An example of a complete computation involving all the radionuclides con­tained in the spent fuel is shown in figure I.1. It has been done by ANDRA as

Table I.2. Mass, radioactivity and half-life of the main radionuclides present in 20 000 metric tons of irradiated fuel at discharge.

Figure I.1. Doses at the outlet as a function of time for various nuclides. The source is composed of 21 600 metric tons of non-processed irradiated fuel. Computation made by ANDRA.

part of the verification of the characteristics and performance of the geo­logical barrier on the Meuse/Haute-Marne site. As a consequence, zero efficiency was ascribed to the man-made containment devices that are to surround the packages, whose purpose is, on one hand, to delay the establish­ment of water contact with the packages, and subsequently, on the other hand, to hold back and delay the migration of any radionuclides released. This migration delay is obtained either by inserting strongly absorbing modified natural materials (bentonite) or by inserting concrete, thus creating an alkaline environment to decrease the solubility of many compounds. The source term includes the following.

• A fraction of the radionuclides are assumed to be unstable, i. e. they are released as soon as water reaches the fuel; this concerns in particular 15% of the iodine and 20% of the niobium and the nickel in the inventory.

• The solubility of the elements is taken into account right at the beginning of the fuel dissolving step.

• The computation is done for 21 600 metric tons IF. This value takes into account the present IF French reprocessing status and fits into a larger framework of complete computation, including the contributions due to vitrified wastes, hulls and endcaps. It corresponds to that portion of the fuel that will not have been reprocessed.

The maximum flow of activity is due to iodine, with a peak of 107 Bq/ year at 2 x 106 years. The dose, too, is dominated by iodine. Its behaviour being governed by diffusion, the appearance of iodine at the outlet will be roughly proportional to the source term, that means to the inventory stored. It follows that, in order to compare results for a larger volume stored, the activity flows should be multiplied by the corresponding factor, a maximum of about 5 for 100000 metric tons of IF stored.

Niobium is limited both by its diffusion and by its precipitation. Tech­netium, like selenium, is limited by the precipitation of solid compounds; the dose associated with these two is thus not very sensitive to the source term but, rather, proportional to the area of penetration of the radioelements in the clay layer.