Investigation of critical safety function "Heat sink" at low power and cold condition for Kozloduy Nuclear Power Plant WWER-1000/V320

Highlights: ► In this study we investigated three scenarios with loss of CSF "Heat Sink". ► The reference power plant for the analyses is Unit 6 at Kozloduy NPP. ► The RELAP/MOD 3.2 computer code is used in performing the analyses. ► The results are used for analytical validation of EOP at low power. - Abstract: This paper presents the results of thermal–hydraulic calculation of accident scenarios that involve the loss of critical safety function (CSF) "Heat sink" for WWER-1000/V320 units at Kozloduy Nuclear Power Plant (KNPP), done during the development of Symptom Based Emergency Operating Procedures (SB EOPs) for this plant at low power and cold condition. The main purpose of this analysis is to provide the response of monitored plant parameters to identify symptoms available to the operators and define timing for reaching the following stages during the development of processes in the reactor system: •Reaching the saturated temperature at the outlet of the assembly. •Beginning of reactor core uncovery. •Heating up of fuel. •Defining the transition time between EOPs and SAMG at temperature of 923 K. •Restoring of water level in the core. •Defining the CSF "Heat sink" status and the time of its loss. The results of the thermal–hydraulic analyses have been used to assist KNPP specialists in analytical validation of EOPs at low power and cold condition. The principal acceptance criteria for EOPs are averting the onset of core damage. The RELAP5/MOD3.2 computer code has been used in performing the analyses in a WWER-1000 Nuclear Power Plant (NPP) model. A model of WWER-1000 based on Unit 6 of Kozloduy NPP has been developed for the systems thermal–hydraulics code RELAP5/MOD3.2 at the Institute for Nuclear Research and Nuclear Energy–Bulgarian Academy of Sciences (INRNE–BAS), Sofia. The low power and cold condition and the modifications after the modernization program are taken into account.