Chernobyl reactor. Reactor hall of the Chernobyl nuclear power station, Ukraine. The floor squares in the foreground are located over the reactor's fuel channels. Four RBMK reactors were completed at Chernobyl during 1978-84; this is reactor 3. Each generates 925 megawatts of electricity. RBMK reactor cores use graphite to promote the nuclear chain reaction, and use water for cooling the vertical fuel channels. The RBMK design is flawed because its reactivity, and hence its power output, increases as coolant is lost from the fuel channels. This positive void coefficient was partly responsible for the major accident involving Chernobyl's Reactor 4 in 1986.