Thermodynamic Modelling of the U-Dy-O System

Abstract

Among the important fission products dissolved in UO₂ are several members of the lanthanide series. Particular lanthanide sesquioxides (Ln₂O₃), notably Dy₂O₃, are of interest as burnable neutron poisons in advanced CANDU fuel bundle designs. For these reasons, a systematic evaluation of the existing thermodynamic knowledge of known UO₂ - Ln₂O₃ systems is being undertaken. A proposed binary phase diagram for UO₂-Dy₂O₃ is based on the generally similar chemical behaviour of all Ln₂O₃ as well as Y₂O₃ systems involving UO₂.

Uranium and lanthanide oxides form rhombohedral compounds of the form ULn₆O₁₂ under oxidizing conditions. The significance in understanding the phase equilibrium in the ternary U-Dy-O system at 1600 C will be discussed. Computed phase diagrams, based on some estimated data, draw attention to the potential importance of oxygen partial pressure. The computed diagrams provide the rationale for sample preparation in ongoing experimental studies.