Bruce NGS-A, Unit 4 Fuel Chanel Feeder Coupling Leakage

Abstract

Candu Reactor fuel channels are connected to the Primary Heat Transport System by mechanical joints known as feeder couplings (feeder connections) which overall have performed well in many reactors over many years of operation. Occasionally, minor leaks have occurred in the couplings. In 1991 three feeder coupling leakages were discovered in Bruce A Unit 4. These leakages became large enough to exceed the operating limits and required the unit to be shut down for repair. Although existing repair methods are available to eliminate the leakage, an investigation, coupled with lab tests, was undertaken to gain an understanding as to the possible cause(s) of the leakage in that unit in order to eliminate or reduce future leaks, The investigation has identified that the leakage at the feeder coupling was caused by a small "separationw of the couplings at the connections. One or more of the following factors might have been involved in causing the separation: a) marginal installation torque of capscrews, b) variation of capscrew preload for a given installation torque, c) alignment of feeder pipe with respect to end fitting port, d) decrease in capscrew load (capscrew loosening) during reactor operation and e) seal ring lip relaxation. In the beginning of 1992, six more leaks which did not exceed the operating limits, were discovered in the same reactor unit. This paper covers only the- investigation of the three leaks in 1991. The investigation of the six leaks found in 1992, which is still on-going, is to find out if there are similarities among all leakages.