Distributions of volatile fission products in or near the fuel-cladding gap of the FBR MOX fuel pins irradiated to high burn-up

Distributions of volatile fission products and microstructures were studied at different axial positions in or near the fuel-cladding gap of FBR MOX fuel pins irradiated up to pin average burn-up of 110.6 GWd/t. It was found that most of the volatile fission products such as Cs, Mo, and Te were released from the fuel matrix into the fuel-cladding gap, and then they axially migrated up and down to a low temperature region along the temperature gradient. As a result, agglomerates of fission products, known as JOG (Joint Oxyde Gaine), were dispersed in the high temperature axial gap region and were densely precipitated in the low temperature axial gap region. The microstructures of MOX fuel near the gap were restructured by the irradiation as follows. In the low temperature rim region, a re-crystallized sub-micron grain structure was observed just like in irradiated LWR fuel. In the high temperature rim region, a white ring region developed which was free from voids and had large sized grains.