Observation of atomic redistribution in PWR cladding after neutron and self-ion irradiation

Microstructures of neutron-irradiated and ion-irradiated PWR cladding were observed by atom probe and transmission electron microscopy in order to investigate the irradiation induced redistribution of alloying elements, which is important to clarify the mechanism of corrosion and hydrogen absorption phenomena of cladding at high burnup. The results were compared with those of ion-irradiated Zr-Nb binary alloys to estimate the relationship between Nb and other additives. The matrix Nb concentration in solid solution decreased after irradiation, and irradiation-induced precipitates of Nb nanoparticles and Nb-rich precipitates were identified in the matrix phase and grain boundaries. There was no significant difference in the Nb concentration between the PWR cladding matrix and the Zr-Nb alloy matrix after irradiation, and the addition of Sn had no effect on the Nb concentration in solid solution. Two types of Zr-Fe-Cr-Nb precipitates were identified as initially formed precipitates in the PWR cladding. In both types of precipitates, most of the Fe dissolved into the matrix phase in the early stage of irradiation, while Cr dissolution was slow and remained in the precipitates after irradiation. The dissolved Fe and Cr formed nanoclusters around the precipitates. The irradiation behavior of these alloying elements showed the same trends as that of Zircaloy cladding and Nb-doped cladding. (author)