Containment penetrations — Flexible metallic bellows: Testing, safety, life extension issues

Abstract The performance and long-term operational integrity of containment systems and components is being challenged as many of the world's nuclear plants progress into the second half of their design lives. As time in service increases, so does the likelihood of component failure due to long term degradation. By observing trends in containment degradation, potential weaknesses can be anticipated and corrected, minimizing interruptions in operations, increasing safety and improving the plant's life extension outlook. One such trend, affecting containment penetration flexible metallic bellows that are subject to cyclic loading, is beginning to appear in some BWR plants. Although the overall performance of flexible metallic bellows penetrations has appeared to be acceptable for approximately 20 years, aging and degradation of these components has been recently identified. Corrosion and fatigue mechanisms have lead to cracking and subsequent leakage through the stainless steel bellows elements. Although periodic testing in accordance with 10CFR50, Appendix J has been performed, fabrication features of some expansion joint assemblies has made it difficult to collect reliable leakage rate data. In fact, recent observations suggest that it is not possible to perform valid Local Leak Rate Testing (LLRT) of certain two-ply bellows elements. This paper presents a description of efforts made to characterize the nature and significance of the bellows degradation observed. Field pressure testing and laboratory examinations are discussed, as well as evaluations to determine leak areas and leakage rates. Crack growth from both corrosion and fatigue mechanisms are examined, and methods for predicting remaining useful life are discussed. Repair and replacement methods will be summarized as well as testing to demonstrate replacement effectiveness.