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 14·10 22 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.The unique nuclear-physical and physical-mechanical properties of beryllium make this element irreplaceable as a material for neutron moderators and reflectors. Numerous studies of beryllium articles and specially prepared samples irradiated to neutron fluence >10 23 cm -2 (E > 0.1 MeV) have now been performed. The changes in the material properties, information on which is needed to establish safe service lifetimes, have been studied. One such property is swelling -an increase of the volume of an article and decrease of the pycnometric density.It is well known that the swelling of irradiated materials can be substantial. Under irradiation, the density AlGdO 3 , Dy 2 TiO 5 , and BN changes by 6.5, -4.2, and ~30%, respectively [1]. The pycnometric density of diamond decreases to 47% and the unit-cell volume increases by 20% (up to the moment of amorphization) [2]. The masonry graphite at the Leningradskaya nuclear power plant showed a 13% reduction of the pycnometric density and the density measured according to the change in volume of the unit cell. It is known that corrosion resistant steel can swell by 20% under certain conditions. Aside from structural changes swelling can be due to vacancy accumulation and a change of the coordination number and therefore the character of the bonding force at the site of a single defect (vacancy or interstitial), for example, the model of the appearance of "graphite structure motifs" in diamond [3]. In these cases, large swelling was determined by the intrinsic defects of the material -components of Frenkel pairs, which arise as a result of the interaction of fast neutrons with the crystal lattice. Aside from Frenkel pairs, helium and tritium form in beryllium-containing materials. For example, the concentration of helium atoms is 12000 ppm, i.e., 1.2%, under fluence 6·10 22 cm -2 (E > 0.1 MeV) and increases to 1.7% under fluence 13·10 22 cm -2 [4]. Where is this helium? How does it affect swelling?Researchers studying beryllium oxide, which can be used as a reflector and moderator, were also faced with this problem. Extensive research done on BeO at the Research Institute for Atomic Reactors (NIIAR) has shown that 16% swelling under irradiation to fluence 1·10 22 cm -2 (irradiation temperature <200°C) is "solid," because the relative increase of the volume of the material is equal to the relative increase of the unit-cell volume. In addition, there are no grounds for attributing it to helium accumulation. Figure 1 sh...