Abstract
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. On the basis of comparison of TEM results on damage microstructures in molybdenum uniformly doped with helium by alpha-particle bombardment and those of neutron irradiated, the effect of helium state on molybdenum lattice parameter change has been extracted. It has been shown that at relatively low concentrations helium forms complexes with vacancies, HemVn, where m < n. As the concentration increases, this correlation between vacancy and helium atoms numbers changes to m>n, leading to considerable expansion of the lattice.