Axial crack on high burnup BWR fuel cladding caused by power ramp test

Lead use assemblies (LUAs) of high burnup 8x8 fuel design for Japanese BWRs were irradiated up to 5 cycles in Fukushima Daini Nuclear Power Station No. 2 Unit. Segment rods were installed in LUAs and used for power ramp tests in Japanese Material Test Reactor. Post irradiation examinations (PIEs) of segment rods were carried out at Nippon Nuclear Fuel Development Co., Ltd. before and after ramp tests. One segment rod irradiated for 3 cycles failed at the ramp terminal power of 610 W/cm by PCI/SCC mechanism, while some segment rods irradiated for 4 and 5 cycles failed at about 550 and 420 W/cm, respectively, and showed axial cracks starting in the outer surface of cladding tubes. Following characteristics on failure segment rods were observed through detailed PIEs before and after ramp tests. Radial hydrides were observed at the outer rim of the cladding tubes irradiated for 4 and 5 cycles and ramp-tested rods, while no radial hydrides were observed before ramp tests. The fractography showed that the fracture surface is composed of three regions. The region near outer surface is strongly brittle, but that of the near inner surface is ductile while the middle region is macroscopically brittle but microscopically the mixture of brittle and ductile features. The width of strongly brittle region is about 50 to 80 micro-meter and that of macroscopically brittle crack is about 700 micro-meter from outer surface. Many hydrides perpendicular to the crack were found on both sides of the crack. Small hydrides along crack propagation direction were observed at the crack tip. Estimated failure mechanism by axial crack is as follows. Hydrogen contents in the cladding tubes increased with burnup and exceeded solubility limits during base irradiation of segment rods. During power ramp tests, some hydrogen diffused from inner to outer region in the cladding tube due to temperature gradient. Diffused hydrogen precipitated radially at the outer region due to coupled effects of high hydrogen content and high tensile stress by PCMI. Radial hydride was cracked by high stress and/or strain. New, small hydride precipitated at the crack tip due to stress concentration and cracked repeatedly. When stress exceeded the yield strength of residual cladding thickness, it failed ductilely. (author)