Surface area and particle size distribution of plutonium oxides derived from the direct oxidation of Pu metal

The intent of this paper is to interrogate the physical characteristics of PuO₂ powders and aggregates produced by direct metal oxidation to examine possible relationships between process parameters and the physical characteristics of the product. The majority of the PuO₂ samples considered were generated during the pre-production and production phases of the Advanced Recovery and Integrated Extraction System (ARIES) program at Los Alamos National Laboratory (LANL). Oxide formed passively on Pu metal surfaces at ambient atmospheric conditions is also characterized. Plutonium oxide powders from the ARIES Direct Metal Oxidation (DMO) furnace and muffle furnace process lines are very consistent in terms of their bulk and tapped density, specific surface area (SSA) and particle size distribution (PSD). The SSAs of calcined PuO₂ generated in the ARIES DMO-2 and muffle furnaces are between 0.1 and 0.5 m²/g. Oxides from both the DMO-2 and muffle furnaces also had similar tri-modal PSDs with maxima at 0.73–3.16 μm, 10.9–18.5 μm, and 41.3–63.0 μm. The PSDs of the oxide formed passively on Pu metal surfaces at ambient conditions were unimodal, with near-Gaussian distributions and D₅₀ close to 1 μm. The oxide formed passively at ambient conditions also had significantly higher SSAs (6–8 m²/g). A documented process upset that originated from a heater element failure in DMO-2 resulted in significantly lower oxidation temperatures, and produced oxide having a slightly greater SSA, and lower bulk and tapped densities. Macroscopic (by quantitative sieving) and microscopic (by laser diffraction of aqueous suspensions) PSDs of the oxide produced during this interval are characterized by depleted large-diameter particle populations. We speculate that the lack of large particles in the aggregate emerging from DMO-2 during the temperature anomaly reflects a change in the thermal regime during oxidation which favors the generation of finer-grained aggregate.