We have studied the energetics and mobility of hydrogen in the bulk and along the rhombohedral Sigma 7 grain boundary (GB) of alpha-Al2O3 via first-principles calculations based on the projector-augmented wave method. The temperature-dependent diffusivities D(T) in the alpha-Al2O3 bulk and along the Sigma 7 GB are derived. The ratio shows that the diffusivity along the GB is 2-5 orders of magnitude greater than in the bulk at temperatures in the range of 273-973 K, revealing that the diffusion along the GB is the underlying reason for the experimentally observed permeation reduction factor being much lower than the anticipated value. Moreover, the calculations also reveal that radiation-induced O vacancies tend to aggregate to the GB plane, thereby forming a zigzag O vacancy chain. In such circumstances, however, the calculated migration energy for hydrogen diffusion along the O vacancy chain is 2.58 eV, which is much greater than that of 1.10 eV and 0.81 eV for H diffusion in the bulk and along the GB, respectively. This finding suggests that O vacancies in the GB trap hydrogen atoms and prevent their diffusion along the GB. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.