The Rajasthan Atomic Power Station Unit-2 (RAPS-II) has reached a stage of operation where the contacting pressure tubes are prone to failure by the above mechanism. To study and assess the safety of these pressure tubes, pressure tubes believed to be in contact with the calandria tubes, have been removed from the reactor and brought to the Radiometallurgy Division for detailed post-irradiation examination. Visual examination of one of the tubes viz. K-7 has revealed marks in the mid-length region where the tube has been suspected to be in contact with the calandria tube. These marks were over a length of 12.5 cm. The most prominent mark had a dumb-bell shape and was 4 cms in length. This location corresponds to 2.4 meters from the inlet end. Since blister formation at the PT/CT contact region is of concern for pressure tube safety assessment, this region was subjected to detailed evaluation by various non-destructive and destructive tests. Neutron radiography is known to be capable of detecting inclusions of low atomic numbers elements in a matrix of relatively higher atomic number materials. Presence of zirconium hydride blisters of massive concentration of hydride platelets in zircaloy matrix can therefore be detected by neutron radiography.

The contact location of K-07 pressure tube was cut to a size of about 29 mm x 55 mm containing two most prominent visible marks. A special handling cask was made along with the arrangement to hold the active sample. This was radiographed by thermal neutrons using transfer technique at APSARA. The neutron radiography was carried out using 100 µm thick Dysprosium foil with 70 minutes exposure time. The image from the activated Dysprosium foil was transferred to Structurix D7 X-ray film by 4 set of transfers of 17.5 minutes each.

The contact location was radiographed simultaneously along with a strip of unirradiated strip of similar thickness, containing four artificially grown blisters obtained from Atomic Fuels Division. These blisters are the smallest, that could be grown in the laboratory. The radiographs were further analysed using microdensitometry measurements and computerised image enhancements. The radiographs of the contact location of the active K-07 sample show no area of possible massive hydride concentrations, indicating no hydride blisters in the PT/CT contact location. However, all the four artificial blisters in the zircaloy-2 strip are clearly discernible on the same radiograph.

This paper describes the work and the results of neutron radiography on the irradiated zircaloy-2 pressure tube of RAPS-2.