Neptunium Concentrations in Solutions Contacting Actinide-Doped Glass

To help predict concentrations of neptunium leached from nuclear waste repositories in geologic environments, the solubility of neptunium in a neptunium-doped borosilicate glass, which simulates a high-level waste glass, was investigated. The concentrations of neptunium in solutions contacting the crushed doped glass were found to be controlled by a neptunium solid phase that is similar to crystal-line(c)´NpO₂ in solubility. Thus, the maximum concentration of the neptunium leached from this waste form can be predicted from the solubility of NpO₂(c). This conclusion is based on similar neptunium concentrations in solutions contacting neptunium-doped glass, neptunium-doped glass plus NpO₂(c), and NpO₂(c) alone, under controlled redox potentials and a range of pH values. The quinhydrone used in this study was found to be a very effective redox buffer (the approximate pe + pH = 11.8). The predictions based on the thermodynamic data and the solvent extraction tests showed Np(V) to be the primary oxidation state in solution.