Ostwald Ripening and Its Effect on PuO2 Particle Size in Hanford Tank Waste

摘要

Between 1944 and 1989, the Hanford Site produced 60 percent (54.5 metric tons) of the United States weapons plutonium and produced an additional 12.9 metric tons of fuels-grade plutonium. High activity wastes, including plutonium lost from the separations processes used to isolate the plutonium, were discharged to underground storage tanks during these operations. Plutonium in the Hanford tank farms is estimated to be (approx)700 kg but may be up to (approx)1000 kg. Despite these apparent large quantities, the average plutonium concentration in the (approx)200 million liter tank waste volume is only about 0.003 grams per liter ((approx)0.0002 wt%). The plutonium is largely associated with low solubility metal hydroxide/oxide sludges where its low concentration and intimate mixture with neutron-absorbing elements (e.g., iron) are credited in nuclear criticality safety. However, concerns have been expressed that plutonium, in the form of plutonium hydrous oxide, PuO(sub 2) (center dot) xH(sub 2)O, could undergo sufficient crystal growth through Ostwald ripening in the alkaline tank waste to potentially be separable from neutron absorbing constituents by settling or sedimentation.