E. V. Belova

Radiation resistance of hydrocarbon diluents of tributyl phosphate in a two-phase system and carbonate regeneration of the solvent

A comparative study in the cyclic mode of the radiation resistance of TBP solutions in Isopar-M isoparaffin diluent and in C-13 narrow n-paraffin fraction, which is the major factor determining the stability of the hydrodynamic parameters of the extraction system, revealed certain advantages of Isopar-M, despite higher yield of carboxylic acids. Also, the systems with C-13 exhibit considerably higher hydrodynamic stability compared to the systems with RED-1 n-paraffin diluent. The possibility of extending the diluent operation life to a dose of 400 kGy by using a weakly alkaline carbonate solution for intracycle solvent regeneration was demonstrated.

Thermochemical Oxidation of Components of Extraction Solutions and Threshold Conditions of Thermal Explosion: 4. Kinetics of Reaction of TBP Solutions in Dodecane with Nitric Acid

The reaction of 30% solutions of TBP in dodecane with HNO3 at the initial concentration of the acid of 1.4 M was studied in the temperature range 75-150°C. The kinetic parameters of acid consumption, gas evolution, and accumulation of liquid-phase products were determined. The effect of γ-preirradiation on the kinetics of oxidation processes was studied. From the kinetic data the threshold temperature conditions for transformation of oxidation processes into the thermal explosion were estimated and the presumable mechanism of initiation and development of oxidation reactions was discussed.

Thermochemical Oxidation of Extraction System Components and the Boundary Parameters of a Thermal Explosion: 5. Gas Evolution in Thermolysis of Two-Phase Systems in Open Vessels

Gas evolution dynamics in thermal oxidation of TBP and its dodecane solutions as part of two-phase systems containing HNO3 were studied in open vessels at different temperatures (75-110°C) and acid concentrations (3-12 M) in aqueous phases. The gas evolution kinetics in subsequent thermolysis were studied as influenced by irradiation of two-phase TBP/dodecane-HNO3 systems. The specific features of gas evolution in the two-phase systems are discussed as compared to those in single-phase organic systems.

Effect of γ-irradiation on the thermal decomposition of nitric acid saturated solutions of TBP in С13 diluent

The dynamics of gas evolution in the course of thermal oxidation of 30% solutions of tri-n-butyl phosphate (TBP) in C13 diluent, equilibrated with 12.9 M HNO3, was studied in open vessels in the temperature interval from 70 to 150°С. The total volumes of the released gases were measured, and the effect of the preliminary γ-irradiation of the TBP–С13–HNO3 system on the thermolysis of nitric acid and accumulation of liquid products of TBP decomposition was studied. The boundary temperature parameters for the development of the oxidation process in the autocatalytic mode were estimated. On heating of the single-phase systems in open vessels, the conditions for the development of the autocatalytic oxidation are not created.

New potentialities of the UNEX process using polyheterocyclic diamides

The possibilities of using 2,2′-bipyridine-6,6′-dicarboxamides in modified versions of the UNEX process were examined. The stability constants of complexes of Eu(III), Am(III), Th(IV), U(VI), and Np(V) ions with certain diamides substituted in the amide moiety or pyridine ring in acetonitrile were determined. The logarithms of the stability constants exceeded 7 for Eu(III) and reached 9 for 5f elements. The ability of the selected diamides in nitrobenzene and trifluoromethyl phenyl sulfone (FS-13) diluents to extract Am(III), Th(IV), U(VI), and Eu(III) was studied.

Kinetics of oxidation of 30% TBP solutions in С13 diluent with nitric acid: Effect of temperature and γ-irradiation

The behavior of the two-phase system consisting of a 30% solution of tri-n-butyl phosphate (TBP) in a paraffin diluent (C13) and a 12 M aqueous HNO3 solution was studied in the temperature interval from 70 to 110°С. The parameters of thermolysis of TBP solutions in C13 in the reaction with HNO3 in open vessels were determined with the aim of evaluating the explosion hazard of the system in the course of extraction reprocessing of high-level radioactive materials. Solutions of TBP in C13 in the two-phase systems are characterized in the examined temperature interval by more intense gas evolution compared to that from the singlephase systems.

