Nuclear meltdown
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A nuclear meltdown occurs when the core of a nuclear reactor melts. In such an event, there is the alarming possibility that the molten reactor core might form an uncontrolled critical mass (a recriticality); or continue generating enough heat through radioactive decay ('decay heat') to maintain its temperature. Even if this does not occur, large explosions and general destruction are likely to occur when the molten mass encounters water or air.
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Causes
In pressurized water reactors, boiling water reactors, and breeder reactors, the core can melt as a result of 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 can cause the reactor core to melt within an hour after coolant flow is stopped.
Other sources of heat may be present in a nuclear reactor core. If the containment has been breached and air enters the reactor, core material such as graphite or zirconium may burn, sharply heating the core (as in the Chernobyl accident and many others). If the reactor contains graphite and appropriate care is not taken, Wigner energy may accumulate, to be suddenly released (as occurred in the Windscale fire).
Sequence of events
What happens when a reactor core melts is the subject of conjecture and, perhaps fortunately, little actual experience.
Before the core of a nuclear reactor can melt, a number of severe accidents must already have happened. However, once the core melts, it will almost certainly destroy the contents of the reactor. If this brings it in contact with liquid water (for example, coolant or moderator), a steam explosion is likely. If air is available, any exposed flammable substances will probably burn fiercely. But the liquid nature of the molten core poses special problems.
In the worst case scenario, the molten reactor core could penetrate the containment vessel and hit ground water. The combination of molten radioactive material and water could cause an enormous steam explosion which would spread radioactive contamination over a large area. In addition, the ground water itself would likely be severely contaminated, and its flow could carry the contamination far afield.
In the best case scenario, the containment vessel would hold the molten material, limiting most of the damage to the reactor itself. This is what has happened in the majority of core melt incidents, where only part of the core melted.
It seems to be an open question to what extent a molten mass can remain critical as it melts its way down through a structure. It is possible that, as in the Chernobyl accident, the molten mass might mix with any material it melts, diluting itself down to a non-critical state. In the basement of the reactor at Chernobyl, a large "elephant foot" of congealed core material was found in which this process had occurred.
Effects
The effects of a nuclear meltdown depend on the safety features designed into a reactor. A modern reactor is designed both to make a meltdown exceedingly unlikely, and to contain one should it occur.
In a modern reactor, a nuclear meltdown, whether partial or total, will be contained inside the reactor containment structure. Thus (in the unlikely event that no other disasters occur) while the meltdown will severely damage the reactor itself, contaminating the whole structure with highly-radioactive material, a meltdown alone will generally not lead to significant radiation release or danger to the public. The effects are therefore primarily economic (see [1] (http://www.nucleartourist.com/events/part-melt.htm)).
In practice, however, a nuclear meltdown is usually part of a larger chain of disasters. For example, in the Chernobyl accident, by the time the core melted, there had already been a large steam explosion and major release of radioactive contamination.
Reactor design
Although pressurized water reactors are susceptible to nuclear meltdown in the absence of active safety measures, this is not a universal feature of civilian nuclear reactors. Much of the research in civilian nuclear reactors is for designs with passive safety features that would be much less susceptible to meltdown, even if all emergency systems failed. For example, pebble bed reactors are designed so that complete loss of coolant for an indefinite period does not result in the reactor overheating.
Fast breeder reactors are more susceptible to meltdown than other reactor types, due to the larger quantity of fissile material and the higher neutron flux inside the reactor core, which makes it more difficult to control the reaction.
Popular awareness
A nuclear meltdown is also colloquially known as the China syndrome, from the humorously exaggerated notion that molten reactor material would burrow from the United States through the center of the earth and emerge in China, as popularized by the 1979 film, The China Syndrome. This has usually been meant jokingly (including in the film): to bore through the Earth to China, the molten fissile material would have to go both with gravity and then against gravity, and also somehow manage to withstand the hotter material at the core of the planet. In reality, a melting reactor is estimated to be able to sink at most 15 meters; once radioactive slag reached the water table beneath the reactor building, the enormous steam release could throw the material into the air, for it to land as fallout across a wide area.
Meltdowns
A number of Russian nuclear submarines have experienced nuclear meltdowns. The only known large scale nuclear meltdown at a civilian nuclear power plant was in the Chernobyl accident at Chernobyl, Ukraine, in 1986, although there have been several partial core meltdowns, including accidents at:
- NRX, Ontario, Canada, in 1952
- EBR-I, Idaho, USA, in 1955
- Windscale, Sellafield, England, in 1957
- Enrico Fermi Nuclear Generating Station, Michigan, USA, in 1966
- Three Mile Island, Pennsylvania, USA, in 1979
Not all of these were caused by a loss of coolant and in several cases (the Chernobyl accident and the Windscale fire, for example) the meltdown was not the most severe problem.
See also
External link
- Partial Fuel Meltdown Events (http://www.nucleartourist.com/events/part-melt.htm)de:Kernschmelze