Research on Fuel Pebble Modeling Methods Comparison and Eigenvalue Analysis of the Running-in Phase of HTR-PM

Abstract The High-Temperature Gas-Cooled Pebble bed Reactor (HTR) utilizes fuel pebbles that consist of an outer graphite shell and a fuel region with thousands of TRi-structural ISOtropic (TRISO) particles dispersing in a graphite matrix, causing the double heterogeneity effect, and presenting computational challenges for neutronics simulations. The Monte Carlo (MC) method is considered a powerful tool for neutronics analysis of complicated reactor design, but it is computationally expensive and inefficient for large-scale systems. This study calculates and compares different fuel pebble modeling methods to balance accuracy and efficiency. Models ranging from high-fidelity to various homogenized approaches are assessed. Results indicate the Reactivity-equivalent Physical Transformation (RPT) model optimally captures heterogeneous effects while reducing the computational cost. Based on RPT, eigenvalue analysis is performed to simulate the running-in phase of the HTR-PM. A linear correlation is observed between the descending surface height and keff, which is consistent with the phenomenon in reactors of smaller scales like HTR-10. This comparative modeling and analysis provide insights into fuel pebble homogenization and support the feasibility of applying proper fuel pebble modeling to full-core HTR design and simulation.