Том 59, № 5 (2017)

Electrochemical properties of the intermetallic compound URh₃ in 0.5–8 M HNO₃ solutions were studied by linear voltammetry. The electrochemical characteristics of URh₃ in nitric acid solution were determined for the first time using the Tafel equation. URh₃ is highly resistant to both chemical and anodic dissolution, which is due to formation of passive films on the electrode surface. All the anodic oxidation processes observed on the electrode led to secondary passivation of the alloy and not to its dissolution. The conclusions based on the electrochemical data were confirmed by experiments in 8 M HNO₃.

The stability of Np(VI) in 5–200 mM iminodiacetic acid (H₂IDA) solutions at 23.5–55°С was studied by spectrophotometry. In a solution with pH 2 and excess Np(VI), 1 mol of H₂IDA reduces 2 mol of Np(VI) to Np(V). In 1 and 0.5 M HClO₄ solutions containing 200 mM H₂IDA and 1 mM Np(VI), no more than 36 and 65% of Np(VI), respectively, is reduced at 44.5°С. Complete reduction of Np(VI) is observed in solutions containing 0.2 M HClO₄ and less. In the examined ranges of H2IDA concentrations and temperatures, Np(VI) is consumed in accordance with the first-order rate law. The reduction mechanism involves formation of a Np(VI) iminodiacetate complex, which is followed by intramolecular charge transfer. The generated radical reduces Np(VI). The activation energy is 107 ± 3 kJ mol^–1.

The possibilities of calculating several successive perturbations without time restriction caused by reaching the steady state and of taking into account the uranium extraction efficiency in the steady state of the cascade operation in adjacent steps of the uranium concentration front were introduced into the algorithm for calculating the transient extraction regime in the Purex process. The results of long-term experiments concerning transient regime of the head mixer–settler unit of the RT-1 complex were reconsidered as applied to reprocessing of spent nuclear fuel (SNF) from WWER (water-cooled water-moderated energy reactor), and good agreement of the experimental data and calculation results for beyond-design-basis regime was reached using the model of ideal mixing in a settling chamber without corrections for the efficiency of the step. The effect of significant extraction of Pu at limiting loading of the solvent with uranium was confirmed. Consideration of this effect allowed adequate description of the Pu accumulation in the uranium-free zone of the extraction–scrubbing unit. The adequacy of the ideal mixing model for a unit of centrifugal contactors was demonstrated as applied to reprocessing of SNF from fast reactors, although the difference between the calculation results obtained using the ideal mixing and plug-flow models becomes considerably smaller. A separate experiment has shown that the Pu accumulation in the uranium-free zone does not occur in this case because of higher acidity of the aqueous phase and higher process temperature, and the Pu distribution between the contactor steps correlates with the U distribution. Comparison of the result of the calculation using the corrected model with the results of step-by-step sampling demonstrated the possibility of complete recovery of Pu(IV) in the beyonddesign- basis (with respect to U) regime at lower values of the HNO3 concentration and temperature than it was assumed previously on the basis of published data.

Extraction of microamounts of U(VI), Th(IV), and REE(III) from HNO₃ and HClO₄ solutions with solutions of (α-pyridyl)tetraphenylmethylenediphosphine N,P,P'-trioxide in dichloroethane was studied. The stoichiometry of the extractable complexes was determined, and the influence of the extraction structure and aqueous phase composition on the efficiency and selectivity of the extraction of U(VI), Th(IV), and REE(III) into the organic phase was studied. Introduction of the pyridine N-oxide fragment into the methylene bridge of the tetraphenylmethylenediphosphine dioxide molecule to obtain (α-pyridyl)tetraphenylmethylenediphosphine N,P,P'-trioxide leads to a decrease in its ability to extract U(VI), Th(IV), and REE(III) from nitric acid solutions, whereas the U/REE separation factors increase. The REE(III) extraction efficiency considerably increases in going from nitric acid to perchloric acid solutions.

Phosphate-grafted chitin (PGC) was synthesized for thorium adsorption from acid solutions. FTIR analysis shows that the phosphate and amide groups of PGC are involved in the adsorption. The Th(IV) adsorption is pH-dependent and is maximal at pH 3.5. The adsorption kinetics is described by a pseudo-second-order kinetic model. The maximum monolayer adsorption capacity obtained from the Langmuir model is 50.5 mg Th(IV)/g at 25°C. Thermodynamic analysis shows that the adsorption is spontaneous and exothermic. The phosphate-grafted chitin can be reused by activating its surface with 0.5 M HCl–0.1 M EDTA.

Causes of initiation of exothermic self-accelerating oxidation processes in mixtures of TBP with HNO₃ are analyzed. The main condition is heating of the mixtures to temperatures (Т_st) above which the heat release in the chemical reaction starts to exceed the heat removal from the system. After the initiation of selfaccelerating exothermic reactions, their development and intensity depend on whether the vessels are sealed, on the HNO3 concentration, and on the heating time. Heat treatment and radiolysis of the extractant slightly decrease Т_st of its mixtures with HNO₃ compared to the values for the “fresh” extractant.

^99mTc-tricarbonyl-labeled chenodiol (chenodeoxycholic acid, CDCA) intended for hepatobiliary imaging was prepared by 30-min heating at 100°C with a radiochemical yield of >98%. The radiolabeling yield and stability of the complex were evaluated using different chromatographic techniques. Biodistribution studies in Albino Swiss mice at different time intervals after administration of the complexes showed that the maximum uptake of the complexes in the liver was 35.5 ± 0.25% ID/g at 30 min post injection. ^99mTc-tricarbonyl- CDCA shows promise for hepatobiliary imaging.

The effect that the ionizing radiation caused by radioactive contamination of soil exerts on the surface of carbon steel 20 samples, on their corrosion resistance, and on operation reliability according to lowcycle fatigue tests (taking into account that fatigue is one of the main causes of failures resulting from corrosion and mechanical damage of thin-wall metal structures, in particular, of pipelines) was studied. It was shown that even low activity levels (below maximum permissible levels) caused corrosion loss of the metal and a decrease in the operation reliability. Synergistic protective compounds suppressing the destructive radiation effects and ensuring reliable operation of metal structures due to chelation of the surface metal atoms were developed using computer simulation.