A. Yu. Shadrin

Solubility and coprecipitation of barium and strontium nitrates in HNO3 solutions and multicomponent systems

The solubility of Ba(NO3)2 and Sr(NO3)2 in HNO3 solutions at 25–95°C is characterized by the power dependence on the total concentration of the nitrate ion with the exponent for Ba(NO3)2 equal to −2 in ∼9 M HNO3 and −6 in more concentrated acid solutions. The latter exponent is also characteristic of the more soluble Sr(NO3)2 throughout the examined range of HNO3 concentrations. In strongly acidic solutions, Ba(NO3)2 coprecipitates with Sr(NO3)2. The solubility curve for Ba(NO3)2 in NH4(Na)NO3 solutions suggests formation of a double salt, whereas in UO2(NO3)2 solutions the dependence is the same as in HNO3 solutions.

New approaches to reprocessing of oxide nuclear fuel

Dissolution of UO2, U3O8, and solid solutions of actinides in UO2, including those containing Cs, Sr, and Tc, in weakly acidic (pH 0.9–1.4) aqueous solutions of Fe(III) nitrate or chloride was studied. Complete dissolution of the oxides is attained at a molar ratio of Fe(III) nitrate or chloride to uranium of 1.6 or 2.0, respectively. In the process, actinides pass into the solution in the form of U(VI), Np(V), Pu(III), and Am(III). At 60°C, actinide oxides dissolve in these media faster than at room temperature. In the solutions obtained, U(VI) and Pu(III) are stable both at room temperature and at elevated temperatures (60°C), and also at high U concentrations (up to 300 mg ml−1) typical of process solutions (6–8 M HNO3, ∼60–80°C). After the oxide fuel dissolution, U and Pu are recovered from the solution by peroxide precipitation. In so doing, the content of Fe, Tc, Cs, and Sr in the precipitate does not exceed ∼0.05 wt %. From the solution after the U and Pu separation, the fission lanthanides, Tc, Cs, and Sr can be recovered by precipitation of Fe hydroxides in the presence of ferrocyanide ions and can be immobilized in appropriate matrices suitable for long-term and environmentally safe storage.

Dissolution of UO2 in the N2O4-H2O system

The kinetics of the UO2 dissolution in the N2O4-H2O system was studied. At 25°C, the process is kinetically controlled, whereas at 55°C the process occurs initially under kinetic control (3 min) and then under diffusion-kinetic control. At 80°C, the process occurs exclusively under diffusion-kinetic control. The apparent activation energy was estimated at ∼39 kJ mol−1.

Deactivation in Sub- and Supercritical Carbon Dioxide

Solutions of hexafluoroacetylacetone and a modifier such as, e.g., pyridine in supercritical CO2 allow 97-99% removal of actinides from the stainless steel surface. The deactivation efficiencies were compared for liquid and supercritical CO2. Single treatment run with solutions of HDEHP and DCH18C6 in liquid CO2 removes 70-80% of transuranium elements and over 50% of strontium and cesium from the stainless steel surface. Deactivation of real contaminated radioactive samples was studied. Methods such as supercritical fluid extraction and extraction with liquid CO2 are suitable for deactivation of surfaces and porous materials.

Supercritical Fluid Extraction of Actinide Complexes: I. SCE of Adduct of Uranyl Trifluoroacetylacetonate with Pyridine

A number of procedures for determining the solubility of metal complexes in supercritical carbon dioxide (SC-CO2) were compared. The solubility of adduct of uranyl trifuoroacetylacetonate with pyridine at a pressure of 300 atm and a temperture of 60°C exceeds 100 g l-1 (26 mg of U per 1 ml of SC-CO2). Partial degradation of the adduct under these conditions was observed.

Formation of molybdenum and zirconium precipitates in concentrated uranyl nitrate solutions

The influence of the keeping time, temperature, and concentrations of U and HNO3 on the residual content of Mo and Zr in the solution was examined. An increase in the preliminary keeping time favors more complete precipitation. An increase in the uranium concentration to 800 g l−1 and/or in the temperature to 105°C leads to growing formation of precipitates of molybdic acid (in the absence of Zr) or zirconium molybdate. The presence of zirconium promotes, and an increase in the acidity impedes the process.

Extraction of Eu and Am with extractants based on carbamoyl phosphine oxide (CMPO)

The possibility of adding an HDEHP solution as solubilizer to a CMPO-diluent system was examined. In the presence of Zr in the organic phase, mixed complexes are formed in this case. They exert no appreciable effect on the dependence of the Eu extraction on the acid concentration but noticeably increase the extractant capacity.

Supercritical Fluid Extraction of Actinide Complexes: II. SFE of Actinide β-Diketonates

The solubility of uranium, plutonium, neptunium, and americium β-diketonates in supercritical carbon dioxide (SC-CO2) was studied. The concentration of actidine β-diketonates in SC-CO2 can be as high as 10-100 g l-1. The complexes with dipivaloylmethane, trifluoroacetylacetone, and hexafluoroacetylacetone and especially their adducts with tributyl phosphate and water are the most soluble. The residues after treatment of the complexes with SC-CO2 were studied by IR spectroscopy.

Precipitation of 137Cs with microorganisms from aqueous industrial process solutions

The principal possibility of using natural microorganisms for binding and subsequent precipitation of 137Cs from low-level aqueous industrial process solutions was examined.

Interaction of Hexafluoroacetylacetone with Metals and Their Alloys in the Medium of Supercritical Carbon Dioxide in the Processes of Equipment Decontamination

In the course of treatment of stainless steel with supercritical CO2 containing β-diketones in combination with pyridine and water, these agents react mainly with the oxide films. Therefore, in industrial use of such media, e.g., for decontamination of the equipment, the corrosion is expected to be low. Under these conditions, uranium does not dissolve from the surface of metallic uranium, and iron does not dissolve from the surface of pure metallic iron. In treatment of a copper or nickel surface with supercritical CO2 containing β-diketons and modifiers, both the oxide film and metals are involved in the reaction. It is presumed that water facilitates ionization of the complexing agent, and pyridine (or another amine with pKa 5-6) creates a medium favorable for complexation. Furthermore, pyridine molecules may be incorporated in the complex as additional ligands.