In theory, the future disposal site should hold the vitrified reprocessed irradiated fuel, the UOx fuel that has not been reprocessed and, possibly, MOx irradiated fuel. Globally, reprocessing decreases the long-term radio­toxicity of the waste by significantly decreasing the amounts of plutonium and iodine it contains. Here, we will consider only the non-reprocessed fuels, i. e. the maximum potential radioactivity losses.

In table I.2, the composition of 20 000 metric tons of irradiated fuel (IF) at the reactor discharge time is given. This amount corresponds to approxi­mately 10 years’ operation of the reactors that are running today in France. It shows that the largest activities at discharge are those of 90Sr, 137Cs and 241Am which have half-lives shorter than 50 years. These activities will be sharply reduced 100 years after discharge. This is why an interim storage with at least such duration is, almost always, thought necessary before deep underground storage. Such interim storage would also reduce consider­ably the activity of 244Cm, which is known to be a strong neutron emitter.