Uranium Secondary Phase Formation During Anoxic Hydrothermal Leaching Processes of UO2 Nuclear Fuel

Mobilisation of uranium in geologic environments from UO2 solid phases usually takes place by oxidative dissolution involving a change of U oxidation state from IV to VI; however, anoxic or reducing geochemical conditions are expected in many of the planned European disposal sites. This work investigates potential alteration mechanisms of UO2 in contact with groundwater ions (Ca2, CO2 3 , and silicate) under anoxic conditions, at ambient (25 C) and hydrothermal (180 C) temperature conditions. SEM–EDX analysis detected (in the case of treatment at 180 C in high silicate content solutions) a compound with U:Si ratio of 1:1 on the UO2 surfaces after leaching. Minor quantities of phases containing U, Ti, Fe, Si, and Ca were formed, these could not be characterized completely. A further experiment, performed in the presence of dissolving CaO:TiO2:SiO2:Fe(0)–Fe2O3, formed a compound with U:Si:Ca of 1:2:8, a ratio not matching any known uranyl compound. The two phases, possibly identical with coffinite, USiO4, and U-bearing ekanite, UCa2Si8O20, were found to form at different (Ca):(Si) conditions. The implications upon the final geologic storage of actual, heterogeneous spent fuel are discussed.JRC.E.3-Materials researc

A Kinetic Study of UO2 Dissolution and H202 Stability in the Presence of Groundwater Ions

Current reactors spent nuclear UO2 fuel that will probably be directly disposed in geologic formations is assumed to get into contact with groundwater of a repository earliest after several thousands of years. After this time, its alpha activity will be still high enough to induce a process of water radiolysis that leads to production of oxidants and, probably, to the oxidative dissolution of the material, that would start a mobilisation of radiotoxic nuclides. The main oxidant produced during this process would be hydrogen peroxide (H2O2), which is thermodynamically unstable and can decompose during reactions with certain groundwater ions. Information on the stability of H2O2 is therefore important for assessing the behaviour of the material in the repository.JRC.E.3-Materials researc