Oxidation of UO2 in molten alkali-metal carbonate mixtures: formation of uranates and diuranates

Abstract

The oxidation of UO, in a variety of alkali-metal carbonate melts has been studied in the temperature range 723-1123 K. UO, reacts with carbonate in the presence of oxygen to produce a mixture of mono-and di-uranates. The monouranate is favoured at high temperature. In the presence of alkali-metal nitrate salts the reaction rate is increased markedly: 100% oxidation of UO, is possible at 723 K in 1 h. Oxygen stirring of the melt also increases the rate of oxidation over air stirring and simple stirring. The mechanism of oxidation of UO, is dependent upon generation of peroxide and superoxide ions in the carbonate melt.Reactions of alkali-metal carbonates with UO, have generally been used for the synthesis and characterization of exotic uranate species. A typical reaction' is that of carbonate, M2C03, (M = Li, Na or K) with UO, at 1073 K in an oxygen atmosphere, yielding monouranate, M,UO, :The diuranates Li,U,O, and Na,U,O, decompose at 1000°C, under vacuum, to form the monouranate species.' However, Spitsyn et ~1 . ~9 ~ have reported that diuranates are thermally stable in air and monouranates decompose above 1200°C, and the final decomposition products of Na,UO, and K,U04 are their respective diuranates and alkali-metal oxides :When the monouranates Li,UO, or Na,UO, are heated with UO,, under vacuum reduction of Uv' to Uv occurs, but surprisingly this reaction does not occur with K 2 U 0 4 : Li,UO,(s) + UO,(s) -+ 2LiU03(s)(3)The uranates NaUO,, Na,UO, and NallU5016 have all been prepared from UO, and Na,CO, above 1200 K at various oxygen partial pressures for varying time period^.^*^ K2U4013 has been prepared6 upon heating UO, and K2C03 in air, but it loses oxygen on heating in an inert atmosphere: this reaction is reversed upon reheating in oxygen.Kryukova et aL7 have examined the kinetics of the oxidation of UO, in the ternary (Li,CO,-Na,CO,-K,CO,) eutectic between 683 and 773 K as a function of oxygen partial pressure, the greater the pressure the greater the yield of diuranate. The addition of 5-15 mol% NaNO, to the melt was also examined. Diuranate yields were 74% with 15 mol% NaNO, present in the melt, 28% in an oxygen containing atmosphere and only 18% when the reaction was carried out in an inert atmosphere. Kryukova et aL8 have constructed a

Crystal growth of K2UO4 and Na4UO5 using hydroxide fluxes

“…Both title compounds, K 2 UO 4 and Na 4 UO 5 , have been studied as part of such general investigations of uranium oxides [2,4,[7][8][9]. In these studies, the oxides were synthesized as polycrystalline powders using solid-state routes, although carbonate melts have been considered as synthetic media [6,10,11].…”

Reprocessing spent nuclear fuel using molten carbonates and subsequent precipitation of rare earth fission products using phosphate

“…200 ppm [7,8]. The details of this conversion process for uranium, and its optimization, have been published [1][2][3][4].…”

“…In the United Kingdom we have been investigating various aspects of the chemistry of UO 2 in molten alkali metal carbonates [1][2][3][4][5][6][7][8][9][10][11]. We have also commenced a study on the removal of compounds of the fission product elements from carbonate melts, and so far have investigated Cs, Mg, Sr, Ba, lanthanides (La to Dy), Zr, Cr, Mo, Mn, Re (to simulate Tc), Fe, Ru, Ni, Cd, Bi and Te in chloride melts and the efficiency of their precipitation as phosphates.…”

The feasibility of using molten carbonate corrosion for separating a nuclear surrogate for plutonium oxide from silicon carbide inert matrix

“…The oxygen flux increases with a decreasing oxygen diffusion distance, an increasing diffusion coefficient and an increasing solubility of the oxygen which largely depends upon the partial pressure of oxygen in the gas phase. Maru et al [15,16] summarized that the solubility of oxygen within a temperature range of 600-800°C in molten carbonates is controlled by the reactions with the melt to generate peroxide (O 2À 2 ), superoxide (O 2À ) ions and carbonate dioxide (CO 2 ). The solubility of oxygen in the molten carbonates will be enhanced with a decreasing partial pressure of CO 2 .…”