An improved quality assessment of fuel pin end plug welds using digital X-ray radiography

Abstract

In fast breeder reactors, the quality of fuel pin end plug welds is very important because the presence of any weld defect can cause fission products to be released into the coolant circuit. Among the various defects that can form during welding, a root defect at the end plug weld joint is the most prominent type observed in fuel pins. A conventional X-ray film radiographic procedure for end plug welds of fuel pins inserted in a stainless steel (SS) shape correction block with a thickness of 7 mm will limit the minimum detectable wall thickness difference to 1.1% of the specimen thickness due to film gradient and film granularity factors. In the present work, with a standard X-ray digital radiographic data acquisition procedure and optimised radiographic exposure conditions (kV, mAs, etc), the normalised signal-to-noise ratio (SNRn) achieved is 140, which limits the detectable wall thickness difference to 2.8% of the specimen thickness. Hence, a modified digital X-ray radiographic inspection procedure is proposed to assess the quality of end plug welded fuel pins using a suitable X-ray detector calibration technique. The structural noise and pixel intensity variations between the flat-panel pixel elements are minimised using this modified procedure, which results in an order of magnitude increase in the SNRn of the radiographic images. Following implementation of this procedure with end plug welds, the X-ray images show that the achievable wall thickness difference is enhanced to 0.9% of the specimen thickness and microdefects are clearly observed in the welds.