RBMK Reactor

The RBMK is a graphite moderated nuclear power reactor designed and built by the Soviet Union, using individual pressure tubes around each fuel assembly instead of one large pressure vessel, an arrangement that makes it broadly similar in principle to a boiling water reactor: water boiling inside the pressure channels generates the steam that drives the turbines, while the surrounding graphite moderates neutrons well enough that the reactor can run on uranium enriched to only 1.8 to 2.0 percent, lower than most Western reactor designs need. The design was finalized in 1968, construction of the first unit began at Leningrad in 1970, and Leningrad Unit 1 began operating in 1973, with further units built through the 1970s and 1980s; as of December 2025 there are 26 RBMK reactor blocks in total, of which 7 remain operational, 9 have been decommissioned, 9 were cancelled, and 1 was destroyed in an accident. That accident was the 1986 Chernobyl disaster, in which an uncontrolled chain reaction caused a steam and hydrogen explosion, exposing design flaws including a large positive void coefficient and weaknesses in the control rod design; despite calls worldwide for the type to be retired, several RBMK reactors continue operating today after post accident modifications.