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Regional Basic Professional Training Course in Korea

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Assessment -vs- demonstration.<br />

PSA general objectives<br />

❙ 609 ❙<br />

9. Probabilistic Safety Analysis<br />

A PSA highlights the weakest po<strong>in</strong>ts of a plant, so it can help to prioritise and choose the best<br />

actions <strong>in</strong> order to improve safety if necessary, know<strong>in</strong>g the associated risk. It can also<br />

identify some conservatism, and eventually relax useless constra<strong>in</strong>ts. This objective is<br />

consistent with a basic pr<strong>in</strong>ciple of the risk-<strong>in</strong>formed regulation be<strong>in</strong>g developed <strong>in</strong> the USA.<br />

Probabilistic goals<br />

INSAG-3 lists technical safety goals for exist<strong>in</strong>g plants:<br />

The overall probability of severe core damage (Core Damage Frequency) should be<br />

under 10-4 per reactor year. This objective <strong>in</strong>tegrates both <strong>in</strong>ternal and external<br />

events;<br />

No accident sequence should contribute excessively to risk, <strong>in</strong> comparison to others<br />

(well balanced risk);<br />

Severe accident management should decrease at least by a fact 10 the probability of<br />

large radioactive releases need<strong>in</strong>g immediate countermeasures outside the plant<br />

(Large Early Release Frequency).<br />

National safety authorities sometimes request stronger objectives than these <strong>in</strong>ternational<br />

values.<br />

Reference PSA model<br />

Application of PSA for safety analysis needs to have a large scope, <strong>in</strong>tegrated PSA model<br />

of the plant, at least of level 1 (Core Damage Frequency) and for <strong>in</strong>ternal events. This<br />

reference model has to be revised accord<strong>in</strong>g to the evolution of the plant (liv<strong>in</strong>g PSA<br />

pr<strong>in</strong>ciple). For specific issues, the analysis sometimes needs a more detailed study of a<br />

limited number of <strong>in</strong>itiators.

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