On a model for the codiffusion of isotopes

We consider a model for the distribution of radionuclides in the ground water around a deep repository for used nuclear fuel, based on the assumption that different isotopes of the same chemical element A contribute jointly to the chemical potential of A. In this hypothesis, the total flux Ji of a particular isotope Ai of an element A has two components. The corresponding problem consists in a parabolic system strongly coupled. In the physically relevant assumption that one of these components is negligible, the model reduces to a parabolic equation for the total concentration of the element A, possibly coupled with hyperbolic equations for the concentrations of the single isotopes.