Published online by Cambridge University Press: 15 March 2011
Density functional theory and Monte(Carlo methods were used to investigate the solid( solution behavior of actinide dioxides (AcO2). The end(members of interest include: ZrO2, ThO2, UO2, NpO2, and PuO2; all have the isometric fluorite structure. Ab initio and subsequent Monte( Carlo simulations are used to calculate the excess enthalpy of mixing (ΔHexcess), excess Gibbs free energy of mixing (ΔGexcess), and excess configurational entropy (ΔSexcess) for the above solid(solution series. From ΔGexcess, phase diagrams are derived and miscibility gaps identified. All of the binaries of the aforementioned end(members were studied; however, this paper focuses on the U1(xZrxO2 and Np1(xUxO2 binaries. About 25 at.% Zr can be in solid solution with the UO2 matrix above 1500 K, while Np is completely miscible in the UO2 matrix. Partial cation ordering was observed at all temperatures for the U1(xZrxO2 binary. The Np1(xUxO2 binary approaches perfect cation disorder at high temperatures (2000 K). The cation ordering scheme is not identified in this study because the number of cation(cation interaction parameters was limited by the single unit cell from the ab initio calculations.