(309c) Large Cemented Gibbsite Agglomerates in Alkaline Nuclear Waste at the Hanford Site and the Impacts to Remediation

Jet-sluicing has been the primary method of moving nuclear waste sludge between storage tanks at sites such as Hanford in the United States. Recent experience at an area of Hanford comprising twelve, 530,000-gallon underground tanks (C Farm), with waste that had aged in place for decades, showed that jet sluicing left a small heel of waste in the tanks. This study sought to determine why this material was left behind, focusing on Hanford tank 241-C-109. The waste heel in this tank was sampled and then analyzed by X-ray diffraction and scanning electron microscopy (SEM) with energy dispersive spectroscopy. Large gibbsite [Al(OH)₃] particles were found to be the major component in the waste heel. Individual, primary gibbsite particles were to 20 to 100 microns in diameter but were cemented together to form agglomerates as big as seven centimeters in diameter that were difficult to crush with a laboratory mortar and pestle. SEM showed that either amorphous aluminum hydroxide or gibbsite itself was the cementing agent. While large cemented gibbsite nodules have been reported in acid soils in nature, this study shows that similarly large gibbsite agglomerates can form in alkaline environments when aged for sufficiently long time, here for over 40 years prior to waste retrieval. Fluid dynamics calculations indicate that these cemented particles would be difficult to mobilize with standard jet slurry technologies. Even if successfully mobilized, the particles would be difficult to keep suspended and would not fit into transfer lines that are commonly 5-cm pipe diameter. Thus, these large gibbsite particles explain some of the difficulty in suspending aged alkaline nuclear waste with standard sluicing technologies.