Failure mechanism of NbO doped PWR fuels under power transient

Yanagisawa, Kazuaki

(1) The niobia doped PWR fuel (test specimen) did not fail below 260 cal/g defined as the failure threshold for RIA. (2) The fuel failure occurred by the cladding melt-brittle mechanism irrespective to the doping. (3) The test specimen caused a significant axial PCMI to the magnitude of 18%. (4)Above 276 cal/g, the test specimen restructured and formed the bonding, the small radial cracks, the metallic agglomeration and inclusion at the very narrow ring area. The grain size of the test specimen at the ring was reduced from original 31 to 10-21 microns. Meanwhile, undoped fuel grew the grain size from original 9 to 60 microns at the similar ring because of the recrystallization.