Chernobyl Nuclear Power Station

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Chernobyl nuclear power station

On April 26, 1986, at precisely 1:24 A.M., the Chernobyl nuclear power plant exploded, releasing large amounts of radioactivity into the environment . The power station is located 9 mi (14.5 km) northwest of the town of Chernobyl, with a population of 12,500, and less than 2 mi (3.2 km) from the town of Pripyat, which contains 45,000 inhabitants. The explosion and its aftermath, including the manner in which the accident was handled, have raised questions about the safety and future of nuclear power.

The Chernobyl accident resulted from several factors: flaws in the engineering design, which were compensated by a strict set of procedures; failure of the plant management to enforce these procedures; and finally the decision of the engineers to conduct a risky experiment. They wanted to test whether the plant's turbine generator—from its rotating inertia—could provide enough power to the reactor in case of a power shutdown. This experiment required disconnecting the reactor's emergency core cooling pump and other safety devices.

The series of critical events, as described by Richard Mould in Chernobyl, The Real Story, are as follows: At 1:00 A.M. on April 25, power reduction was started in preparation for the experiment. At 1:40 A.M. the reactors's emergency core cooling system was turned off. At 11:10 P.M. power was further reduced, resulting in a nearly unmanageable situation. At 1:00 A.M. on April 26, power was increased in an attempt to stabilize the reactor; however, cooling pumps were operating well beyond their rated capacity, causing a reduction in steam generation and a fall in stream pressure. By 1:19 A.M., the water in the cooling circuit had approached the boiling point. At 1:23 A.M., the operators tried to control the reaction by manually pushing control rods into the core; however, the rods did not descend their full length into the reactor since destruction of the graphite core was already occurring. In 4.5 seconds, the power level rose two thousandfold. At 1:24 A.M., there was an explosion when the hot reactor fuel elements, lacking enough liquid for cooling, decomposed the water into hydrogen and oxygen. The generated pressures blew off the 1,000-ton concrete roof of the reactor, and burning graphite, molten uranium , and radioactive ashes spilled out to the atmosphere .

The explosion that occurred was not a nuclear explosion such as would occur with an atomic bomb but its effects were just as devastating. In order to put the expulsion of radioactive material from the Chernobyl reactor into perspective, almost 50 tons of fuel went into the atmosphere plus an additional 70 tons of fuel, and 700 tons of radioactive reactor graphite settled in the vicinity of the damaged unit. Some 50 tons of nuclear fuel and 800 tons of reactor graphite remained in the reactor vault, with the graphite burning up completely in the next several days after the accident. The amount of radioactive material that went into the atmosphere was equivalent to 10 Hiroshima bombs.

Officials at first denied that there had been a serious accident at the power plant. The government in Moscow was led to believe for several hours after the explosion and fire at Chernobyl that the reactor core was still intact. This delayed the evacuation for a critical period during which local citizens were exposed to high radiation levels. The evacuation of Chernobyl and local villages was spread out over eight days. A total of 135,000 persons were evacuated from the area, with the major evacuation at Pripyat starting at 2:00 P.M., the day after the explosion. Tests showed that air, water, and soil around the plant had significant contamination. Children, in particular, were a matter of concern and were evacuated to the southern Ukraine, the Crimea, and the Black Sea coast.

At the time of the accident, and for several days thereafter, the winds carried the radioactive waste to the north. The radioactive cloud split into two lobes, one spreading west and then north through Poland, Germany, Belgium, and Holland, and the other through Sweden and Finland. By the first of May, the wind direction changed and the radioactive fallout—at a diminished rate—went south over the Balkans and then west through Italy. Large areas of Europe were affected, and many farmers destroyed their crops for fear of contamination. Forests have been cleared and large amounts of earth were removed in order to clean up radioactivity. Plastic film has been laid in some areas in an effort to contain radioactive dust.

Officially 31 persons were reported to have been killed at the reactor site by a combination of the explosion and radiation exposure ; another 174 were exposed to high doses of radiation which resulted in radiation sickness and long-term illnesses. The maximum permissible dose of radiation for a nuclear power operator is 5 roentgens per year and for the rest of the population, 0.5 roentgens per year. At the Chernobyl plant, the levels of radiation ranged from 1,000 to 20,000 roentgens per hour. One British report estimates that worldwide, the number of persons afflicted with cancer which can be attributed to the Chernobyl accident will be about 2,300. Others argued that the number will be much higher. In Minsk, the rate of leukemia has more than doubled from 41 per million in 1985 to 93 per million in 1990.

Many heroic deeds were reported during this emergency. Fire fighters exposed themselves to deadly radiation while trying to stop the inferno. Every one eventually died from radiation exposure. Construction workers volunteered to entomb the reactor ruins with a massive concrete sarcophagus. Bus drivers risked further exposure by making repeated trips into contaminated areas in order to evacuate villagers. Over 600,000 people were involved in the decontamination and clean up of Chernobyl. The health effects on them from their exposure are not completely known. The Chernobyl accident focused international attention on the risks associated with operating a nuclear reactor for the generation of power. Public apprehension has forced some governments to review their own safety procedures and to compare the operation of their nuclear reactors with Chernobyl's. In a review of the Chernobyl accident by the Atomic Energy Authority of the United Kingdom, an effort was made to contrast the design of the Chernobyl reactor and management procedures with those in practice in the United States and the United Kingdom.

Three design drawbacks were noted of the Chernobyl nuclear power plant:

The reactor was intrinsically unstable below 20% power and never should have been operated in that mode. (U.S. and UK reactors do not have this design flaw).
The shut-down operating system was inadequate and contributed to the accident rather than terminating it. (U.S. and UK control systems differ significantly).
There were no controls to prevent the staff from operating the reactor in the unstable region or preventing the disabling of existing safeguards.

In addition, the Chernobyl management had no effective watchdog agency to inspect procedures and order closure of the facility. Also in years prior to the accident there was a lack of information given the public of prior nuclear accidents, typical of the press censorship and news management occurring in the period before glasnost. The operators were not adequately trained nor were they themselves fully aware of prior nuclear power accidents or near accidents which would have made them more sensitive to the dangers of a runaway reactor system.

Unfortunately in the former Soviet block nations there are several nuclear reactors that are potentially as hazardous as Chernobyl but which must continue operation to maintain power requirements; however, the operational procedures are under constant review to avoid another accident. Clearly the Western world will have to assist the former Soviet block to bring reactor operating equipment and standards up to a much higher level of safety to avoid a similar and possibly more disastrous accident.

[Malcolm T. Hepworth ]