RBMK is an acronym for the Russian "reaktor bolshoy moshchnosti kanalniy" ( _ru. Реактор Большой Мощности Канальный) which means "High Power Channel Type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl accident. In 2008, there are at least 12 RBMK reactors still operating in Russia and Lithuania, but there are no plans to build new RBMK type reactors (the RBMK technology was developed in 1950s and is now considered obsolete) and there is international pressure to close those that remain.

The RBMK was the culmination of the Soviet program to produce a water-cooled power reactor based on their graphite-moderated plutonium production military reactors. The first of these, AM-1 ("Атом Мирный", Russian for "Atom Mirny", "peaceful atom") produced 5 MW of electricity (30 MW thermal) and delivered power to Obninsk from 1954 until 1959.

Using light water for cooling and graphite for moderation, it is possible to use natural uranium for fuel. Thus, a large power reactor (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 today) can be built that requires no separated isotopes, such as enriched uranium or heavy water.

The reactors were used in nuclear power plants to produce nuclear power from nuclear fuel.


An RBMK employs long (7 metre) vertical pressure tubes running through a graphite moderator and cooled by water, which is allowed to boil in the core at 290 °C, much as in a boiling water reactor. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.65 metres long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorption without inhibiting the fission reaction, so the reactor can have a large positive void coefficient, and a positive feedback problem can arise, as with the disaster at Chernobyl.

Because the water used to remove heat from the core in a light-water reactor absorbs some of the free neutrons normally generated during operation of the reactor, the concentration of the naturally fissionable U-235 isotope in uranium used to fuel light-water reactors must be increased above the level of natural uranium to assist in sustaining the nuclear chain reaction in the reactor core: the remainder of the uranium in the fuel is U-238. Increasing the concentration of U-235 in nuclear fuel uranium above the level that occurs in natural uranium is accomplished through the process of enrichment.

The fuel core for a light water reactor can have up to 3,000 fuel assemblies. An assembly consists of a group of sealed fuel rods, each filled with uranium oxide (UO2) pellets, held in place by end plates and supported by metal spacer-grids to brace the rods and maintain the proper distances between them. The fuel core can be thought of as a reservoir from which heat energy can be extracted through the nuclear chain reaction process. During the operation of the reactor, the concentration of U-235 in the fuel is decreased as those atoms undergo nuclear fission which creates heat energy. Some U-238 atoms are converted to atoms of fissile Pu-239, some of which will, in turn, undergo fission and produce energy. The products created by the nuclear fission reactions are retained within the fuel pellets and these become neutron-absorbing products, also called nuclear poisons, that act to slow the rate of nuclear fission and heat production. As the reactor operation is continued, a point is reached at which the declining concentration of fissile nuclei in the fuel and the increasing concentration of poisons result in lower than optimal heat energy generation. The RBMK has a refueling machine that can change the fuel on-load, while the reactor is still producing power.

High Positive Void Coefficient

Light water (the ordinary H2O) is both a neutron moderator and a neutron absorber. This means that not only can it slow down neutrons to velocities in equilibrium with surrounding molecules ("thermalize" them and turn them into useful sources of energy), but it can also absorb them outright. Heavy water is also a good neutron moderator, but does not absorb neutrons as easily.

In RBMKs, light water was used as a coolant; moderation was instead carried out by graphite. As graphite already moderated neutrons, light water had a lesser effect in slowing them down, but could still absorb them. This means that the reactor's moderation level (adjustable by appropriate neutron-absorbing rods) had to account for the neutrons absorbed by light water.

In the case of evaporation of water to steam, the place occupied by water would be occupied by water vapor, which has a density hundreds of times smaller than that of liquid water (the exact number depends on pressure and temperature; at standard conditions, steam is about 1350 times lighter than liquid water). Because of this lower density (of mass, and consequently of atom nuclei able to absorb neutron), light water's capability of absorbing neutrons would practically disappear.

In RBMK's, generation of steam in the coolant water would then in practice create void, a bubble that does not absorb neutrons; the reduction in moderation by light water is irrelevant, as graphite is still moderating the neutrons, enabling them to be absorbed more easily to continue the reaction. This event would dramatically alter the balance of neutron production, causing a runaway condition in which more and more neutrons are produced, and their density grows exponentially fast. Such a condition is called a "positive void coefficient", and it is particularly high for RBMK reactors.

A high void coefficient does not automatically make a reactor unsafe. Some RBMK designs did include control rods on electromagnetic grapples, thus controlling the reaction speed and, if necessary, stopped the reaction completely. The RBMK at Chernobyl, however, had manual control rods.

After the Chernobyl disaster, all RBMKs in operation underwent significant changes, lowering their void coefficients to +0.7 b. This new number precludes the possibility of a low-coolant meltdown.


