RBMK
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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 2004 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 5MW (30MW thermal) and delivered power to Obninsk from 1954 until 1959. In spite of their name, they were designed to be able to produce plutonium for weapons as well as energy.
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. Unfortunately, such a configuration is also unstable.
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Design
An RBMK employs long (7 metre) vertical pressure tubes running through graphite moderator, and is 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 3.5 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 (such as at Chernobyl, which was a RBMK reactor).
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 nuclear power reactor can have up to 3,000 fuel assemblies. An assembly consists of a group of sealed fuel rods, each filled with 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 to create 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 (called "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.
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.
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 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 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, RBMK reactors were designed to allow fuel rods to be changed without shutting down, both for refueling and for plutonium production (for nuclear weapons). This required large cranes above the core. As a result, 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.
Improvements since the Chernobyl accident
Since the Chernobyl accident, remaining RBMKs have been operated with a reduced number of fuel elements containing more highly enriched fuel, enabling them to operate relatively safely but defeating the original concept. Control systems have also been improved, in particular to eliminate the graphite tips on the control rods which produced an immediate increase in power when the rods were first inserted. This design feature is blamed for triggering the first actual explosion when the emergency shutdown button was pressed in an attempt to shut down the already out of control reactor during the Chernobyl disaster.
References
- Technical data on RBMK-1500 reactor (http://www.iae.lt/inpp_en.asp?lang=1&subsub=29) Ignalina nuclear power plant
- Chernobyl - A Canadian Perspective (http://canteach.candu.org/library/19910101.pdf) (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 the CANDU organization.
- The Chernobyl Disaster (http://www-formal.stanford.edu/jmc/progress/chernobyl.html) and how the RBMK's design made it possible.nl:RBMK