The solution to equation (I.5) in a semi-infinite environment defined by an interface at x = L, and for N nuclei, gives the current at the interface:

For stable nuclei (A = 0), the flow has a maximum at time:

L2

Ts = 6D

which can be considered as a delay to the egression of the radioelements. The width of the egression time distribution is about

It appears that the clay layer plays a twofold role: it delays biosphere contamination (thus leaving time for many isotopes to decay); it spreads the contamination over a duration that is proportional to the delay. The
maximum value determines the amplitude of the risk for the critical popula­tion. The risk, then, is proportional to the diffusion constant and inversely proportion to the square of the layer thickness. The quadratic dependence on the clay layer thickness emphasizes its importance and, as a consequence, that of a judicious selection of the geological formation used for disposal.

These formulas are easily modified so as to take the decay constant of the radioelements into consideration. Similarly, the time spread of radio­activity release from the fuel elements can be taken into consideration.