Uranium Mineralogy and Neptunium Mobility

Spent nuclear fuel, mainly UO2, is chemically unstable under oxidizing conditions. Alteration occurs by oxidation and hydration and can be rapid and substantial. Uranyl minerals, containing U6+, form when commercial spent fuel is altered in a moist, oxidizing environment. During the alteration and dissolution of the UO2, fission-product radionuclides and transuranium elements are released. Uranyl minerals that form locally as alteration products in a geologic repository may incorporate many of these radionuclides, thereby immobilizing them for lengthy periods and thus improving the long-term performance of the repository. Here we discuss one very important radionuclide, 237Np (half-life = 2.14 million years), and focus on mineralogical studies that probe the potential impact of uranyl minerals on neptunium mobility.