Investigation of nuclear power plant behaviour at low power and cold conditions during an overpressurization in primary circuit

The performed analysis is based on a previously used bounding approach in analytical validation of Symptom Based EOPs (SB EOPs). The presented thermal-hydraulic calculations of the accident scenarios involve the loss of Critical Safety Function (CSF) “Integrity” for WWER-1000/V320 units at Kozloduy Nuclear Power Plant (KNPP). During the analysis is also discussed the behaviour of CSF “Core cooling”. The most specific in this analysis compared to the analyses of NPP accidents at full power is the lack of some important safety systems due to NPP regulations. Based on this approach a list of scenarios has been performed, involving a different number of safety systems with or without operator actions. The other specific characteristic of the accidents in NPP at low power and cold conditions is that even though all process are progressing significantly slower compared to full power, the operator actions should always bring the reactor system under safety conditions.