Fuel – cladding chemical interaction in MOX fuel rods irradiated to high burnup in an advanced thermal reactor

The performance of MOX fuel irradiated in the advanced thermal reactor, FUGEN, to a burnup of 47.5 GWd/t, was investigated by using a telescope, optical microscope, SEM and EPMA. Observations focused on elucidating the corrosion behavior of the cladding inner surface. A reaction layer was observed at burnups higher than about 35 GWd/t. The relationship between the thickness of the reaction layer and burnup was similar to that reported in the literature for conventional UO2 fuel and other MOX fuels. The existence of a plutonium spot near the outer surface of the fuel pellet had no significant effect on the thickness of the reaction layer. A bonding layer was observed on the cladding inner surface. Its morphology and elemental distributions were not so different from those in BWR UO2 fuel rods irradiated to high burnup, in which the fission gas release rate is high. In addition, the dependences of bonding layer formation on the burnup and linear heat rating were similar to results of UO2 fuel rods. It was, thus, suggested that the bonding layer formation mechanism was similar in both UO2 and MOX fuel rods.