Large-scale density functional theory simulations of defects and hydrogen incorporation in PuO2

We have examined a range of point defects, Frenkel pairs, Schottky defects, and hydrogen-related defects in the PuO2 system (supercells of 96 and 768 atoms) using the onetep linear-scaling density functional theory code. Vacancy point defects related to oxygen are found to be more stable than those related to plutonium. The oxygen in the octahedral interstitial is higher in the formation energy than the plutonium in the same octahedral site, although the difference is less than 1 eV. We were also able to identify a stable peroxide species (1.57–2.67 eV) with a O-O distance of 1.46 Å. Of the Frenkel defects we studied, we found that the oxygen is more stable than the plutonium, whereas the Schottky stability changes as a function of supercell size. Finally, we examined a number of likely hydrogen sites in the PuO2 lattice: octahedral interstitial, oxygen edge, hydroxyl, oxygen vacancy, and plutonium vacancy. We report hydrogen which exists as a hydride at oxygen and plutonium vacancies to be relatively high in energy (2.69–3.81 and 13.71–15.54 eV, respectively). The hydrogen was found to exist as a radical at the octahedral interstitial site (2.43–3.38 eV) and which is somewhat higher formation energy than other studies find. We find that the hydrogen at the oxygen edge (as a H+ cation) and at the oxygen cube corner (as a hydroxyl) are both lower in energy (1.14–1.40 and 1.17–1.56 eV, respectively) as opposed to hydrogen in the octahedral interstitial site but again higher than found by other studies. We discuss the data in the context of potential hydrogen transport pathways and how that might be modified by radiation damage