2018 | ANNUAL REPORT 123 Figure 3. U-Zr foil temperature as a function of power for 1.5 W/g gamma heating. The black horizontal lines represent target specimen irradiation temperatures. The gap refers to the gas gap between specimen holder and vehicle wall in inches. experiment. Fuel compositions for U-Mo include 7, 10, and 12 weight percent Mo, fabricated as discs (3 mm in diameter and 250 μm in height). The compositions for U-Zr include 6, 10, 20, and 30 weight percent Zr, fabricated as foils (180 μm thick, 72.75 mm long, and 8.9 mm wide), with the first irradiation including only U-10Zr and U-30Zr. The U-Zr foil specimen holder design allows for a discrete sample temperature to be obtained by varying the insulating gas gap between the holder and vehicle wall.This feature allows for otherwise identical fuel samples to reach different temperatures without modifications to power, specimen geometry, alloy composition, or enrichment. Figure 3 indicates the ideal gas gap required for a target temperature and a given power. In contrast, the desired irradiation temperature for the U-Mo discs is obtained by optimizing the spacer material surrounding the specimens with varying thermal conductivities to obtain desired irradiation temperatures. This method allows for higher power densities to be achieved at lower temperatures due to the contact of the specimen holder with the outer wall