Give it time was one of François Mitterrand's favourite maxims. Far from the cut and thrust of politics, this maxim really does have to be followed when it comes to managing radioactive waste.
When a radioactive nucleus emits radiation, it loses some of its radioactivity. A fission product, for example, turns into a stable nucleus. The shorter the half-life of a radioactive nucleus, the quicker it becomes stable. After around ten half-lives the radiation emitted is reduced by a factor of 1000. If we can wait that long or longer, this is how much the radiological risk from this radiation will have decreased.
The most active elements are the ones that disappear quickest. A good illustration of this is given by the example of four radioelements representative of the spent fuel from reactors. These radioactive nuclei, which have very different half-lives, are iodine-131 (half-life of 8 days), caesium-137 (30 years), technetium-99 (210,000 years) and the actinide americium-241 (433 years). Because of the disparity of these half-lives, iodine-131 is 1300 times more radioactive than caesium-137 for an equal number of atoms. Caesium-137 is 14 times more active than americium-241, which is itself 485 times more active than technetium-99.
Because of its extremely high level of activity, a few months after the reactor is stopped the iodine-131 will have completely disappeared. On the other hand the caesium-137, an abundant fission product, will have time to pass through a reprocessing plant and be contained in a glass matrix, though our distant descendants would find that it had disappeared from the glass package within a few centuries. An actinide like americium-241 will take several millennia to disappear. In principle it should remain captive in the glass for the whole of this time.
Finally, with the technetium, which has a half-life of 210,000 years, we can no longer count on the glass being intact. But this long half-life is offset by its low level of activity: in 100 years only one technetium atom in 3300 decays!
Letting time do its work is good for the most radioactive elements with short or medium half-lives. That is why spent fuel assemblies are stored in the pools of reactors and reprocessing plants, and why vitrified waste is stored in ventilated shafts until a decision is made about its fate.
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