Increased oxidation of UO2in molten alkali-metal carbonate based mixtures by increasing oxygen solubility and by controlled generation of superoxide ions, and evidence for a new sodium uranate

Abstract

The oxidation of uranium dioxide to uranates in the ternary alkali-metal carbonate melt (Li–Na–K)₂CO₃ containing added chlorides or sulfates has been studied in the range 723–1023 K and a variety of uranium(VI) species was obtained. Increased oxygen solubility in fused carbonates was achieved by adding alkali-metal chloride or sulfate. The yield of uranates in chloride-containing melts decreased with increasing radius of the alkali-metal chloride cation, Li Na > K > Cs. When UO₂ was oxidised in alkali chloride containing carbonate 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 superoxide at 723 K were inefficient owing to thermal instability of the reagent, but UO₂ oxidation was enhanced, by more than 10%, by superoxide formed insitu by the reaction of peroxide and nitrate, compared with oxidation by peroxide or nitrate individually. Optimum conditions for complete oxidation at 723 K required a peroxide:nitrate mole ratio of ca. 3. The major reaction pathway in melts containing nitrate and peroxide, at various concentrations of these reactants, was determined. At a mole ratio of oxidiser to UO₂ of 0.3, the percentage of UO₂ oxidised and the yield of uranates, increased in the order Na₂O₂ ≈ KO₂ < KNO₂ < KNO₃ < KNO₃ + Na₂O₂. Evidence has been obtained for a new sodium uranate, Na₂O(UO_3−y)_x (1 < x <2 ; y < 0.02).