Minor leakage in brazed joints is occasionally detected in nuclear power plant Class 3 piping systems such as emergency service water cooling lines for equipment room and containment area coolers. Detection of such leakage often causes the system to be taken out of service for performance of repair/replacement activities to restore the piping to its original condition. This can lead to plant shutdowns or outage delays which are very costly, and often at increased safety risk, to accomplish the repairs. To avoid such costs and increased risk, a method is needed to establish structural adequacy of leaking brazed joints so that leakage can be controlled and monitored until the next planned opportunity for replacement of the brazed joint.
During industry testing performed in the late 1950s, over 1200 lap joint tensile test specimens were brazed in a round-robin series of tests performed by 10 laboratories. The results of these tests showed that very little overlap is needed in a brazed lap joint to obtain full strength in the joint. In fact, braze metal shear strength exceeded piping collapse strength in all cases when the braze overlap was only 2.3 times the thickness of the members being brazed. Similar testing was performed by the U.S. Navy, and resulted in development of a NAVSEA document that established a conservative percentage of bond required for MIL F-1183 brazed fittings to prevent structural failure of a leaking brazed joint.
This paper discusses an ASME Section XI Code Case that has been developed to incorporate the results of this testing into a methodology for temporary acceptance of leakage of brazed lap joints in copper, copper-nickel, and nickel-copper ASME Code Class 3 nuclear piping systems.
