Investigation of ex-vessel core catcher for SBO accident in VVER-1000/V528 containment using MELCOR code

Abstract

To mitigate consequences of core melting, an ex-vessel core catcher is investigated in this study. Instructions should be obeyed to cool down the corium caused by core melting. The corium destroys the reactor containment and causes radioactive materials to be released into the environment if it does not cool down well. It is important to build a core catcher system for the reception, localization, and cool down of the molten corium during a severe accident resulting from core melting. In this study, the role of a core catcher in the VVER-1000/v528 reactor containment during a station black out (SBO) accident is evaluated using the MELCOR1.8.6 code. In addition, parametric analyses of the SBO for (i) SBO accidents with emergency core cooling system (ECCS) operation, and (ii) without ECCS operation are performed. Furthermore, thermal–hydraulic analyses in dry and wet cavities with/without water are conducted. The investigations include the reduction of gases resulting from molten–corium–concrete interactions (H2, CO, CO2). Core melting, gas production, and the pressure/temperature in the reactor containment are assessed. Additionally, a full investigation pertaining to gas release (H2, CO, CO2) and the pressure/temperature of the core catcher is performed. Based on MELCOR simulations, a core cavity and a perimeter water channel are the best options for corium cooling and a lower radionuclide release. This simulation is also theoretically investigated and discussed herein. The simulation results show that the core catcher system in addition to an internal sacrificial material reduces the containment pressure from 689 to 580 kPa and the corresponding temperature from 394 to 380 K. Furthermore, it is observed that the amount of gases produced, particularly hydrogen, decreased from 1698 to 1235 kg. Moreover, the presence of supporting systems, including an ECCS with a core catcher, prolonged the core melting time from 16,430 to 28,630 s (in an SBO accident) and significantly decreased the gases produced.