Multiphysics analysis of hydrogen kinetics and their effects in UZrHx-fueled reactors

Hydrogen kinetics within the Uranium–zirconium hydride (UZrH x ) fuel, including redistribution, dissociation, and permeation loss, can alter local moderating capability and material properties, thereby affecting core neutronics and safety characteristics. In this work, a hydrogen kinetics model was developed and integrated into the deterministic code system SARAX to establish a coupled neutronic-thermal-hydrogen kinetics analysis tool. Hydrogen redistribution, dissociation, and permeation loss were simulated at the whole-core scale over the full reactor lifetime and their effects on neutron science and thermal parameters were quantitatively evaluated. The results show that temperature gradients drive pronounced hydrogen migration from hotter regions to colder regions, leading to a nonuniform hydrogen distribution and a corresponding change in local moderating capability. Axial hydrogen redistribution shifts the axial power profile toward the lower core region with higher hydrogen concentration. Radial redistribution increases the H/Zr ratio at the outer solid–gas interface of the fuel pellet, thereby increasing the local equilibrium hydrogen pressure and promoting subsequent permeation loss. The core-averaged H/Zr ratio decreases nearly linearly from 1.6 to 1.5763 by the end of life, while the minimum local H/Zr ratio remains within the δ-ZrH x phase field under the corresponding temperature condition. Axial hydrogen redistribution alone increases k eff by approximately 49 pcm, whereas hydrogen permeation loss decreases k eff by approximately 489 pcm. Under their combined effects, k eff decreases by approximately 447 pcm over the reactor lifetime. The present results provide a computational basis for the design and safety evaluation of UZrH x -fueled microreactors.