Influence of a high pressure injection system on the large break loca event tree

Performing a Level 1 PSA analysis for the Krsko NPP, the authors have constructed an event tree for a large break loss of coolant accident (LOCA) according to assumptions in the Final Safety Analysis Report. Because of conservative assumptions one complete emergency core cooling system train is necessary to mitigate the consequences. On the basis of a deterministic conservative analysis, the authors have, however, tried to find out what the minimum success criteria requirement would be. In the first part of the analysis, the influence of high pressure injection on the large break LOCA transient has been tested. It was shown that the cladding temperatures without high pressure safety injection (HPSI) system were approximately 50 K higher than with HPSI operating. The second part of the analysis showed that, for the worst case combination of plugging, which for Krsko NPP is 10% plugging and 35% break size, the peak cladding temperature exceeds the allowed limit of 1478 K. The lesson learnt is that the plugging of steam generators demands HPSI to mitigate a large break LOCA. (author). 3 refs, 4 figs