Quantitative Microstructural Characterization of Plutonium Oxalate Auto‐Degradation and Evidence for PuO2 Nanocrystal Formation

It has been known since the 1950s that plutonium oxalate powders change color and lose mass over time when stored at room temperature in air. Despite several studies monitoring these changes, there are still discrepancies in the literature regarding the speciation of intermediate and final products that result from this decomposition. Presented here for the first time is a comprehensive series of time‐resolved powder X‐ray diffraction experiments coupled with solid‐state optical spectroscopy and electron microscopy of aged plutonium (III) and (IV) oxalate powders. These data provide fresh insight into the chemical and structural changes that occur in these solids over time at room temperature and represent new evidence suggesting both plutonium (III) and plutonium (IV) oxalates auto‐decompose to form nanocrystalline plutonium oxide in the solid state. Fresh batches of plutonium(III) and plutonium(IV) oxalate hydrates were stored at room temperature in air and monitored by powder X‐ray diffraction, optical spectroscopy, and electron microscopy up to 4 years. Rate constants for microstructural changes are reported, and diffraction data along with optical spectra collected over time suggest that the decomposition product of aged plutonium(III) and (IV) oxalates shares properties with plutonium oxide nanocrystals.