In this work, we have studied the oxidation behavior of a fresh UO 2 surrogate in representative conditions of a dry interim storage for SNF, both in terms of temperature (200–400 °C) and oxygen partial pressure (0.1–21% O 2 ). The reaction has been studied in-situ by TGA and ex-situ by autoclaves. XRD and Rietveld's refinement has been used for quantification of uranium oxidized phases, with particular interest in U 3 O 8 . The predominance of temperature over oxygen concentration in the formation of U 3 O 8 has been confirmed. TGA results have shown that, at 200 and 250 °C, no U 3 O 8 has been detected after 10 h of thermal treatment, not even in air (21% O 2 ). Only at 275 °C, U 3 O 8 has been detected at 21 and 1% O 2 . This result allows better defining the temperature threshold of U 3 O 8 formation in unirradiated UO 2 . On the upper limit, 350 and 400 °C, the reaction proceeds up to quantitative formation ( i.e., more than 90%) of U 3 O 8 , even with very low oxygen content (as low as 0.1% O 2 ). Finally, at intermediate temperatures ( i.e., 300 °C), oxygen partial pressure starts playing a role, being the extent of the oxidation lower with lower oxygen concentration. This effect has been seen when studying the reaction at longer times by using ex-situ autoclaves. However, given enough time, all the fuels end showing a majority of U 3 O 8 , which is between 6 and 24 h for 10 and 1% O 2 and between 24 h and 1 week for 0.1% O 2 . In all the experiments, 21 and 10% O 2 behave in a very similar way for all the temperatures, but there is a substantial change when decreasing oxygen to 1% O 2 , being in this case the oxidation much more limited. This could be an indication of an oxygen concentration threshold that could trigger the nucleation and growth of U 3 O 8 in a greater extent. Based on the experimental work and previous modelling studies, an analytical methodology to assess the cladding integrity during UO 2 oxidation has been developed by adapting FRAPCON-xt, a home-extended fuel performance code to dry storage.
Milena-Pérez et al. (Tue,) studied this question.