  UO2 and (U,Pu)O2 solid solutions (the so-called MOX) nowadays are used as
commercial nuclear fuels in many countries. One of the safety issues during the
storage of these fuels is related to their self-irradiation that produces and
accumulates point defects and helium therein. We present density functional
theory (DFT) calculations for UO2, PuO2 and MOX containing He atoms in
octahedral interstitial positions. In particular, we calculated basic MOX
properties and He incorporation energies as functions of Pu concentration
within the spin-polarized, generalized gradient approximation (GGA) DFT
calculations. We also included the on-site electron correlation corrections
using the Hubbard model (in the framework of the so-called DFT+U approach). We
found that PuO2 remains semiconducting with He in the octahedral position while
UO2 requires a specific lattice distortion. Both materials reveal a positive
energy for He incorporation, which, therefore, is an exothermic process. The He
incorporation energy increases with the Pu concentration in the MOX fuel.
