Microstructural developments in the end cap weld region of failed PHWR fuel pins

Post irradiation examination of a number of PHWR fuel bundles in the past had shown that the failure site in a fuel pin was close to the end cap weld region. Leaking pinholes and circumferential cracking were also observed in the cladding near the weld. In a few cases end cap remained attached to the end tie plate, while remaining portion of the fuel pin was detached at end plug weld exposing fuel to the coolant. Secondary failure in the form of axial cracks in the cladding slightly away from the end plug weld was also observed. Rise in the fuel temperature resulted in restructuring of fuel and degradation of physical and chemical state of cladding. Due to oxidation and hydriding of the cladding, several incipient cracks with hydride platelets at the crack tips were observed indicating crack propagation by delayed hydride crack (DHC) mechanism. The post defect, secondary and tertiary affects like blistering and cracking obscure the primary cause of fuel failures. To find out the primary cause of fuel failures, ultrasonic testing technique was developed and deployed to inspect the end cap weld region of the irradiated fuel pins. Lack of fusion (incomplete weld) defect or root crack may be present from the fabrication stage itself, which could open up at higher temperature during operation. These flaws are further assisted by hydriding or fission product attack. It was observed that hydrogen migrates to the colder end cap weld region forming harmful radial hydrides. Incipient root cracks may also get created in reactor by DHC, in the cladding tube near the weld with massive accumulation of radial hydride platelets. Radial hydride platelets form under tensile residual stress coming from fabrication stage. This paper presents the microstructural details leading to fuel failures arising from different weld defects. (author)