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

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.