In a reactor accident, an event occurs which causes a mismatch between the cooling of the fuel and the energy generated in the fuel. Even if the fission process is stopped by the insertion of neutron absorbing control rods into the reactor, energy is still released by the radioactive decay of the fission products. These fission products continue to generate large amounts of energy days and even weeks after shutdown of the reactor. If the fuel is not adequately cooled and this energy is not removed, the fuel will overheat, eventually melting and releasing the more volatile fission products from the fuel.
The classic reactor accident occurs when one of the large pipes that carry cooling water to the reactor breaks, causing the cooling water to be lost. The reactor is shutdown by the reactor protective system and large pumps are started to supply cooling water. If for some reason, the cooling is insufficient as happened during the accident at Three Mile Island (TMI) [9, 10]; the fuel will overheat, melt, and release the fission products to the cooling system and eventually outside of the reactor through the break in the piping. The containment building provides the final barrier to release of the radioactive fission products to the environment. The building is a steel reinforced concrete structure designed to withstand the pressures and temperatures that occur during an accident. Although the reactor at Three Mile Island saw significant fuel melting, the containment building prevented any large-scale release of radioactive material to the environment.
Critics are concerned that a more severe event than occurred at Three Mile Island could lead to failure of the containment building and widespread contamination of the surrounding area with fission products. Such an accident did occur at the Russian designed nuclear power plant at Chernobyl in the Ukraine. This reactor did not have a containment building and when the reactor failed catastrophically, it contaminated much of the surrounding area .
Fig. 4.4 Reactor containment building (Courtesy US DOE)
Containment Building 3" Thick Reinforced Concrete
Reactor Vessel 8"-10" Thick Steel
Fuel Rod, Length 12" Diameter 3/4" Containing a Stack of Ceramic Uranium Dioxide Pellets Sealed in a Zirealoy Tube
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