PNNL

Abstract

Plutonium (Pu) has been previously released to the subsurface worldwide including the release of approximately 1.85 x 1015 Bq at the Hanford SiteSite. The subsurface mobility of Pu is influenced by complex interactions with sediments and co-contaminants within waste streams. Previous investigations at Hanford have suggested that Pu may be associated with discrete particles as PuO2 before or after disposal, as secondary phases formed from waste interactions, or with sediments as adsorbed/incorporated species. In this work, evidence is presented for the existence PuO2, PuO2-Bi2O3 compositeSites and burnt Pu metal particles in near-surface sediments where acidic waste was disposed. Further, Pu is found to be homogenously distributed in sediments near the surface of a crib for neutral pH waste disposals and is immobile. For acidic waste disposal, Pu and americium (Am), the long-lived daughter product of Pu-241, moved deeper into the subsurface. Pu and Am exhibit different behavior with depth, suggesting different mobility controlling phases. Pu and Am are relatively insoluble in groundwater (initially pH 8) extractions, with Am less soluble than Pu. Although the phase controlling Am solubility is not explicitly identified, these results suggest that the more mobile fraction of Am has been transported out of the system or is present in a phase (e.g. Am-phosphate) different from Pu-containing phases in the deeper sediment samples. From these preliminary results, it is suggested that Pu and Am have limited mobility within the soil profile.Th