Selective Extraction of Uranium from a Mixture of Metal or Metal Oxides by a Tri-n-butylphosphate Complex with HNO3 and H2O in Supercritical CO2

Abstract

Several studies have been performed to extract actinides from the spent nuclear fuels using supercritical fluids. [1][2][3][4][5][6][7][8] Among these studies, a method has been studied in which decladded spent fuels are "directly" treated by the supercritical fluid, a single-phase mixture of HNO3, H2O and tri-n-butylphosphate (TBP) in supercritical carbon dioxide (SC-CO2). [9][10][11] Here, "directly" means "without a dissolution process". For the decladding process, the spent fuel is treated thermally in air and is pulverized by the stress of the swelling through oxidation of UO2 to U3O8 during the cladding. Uranium and plutonium in the solid fuel can be directly extracted into the supercritical fluid. The single-phase mixture of HNO3, H2O and TBP in SC-CO2 is reactive with U3O8. 8,12,13 As a result of the reaction, the uranium oxide is converted into UO2(NO3)2 and extracted as UO2(NO3)2·2TBP in the supercritical fluid. The dissolution and extraction simultaneously proceed in this series of reactions. Because uranium and plutonium are extracted from the spent fuel into the supercritical fluid, the fission products in the spent fuel can remain as solids. Therefore, the direct extraction can decrease the amount of secondary radioactive wastes by omitting the acid dissolution processes, which usually generates a large amount of highly radioactive liquid waste. This technique investigated in these studies can be useful for analytical purposes, because the minimization of the waste solution in the chemical analyses for process management becomes very important in spent nuclear fuel reprocessing plants.In an extraction process for nuclear fuel reprocessing, a higher extraction capacity and higher decontamination ability are desirable. For the former, the contact time of the spent fuel and supercritical fluid is long enough, over 2 min, and an equivalent extraction capacity for the TBP complex is expected. 9 For the decontamination nature, which is more sensitive to the chemical conditions during the direct extraction, we performed some experiments to evaluate the decontamination nature of the direct extraction process using a single-phase mixture of HNO3, H2O and TBP in SC-CO2. Prior to the experiments under high pressure, we performed the experiments at ambient pressure without SC-CO2 in order to determine the effect of SC-CO2 on the decontamination. Experimental ChemicalsAs simulated materials for the spent fuel, we prepared a mixture of U3O8 and metal or metal oxides as the fission products. The composition of the simulated fuel used in our experiments is as follows: U3O8 (92.22), SrO (0.40), ZrO2 (0.85), MoO3 (1.15), RuO2 (0.99), Pd (0.85), CeO2 (1.38) and Nd2O3 (2.16). The values in parentheses indicate the weight fraction of each chemical. The fraction of each element was proportional to the calculated results based on the ORIGEN-II computer code for PWR fuel of 45000 MWd t -1 . Based on the condition of the elements in the spent nuclear fuel, SrO, ZrO2, MoO3, RuO2, Pd, CeO2 and Nd2O3 were selected; these were...

Characterization and remediation of soils contaminated with uranium

“…Shimada et al [240] studied the decontamination nature of the reactive extraction of uranium in the presence of some metal chemicals using a single-phase mixture of HNO 3 , H 2 O and tri-nbutylphosphate in supercritical carbon dioxide. They measured the decontamination factors (DFs) of the metals from their mixture with U, as a function of the molecular ratio of U to TBP in the extracted complex.…”

Supercritical fluid extraction and purification of uranium from crude sodium diuranate

“…Several studies have been reported on SFE of actinides and lanthanides with TBP·HNO 3 adduct from various matrices viz. assorted radioactive waste matrices and spent nuclear fuels from different nuclear reactors (Shadrin et al, 2008;Zhu et al, 2012;Shamsipur et al, 2001;Shimada et al, 2006). Uranium present in spent nuclear fuel has been selectively recovered with good separation factors from fission products by SFE followed by counter current chromatographic separation (Myasoedov et al, 2009).…”

Studies on solubility of uranyl complexes in supercritical carbon dioxide and its controlling factors using UV-visible and17O- and19F-NMR spectroscopy

“…Several studies have demonstrated that MA(III), Ln(III), and uranyl ion (U(VI)) can be extracted from solid or liquid materials to scCO 2 as soluble complexes with chelating ligands such as organophosphorus compounds and b-diketones [7][8][9][10][11][12]. The solubility of various metal complexes including U(VI) in scCO 2 has been investigated, and the results have indicated that fluorination of coordinated ligands drastically enhances the solubility of metal complexes by several orders of magnitude compared with non-fluorinated analogues [13][14][15][16][17][18][19].…”