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+, CO32-, 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.