Numerical Analysis of Nuclide Migration Through Fissured Geological Media

A computational analysis of nuclide migration through fissured geological formations was performed. The migration behavior can be described by convective transport in the fissures, diffusive transport with radioactive decay chain in the bulk rock, and sorption on the fissure wall. The mathematical model employed is based on the finite element method (FEM) solution of transport equations, taking into account the interfissure two-dimensional diffusion. The decay chain, ²³⁴U → ²³⁰Th → ²²⁶Ra, was examined to illustrate the migration behavior. The FEM solution was in good agreement with the analytical solution using simpler assumptions. Numerically investigated were the effects of (a) the decay chain in pores, (b) two-dimensional diffusion in pores, (c) the axial dispersion in fissures, (d) the interaction between fissures, and (e) the fissure wall sorption. As a result, it can be said that the effect of the decay chain in pores is especially important in order not to have overestimates in terms of safety and that the fissure wall sorption is an important factor for realistic estimates because it has a remarkable effect on the extent of nuclide confinement within the geological media.