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Titanium beryllide, Be 12 Ti, has been proposed as a prospective neutron multiplier in fusion reactors. First-principles calculations have been performed to investigate the nucleation mechanism of a He bubble in bulk Be 12 Ti. Meanwhile, the influence of the presence of H atoms on the nucleation of the He bubble, i.e. , the synergistic effect of He and H atoms, has also been investigated. It has been found that the He bubble will initially nucleate around a monovacancy (V Be2 ). When more He atoms have been implanted, two newly induced vacancies (V Be1 and V Be3 ) could be successively observed. The nucleation of the He bubble will occur around the divacancy of V Be2 V Be1 and the trivacancy of V Be2 V Be1 V Be3 . Dumbbell structures in the He bubble evolve with the number of implanted He atoms and finally disappear. The presence of H atoms will significantly influence the nucleation of the He bubble. It is interesting that some tetrahedral and octahedral structures have also been observed. The maximal number of H atoms trapped by a He bubble has been obtained. These phenomena could be further explained by the continuous shrinking of the isosurface of charge density. The present results provide a microscopic physical foundation to understand the mechanism of He and H atoms retention in neutron multiplier materials. This investigation could be helpful for the design and fabrication of more promising beryllides which could withstand a severe external environment., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)