The more a nucleus is long-lived, the less radioactive it is. This obvious statement is often overlooked, but this is the case of long-lived fission products whose lifetimes are of the order of hundreds of thousands or a million years. For waste management, a radioelement is said long-lived if its radioactive half-life (or period) is beyond 30 years. These fission products, which represent about 6% of the total, are troublesome mainly because they are slow to disappear. It should be noted that the drawback of extremely long lives is offset by very low radioactivity.
After an hundred years of age, the activity of fission products in the nuclear spent fuel from drops with the disappearance of medium-lived elements. After 500 years, fission products represents only a thousandth of that of plutonium and minor actinides. This activity is more than fifty-thousandth of what it was at the time of discharge from the reactor.
After a dramatic decrease during the first 500 years, the radioactivity of fission products reached a plateau that will last for hundreds of thousands years, that is to say the time needed for long-lived elements to disappear in their turn. During this time, about as long as the Quaternary period, the activity of fission products will be only a few thousandths or hundredths of those actinides, whose activity has also decreased considerably.
This residual radioactivity of fission products is due to a handful of long-lived isotopes. Key members of this group are technetium-99, cesium-135, iodine-129, palladium-107 and zirconium-93. Zirconium and palladium are chemically not very mobile, which leaves cesium, iodine and technetium as potential troublemakers. These elements are more mobile than actinides. Iodine-129 and cesium-135 are known for their proneness to leave the chemical structures in which we try to keep them and their tendency to spread in the environment.
Technetium-99 is the main nuclide of this trio, being the most abundant and the most active with the shortest period of 215,000 years. It is possible to destroy it by transmutation, the neutron capture transforming technetium-99 within a few minutes into a stable nucleus. This nuclear reaction, which requires to back to a nuclear reactor, however, is difficult to achieve with a good yield.
Iodine-129 is the most toxic if swallowed, but its very long period of 15.7 million years makes it very little active. By comparison, it is 240 million times less active than its short-lived and dangerous companion, iodine-131 which was the cause of thyroid cancer around Chernobyl. The effects of cesium-135 are the same as those of cesium-137 (the main legacy of radioactivity at Chernobyl 20 years later), but with a period of 2.3 million years, its activity is 77 000 times lower.
The risks presented by these few tough radioelements are minimal. There are plans to transmute, as stated above, elements such as technetium. This shows the progress made towards the elimination of some radioelements from nuclear waste (see note).
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