At the time of the Chernobyl accident, on 26 April 1986, the Soviet nuclear power programme was based mainly upon two types of reactor, the WWER, a pressurized light-water reactor, and the RBMK, a graphite moderated light-water reactor. The RBMK reactors were developed in order to produce both electricity and plutonium for military purposes. For that reason they were built only within the Union.

The Chernobyl power complex, lying about 130 km north of Kiev, Ukraine, consisted of four nuclear reactors of the RBMK-1000 design, Units 1 and 2 being constructed between 1970 and 1977, while Units 3 and 4 of the same design were completed in 1983 (IA86). At 01:23 hr on Saturday, 26 April 1986, reactor 4 exploded. We want to give here a short description of the reactor, especially of the features which explain why the accident was not only possible, but probable, as well as the sequence of events that led to the explosion. This appendix is based on the reports made public on the World Wide Web by the IAEA, the NEA and the IRSN. A thorough description of the RBMK reactors and the circumstances of the accident can be found in Libmann [60].