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.