CRACKING OF CLADDING TUBES CAUSED BY POWER RAMPING AND BY LABORATORY STRESS-CORROSION EXPERIMENTS

The report describes experiments focused on improving the understanding of the phenomena leading to PCI failures. The first part treats out-of-reactor iodine stress corrosion experiments in which the stress, strain or stress intensity were carefully controlled. Crack initiation was found to be a function of stress, while crack growth was a function of stress intensity. Conditions leading to transgranular and intergranular cracking, and to a large mechanical rupture zone are clarified. The second part summarizes the results from studies conducted at the Halden Project to correlate the occurrence of PCI/SCC failures with fuel design and operating parameters. The following parameters and their influence on fuel failure propensity have been studied: pellet cladding interaction factors derived from in-reactor elongation/deformation measurements, fuel temperature and cladding strain, derived from correlations based on cladding deformation and fuel centre line temperature measurements and transient fission products released during a power ramp. The analysis of the out-of-reactor and in-reactor experiments has provided an improved understanding of the critical events in the PCI/SCC processes. © 1979.