The release of helium in irradiated uranium dioxide plays an important role in the mechanical stability of the nuclear fuel during and after its use in nuclear power plants. A large amount of helium is formed during the long-term storage of the fuel due the alpha decay of transuranium nuclei, which are created during in-reactor operation. The migration mechanisms of helium in UO₂ matrix were already the subject of theoretical studies but there is a lack of experimental data on the possible helium positions in the crystal lattice.

The behavior of helium atoms implanted at low fluence in UO₂ single crystals was investigated before and after thermal annealing in the temperature range 873-1073 K. UO₂ single crystals were implanted with ³He ions and the location of helium in the lattice was revealed by NRA with deuterons in the channelling mode by recording angular scans across major axes and planes. The uranium sublattice was analysed by the classical RBS method, while the presence of ³He was probed by the nuclear reaction ³He(d,p)⁴He.         

Results show that the majority of helium atoms occupy octahedral sites of the fluorite-type lattice after implantation. These experimental results were confirmed by Monte Carlo simulations of the channeling dips and crystal visualizations with the Diamond software. This study also shows that helium atoms are mainly located in octahedral sites up to a temperature of 873 K. At higher temperatures (1073 K) helium atoms appear to be randomly distributed in the crystal lattice. This behavior may be a signature of the formation of helium clusters.