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

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.