Barrier properties of crystalline rocks in radionuclide migration

Abstract

The parameters of the diffusion–sorption interaction of rocks with Sr, Cs, U, Np, Pu, and Am isotopes dissolved in simulated groundwater were determined by in- and through-diffusion methods for monolithic rock samples (gneisses, dolerites, lamprophyres) from the Yeniseisky site of the Nizhnekansy massif. Despite heterogeneous mineral composition and structure of the rocks, these parameters mainly depend on the physicochemical properties of the element and on the open porosity of the rock. The effective diffusion coefficients D_e vary in the order Sr = Cs ≫ U > Np > Am ≥ Pu, dividing these elements into two groups, the difference between which is 1.5–2 orders of magnitude. With respect to the apparent diffusion coefficient D_a, the elements are also subdivided into two groups, Sr = Cs > Np ≥ U ≫ Am > Pu, differing by 5 orders of magnitude because of different sorbability of the elements: Pu > Am ≫ Cs = Sr > U > Np. The end members of this series differ in the sorption capacity coefficient of the rocks, α = D_e/D_a, by 4.5 orders of magnitude.