Verification of AP1000® low-margin PRA sequences based on best-estimate calculations

The Westinghouse AP1000® reactor is an advanced design whose safety systems are mainly passive safety systems. Due to the passive nature of the safety related systems and its dependency on small changes on certain variables (e.g. pressure, friction coefficients) together with the use of a simplified code like MAAP in Probabilistic Risk Assessment (PRA) analyses, it makes necessary to confirm that when core cooling is achieved, thermal-hydraulic (T/H) uncertainties are bounded. The T/H uncertainty evaluation process performed by Westinghouse Electric Company (WEC) identified the low-margin sequences (core uncovery) by expanding PRA Event Trees (ETs). The expanded ETs allowed finding the low-margin risk-important sequences and then a set of low-margin bounding sequences was selected. Then detailed safety analysis methodologies were applied in order to evaluate the bounding sequences demonstrating that the T/H uncertainties were bounded. The Universidad Politécnica de Madrid group has verified the low-margin bounding sequences obtained by WEC with the best-estimate TRACE code in order to verify the previous results and also to study the phenomenology of such sequences through a best-estimate code. This paper presents the results obtained for short-term low-margin bounding sequences. In general, TRACE results do not present important discrepancies with respect to NOTRUMP and WCOBRA/TRAC results although TRACE presents lower values for Peak Cladding Temperature. This analysis has allowed to verify the AP1000 thermal-hydraulic bounding evaluation process performed by WEC for the low-margin risk-important sequences with TRACE code.