Accommodation of helium in PuO2±x 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 PuO2-x as the material ages.

Automated summary

This study investigates the incorporation of helium in plutonium dioxide using density functional theory and a point defect model. The simulations suggest that helium is mainly accommodated as an interstitial in the lattice, and its preference for oxygen vacancy sites increases with material ageing.