Post-irradiation examination of KKB special fuel rods

In a cooperation between Kernkraftwerk Brunsbüttel (KKB), which is a boiling water reactor (BWR), and Studsvik Nuclear AB, a post-irradiation examination (PIE) project is carried out on 13 Special Fuel Rods at the Studsvik Hot Cell Laboratory. The rods are considered Special Fuel Rods, because they have different defects. The research project includes three focus areas: Basic PIE and characterization of the conditions of the Special Fuel Rods, a study of the drying process for defect rods and determination of the residual moisture content, and determination of the critical stress for hydride reorientation during dry storage. The set of 13 Special Fuel Rods is composed of rods of different fuel rod designs and manufactured by different vendors, which is representative of the fuel used in KKB over its operational life. The 12 defect rods represent a wide range of several important failure mechanisms, including classical pellet cladding interaction (PCI), PCI with secondary degradation, fretting and fretting with secondary degradation. The defects observed are tight cladding breakthroughs, small penetrations, axial cracks and circular cracks. Two rods have also experienced significant fuel loss, one at a transversal break of the rod and another with a large loss of cladding which exposed the fuel to the coolant. The 13 Special Fuel Rods were shipped to Studsvik Hot Cell Laboratory in February 2018, which marked the start of the planned 3.5-year PIE project. Since the start of the project, basic non-destructive PIE and some destructive characterization of the fuel rods and of the failure sites have been performed. This paper reports on the progress and results obtained so far in the KKB Special Fuel Rods PIE program.