RBMK

The RBMK (Russian: Реактор большой мощности канальный, РБМК; reaktor bolshoy moshchnosti kanalnyy, "high-power channel-type reactor") is a class of graphite-moderatednuclear power reactor designed and built by the Soviet Union. It is somewhat like a boiling water reactor as water boils in the pressure tubes. It is one of two power reactor types to enter serial production in the Soviet Union during the 1970s, the other being the VVER reactor. The name refers to its design where instead of a large steel pressure vessel surrounding the entire core, the core is surrounded by a cylindrical annular steel tank inside a concrete vault and each fuel assembly is enclosed in an individual 8.4 cm (inner) diameter pipe (called a "technological channel"). The channels also contain the coolant, and are surrounded by graphite.

The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation. Certain aspects of the original RBMK reactor design had several shortcomings, such as the large positive void coefficient, the 'positive scram effect' of the control rods and instability at low power levels—which contributed to the 1986 Chernobyl disaster, in which an RBMK experienced an uncontrolled nuclear chain reaction, leading to a steam and hydrogen explosion, large fire, and subsequent core meltdown. Radioactive material was released over a large portion of northern and southern Europe—including Sweden, where evidence of the nuclear disaster was first registered outside of the Soviet Union, and before the Chernobyl accident was communicated by the Soviet Union to the rest of the world. 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 with the aggregate power of operational units at almost 7 GW of installed capacity. Most of the flaws in the design of RBMK-1000 reactors were corrected after the Chernobyl accident and a dozen reactors have since been operating without any serious incidents for over thirty years.

RBMK reactors may be classified as belonging to one of three distinct generations, according to when the particular reactor was built and brought online:

• Generation 1 – during the early-to-mid 1970s, before OPB-82 General Safety Provisions were introduced in the Soviet Union.
• Generation 2 – during the late 1970s and early 1980s, conforming to the OPB-82 standards issued in 1982.
• Generation 3 – post Chernobyl accident in 1986, where Soviet safety standards were revised to OPB-88; only Smolensk-3 was built to these standards.

Beside RBMK, there are several other graphite-moderated reactors. A graphite-moderated Magnox reactor exists in North Korea at the Yongbyon Nuclear Scientific Research Center. While the gas cooled Magnox, AGR and pebble bed reactors (Such as the Dragon reactor at Winfrith) use graphite as moderators their use of gases (carbon dioxide for Magnox and AGR, while helium for Dragon) as heat transfer fluids causes them to have no void coefficient. 4 EGP-6 graphite water reactors which are a scaled down version of the RBMK were operating at the world's second northernmost nuclear power plant i.e. the Bilibino Nuclear Power Plant. One EGP-6 was shut down permanently in 2020. The remaining three EGP-6 were shut down permanently in December 2025.