## PROBABILITY OF UNACCEPTABLE RADIOACTIVE RELEASE BEYOND THE PLANT BOUNDARY

Probabilistic safety parameters determined in the probabilistic risk assessment (PRA) of a floating NPP with KLT-40S reactors are prescribed by a top level Russian regulatory document, the OPB-88/97 [I-5]. The parameters include core damage frequency and the probability of a large (limited) radioactivity release in accidents.

According to OPB-88/97, the PRA goal is to demonstrate that cumulative core damage frequency does not exceed 10-5 per reactor year, and the probability of a large radioactivity release is not higher than 10-7 per reactor year.

Level 1 PRA has been performed for a floating NPP with KLT-40S nuclear installations. According to its results, point estimate of the resulting core damage frequency of the KLT-40S under internal initiating events is about 10-7 per reactor year for initial reactor conditions, corresponding to normal power operation. Uncertainty analysis of probabilistic safety attributes, performed using a method of statistical testing (Monte Carlo method), has shown an upper confidence boundary (95 % quantile) that core damage frequency will not be higher than

[31]-6 per reactor-year.

Low probability of a severe accident with core damage is conditioned by inherent safety features (selfprotection) and other design features of this modular reactor design, as well as by redundancy and diversity of safety systems in the NPP Both active and passive safety systems are incorporated in the KLT-40S; these systems are based on components with high reliability proven by multi-year operating experience of prototype (marine) reactors.