Helium is produced in the fuel pellets/bars during irradiation via three mechanisms: (i) ternary fission; (ii) neutron capture by 1 6O (in oxide fuel); and (iii) alpha decay of transuranic isotopes, principally 242Cm (Kamimura et al., 1999). The generated helium atoms can then diffuse to intra-granular and grain face fission gas bubbles and to fuel surfaces in the same way as xenon and krypton fission gas atoms. However, due to the small size of the helium atoms, diffusion (at least in oxide fuels) is more rapid than that of fission gas, and so the fraction of the generated helium that is released is larger than that for fission gas. In plutonium-free fuels the helium generation due to (iii) is negligible and so helium generation and release can generally be ignored. In contrast, in plutonium-bearing fuels the helium generation due to (iii) can be significant, and helium release can provide an important contribution to the pressurisation of the fuel pin.