Effect of intrinsic 239Pu α-radiation on its extraction with a 40% solution of Tri-n-butyl phosphate in formal n2

In extraction regeneration of spent nuclear fuel, namely, in refining of solutions containing 0.2 M Pu and more, the radiolytic effects caused by intrinsic Pu α-radiation become noticeable. To examine this effect, Pu extraction from an aqueous solution containing 0.21 M Pu(IV) nitrate and 3 M HNO3 with a 40% solution of TBP in bis(octafluoroamyl) formal, CH2[OCH2(CF2)3CF2H]2, was performed without intercycle treatments. As a result, after 2-month contact with aqueous Pu solution, the organic phase got an irradiation dose of 470 kGy. This led to a decrease in the Pu distribution ratio by a factor of 4.1 and in the phase separation rate by a factor of 1.3 and to an increase in the viscosity by 12%; the density varied within 1–3%.

Thermal stability of mixtures of the extractant and nitric acid at temperatures from 90 to 120°С

The thermal stability of mixtures of tri-n-butyl phosphate (TBP) with HNO3 was studied in the temperature range from 90 to 125°C. In mixtures with the irradiated extractant at 110°C, intense exothermic processes initiated by oxidation of extractant radiolysis products are possible, whereas at 90°C oxidation of the irradiated extractant is accompanied by weak heat and gas evolution. In the temperature range from 110 to 120°C, exothermic oxidation processes in nonirradiated mixtures start after long induction period (hours), develop gradually, and do not have an avalanche character. Below 110°C, heating of mixtures of the extractant with HNO3 at the HNO3 concentration from 8 to 15.7 M is accompanied by gas evolution without exothermic effects. On the whole, oxidation processes in extraction mixtures at temperatures below the “start” parameters of thermal explosion are not dangerous from the viewpoint of the probability of thermal explosion, even at prolonged heating.

Kinetics of thermochemical reaction of TBP solutions in HCBD with nitric acid in single-phase systems

The reaction of 30% solutions of TBP in hexachlorobutadiene (HCBD) with dissolved HNO3 at its initial concentration of 1.2 M was studied in the range from 90 to 150°C. The kinetic parameters of HNO3 consumption, gas release, and accumulation of dibutyl hydrogen phosphate were determined. The boundary temperature conditions for transition of oxidation processes to thermal explosion mode under conditions of extraction reprocessing of high-level radioactive materials were estimated from the kinetic data.

Kinetics of Thermal Degradation of VP-1AP Anion-Exchange Resin in the Nitrate Form

Thermal degradation of VP-1AP anion-exchange resin in the nitrate form was studied by DSC–TG. The degradation occurs in steps and is accompanied by exothermic effects in the temperature ranges 170–190, 200–280, 280–390, and 400–550°С. The first two exothermic peaks are associated with thermal degradation of the sorbent, and all the other peaks, with oxidation of the matrix with atmospheric oxygen. The kinetic parameters of separate steps were determined. A mathematical model of degradation of VP-1AP anion-exchange resin in the nitrate form was developed. It can be applied to evaluation of the safety in handling this form of the sorbent. Principal possibility of constructing a kinetic model of degradation of ion-exchange materials on the basis of the results of DSC–TG experiments performed with milligram amounts of samples, applicable to simulation of enlarged installations, was demonstrated. An experimental cell suitable for ion-exchange materials was developed for validation of the degradation kinetic models constructed on the basis of the DSC data. The thermal explosion of VP-1AP in the nitrate form in the experimental cell is observed at a thermostat temperature of 245°С, but is not observed up to 230°С. The calculated critical temperature of thermal explosion of VP-1AP anion-exchange resin in the nitrate form under the experimental conditions is in the interval 237–242°С.