Status and Future of Corrosion in PWR Steam Generators

Abstract

This review considers the broad set of corrosive degradations that can occur in PWR steam generators. The general status is reviewed, and potential future problems are identified. Recommendations for future work in each category are given. Main categories in this review include thick sections, tubing, and water chemistry. Tubing materials included are Alloys 600MA, 600TT, 690TT, 800NG, and Type 321 stainless steels. It appears that the Alloys 690TTand 800 give the best performance in vertical SGs and that the Type 321 gives adequate lifetimes for the horizontal steam generators. For tubing, the most likely location for degradation, mainly by stress corrosion cracking (SCC), is at the top of the tubesheet (ITS) of vertical steam generators, especially from the secondary side owing to both the superheated crevices and in some cases denting. These locations are the most likely to concentrate impurities, especially lead and possibly reduced sulfate species, which are inimical to both Alloys 690TT and 800. Available data from operating plants and from laboratory studies of these two alloys suggest that low potential stress corrosion cracking (LPSCC) is not a major concern at present although the presence of cold work, due to surface abuse or bulk deformation, may produce such SCC at longer exposure times. The line contact tube supports are expected, in principle, to be improvements over ones with drilled holes. However, in the long term this degree of improvement is not so clear since the interstices in some plants can fill with deposits; the possible concentration of impurities here is not clear but should be expected. Special attention should be paid to Alloys 690TT and 800 when exposed to lead since, in addition to their possible (neither has cracked in service so far) susceptibility to SCC, based on laboratory data, these alloys produce relatively thick scales when exposed to nominal concentrations of lead.There appears to be little question that degradation could occur in the future in modern steam generators. Only the mode of such degradations may differ from those in the past. There is substantially insufficient information upon which to predict future performance; however, the directions and types of work required are quite clear.