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

This paper discusses the results obtained during an investigation of WWER-1000 Nuclear Power Plant (NPP) behaviour at low power and cold conditions during an overpressurization in the primary circuit. The reference nuclear power plant for this analysis is Unit 6 at Kozloduy NPP (KNPP) site. The systems and equipment of the KNPP, Unit 6 operate according to the design requirements for the corresponding level of reactor power. In this paper is also presented an analysis of the additionally installed equipment for situations such as cold overpressurization. This equipment was mounted during the Modernization program in KNPP. In the paper is analysed the response of the new equipment together with other NPP available safety systems during the postulated transient in shutdown state. For the purpose of this analysis a RELAP5/MOD3.2 computer code has been used to simulate the transient for WWER-1000/V320 NPP model. A model of WWER-1000 based on Unit 6 of Kozloduy NPP has been developed for the RELAP5/MOD3.2 code at the Institute for Nuclear Research and Nuclear Energy - Bulgarian Academy of Sciences (INRNE-BAS), Sofia. The low power and cold conditions and the modifications after the modernization program, have been taken into account. The main purpose of this analysis is to estimate the parameters of the monitored plant which are used to identify symptoms that are used by operators to identify the plant's state and the Critical Safety Function (CSF). The analysis is also used to define the timing needed to reach the following stages during the progression of processes in the reactor system: Reaching the saturated temperature at the outlet of the assembly. Beginning of reactor core uncover. Heating up of fuel. Defining the transition time between Emergency Operating Procedures (EOPs) and Severe Accident Management Guidance (SAMG) at temperature of 923.15 K. Restoring of water level in the core. Defining the CSF "Integrity" status and the time of its loss. The results of the thermal-hydraulic analysis have been used to assist KNPP specialists in analytical validation of EOPs at low power and cold conditions. 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. (C) 2013 Elsevier Ltd. All rights reserved.