Non-destructive evaluation of ultrasonic under-sodium scanner (USUSS) shield plug of prototype fast breeder reactor by gamma radiometry

摘要

Radiometric testing is one of the non-conventional non-destructive evaluation methods that has found wide applications, especially in the nuclear industry for evaluation of the integrity of components in nuclear reactors and fuel cycle facilities. In this method, the transmitted intensity of radiation is measured and then correlated to dimensional variations or gross defects, such as voids in materials. India has embarked on a three-stage nuclear programme, with the second stage focusing on sodium-cooled fast reactors. At Kalpakkam, an indigenously-designed 500 MWe prototype fast breeder reactor (PFBR) using mixed oxide as fuel and sodium as coolant is under construction. This is a pool-type reactor in which the entire in-core components will be immersed in sodium. As liquid sodium is opaque to visible light and conventional visual methods based on charge-coupled devices (CCD) cannot be used, ultrasound waves are used to visualise the components immersed in sodium. It is essential to know the position and protrusion of fuel sub-assemblies inside the core. An innovative, ultrasonic under-sodium scanner (USUSS) has been developed for this purpose. USUSS is a slender structure consisting of two detachable parts known as the lower and upper parts. This shield plug is designed to protect personnel and components from the intense gamma radiation. In this paper, the authors focus on the application of radiometric testing for evaluating the integrity and shielding effectiveness of the shield plug. A ~(60)Co source of strength 31.5 MBq was used as the source and measurements were made using a Nal(TI)-based portable gamma spectrometer. Radiometric testing confirmed that all dimensional variations were well within the 3% tolerance limits and the material was free from major voids and defects.