UO2 and MOX fuel rodlets are examined by optical microscopy after 5 years leaching in water to correlate the irradiation history of the fuels and of the fuel behaviour and with the release data for fission products and actinides. Metallographic observation revealed a strong intergranular attack of the MOX fuel in contrast to UO2. The latter effect could be responsible for the particularly high releases from the MOX fuel (upto 12% of the Cs inventory), compared to the UO2 fuel (only 0.6% of the Cs inventory). The large inventory of volatile fission products at grain boundaries can be explained by the high linear power rating (350 W/cm) with extremely high central temperatures in the MOX fuel during the irradiation.

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