DIFFUSION OF TRITIUM IN ARID DISPOSAL SITES

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 tr itium redistribution should be modeled recognizing transfer ''in paral lel'' in both phases. The diffusion equation cast in spherical coordin ates, and taking into account radioactive decay, then provides a basis for design of field experiments and predicts the long-term fate of tr itiated water released in these experiments. We calculate the evolutio n of profiles of tritium concentration, within and external to the sph ere of released solution, assuming the initial concentration to be uni form. We also predict the speed and the envelope of the tritium maximu m as it advances and attenuates in the soil. We briefly discuss effect s of variation in the volume fractions of soil water and air on the ef fective diffusion coefficient of tritium in soil and comment on possib le effects of convective transfer by moving water and air.