Diffusion of the 241Am → 237Np Decay Chain Limited by Their Elemental Solubilities in the Artificial Barriers of High-Level Radioactive Waste Repositories

Abstract

A mathematical analysis of the diffusion of the [sup 241]Am [r arrow] [sup 237]Np decay chain in the artificial barrier of a high-level radioactive waste repository is presented. First, analytical solutions obtained are for the space- and time-dependent concentration of [sup 241]Am in the artificial barrier and the time-dependent amount of americium precipitated at the surface of the waste glass, based on the assumption of the congruency of the radionuclides with solubility-limited dissolution of the glass matrix. The effects of solubility sharing with coexisting [sup 243]Am are considered. Transport and precipitation of [sup 237]Np in the artificial barrier are analyzed by dividing the time domain into a small time domain, where the [sup 241]Am concentration is so large that [sup 237]Np precipitation is dominant, and a large time domain, where the [sup 241]Am becomes negligible and the precipitation region shrinks by diffusion from the precipitation front. The equation for the movement of the precipitation front is obtained. As the overpack lifetime increases, the effect of neptunium precipitation becomes less significant. With a lifetime longer than [approximately]6000 yr, an earlier model, where neptunium is treated as a mother nuclide and the precipitation occurs only at the glass surface, can be used.