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Unformatted text preview: 11-7disadvantages:.As a gas, it has negligible affect on the neutrons in the core. Thus,another material must be used as a moderator. In He cooled reactors,graphite is usually used as the moderator..Helium is moderately expensive..For accidents involving the breach of primary the primary flow, theremust be special fuel designs to prevent release of fission products re-sulting from fuel failure. This usually means the fissile fuel mustbe small microspheroids encased in sufficient graphite to absorb anyresidual heat following an emergency shutdown or loss of coolant ac-cident.10. During the April 1986 accident in the 1000-MWe RMBK Chernobyl reactor,the water in the cooling tubes of the graphite-moderated reactor was allowed,through operator error, to boil into steam and cause a supercritical, run-awaychain reaction. The resulting energy excursion resulted in the destruction ofthe reactor containment and a large amount of the fission products in the fuelIelements were released to the environment as the reactor containment ruptured.to the environment as the reactor containment ruptured....
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- Spring '06