Phase stability of U5Si4, USi, and U2Si3 in the uranium–silicon system

Tashiema L. Ulrich, Sven C. Vogel, Denise A. Lopes, Vancho Kocevski, Joshua T. White, Elizabeth S. Sooby, Theodore M. Besmann

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Abstract

The intermetallic compound U3Si2 has received interest in recent years for use as accident tolerant fuels in light water reactors due to its combination of high thermal conductivity, reasonably high melting point, and high uranium density compared to UO2. Although U3Si2 is well characterized there are ucertainties about the remaining phases in the U–Si system which could be potential fission products of the silicide fuel. In an effort to better detail the U–Si binary phase diagram, the 44–60 at.% Si region of the U–Si phase diagram was investigated experimentally by analyzing samples with nominal compositions U/Si = 5/4, U/Si = 1/1 and U/Si = 2/3 using SEM-EDS, XRD, and high temperature time-of-flight neutron diffraction, supported by DFT calculations. Both the U/Si = 5/4 and the U/Si = 2/3 samples were composed of two phases, U3Si2 + USi and USi + U3Si5 for U/Si = 5/4 and U/Si = 2/3, respectively. Both U5Si4 and U2Si3 were found from DFT calculations to have imaginary phonon frequencies and an enthalpy of formation above the U–Si convex hull, and thus likely to be thermodynamically unstable compared to the other phases, consistent with the experimental results. In the literature there are three different structures that are proposed for the USi phase, and for the first time, high temperature neutron diffraction measurements were collected on the compound synthesized with the U/Si = 1/1 composition to verify the correct structure. The structure of the USi phase was confirmed to be tetragonal with the I4/mmm space group from room temperature to 1100 °C. The USin crystal structure has 6 U atom sites and 8 Si atoms sites of which two are partially occupied.

Original languageEnglish
Article number152353
JournalJournal of Nuclear Materials
Volume540
DOIs
StatePublished - Nov 2020
Externally publishedYes

Funding

This research is being performed using funding received from the DOE Office of Nuclear Energy’s Nuclear Energy University Programs and has benefitted from the use of the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract number 89233218NCA000001. Tashiema Ulrich was supported by a fellowship from the Nuclear Regulatory Commission . The computational work was performed using the Hyperion cluster supported by the information technology division at the University of South Carolina.

FundersFunder number
U.S. Department of Energy89233218NCA000001
U.S. Nuclear Regulatory Commission
Office of Nuclear Energy
National Nuclear Security Administration
Los Alamos National Laboratory

    Keywords

    • Crystal structure
    • Nuclear fuel
    • Phase stability
    • Uranium silicide

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