NUREG/CR-6726, Aging Management and Performance of Stainless Steel Bellows in Nuclear Power Plants

In commercial nuclear power plants, many containments use stainless steel bellows at piping penetrations, in dry well-to-wetwell vent lines, or as a part of the fuel transfer tube. Bellows are an integral part of the primary contain ment pressure boundary in nuclear power plants. These bellows allow for thermal expansion and contraction of the pipe, vent line, or fuel transfer tube, and accommodate relative motion between the containment and other structures, such as a shield wall. Ensuring that bellows in operational plants remain leak tight is an important safety issue. A number of bellows have been replaced when transgranular stress corrosion cracking caused them to leak. Determining how to evaluate bel lows for age-related damage is a critical issue that must be addressed for plants operating under their original license, as well as for plants applying for license extension. The types of bellows, locations in a containment, and required bellows leak tests are described for major categories of boiling-water reactor (BWR) and pressurized-water reactor (PWR) power plants. Compilation of instances of bellows cracking, excessive leakage, or structural integrity questions, as well as testing issues and other degradation problems, are reviewed and summarized. The aging mechanisms that caused the degradation are discussed. Inspec tion methods currently in use, and a typical industry approach to managing bellows aging are described. Degradation that resulted from corrosion, misalignment, and other damage was considered. The risk and regulatory significance of containment bellows when subjected to Current License Basis (CLB), degraded, and severe accident (SA) condi tions are also addressed.