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Graphite is widely used in the atomic industry and nuclear power engineering. In the next 10–15 years, the resource of most blocks in the uranium–graphite reactors in Russia will be exhausted with allowance for the elongation of the service life, as follows from the results of repair-and-renewal operations. High-activity graphite can be processed by flameless oxidation in molten salts. The purpose of this work is to study the behavior of radionuclides during the flameless oxidation of radioactive graphite in the CuO–NaCl–KCl–Na2CO3–K2CO3 melt in water vapor using a thermodynamic simulation and the Terra software package. A thermodynamic simulation is an effective and unique method used to investigate diverse high-temperature processes of thermal decomposition, reduction, and synthesis. The calculation is performed using a reference datable of the properties of individual substances. Graphite burns up at 573 K. The condensed compounds of cesium, chlorine, and uranium evaporate at 1673 K. An increase in the temperature to 1973 K leads to the vapor pressure of the condensed compounds of beryllium, calcium, and strontium. The condensed oxide of europium(III) and the condensed oxide of plutonium(IV) transform into a vapor state at 2173 K. The condensed oxide of nickel transforms into a vapor state at 2473 K. A further increase in the temperature to 2773 K leads to the transformation of the condensed oxide of americium(III) into a vapor state. Only a vapor–gas phase is present in the isolated system in the temperature range from 2773 to 3273 K. [ABSTRACT FROM AUTHOR]

More than 100 power reactors, reactors for plutonium production, and research reactors are presently available in the world. In these reactors, graphite is used as a reflector, moderator, and cladding of fuel elements. Flameless combustion is a promising method for reducing the amount of solid radioactive waste. The method is based on the oxidation of solid radioactive waste in oxide–carbonate melts and makes it possible to reduce the radioactive graphite volume considerably. Thermodynamic simulation and analysis of the oxidation of radioactive graphite in molten Na2CO3–K2CO3–Sb2O3 in a carbon dioxide atmosphere are performed using the TERRA software package. The thermodynamic simulation is conducted at a pressure of 1 atm and initial and final temperatures of 273 and 3273 K, respectively. The step of temperature changing is 100 K. Based on these data, the distributions of elements between condensed and gas phases are examined. The simulation results show that carbon disappears at a temperature of 873 K. Heating of the system up to 1073 K leads to the evaporation of condensed antimony compounds. Heating of the system up to 1673 K leads to the evaporation of condensed potassium, sodium, chlorine, uranium, and cesium compounds. Heating of the system up to 2273 K leads to the evaporation of condensed nickel compounds. Heating of the system up to 2573 K leads to the evaporation of condensed calcium, plutonium, beryllium, strontium, americium, and europium compounds. Only a vapor–gas phase is observed at temperatures above 2573 K. [ABSTRACT FROM AUTHOR]

Abstract—Graphite is used as a neutron moderator and reflector. Moreover, graphite can be used as sealants and bearings in reactors. The graphite mass in a reactor is 1-2 ths t. A large amount of radioactive graphite wastes forms when graphite reactors are taken out of service. The existing methods of processing radioactive graphite are based on its isolation from the environment. These methods cannot substantially decrease the volume of radioactive graphite wastes. As a result, the processing of irradiated reactor graphite by oxidation in molten salts can be considered as an alternative reclamation method, which can decrease the volume of radioactive graphite wastes. The oxidation of radioactive graphite in the Na2CO3-K2CO3-Sb2O3 melt in an argon atmosphere is thermodynamically simulated using the TERRA software package. The data obtained are used to analyze the distribution of elements over condensed and gas phases. Heating of the system to 1073 K is found to cause the evaporation of the condensed compounds of antimony and cesium. Upon heating to 1273 K, the condensed compounds of potassium, sodium, and chlorine evaporate. Heating to 1373 K leads to the evaporation of the condensed compounds of nickel. Heating to 1673 K brings about the evaporation of the condensed compounds of uranium, calcium, and strontium. Heating to 1773 K causes the evaporation of the condensed compounds of plutonium, beryllium, americium, and europium. At temperatures above 1773 K, only a vapor-gas phase exists in the system. [ABSTRACT FROM AUTHOR]

Abstract: Graphite occupies a specific place among the entire mass of accumulated radioactive wastes. The problem of reclamation of spent graphite has not yet been solved in the world. The behavior of radioactive graphite in the multicomponent CuO-NaCl-KCl-Na2CO3-K2CO3 melt in an argon atmosphere in the temperature range 373-3273 K is studied by thermodynamic simulation with the Terra software package, which is intended for calculating the phase compositions and the thermodynamic and transport properties of arbitrary systems. The calculation was performed using a database on the properties of individual substances. At 1273 K, graphite burns to form carbon monoxide and carbon dioxide. The condensed compounds of cesium, chlorine, and uranium evaporate at 1573 K. An increase in the system temperature to 1873 K leads to the vapor pressure of condensed nickel. Condensed strontium oxide transforms into a vaporized state at 2273 K. The condensed compounds of plutonium, calcium, and europium transforms into a vaporized state at 2373 K. Condensed beryllium oxide evaporates at 2473 K. A further increase in the temperature to 2673 K leads to the evaporation of condensed americium(III) oxide. Only a vapor-gas phase is present in the isolated system in the temperature range 2673-3273 K. [ABSTRACT FROM AUTHOR]