V. M. Chistyakov

Certification of the Reference Sample of Plutonium Dioxide

An algorithm of certification of plutonium dioxide by the coulometric method is described. The results of measuring the mass fraction of Pu in PuO2, obtained in certifying and confirming measurements in the course of the development of the state standard sample, are given. The mean mass fraction of Pu in the standard sample material and the uncertainty of the certified value are determined. Based on the results obtained and metrological expertise of the documentation in accordance with GOST (State Standard) 8.315–97, the type of plutonium dioxide composition, SS Pl-tot, is approved, included into the state register, and accepted for use in the Russian Federation.

Behavior of Am(V) in concentrated HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The Am(V) disproportionation in (2–10) × 10−3 M K10P2W17O61 (KPW) solutions in 1–7 M HNO3 was studied spectrophotometrically. Under the experimental conditions, Am(VI) and Am(III) are the final products of Am(V) transformation; the process is described by the rate equation of a reversible reaction. The direct reaction follows the first-order rate equation with respect to Am(V) concentration, and the reverse reaction, the zero-order equation.

Methodical factors in coulometric determination of the sum of uranium and plutonium weight fractions in MOX fuel

The influence exerted on the accuracy of coulometric titration by the composition of the supporting electrolyte solution in the titration step, by the way of stirring of the supporting electrolyte solution, by the weight of the aliquot of the solution being titrated, by the weight of the substance being titrated in the aliquot, by the kind of reductant, and by the solution compositions in the steps of oxidation and reduction of U-Pu mixtures was studied by spectrophotometry and coulometry. The optimum conditions for performing the analysis were found.

Electrochemical Oxidation of Am(III) Ions in HNO3 Solutions

Formal oxidation potentials (E0p) of the Am(IV)/Am(III) couple were measured and the kinetics of electrochemical oxidation of Am(III) on platinum electrode in concentrated solutions of nitric acid (1-6 M) containing potassium phosphotungstate K10P2W17O61 (KPW) was studied. The formal potential E0p only slightly depends on the concentration of HNO3 and is shifted toward the negative region by ≈1.0 V as compared with the standard values. The extent of Am(III) oxidation increases with increasing KPW concentration and decreasing concentration of nitric acid. Electrochemical oxidation of Am(III) is accompanied by radiochemical reduction of Am(IV) and is described by the equation -dCAm(III)/dt = (k + k1)CAm(III) - k1C0 - k0, where k is the apparent rate constant of electrochemical oxidation of Am(III), k1 is the apparent rate constant of Am(IV) reduction, and k0 is the constant of radiation-chemical reduction of Am(IV).

Thermodynamics of actinide redox reactions in potassium phosphotungstate solutions

Abstract The paper represents the results of the investigation of kinetics and thermodynamics of the M(IV) reduction reactions (M=Am, Cm, Bk, Cf) and also those of the Am(V) disproportionation reaction and the reaction between Am(V) and Am(III) in aqueous solutions of potassium phosphotungstate, K 10 P 2 W 17 O 61 (KPW). From experimental data the thermodynamic parameters of activated complex formation for the M(IV) ions reduction process through the chemical route have been calculated. Linear correlation has been ascertained between the Δ G 298 ≠ values and the oxidation potentials of the M(IV)–M(III) pairs in KPW solutions and on this basis the estimation of the Δ G 298 ≠ values has been made for U, Pu and Np. In the case of the Am(V) disproportionation and the reaction of the interaction between Am(V) and Am(III) ions the enthalpy component introduces the main contribution into the Δ G 298 ≠ change. At the same time the contribution of the entropy component into the Δ G 298 ≠ change in KPW solutions is considerably lower than for perchloric acid solutions.

