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Revision as of 04:38, 29 December 2002 by Roadrunner (talk | contribs)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)A nuclear meltdown occurs when the core of a nuclear reactor melts. In pressurized water reactors and boiling water reactors, this can occur after a loss of coolant accident in which emergency cooling systems fail. Although the emergency systems are designed to reinsert the control rods and stop the fission reaction in the event of an emergency, radioactive decay from the reaction products will continue to generate heat in the absence of coolant and fission reactions. This heat will cause the reactor core to melt.
What happens at this point is the subject of conjecture and perhaps fortunately, little actual experience. The worst case scenario would be if the molten reactor core penetrates the containment vessel and hits ground water. The combination of molten radioactive material and water may cause a chemical explosion which would spread radioactive material over a large area. The best case scenario would be if the containment vessels held the molten material.
Although, pressurized water reactors are susceptiable to nuclear meltdown in the absence of active safety measures, this is not a universal feature of civilian nuclear reactors, and much of the research in civilian nuclear reactors are for "passive safety" designs that would be much less susceptible to meltdown even if all emergency systems failed.
The only known large scale nuclear meltdown in history was at Chernobyl, Ukraine, in 1986, although there was a partial core meltdown at Three Mile Island, Pennsylvania in 1979.