IRRADIATION BEHAVIORS IN MOLYBDENUM AND URANIUM-10WT.%MOLYBDENUM ALLOYS FROM THE ATOMISTIC SCALE TO THE MICROSCALE

Low enriched uranium (LEU, < 20 % 235U)-molybdenum (U-Mo) alloy is the primary nuclear fuel candidate for research and test reactors, and it is also considered one of the fuel candidates for fast reactors. Furthermore, U-Mo monolithic fuel is currently undergoing a qualification process to replace highly-enriched uranium (≥ 20 % 235U) fuel for high-performance research and test reactors. As part of the fuel qualification process, it is critical to examine the microstructural evolution in the final form of U-Mo fuel under irradiation. However, there is a lack of knowledge on the microstructural evolution of the rolled U-Mo alloy foil, which is a proposed geometry for research and test reactors/high-performance research and test reactors, as well as the U-Mo alloy fuel that is cast into a slug form without rolling, which is a more suitable geometry for other advanced reactor fuel types. The effects of the fabrication methods, specifically arc-casting and cold-rolling, on the phase decomposition in U-10Mo alloy subjected to low neutron fluence (0.01 displacements per atom (dpa) and 0.1 dpa) in the temperature range of 150–350oC are evaluated using synchrotron X-ray techniques and scanning electron microscopy (SEM). The as-cast U-10Mo alloys demonstrated better irradiation performance than the U-10Mo alloy foils in research reactor conditions for the investigated regimes. This study will help optimize fuel fabrication techniques to tune phase decomposition under irradiation. 

Additionally, irradiation behavior in Mo, a critical element in U-Mo fuel, as well as a candidate for cladding material for the next generation of nuclear power plants, is investigated at the atomistic scale following low neutron fluence regimes (0.01 dpa and 0.1 dpa) in the temperature range of 150–800oC using synchrotron X-ray techniques. Lattice contraction was observed in irradiated Mo by synchrotron XRD, indicating that interstitial diffusion is faster than vacancy diffusion in Mo. More interstitials diffuse into the sinks, such as grain boundaries, while fewer vacancies diffuse into the sinks due to slow diffusion, resulting in a higher steady-state concentration of vacancies than that of interstitials under irradiation. The synchrotron PDF also supported the synchrotron XRD results by demonstrating a decrease in the atomic distances under irradiation.