Investigation of failure of high burn up carbide fuel pin of fbtr through post-irradiation examination

A fuel pin failure was identified for the first time in the mixed carbide fuelled Fast Breeder Test Reactor (FBTR) at a burn-up of 148.3 GWd/t. Post irradiation examinations were carried out on the failed fuel subassembly in the hot cells to investigate and analyse the causes of failure. The investigations included a series of non-destructive examination techniques like X ray & neutron radiography, axial gamma scanning, and profilometry on select intact pins as well as the failed pin. Based on the results, a few fuel pins were selected for fission gas analysis, metallography, high temperature tensile testing of cladding and clad swelling measurements. The results obtained were compared with that of fuel pins which had reached a burn-up of 155 GWd/t without failure under similar irradiation conditions. The failure of the cladding in the form of a longitudinal rupture is attributed to be the combined effect of high radial and axial stresses in the clad resulting from high Fuel Clad Mechanical Interaction (FCMI) and local bending of a few pins in the failure region. The failed pin and adjacent pins have evidently been subjected to high temperatures, presumably due to local coolant flow reduction between two pins that came closer as a result of local bending.