Investigation of primary to secondary leakage at low power during “Hydro-test” for VVER440/V230 at Kozloduy NPP

Abstract

This paper presents the thermal–hydraulic investigation “Primary to secondary leakage at low power” for analytical validation of Symptom Based Emergency Operating Procedures (SBEOPs) at low power for VVER440/V230 units 3 and 4 at Kozloduy NPP. 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, after primary to secondary leakage: • Reaching the saturated temperature at the outlet of the assembly. • Beginning of reactor core uncovery. • Define the time for reaching maximum level in the damaged SG. • Termination of natural circulation after decreasing of water level in reactor vessel below the hot nozzles. • Defining the time for opening SVs of damaged SG. • Define the CSF status and the time of loss of Critical Safety Functions. The thermal–hydraulic analysis for analytical validation of Symptom Based Emergency Operating Procedures (SBEOPs) at low power of reactor for VVER440/V230 units 3 and 4 at Kozloduy NPP is performed with RELAP5/MOD3.2 computer code. The RELAP5/MOD3.2 model of Kozloduy NPP VVER-1000 for investigation of operational occurrences, abnormal events, and design basis scenarios have been developed and validated in the Institute for Nuclear Research and Nuclear Energy – Bulgarian Academy of Sciences (INRNE–BAS) Sofia, and Kozloduy NPP. The model provides a significant analytical capability for the specialists working in the field of NPP safety.