This project had the following objectives: 1. Develop and customize in situ ion beam analysis tools to observe and characterize the nucleation and growth of gas bubbles in advanced U-TRU-Zr alloy fuel materials. 2. Quantify and model the fundamental phenomena relevant to fission gas induced swelling in advanced U-TRU-Zr alloy fuel materials. 3. Validate and benchmark the new multi-scale fuel swelling models using critical experiments performed using new in situ ion beam methods and post-test analysis. To achieve this objective, a comprehensive strategy is being implemented to provide insight and validation for the fundamental models that will enable multi-scale fuel performance predictions from first principles. The development experiments will focus on fission gas bubble nucleation and swelling in uranium-zirconium alloys containing transuranic (TRUs); however, the fundamental models developed for diffusion and fission gas mobility in the various phases present in U-TRU-Zr nuclear fuels will enable the modeling of other critical fuel performance phenomena in the future such as creep, component redistribution, and fuel-cladding mechanical or chemical interactions.