b'Accomplishments:The work supported under this work package has resulted in three mile-stone reports, M3FT-22LA020201018 (submitted March 2022), M3FT-22LA020201013 (submitted March 2022) and M2FT-22LA020201011 (to be submitted September 2022). The work accomplished here involves sintering of Cr-doped UO2 pellets with Cr concentrations ranging from 750 to 7800 ppm Cr. Inductively coupled plasma mass spectrometry (ICP-MS) was collected before and after sintering to determine any Cr loss during sintering. The sintered pellets have densities 94.5 percent theoretical density (%TD) and post sintering XRD patterns show only peaks from UO 2 . There is a small shiftrevealed grain sizes of 28.22.3 mFigure 2. Thermal conductivity towards lower angles with increasingand 41.53.8 m, respectively. Forevolution of 750 ppm and 2500 ppm Cr content, which indicates a contrac- comparison, the average grain sizeCr-doped UO2 from 50 to 1000 Ction of the lattice with higher Crof standard, undoped UO 2is ~10 contents. This indicates that Cr ism. The sintered pellets were used being dissolved into the UO 2latticeto measure heat capacity, thermal as opposed to forming a secondaryexpansion, density, and thermalCr-doped UO2 shows phase. Scanning electron microscopydiffusivity of the Cr-doped materials.an increase in grain (SEM) images and energy dispersiveThese measurements were used tosize and also shows X-Ray (EDX) spectroscopy show thatcalculate the thermal conductivitychanges in the thermal at higher Cr contents, Cr 2 O 3doesas a function of temperature andconductivity relative precipitate out as a secondary phasewere compared to that of standardto UO 2 , which will be and indicates that Cr is only dissolvedUO 2published in 2008 by Fink.used to model in-pile into the lattice up until a thresholdThe results show that the thermalperformance.level, after which the secondary phaseconductivity decreases at higher is seen. The precipitates are foundtemperature and likely results within the grains as opposed to beingfrom the phonon scattering due to located at the grain boundaries. Cr dopants in the UO 2lattice. The The average grain size in the dopedresults are shown in the attached UO 2sintered pellets was measuredFigure 2. Doped and undoped and show increases in the grainlarge grain UO 2were fabricated sizes with increasing Cr content,and shipped to High Flux Isotope as is shown in Figure 1. The 750Reactor (HFIR) for irradiation tests.and 2500 ppm Cr-doped pellets 2022|AFC ACCOMPLISHMENTS 25'