On April 26, 1986, at 1:23 A.M., an accident occurred during a test of a turbine generator on the Unit 4 reactor at the Chernobyl nuclear power station in the Ukraine. The accident resulted from the improper withdrawal of control rods and the inactivation of important safety systems—in violation of the operating rules—which caused the reactor to overheat, explode, and catch fire. Because the facility lacked an adequate containment structure, the damage to the reactor core and control building allowed large quantities of radiation and millions of curies of krypton-85, xenon-133, iodine-131, tellurium-132, strontium-89, strontium-90, plutonium-240, and other radionuclides from the rector core to be released during the ensuing ten days, necessitating the evacuation of tens of thousands of people and farm animals from the surrounding area and resulting in radiation sickness and burns in more than two hundred emergency personnel and firefighters, thirty-one of whom were injured fatally.
The heaviest contamination occurred in the vicinity of the reactor itself and, to a lesser extent, in neighboring countries of eastern Europe. Those living in the vicinity of the reactor were given potassium iodide preparations to inhibit the thyroidal uptake of radioactive iodine, but infants in a number of areas elsewhere in eastern Europe are estimated to have received sizeable radiation doses to the thyroid gland, largely through ingestion of radioiodine via cow's milk, and the incidence of thyroid cancer in such persons has since risen dramatically in Belarus and in Ukraine. In areas outside Belarus, Russia, and Ukraine, organs other than the thyroid typically received only a small fraction of the radiation dose normally accumulated each year from natural background radiation. For example, the highest average effective dose in such areas during the first year was received in Bulgaria, where it is estimated to have approximated slightly less than one-third of the average annual effective dose received from natural sources. Because of the small magnitude of the average dose to a given individual, the ultimate health impacts of the accident cannot be predicted with certainty. However, nonthreshold risk models for the carcinogenic effects of radiation imply that the collective dose to the population of the northern hemisphere may cause up to thirty thousand additional cancer deaths during the next seventy years.
The accident, by far the worst nuclear reactor accident to date, highlighted flaws in the design, as well as the operation, of the Chernobyl reactor. The reactor's lack of adequate containment and its positive void coefficient, which made the reactor potentially unstable at low-risk power levels, prompted the International Atomic Energy Agency subsequently to recommend the decommissioning of all Chernobyl-type reactors, a recommendation yet to be fully implemented throughout eastern Europe. In spite of the fact that the accident also prompted reassessment and upgrading of the safety of nuclear power systems everywhere, nuclear power has fallen into disfavor in most countries because of the magnitude of the disaster. To many experts, nevertheless, nuclear power still compares favorably with other sources of energy in its impact on human health and the environment, and it is expected to continue to play a role in helping to meet the world's rapidly growing demands for energy.
ARTHUR C. UPTON
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