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wiki:user:the_chernobyl_disaster [2018/07/29 01:22] (current)
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 +==The Chernobyl Disaster==
  
 +The April 1986 disaster at the Chernobyla nuclear power plant in Ukraine was the product of a flawed Soviet reactor design coupled with serious mistakes made by the plant operatorsb. ​ It was a direct consequence of Cold War isolation and the resulting lack of any safety culture.
 +The accident destroyed the Chernobyl 4 reactor, killing 30 operators and firemen within three months and several further deaths later. One person was killed immediately and a second died in hospital soon after as a result of injuries received. Another person is reported to have died at the time from a coronary thrombosisc. Acute radiation syndrome (ARS) was originally diagnosed in 237 people on-site and involved with the clean-up and it was later confirmed in 134 cases. Of these, 28 people died as a result of ARS within a few weeks of the accident. Nineteen more subsequently died between 1987 and 2004 but their deaths cannot necessarily be attributed to radiation exposured. Nobody off-site suffered from acute radiation effects although a large proportion of childhood thyroid cancers diagnosed since the accident is likely to be due to intake of radioactive iodine falloutd. Furthermore,​ large areas of Belarus, Ukraine, Russia and beyond were contaminated in varying degrees. The Chernobyl disaster was a unique event and the only accident in the history of commercial nuclear power where radiation-related fatalities occurrede. However, the design of the reactor is unique and in that respect the accident is thus of little relevance to the rest of the nuclear industry outside the then Eastern Bloc. However, it led to major changes in safety culture and in industry cooperation,​ particularly between East and West before the end of the Soviet Union.
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 +- The Chernobyl accident in 1986 was the result of a flawed reactor design that was operated with inadequately trained personnel.
 +
 +- The resulting steam explosion and fires released at least 5% of the radioactive reactor core into the atmosphere and downwind – some 5200 PBq (I-131 eq).
 +
 +- Two Chernobyl plant workers died on the night of the accident, and a further 28 people died within a few weeks as a result of acute radiation poisoning.
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 +- UNSCEAR says that apart from increased thyroid cancers, "there is no evidence of a major public health impact attributable to radiation exposure 20 years after the accident."​
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 +- Resettlement of areas from which people were relocated is ongoing.
 +
 +
 +==The Chernobyl Site and Plant==
 +The Chernobyl Power Complex, lying about 130 km north of Kiev, Ukraine, and about 20 km south of the border with Belarus, consisted of four nuclear reactors of the RBMK-1000 design. units 1 and 2 being constructed between 1970 and 1977, while units 3 and 4 of the same design were completed in 1983. Two more RBMK reactors were under construction at the site at the time of the accident. To the southeast of the plant, an artificial lake of some 22 square kilometres, situated beside the river Pripyat, a tributary of the Dniepr, was constructed to provide cooling water for the reactors.
 +This area of Ukraine is described as Belarussian-type woodland with a low population density. About 3 km away from the reactor, in the new city, Pripyat, there were 49,000 inhabitants. The old town of Chornobyl, which had a population of 12,500, is about 15 km to the southeast of the complex. Within a 30 km radius of the power plant, the total population was between 115,000 and 135,000.
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 +[[Chernobyl diagram|{{http://​www.world-nuclear.org/​uploadedImages/​org/​info/​Safety_and_Security/​Safety_of_Plants/​rbmk%201000.gif}}]]
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 +The RBMK-1000 is a Soviet-designed and built graphite moderated pressure tube type reactor, using slightly enriched (2% U-235) uranium dioxide fuel. It is a boiling light water reactor, with two loops feeding steam directly to the turbines, without an intervening heat exchanger. 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. The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium alloy clad uranium dioxide fuel around which the cooling water flows. The extensions of the fuel channels penetrate the lower plate and the cover plate of the core and are welded to each. A specially designed refueling machine allows fuel bundles to be changed without shutting down the reactor.
 +The moderator, whose function is to slow down neutrons to make them more efficient in producing fission in the fuel, is graphite, surrounding the pressure tubes. A mixture of nitrogen and helium is circulated between the graphite blocks to prevent oxidation of the graphite and to improve the transmission of the heat produced by neutron interactions in the graphite to the fuel channel. The core itself is about 7 m high and about 12 m in diameter. In each of the two loops, there are four main coolant circulating pumps, one of which is always on standby. The reactivity or power of the reactor is controlled by raising or lowering 211 control rods, which, when lowered into the moderator, absorb neutrons and reduce the fission rate. The power output of this reactor is 3200 MW thermal, or 1000 MWe. Various safety systems, such as an emergency core cooling system, were incorporated into the reactor design.
 +One of the most important characteristics of the RBMK reactor is that it it can possess a '​positive void coefficient',​ where an increase in steam bubbles ('​voids'​) is accompanied by an increase in core reactivity (see information page on RBMK Reactors). As steam production in the fuel channels increases, the neutrons that would have been absorbed by the denser water now produce increased fission in the fuel. There are other components that contribute to the overall power coefficient of reactivity, but the void coefficient is the dominant one in RBMK reactors. The void coefficient depends on the composition of the core – a new RBMK core will have a negative void coefficient. However, at the time of the accident at Chernobyl 4, the reactor'​s fuel burn-up, control rod configuration and power level led to a positive void coefficient large enough to overwhelm all other influences on the power coefficient.
