Leaching and migration of neptunium in a simulated engineered barrier system consisting of HLW glass and compacted bentonite

In a simulated engineered barrier system consisting of a simulated HLW glass doped with 237Np, a compacted bentonite and water under reducing conditions, leaching of Np from the glass and diffusion of the leached Np through the bentonite were measured systematically. The following results were obtained: (1) The concentration of dissolved species of Np in the leachate is 10−7–10−8 M, which indicates that Np(IV) is the dominant oxidation state in the system and the concentration may be controlled by solubility of NpO2xH2O (am). (2) The presence of compacted bentonite facilitates reduction of Np(V) to Np(IV) in the system. (3) The apparent diffusion coefficient of the leached Np through the bentonite is evaluated to be lower than 1.5×10−13 m2/s, showing that the diffusivity of Np(IV) is more than one order of magnitude lower than that of Np(V).