OXIDATION OF CERAMIC URANIUM-DIOXIDE IN ALKALI-METAL CARBONATE-BASED MELTS - A STUDY USING VARIOUS OXIDANTS AND COMPARISON WITH UO2 POWDER

The oxidation of broken sintered ceramic UO2 pellets has been studied between 723 and 1073 K in a range of carbonate-based melts and under a variety of oxidising conditions. Comparison with the oxidation of UO2 powder under corresponding conditions showed that the reactivity of t hese media with ceramic UO2 was less, due to a lower relative surface area and a lower chemical activity of the sintered material. Pure carb onate melts, and with added alkali metal chlorides, produced essential ly no reaction with ceramic UO2, Bubbling the melt with air or oxygen for many hours also had little effect. Addition of nitrate enabled the rate and extent of the reaction to be controlled, by varying the conc entration of the oxidising agent and keeping the nitrate-to-UO2 mole r atio below similar to 0.4. Above this ratio the available surface area was the main controlling factor of the reaction. Controlled in situ g eneration of superoxide ions increased significantly the uranate yield at 723 K. A minimum concentration of oxidising agent is thus necessar y before reaction commences. Complete oxidation of ceramic UO2 was ach ieved in similar to 2 h at 873 K in carbonate melts that contained KNO 3 and were bubbled with oxygen. (C) 1998 Elsevier Science B.V. All rig hts reserved.