Hydrogen is considered one of the major problems for ferritic and martensitic steel structures in future fusion reactors. In contrast to hydrogen from other sources, hydrogen produced by nuclear transmutations cannot be kept away by barriers but must be drained off through the surfaces. An upper limit of the diffusion distance is derived at which the stationary concentration of hydrogen stays below the critical concentration for hydrogen embrittlement. In addition a lower limit for the effusion time is given that is needed to reduce the hydrogen concentration below a certain level during shutdown periods. Similar considerations are applied to the target of a planned spallation neutron source.