Fission product behaviour in fast breeder fuel pins

How a fission product behaves depends on its chemical and physical state during reactor operation. Where it is found in postirradiation examinations depends in some cases also on the state of its precursors. Fission products in the gaseous state during reactor operation migrate down the temperature gradient from the hot inner region to the cooler outer region of the fuel. They are easily released to the fuel cladding gap. Fission products in the condensed state not in solid solution with the fuel are transported up the temperature gradient, from large metallic and ceramic inclusions in the columnar grain region, and sometimes even ingots in the central void. The motor behind the fission product movement in the inner part of the fuel that leads to the observed redistribution effects is pore migration. It provides for the transport vehicles up the temperature gradient and for the release paths down the temperature gradient. In the outer, cooler part of the fuel, fission product movement is much slower. Its driving force is very probably grain boundary movement due to resintering and recristallisation after mechanical interactions between fuel and cladding. (orig.)