PHOTOTHERMAL DEFLECTION SPECTROSCOPY INVESTIGATIONS OF URANIUM-DIOXIDE OXIDATION

Photothermal deflection spectroscopy (PDS), has been applied to the st udy of uranium oxide electrochemistry. PDS measures the optical absorp tion of the sample surface and concentration gradients formed in the e lectrolyte. Both these measurements are performed in situ under dynami c conditions. The combination of these two measurements provides infor mation that can be used to infer the mechanism of the UO2 surface chem istry. These studies of the uranium dissolution mechanism are performe d in pH 10.5 sodium sulfate electrolytes at 22-degrees-C. The electrol ytes are free from oxygen and complexing species. Our results suggest that dissolution of UO2 can occur at oxidizing potentials as low as -3 00 mV/SCE. The optical absorption and concentration gradient results p rovide evidence for a surface recrystallization reaction that occurs a t an oxidation potential of +300 mV. The results show that the surface layer formed by the recrystallization reaction dissolves slowly by a non-electrochemical reaction.