2017 | AFC ACCOMPLISHMENTS 67 Synergy of ORNL characterization methods and LANL fabrication of high density fuels leads to new insights regarding native oxide structure of U-Si compounds. the native oxide structure present on U3Si2 samples is similar to the structure found following elevated temperature oxidation. An important finding was the limited thickness of the affected region.The surface U-O structure (identified as crystalline and likely UO2 but not indepen- dently confirmed in this study) is roughly 10 nm thick.The region of Si-enrichment is roughly 15 nm thick.These behaviors are show in Figure 2, a single line scale of the EDS data.The three regions are identified in the figure. The native oxide structure of U3Si5 was characterized in the same manner.While U3Si2 and U3Si5 exhibit similar chemical evolutions following oxidation at high temperature, the native oxide structure observed in U3Si5 was distinct. No crystalline oxide was found. Instead, an amor- phous U-Si-O region roughly 20 nm in thickness was observed.This result was unexpected and suggests that Si content may play a larger role in the formation of surface oxides for compositions in the U-Si binary than previously believed based on thermo- dynamics alone. The results of this study will be expanded to include other U-Si binary compounds in the near future to better understand the transition between the well-ordered surface oxide observed in U3Si2 to the disor- dered and chemically homogeneous behavior of U3Si5.These findings will help improve our understanding of how oxidation progresses in the U-Si system and may lead to methods of mitigating oxidation during fabrication or service for high density U3Si2 fuels.