Modeling of H/D isotope-exchange in crystalline beryllium. (August 2019)

• Record Type: Journal Article
• Authors: Matveev, D.; Hansen, P.; Dittmar, T.; Koslowski, H.R.; Linsmeier, Ch.
• Is Part Of: Nuclear materials and energy. Volume 20(2019)
• Publication Date: 2019-08
• Subjects: Beryllium -- Isotope-exchange -- Hydrogen retention -- Reaction-diffusion -- TDS; Nuclear energy -- Periodicals; Nuclear fuels -- Periodicals; Nuclear reactors -- Materials -- Periodicals; Radioactive substances -- Periodicals; 621.4833
• DOI: 10.1016/j.nme.2019.100682
• Languages: English
• ISSNs: 2352-1791

Abstract

Highlights: Isotope-exchange experiments on polycrystalline beryllium are performed. Second implanted isotope dominates the low temperature desorption stage, both isotopes are rather equally present in the high temperature desorption stage. Reaction-diffusion modelling with multiple-trapping and surface effects for two isotopes is described. Modelling results give qualitative picture of possible mechanisms responsible for experimentally observed isotope-exchange effects. Abstract: A reaction-diffusion model with surface occupation dependent desorption [D. Matveev et al., Nucl. Instr. Meth. B 430 (2018) 23–30] has been updated to handle multiple hydrogen species to simulate hydrogen/deuterium isotope-exchange experiments performed on polycrystalline beryllium samples under ultra-high vacuum laboratory conditions. In the experiments subsequent exposures of a sample to hydrogen and deuterium ion beams in direct and reverse implantation order were followed by thermal desorption spectroscopy measurements under a constant heating rate of 0.7 K/s. The recorded signals of masses 2 to 4 (H2, HD and D2 ) indicate that the second implanted isotope dominates clearly the low temperature release stage ( ≈ 450 K), while both isotopes show a comparable contribution to the high temperature desorption stage ( ≈ 700 K) with only minor effect of the implantation order attributed to a slightly deeper penetration of deuterium compared to hydrogen. Simulations of the implantation and subsequent thermal desorption of hydrogen isotopes are performed to assess the atomic processes behind the isotope-exchange. Simulations were performed under the assumption that the low temperature release stage is attributed to hydrogen/deuterium atoms retained on effective open surfaces (e.g. interconnected porosity) represented in the simulations by a surface with an effective surface area exceeding the nominal exposed surface area by a factor up to 100. Kinetic de-trapping from vacancies with multiple trapping levels and enhanced desorption at surface coverages close to saturation are addressed in the model as possible mechanisms promoting the isotope-exchange. Simulation results suggest the applicability of the model to describe isotope-exchange processes in crystalline beryllium and give a qualitative explanation of the observed experimental facts.