2018 | AFC ACCOMPLISHMENTS 60 The six materials were also tested by thermally ramping from 150°C to 1000°C under a steam atmosphere. The thermograms for this testing are shown in Figure 1.The large sudden drops in mass gain are due to material ejection from the crucible. These material ejections are only seen in the 5 and 10 volume percent composite pellets, but the pellets did not completely pulverize. The higher UN containing composites and the monolithic UN pellets did pulverize under these conditions, as seen in Figure 2. Similarly to the 350°C isotherm, the calculated O/U ratios following oxidation were lower than expected, but to a lesser extent, suggesting incomplete oxidation of UN.The oxidation onset is delayed in temperature for the composite materials compared to monolithic UN and is much more gradual in rate. These ramps show some protection given to UN from the UO2 matrix.This is supported by the higher oxidation onset temperature of the composite material compared to monolithic UN and the presence of UN in the resulting diffraction patterns. The lower UN composite materials did not show significant oxidation and the pellets did not pulverize. Loss of material due to ejection is still under investigation, but similar features have been observed in other high U-density fuels under similar conditions. Each of the four composite materials were tested in autoclave conditions at 300°C and pressures of 86 bar for either 2, 5, 10, or 20 days. Under these conditions, UO2 is expected to oxidize to O/U ratios greater than 2.25, leading to pulverization and washout. Following autoclave testing, the pellets were visually inspected for pulverization and a mass obtained, where possible. It was not possible to determine O/U ratios due to material that was lost to the autoclave.The 5 volume percent pellets did not pulverize at any of the times tested, and did not show any appreciable mass change. Ten volume percent pellets did not pulverize up to 10 days. All 5 and 10 volume percent pellets (with the exception of the anomalous 20-day 10 volume percent pellet) had diffraction patterns consisting only of UO2. The 20 and 30 volume percent pellets pulverized at all times tested and the resulting powder consisted of UN, UO2, and higher oxide phases.This was consistent with the isothermal data taken at 350°C for 12 hours, and again further suggest oxidation protection of UN from UO2 for the lower UN composites (5 and 10 vol. %). Using a composite fuel form consisting of uranium nitride in a uranium oxide matrix shows promise for increasing thermal conductivity and uranium atom density, compared to UO2, while increasing the oxidation resistance of monolithic UN.The results here indicate that further studies are needed to determine the amount of UN that can be incorporated within a UO2 matrix, before having detrimental effects on the oxidation behavior. The results of these studies suggest the possibility to use UO2 to protect UN from oxidation and prevent the pulverization of the pellet under certain conditions for some composite materials.