There are other mechanisms, such as inhalation of radioactive material that has been deposited on the ground and then been resuspended, but these are generally of lesser importance.
The direct irradiation from the cloud will usually be dominated by the inert gases such as xenon and krypton. Irradiation from deposited fission products, such as caesium and iodine, will, at least in the early stages of the accident, be of minor significance.
Inhalation of the radioactivity will result in a dose to the lungs and also absorption of some radioactive materials into the bloodstream and hence to other organs of the body. Of particular importance is the inhalation of radioactive iodine since this will concentrate in the thyroid gland. This particular radiological hazard is likely to be the most important following a major accident. Other inhalation hazards, for example a dose to the lungs due to ruthenium, may also be important.
Ingestion of radioactivity, by eating contaminated foodstuffs or (less likely) drinking contaminated water, could be a significant radiological hazard. It is also, of course, a mechanism by which people who were outside the affected area could be subject to a radiological hazard from the accident. The concentration of radioactive iodine in milk is of particular importance. This is because iodine deposited at low concentrations over a wide area of pasture land could be concentrated by grazing cows into a few tens of litres of milk. Fortunately the most significant type of iodine is iodine-131 which has a half-life of only eight days. Even in the event of a major accident the iodine problem is unlikely to remain for more than a few weeks. The hazards can, of course, be avoided provided adequate steps are taken to prevent the eating and drinking of contaminated foodstuffs.
4.5 Experience of Nuclear Reactor Accidents
In 1942 the first self sustaining nuclear chain reaction was achieved in a nuclear pile constructed by the physicist Enrico Fermi on a converted squash court at the University of Chicago, USA. Since that time numerous experimental, military and commercial nuclear reactors have been constructed and operated. As with the development of any major new technology there have been a small number of incidents and accidents over the years. It is probably worthwhile highlighting three of these accidents, viz, Windscale 1957, Three Mile Island 1979 and Chernobyl 1986. These accidents have gained an exceptional infamy. Their radiological consequences are discussed in this chapter.
4.6 Windscale
In the 1950's two nuclear reactors were constructed at Windscale in Cumbria, England. These reactors were designed to produce plutonium for nuclear weapons manufacture. The heat produced was purely adventitious and cooling was by a forced draught of air passing directly to the atmosphere via a filtered, 125 m high, stack. Graphite blocks were used as moderators. Irradiation of the graphite can lead to small dimensional changes in the block which causes tensions (Wigner energy) to be built up in the graphite. It was routine practice to release these internal tensions in the graphite blocks at regular intervals. This process involved using the nuclear heat produced in the reactor to heat the graphite to above its normal operating temperature, and it was controlled by observing. temperature changes within the graphite. In October 1957 this process was being carried out. The nuclear heat was applied to the core twice resulting in two fuel elements failing in a part of the core where the temperature was not being monitored. The exposed uranium smouldered and the fire eventually spread to about 50 channels of the core. The fire was extinguished by pumping water through the core, but not before some gaseous and volatile fission. products had been released via the filtered stack into the atmosphere.
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