Study on Hydrogen Redistribution in Fuel of Xi'an Pulsed Reactor Based on RMC and COMSOL

[Background] Core simulation is essential in nuclear reactor engineering design, involving coupled multiphysics processes such as neutron transport, fluid flow, and heat transfer. The Xi'an Pulsed Reactor (XAPR) utilizes uranium-zirconium hydride (UZrHₓ) fuel, in which high-temperature hydrogen behavior significantly affects neutronic and thermal performance. However, existing studies have not sufficiently addressed the coupling effects between hydrogen migration and core physical properties, especially under combined temperature and concentration gradients, which limits accurate safety assessment and fuel optimization. [Purpose] This study aimed to develop a high-fidelity multiphysics framework coupling neutronics, thermal-hydraulics, and hydrogen diffusion to investigate hydrogen redistribution behavior in XAPR fuel and its feedback effects on reactor performance. [Methods] A coupled modeling approach was established by integrating the Reactor Monte Carlo Code (RMC) for neutron transport simulation with COMSOL Multiphysics for thermal-hydraulic and hydrogen diffusion analysis. The core was spatially discretized into 3030 units. A one-dimensional single-channel model was used for coolant flow, and a three-dimensional heat conduction model was applied to the solid fuel  region. Hydrogen diffusion in UZrHₓ was modeled using a simplified flux equation incorporating Fickian diffusion and Soret thermodiffusion. The model was validated against classical experimental data from Huangs et al. (2000) and compared with CFD simulations for a 3×3 fuel assembly.[Results] Hydrogen redistribution was found to be jointly governed by temperature and concentration fields. Under 2 MW steady-state operation, hydrogen migrated from high-temperature regions toward cooler areas, resulting in a radial H/Zr ratio ranging from 1.46 at the fuel center to 1.96 near the cladding. The effective multiplication factor (kₑff) decreased from 1.013 817 to 1.001 202 after one coupling iteration, indicating a clear negative reactivity feedback due to hydrogen migration. The maximum fuel temperature decreased by 3.1%, and the calculated temperature distribution in the instrumented fuel element C10 agreed with experimental data within a 1.2% deviation. [Conclusions] Hydrogen redistribution in UZrHₓ fuel is a coupled thermodiffusion process that significantly influences local neutronic and thermal-hydraulic behavior. The established RMC-COMSOL framework provides an effective tool for evaluating hydrogen management strategies in hydride-moderated reactors and offers critical theoretical and data support for fuel design optimization and safety assessment under long-term high-power operating conditions.