Vaporization chemistry of hypo-stoichiometric (U,Pu)O2

Calculations were performed on hypo-stoichiometric uranium plutonium di-oxide to examine its vaporization behavior as a function of O/M (M=U+Pu) ratio and plutonium content. The phase U(1−y)PuyOz was treated as an ideal solid solution of (1−y)UO2+yPuO(2−x) such that x=(2−z)/y. Oxygen potentials for different desired values of y, z, and temperature were used as the primary input to calculate the corresponding partial pressures of various O-, U-, and Pu-bearing gaseous species. Relevant thermodynamic data for the solid phases UO2 and PuO(2−x), and the gaseous species were taken from the literature. Total vapor pressure varies with O/M and goes through a minimum. This minimum does not indicate a congruently vaporizing composition. Vaporization behavior of this system can at best be quasi-congruent. Two quasi-congruently vaporizing compositions (QCVCs) exist, representing the equalities (O/M)vapor=(O/M)mixed−oxide and (U/Pu)vapor=(U/Pu)mixed−oxide, respectively. The (O/M) corresponding to QCVC1 is lower than that corresponding to QCVC2, but very close to the value where vapor pressure minimum occurs. The O/M values of both QCVCs increase with decrease in plutonium content. The vaporization chemistry of this system, on continuous vaporization under dynamic condition, is discussed.