MECHANISMS AND KINETICS OF LEACHING OF UO2 IN WATER

Interaction of UO2 with water leads to the formation of nonstoichiometric oxides and is of interest both in mineralogy and for the envisaged technique of disposal of spent nuclear fuel elements in deep geological repositories. In the present study, UO2 single crystals and sinters were leached in distilled water between room temperature and 200-degrees-C and for time periods between about 1 and 1000 h. Rutherford backscattering of 2 MeV He+-ions, with and without the channeling effect technique, was used to determine thickness and O/U-ratio of the surface layers, and elastic recoil detection analysis (ERDA) was used to measure the hydrogen concentration of the surface layers. Following a first formation of thin (< 0.2-mu-m) layers of essentially water-free oxides of compositions between U3O7 and U2O5, thick layers Of UO3.nH2O were formed at T greater-than-or-equal-to 150-degrees-C. The value of n was low indicating that the expected equilibrium composition of schoepite was either not formed or that schoepite was not covering all of the surface. Islands of basaltic-type crystals of redeposited uranium oxide were also found. The kinetics of U3O7 or U2O5 layer formation was indicated to be either controlled by the reaction at the UO2/U3O7-U2O5 interface or by oxygen diffusion through the oxidized layer, depending on the temperature and pH-value.