[en] The removal of fission product elements from molten salt wastes arising from pyrochemical reprocessing of spent nuclear fuels has been investigated. The experiments were conducted in LiCl-KCl eutectic at 550 deg. C and NaCl-KCl equimolar mixture at 750 deg. C. The behavior of the following individual elements was investigated: Cs, Mg, Sr, Ba, lanthanides (La to Dy), Zr, Cr, Mo, Mn, Re (to simulate Tc), Fe, Ru, Ni, Cd, Bi and Te. Lithium and sodium phosphates were used as precipitants. The efficiency of the process and the composition of the solid phases formed depend on the melt composition. The distribution coefficients of these elements between chloride melts and precipitates were determined. Some volatile chlorides were produced and rhenium metal was formed by disproportionation. Lithium-free melts favor formation of double phosphates. Some experiments in melts containing several added fission product elements were also conducted to study possible co-precipitation reactions. Rare earth elements and zirconium can be removed from both the systems studied, but alkaline earth metal fission product elements (Sr and Ba) form precipitates only in NaCl-KCl based melts. Essentially the reverse behavior was found with magnesium. Some metals form oxide rather than phosphate precipitates and the behavior of certain elements is solvent dependent. Caesium cannot be removed completely from chloride melts by a phosphate precipitation technique

[en] The high temperature reactions of molybdenum and its oxides with chlorine and hydrogen chloride in molten alkali metal chlorides were investigated between 400 and 700 deg. C. The melts studied were LiCl-KCl, NaCl-CsCl and NaCl-KCl and the reactions were followed by in situ electronic absorption spectroscopy measurements. In these melts Mo reacts with Cl₂ and initially produces MoCl₆^2- and then a mixture of Mo(III) and Mo(V) chlorocomplexes, the final proportion depending on the reaction conditions. The Mo(V) content can be removed as MoCl₅ from the melt under vacuum or be reduced to Mo(III) by Mo metal. The reaction of Mo when HCl gas is bubbled into alkali chloride melts yields only MoCl₆^3-. MoO₂ reacts in these melts with chlorine to form soluble MoOCl₅^2- and volatile MoO₂Cl₂. MoO₃ is soluble in chloride melts and then decomposes into the oxychloride MoO₂Cl₂, which sublimes or can be sparged from the melt, and molybdate. Pyrochemical reprocessing can thus be employed for molybdenum since, after various intermediates, the end-products are chloride melts containing chloro and oxychloro anions of molybdenum plus molybdate, and volatile chlorides and oxychlorides that can be readily separated off. The reactions were fastest in the NaCl-KCl melt. The X-ray diffraction pattern of MoO₂Cl₂ is reported for the first time

[en] One of the possible pyrochemical reprocessing procedures for spent ceramic nuclear fuels may involve the oxidation of UO₂ in alkali metal carbonate and carbonate-based melts, and this is controlled by the level of dissolved oxygen in the melt. A quantitative relationship has been derived between the extent of UO₂ oxidation and the concentration of oxygen (peroxide/superoxide) species formed upon oxygen dissolution in carbonates. A novel sensitive method for determining oxygen solubility in molten carbonates and carbonate-based melts has thus been developed. The concentrations of the alkali metal uranates(VI) formed can then be accurately determined without interference from unreacted UO₂. Oxygen solubilities at temperatures from 450 deg. C to 800 deg. C have been determined. The solubility of oxygen in a range of carbonate-chloride melts was also determined and found to increase with decreasing radius of the cation of the alkali metal chloride added. Measurements at various partial pressures of oxygen allowed the determination of the predominant oxygen species formed in the melt, and preliminary experiments showed that in the ternary carbonate melt, at 450 deg. C, oxygen dissolves forming mainly superoxide ions. The applicability of Henry's Law in this situation is examined

[en] The chlorination of uranium metal or uranium oxides in chloride melts offers an acceptable process for the head-end of pyrochemical reprocessing of spent nuclear fuels. The reactions of uranium metal and ceramic uranium dioxide with chlorine and with hydrogen chloride were studied in the alkali metal chloride melts, NaCl-KCl at 973K, NaCl-CsCl between 873 and 923K and LiCl-KCl at 873K. The uranium species formed therein were characterized from their electronic absorption spectra measured in situ. The kinetic parameters of the reactions depend on melt composition, temperature and chlorinating agent used. The reaction of uranium dioxide with oxygen in the presence of alkali metal chlorides results in the formation of alkali metal uranates. A spectroscopic study, between 723 and 973K, on their formation and their solutions was undertaken in LiCl, LiCl-KCl eutectic and NaCl-CsCl eutectic melts. The dissolution of uranium dioxide in LiCl-KCl eutectic at 923K containing added aluminium trichloride in the presence of oxygen has also been investigated. In this case, the reaction leads to the formation of uranyl chloride species. (author)

[en] Four thallium(I) uranates(VI), Tl₄UO₅, Tl₂UO₄, Tl₂U₂O₇ and Tl₂U₃O₁₀, were prepared and their IR, and, for the first time, their Raman, electronic and X-ray absorption (EXAFS) spectra have been measured. These uranates are thermally stable under nitrogen up to 660 deg. C but above this temperature they decompose and their uranium content increases, due to loss of Tl₂O. The thermal stability of these uranates increases with decreasing thallium content. Comparison with the reported values for the standard enthalpies of formation of the corresponding alkali metal uranates, which are essentially cation independent, allows suggested values for the above uranates, respectively, of -2437, -1900, -3182, and -4437 kJ mol^-1. Structural information and atom arrangements were obtained from analysis of powder XRD and EXAFS spectroscopy measurements. U-O distances, both primary and secondary, were obtained from EXAFS measurements and these were essentially identical with those calculated from vibrational IR spectra

[en] Uranium (and plutonium) can be separated from spent fuel by treatment in molten carbonates and air sparging. UO₂ is converted into insoluble uranate species that can be filtered off. The rare earth and other fission product elements remaining in solution can be later precipitated using phosphates as precipitants. A sequence is described that can (in principle) be used to recover and recycle the carbonate melt used. Molten chloride eutectics are more convenient solvents than carbonates for obtaining the detailed chemistry of fission products in molten salts and their selective precipitation as phosphates. We have investigated the behavior of: Cs, Mg, Sr, Ba, lanthanides (La to Dy), Zr, Cr, Mo, Mn, Re (to simulate Tc), Fe, Ru, Ni, Cd, Bi and Te, but here report results for the rare earths. The distribution coefficients of these elements between chloride melts and precipitates were determined. Lithium-free melts favored formation of double phosphates. Rare earth elements and zirconium can be removed from chloride melts but Sr, Ba and Mg are melt cation specific. EXAFS and XANES measurements have established the speciation of both transition and non-transition elements in molten chlorides. EXAFS is largely model dependent, and to ensure the correct coordination number requires absorption spectroscopy data. EXAFS provides the element-Cl distance in the melt, not previously available. At high concentrations, the presence of bridging chlorine atoms can be established by EXAFS. The effect of diluting such melts and preliminary data on the solubilities of rare earth fission product elements in carbonate melts are reported