Modeling of H/D isotope-exchange in crystalline beryllium

•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.