Diffusion of tritium in arid disposal sites

Abstract

Tritiated water, released at a point in a uniform, relatively dry soil, diffuses in both the liquid and vapor phases. A model which accords well with published data indicates that the flux density of tritium in the liquid exceeds that in the vapor phase provided the water content is greater than approximately 20% of the total soil porosity. Thus tritium redistribution should be modeled recognizing transfer “in parallel” in both phases. The diffusion equation cast in spherical coordinates, and taking into account radioactive decay, then provides a basis for design of field experiments and predicts the long‐term fate of tritiated water released in these experiments. We calculate the evolution of profiles of tritium concentration, within and external to the sphere of released solution, assuming the initial concentration to be uniform. We also predict the speed and the envelope of the tritium maximum as it advances and attenuates in the soil. We briefly discuss effects of variation in the volume fractions of soil water and air on the effective diffusion coefficient of tritium in soil and comment on possible effects of convective transfer by moving water and air.