Modeling hydrogen isotope transport behavior in the first wall: role of the bonding region in W-RAFM steel with an improved evaluation of transport coefficients

Understanding the hydrogen isotope transport behavior (HITB) in the first wall (FW) is significant whereas simulations have been extensively employed. Most previous simulations use hydrogen transport parameters of pure tungsten (W) and reduced activation ferritic/martensitic (RAFM) steel, while largely neglecting hydrogen diffusion at their interface. Joining W and RAFM steel inevitably leads to the formation of a bonding region characterized by elemental interdiffusion, Fe-W binary phase precipitates, and increased densities of dislocations and microvoids. The HITB of the bonding region where complex microstructural features and trapping mechanisms coexist remains insufficiently understood. Therefore, this work develops a comprehensive approach to quantify the influence of the bonding region on the HITB in the FW by explicitly incorporating both diffusion and trapping effects. The diffusivity of the bonding region is obtained by minimizing the deviation between gas-driven permeation (GDP) experimental data and simulation results, while trap properties are evaluated using macroscopic rate equation modeling based on thermal desorption spectroscopy (TDS) data. Results show that the diffusivity of the bonding region lies between that of pure W and RAFM steel, and its trap density is at least an order of magnitude higher than that of pure W and RAFM steel. The permeation flux is delayed by the bonding region. Simulated retention indicates about 20% higher retention as considering the bonding region in the FW model, however the overall retention in the FW is not significantly affected due to the limited thickness of the bonding region. Additionally, to improve the predictive accuracy of the HITB model, the quasi-intrinsic diffusion coefficient () of RAFM steel is obtained. This is achieved by employing an error minimization function that adjusts the value of to minimize the discrepancy between the simulated permeation flux and the experimental flux obtained from GDP measurements. This study establishes a refined HITB model for the FW.