V. F. Peretrukhin

Bioaccumulation of Tc, Pu, and Np on Bottom Sediments in Two Types of Freshwater Lakes of the Moscow Oblast

A laboratory study is made of Tc, Np, and Pu accumulation by natural and sterilized silts collected from two typical eutrophic and dystrophic lakes of the Moscow oblast. After a short inductive period (1-3 days), the rate of Tc uptake by the eutrophic lake silt is about twice that by the dystrophic lake silt (at a dry solid to liquid phase ratio of 0.033 g ml-1). The rate is only slightly temperature-dependent, being practically constant (at a level of about 3% per day) under certain conditions. The distribution coefficients after a month of contact were found to be 1700±50 and 56±5 ml g-1 for the eutrophic and dystrophic lake silts, respectively. Sulfate and nitrate have an inhibiting effect on the Tc uptake. Efficient desorption of Tc from bottom sediments can be realized only in the presence of an oxidant (H2O2), suggesting the reductive mechanism of Tc sorption on silts, complicated by either complexation with microorganism cell components or by intracellular deposition.

Transmutation of Metallic 99Tc into Ruthenium under Irradiation in High-Flux SM Reactor

Transmutation of 99Tc upon irradiation of metallic technetium discs 6 mm in diameter and 0.3 mm thick in a neutron trap of high-flux SM reactor for 72.7 effective full-power days (100 mW thermal power) was studied. At a total neutron fluence of 8 × 1021 cm-2 with the thermal component of 7.3 × 1021 cm-2, about 34% of 99Tc is convereted to yield 0.8 g of ruthenium, which agrees within the limits of the determination error with the results of Monte-Carlo calculations. For ruthenium formed under these conditions to be used in practice without any restrictions, the target should be cooled for three years to decrease the 103Ru activity and the decontamination factor with respect to 99Tc should be about 3 × 108.

Transmutation of 99Tc and preparation of artificial stable ruthenium: I. transmutation of metallic 99Tc in a high-flux SM reactor

Technetium transmutation into ruthenium in a high-flux SM reactor was numerically simulated. The results were compared with the experimental data on isolation of technogenic stable Ru form irradiated technetium targets.

Anodic dissolution of Tc metal in HNO3 solutions

Corrosion and dissolution of Tc metal in 0.5–6.0 M HNO3 without external sources of the redox potential and under the conditions of constant-current electrolysis were studied. The dissolution rates of Tc metal without external potential in relation to the HNO3 concentration and the rates of anodic dissolution of Tc in relation to the anodic current density and HNO3 concentration were determined. The Tc(VII) current efficiencies in 0.5–6.0 M HNO3 solutions were measured. The anodic dissolution of Tc is characterized by the linear growth of the concentration of the dissolved Tc species in solution with time. Examination of the sample surfaces by scanning electron microscopy allows a conclusion that the corrosion degradation has intercrystallite character.

Electrochemical study of corrosion and dissolution of Tc metal in 0.5–6.0 M HNO3

Electrochemical dissolution of Tc metal in 0.5–6.0 M HNO3 was studied. The quantitative characteristics of corrosion and dissolution were determined. At the HNO3 concentration in the electrolyte below 2.0 M and potentials below 650 mV (vs. Ag/AgCl), the metal surface is passivated owing to the formation of difficultly soluble hydrated Tc(IV) dioxide. An increase in the potential leads to the transition of Tc metal to the transpassive state. In solutions containing more than 2.0 M HNO3, hydrated Tc(IV) oxide loses passivating properties. The oxidation of Tc(IV) to Tc(V) is the main reaction controlling the corrosion rate. The results of corrosion rate measurements show that Tc metal can be quantitatively dissolved in HNO3 of concentration higher than 4.0–6.0 M. The dissolution rate at room temperature is 0.3 mg cm−2 h−1.

Transmutation of 99Tc and preparation of artificial stable Ruthenium: III. Isolation of artificial metallic Ruthenium from irradiated technetium

Dissolution of Tc-Ru alloys prepared by reactor irradiation of metallic Tc and isolation and purification of stable Ru were studied. The first sample of artificial metallic Ru was prepared.

Uranium(III) in aqueous solutions: Preparation, properties, synthesis of solid compounds

Published data on the preparation procedures, stability, and complexation of U(III) in aqueous solutions are summarized and correlated. Reactions with inorganic and organic free radicals studied by the flash radiolysis method, the spectroscopic properties, the extraction and ion-exchange behavior of U(III), and methods for isolation of solid U(III) compounds from aqueous solutions are discussed.

Oxidation of U(III) with water in the course of precipitation of its solid compounds from aqueous solutions

U(III) was precipitated from solution as a yellowish-brown precipitate under the action of NH4OH or Na2B4O7 solutions at pH ∼7.5. The electronic spectra showed the presence of substantial amounts of U(OH)3 in the precipitate, decreasing within a few minutes. Therefore, pure U(OH)3 can hardly be obtained. A brown precipitate is formed when a U(III) solution is added to an NH4F solution. An X-ray phase analysis shows that anhydrous UF3 is the only crystalline compound in the precipitate. The synthesis of U(III) phosphate was unsuccessful, because uranium was completely oxidized to U(IV). Thus, preparation of U(III) phosphate compounds from aqueous solutions is hardly possible.

An electrochemical study of the corrosion and dissolution of Tc-Ru alloys in HNO3 solutions

Electrochemical oxidation of Tc-Ru alloys in 0.5–6.0 M HNO3 solutions was studied by linear voltammetry. Quantitative characteristics of the corrosion and dissolution processes were determined, namely: corrosion potentials Ecorr, exchange current densities je and the corresponding corrosion rates vcorr, and transpassivation potentials Etp. The electrode reactions occurring in the course of electrochemical dissolution of Tc-Ru alloy differ essentially from those occurring on the surface of Tc metal electrodes. The exchange current density je and corrosion rate vcorr drastically decrease on alloying technetium with ruthenium. The corrosion potential Ecorr of Tc-Ru alloys in HNO3 solutions increases with an increase in the HNO3 concentration and, at the same time, is virtually independent of the alloy composition. Comparison of the transpassivation potentials of Tc-Ru alloys with those on electrodes made of pure Tc and Ru shows that transpassivation potentials of Tc-Ru alloys were closer to Etp of Ru metal throughout the examined range of HNO3 concentrations. Thus, alloying of technetium with ruthenium enhances the resistance of the alloy to electrochemical oxidation in HNO3 solutions.

Oxidation of U(VI) with oxygen in weakly acidic and neutral solutions

The kinetics of U(IV) oxidation with atmospheric oxygen in solutions with pH 2–7 was studied. In the kinetic curves there is an induction period, which becomes shorter with increasing pH. The induction period is caused by accumulation of U(VI), whose initial presence in the working solution accelerates oxidation. The pseudo-first-order rate constants and bimolecular rate constants of U(IV) oxidation with oxygen were evaluated. The mechanism of U(IV) oxidation is considered. At pH higher than 3, formation of a polymer of hydrolyzed U(IV) with U(VI) plays an important role in oxidation of U(IV), since this prevents formation of U(V). Heating accelerates oxidation of U(IV) at pH 2–2.5, but at a higher pH the process becomes difficultly controllable.