THE VOLATILITY OF UO$sub 1$ $sub 2$+-x AND PHASE RELATIONS IN THE URANIUM- OXYGEN SYSTEM

The volatility of UO/sub 2 plus or minus x/ and the phase relations in the uranium-oxygen system were studied using thermogravimetric techniques. Chemical reactions describing the loss of uranium from UO/sub 2 plus or minus x/ at temperatures between 1100 and 2200 deg C in oxygen pressures between approximately 10 and 10/sup -6/ torr are proposed. The uranium-bearing vapor species above UO/sub 2 plus or minus x/ appears to be UO/sub 4/(g). Evidence supporting the existence of UO/sub 4/(g) included the volatilization of material with an oxygen-to-uranium ratio of 4 during the decomposition of UO/sub 2 plus or minus x/ (0.2 > x > 0) to near-stoichiometric UO/sub 2/ in vacuum above 1500 deg C and the dependence of the evaporation rate of the uranium dioxide on the oxygen pressure between 1200 and 1500 deg C. The equilibrium oxygen pressures over compositions between UO/sub 2.02/ and UO/sub 2.63/ in the UO/sub 2+x/ and U/ sub 3/O/sub 8/- regions and over the boundary between these phases were measured between 1000 and 1600 deg C. The equilibrium oxygen-to-uranium ratio of UO/sub 2 plus or minus x/ was less than 2 above 1700 deg C in vacuum. (auth