Modeling for migration of a redox-sensitive radionuclide in engineered barriers

In this paper numerical results are presented for an analysis of diffusion of {sup 237}Np, a redox-sensitive radionuclide, in engineered barriers consisting of overpack and bentonite-filled buffer regions, with oxidation-reduction kinetics of neptunium with iron and dissolved oxygen. Steady-state distributions of Fe(II) and dissolved oxygen are first obtained by considering the oxidation reaction of Fe(II) with oxygen. Based on these profiles, the neptunium diffusion models for pH 6.5 and 9.,0 are established. Analytical or finite element solutions are obtained for the corresponding mathematical problems. At pH 9.0, even if dissolved oxygen intrudes on the buffer region from the rock/buffer interface, the penetration of oxygen into the overpack region and to the surface of the waste solid can be avoided. Thus, less soluble, strongly sorbing Np(IV) is released from the waste solid. At pH 6.5, dissolved oxygen can reach the waste surface. Weakly sorbing, soluble Np(V) is released from the waste solid. Although the released Np(V) is reduced to Np(IV) by Fe(II) in the overpack region, the Np(IV) is quickly oxidized by FE(III) and dissolved oxygen at the overpack-buffer interface.