b'to accommodate excess Si atomssamples were heated to ~1150C without partitioning of secondaryto assess possible differences in phases (Andersson et al., 2018).the crystal structure evolution as a Ultimately, experimental data isfunction of temperature. Differences needed to assess this issue andin the thermal expansion for this research provides the data.different chemical compositions Twelve hours neutron countcould lead to thermal stresses,times (compared to the usualwhich ultimately may lead to 30-120 minutes) were applied tocracking of pellets. characterize stoichiometric U 3 Si 2.00 Accomplishments: and hyper-stoichiometric U 3 Si 2.01The stoichiometric U 3 Si 2.00and at room temperature. The ultra-highhyper-stoichiometric U 3 Si 2.01were statistical quality of the diffractionsynthesized at LANLs Fuel Research data allowed researchers to assessLab (FRL) by J. White (LANL) and whether secondary phases wereE. Sooby Wood (now UT Austin) by present at a level of arc-melting of weighed amounts of >0.1 wt. %. Crystallographicthe constitutive elements. Because techniques such as differenceuranium metal readily oxidizes, Fourier maps applied to thesegreat care was taken to control the data sets allow to identify sites ofoxygen in the atmosphere, e.g., by scattering density unaccounteduse of oxygen getters. The ingot for by the standard U 3 Si 2crystalwas crushed into a powder, sieved, structure. These sites are indicativeannealed, and then loaded into vana-of locations of excess Si atoms,dium cans for the neutron diffrac-which can subsequently betion experiments (vanadium has compared to DFT predictionsa typically negligible contribution of these sites. Adding small siteto the diffraction signal). Neutron occupations of Si atoms to thediffraction data was collected at data analysis model allows thenambient conditions for ~12 hours to identify which site fits theper sample as well as in a vanadium experimental data best. The same 2019|AFC ACCOMPLISHMENTS 47'