In this paper, some results of microstructure analyses on spent MOX fuel with burnup of about 20 GWd/tM, stored for an extended period, are presented. The samples are taken from fuel that has been stored for 25 years either in intact cladding or exposed to dry air. The results of this investigation are compared to the microstructure observations made in the post-irradiation campaign, conducted on the same or sibling fuel rods shortly after reactor unloading (i.e. about twenty years ago).
The fuel from the long term stored intact rods demonstrated an identical behaviour, compared to its state shortly after reactor unloading. Segments of fuel rods that were exposed to dry air showed an alteration of the grain boundary cohesion of the UO2 grains, while apparently the Pu-rich zones were unaffected. These findings are coherent with the results found in accelerated ageing experiments. On the other hand, the formation of secondary phases with higher oxidation states of uranium, often observed during accelerated ageing experiments, could not be confirmed here.