The RBMK ( Reaktor Bolshoy Moshchnosti Kanalnyy, "High Power Channel-type Reactor") is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union.
The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the RBMK reactor design, such as the positive void coefficient properties, the graphite-tipped control rods and instability at low power levels, contributed to the 1986 Chernobyl disaster, in which an RBMK exploded during a mishandled test, and radioactivity was released over a large portion of Europe. The disaster prompted worldwide calls for the reactors to be completely decommissioned. However, there is still considerable reliance on RBMK facilities for power in Russia. While nine RBMK blocks under construction were cancelled after the Chernobyl disaster, and the last of three remaining RBMK blocks at the Chernobyl Nuclear Power Plant was finally shut down in 2000, as of 2013 there are still 11 RBMK reactors operating in Russia – though all 11 have been retrofitted with a number of safety updates.