MICROSTRUCTURAL EXAMINATION OF Zr-2.5%Nb PRESSURE TUBE S-07 FROM KAKRAPAR ATOMIC POWER STATION UNIT-2

摘要

The Indian pressurized heavy water reactors (PHWRs) use Zr-2.5% Nb pressure tubes (PT) and the in-reactor performance of these tubes, such as irradiation creep and growth behaviour, depends strongly on their microstrucrure. Microstrucrural characterization of off cuts of unirradiated Zr-2.5% Nb PT was carried out using transmission electron microscopy (TEM). The observations show lamellar morphology of the α-Zr along with the β-phase present as stingers between two α-Zr lamellae / laths as well as globules. It could also be seen that in several instances, large number of coarse (3-phase had precipitated within the α-Zr laths. The size of α-Zr lamellae was found to be in the range from 0.17 to 0.2,1.8 to 2.4 and 1.7 to 2.8 μm in the radial, circumferential and axial directions respectively, resulting in an aspect ratio of approximately 1:7:8. Energy dispersive spectroscopic analysis confirmed the composition of the stringers, small globules and the large globules of the (3 phase to be of the order of 8-15, 15-25 and 30-50 wt% Nb respectively. An irradiated Zr-2.5% Nb PT removed from the Kakrapar Atomic Power Station Unit-2 (KAPS-2) after 8 effective full power years (EFPYs) of operation, was subjected to microstrucrural examination. The irradiated samples obtained from 13 locations along the length of the tube showed average width, length and aspect ratio of the a-grains are in the range of 0.17-0.27 μm, 1.7-2.3 μm and 7.1-8.5 respectively. Extensive modification in β-phase morphology could be seen at the high flux and high temperature regions of the PT. The β phase was observed to have globulised completely in many regions and was present at the interface of α-Zr laths as well as within the laths. The Nb concentration of the β phase appeared to have increased along with the reduction in the volume fraction.