Nuclear Science User Facilities 54 Project Description The technical objective of this study was to probe an aged material using an instrument capable of giving information on structure and chemistry.The two-phase microstructure of the U-6Nb alloy undergoes aging decomposition during heat treatments at temperatures used in reactors and is a simple alloy to probe phase transformations, kinetics, and age‑hardening mechanisms. Metallic uranium fuel will be necessary in advanced reactors to meet the fissile‑atom density requirements of reactors while maintaining low enrichment that policy demands. Understanding decomposition mechanisms and underlying kinetics within the operating temperature regime is necessary for performance prediction for reliability, safety, and life extension for new reactors. Accomplishments ClarissaYablinsky was able to accomplish the microscopy and atom probe for one of the four samples proposed, rounding out a set of 3 aging conditions: 10 min at 500°C (completed this fiscal year [FY]), 100 min at 500°C, and 10000 minutes at 500°C (completed in previous year outside of Nuclear Science User Facilities [NSUF]). General trends were observed with aging: the Nb segregation to the matrix was greater with increased aging and the U precipitate increased in concentration as the Nb segregated.This increase in partitioning relates to the energy released during the decomposition; thus, the same trend is seen for the extent of partitioning, delta C, where delta C increases with aging time.The Center for Advanced Energy Studies (CAES) staff was instrumental in Atom Probe Characterization of Phase Separation During Age Hardening of a U-6wt.%Nb Alloy ClarissaYablinsky – Los Alamos National Laboratory – email@example.com Understanding decomposition mechanisms in metallic fuel will help with performance prediction over the lifetime of a reactor. Figure 1. Tip reconstructions for aging times of 10 minutes at 500°C, 100 minutes and 500°C, and 10000 minutes at 500°C. The non-lamellar precipitate structures all contain interconnected alpha-U precipitates, which coarsen with age, of varying widths and orientations.