Single-circuit, graphite-moderated, water-cooled nuclear reactor of Soviet design. Also known as the light water graphite reactor (LWGR). These reactors are used exclusively in the territory of the former Soviet Union. In 2023, 11 of them were in operation. The reactor core consists of graphite blocks arranged around cooling channels through which water flows around the fuel rods. The water is boiled directly in the channel, from where dried steam flows to the turbine. The reactor has several design flaws, such as the control rod design, and a positive void coefficient.

An infamous representative of the RBMK type was the Chernobyl reactor, where a nuclear accident occurred after a series of fatal operator errors.