Assessment of multiscale hydrogen desorption models from (0001) Be surfaces

Beryllium is a proposed neutron multiplier material in future fusion devices like ITER and DEMO. As such, numerous studies of neutron-irradiated beryllium specimen have shown the appearance of facetted bubbles with considerable tritium accumulation at their (0001) surfaces. In this work, we thus simulate hydrogen isotope desorption from (0001) beryllium surfaces using a multiscale modeling approach. We first establish that very fast surface diffusion of hydrogen adsorbed in FCC and HCP sites can be studied independently from the bulk by means of static DFT calculations, as diffusion into the bulk is inhibited by multiple diffusion processes with an accumulated energy barrier of almost 3 eV. Static saddle point searching dimer DFT calculations also reveal energy barriers for associative desorption in a range between 0.94 eV and 1.52 eV due to varying amounts of co-adsorbed hydrogen isotopes in the immediate vicinity of the desorbing pair. A simple rate equation-based desorption model incorporating these associative desorption processes is constructed and compared to thermal programmed desorption (TPD) spectra of earlier exposure experiments. It is found that the simple rate equation-based desorption model seemingly already explains some characteristic aspects of the TPD spectra like a peak shift to lower temperatures by almost 100 K and considerable broadening with increasing coverage. To asses the merit of this model, a refined model effectively taking the repulsion among adsorbed hydrogen isotopes into account by means of relevant correlations from kinetic Monte Carlo (KMC) simulations using a cluster expansion (CE) truncation is considered as well. It is found that the simple model’s reproduction of characteristic features of experimental spectra is the result of error cancellation from considering an incomplete set of desorption processes on one hand and neglecting repulsion among adsorbed hydrogen on the other hand. We conclude that the experimental TPD spectra can not be fully explained by either of the two models and further research particularly considering additional adatoms like oxygen are necessary to understand the TPD spectra of the corresponding experiments.