Diagnosis of cracking in the boiler shell seam welds at Sizewell A Power Station

Following a period of satisfactory operation, as part of the Periodic Safety Review the seam welds of the low alloy steel (BW87A) boiler shells at Sizewell A Power Station were non-destructively inspected in 1996. These inspections revealed crack-like defects in the circumferential butt welds which join the plates. A range of diagnostic investigations was undertaken, both at site and on excavated samples, to identify the factors that led to the formation of these cracks. The cracks were found to be within the heat affected zone of the weldments. From their morphology, associated cavitation damage accumulation on the grain boundaries together with their spatial through wall extent and oxide formation it has been possible to establish that (i) the cracks were present at the time of entering service (ii) they formed by a creep related process and (iii) they were consistent with stress relief cracking formed during the stress relief heat treatment cycle during fabrication. The results are discussed with respect to the detail of this cracking mechanism and implications for the repair strategy. (C) 2002 Elsevier Science Ltd. All rights reserved.