RBMK is an acronym for the Russian reaktor bolshoi moshchnosty kanalny which means "reactor (of) large power (with) channels", and describes a now-obsolete class of nuclear power reactor which was built only in the Soviet Union. In 2003 several were still operating but there were no plans to build any more, 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 reactors. The first of these, AM-1 (for Atom Mirny, Russian for "peaceful atom") was designed to produce 5MWe (30MW 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 can be built that requires no separated
isotopes, such as
enriched uranium or heavy water, and the RBMK remains the only large power
reactor yet built without needing these expensive materials. But unfortunately, such a configuration is also unstable.
1 Positive void coefficient
Ordinary (light) water absorbs neutrons fairly readily, and so removing water from the core (such as happens when it boils and is replaced by steam) tends to increase the rate at which the nuclear reaction proceeds. In a water-moderated reactor, this effect is countered by the reduction in moderation, but in the RBMK the moderating effect of the water is small compared to that of the graphite, so the overall effect is positive. This is called a "positive void coefficient". The RBMK as designed also had a "positive power coefficient", meaning that an increase in reactor power tends to further increase the rate of reaction. Large positive void and power coefficients can produce runaway conditions and have not been permitted in other reactor designs, but it was not possible to eliminate them from the RBMK if natural uranium fuel was to be used.
The RBMK was also intended to use recycled uranium from reprocessed PWR fuel, which has a low remaining enrichment. In this configuration it was also unstable. These characteristics brought the RBMK to the world's notice in 1986, when one of the four RBMK reactors at Chernobyl exploded in the worst civilian nuclear accident to date.
2 Containment
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 CelsiusThe degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius ( 1701 1744), who first proposed it in 1742. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees, and the boiling po). There is also a large amount of shielding to absorb radiationRadiation generally means the transmission of objects or information from a source into a surrounding medium or destination. Within physics, related concepts are: Ionizing radiation is a stream of particles (photons or other particles) with sufficient ene from the reactor core. This includes a concrete slab on the bottom, sand and concrete areound 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 Islandnuclear reactors each with its own containment building and cooling towers. TMI-2, which sufferred a partial meltdown, is in the forground. Three Mile Island is an island in the Susquehanna River in Dauphin County, Pennsylvania, near Harrisburg, of area 3 incident, RBMK design also includes a partial containment structure 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, an RBMK reactor is very tall (about 70 metres), so due to the cost and difficulty of building such a heavy containment structure, pipes on top of the reactor have no additional emergency containment structure. Unfortunately, in the Chernobyl accident, when the pressure rose enough, the top blew off the reactor, breaking open all these top pipes.
