Relap5/mod3.2 investigation of main loop isolating valves in case of SGTR in VVER440/V230

During the development of symptom-based emergency operating procedures (SB-EOPs) for VVER440/V230 units at Kozloduy NPP, a number of analyses have been performed using the RELAP5/MOD3.2 computer code (RELAP5/MOD3 Code Manual. Userʹs Guidelines. NUREG/CR-5535, INEL-95/0174, vols. 1–4, Rev. 1, 1990). Some of them discuss advantages and disadvantages of the use of Main Loop Isolating Valves (GZZs) in case of steam generator tube rupture (SGTR) transient. Use of the GZZs as a part of EOP mitigation strategies for SGTR accidents is analyzed in this paper to identify the behavior of important VVER parameters. A double-ended single-pipe break in SG #6 was chosen as representative. The results of these analyses demonstrate that sometimes GZZs – installed on the hot and cold legs of the primary loops – could provide safety function but sometimes their closing and re-opening could make the situation worse. RELAP5/MOD3.2 computer code has been used to simulate the SGTR transient in VVER 440 NPP model [RELAP5/MOD3 Code Manual. Userʹs Guidelines. NUREG/CR-5535, INEL-95/0174, vol. 5, Rev. 1, 1995]. This model was developed at the Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences (INRNE-BAS), Sofia, for analyses of operational occurrences, abnormal events and design bases scenarios. The model provides a significant analytical capability for specialists working in the field of NPP safety.