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.

AMME Marcus;  WISS Thierry;  THIELE Hartmut;  BOULET Pascal;  LANG Hauke; 

2005-09-08

Elsevier B.V.

JRC30456

https://publications.jrc.ec.europa.eu/repository/handle/JRC30456,   

10.1016/j.jnucmat.2005.02.004,