INCREASED OXIDATION OF UO2 IN MOLTEN ALKALI-METAL CARBONATE BASED MIXTURES BY INCREASING OXYGEN SOLUBILITY AND BY CONTROLLED GENERATION OF SUPEROXIDE IONS, AND EVIDENCE FOR A NEW SODIUM URANATE

The oxidation of uranium dioxide to uranates in the ternary alkali-met al carbonate melt (Li-Na-K)(2)CO3 containing added chlorides or sulfat es has been studied in the range 723-1023 K and a variety of uranium(v r) species was obtained. Increased oxygen solubility in fused carbonat es was achieved by adding alkali-metal chloride or sulfate. The yield of uranates in chloride-containing melts decreased with increasing rad ius of the alkali-metal chloride cation, Li greater than or equal to N a > K > Cs. When UO2 was oxidised in alkali chloride containing carbon ate melts no intermediate uranium chloride complexes were observed The temperature required for complete oxidation of UO2 can be lowered by 100-150 K by the addition of alkali-metal chlorides to carbonate melts . The addition of chloride and aluminium ions to form AlCl4--effected UO2 oxidation by a different mechanism, through intermediate formation of uranyl complexes, but the yield of uranates was not significantly altered. Attempts at oxidation by direct addition of potassium superox ide at 723 K were inefficient owing to thermal instability of the reag ent, but UO2 oxidation was enhanced, by more than 10%, by superoxide f ormed in situ by the reaction of peroxide and nitrate, compared with o xidation by peroxide or nitrate individually. Optimum conditions for c omplete;oxidation at 723 K required a peroxide :nitrate mole ratio of ca. 3. The major reaction pathway in melts containing nitrate and pero xide, at various concentrations of these reactants, was determined. At a mole ratio of oxidiser to UO2 of 0.3, the percentage of UO2 oxidise d and the yield of uranates, increased in the order Na2O2 approximate to KO2 < KNO2 < KNO3 < KNO3 + Na2O2. Evidence has been obtained for a new sodium uranate, Na2O(UO3-y)(x)(1 < x < 2; y<0.02).