The RBMK (Reactor Bolshoi Moschnosti Kanalynyi/Channelized Large Power Reactor) reactor, designed in the Soviet Union, is a nuclear fission reactor that uses ordinary water as coolant and graphite as the moderator, a combination not used in any other power reactor as of February 2002. It was derived from a design for plutonium productrion and was intended and used to produce both plutonium and power. The water/graphite coolant/moderator combination results in instability at low power levels, due to the control rod design and a positive void coefficient (SEE: Chernobyl). The graphite moderator, however, allows for use of fuel unsuitable for use in water-moderated reactors. Design changes have been made to address these problems.

The reactor uses fuel assemblies composed of eighteen 0.5inch fuel rods cylindrically arranged in a carriage, made of slightly enriched uranium oxide fuel pellets enclosed in a 3.65m zirconium (2.5% niobium) tube. Each pressure tube contains two of these fuel assemblies end on end, in their own 7m vertical channel. Each channel is individually cooled by pressurised (1000psi) water from two separate systems with four pumps each, which emerges from the channel as steam at about 290°C. There are 1661 3.5 inch coolant tubes in the core. This steam produced is separated and is used in turbines to produce electricity, after which it is recondensed back into the cooling system. The steam separators delay the steam before sending it to the turbines, to reduce radiation levels produced by the radioactive steam near the turbines. An emergency cooling system exists in case either coolant system is interrupted. With isolated channels, the reactor can be refueled while in operation by lifting the fuel assemblies out of and into the reactor. The pressure tubes are separated by a series of graphite blocks, which slow down the neutrons released during fission to maintain a continuous fission chain reaction. Heat is conducted between the blocks by a mixture of helium and nitrogen gas. The rate of fission is slowed down by boron carbide control rods. Short rods inserted upwards from the bottom of the core maintain even power distribution, while the main rods are inserted from the top to provide automatic (regulated by feedback from in-core detectors), manual, or emergency control. By Western standards, there is no secure containment, only a concrete lined cavity that is used as a radiation shield. An upper shield (pile cap) made of steel is above the core, supporting the fuel assemblies. The steam separators in the coolant systems have their own concrete shields.

After the incident at Chernobyl unit 4, all operating RBMK reactors had their manual control rod count increased from 30 to 45, 80 additional absorbers were installed in the core to inhibit low power operation, fuel enrichment was increased from 2% to 2.4% (reducing reliance on cooling water), scram rod insertion time was cut from 18 seconds to 12, the control rods were redesigned, and precautions against unauthorised access to the emergency safety systems were installed. The modifications have reduced the positive void coefficient from +4.5 b to +0.7 b. The redesigned control rods had graphite "riders" installed on their bottom ends, which removes water from the lower part of the core when the rods are lowered, increasing reactivity, and hence increasing power (which helps reduce the problem of low power instability).

As of February 2002, there are 13 RBMK reactors in the world, all began operation between 1973 and 1990, all in Russia and Lithuania, with another Russian (Kursk-5) one due for operation in 2003the future. The net capacity of these reactors is 12,545MWe. The 13 are: Ignalina-1 (due for shutdown May 2004), -2 (August 2018), Kursk-1 (October 2007), -2 (August 2009), -3 (March 2014), -4 (February 2016), Leningrad-1 (November 2004), -2 (February 2006), -3 (June 2010), -4 (August 2011), Smolensk-1 (December 2013), -2 (July 2015), and -3 (July 2023).

jasstrong says re RBMK: in rbmk you state that graphite moderators aren't used in water-cooled reactors outside the RBMK; this isn't true, the Calder Hall reactors used that combination.
I don't really know anything about Calder Hall reactors, so I'll have to take jasstrong's word on that.

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