This paper discusses the results obtained during an investigation of WWER-1000 Nuclear Power\nPlant (NPP) behavior at shutdown reactor during maintenance. For the purpose of the analysis is\nselected a plant operating state with unsealed primary circuit by removing the MCP head. The reference\nnuclear power plant is Unit 6 at Kozloduy NPP (KNPP) site. RELAP5/ MOD3.2 computer\ncode has been used to simulate the transient for WWER-1000/V320 NPP model. A model of\nWWER-1000 based on Unit 6 of KNPP has been developed for the RELAP5/MOD3.2 code at the Institute\nfor Nuclear Research and Nuclear Energy-Bulgarian Academy of Sciences (INRNE-BAS), Sofia.\nThe plant modifications performed in frame of modernization program have been taken into account\nfor the investigated conditions for the unsealed primary circuit. The most specific in this\nanalysis compared to the analyses of NPP accidents at full power is the unavailability of some important\nsafety systems. For the purpose of the present investigation two scenarios have been studied,\ninvolving a different number of safety systems with and without operator actions. The selected\ninitiating event and scenarios are used in support of analytical validation of Emergency\nOperating Procedures (EOP) at low power and they are based on the suggestions of leading KNPP\nexperts and are important in support of analytical validation of EOP at low power.
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