URANIUM RELEASE FROM ACIDIC WEATHERED HANFORD SEDIMENTS

Past nuclear weapon-production activities at Hanford, WA, USA resulted in approximately 1.7 trillion L (450 billion gal) of liquid waste, with some portion released to the subsurface vadose and saturated zones. Most of Hanford waste is basic, but the 216-U-8 and U-12 Cribs located in the 200 West Area received acidic process condensate from different sources. Due to potential secondary source for uranium contamination in subsurface groundwater, a full understanding of uranium release from the acidic weathered Hanford vadose zone sediment was investigated using the reacted Hanford fine sand with simulated waste stream. Two different waste simulants were prepared with and without phosphate ion. After 3-15 months reaction, the reacted sediment was characterized using XRD, TRLFS, NMR, and microwave digestion. The results showed that the precipitate in the simulant with phosphate was identified as independent phase of meta-ankoleite (K[UO₂][PO₄]•3H₂O). More sensitive TRLFS analysis showed that meta-ankoleite was the major uranium species in the reacted sample. However, the TRLIF spectra for the reacted sediment with no-phosphate simulant indicates that the uranium-containing mineral in the reacted sediment could be becquerelite [Ca(UO₂)₆O₄(OH)₆•8(H₂O)]. In addition, the flow-through cell test was conducted to assess the amounts of uranium from uranium-containing acidic waste stream weathered Hanford sediments using synthetic Hanford background pore water (BPW). For the reacted sample with no-phosphate simulant, there was no significant differences in uranium release between samples reacted for 90 and 450 days, implying that uranium phase stabilization occurred by day 90 of weathering. However, for the reacted sample with phosphate simulant, the uranium release from the 450-day sample was ~40% lower than the 90-day sample, implying either larger particle size of formed uranium mineral or higher crystallinity of meta-ankoleite with longer weathering time. These results show that insoluble uranium phosphate phases (meta-ankoleite) strongly retained uranium in the sediments, while, uranium release in phosphate-free systems was likely from dissolution of oxyhydroxide minerals precipitated in batch weathering.