Modeling of Radioactive Graphite Oxidation in Molten Salts: Computer Experiment
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Modeling of Radioactive Graphite Oxidation in Molten Salts: Computer Experiment Nikolai M. Barbin1,2, Dmitri I. Terentiev1, Sergei G. Alekseyev1, Marat A. Tuktarov3, A. A. Romenkov3 1 Ural Institute of State Fire Fighting Service, 22 Mira St., Ekaterinburg, 620062 Russia 2 Institute of High-Temperature Electrochemistry, Ural Branch RAS, Ekaterinburg, Russia 3 N.A. Dolezhal R&D Institute of Power Engineering, Moscow, Russia ABSTRACT Graphite is used as the neutron moderator and reflector in many nuclear reactors. Obsolete graphite nuclear reactors are put out of operation, leading to formation of a large quantity of radioactive graphite waste. It is proposed that irradiated reactor graphite is processed by high-temperature chemical oxidation in salt melts with an oxidant, which is part of the salt melt, leading to formation of exhaust gases: gaseous compounds of carbon and oxygen (CO2 and CO). This study deals with carbon oxidation and physical-chemical transformations of radioactive elements during the interaction between graphite waste of the atomic power industry and salt melts. The method of thermodynamic simulation is used. The carbon melt decreases the transfer of radionuclides to the gaseous phase as compared to incineration of graphite in the atmosphere. INTRODUCTION Graphite is used in many nuclear reactors. A large quantity of radioactive graphite waste is formed when obsolete graphite nuclear reactors are put out of operation. Most of the existing technologies used for processing of nuclear graphite waste are based on isolation of radioactive graphite from the environment. However, they cannot provide a considerable reduction in the waste store. Therefore technologies of high-temperature thermal treatment, such as incineration, are viewed as an efficient alternative since they considerably reduce the waste store. Radioactive graphite waste contains various radionuclides. Radioactive elements cannot be destroyed by incineration. They either are retained in noncombustible waste or evaporate depending on their volatility. It is proposed to use oxidation of carbon with lead oxide in a Na2CO3-K2CO3 melt for processing of radioactive graphite. In addition to impurities of the activation origin, specifically 14C, graphite contains fuel spillages, minor actinides and fusion products (see Table I). This study deals with the behavior of radionuclides during the interaction of radioactive graphite with an oxide-carbonate condensed medium. The method of thermodynamic simulation was used. EXPERIMENTAL DETAILS The thermodynamic simulation (TS) consists in thermodynamic analysis of the equilibrium state of systems as a whole (the full thermodynamic analysis). Thermodynamic systems imply conditionally isolated material regions whose interaction with the environment is reduced to the exchange of heat and work. The use of TS provides for qualitative modeling and prediction of the composition and properties of complex heterogeneous, multicomponent and
Table I. Radionuclides present in radioactive graphite Radionuclides 10
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