Atomistic Ordering in Body Centered Cubic Uranium-Zirconium Alloy
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Atomistic Ordering in Body Centered Cubic Uranium-Zirconium Alloy Alex P. Moore1, Ben Beeler1, Michael Baskes2,3, Maria Okuniewski4, and Chaitanya S. Deo1 1 Nuclear and Radiological Engineering Program, George W Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 770 State Street, Atlanta, GA 30332, USA 2 University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA 3 Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545, USA 4 Idaho National Laboratory, PO Box 1625, Idaho Falls, ID 83415, USA ABSTRACT The metallic binary-alloy fuel Uranium-Zirconium is important for the use of the new generation of advanced fast reactors. Uranium-Zirconium goes through a phase transition at higher temperatures to a (gamma) Body Centered Cubic (BCC) phase. The BCC high temperature phase is particularly important, since the BCC phase corresponds to the temperature range in which the fast reactors will operate. A semi-empirical MEAM (Modified Embedded Atom Method) potential is presented for Uranium-Zirconium. The physical properties of the Uranium-Zirconium binary alloy were reproduced using Molecular Dynamics (MD) simulations and Monte Carlo (MC) simulations with the MEAM potential. This is a large step in making a computationally acceptable fuel performance code. INTRODUCTION Metal alloy fuels demonstrate superior performance (over ceramic fuels) in that they behave in a benign manner during core off-normal events, maintain integrity in high burn-up conditions, have low-loss fuel recycling during reprocessing, have a high thermal conductivity, and have a fairly isotropic neutron cross-sections. Uranium-Zirconium (U-Zr) is a metal alloy that looks to be a promising option as a nuclear fuel. However, for it to be used in a commercial setting, a computational fuel performance code must first be made. Uranium-Zirconium has a Body Centered Cubic (BCC) structure for reactor operating temperatures; therefore, the BCC structure is particularly important to analyze. It is also important to note that the Uranium-Zirconium alloy goes through a δ (C32 Crystal Structure) to γ (BCC) phase transition for 65%-75% Zirconium around 890 Kelvin. The most recent phase diagram was constructed by H. Okamoto [9] and was made from a compilation of experimental papers on Uranium-Zirconium. THEORY MEAM Potential The MEAM (Modified Embedded Atom Method) inter-atomic potential is a semiempirical potential proposed by Baskes et al. [11] that has been successfully used to reproduce the physical properties of various metals with different crystal structures. The MEAM potential is useful because it has the ability to replicate physical properties while keeping the computational power and time, which are necessary to complete the simulations, down to an acceptable level. This is a large step in making a fuel performance code and an accurate irradiation damage code.
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The MEAM potential for a single element contains 14 adjustable parameters used to obtain the physical properties seen by experiments or ab-initi
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