Research on thermo-hydro-mechanical coupling characteristics in lead-bismuth cooled inverted fuel assemblies

As an innovative geometry for nuclear reactor cores, the inverted fuel assembly meets key design requirements for small modular lead-bismuth fast reactors, such as long lifetime, high maneuverability, and light weight. The unique flow and heat transfer behaviors of lead-bismuth coolant, combined with the inverted assembly configuration, induce complex thermo-hydro-mechanical coupling effects. A thorough understanding of these interactions is essential for ensuring reactor safety, improving thermal efficiency, and supporting long-life operation. This study investigates the thermo-hydro-mechanical characteristics of an inverted fuel assembly designed for the URANUS core. By utilizing geometric and material models along with a lead-bismuth flow model, this study establishes a steady-state one-way coupling framework in ANSYS Workbench to compare vertical and horizontal assembly configurations in terms of temperature, flow velocity, and thermal stress. Results show that assembly orientation has little influence on fuel temperature distribution: the peak fuel temperature is 1161K in the vertical assembly and 1150K in the horizontal one. However, buoyancy causes noticeable temperature and velocity stratification in the horizontal assembly, where the lowest temperature and highest velocity points at the outlet shift downward by about 50% of the pipe radius relative to the pipe axis. In contrast, both points remain on the axis in the vertical case. This stratification leads to a coolant temperature difference of 133K in the horizontal assembly, significantly larger than the 86K observed in the vertical one. Thermal stress analysis shows that the maximum deformation of the horizontal assembly is 0.90mm, which is greater than the 0.61mm of the vertical assembly, and the maximum stress in both assemblies is concentrated at the junction of the fixed section and the free section. This work provides valuable data for optimizing the design and supporting the engineering applications of lead-bismuth cooled inverted fuel assemblies.