The RBMK design includes several kinds of containment needed for normal operation. There is a sealed metal containment structure filled with inert gases surrounding the reactor to keep oxygen away from the graphite (which is normally at about 700 degrees Celsius). There is also a large amount of shielding to absorb radiation from the reactor core. This includes a concrete slab on the bottom, sand and concrete around the sides, and a large concrete slab on top of the reactor. Much of the reactor's internal machinery is attached to this top slab, including the water pipes.

Initially, the RBMK design focused solely on accident prevention and mitigation, not on containment of severe accidents. However, since the Three Mile Island accident, RBMK design also includes a partial containment structure (not a full containment building) for dealing with emergencies. The pipes underneath the reactor are sealed inside leak-tight boxes filled with a large amount of water. If these pipes leak or burst, the radioactive material is trapped by the water inside these boxes. However, RBMK reactors were designed to allow fuel rods to be changed without shutting down (as in the pressurized heavy water CANDU reactor), both for refueling and for plutonium production (for nuclear weapons). This required large cranes above the core. As the RBMK reactor is very tall (about 7 metres), the cost and difficulty of building a heavy containment structure prevented building of additional emergency containment structure for pipes on top of the reactor. In the Chernobyl accident, the pressure rose to levels high enough to blow the top off of the reactor, breaking open these pipes in the process.

Improvements since the Chernobyl accident

Since the Chernobyl accident, all remaining RBMKs have been retrofitted with a number of updates for safety. The largest of these updates fixes the RBMK control rod design. Previously the control rods were designed with graphite tips, which when initially inserted into the reactor sped up the reaction, instead of slowing or stopping it. This design flaw contributed to the first explosion of the Chernobyl accident, when the emergency button was pressed to stop the reactor. The updates are

* An increase in fuel enrichment from 2% to 2.4% to compensate for control rod modifications and the introduction of additional absorbers.
* Manual control rod count increased from 30 to 45.
* 80 additional absorbers inhibit operation at low power, where the RBMK design is most dangerous.
* SCRAM (rapid shut down) sequence reduced from 18 to 12 seconds.
* Precautions against unauthorized access to emergency safety systems.


Of the 17 RBMKs built (one is still under construction at the Kursk Nuclear Power Plant), all three surviving reactors at the Chernobyl plant have now been closed (the fourth having been destroyed in the accident) and one of the two reactors at Ignalina in Lithuania has shut down with the second due to close by 2009. [http://www.uic.com.au/nip56.htm] . The others built are still operational at Saint Petersburg (4 RBMK-1000), Smolensk (3 RBMK-1000) and Kursk (4 RBMK-1000) [http://www.iaea.org/programmes/a2/index.html] .


A development of the RBMK is the MKER (Russian: "МКЭР", ногопетлевые Канальные Энергетические Реакторы " [Mnogopetleye Kanalynye Energeticheskie Reaktory] wich means "Multiloop pressuretube power reactor") of security has improved and a containment. [ [http://world-nuclear.org/info/inf45.html World Nuclear Association - Nuclear Power in Russia] ] [ [http://www.nikiet.ru/eng/structure/presstube.html NIKET - Department of Pressure-Tube Power Reactors] ] The prototype of the MKER-1000 is the 5th unit of the Kursk nuclear power plant. [ [http://www.lnpp.ru/new_lnpp/eng-htm/cont/proizv/perspek/dolgo/dolgo2.htm LNPP - The proposed NPP design meets the following requirements] ] A MKER-800, MKER-1000 and MKER-1500 planned for the Leningrad nuclear power plant. [ [http://www.lnpp.ru/new_lnpp/eng-htm/cont/proizv/perspek/dolgo/dolgo.htm LNPP - LNPP REPLACING CAPACITIES] ] [ [ THE PRESSURE-TUBE LINE IN RUSSIAN NUCLEAR ENGINEERING] ] [ [http://www.bellona.org/english_import_area/international/russia/npps/leningrad/9358 Bellona - Statistics from Leningrad Nuclear Power Plant] ]


* [http://www.iae.lt/inpp_en.asp?lang=1&subsub=29 Technical data on RBMK-1500 reactor] at Ignalina nuclear power plant - one of the remaining working RBMK reactors
* [http://canteach.candu.org/library/19910101.pdf Chernobyl - A Canadian Perspective] (PDF 405KB) - A brochure describing nuclear reactors in general and the RBMK design in particular, focusing on the safety differences between them and CANDU reactors. Published by Atomic Energy of Canada Limited.
* [http://www-formal.stanford.edu/jmc/progress/chernobyl.html The Chernobyl Disaster] - how the RBMK's design made it possible.

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