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.