Modeling stress-driven diffusion of hydrogen in irradiated liner claddings using OFFBEAT

To ensure the safe long-term dry storage of used nuclear fuel in Switzerland, PSI investigates hydrogen behavior in Zircaloy liner claddings, commonly used in Swiss reactors. This study focuses on irradiated DX D4 cladding, consisting of a Zircaloy-4 substrate and an outer liner with low tin concentration, and examines hydrogen redistribution during thermo-mechanical treatments using the OFFBEAT fuel performance code. In this work, two irradiated samples of the cladding were simulated and compared to experimental results: one submitted to a thermal treatment, and the other additionally subjected to stress under mechanical loading. While the thermal-only case aligned with experimental data, the stress-influenced simulation diverged. To identify causes, parametric studies varying mechanical properties, hydrogen solubility (TSS), and diffusion parameters were carried out. Results indicate that TSS correlations have a greater impact than mechanical assumptions. Despite hydrogen profile mismatches in liner, predictions in the substrate remained robust. Moreover, the characteristic migration of hydrogen toward the liner is well captured. These findings contribute to understanding hydrogen behavior in duplex claddings and guide future model development for fuel performance under extended dry storage.