When boron carbide is used as a neutron absorber in nuclear power plants, large quantities of helium are produced. To simulate the gas behaviour, helium implantations were carried out in boron carbide. The samples were then annealed up to 1500 °C in order to observe the influence of temperature and duration of annealing. The determination of the helium diffusion coefficient was carried out using the 3 He(d,p) 4 He nuclear reaction (NRA method). From the evolution of the width of implanted 3 He helium profiles (fluence 1 Â 10 15 /cm 2 , 3 MeV corresponding to a maximum helium concentration of about 10 20 /cm 3) as a function of annealing temperatures, an Arrhenius diagram was plotted and an apparent diffusion coefficient was deduced (E a = 0.52 ± 0.11 eV/atom). The dynamic of helium clusters was observed by transmission electron microscopy (TEM) of samples implanted with 1.5 Â 10 16 /cm 2 , 2.8 to 3 MeV 4 He ions, leading to an implanted slab about 1 mm wide with a maximum helium concentration of about 10 21 /cm 3. After annealing at 900 °C and 1100 °C, small (5–20 nm) flat oriented bubbles appeared in the grain, then at the grain boundaries. At 1500 °C, due to long-range diffusion, intra-granular bubbles were no longer observed; helium segregates at the grain boundaries, either as bubbles or inducing grain boundaries opening.