Compounds of Hexavalent Uranium and Dibutylphosphate in Nitric Acid Systems

Abstract

Storage tanks at the Savannah River Site (SRS) contain highly enriched uranium (U) and dibutyl phosphoric acid (HDBP), formed from the hydrolysis and radiolysis of tributylphosphate (TBP) in the PUREX process. Highly enriched dibutyl phosphoric acid, a relatively strong acid with a pKa (acid dissociation constant) of about one, forms insoluble compounds with U and other actinides in acid solutions. Accumulation of solids in these storage tanks presents a criticality hazard and must be avoided. In this work, U and HDBP compounds were formed from batch reactions in 0.2-6.0 M nitric acid (HNO 3 ) and two HDBP concentrations, forming either a homogenous or heterogeneous solution based on solubility. A 4 : 1 HDBP : U ratio was used in all experiments. The chemical composition of the compounds was characterized using liquid scintillation, ion chromatography, and stannous chloride reduction. The physical characteristics of the U and HDBP compounds were characterized using a combination of infrared spectrometry (IR), electronic spectrophotometry (UV=Vis), and Phosphorous-31 Nuclear Magnetic Resonance ( 31 P NMR). The physical appearance of the solids ranged from a pale yellow powder at 0.2 M HNO 3 loading to a bright yellow gel at 6.0 M HNO 3 , with constant modifications as the acid concentration increased. The U and HDBP precipitate from 0.2 M HNO 3 was characterized as UO 2 (DBP) 2 (a polymer with ten units) in homogenous U and HDBP solutions and UO 2 (DBP) 2 (HDBP) x (where x ¼ 1 or 2) in heterogeneous U and HDBP solutions. Indications of nitrate complexation appeared in precipitates from 0.8 M HNO 3 for both HDBP loadings, however a 1 : 1 uranium : nitrate ratio was not reached at that acid concentration. UO 2 (NO 3 )(H(DBP) 2 )(HDBP) 2 precipitated from ! 3.0 M HNO 3 with high HDBP concentrations. This compound was not formed until 6.0 M HNO 3 was used in lower HDBP concentrations. It is assumed that the change in physical characteristics and addition of nitrate coincides with the breakdown of the UO 2 (DBP) 2 polymer. Supporting data for these findings are presented and discussed.

Characterization of Actinides Complexed to Nuclear Fuel Constituents Using ESI-MS

“…Furthermore, at the higher cone voltage many dimerized uranyl-DBP complexes were observed. In solution, uranyl and DBP can polymerize in a high nitric acid media . While the initial solution conditions did not contain a high nitric acid content, following the ESI process and evaporation of the solvent on the formed droplets, a high nitric acid media would be present possibly favoring the formation of these polymeric species.…”

“…Using eq , the apparent gas phase stability constant was double the previously reported value. However, DBP readily forms dimers in solution, which are observed in the gas phase by the ion at m / z = 421.215 Da. If the 1:4 complex is composed of two DBP dimers, then the log β 4 (dimer) can be calculated according to the reaction From this reaction, the log β 4 (dimer) is found to be 8.1 ± 0.7, which is in agreement with the previously reported values (Table ).…”

Synthesis and evaluation of N,N-di-alkyl-2-methoxyacetamides for the separation of U(VI) and Pu(IV) from nitric acid medium

“…The extractant, TBP, has been the reagent of choice in the nuclear reprocessing industry for more than the past 50 years, in spite of the fact that there were few drawbacks [4][5][6]. One of the major limitations of using TBP is the formation of deleterious degradation products that affects the performance of solvent extraction in high radiation fields and recycling [6][7][8]. Moreover, TBP is not completely incinerable and thus produces a large volume of radioactive secondary waste in the form of inorganic phosphates during waste treatment [9].…”

Facilitated transport of dibutylphosphate across fixed-site membrane

“…Hence, the only possibility is dimerization of DBP by hydrogen bonding on increasing its concentration. DBP is known to form dimer in certain solutions [33].…”