Nuclear fuel UO2 fragmentation in oxidizing medium: kinetic and phenomenological study

One of the considered way to bring flexibility to spent fuel UO2 management is its dry storage. This management way sets in particular the problem of the fuel resistance in case of an accidental scenario which led to the venting of the UO2 pellets. The oxidation kinetics and the phenomenology of the UO2 fragmentation between 250 and 330 C have been studied from aging under O2 and air of monocrystals and fragments of non irradiated fuel pellets. In an oxidizing environment, the initiation of the cracking seems to be due to the relaxation of the tensile stress induced by the contraction of the crystal lattice during the growth of a U4O9/U3O7 layer in surface of the UO2. Then, an incubation period before cracking, whose period of time decreases with the increase of the temperature, has been revealed. During their oxidation, monocrystals and fragments present a progressive cracking leading to their subdivision until a sub-micron grain size. (O.M.)