b'Figure 2. a) Average CTE for U 3 Si 2(red) and U 3 Si 2+x(blue) compared reported values. (b) Recommended average CTE foe U 3 Si 2 .Accomplishments: developed using the results from this The objective of the project is towork and the assessed literature (see provide datasets such as lattice param- Figure 2). DFT calculations by David eters, thermal expansion, compositionAndersson were used to predict that limit and defect chemistry neededa low defect formation energy and a as input for fuel performance codeshigh entropy for Si interstitial defects through experimental and theoreticalgive rise to Si-rich non-stoichiometry research. Samples were fabricated byin U 3 Si 2at elevated temperatures, in Joshua White at the Los Alamos Fuelsagreement with the experimental Research Laboratory and neutronfindings. The enthalpy of formation diffraction data of the U 3 Si 2and USialong with the entropy of each point compounds in the U-Si system weredefect were used to calculate the collected the at Los Alamos Neutronnon-stoichiometry in U 3 Si 2providing Science Center (LANSCE) by Sveninformation on solubility limit. These Vogel. The analysis of the data wasresults provide experimental evidence done by Tashiema Ulrich and Sventhat hyperstoichiometric U 3 Si 2+xexists Vogel while Tashiema Ulrich wasat temperatures as low as ambient under the advisement ofTheodorecondition, whereas previous model-Besmann at the University of Southling predicted that U 3 Si 2is a line Carolina. The diffraction data analysiscompound and excess Si would lead to found that the crystal structure inprecipitation of additional phases. The U 3 Si 2(tetragonal P4/mbm spacelatter would be potentially detrimental group) is stable up to the maximumto the mechanical integrity of fuel temperature investigated, 1373 Kpellets due to different thermal expan-(see Figure 1 for visualizations of thesion coefficients of different U-Si structure included anisotropic thermalphases causing mechanical stresses.motion). An equation for the coef-ficient of thermal expansion (CTE) as a function of temperature was 2020|AFC ACCOMPLISHMENTS 45'