Advances in the Experimental Determination of the Uranium–Oxygen Phase Diagram at High Temperature

Abstract

Due to its complex phase diagram and the employment of UO₂ as a nuclear fuel, the binary system U–O is of great interest both scientific and technological. Numerous experimental and theoretical studies have been carried out in the last 45 years in order to determine the properties of this system, leading to a precise definition of a considerable part of the state diagram in the region ranging from pure uranium to stoichiometric UO₂, and at temperatures lower than 1500 K, up to the oxide U₄O₉. However, due to the poor chemical stability of O–U compounds with high oxygen content at high temperature (O/U > 2, T > 2000 K), an important part of the phase diagram still lacks experimental data. In this work measurements are presented on the melting transition of the stoichiometric and hyperstoichiometric dioxide UO_2+x up to x=0.21, and on the melting point of the higher oxide U₃O₈. These measurements were performed under buffer gas pressures varying between 10 and 250 MPa, using a method based on subsecond laser heating developed to overcome experimental difficulties encountered by previous researchers.