Development of EMT Equipment for High Burn-up Cladding

Since the 1960's, fractures of cladding induced by a pellet-cladding interaction (PCI) in BWR and CANDU-type reactors have been reported. With the changes in pellet shapes and the development of a barrier cladding using an inner layer of pure zirconium in BWR fuel design in the 80's, PCI has since received little attention, except for power transient conditions in a PWR. Indeed, it is well known that a cladding failure by an iodine-induced stress corrosion cracking (ISCC) may occur under PCI conditions during power transients in PWRs. Under PCI conditions, ISCC then results from the synergic effect of (1) the hoop tensile stress imposed on the cladding by fuel thermal expansions during power transients and (2) the corrosion by iodine released from the UO2 fuel as a fission product. Now, many power plants adopt a high burn-up operation, which includes a high power, enlarged fuel cycle, and so on. In high burn-up, the cladding becomes more brittle because of the hydride formed by waterside corrosion and the possibility of a fracture by PCI is increased. To evaluate the PCI resistance of high burn-up cladding, ramp tests in research reactors or the expanding mandrel test (EMT) in laboratory tests have been used. The fuel rod ramp tests are both time consuming and costly. Furthermore, expanding mandrel tests have been used to supplement fuel rod ramp test data to understand the PCI phenomenon. The segmented expanding mandrel test (SEMT) has been used for studying the PCI phenomenon and for selecting the high PCI resistant Zircaloy cladding. Because of the segmented and complex structure of the cells, reproducibility of the SEMT test results has been very limited. Therefore, in this study, we focused on the development of new EMT equipment which has greater reproducibility and can do on-line monitoring for radial strain change. The rupture behavior of Zircaloy-4 cladding in the range of room temperature to 350 . deg. C was evaluated and compared