The flow of radioactive nuclei exiting the layer being known, it is possible to determine the maximum dose delivered to the critical population. The

assumption is that the radioelements coming out of the clay layer are dispersed in the ground water used by the critical population. Let Jmax be the maximum value of the flow of nuclei. Admitting that an equilibrium of the flows is reached, an equal flow is obtained at the outlet of the ground water. Each member of the population ingests a fraction a of the water available so that the flow of radioelements ingested is aJmax. Only a fraction f of radioelements ingested settles in the body. The radioelement is character­ized by its biological period Tb that corresponds to the time during which it remains in the body and, hence, a biological half-life Ab = 0.693/Tb. The amount r of radioelement present in the body is thus such that, at equilibrium, rAb = afJmax. The radioactivity inside the body due to the radioelement is Ar = AafJmax/Ab. The radioelement is assumed to settle in target organs whose mass is m,. If the radioelement’s decay energy (neutrinos excluded) is є, the annual dose received is A = A"(afJmaxv/m, Ab) where, time being measured in seconds, v is the duration of a year. If the energies are expressed in Joules and masses in kg, the absorbed dose is in Gray/year. To account for the differing biological effectiveness of radiations, each is given a quality factor, which is 1 for photons and electrons and approximately 20 for a particles. The committed dose is then expressed in Sieverts. Moreover, as the radioelement settles preferentially in one or several organs, the mean dose delivered to the whole body is less than that delivered to the organ(s) considered. To account for this effect, a weighting factor w is associated with each organ. Finally, the effective committed dose is