Ambient Melting Behavior of Stoichiometric Uranium-Plutonium Mixed Oxide Fuel

Mixed oxides of uranium and plutonium (MOX) are currently considered as a reference fuel for the new generation of fast breeder reactors such as ASTRID. The key factor determining the performance and safety of a fuel such as MOX is its operational limits in the application environment which are closely related to the material's structure and thermodynamic stability. They are in turn closely related to the ambient (zero pressure) melting point (T-m); thus, T-m is an important engineering parameter. Furthermore, PuO2 and UO2 are two endpoints of the phase diagram of MOX; therefore, their ambient T(m)s are fundamental reference points. However, the current knowledge of the T-m of MOX is limited and controversial as several studies available in the literature do not converge on the unique behavior of T-m as a function of x. Specifically, some studies produced T-m as a monotonically decreasing function of x such that, with T-m of UO2 (x = 0) of 3150 K, T-m of PuO2 (x = 1) is similar to 2650 K, while other studies resulted in T-m having a local minimum at 0.5 < x < 1 such that T-m of PuO2 is similar to 3000 K, so that the difference between the two values of T-m is as high as 350 K. In this study, using the ab initio Z method implemented with the Vienna Ab Initio Simulation Package (VASP), we carry out a suite of quantum molecular dynamics simulations to obtain the ambient T-m of MOX at several values of x, 0 < x < 1, including the two end points (x = 0, x = 1). Our results agree with the behavior of T-m of MOX as a function of x having a local minimum at x = 0.7 and T-m of PuO2 of 3050 K. Our study suggests potential ambient density-melting point systematics of MOX which may be useful in subsequent research on MOX such as its thermoelasticity modeling.