The flexible metallic bellows expansion joints in containment systems at Nuclear units have been the focus of increasing examination the past 15 years. As these plants enter the second half of their design lives, failure due to aging components is a concern. One point of potential weakness lies in the bellows which form an integral part of many containment designs.

The bellows present at least three major difficulties. Their thin gauge stainless steel is susceptible to degradation and leakage due to transgranular stress corrosion cracking (TGSCC). Further, these thin-walled joints are a weak link compared to the concrete-reinforce steel containment vessel. Finally, the configuration of existing bellows renders common Local Leak Rate Testing (LLRT) methods invalid, failing to satisfy Type B test requirements under 10CFR50, Appendix J.

Replacement of existing stainless steel units with fully testable systems appears the best solution, especially considering proposed changes to Federal regulatory testing requirements. These changes would allow less frequent integrated leak rate testing (ILRT) at NRC-approved plants - an approval more likely when testing uncertainties are eliminated.

Domestic nuclear power plants have successfully followed at least three options: and Outer Bellows design; a Bellows Test Enclosure; and a Bellows Test Assembly. Complexity and cost vary, but all achieve valid testability.

Outer Bellows

The outer bellows design replaces the two bellows plies with "stand-off" rings separating the bellows while allowing unimpeded flow of the test medium. TGSCC is countered with corrosion-resistant Inconel 625 alloy steel. However, this bellows is a permanent fixture which cannot be removed and re-used. In addition, the clamshell design must be qualified by prototype testing under ASME Code Case N-315. Finally, the replacement bellows must be welded by an experienced specialty welder.

Bellows Test Enclosure

A Bellows Test Enclosure (BTE) successfully installed at two domestic plants overcomes many of these disadvantages. A reusable component requiring less specialized installation skills, the BTE is added to the existing systems so no prototype testing and approval process is required. Finally the BTE provides both a test boundary for the bellows and a protective cover for the containment pressure boundary bellows, eliminating the need for an independent cover.

The most critical aspect of the design process is ensuring that it is 100% airtight under LLRT pressures to 48 psig by injecting a patented sealant at high pressure. This elastomeric expansion joint absorbs movement but maintains an internal stiffness an order of magnitude less than that of the corresponding bellows, leading to lower reaction loads on attached structures.

Bellows Test Assembly

The BTE can be modified to create a Bellows Test Assembly (BTA) in situations with minimal clearance. This has proved particularly useful for testing of Inclined Fuel Transfer Systems (IFTS) bellows. An elastomer BTE expansion joint and two custom flanges are welded to the fuel transfer tube (FTT) and to a concentric guard pipeline. An interruption in the guard pipeline several feet from the bellows allows internal pressurization by introducing a test boundary that seals off the annular volume.

Conclusion

Bellows Test Enclosures (BTE) and Bellows test Assemblies (BTA) have proven efficient, economical solution to nuclear power plants' system degradation and testing dilemmas with regard to primary containment penetration flexible metallic expansion joints. BTEs provide reusable components with relatively simple installation requirements, while the BTA design modification fits this approach to situations with limited access. The Outer Bellows design, while having significant drawbacks, does provide a solution where existing bellows must be replaced.

For more information visit Expansion Joint Leaks Section under Onstream Leak Repairs