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Regional Basic Professional Training Course in Korea

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examples of useful <strong>in</strong>sights of PSAs for operation.<br />

9.6.2. General pr<strong>in</strong>ciples for us<strong>in</strong>g PSAs<br />

❙ 605 ❙<br />

9. Probabilistic Safety Analysis<br />

This paragraph recalls the ma<strong>in</strong> limits and advantages of PSAs, and suggests some good<br />

practices to use PSAs <strong>in</strong> the mak<strong>in</strong>g decision process.<br />

9.6.2.1. Limits<br />

PSAs are abstract, global and simplified models. There are many k<strong>in</strong>ds of limits that are<br />

necessary to know <strong>in</strong> order to understand the advantages and real contributions to safety<br />

assessments.<br />

Limits due to PSA <strong>in</strong>put events: Uncerta<strong>in</strong>ty and <strong>in</strong>accuracy <strong>in</strong> statistics when<br />

events occur (ex:component failure rates), or do not occur (ex: rare <strong>in</strong>itiat<strong>in</strong>g<br />

events, common cause failures), or limits on human reliability assessment. For<br />

these limits, it is possible to build uncerta<strong>in</strong>ty models, even if it is difficult for rare<br />

events and HRA.<br />

Limits due to PSA sequence and system models: Physics phenomena<br />

(simplifications and uncerta<strong>in</strong>ties), fault-trees and event-trees modell<strong>in</strong>g<br />

(simplification of comb<strong>in</strong>ations, static representation), software, <strong>in</strong>strumentation<br />

(many times not modelled), human factors, and organizational factors (generally<br />

not approached). For these limits, it is not possible to build uncerta<strong>in</strong>ty models.<br />

Limits due to the safety of the plants: Some features are due to the fact that it is<br />

difficult to analyse safety of plants. Sequences are rare, model verification us<strong>in</strong>g<br />

real feedback is impossible, and results are often very sensitive to details. It<br />

requires that PSAs are as realistic as possible. Models often have to be very

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