Evaluation of non-regular means for heat removal from RBMK reactor core in case of BDBA

Abstract

The most dangerous BDBAs for RBMK reactors leading to the worst consequences are related with loss of long-term heat remval from the core.It can occur due to large break in reactor cooling system with failure of emergency core cooling system, or in case of station blackout. Due to specific design of RBMK there are a few possibilities for heat removal from reactorcore by non-regular means: depressurisation of reactor cooling system 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 control rods cooling circuit, etc 211 control rods are placed into special channels and distributed evenly in the reactor core.These channels are filled with water supplied by the low pressure system totally independent from the reactor cooling system. The heat from fuel channel, where heat is generated, is transferred in radial direction through graphite bricks to cooled control rods channels. This paper presents the analysis of such heat removal by employing RELAP5, RELAP5-3D and RELAP/SCDAPSIM codes. The analysis was performed for Ignalina NPP RBMK-1500 reactor.The results of analysis show that such cooling enables to remove 10-30 MW of the generated heat from the reactor core.This amount of removed heat is comparable with reactor decay heat in long-term period.Due to high thermal inertia of graphite this heat removal is very slow, however it allows to slowdown the core heat-up process and prevents fuel melting.