Influences on Subsurface Plutonium and Americium Migration

Abstract

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.

Evolving Defect Chemistry of (Pu,Am)O2±x

“…The Am (+IV)/Am (+III) ratio increases with decreasing temperature, and at low temperatures, the Am in (Pu 1– y Am y )O 2± x is composed entirely of Am (+IV). This is supported by the recent finding of Emerson et al 8 who measured the Am L 3 -edge X-ray absorption near-edge structure (XANES) spectrum of aged PuO 2 samples finding a spectrum characteristic of Am 4+ O 2 .…”

“…Globally, significant amounts of Pu exist in the environment, a proportion of which is the form of PuO 2 . The subsurface mobility of the material is very complex and is likely impacted by the accumulation of Am, which has a different environmental mobility. , 238 PuO 2 is also the most commonly used isotope in radioisotope thermoelectric generators and heating units for space applications .…”

“…Globally, significant amounts of Pu exist in the environment, a proportion of which is the form of PuO 2 . The subsurface mobility of the material is very complex and is likely impacted by the accumulation of Am, which has a different environmental mobility. , 238 PuO 2 is also the most commonly used isotope in radioisotope thermoelectric generators and heating units for space applications . A better understanding of how PuO 2 continues to evolve and accommodates Am growth is, therefore, of widespread interest.…”

Subsurface Transport of Plutonium in Organic and Aqueous Acidic Processing Wastes at the Hanford Site, USA

“…25,27,28 Pu particles in the shallow sediments beneath the trench are likely undissolved Pu scrap metal, precipitated PuO 2 , and Pu iron phosphates. 3,12,19,27 Core samples show the presence of a pH gradient beneath the trench, with low pH (pH ∼ 2), acid-weathered sediments in the shallow region beneath the trench, and unaltered sediments (pH 7−9) deeper in the cores. The cores also show that Pu has migrated in the vadose zone to depths of at least 37 m below ground surface (bgs) and that high Pu concentrations (4 × 10 9 Bq/kg soil) exist in the sediments with the lowest pH (pH ∼ 3.9), and in locations where TBP is also present.…”

“…Early studies gave assurance that Pu migration was limited but increased with decreasing pH and in the presence of TBP and DBBP. , However, the factors that influence both Pu mobility and immobility are complex, and the ability to accurately predict the movement of Pu in a specific hydrogeologic environment remains a significant challenge. Pu mobility is influenced by ionic strength, pH, presence of complexing ligands, oxidation state, aqueous speciation, solubility, host rock composition, and groundwater transport behavior. − Pu mobility can be limited by precipitation, adsorption, and incorporation into secondary mineral phases ,− and can be enhanced by the presence of mobile intrinsic or pseudocolloids − and organics. , …”