a method for fabricating a steam generator and a steam generator wherein primary coolant tubes are sleeved in and in the vicinity of a tubesheet . a smooth transition region is formed between stock primary coolant tubes and sleeved portions thereof in such a manner that no sites for accelerated corrosion are formed . in addition , an adequate volume of material is provided in the transition region to withstand thermal and mechanical stresses and chemical attack . the sleeved tube is preferably thermally treated for improved corrosion resistance .