An increased knowledge of the chemistry of PuO2 is imperative for the design of procedures to store, dispose, or make use of PuO2. In this work, point defect concentrations in...

Plutonium (Pu) has been released to the environment worldwide, including approximately 1.85 × 1015 Bq (200 kg) of Pu from process waste solutions to unconfined soil structures at the Hanford Site in Washington State. The subsurface mobility of Pu is influenced by complex interactions with sediments, groundwater, and any co-contaminants within the waste stream. Previous investigations at Hanford have shown that Pu exists as discrete PuO2 particles forming before or after disposal, as secondary solid phases formed from waste interactions with sediments as adsorbed/incorporated species, and/or as dissolved species. In this research, new evidence is presented for the existence of PuO2, PuO2-Bi2O3 composites, and particles from burnt Pu metal in near-surface sediments where Pu-laden acidic process waste was disposed to sediments. Pu and americium (Am) L 3 X-ray absorption spectroscopy and density functional theory suggest that, in larger, more crystalline PuO2 particles, Am formed from radioactive decay is retained in the PuIVO2 structure as AmIV. The Pu and Am that were disposed of in an acidic waste stream have since migrated deeper into the subsurface with detection to at least 37 meters below ground surface. In contrast, Pu deposited near the ground surface from neutral pH waste is found to be homogeneously distributed and relatively immobile. Groundwater extractions performed on contaminated sediments indicate that both Pu and Am are recalcitrant, with Am being fractionally less extractable than Pu on a molar basis. These results suggest that the more mobile fraction of Am has migrated from the near-surface and may be present in the deeper sediments as a different phase than Pu. From these results, it is suggested that Pu and Am deposited from acidic wastes were initially mobile and became significantly less mobile as wastes were neutralized within the soil profile.

The β decay of 241 Pu to 241 Am results in a significant ingrowth of Am during the interim storage of PuO 2 . Consequently, the safe storage of the large stockpiles of separated Pu requires an understanding of how this ingrowth affects the chemistry of PuO 2 . This work combines density functional theory (DFT) defect energies and empirical potential calculations of vibrational entropies to create a point defect model to predict how the defect chemistry of PuO 2 evolves due to the incorporation of Am. The model predicts that Am occupies Pu sites in (Pu,Am)O 2± x in either the +III or +IV oxidation state. High temperatures, low oxygen-to-metal (O/M) ratios, or low Am concentrations favor Am in the +III oxidation state. Am (+III) exists in (Pu,Am)O 2± x as the negatively charged (Am Pu 1– ) defect, requiring charge compensation from holes in the valence band, thereby increasing the conductivity of the material compared to Am-free PuO 2 . Oxygen vacancies take over as the charge compensation mechanism at low O/M ratios. In (Pu,Am)O 2± x , hypo- and (negligible) hyperstoichiometry is found to be provided by the doubly charged oxygen vacancy (V O 2+ ) and singly charged oxygen interstitial (O i 1– ), respectively.

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...

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