State of helium in irradiated beryllium

Experimental data are presented on the change of the pycnometric density and unit-cell volume of beryllium irradiated in an SM reactor to neutron fluence 14·10²² cm^–2. The behavior of the unit-cell volume as a function of the neutron fluence is similar to the data for other metals. This and data on the volume change in beryllium oxide, where a large amount of gas is also formed during irradiation, make it possible to evaluate critically the assertion that helium forms a solid substitution solution. It is proposed that helium is confined in small accumulations.