PRELIMINARY AND FINAL SAFETY ASSESSMENT

This section deals with the evaluation of the overall safety of a facility in relation to the basic safety objectives set forth in Sections 2.2 and 2.3. With reference to the design tasks discussed in Section 3, the proposed approach to safety assessment may be used in two steps, namely  

It is preferable to evaluate the external event hazard on a probabilistic basis. The frequency of occurrence of the parameters describing the severity of the external hazard (such as earthquake ground acceleration, wind speed, water elevation) is estimated by probabilistic methods. Statistical parameters used for extreme events include return period and annual probability of exceedance. The hazard from other rare external events such as accidental aircraft crashes or explosions reflects the frequency of occurrence of an event with postulated characteristics (quantity of explosive material, weight and velocity of the missile, etc.), as proper statistics may not be available for the area of interest. Performance goals depend on the external event categorization as defined in Section 2. For practical use they can be approximated by deriving the product (for continuous hazard levels it can be the convolution) between the annual probability of exceedance of an external event and the Pp induced by that specific external event. Probability values for performance 

In general, different combinations of hazard level and design class can be used to achieve the performance goal. Once a suitable combination of hazard level and design class is selected, siting, design and qualification of structures, systems and components can take place, as in the following sequence  

At the end, as confirmation that the basic safety objectives for the facility have been met, a safety assessment for the facility can be carried out based on the plant safety analysis, as described in the following. This assessment is expected to be included in the plant s safety analysis report. A simplified sequence for calculation of the Pp is shown in Fig. 2. 

The fragility of a structure, system or component is defined as the conditional Pp (unacceptable performance) versus the selected hazard parameter. It is usually acceptable to assume that the component failure is lognormally distributed. The fragility can be expressed in a simplified way either through its median capacity Cjoo/ or HCLP/ capacity and variability parameter p. For preliminary estimation, p = 0.3 can be used for the entire facility (Fig. 3). 

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