Examination of cold leg roll transition zones at Ringhals 2

Ringhals 2 is a PWR of Westinghouse design put into operation in 1974. The unit has three Westinghouse Series 51 steam generators with mill annealed Alloy 600 tubing. The unit was operated for a few months at low power with a phosphate secondary water chemistry. This was changed in September 1974 to all volatile treatment (AVT). In 1981, cracks on the primary side at roll transition zones on the hot leg side were detected. Examination of 15 tubes removed in 1983 revealed that a majority of the roll transition zones on the hot leg side have such cracks. This examination established the importance of carbide distribution and geometry of roll transitions on this type of cracking. In 1984, five tubes were removed from the cold leg side as part of a life expectancy study. Examination of these tubes revealed primary side cracks in the roll transition zone on three of the five tubes. The microstructural characterization of the tube materials established that in all five tubes the carbide distribution was intragranular. The insides of the tube walls have been examined well away from the expanded part of the tubes. Intergranular attack was found on the hot leg. This attack is about one grain diameter deep and on the cold leg side the attack is very shallow. Nevertheless, the observations show a very slow intergranular attack on the primary side of the tubes