| Abstract Scope |
U-7Mo is a representative low-enriched uranium (LEU) alloy widely applied in research and test reactors. The thermal conductivity and mechanical integrity of U–7Mo strongly depend on grain size and distribution, necessitating a precise understanding of grain growth behavior. In practical fuel design, U–7Mo exists as dispersed particles embedded in an Al matrix. During annealing, interdiffusion between U–7Mo and Al produces interaction layers that degrade fuel performance. To mitigate formation of interaction layers, protective Si or N coatings are employed. However, diffusion of Si and N into U–7Mo generates second-phase particles that exert a Zener pinning effect, retarding grain boundary migration. Experimental observations indicate that annealing at 1000 ◦C for 1 h results in grain growth from 3 μm to 32 μm in uncoated U–7Mo particles, while Si-coated particles show limited growth up to 15 μm. Furthermore, atomization during fuel fabrication induces heterogeneous cooling rates, frequently leading to a bimodal initial grain size distribution. Such bimodal structures are expected to significantly modify the interaction between grain boundaries and second-phase particles, thereby influencing the kinetics of grain growth under Zener pinning conditions. In this study, a phase-field model incorporating the effects of second-phase particles is developed to simulate grain growth in U–7Mo particles with bimodal initial grain size. The simulations provide mechanistic insight into the interplay between bimodal grain size distribution and Zener pinning, offering guidance for microstructural optimization strategies aimed at improving the stability and performance of U–7Mo nuclear fuel. |