The Specifics of RBMK Core Cooling at Overheated Conditions

Abstract

Reactor RBMK-1500 of Ignalina NPP is a boiling light water reactor with graphite moderator. Several important design features of RBMK-1500 are unique and extremely complex in respect to western reactors: the fuel assemblies are loaded into individual channels rather than a single pressure vessel; the plant can be refueled on-line; the neutron spectrum is thermalized by a massive graphite moderator. The reactor coolant system consists of two loops, each having flow length of more than 200 m. There are 1661 of vertical parallel fuel channels and numerous components, such as headers, pumps, valves, etc. The fuel channels and fuel claddings are made of Zirconium-Niobium alloy. From the point of view of safety barriers, each fuel channel in RBMK-type reactor corresponds to a pressure vessel of vessel-type reactors. Thus, the fuel channels are the most important element in reactor cooling system. However, in case of beyond design basis accidents with loss heat removal from the core the integrity of fuel channels could be challenged as they are not so strong as the pressure vessel. The paper presents the analysis of different possibilities to cooldown the core of RBMK-type reactors. Injection of water to RCS is considered as main strategy. Such “bleed and feed” procedure is used for vessel type reactors, but at present is not considered at RBMK-1500.