Evidence for Particle Attachment of Plutonium Solids in TX-118 Hanford Tank Waste

The accumulation of plutonium particles in waste tanks has been suggested as a process that could lead to a nuclear criticality event during the processing of wastes at the Hanford site, WA, USA. This concern was bolstered by the apparent discovery of a bismuth-plutonium phase in the wastes by Reynolds et al. Initial analyses of TX-118 solids with SEM-EDS matched the findings of Reynolds and co-workers with the observation of similarly large 5 to 10 micrometer particles of Pu-bearing particles that also appeared to contain bismuth and phosphorus. However, it was not clear from the SEM-EDS, how these elements were incorporated into the Pu-phase. We used Scanning Electron Microscopy, (SEM), Scanning Transmission Electron Microscopy (STEM) and found no evidence for a distinct plutonium (Pu) -bismuth (Bi) and phosphate (P) phase. The plutonium was present as PuO₂ with closely associated minor levels of Bi and P. Plutonium-bearing particles in Waste Tank TX-118 appeared to consist of agglomerates of nanoparticles of PuO₂ with other metal oxide nanoparticles that were both crystalline and amorphous. We propose a particle-particle attachment mechanism for the formation of the plutonium agglomerates in TX-118 wastes. Low solubility Pu-bearing solids formed in the highly alkaline tank waste conditions would have had extremely low particle growth rates through Ostwald ripening or similar mechanisms. We performed precipitation experiments and examined the resulting agglomerates with STEM showing a possible particle attachment mechanism of formation.