Kinetics of the high temperature oxygen exchange reaction on ²³⁸PuO₂ powder

Mechanisms and kinetics of the oxygen exchange reaction were studied on PuO₂. Results indicate that the oxygen exchange behavior of PuO₂ is quite complex. Exchange rates are influenced by three different mechanisms: a chemical reaction that is occurring within the bulk and not at the material surface, the surface mobility of active species/defects, and the actual exchange of surface adsorbed oxygen with lattice oxygen ions. Determining which mechanism dominates the overall exchange rate appears to be a complex function including at least temperature and specific surface area. The fastest exchange rate is controlled by an internal chemical reaction and can be obtained when the specific surface area of the PuO₂ and exchange temperatures are high. The rate of this internal chemical reaction should be independent of most particle and atmospheric characteristics, including: specific surface area, oxygen partial pressure, total pressure, particle size, and grain size. As temperature and/or specific surface area decrease, eventually the surface mobility mechanism becomes slow enough to control the exchange rate. As temperature and/or specific surface area decrease further, the exchange rate will enter a regime where the surface mobility and surface exchange mechanisms become competitive, and eventually the surface exchange mechanism will become slow enough to completely dominate the exchange rate. These two surface mechanisms are observed to have a dependence on the specific surface area of the PuO₂. Strong similarities are observed between the oxygen exchange behavior of CeO₂ and PuO₂. This suggests that CeO₂ is a very good surrogate for PuO₂ in this regard and means that the extensive oxygen exchange experiments performed on CeO₂ can be used to predict the behavior of PuO₂. Exposure of the PuO₂ to high temperatures (≥ 600 oC) may cause a reduction in the specific surface area of the material due to initial phase sintering. Upon exposure to 1000 oC for ∼3 hours, the surface of the PuO₂ appears to have stabilized enough that exposure to lower temperatures does not have a significant impact on the surface over the short term. Activation energies are obtained for the internal chemical reaction and the surface mobility reaction.