Regional Basic Professional Training Course in Korea

❙ 591 ❙
9. Probabilistic Safety Analysis
Modelling adequacy. Even for those scenarios that have been identified, the event
sequence and system logic models do not precisely represent reality. There are
uncertainties introduced in the definition of success criteria, in the assessment of
physical phenomena, of behaviour of equipment, and of recovery actions. Although
it is possible to quantify at least a part of these uncertainties (see NUREG 1150),
the usual approach is to perform sensitivity studies to assess the impact of these
modelling uncertainties and their relative importance.
Input parameters uncertainty. The input parameters are not exactly known, due to
scarcity or lack of data. This is especially the case of parameters related to CCFs,
HRA or rare IEs. It is possible to assess the effect of these uncertainty on the
overall PSA results by estimating the uncertainty related to each basic parameter
and to propagate these uncertainties through the PSA model by a Monte-Carlo
method.
Nevertheless, it should be emphasized that the uncertainty is not inherent in the
probabilistic safety assessment, but essentially stems from limited knowledge. This being
the case, one of the important features of the PSA is that it indicates the fields in which
obtaining further knowledge would be particularly useful.