Evaluation of safety for VVER-1200-based nuclear power plants during main steam-line break accidents

Abstract

In this research project, main steam-line break accidents (MSLBs) in a VVER-1200 nuclear power plant were investigated. Three different break sizes, 100, 200 and 400 cm2, were considered to observe the effect of break size on the transient response of the safety systems. It was assumed that the break was located inside the containment building and thus there was no emission of radioactive substances unless there was a containment failure. The simulations were performed for two cases, one in which the SCRAM system was initiated depending on the trip conditions specified for VVER-1200, and another in which there was no SCRAM i.e., anticipated transient without SCRAM (ATWS) situation. The simulations were performed for 12 hours to observe the performance of the safety systems to control the situation when emergency response is unavailable. The accident scenarios were simulated in PCTRAN (Personal Computer Transient Analyzer) VVER-1200 module. Results revealed that SCRAM was triggered due to low SG level signal for 100 and 200 cm2 MSLBs while it was triggered by low sub-cooling margin (SCM) for 400 cm2 MSLB. In absence of SCRAM, however, fuel temperature coefficient (FTC) and negative moderator temperature coefficient played a crucial role in keeping reactor thermal power under control. Nevertheless, sharp spikes in reactor power were observed for ATWS cases between 400-600 seconds due to the simultaneous effect of control rods and moderator temperature on reactivity. For both scenarios considered, fuel and clad temperatures did not rise above 1875 and 625oC respectively, indicating that both fuel meltdown and cladding failure is extremely unlikely. The containment pressure was significantly below 5 bar for all the scenarios, eliminating the possibility of containment failure. However, MSLBs with ATWS cases recorded bubble condenser membrane rupture due to overpressure. Finally, DNBR was always greater than unity for MSLBs with SCRAM. They were well above 4.5 and 3.0 for 100 and 200 cm2 MSLBs with ATWS, respectively. However, DNBR went below unity for 400 cm2 MSLB with ATWS after 2705 seconds, which is an indication of boiling crisis.