238PuO2, the fuel used in Radioisotope Power Systems for space exploration, undergoes a secondary (a,n) reaction with the naturally occurring 17O and 18O found in the oxide. This (a,n) is the primary source of worker dose rates and can have a detrimental impact on spacecraft components. Currently, isotopic exchange with 16O is used to minimize the (a,n) reaction, but the kinetics and mechanism of this reaction are not well understood. Previously, the kinetic behavior of the exchange reaction was studied at 1000 oC between CeO2, a common cold surrogate for PuO2, and 16O2(g). Here we present additional kinetic and mechanistic data which suggests that the exchange rate is not limited by a chemical reaction at the CeO2 surface, but is most likely small polaron hopping occurring throughout the bulk of the material. In the CeO2? system, small polaron hopping can alternatively be described as electron transfer from the Ce(III) generated by oxygen vacancies to nearby Ce(IV) atoms. This information suggests the very surprising conclusion that the exchange rate is independent of atmospheric composition and most particle characteristics.Results from CeO2 oxygen exchange allows old PuO2 data to be reevaluated and mined for a better understanding of the kinetic behavior of PuO2 oxygen exchange. Activation energies and rate constants obtained from this reevaluation show that the exchange behavior of CeO2 and PuO2 are very similar, and mechanistic data suggests that the exchange rate in PuO2 is also limited by small polaron hopping throughout the bulk of the material. This comparison strongly suggests that CeO2 provides an excellent approximation of PuO2? oxygen exchange behavior.Plans to perform new oxygen exchange experiments on PuO2? are underway. These results will also be discussed if possible.

Evidence of small polaron hopping as the rate limiting step in PuO<sub>2</sub> oxygen exchange reactions at high temperature

246th National Meeting (2013)

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Evidence of small polaron hopping as the rate limiting step in PuO2 oxygen exchange reactions at high temperature

Evidence of small polaron hopping as the rate limiting step in PuO₂ oxygen exchange reactions at high temperature