Microstructural and crystallographic analysis of hydride reorientation in a zirconium alloy cladding tube

Abstract

Understanding the hydride platelet reorientation in zirconium (Zr) alloy fuel claddings that potentially occurs during the dry storage process of spent nuclear fuel is of great technological importance for the cladding integrity. In this study, detailed microstructural and crystallographic analysis of the circumferential and reoriented radial hydrides in a Zr–Sn–Nb cladding tube has been performed. The results show that the crystallographic nature of the reoriented radial hydrides in the material remains the same as that of the circumferential hydrides. The radial hydrides remain to be the δ-phase precipitated along the {10–17} habit plane of the α-Zr matrix and follow the {0001}α//{111}δ and <11–20>α//<110>δ orientation relationship with the matrix. The local misfit strain between the matrix grains and the circumferential/radial hydrides is accommodated by the development of {01–10}α <a> type misfit dislocations in the matrix grains as well as the dislocations and twinning in the hydrides. It is confirmed that the hoop stress applied on the cladding tube during the hydride precipitation only changes the growth direction of the macroscopic hydrides from the circumferential direction to the radial direction of the tube material. A qualitative micromechanical stress analysis shows that the shear stress on the basal plane of α-Zr grains promotes the activation of {01–10}α <a> type dislocations slip in the matrix. In addition, to accommodate the c-axis stretching of the matrix unit cell due to the application of hoop stress, the precipitated hydride sub-platelets stacks tend to grow along the direction perpendicular to the direction of the applied hoop stress, leading to the eventual formation of macroscopic radial hydrides (i.e., the occurrence of the stress reorientation).