High-Temperature Oxidation of Simulated Irradiated UO₂ Fuels Characterized by Raman Spectroscopy

In order to investigate the molecular structure of spent fuels and their oxidation behavior at high temperature, micro-Raman spectroscopy is used to characterize four simulated fuels of typical light water reactors(LWR) with different burnup values(0, 15, 33 and 50 GWd/tU) to deeply understand the effects of fission products generated during burnup deepening on the molecular structure of UO₂ fuels and the resistance to high-temperature oxidation corrosion. Results indicate that the pre-irradiation UO₂ pellets show a remarkable T_2g band characteristic of the fluorite structure, a 1LO band which is activated due to the presence of disorder in the crystal and an intense 2LO band which can be used as an indicator of the extent of UO₂ oxidation. With the increase of the burnup, the presence of fission products induces lattice distortion, and increases the degree of oxygen non-stoichiometry. The defect structures lead to a decrease in perfect fluorite character and a shift of T_2g band towards higher frequency. Meanwhile, the value of I_T_2\mathrmg/I_2\mathrmLO increases almost linearly. High-temperature oxidation experiments show that simulated irradiated UO₂ pellets have a better oxidation resistance when the doping content of fission products reaches a certain level. Compared with pure UO₂ pellets, simulated irradiated UO₂ with the burnup of 50 GWd/tU possesses a better ability to maintain the fluorite crystal structure, therefore, the formation of α-U₃O₈ phase is delayed. Based on the advantage of Raman spectroscopy in situ analysis, real-time high temperature corrosion tests were carried out at 20-350 ℃ and attained a sequential acquisition of the characteristic Raman spectra of the different oxides involved in the oxidation from UO₂ to U₃O₈. The simulated irradiated UO₂ with the burnup of 50 GWd/tU was observed to undergo two phase transitions. In the first stage, UO₂ is gradually oxidized to U₄O₉ during the temperature rises from 20 ℃ to 245 ℃. In the second stage, U₄O₉ gradually transforms into U₃O₈ when the temperature rises from 245 ℃ to 345 ℃. This result reveals an oxidation pathway of irradiated UO₂ containing a large amount of fission products. Raman spectroscopy plays an important role in molecular structure analysis and is expected to be a powerful tool for nondestructive characterization of irradiated fuel samples.