 +
 +==The Disaster==
 +
 +In the morning of 25th April, operators began the power reduction procedure.Between 13:00 and 23:00, the reactor was held at half-power, contrary to the initial test schedule, at the request of the electric power distribution centre. At about 23:00, power reduction was resumed. However, the reactor state was now inappropriate for the test to be performed. The core was very difficult to control with the systems available. The reactor should have been stabilized at this stage. However, operators were in a hurry to catch up the delay in the schedule, and decided to perform the test regardless.On 26th April at 01:​23:​04,​the test was launched, and the turbine steam supply valves were closed. The temperature rose in the core, causing reactivity to increase.
 +
 +The reactor started to go critical and out of control. At this point, the operators realized the seriousness of the situation.At 01:23:40, the chief operator ordered an emergency shutdown. All control rods began to enter the core, but did not have time to stop a runaway chain reaction. At 01:23:44, power peaked, exceeding the reactor’s nominal power by a factor of more than 100. The high pressures reached in the pressure tubes caused them to rupture. An explosion lifted the upper reactor cover, weighing about 2000 tonnes, off the reactor. The top of the reactor core was exposed to the open air. The graphite ignited, and a number of fires broke out in the facility. It The fuel rods in the core broke up. The overheated uranium oxide pellets exploded. It took fire-fighters three hours to extinguish the fires. The graphite fire reignited. It was not fully extinguished until 9 May.
 +
 +The release of radioactive materials to the atmosphere consisted of gases, aerosols and finely fragmented nuclear fuel particles. This release was extremely high in quantity, involving a large fraction of the radioactive product inventory existing in the reactor, and its duration was unexpectedly long, over a 10-day period, with varying release rates. The duration and high altitude (about 1 km) reached by the release were largely due to the graphite fire which was difficult to extinguish until day 10, when the releases dropped abruptly, thus ending the period of intense release. For these reasons and the concomitant frequent changes of wind direction during the release period, the area affected by the radioactive plume and the consequent deposition of radioactive substances on the ground was extremely large, encompassing the whole Northern hemisphere, although significant contamination outside the former Soviet Union was only experienced in part of Europe.
 +
 +The pattern of contamination on the ground and in food chains was, however, very uneven in some areas due to the influence of rainfall during the passage of the plume. This irregularity in the pattern of deposition was particularly pronounced at larger distances from the reactor site. Since the last report we have a better view of the behavior of radionuclides in the contaminated areas, and we know now that the natural decontamination processes have reached an environmental equilibrium state. The decrease of contamination levels from now on will be mainly due to radioactive decay indicating that radioactive cesium will be present for approximately 300 years.
 +
 +==Health Impact==
 +
 +The health impact of the Chernobyl accident can be described in terms of acute health effects (death, severe health impairment),​ late health effects (cancers) and social/​accident effects that may affect health. The acute health effects occurred among the plant personnel and the persons who intervened in the emergency phase to fight fires, provide medical aid and immediate clean-up operations. A total of 31 people died as a consequence of the accident, and about 140 people suffered various degrees of radiation sickness and radiation-related acute health impairment. No members of the general public suffered these kinds of effects.
 +
 +As far as the late health effects are concerned, namely the possible increase of cancer incidence, since the accident there has been a real and significant increase of carcinomas of the thyroid among the population of infants and children exposed at the time of the accident in the contaminated regions of the former Soviet Union. This should be attributed to the accident until proved otherwise. There might also be some increase of thyroid cancers among the adults living in those regions. From the observed trend of this increase of thyroid cancers it is expected that the peak has not yet been reached and that this kind of cancer will still continue for some time to show an excess above its natural rate in the area.
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 +On the other hand, the scientific and medical observation of the affected population has not to date revealed any significant increase in other cancers, leukemia, congenital abnormalities,​ adverse pregnancy outcomes or any other radiation induced disease that could be attributed to the Chernobyl accident. This observation applies to the whole general population, both within and outside the former Soviet Union. Large scientific and epidemiological research programmes, some of them sponsored by international organisations such as the WHO and the EC, are being conducted to provide further insight into possible future health effects. However, the population dose estimates generally accepted tend to predict that, with the exception of thyroid disease, it is unlikely that the exposure would lead to discernible radiation effects in the general population above the background of natural incidence of the same diseases. In the case of the liquidators,​ increases in cancer have not been observed to date, but a specific and detailed follow-up of this particular group might better reveal increasing trends should they exist.
 +
 +An important effect of the accident, which has a bearing on health, is the appearance of a widespread status of psychological stress in the populations affected. The severity of this phenomenon, which is mostly observed in the contaminated regions of the former Soviet Union, appears to reflect the public fears about the unknowns of radiation and its effects, as well as its mistrust towards public authorities and official experts, and is certainly made worse by the disruption of the social networks and traditional ways of life provoked by the accident and its long-term consequences. These accident related effects have resulted in a general degradation of the health of the population living in the contaminated territories. Illnesses that have been observed are not typically related to radiation exposure. Further study of those effects should continue.

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