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Summary:
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In this preliminary study, the air-oxidation studies were performed with 6 wt% Gd2O3 doped UO2 at high temperature (225 - 450 ?C) in order to test conditions for the preparation of the (Gd, U)O2+x oxide, that has a cubic U4O9+x crystal lattice and the O/U ratio close to 2.3. The temperature effects
were studied with the help of the thermogravimetric analysis method (TGA) and the oxidation tests in a laboratory furnace. The preparation of the (Gd, U)O2+x oxide was followed by preliminary dissolution tests with this solid phase in 0.1 M HCl and 10 mM NaHCO3 under anoxic conditions.
These leachants were chosen in order to accelerate dissolution reaction and are not relevant under the disposal conditions of spent fuel. The main objective was to find out the sensitivity of the experimental methods for studying the oxidation and dissolution mechanisms of Gd-doped UO2,
especially how accurately the changes in the crystal lattice of U4O9+y caused by dissolution reaction can be seen by measuring the cubic lattice parameter with X-ray powder diffraction.
The homogeneity of the oxidized samples caused problems in the TGA tests (350 - 450 ?C). In the subsequent air-oxidation tests in a laboratory furnace at lower temperature (225, 305 ?C), the homogeneity was improved. The solid phase for the dissolution tests was oxidized at 305 ?C. A
preliminary calibration curve was measured for the cubic lattice parameter vs. O/U. The presence of original, unoxidized UO2 in the final products at lower O/U ratios (< 2.14) complicated the calculation of the parameter. Further tests are needed to eliminate the effect of unoxidized material.
The dissolution tests of the (Gd, U)O2+x phase, O/U= 2.32, in NaHCO3 solutions under anaerobic conditions (N2) showed a slow increase in the lattice parameter of the solid phase as a function of reaction time, thus suggesting preferential removal of oxygen from the crystal lattice.
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Keywords:
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Uranium dioxide, UO2, UO2 (6% Gd2O3), air-oxidation, dissolution, dissolution mechanisms
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