b'Figure 1. Example FAST geometry pellets for (a) UO 2and (b) UN. The UO 2pellets are 90% dense, while theUN pellets achieved 92.5%of the theoretical density ofthe material.at elevated temperature of smallwhich is related to difficulties in specimens have been conductedcontrolling the pressing pressures using a Hysitron Triboindenterat these small geometries. Defects outfitted with a XSOL 800 hot stage.were also observed in some of the Measurement of the mechanicalannular pellets resulting from the properties is limited to 800C andejection of the central punch. Progress evaluated dislocation flow on (U,Ce) towards large grain sized UO 2was O 2as a surrogate for mixed oxide fuel. also achieved yielding undoped pellets Accomplishments: with grains on the order of 30 m, The first aspect of the research was tosimilar to the doped UO 2ATF grain demonstrate the ability to fabricatesizes produced in industry. UO 2pellets new geometries for FAST irradiationswith FAST geometries and large as well as sinter different accidentgrain sizes will be demonstrated in a tolerant fuel concepts of interest. Anmilestone due at the end of this FY example of geometries is shown into allow for future irradiation tests to Figure 1 (a-b), which highlights theevaluate the fission gas release (FGR) both UO 2as well as UN pellets. Pelletseffects with enhanced grain sized UO 2 , ranging from reference 10 mm downelucidating the effects that defective to 3 mm diameter have been achieved(doped) UO 2has on FGR.on UO 2to 90-97% of the theoreticalProgress towards measuring density, while UN pellets werethermophysical properties was also densified to 92-95% density. Pelletsadvanced in the last fiscal year. The down to the size of 3 mm by 3 mminstallation of an LFA467 HyperFlash have been demonstrated along withinstrument was accomplished. This annular variants with a 1 mm holeunit has a Xe flash bulb with a light in the center, shown in Figure 2 (b).pulse that is 10 times faster than a Of note is that the smaller geometriesstandard laser unit. This facilitates the tend to have lower densities comparedmeasurement of thin samples, which to the larger monolithic variants,was demonstrated on UO 2and U 3 Si 22020|AFC ACCOMPLISHMENTS 47'