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Regional Basic Professional Training Course in Korea

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<strong>Regional</strong> <strong>Basic</strong> <strong>Professional</strong> <strong>Tra<strong>in</strong><strong>in</strong>g</strong> <strong>Course</strong> (BPTC) on Nuclear Safety<br />

9.4.6. Treatment of uncerta<strong>in</strong>ties<br />

Uncerta<strong>in</strong>ties will arise <strong>in</strong> the level 2 PSA analysis as the result of several factors,<br />

<strong>in</strong>clud<strong>in</strong>g:<br />

Completeness: there is no guarantee that all possible scenarios have been identified<br />

and properly assessed; this lack of completeness can be reduced by extensive peer<br />

review;<br />

Modell<strong>in</strong>g adequacy: uncerta<strong>in</strong>ties <strong>in</strong> the modell<strong>in</strong>g are either due to <strong>in</strong>complete<br />

knowledge of the phenomena, or to <strong>in</strong>adequacies or simplifications <strong>in</strong>troduced <strong>in</strong><br />

the modell<strong>in</strong>g of the phenomena;<br />

Uncerta<strong>in</strong>ties <strong>in</strong> <strong>in</strong>put parameters: the parameters of the various models used <strong>in</strong> the<br />

PSA are not exactly known because of scarcity or lack of data, and the models<br />

<strong>in</strong>clude assumptions made by experts; the group<strong>in</strong>g of level 1 sequences <strong>in</strong>to PDSs<br />

as the <strong>in</strong>put to level 2 analysis <strong>in</strong>troduces uncerta<strong>in</strong>ties <strong>in</strong> addition to those<br />

associated with the specific level 2 PSA parameters.<br />

There is no universally accepted approach to uncerta<strong>in</strong>ty analysis. The analysis should<br />

make use of all the <strong>in</strong>formation available <strong>in</strong> a structured fashion to arrive at a conclusion<br />

about both the relative probabilities of accident progression sequences, and the<br />

radiological source terms associated with them.<br />

The format and range of uncerta<strong>in</strong> parameters of each issue identified must either be<br />

def<strong>in</strong>ed by probability distribution, or, more simply, represented by sensitivity bounds.<br />

The use of probability distributions, represent<strong>in</strong>g a mathematically more rigorous<br />

approach, will facilitate the propagation of uncerta<strong>in</strong>ties <strong>in</strong> the PSA models. Whichever<br />

format is adopted, judgment on the formulation of uncerta<strong>in</strong>ties/sensitivities is supported<br />

❙ 598 ❙

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