Effect of uncertainties in best-estimate thermal hydraulic analysis on core damage frequency for PSA

Generally, thermal hydraulic (TH) analyses have been performed as part of a probabilistic safety assessment (PSA) to construct event trees and to evaluate success criteria. Even though an accident scenario in an event tree for PSA is exceedingly dependent on many uncertainty parameters, TH analysis in PSA, up to now, has been performed without considering the uncertainties for the important parameters. In the present study, TH analysis was carried out using the MARS code to simulate the large break loss of coolant accident (LBLOCA) which is one of the event sequences of level 1 PSA in an optimized power reactor 1000 MWe (OPR1000). First, the phenomena identification and ranking table (PIRT) for LBLOCA were established, and the candidate parameters were set-up. Once the input file for the MARS code was made with consideration of the uncertainties of the candidate parameters, and a parameter assessment was carried out with the MARS code to rank the candidate parameters according to the effect on peak cladding temperature (PCT). For the five highest-ranking parameters resulting from parameter assessment, the probability density function (PDF) of PCT was derived by the response surface method (RSM), and comparative Monte Carlo calculations were also performed to assess the accuracy of the RSM. As a result, it was shown that by considering the uncertainties of the TH analysis, the accident sequence, which had filed in the PSA result in the established PSA results, had a possibility of succeeding, and thus, be able to modify the core damage frequency (CDF).