Chernobyl's reactor. Model of the ruined RBMK reactor core of the Chernobyl power station in the Ukraine after the accident which occurred on 26 April 1986. The reactor's 1000-tonne circular lid (centre right) is lying tilted at an angle over the reactor. Reactor 4 overheated during a low- power experiment & caused a steam explosion which released over a billion billion becquerels of radioactive nuclides into the atmosphere. RBMK reactors use graphite to promote the nuclear chain reaction, & use water for cooling the vertical fuel channels. The RBMK design is flawed because its reactivity, & hence its power output, rises as coolant is lost from the fuel channels.