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i21

Severe accidents

Meltdown

Severe accidents, involving the uncontrolled release of massive amounts of radioactivity, occur in case of the meltdown of a reactor core or of spent fuel in a storage pool. When the cooling of a reactor or spent fuel pool fails, the fuel elements go heating up rapidly and will melt due to the residual heat of nuclear fuel. The hot zirconium hulls of the fuel elements react with water generating hydrogen. As the hydrogen gets mixed with air, the mixture will explode. This scenario happened at Chernobyl in 1986 and at Fukushima in 2011. As a result of the explosion a large part of the radioactive content of the reactor and/or spent fuel pool will be dispersed into the environment.

Spent fuel pools

Especially the spent fuel pools entail high risks because these contain usually the spent fuel of a number of years with a radioactivity of thousands of nuclear bomb equivalents. In addition the spent fuel pools are not situated in a heavy safety contaiment like the reactor. Meltdown of a reactor is highly hazardous, because the molten nuclear fuel may become easily critical again, starting an uncontrolled fission process generating more heat and radioactive fission products. The molten fuel in a boiled-dry fuel pool can also become critical, as happened at Fukushima. Scenarios are conceivable of explosions at spent fuel storage facilities with consequences that could dwarf the Chernobyl and Fukushima disasters. One operating reactor contains about 1000 bomb equivalents of radioactivity, some storage pools contain many tens of thousands of bomb equivalents.

Consequences

The consequences of a Chernobyl/Fukushima-like disaster are very serious. Large regions, tens of thousands of square kilometers, are so heavily contaminated with radioactive materials, that these areas become in fact inhabitable forever. Some kinds of radionuclides, with short half-lifes, will decay within weeks or months to 'low' levels, other will remain for thousands of years. Many kinds of dangerous radionuclides are difficult to detect and enter the food chain undetected.

Contamination

How are 'low' levels defined? Who determines how harmless a given 'low' radioactivity level is to people living in a contaminated region and who are chronically exposed to the radioactivity? Besides, several radionuclides tend to accumulate in living organisms, in food. So it can happen that the exposure to harmful radioactivity is far higher than predicted on base of the average contamination of an area. In addition a number of biologically active radionuclides are not easily detectable and generally are not monitored and are left out of consideration [more i24].

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chernobylcontams

Figure 21-1. Extent of a large-scale nuclear accident

The spread of caesium-137 (Cs-137), a fission product, after the Chernobyl disaster in 1986, according to UNSCEAR. The darkest colored areas have become in fact inhabitable due to radioactive contamination. In the blanc areas insufficient data were available. The lightest colored areas did not get an appreciable radioactive deposition. The spread of the many other radionuclides escaped from the exploded reactor is not necessarily the same as that of cesium-137.

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Figure 21-2. Boiling water reactor.

The spent fuel cooling basin of this type of reactors are outside of the containment structure around the reactor, as is with all other types of nuclear power plants.

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Figure 21-3. Areas at risk in Europe.

Chart with the nuclear power plants of Europe. The darkest colored areas are within 30 km of an NPP and are the areas to be evacuated in case of an accident releasing nuclear fuel. The risks posed by accidents involving the interim storage of spent fuel might be greater than reactor accidents. Most interim storage facilities are located at the reactor sites.

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Figure 21-4. Spent fuel cooling basin at a reprocessing plant in the UK

The blue light is the result of the interaction of the radiation of the fuel elements with the cooling water. The cooling ponds of nuclear power stations are smaller than this one, but of the same design.

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Figure 21-5. Explosion of a spent fuel cooling basin.

Explosion of the spent fuel cooling basin of reactor 3 at the Fukushima Daiichi plant on March 14, 2011. As a result of the breakdown of the cooling of the basin, the spent fuel partially melted and reacted with the remaining water. The hydrogen generated by this reaction exploded, initiating a criticality incident in the (partially) molten nuclear fuel.