Assessment of weld joints of steam generator of prototype fast breeder reactor by microfocal radiography

Abstract

The tube to tube sheet (TTS) welds of steam generator of Prototype Fast Breeder Reactor (PFBR) are quite critical. Sodium flows on shell side and water on tube side. Any failure would thus be catastrophic. Apart from defects such as porosities, wall thinning due to concavity is endemic in such joints and needs to be detected. Earlier the authors have successfully developed procedures based on microfocal radiography and adopted the same in shop floor for the evaluation of these welds for volumetric defects. Presently, replica measurements are the only reliable technique for quantification of internal concavity. However, such measurements are costly and time consuming making it a highly unviable solution for shop floor inspection. This paper presents the experimental results of the procedure developed for quantitative evaluation of wall thinning due to concavity in the TTS welds by microfocal radiography. Internal concavity in the range of 0.1 mm – 0.4 mm was created in tubes using electro discharge machining. The concavity was calibrated by replica technique. The tubes with concavity were subjected to microfocal radiography. Density measurements were made on the radiographs and a calibration graph correlating the density as a function of concavity was established. The experimental values were also found to match the theoretical prediction for the minimum wall thinning that can be detected by microfocal radiography and the density difference corresponding to a wall thinning of 0.2 mm. The results were correlated with concavity in five tube to tubesheet weld joints of evaporator which was also subjected to replica technique. Very good correlation was observed between radiographic measurements and replica values thus validating the methodology developed.