The high-energy, high-intensity neutron fluxes produced by the fusion plasma will have a significant life-limiting impact on reactor components in both experimental and commercial fusion devices. As well as producing defects, the neutrons bombarding the materials initiate nuclear reactions, leading to transmutation of the elemental atoms. Products of many of these reactions are gases, particularly helium and hydrogen, which cause swelling and embrittlement of materials. This paper investigates, using both neutron-transport and inventory calculations, the variation in nuclear transmutation and gas production rates at various locations of a conceptual design of the next-step fusion device DEMO. Modelling of grain structures and gas diffusion rates illustrates that the timescale for susceptibility to helium embrittlement varies widely between different materials, and between the same materials situated at different locations in the DEMO structure.