Research Program to Elucidate Outside-in Failure of High Burnup Fuel Cladding

Among a series of power ramp tests on 25 Zr-lined segment rods of burnup ranging from 43 to 61 GWd/t, five segment rods failed during the power ramp tests. One segment rod irradiated for 3 cycles (43 GWd/t) failed with a pinhole due to PCI/SCC. The rest of higher burnups failed with an axial crack on the outer surface. The failure threshold power tended to decrease as burnup increases. Post irradiation examinations revealed increased cladding hydrogen absorption and its precipitates in the cladding outer rim after 4 and 5 cycle irradiations, in contrast to a uniform hydride distribution and a small hydrogen content after 3 cycle irradiation. Metallographic observations suggested an axial crack failure mode induced by the combined effects of high stress and hydrides precipitated in a radial direction during power ramp. The axial crack failure during the power ramp is supposed to be initiated by a cracking of radial hydride formed by hydride re-distribution and re-orientation at the cladding outer rim and to propagate through a process of hydride concentration and precipitation at the crack tip. Research programs of experimental and analytical studies to clarify the conditions of such mechanism are on-going focusing on the hydrogen behavior and mechanical performance of the irradiated cladding.