Knowledge of the state and behaviour of helium in reactor components is needed in order to understand such phenomena as void swelling and high temperature radiation embrittlement. The X-ray diffraction method of lattice parameter measurement can provide useful information about the state of helium in radiation damaged lattices of metals if the effects of irradiation induced lattice defects—vacancies and interstitials as well as their agglomerates—on lattice parameter changes are taken into account.