b'crystal structure. Adding Sitherefore warranted. The detailed atoms on those sites, with partialexperimental and simulation results occupancy, allowed to identify thewere submitted to Journal of thebest match to the experimentalApplied Crystallography.data.Occupation of the 4e siteCitations:(, , 0.262) was identified as theD. A. Andersson, X.-Y. Liu, B. Beeler, S. most likely site of excess Si atomsC. Middleburgh, A. Claisse, C.R. Stanek, in the U 3 Si 2structure. D. AnderssonDensity functional theory calculations (LANL, in collaboration with theof self- and Xe diffusion in U 3 Si 2 , Nuclear Energy Advanced ModelingJournal of Nuclear Materials 515 and Simulation (NEAMS) and the(2019) 312-325.Consortium for Advanced Simulation of LWRs (CASL) programs predictedS. C. Middleburg, R. W. Grimes, E. J. the energy of various interstitialLahoda, C. R. Stanek, D. A. Andersson, sites using DFT calculations,Non-stoichiometry in U 3 Si 2 , Journal which allowed researchers toof Nuclear Materials 482 (2016) confirm the experimental finding.300-305.Identification of a site where U 3 Si 2can accommodate excess Si can explain why precipitation of additional phases is not observed. Absence of this precipitation due to excess Si makes the U 3 Si 2system more robust against deviations from stoichiometry, an important finding for the application as a nuclear fuel. However, the observation that the thermal expansion changes significantly if the stoichiometry varies could lead to thermal stresses within a pellet that could ultimately lead to cracking. Further investigations are 2019|AFC ACCOMPLISHMENTS 49'