G. F. Egorov

Radiation-Chemical Behavior of TBP in Hydrocarbon and Chlorohydrocarbon Diluents under Conditions of Reprocessing of Spent Fuel from Nuclear Power Plants

Experiments were performed for comparative assessment of the effect of ionizing radiation on the service characteristics of 30% TBP solutions in various diluents: hexachlorobutadiene (HCBD), mixtures of n-paraffins (RED diluent), and fraction of naphthenic hydrocarbons (RZh diluent). The following main parameters were chosen for comparative assessment of the quality of irradiated extractant solutions: composition and behavior of diluent radiolysis products, effect of diluents on the TBP radiolysis, and aggregative stability of emulsions in the main operations of the extraction cycle. The service life of the extraction mixtures was estimated from variation of the above parameters with the irradiation dose. Principles for choosing solvents with increased service life under irradiation were substantiated.

Regeneration of Spent TBP-Diluent Solvent by Steam Distillation

Regeneration of irradiated and actual spent solvent (30% TBP in a hydrocarbon diluent) by overheated steam distillation was studied in batch experiments. The hydrodynamic and extraction-backwash parameters of thus regenerated solvent are practically the same as those of the fresh solvent. The bottoms comprise 3-7% of the initial volume and consist essentially of TBP. Large-scale rig trials of the single- and two-step continuous processes of solvent distillation are performed. Single-step distillation with overheated steam in a vacuum or with saturated water vapor at atmospheric pressure provides transfer into the distillate of the whole amount of the solvent, permitting the use of any diluent. The two-step process allows separation of the solvent into the TBP and diluent fractions and refining of each of them in the case when the diluent boiling point is below that of TBP by 50-70°.

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.

Distribution of HDBP in two-phase systems consisting of aqueous nitric acid, organic diluent (with or without TBP), and HDBP or its zirconium salt

The distribution of HDBP between 0–5 M aqueous HNO3 and various solvents or solutions of acidic zirconium salt of HDBP (HDBP ZS, HDBP : Zr ratio 6–18) in the same solvents [decane, xylene, CCl4, m-nitrobenzotrifluoride (F-3), 30 vol % TBP in decane and xylene] was studied. The HDBP concentration in aqueous solutions decreases with an increase in the HNO3 concentration. At HDBP concentrations in the organic phase below 0.1 M, its distribution follows the Nernst equation; in the range 0.1–0.4 M, its concentration in the aqueous phase at a constant acidity is approximately constant; and with a further increase in the HDBP concentration in the organic phase, its concentration in the aqueous phase starts to grow again. When the organic solvent is TBP in diluent, the HDBP concentration in more concentrated HNO3 solutions decreases by almost an order of magnitude compared to the systems without TBP, whereas with water and dilute HNO3 solutions the difference from the TBP-free systems is insignificant. The presence of Zr somewhat increases the HDBP distribution ratio. The extraction of HNO3 with a solution of HDBP in xylene and CCl4 apparently involves the HDBP dimer. A mathematical description of the HDBP and HNO3 distribution between aqueous nitric acid solutions and xylene or CCl4, based on equations of chemical reactions between the components, was suggested.

Radiation-Chemical Resistance of Anion Exchangers and Safety of Sorption Processes in Nitric Acid Solutions: III. Radiation-Chemical Resistance of VP-1AP Anion Exchanger

Variation of physicochemical characteristics of VP-1AP anion-exchange resin in HNO3 solutions on heating and under the action of external γ-radiation up to the absorbed dose of 5 MGy was studied. It was shown that at thermal treatment of anion exchanger with increasing HNO3 concentration and temperature and under the action of ionizing radiation the total exchange capacity (TEC) and capacity with respect to strongly basic groups (Csbg) decrease. The swelling coefficients Ksw noticeably vary only in treatment in 12 M HNO3 solutions and reach a maximum of 1.4. In this case, the specific surface area of the sorbent decreases from 15.7 to 1.2 m2 g-1. Under the action of ionizing radiation the specific surface area of the irradiated anion exchanger somewhat increases and reaches 18 m2 g-1.

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.

Simulation of the distribution of radionuclides in the reservoir bed for deep-well injection disposal of acid liquid radioactive waste

A mathematical model was developed for describing the changes in the state of the reservoir bed for dee-well injection disposal of acid liquid radioactive waste. The model considers the multicomponent filtration of the solution in the heterogeneous bed, sorption-desorption of radionuclides, taking into account the dependence of the distribution coefficient on the temperature and pH, as well as radioactive decay, interaction of acids with minerals, radiation-chemical and thermochemical decomposition of the acids, and dynamics of the temperature field, taking into account the convective heat transfer, thermal conductivity, and radiogenic heat release. The results of the simulation of the migration of radionuclides were reported, as well as of the distribution of the acids and the dynamics of the temperature field in the vicinity of the injection well of the site for deep-well injection disposal of the waste from Siberian Chemical Combine. A technogenic barrier is formed in the vicinity of the injection well, hindering the spread of radionuclides in the reservoir bed.

Effect of radiation and heating on acid products of acetic acid radiolysis in concentrated nitrate solution

Radiolysis and thermolysis of glyoxalic, glycolic, and succinic acids in aqueous nitrate and nitrate-nitric acid solutions were studied. The radiolysis of the acids occurs with the participation of nitric acid radical species, precursors of stable NO2− ions. Thermolysis of nitrate-nitric acid solutions of acids involves rapid decomposition of HNO3, intensifying the decomposition of the organic acids owing to their reaction with NO2, major product of HNO3 thermolysis. Accumulation of NO2− ions in irradiated and heated (150°C) solutions, with an increase in pH, was detected.

Radiation-Chemical Resistance of Anion Exchangers and Safety of Sorption Processes in Nitric Acid Solutions: IV. Thermal Gravimetric Study of VP-1AP Anion Exchanger in Nitrate Form

Thermal gravimetric analysis of the samples of VP-1AP anion exchanger in sulfate and nitrate forms was carried out. In the derivatograms of the anion exchanger in the nitrate form there are one endothermic and two exothermic peaks (∼100, 160-180, and 200-220°C). Heating of dry samples of VP-1AP anion exchanger in the nitrate form at 130-230°C induces oxidative degradation of the sorbent matrix with participation of nitrate ions. In the presence of products of anion exchanger degradation and sorbed HNO3 in the anion exchanger phase, the weight loss of the samples abruptly increases at temperatures of approximately 180-190°C. The action of ionizing radiation strongly affects the heat resistance of the anion exchanger. At absorpbed dose of 5 MGy the first exothermic peak is shifted by 15-20°C; in this case, both exothermic peaks coalesce and the oxidation can be considered as single-stage process starting at 110°C.