Caustic Neutralization and Precipitation of Acidic Dissolved Simulated Stainless Steel–Clad Plutonium and Plutonium/Uranium Nuclear Fuel

Simulated dissolved stainless steel–clad plutonium (Pu) and plutonium/uranium nuclear fuel in nitric acid (HNO₃) was neutralized to a free hydroxide (OH⁻) concentration of 0.6 M. A thermal neutron poison, gadolinium (Gd), was added to the simulants at concentrations of either ~3 to 6 g/L or ~37 to 38 g/L. The supernate Pu concentration on the day of neutralization ranged from 0.48 to 8.75 mg/L. The supernate Pu concentration of a simplified simulant neutralized to 0.6 M OH⁻ above precipitated solids containing Pu was demonstrated to decrease over 18 days. A significant portion of precipitated Pu was found to be insoluble in 8 M HNO₃ at ambient temperature, but essentially quantitative Pu dissolution was achieved in 11.5 M HNO₃/0.1 M potassium fluoride at 100°C. The difficulty in dissolving the Pu precipitate is believed to be due to the formation of refractory PuO₂•xH₂O during the neutralization process. Initial Gd concentrations of ~37 to 38 g/L were found to result in a greater aluminum precipitation when neutralized to 0.6 M OH⁻ than initial Gd concentrations of ~3 to 6 g/L. Physical properties of the resultant slurries were measured and used to calculate limiting flow rates and slurry velocities by gravity only in transfer piping between the Savannah River Site’s H-Canyon Facility and the Concentration, Storage, and Transfer Facility. These results were compared to calculated deposition velocities to predict if solids would settle during the transfer. The Newtonian model was found to be reasonable for each diluted slurry evaluated. Deposition velocities of Pu-containing slurries are lower than nuclear fuel slurries primarily composed of uranium because of the high density of Pu solids. Dilution of slurries reduces the margin between the slurry and the deposition velocities due to the reduction in viscosity because higher viscous forces on the particles promote maintained suspension.