Decreasing Thermal Stresses in Steam Generator Collector Weld's Area Using External Cooling

Abstract

Numerous WWER-440 steam generators showed a very serious problem of cracking in weld joints which are connecting primary collectors to the steam generator's vessel. As a cause of this cracking was determined stress corrosion cracking mechanism. The weld is connecting austenitic steel collector to the carbon steel vessel, so it is a dissimilar metal weld. There are two approaches how to lower the possibility of stress corrosion cracking in the weld. First one is to improve the secondary water chemistry in the weld's surrounding to stop the corrosion and second one is to decrease presented stresses in weld's area. It is better to combine both approaches. This paper deals with the second approach of possibilities to decrease presented stresses. Inspired by Russian study of external cooling on WWER-1000 steam generators, it was an attempt to adapt the external cooling idea on the WWER-440 steam generators and their problem of weld cracking. It was shown that by using the external cooling it is possible not just lower the presented tensile stresses but to change the tensile stresses into compressive stresses. This will of course result in significant reduction of stress corrosion cracking possibility in the steam generator primary collector weld's area.