Fracture toughness of the pressure tube is affected by the hydrogen pick up by the tube and the irradiation damage caused by the neutron flux. Hence, to make a comprehensive study of the properties on the pressure tube, a detailed knowledge of the spatial variation of the flux is needed. In addition, the knowledge of various activation products and their intensity and decay pattern is needed for planning irradiated pressure tube material handling, storage and disposal. As part of the Non-Destructive Examination of these tubes, a spatial gamma scanning on the irradiated tubes is performed to study the flux pattern seen by the tubes during their residence in the reactor.

For gamma scanning, a special lead shielded ring having a collimator of 5 mm dia is used. The detector is NaI(Tl) with a single channel analyser. The window is set for Sb-125 which is a prominent gamma emitter in irradiated pressure tubes. The counting time is set to 10 seconds for uniform comparison of various tubes. The tube is translated in front of the collimator and the counting done along the entire length of the tube. The spatial scan is then plotted with counts on one and distance on the other axis.

Before gamma scanning has been performed on thin rings specially trepanned from irradiated pressure tubes. The active ring is mounted on a special XY stage having translation resolution of 1 micron. The collimator used is a special pin hole type. The detector is NaI(Tl) with a PC based multichannel analyser. Analysis is performed using interactive computer code PLOT3D.

This paper presents the details of the gamma scanning and the gamma spectrometry measurements and analysis carried on the pressure tubes from various PHWRs.