The reasons for the occurrence of exothermic processes of oxidation of nitric acid solutions with reducing agents are analyzed. The main condition is heating the mixtures to temperatures (Tst), above which the release of heat as a result of a chemical reaction begins to exceed its removal from the system. An analysis of the characteristics of exothermic processes obtained experimentally for solutions with reducing agents has been carried out. The effect of irradiation of solutions on the Tst value is small, while other parameters of exothermic processes decrease - heat release, the self-heating value, and the volume of released gases. Necessary and sufficient parameters have been identified to assess the explosion hazard of heating operations of these solutions. Conditions have been established to ensure safety during the evaporation of nitric acid solutions with reducing agents.

The effect of irradiation of a strongly basic vinylpyridine anion exchange resin of the VP-1AP grade in nitrate form on the onset temperatures of exothermic reactions, thermal effects and the composition of gaseous thermolysis products of VP-1AP was studied. It was established that the onset temperatures of exothermic reactions for an irradiated anion exchange resin are reduced by 59–100°C. The total thermal effect of thermolysis of the irradiated sorbent is 67

Исследована термическая стабильность растворов 5-12 моль/л HNO 3 , содержащих 5-50 г/л ацетогидроксамовой кислоты, при атмосферном давлении и давлении выше атмосферного. Установлено, что в растворах при температурах ниже температуры кипения протекают слабые экзотермические процессы окисления, сопровождающиеся газовыделением. На основе полученных характеристик экзотермических процессов анализируется вопрос взрывобезопасности при упаривании азотнокислых растворов, содержащих ацетогидроксамовую кислоту.

The thermal stability of 5–12 M HNO 3 solutions containing 5–50 g/L acetohydroxamic acid was studied at atmospheric and above atmospheric pressure. It was established that at temperatures below the boiling point, solutions undergo weak exothermic oxidation processes, accompanied by gas evolution. The problem of explosion safety in the evaporation of nitric acid solutions containing acetohydroxamic acid is analyzed based on the obtained characteristics of exothermic processes.

The thermal stability of a solution of 12 mol·L−1 nitric acid with 50 g·L−1 acetohydroxamic acid has been studied. It has been established that weak exothermic oxidation processes occur in solutions at atmospheric pressure at temperatures below the boiling point, accompanied by gas evolution. Under these conditions, the specific volume of evolved gases, the onset temperature of the exothermic reaction, the induction period, the duration of the exothermic process, and the self-heating value have been determined. In the adiabatic mode, the kinetic parameters of a thermal explosion have been determined taking into account the thermal energy compensation function, and it has been shown that under these conditions this reaction proceeds in an autocatalytic mode. Such kinetic parameters as activation energy and pre-exponential factor have been calculated. The influence of the heating rate of the solution and its exposure time on the onset temperature of the exothermic reaction has been studied. The values of heat release for the concentration of nitric acid, which simulates the initial stages of evaporation of aqueous solutions, have been determined. It has been shown that even at low concentrations of nitric acid of 1.6 and 3.2 mol·L−1, an exothermic reaction occurs with a noticeable heat release, while the onset temperature of the exothermic reaction of the solution with 3.2 mol·L−1 HNO3 is lower than its boiling point. The data obtained are necessary to assess the safe conditions for the evaporation of nitric acid solutions containing acetohydroxamic acid.

Thermal stability of nitric-acid (0.8–6.6 M) solutions of monoethanolamine (MEA) in the temperature range 90–150°С under atmospheric pressure and pressure exceeding that of the atmosphere (autoclave) has been studied. It was found the interaction of solution components begins at the atmospheric pressure begins at 100–105°С and НNО3 concentration of 1.4 M and is accompanied by the gas evolution without heat release. When solutions are heated in an autoclave, exothermic reactions appear under certain conditions, with gas evolution the intensity of which depends on the component concentration and temperature. The characteristics of the exothermic processes were determined. It was shown that, because of the sharp pressure buildup in the apparatus as a result of the exothermic reactions, evaporation of nitric-acid solution with MEA may be dangerous as regards the appearance of emergencies.

Thermal stability of hydrazine nitrate (HN) in solutions in 4–12 M HNO3 in the temperature interval 70–150°C was studied. Exothermic reactions accompanied by gas evolution arise in such systems under definite conditions. The intensity of such reactions depends on the component concentrations and temperature. The characteristics of the exothermic processes were determined to evaluate the explosion safety of process operations. Because of sharp pressure buildup in apparatuses, exothermic processes in the course of evaporation of HN-containing nitric acid solutions are hazardous from the viewpoint of radiation accidents.

