STUDY ON THE UNDERLYING PHYSICAL MECHANISMS AND INFLUENCING FACTORS OF FISSION GAS SWELLING AND RELEASE IN NUCLEAR FUEL

Fission gas swelling and release behavior is one of the most critical issues in the design of nuclear fuel components, so it is of great significance to elucidate the underlying physical mechanisms through mechanistic modeling and numerical simulation. Based on a multi-field coupled model and computational method integrating the vacancy diffusion field and the mechanical field in polycrystalline nuclear fuel, this study conducts finite element simulations of mesoscale vacancy diffusion, dislocation motion, and creep deformation in nuclear fuel containing grain-boundary bubbles, with the objective of quantitatively describing the interactions among these coupled physical processes. It systematically analyzes the multi-scale correlation mechanisms and key influencing factors for the coupled fission gas swelling-release-creep behavior over a wide range of microstructural parameters and loading conditions. The numerical simulation results reveal that the internal pressure of fission bubbles creates a chemical potential difference between the grain boundaries and the regions surrounding the bubbles, driving vacancies to diffuse from the grain boundaries to the bubble edges. This consequently induces diffusion creep and dislocation creep deformation in the fuel skeleton around the bubbles, characterized by circumferential elongation and radial contraction. It is indicated that the fuel skeleton creep is a key physical mechanism governing the growth and interconnection of fission bubbles, playing a crucial role in fission gas swelling and release behavior. The vacancy diffusion in the skeleton around the bubbles, along with the induced diffusion creep and dislocation creep, is significantly influenced by the internal bubble pressure, macroscopic external pressure, and porosity, which in turn dominates the dynamic evolution of fission gas swelling and release behavior, representing the critical factors to be addressed in theoretical modeling. This study establishes a theoretical foundation for developing mechanistic models of fission gas swelling and release, thereby providing fundamental support for the safety design and performance evaluation of in-core nuclear materials and components.