b'Investigation of stabilityThe uranium-zirconium phase diagram has been corrected, in the uranium-zirconiumenabling accurate interpretation of in-reactor and out-of-reactor intermetallic compound material behavior of uranium-zirconium nuclear fuels.U ranium (U)-zirconium (Zr) alloys, specifically U with 10 weight % Zr (U-10Zr), have been investigated for many years as the nuclear fuel of choice in sodium-cooled fast reactors. A number of phases are present-U, -Zr, -U, -(U,Zr), and -UZr 2 across the cross section of the fuel due to a temperature gradient generated during irradiation. The behavior of the fuel during irradiation is very dependent on microstructure, so our understanding PROJECT NUMBER:of the fuel behavior is very dependent on the U-Zr phase diagram. This diagram 22P1068-011FP has had essentially the same layout and phases present since 1956. -UZr 2is the TOTAL APPROVED AMOUNT:only intermetallic compound in the phase diagram and is a primary component of $150,000 over 1 year U-10Zr fuel. PRINCIPAL INVESTIGATOR:Recent experiments on the UZr 2alloy indicated it may not be thermodynamically Michael Benson stable. The objective of this research was to determine the stability of this phase. To that end, samples of UZr 2were annealed, sealed inside quartz tubes under CO-INVESTIGATORS: vacuum and heated to 500C with samples removed after 1,272 hours, 2,544 hours, Chao Jiang, INL 3,888 hours, 5,088 hours, and 8,760 hours (one full year). These samples were Mukesh Bachhav, INL analyzed using scanning electron microscopy, transmission electron microscopy, Tiankai Yao, INL x-ray diffraction, and atom probe tomography. Density functional theory modeling was also performed to determine formation energies. The scanning electron microscopy images include the sample prior to annealing (as-cast), and the 6 month (3,888 hours) and one year (8,760 hours) annealed samples. Starting with the 1,272 hour sample, precipitates were observed in the samples, increasing with each sample analyzed. The white precipitates are pure U, and the small black precipitates are pure Zr. UZr 2is breaking down with time, indicating this phase is not thermodynamically stable. UZr 2is a long-lived metastable phase and should not be included in the U-Zr phase diagram. This is the first significant change in the U-Zr phase diagram in 67 years. This does not change the irradiation behavior or any of the data collected during post-irradiation examinations, but it does affect interpretation of the data. U-10Zr may be a dispersion fuel, with U and Zr remaining mixed due to irradiation-induced mixing. Scanning electron microscopy images of as-cast UZr 2 , and after annealing for 6 months and one year.140'