Analysis of BDBA in RBMK-1500 reactor with long-term loss of heat removal from the core

Abstract

The Ignalina nuclear power plant (NPP) is a twin-unit with two RBMK-1500, graphite moderated, boiling water, multichannel reactors. The accident management guidelines for beyond design basis accidents (BDBAs) are in a stage of preparation at Ignalina NPP. The most challenging event from BDBAs is the unavailability of water sources for heat removal from fuel channels (FCs). Due to specific design of RBMK, there are a few possibilities for heat removal from reactor core by non-regular means: depressurisation of reactor cooling system (RCS) (if pressure in cooling circuit is high) and supply of water into cooling system from low pressure water sources, removal of heat from graphite stack by reactor gas circuit, removal of heat from reactor core using cooling circuit of control and protection system channels, etc. The possibility to remove the heat using cooling circuit of control and protection system channels looks very attractive, because the channels with control rods are cooled with water supplied by the system totally independent from the reactor cooling system. The heat from fuel channels, where heat is generated, through graphite columns is transferred in radial direction to cooled channels with control rods. Therefore, the heat removal from RBMK-1500 reactor core using control rods cooling circuit can be used as non-regular mean for reactor cool-down in case of BDBAs with loss of long-term heat removal from the core. This article presents the analysis of large loss of coolant accident (LOCA) with emergency core cooling system failure and station blackout cases without any operator intervention and when the cooling of reactor control rods is restored. For the station blackout case the other non-regular measures (depressurisation of reactor cooling system, water injection from hydro-accumulators, deaerators and city water system) are evaluated. The analysis was performed using RELAP5 Mod3.2, RELAP/SCDAPSIM and RELAP5-3D codes. The use of three codes allows to investigate all aspects of accident with long-term loss of heat removal from the core: RELAP5 code allows to perform thermal-hydraulic analysis using detailed RBMK-1500 model; RELAP/SCDAPSIM code is necessary for modelling of core damage progression; RELAP5-3D code with special “multidimensional heat conduction” model allows to model heat transfer from hot fuel channels through graphite column in radial direction. Results of the analysis have shown, that the heat removal from control rods cooling circuit allows to slowdown effectively the core heat-up process.