Ultrasonic Testing as Alternative for Radiography for Evaluation of PFBR Blanket Pin End Plug Weld Integrity

Abstract

Radial blanket pins of Prototype Fast Breeder Reactor (PFBR) are made by encapsulating Deeply Depleted UO2 (DDUO2) pellets in a D9 austenitic stainless steel clad tube. The ends of the tubes are sealed by TIG welding of end plugs at both the ends. This is a critical weld as any failure will lead to spread of radioactivity in sodium coolant circuit of PFBR. At present, integrity of these welds is ensured by 100% radiography by using double wall single image full compensating block tangential radiography method. The present work aims at understanding the nature of defects encountered in the end plug welds and developing an Ultrasonic Testing (UT) methodology in lieu of radiography for qualification of the end plug welds. Typical defects have been examined by metallography and statistical analysis of weld zone has been carried out. A detailed analysis on various parameters affecting detection of tight lack of penetration (LOP) defects in radiography has also been carried out. A UT technique based on B-scan imaging in pulse-echo mode has been explored for inspection of end plug weld. Testing has been carried out with a highly damped spherically point focused 20 MHz transducer to produce 450 shear wave in weld joint area. End plug welds with different types of natural defects have been generated by varying different TIG welding parameters. UT and radiography of the same have been carried out. LOP has been detected with good sensitivity and UT results show good correlation with radiography.