Microstructural Studies of Zr-2.5Nb and Zircaloy-2 Pressure Tubes Irradiated in Indian Pressurized Heavy Water Reactors

The Indian PHWR uses Zr-2.5% Nb pressure tubes and its in-reactor performance mainly irradiation creep and growth depends strongly on its microstructure. A detailed microstructural examination was carried out on unirradiated pressure tubes off-cuts and an irradiated pressure tube S-07 of KAPS-2 (operated for 8 effective full power years (EFPYs)), Microstructural characterization was carried out using transmission electron microscopy. Microstructual observation of un-irradiated off-cuts shows the lamellar morphology of the alpha-Zr along with the beta-phase present as stingers between two alpha laths as well as fine and coarse beta globules. The size of alpha-Zr lamellae was found to be in the range from 0.17 to 0.2 mu m, 1.8 to 2.4 mu m and 1.7 to 2.8 mu m in the radial, circumferential and axial direction respectively (aspect ratio of 1:7:8). TEM-EDS analysis showed composition of the beta phase tin the range of 15-50 wt%Nb. The irradiated pressure tube samples obtained from 13 locations were showing average alpha grain width, grain length and aspect ratio in the range of 0.17-0.27 micron, 1.7-2.3 micron and 7.1-8.5 respectively. Extensive modification in beta morphology could be seen at the high flux and high temperature regions. The beta phase was observed to have globulised completely in many regions. They were present at the interface of alpha-Zr laths as well as within the lath. The Nb concentration of the beta phase appeared to have increased as the volume fraction had reduced. The microstructure details of irradiated and un-irradiated pressure tubes obtained in this study is expected to help in modeling the dimensional change occurring during irradiation in reactor.