Comprehensive modeling of hydrogen transport and accumulation in titanium and zirconium

We developed kinetic models of hydrogen absorption in Ti and Zr. The models comprise series connections of the hydrogen-transport processes of surface dissociative adsorption and recombinative desorption; subsurface transport; and bulk diffusion. Numerical calculations using the models quantitatively reproduce the results of experimental series of time-transient absorption curves at various temperatures, demonstrating the validity of our models. Experimental desorption curves at various temperatures are also reproduced by the same model equations and kinetic parameters, particularly for Zr, demonstrating the dual functionality of our single model for hydrogen-transport directions. We use an effectiveness factor defined as the ratio between the real absorption rate and the virtual rate neglecting bulk diffusion. The transitions of the rate-determining steps of hydrogen transport in Ti and Zr under various conditions – such as temperature, pressure, and metal object size and shape – are systematically analyzed. As a case study to test the applicability of our model, hydrogen accumulation in the fuel claddings of light-water nuclear reactors was simulated to determine the cladding thickness required to prevent hydrogen embrittlement during the practical operation period. Our versatile kinetic models could be a useful tool that can aid the structural design and optimization of nuclear materials and facilities

A Heterothermic Kinetic Model of Hydrogen Absorption in Metals with Subsurface Transport

A versatile numerical model for hydrogen absorption into metals was developed. Our model addresses the kinetics of surface adsorption, subsurface transport (which plays an important role for metals with active surfaces), and bulk diffusion processes. This model can allow researchers to perform simulations for various conditions, such as different material species, dimensions, structures, and operating conditions. Furthermore, our calculation scheme reflects the relationship between the temperature changes in metals caused by the heat of adsorption and absorption and the temperature-dependent kinetic parameters for simulation precision purposes. We demonstrated the numerical fitting of the experimental data for various Pd temperatures and sizes, with a single set of kinetic parameters, to determine the unknown kinetic constants. Using the developed model and determined kinetic constants, the transitions of the rate-determining steps on the conditions of metal-hydrogen systems are systematically analyzed. Conventionally, the temperature change of metals during hydrogen adsorption and absorption has not been a favorable phenomenon because it can cause errors when numerically estimating the hydrogen absorption rates. However, by our calculation scheme, the experimental data obtained under temperature changing conditions can be positively used for parameter fitting to efficiently and accurately determine the kinetic constants of the absorption process, even from a small number of experimental runs. In addition, we defined an effectiveness factor as the ratio between the actual absorption rate and the virtually calculated non-bulk-diffusion-controlled rate, to evaluate the quantitative influence of each individual transport process on the overall absorption process. Our model and calculation scheme may be a useful tool for designing high-performance hydrogen storage systems

Spatially and temporally heterothermic kinetic model of hydrogen absorption and desorption in metals with heat transport

Numerical modelling of hydrogen transport is effective for designing and optimizing various energy systems, including hydrogen storage devices, fuel cells, and nuclear fusion reactors. In the present study, we propose and demonstrate a spatiotemporally heterothermic, autonomous kinetic model of hydrogen absorption and desorption in metals for precise simulations. Our bidirectional transport model comprises elementary mass transfer processes of surface adsorption and desorption, subsurface transport, and bulk diffusion. Also implemented are heat generation and conduction stemming from the absorption enthalpy, to determine the evolution of temperature distribution in the metal body, as well as the hydrogen concentration profile. Simulations by our transport model reproduce experimental hydrogen absorption and desorption curves for various temperature levels and metal scales with a single identical set of numerical equations and kinetic parameters, to thus verify the validity of the model