Thermodynamic study of the behavior of uranium and plutonium during thermal treatment under reducing and oxidizing modes

This study investigated the equilibrium compositions of uranium and plutonium under various thermal treatment conditions using an incineration equilibrium calculation program. The treatment conditions examined included temperature, oxygen level (either reducing or oxidizing), and the existence of chlorine. In a simulation, a selected waste containing either uranium or plutonium was input to the program along with the desired treatment conditions. The program then performed the free energy calculations and searched for the optimum composition which minimizes the total system free energy. The simulation results have indicated that, under a reducing mode, uranium tends to stay in a solid phase as UO2(s) up to 1500°C; however, under an oxidizing mode, it will exist as U3O8(s) up to 1100°C. As the temperature increases, the solid-phase compounds either vaporize or decompose into various vapor-phase compounds. Under a reducing mode, all the preferred compounds will be in vapor phase when the system temperature is above 1900°C; under an oxidizing mode, this temperature is 1450°C. For plutonium, the thermodynamically preferred solid-phase compound is PuO2(s) up to about 1500°C under either a reducing or an oxidizing mode. As the temperature increases, the compound will vaporize mainly into its vapor phase, i.e. PuO2(g), up to about 2000°C. Above this temperature, the system contains only vapor-phase compounds. In addition to equilibrium composition, the effective vapor pressure and the fraction in vapor phase for the two metals have also been evaluated. The existence of chlorine has not been found to affect the simulation results significantly. The simulation results have been compared with those generated from the HSC program and the results have indicated that the HSC program contains inappropriate thermodynamic data for uranium and plutonium simulations.