2018 | AFC ACCOMPLISHMENTS 59 pellet. The UN additions would also increase the uranium atom density, improving the neutron economics of the fuel. The UO2 matrix would improve the oxidation behavior, compared to UN fuels, minimizing washout in accident scenarios. The oxidation behavior of the UN/ UO2 composites was studied by two different techniques; oxidation in simultaneous thermal analyzer equipped with a water vapor furnace and water vapor generator and in autoclaves. These test conditions were chosen to mimic possible Light Water Reactor (LWR) acci- dent conditions, such as a loss of coolant or leaker accident. Testing of dense composite compacts in the simultaneous thermal analyzer was performed under ambient pressure and consisted of isotherms at 350°C and thermal ramps to 1000°C under steam atmospheres ranging from 62% H2O (g) to 82% H2O (g), depending on temperature. The steam atmospheres were chosen such that the UO2 would not oxidize past UO2.25 and would remain in the fluorite phase field. The autoclave testing was performed on dense composite compacts at 300°C and 86 bar, for durations of either 2,5, 10, or 20 days. Solid state Ni/NiO buffers were used to maintain hydrogen levels in the 1-5 ppm range to simu- late LWR coolant chemistry. Accomplishments: Composite materials have been synthesized and the thermophysical, oxidation behaviors, and microstruc- ture have been explored. Composites compacts consisting 5, 10, 20, and 30 volume percent UN in UO2 have been produced with densi- ties approximately 90% theoretical density (TD). The oxidation behavior has been explored in air, under steam atmospheres at ambient pressure, and under pressurized water conditions. Dense compacts of UO 2, UN, and the four composite materials were exposed to steam at an isothermal temperature of 350°C for 12 hours to mimic a leaker type scenario in an LWR. The expected O/U ratio of UO2 at these conditions was greater than 2.25 indicating oxidation of the material to higher oxide phases, leaving the fluorite structure of UO2+x, leading to pulverization of the pellet and material washout. Of the six pellets tested, only the mono- lithic UO2 and the 5 volume percent composite were not pulverized under these conditions. The calculated O/U ratios of the pellets following oxidation were lower than expected, suggesting incomplete oxidation of the UN to oxide phases. This was supported by the presence of UN in the diffraction pattern.