Indeed, the longest lasting legacy of the nuclear age which Fermi inaugurated in December 1942 will be the packages or irradiated wastes which shall persist in a state hazardous to the biosphere for hundreds of thousand years.
These wastes contain radio-active isotopes which have a very long life. For example Plutonium-239, which has a half life of 24,400 years, is dangerous for about quarter million years or 12,000 human generations. It decays to produce uranium-235 which has a half life of710, 000 years, a span of time longer than the parent material. Uranium-235 may decay to produce another radio-active isotope with a long half-life. Thus the problem of nuclear waste disposal acquires almost a permanent character.
Radio-active wastes or irradiated wastes may be grouped into the following two categories:
1. High level irradiated wastes.
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2. Low level irradiated wastes.
(1) High Level Irradiated Wastes:
High level irradiated wastes consist of spent fuel from nuclear reactors and left over material from a nuclear fuel processing units. They contain radioactive isotopes of a large number of elements whose half-life range from fraction of a second to hundreds of thousand years. Each metric ton of spent fuel produces about 180,000,000 curies of radio-activity which gradually declines with time but the rate is very slow. Even after 10,000 years each metric ton of this fuel shall emit 470 curies.
Some countries like U.K. and France reprocess their spent fuel to be used again. The overall impact of which is the production of more high level radio-active wastes. About four hundred and twelve or more commercial reactors all over the world generate about 5% of the world’s power supply but they also produce about 10,000 metric tons of high level nuclear wastes – solids, liquids and gases. It shows the approximate trend in accumulation of high level wastes since the year 1965. At present about 100,000 metric tons of these wastes have already accumulated.
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(2) Low Level Irradiated Wastes:
Low level irradiated wastes are those wastes which have a smaller amount of radio-active material with half-lives about thirty years or less. The material which is regularly irradiated by radiations from nuclear reactors also becomes radio-active. Though there is little radio-activity, these wastes may contain radio-active isotopes with very long half-life – such as those of technitium-94 (half life 210,000 years) and iodine-129 (half life 15.8 million years).
The largest volume of radio-active wastes, however, comes from uranium mines, milling and processing establishments. The uranium ore which also contains a number of radio-active disintegration products of uranium as well as other isotopes, is finely powdered, most of the uranium is extracted and tailings are left behind which possess about 85% of the total radio-activity originally present in the ore.
Low level wastes having little radio-activity are almost as dangerous as high level wastes. Indeed, they do not cause high level injury which occurs in cases of acute exposures. However, both low as well as high level wastes are capable of causing delayed carcinogenic, teratogenic and mutagenic responses. There is no ‘threshold dose’ or ‘dose of no consequences’ for radiation exposures.