COMPARISON OF ACTIVE AND INACTIVE URANIUM DIOXIDE-OXYGEN SYSTEMS

Inactive UO/sub 2/ oxidizes at 180 deg C by tbe formation of a tetragonal oxide (U/sub 3/O/sub 7/), which is unsthble, on heating above 300 deg C. The U/sub 3/C/sub 7/ in compositions below UO/sub 2.25/ reacts with UO/sub 2/ to form UC/sub 2+x/ and U/sub 4/O/sub 9/. At compositions above UO/sub 2.25/, the tetragomal phase is stable to 600 deg C. In the homogenized condition, both U/ sub 3/O/sub 7/ and U/sub 4/O/sub 9/ are n-type conductors, while UO/sub 2+x/ is p- type. The solubility limit of O in UO/sub 2/, as determimed by x-ray diffraction data and electrical measurements, is UO/sub 2.08/ at 25 and 200 deg C, UO/sub 2.11/ at 400 deg C, UO/sub 2.17/at 600 deg C, and UO/sub 2.20/ above 700 deg C. The active UO/sub 2/ oxidation product retains the cubic symmetry of UO/sub 2/ up to the composition UO/sub 2.40/ and is stable to 600 deg C in vacuum. The structural behavior of the active oxide in this composition range is significantly different from that of the inactive oxide. (auth)