The thermal stability of mixtures of TBP and its solutions in diluents with uranyl nitrate (UN) at 150–200°C was evaluated. Exothermic self-accelerating oxidation processes (thermal explosions) arise in mixtures with TBP at 160–170°C. In mixtures of UN with solutions of TBP in diluents, the intensity of the exothermic processes is appreciably lower. The presence of the extractant in UN solutions fed to high-temperature operations gives rise to the hazard of the initiation of intense exothermic processes.

Causes of initiation of exothermic self-accelerating oxidation processes in mixtures of TBP with HNO 3 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 HNO 3 compared to the values for the “fresh” extractant.

The effect of HNO3 on the thermal stability and radiation resistance of trifluoromethyl phenyl sulfone (FS-13) used as extractant diluent in high-level waste partitioning was studied. Heating of FS-13 in the presence of 14 M HNO3 is not accompanied by exothermic effects up to 160°С. The amount of gaseous products released in the course of heating the system consisting of FS-13 and HNO3 and the composition of the major radiolysis products of the system components at irradiation doses of up to 0.5 MGy were determined.

The influence of thermal and gamma radiation effects on the characteristics of the thermal explosion of mixtures of tributyl phosphate (TBP) and nitric acid was studied. The products of sequential radiolysis and pyrolysis of TBP were determined to have little effect on the thermal stability of mixtures of TBP and nitric acid. The onset temperatures of exothermic processes leading to a thermal explosion were slightly decreased only by the absorbed dose of 2 MGy. The thermal stabilities of solutions of TBP in n-dodecane and diluent C-13, consisting of a mixture of saturated hydrocarbons, were investigated. The experimental results indicate that the irradiation decreased the onset temperature of the exothermic processes in mixtures of nitric acid with TBP solutions in a hydrocarbon diluent; the onset temperature decreased by 5 circle C to 7 circle C for n-dodecane and by 9 circle C to 13 circle C for C-13, as compared to unirradiated extractants.

The thermal stability of mixtures of tri- n -butyl phosphate (TBP) with HNO 3 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 HNO 3 at the HNO 3 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.

Thermochemical degradation of two-phase systems TBP-HNO3 and 30% TBP in dodecane-HNO3 (HNO3 concentration 1.5-10 M) on heating in a sealed vessel to 130-170°C is studied. Heating to 130-150°C promotes exothermic oxidation of the organic phase, resulting in its self-heating. The oxidation rate (self-heating and specific volume of gaseous products) increases with increasing HNO3 concentration in the aqueous phase. Irradiation of the organic phase decreases the thermal resistance of the systems, which is reflected in a decrease in the onset temperature of exothermic reactions. In the organic phase 30% TBP in dodecane, irradiated to doses of 1.5-2.4 MGy, in contact with 14 M HNO3, exothermic reactions start at 85-95°C.

Thermal degradation of VP-1AP anion exchanger and its mixtures with HNO 3 was studied in open vessels at 40 to 260°C. The anion exchanger ignites only after drying and heating to 220-260°C. Heating VP-1AP mixtures with 3-12 M HNO 3 to 100°C is accompanied by gas evolution at a rate of 0.6 l min -1 (lsorb) -1 . Removal of the aqueous phase from the mixtures at 130-170°C initiates intensive oxidation. It is accompanied by a temperature jump in the sorbent phase and by gas evolution acceleration and can be regarded as thermal explosion. Mixture preheating and irradiation reduce the thermal explosion "onset" temperatures.

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.

Gas evolution dynamics in thermal oxidation of TBP and its dodecane solutions as part of two-phase systems containing HNO 3 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-HNO 3 systems. The specific features of gas evolution in the two-phase systems are discussed as compared to those in single-phase organic systems.

Variation of physicochemical characteristics of VP-1AP anion-exchange resin in HNO 3 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 HNO 3 concentration and temperature and under the action of ionizing radiation the total exchange capacity (TEC) and capacity with respect to strongly basic groups ( C s b g ) decrease. The swelling coefficients K s w noticeably vary only in treatment in 12 M HNO 3 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 m 2 g - 1 . Under the action of ionizing radiation the specific surface area of the irradiated anion exchanger somewhat increases and reaches 18 m 2 g - 1 .