The RBMK (, РБМК; 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. 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.Chernov D., Sornette D. Man-Made Catastrophes and Risk Information Concealment: Case Studies of Major Disasters and Human Fallibility. Springer. 2015. P. 71
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. [a slide at 00:33:00] 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.
Lifespan
Initially the service life of the RBMK reactor type was expected to be 30 years, but this may be extended to 45 years with mid-life refurbishments (such as fixing the issue of graphite stack deformation in the core), and eventually a 50-year lifetime was adopted for some units (Kursk 1-3 and 1-4, Leningrad 1-3 and 1-4, Smolensk 1-1, 1-2, 1-3). Efforts are underway to extend the license of all the units. In July 2024, Leningrad unit 3's license was extended from 2025 to 2030. In February 2026, the Russian nuclear regulator approved a five year life extension to Leningrad Unit 4 to operate until 2031."Leningrad 4 given five-year life extension licence - World Nuclear News" https://www.world-nuclear-news.org/articles/leningrad-4-given-five-year-life-extension Today all the reactors are operated by Rosatom's subsidiary Rosenergoatom.
In 2026, it was reported that the RBMK units of the Kursk plant had undergone significant safety upgrades reducing risks by almost 100 times.
History The RBMK was the culmination of the Soviet nuclear power program to produce a water-cooled power reactor with dual-use potential based on their graphite-moderated plutonium production military reactors. The first of these, Obninsk AM-1 ("Атом Мирный", Atom Mirny, Russian for "peaceful atom," analogous to the American Atoms for Peace) generated 5 MW of electricity from 30 MW thermal power, and supplied Obninsk from 1954 until 1959. Subsequent prototypes were the AMB-100 reactor and AMB-200 reactor both at Beloyarsk Nuclear Power Station.
The RBMK was a minimalist design that used regular (light) water for cooling and graphite for moderation, making it possible to use fuel with a lower enrichment (1.8% enriched uranium instead of considerably more expensive 4% enrichment). This allowed for an extraordinarily large and powerful reactor that could be built rapidly, largely out of parts fabricated on-site instead of by specialized factories. Because a containment building would have needed to be very large and expensive, doubling the cost of each unit, due to the large size of the RBMK, it was originally omitted from the design. It was argued by its designers that the RBMK's strategy of having each fuel assembly in its own channel with flowing cooling water was an acceptable alternative for containment.
The RBMK was mainly designed at the Kurchatov Institute of Atomic Energy and , headed by Anatoly Aleksandrov and Nikolai Dollezhal respectively, from 1964 to 1966. A top-secret invention patent for the RBMK design was filed by Aleksandrov, who personally took credit for the design of the reactor, with the Soviet patent office. The RBMK was proclaimed by some as the national reactor of the Soviet Union, probably due to nationalism because of its unique design, large size and power output. Meanwhile the VVER (an earlier Soviet reactor design) was called the "American reactor" due to the pressurized water (PWR) design shared with many Western reactors. The RBMK was favored over the VVER by the Soviet Union due to its ease of manufacture, due to a lack of a large and thick-walled reactor pressure vessel and relatively complex associated steam generators, and its large power output, which would allow the Soviet government to easily meet their central economic planning targets.
The RBMK-1000's design was finalized in 1968. At that time it was the world's largest nuclear reactor design, surpassing western designs and the VVER in power output and physical size, being 20 times larger by volume than contemporary western reactors. Similarly to CANDU reactors and the Indian IPHWR reactors it could be produced without the specialized industry required by the large and thick-walled reactor pressure vessels such as those used by VVER reactors, thus increasing the number of factories capable of manufacturing RBMK reactor components.
The initial 1000 MWe design of the RBMK-1000 left room for development into yet more powerful reactors. For example, the RBMK reactors at the Ignalina Nuclear Power Plant in Lithuania were rated at 1500 MWe each, a very large size for the time and even for the early 21st century. For comparison, the EPR has a net electric nameplate capacity of 1600 MW (4500 MWthermal) and is among the most powerful reactor types ever built.
The flaws in the original RBMK design were recognized by others, including from within the Kurchatov Institute before the first units were built, but the orders for construction of the first RBMK units, which were at Leningrad, had already been issued in 1966 by the Soviet government by the time their concerns reached the Central Committee of the Communist Party of the Soviet Union and the Soviet Council of Ministers. This prompted a sudden overhaul of the RBMK. Plutonium production in an RBMK would have been achieved by operating the reactor under special thermal parameters, but this capability was abandoned early on.History of the atomic energy of the Soviet Union and Russia. Issue 3. History of RBMK. Ed. Sidorenko V.A. - Moscow, IzdAT, 2003. . "History of Rosatom" ('История атомной энергетики Советского Союза и России. Вып. 3. История РБМК. Под ред. Сидоренко В. А. — М.: ИздАТ, 2003.. Электронная библиотека «История Росатома» — http://elib.biblioatom.ru/text/istoriya-atomnoy-energetiki_v3_2003/go,0/ ) This was the design that was finalized in 1968. The redesign did not solve further flaws that were not discovered until years later.
No prototypes of the RBMK were built; it was put directly into mass production. Construction of the first RBMK, which was at Leningrad Nuclear Power Plant, began in 1970. Leningrad unit 1 opened in 1973.
At Leningrad it was discovered that the RBMK, due to its high positive void coefficient, became harder to control as the uranium fuel was consumed or burned up, becoming unpredictable by the time it was shut down after three years for maintenance. This made controlling the RBMK a very laborious, mentally and physically demanding task requiring the timely adjustment of dozens of parameters every minute, around the clock, constantly wearing out switches such as those used for the control rods and causing operators to sweat. The enrichment percentage was increased to 2.0%, up from 1.8% to alleviate these issues.
The RBMK was considered by some in the Soviet Union to be already obsolete shortly after the commissioning of Chernobyl unit 1 (completed in 1977). Aleksandrov and Dollezhal did not investigate further or even deeply understand the problems in the RBMK, and the void coefficient was not analyzed in the manuals for the reactor. Engineers at Chernobyl unit 1 had to create solutions to many of the RBMK's flaws such as a lack of protection against no feedwater supply. Leningrad and Chernobyl units 1 both had partial meltdowns that were — alongside other nuclear accidents at Soviet power plants — treated as state secrets, and so were unknown even to other workers at those same plants.
By 1980 NIKIET realized, after completing a confidential study, that accidents with the RBMK were likely even during normal operation, but no action was taken to correct the RBMK's flaws. Instead, manuals were revised, which was believed to be enough to ensure safe operation as long as they were followed closely. However, the manuals were vague and Soviet power plant staff already had a habit of bending the rules to meet economic targets in the face of inadequate or malfunctioning equipment. Crucially, it was not made clear that a minimum number of control rods had to stay in the reactor at all times in order to protect against an accident, as loosely articulated by the Operational Reactivity Margin (ORM) parameter. An ORM chart recorder and display were added to RBMK control rooms after the Chernobyl disaster.
Reactor design and performance
Reactor vessel, moderator and shielding
The reactor pit or vault is made of reinforced concrete and has dimensions . It houses the vessel of the reactor, which is annular, made of an inner and outer cylindrical wall and top and bottom metal plates that cover the space between the inner and outer walls, without covering the space surrounded by the vessel. The reactor vessel is an annular steel cylinder with hollow walls and pressurized with nitrogen gas, with an inner diameter and height of , and a wall thickness of .