Thermodynamic Analysis of the Oxidation of Radioactive Graphite in Molten Na2CO3–K2CO3–Sb2O3 in a Carbon Dioxide Atmosphere

Abstract

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.