Fission gas release in FBR MOX fuel irradiated to high burnup

Fission gas release in FBR MOX fuel pins irradiated to high burnup was studied as a function of burnup. Fuel pellets of different microstructures were fabricated by three methods. The pin averaged burnup of fuel pins ranged from 5.8 to 144 MWd/kgM. The fission gas release mechanism was investigated from results of the puncture test, microstructure observations and EPMA analyses. The fission gas release increased with the increase of burnup. But it depended on the microstructure of as-fabricated fuel below a burnup of 70 MWd/kgM. The difference in fission gas release among fuel pellets of different microstructures, could be attributed to fission gas which was retained in the large size pores of the as-fabricated fuel. In the fuel pin irradiated beyond burnup of 100 MWd/kgM, the fission gas release fraction was significantly large (nearly 80%), and independent of the burnup, the kinds of pellets and LHR. This indicated that little capacity for retention of fission gas is expected in the fuel irradiated to the burnup beyond 100 MWd/kgM and this should be considered in the FBR fuel pin design.