Recovery of Plutonium from Refractory Residues Using a Sodium Peroxide Pretreatment Process

Abstract:

The recycle of plutonium from refractory residues is a necessary activity for the nuclear weapon production complex. Traditionally, high-lired plutonium oxide (PuO₂) was leached from the residue matrix using a nitric acid/fluoride dissolving flow sheet. The recovery operations were time consuming and often required multiple contacts with fresh dissolving solution to reduce the plutonium concentration to levels where residual solids could be discarded. Due to these drawbacks, the development of an efficient process for the recovery of plutonium from refractory materials is desirable. To address this need, a pretreatment process was developed, which utilizes a sodium peroxide (Na₂O₂) fusion to oxidize plutonium from the 4+ to the 6+ valence forming an acid-soluble ternary oxide. The development program utilized a series of small-scale experiments to optimize processing conditions for the fusion process and demonstrate the plutonium recovery efficiency using ceramic materials developed as potential long-term storage forms for PuO₂ and an incinerator ash from the Rocky Flats Environmental Technology Site (Rocky Flats) as test materials.

Process optimization studies using high-fired PuO₂ demonstrated that the efficiency of plutonium recovery increased with the Na₂O₂ fusion temperature (up to 700°C), increasing Na₂O₂ to PuO₂ ratio and a decrease in the Na₂O₂ particle size. Complete recovery of high-fired PuO₂ was obtained by dissolving the fused product in 8 M nitric acid following fusion at 700°C for 2h with 100% excess in the amount of Na₂O₂ required for complete conversion to the ternary oxide.

In the initial demonstration of the process, the plutonium-containing ceramics were fused with Na₂O₂ and dissolved in 8 M nitric acid. Plutonium recovery efficiencies ranged from approximately 95% to essentially complete recovery. The second demonstration of the fusion process was performed using incinerator ash from Rocky Flats.

In experiments with ash containing 17-18wt% plutonium, 90-95% recovery was obtained following dissolution of the fused product in 8 M nitric acid. In subsequent experiments with ash containing only 8-9 wt% plutonium, the recovery efficiencies were significantly less, ranging from 60-90% for experiments performed using similar conditions. The general reduction in the recovery efficiency was attributed to at least a two- to three-fold increase in the silicon dioxide (SiO₂) concentration. Because the ash was predominately SiO₂, a material which reacts with Na₂O₂, it is doubtful that sufficient Na₂O₂ was added to completely oxidize the plutonium.