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 plutoni um under various thermal treatment conditions using an incineration equilib rium calculation program. The treatment conditions examined included temper ature, oxygen level (either reducing or oxidizing), and the existence of ch lorine. In a simulation, a selected waste containing either uranium or plut onium was input to the program along with the desired treatment conditions. The program then performed the free energy calculations and searched for t he optimum composition which minimizes the total system free energy. The si mulation results have indicated that, under a reducing mode, uranium tends to stay in a solid phase as UO2(s) up to 1500 degrees C; however, under an oxidizing mode, it will exist as U3O8(s) up to 1100 degrees C. As the tempe rature increases, the solid-phase compounds either vaporize or decompose in to various vapor-phase compounds. Under a reducing mode, all the preferred compounds will be in vapor phase when the system temperature is above 1900 degrees C; under an oxidizing mode, this temperature is 1450 degrees C. For plutonium, the thermodynamically preferred solid-phase compound is PuO2(s) up to about 1500 degrees C under either a reducing or an oxidizing mode. A s the temperature increases, the compound will vaporize mainly into its vap or phase, i.e. PuO2(g), up to about 2000 degrees 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 simulati on results have been compared with those generated from the HSC program and the results have indicated that the HSC program contains inappropriate the rmodynamic data for uranium and plutonium simulations. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.