazards and their nature. This is followed by identification of all potential event sequences and potential incidents (scenarios) that can result in loss of control of energy or material. Next is an evaluation of both the consequences and the probability. The consequences are estimated by using source models (to describe the... 

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