Investigation of Plutonium and Uranium Precipitation Behavior with Gadolinium as a Neutron Poison

Abstract:

The caustic precipitation of plutonium (Pu)-containing solutions has been investigated to determine whether the presence of 1 :3 Pu : uranium (U) in solutions stored in the H-Canyon Facility at the U.S. Department of Energy’s Savannah River Site (SRS) would adversely impact the use of gadolinium nitrate [GcHNO₃)₃] as a neutron poison. In the past, this disposition strategy has been successfully used to discard solutions containing approximately 100 kg of Pu to the SRS high-level waste (HLW) system. In the current experiments, gadolinium [as Gd(NO₃)₃] was added to samples of a 1 : 3 Pu : U solution, a surrogate 3 g/L U solution and a surrogate 3 g/L U with 1 g/L Pu solution. A series of experiments was then performed to observe and characterize the precipitate at selected pH values. Solids formed at pH 4.5 and were found to contain at least 50% of the U and 94% of the Pu but only 6% of the Gd. As the pH of the solution increased [e.g., pH >14 with 1.2 or 3.6M sodium hydroxide (NaOH) excess], the precipitate contained greater than 99% of the Pu, U, and Gd. After the pH >14 systems were undisturbed for one week, no significant changes were found in the composition of the solid or supernale for each sample. The solids were characterized by X-ray diffraction (XRD) which found sodium diuranate (Na₂U₂O₇) and gadolinium hydroxide [Gd(OH)₃] at pH > 14. Thermal gravimetric analysis (TGA) indicated sufficient water molecules were present in the solids to thermalize the neutrons, a requirement for the use of Gd as a neutron poison. Scanning electron microscopy (SEM) was also performed, and the accompanying back-scattering electron analysis (BSE) found Pu, U, and Gd compounds in all PH >14 precipitate samples. The rheological properties o! the slurries at pH >14 were also investigated by performing precipitate settling rate studies and measuring the viscosity and density of the materials. Based on the results of these experiments, poisoning the Pu-U solutions with Gil and subsequent neutralization is a viable process for discarding the Pu to the SRS HLW system.