Thermodynamic Properties of Actinide Oxide Solid Solutions
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1125-R10-06
Thermodynamic Properties of Actinide-Oxide Solid-Solutions Lindsay C. Shuller1, Niravun Pavenayotin2, Rodney C. Ewing1,2,3, and Udo Becker3 University of Michigan, Department of Materials Science and Engineering 2 University of Michigan, Department of Nuclear Engineering and Radiological Sciences 3 University of Michigan, Department of Geological Sciences 1
ABSTRACT Density functional theory and Monte-Carlo methods were used to investigate the solidsolution 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 MonteCarlo simulations are used to calculate the excess enthalpy of mixing (∆Hexcess), excess Gibbs free energy of mixing (∆Gexcess), and excess configurational entropy (∆Sexcess) for the above solid-solution series. From ∆Gexcess, 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. INTRODUCTION Generation IV reactors and the new strategies for “burning” actinides in advanced burner reactors will require the development of new nuclear fuels, such as mixed-oxide (MOX) or inert matrix fuels (IMF). There are a number of experimental and computational studies of the properties of MOX and IMFs that focus on in-reactor performance, such as investigations of thermal conductivity and the neutronic properties [1,2,3]. There has also been considerable effort devoted to understanding the behavior of fission product elements, both solids and gases in the fuel matrix [4,5]. However, only limited information is available on the thermochemical properties of fuels [6] that consist of solid-solutions of the actinides, e.g., (U,Pu)O2. Much of the experimental work is hindered by the restrictions of dealing with radioactive materials [7]; hence, there is an increased effort to use first-principles calculations to determine thermochemical parameters. There are two fundamental properties that control the performance of nuclear fuels: i.) thermochemical parameters that change due to equilibrium mixing properties of relevant solidsolutions; ii.) the kinetics of processes that are mainly controlled by defect mobility. In this study, the thermodynamic properties of actinide dioxide solid-solutions were examined using a combination of computational techniques. The majority of the previous computational studies related to advanced nuclear fuels have used molecular mechanics and dynamic
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