It was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the emergency equipment and the core cooling water circulating pumps, until the diesel emergency power supply became operative. The Unit 4 reactor was to be shutdown for routine maintenance on 25 April 1986. Events leading to the accident (IA86, IA86a) This was a major factor in the development of the accident. However, at a lower power output of less than 20% the maximum, the positive void coefficient effect is dominant and the reactor becomes unstable and prone to sudden power surges. At the high power level of normal operation, the temperature effect predominates, so that power excursions leading to excessive overheating of the fuel do not occur. The net effect of these two opposing characteristics varies with the power level. However, as the power increases, so does the temperature of the fuel, and this has the effect of reducing the neutron flux (negative fuel coefficient). This means that if the power increases or the flow of water decreases, there is increased steam production in the fuel channels, so that the neutrons that would have been absorbed by the denser water will now produce increased fission in the fuel. The most important characteristic of the RBMK reactor is that it possesses a "positive void coefficient". Various safety systems, such as an emergency core cooling system and the requirement for an absolute minimal insertion of 30 control rods, were incorporated into the reactor design and operation. The power output of this reactor is 3 200 MWt (megawatt thermal) or 1 000 MWe, although there is a larger version producing 1 500 MWe. The reactivity or power of the reactor is controlled by raising or lowering 211 control rods, which, when lowered, absorb neutrons and reduce the fission rate. There are four main coolant circulating pumps, one of which is always on standby. The core itself is about 7 m high and about 12 m in diameter. A mixture of nitrogen and helium is circulated between the graphite blocks largely to prevent oxidation of the graphite and to improve the transmission of the heat produced by neutron interactions in the graphite, from the moderator to the fuel channel. The moderator, whose function is to slow down neutrons to make them more efficient in producing fission in the fuel, is constructed of graphite. A specially designed refuelling machine allows fuel bundles to be changed without shutting down the reactor. The vertical pressure tubes contain the zirconium-alloy clad uranium-dioxide fuel around which the cooling water flows. The water acts as a coolant and also provides the steam used to drive the turbines. Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two 500 MWe turbines. It is a boiling light water reactor, with direct steam feed to the turbines, without an intervening heat-exchanger. The RBMK-1000 (Figure 2) is a Soviet designed and built graphite moderated pressure tube type reactor, using slightly enriched (2% 235U) uranium dioxide fuel. Within a 30-km radius of the power plant, the total population was between 115 000 and 135 000.įigure 1: The site of the Chernobyl nuclear power complex (modified from IA91)įigure 2: The RBMK reactor The RBMK-1000 reactor The old town of Chernobyl, which had a population of 12 500, is about 15 km to the South-east of the complex. About 3 km away from the reactor, in the new city, Pripyat, there were 49 000 inhabitants. This area of Ukraine is described as Belarussian-type woodland with a low population density. To the South-east of the plant, an artificial lake of some 22 km2, situated beside the river Pripyat, a tributary of the Dniepr, was constructed to provide cooling water for the reactors. Two more RBMK reactors were under construction at the site at the time of the accident. The Chernobyl Power Complex, lying about 130 km north of Kiev, Ukraine, and about 20 km south of the border with Belarus (Figure 1), consisted of four nuclear reactors of the RBMK-1000 design, Units 1 and 2 being constructed between 19, while Units 3 and 4 of the same design were completed in 1983 (IA86). While the WWER type of reactor was exported to other countries, the RBMK design was restricted to republics within the Soviet Union. At the time of the Chernobyl accident, on 26 April 1986, the Soviet Nuclear Power Programme was based mainly upon two types of reactors, the WWER, a pressurised light-water reactor, and the RBMK, a graphite moderated light-water reactor.
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