Atom probe tomography of Zircaloy-2 exposed to boiling water reactor operation

Licentiate thesis, 2021

Zirconium alloys are used as cladding tubes for the fuel in nuclear reactors. In boiling water reactors (BWRs), the alloy Zircaloy-2, which contains the alloying elements Sn, Fe, Cr, Ni, and O, is commonly used. In this thesis, results from atom probe tomography studies of Zircaloy-2 exposed to three and nine annual cycles of BWR operation are presented. The alloying elements were observed to be redistributed after reactor exposure. Clusters of Fe and Cr or Fe and Ni were observed at expected positions of radiation-induced <a> type dislocation loops. Regions containing mainly Fe–Cr clusters were much more common than regions containing mainly Fe–Ni clusters. The Fe–Cr clusters appeared dense and spheroidal, whereas the Fe–Ni clusters appeared dilute and disc-shaped. The Fe–Ni discs occupied planes that correspond to all three families of first-order prismatic planes but with deviation from perfect <c> axis alignment. Many grain boundaries were observed, and there were large grain-to-grain variations in cluster number density and cluster composition. Regarding cluster number density, there was on average no difference between three cycles and nine cycles. After nine cycles the clusters were on average larger and contained more Cr. Sn was observed to cluster in regions where there were no Fe–Cr or Fe–Ni clusters. Enrichment of Sn and Fe and small amounts of Cr and Ni was observed at grain boundaries. After nine cycles, but not after three cycles, enrichment of Sn, Fe, and Ni at ring-shaped features interpreted to be radiation-induced <c> component loops was observed. Hydrogen was observed to be preferentially located outside a partially dissolved Zr(Fe,Cr)2 second phase particle that also contained Ni and Si. The present work demonstrates that the well-known dislocation loops have a chemistry, in the form of clusters of different types and shapes, that might substantially affect mechanisms degrading the cladding tubes, e.g. irradiation growth, corrosion, and hydrogen pickup.