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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 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, and the RBMK remains the only large power
reactor yet built without needing these expensive materials. Unfortunately, such a configuration is also unstable.
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
- 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.
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