For clay porosity equal to 20%, measurements of the diffusion constant for water give the value D0 = 1.6 x 10~3m2/year. The diffusion constants for dissolved elements depend on the type of element and reflect its greater or smaller affinity for clay. The sorption phenomena on the mineral surfaces is taken into account through a reduction of the diffusion constant D by the delay coefficient. Table I.1 shows examples of delay coefficients and solubility limits giving the maximum possible concentration in water for each element, according to its chemical properties. Note, in the table, that radium, which is always one of the descendants of actinides, is delayed 100 times less than thorium, uranium, or the minor actinides. Radium could then well play an important role in the transfer of radioactivity due to the actinides. In fact, only Ra, descending from U, has a half-life that is large enough (1600 years).

The delay coefficients are measured in the laboratory, by studying the sorption of an aqueous solution of the element by a clay sample with the

same properties as the layer. These evaluations are done also as a function of the temperature.