Accommodation of helium in PuO_2± and the role of americium

The high alpha-activity of plutonium dioxide (PuO₂) results in significant ingrowth of radiogenic helium (He) in the aged material. To safely store/dispose PuO₂ or use in applications such as space exploration, the impact of He accumulation needs to be understood. In this work, defect energies obtained using a density functional theory (DFT) + U + D3 scheme are used in a point defect model constructed for PuO₂ to predict the method of He incorporation within the PuO₂ lattice. The simulations predict that the preferred incorporation site for He in PuO₂ is a plutonium vacancy, however, the point defect model indicates that helium will be accommodated as an interstitial irrespective of He concentration and across a wide stoichiometric range. By considering the charge imbalance that arises due to incorporation of Am³⁺ ions it is shown that He accommodation in oxygen vacancy sites will dominate in PuO_2- as the material ages.