b'2.2 HIGH-PERFORMANCE LWR FUEL DEVELOPMENTUnderstanding the U 3 Si 2Crystal Structure Evolution as a Functionof Temperature by Neutron Diffraction and SimulationPrincipal Investigator: Sven C. Vogel, LANLTeam Members/ Collaborators: Tashiema L. Ulrich (University of South Carolina - Columbia), Joshua T. White and David A. Andersson (LANL) Elizabeth Sooby Wood (University of Texas - Austin)The combination of high-quality neutron diffraction data and DFT calculations allowed to identify interstitial sites for Si atoms in the U 3 Si 2structures that explains the absence of secondary phases in hyper-stoichiometric U 3 Si 2+1 .U 3 Si 2is actively researched1150 C using time-of-flight neutron as an accident tolerant fueldiffraction on the HIPPO instrument (ATF). Detailed knowledgeat the Los Alamos Neutron Science of the crystal structure evolution,Center (LANSCE) at Los Alamos including knowledge of the aniso- National Laboratory (LANL).tropic tetragonal lattice parameters,Project Descriptionas a function of temperature andUnderstanding the changes chemical composition is of para- in behavior of U 3 Si 2in case of mount importance for predictionsdeviations from the line compound ranging from thermo-mechanicalis imporant for the application stresses to phase transformations toof this system as fuel. Initial crystal lattice sites of fission products.thermodynamic calculations Knowledge of the stability range(Middleburgh et al., 2016) of U 3 Si 2is of great importance toindicated that the material at room understand whether changes of thetemperature cannot accommodate stoichiometry, either from burn-upany significant excess Si atoms, or due to synthesis variations, willresulting in precipitation of lead to undesirable phase decom- secondary phases. Such behavior position. To investigate these topics,could ultimately affect fuel pellet stoichiometric U 3 Si 2.00and hyper- integrity. More recent calculations stoichiometric U 3 Si 2.01were synthe- revised these earlier results and sized and characterized at ambientpredict a significant ability of U 3 Si 2conditions and up to46 2019|AFC ACCOMPLISHMENTS'