α-Radiolysis of the tributyl phosphate–hydrocarbon diluent–nitric acid system

Abstractα-Radiolysis of tributyl phosphate in Sintin n-paraffin diluent in equilibrium with HNO3 solutions at single “internal” irradiation from the extracted 238Pu was studied. The radiation-chemical yields (molecules/100 eV) of butyl hydrogen phosphates (BHP), carboxylic acids, carbonyl compounds, and nitro compounds upon irradiation of 20% TBP in the treated Sintin in equilibrium with 3 M HNO3 were 0.4 (at dibutyl hydrogen phosphate to monobutyl dihydrogen phosphate ratio HDBP: H2MBP = 4.3), 1.4, 0.2–0.3, and 0.2–0.3, respectively. The degradation and oxidation processes occur more deeply than under γ-irradiation. A simple volumetric method for determining carboxylic acids in the extract was developed. In the course of irradiation, the Pu(IV) oxidation state in the extract does not change, and its retention is due to the interaction with BHP at the ratio BHP: Pu = 2 in stripping with 0.02 M HNO3 and BHP: Pu = 4 in stripping with Fe(II). The retention can be eliminated by the displacing action of Np(IV).

Kinetics of Pu(IV) reduction with zinc amalgam in sulfuric acid solutions

The kinetics of Pu(IV) reduction with zinc amalgam under different conditions [H2SO4 concentration 0.5–4.0 M, initial Pu(IV) concentration (1.11–4.80) × 10−3 M, temperature 294.0–323.0 K] was studied by spectrophotometry. The kinetic scheme of the Pu(IV) transformation and the rate law were suggested, and the reduction rate constants were determined. The activation energies Ea and the thermodynamic functions of formation of the activated complex (activation Gibbs energy ΔG≠, enthalpy ΔH≠, entropy ΔS≠) were determined.

Kinetics of neptunium(V) reduction with iron(II) sulfamate in nitric acid solutions

The kinetics of chemical reduction of Np(V) with iron(II) sulfamate in HNO3 solutions was studied by spectrophotometry. The reduction process can be described by the equation NpO2+ + Fe2+ + 4H+ ⇆ Np4+ + Fe3+ + 2H2O and follows the first- and zero-order rate law with respect to the Np(V) concentration. The reaction rate constants were determined in relation to the HNO3 concentration (1–3 M), iron(II) sulfamate concentration [(0.59–2.94) × 10−3 M], and temperature (298.2–319.2 K). The activation energy (Ea) and the thermodynamic functions of formation of the activated complex (activation free energy ΔG≠, enthalpy ΔH≠, entropy ΔS≠) were calculated.

Formal oxidation potentials of the Np(VI)-Np(V) couple and schemes of formal oxidation potentials of Np and am ions in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The formal oxidation potentials Ef0 of the Np(VI)-Np(V) couple in 1–3 M HNO3 solutions containing (1–3) × 10−3 M potassium phosphotungstate K10P2W17O61 (KPW) were measured by the potentiometric method with spectrophotometric identification of the valence states. The potentials slightly decrease with an increase in the HNO3 and KPW concentrations, and the shift of the Np(VI)-Np(V) potential toward negative values relative to its value in 1 M HClO4 reaches 0.08 V. The formal oxidation potentials of the Np(V)-Np(IV) couple were calculated from the experimentally determined equilibrium constants (Keq) of the redox reaction of the Np(V) disproportionation. Under the examined conditions, these potentials also vary insignificantly, and the shift of Ef0 toward positive values relative to its value in 1 M HClO4 is 0.53 V. The schemes of potentials of the Np and Am couples in 1 M HNO3 in the presence of KPW are suggested.

Behavior of Np(VI) in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The behavior of Np(VI) in HNO3 solutions containing potassium phosphotungstate K10P2W17O61 (KPW) at various concentrations of HNO3 (1–3 M) and KPW [(1–3) × 10−3 M] was studied by spectrophotometry. The rate law and the Np(VI) transformation scheme were determined, and the effective rate constants of the Np(VI) reduction and Np(V) reproportionation were calculated.