Leak-before-break: An integrated approach for high energy piping

An integrated approach that involves system design, thermal hydraulics, materials, and fracture mechanics analyses to assure that pipe failure is highly unlikely is described. This approach is based on a leak-before-break (LBB) premise and includes through-wall flaw detectable leakage, through-wall flaw stability, and part-through-wall flaw fatigue crack propagation calculations. A successful application of LBB can reduce the amount of excessive pipe rupture restraint hardware. Assuring LBB not only reduces initial construction, future maintenance, and radiation exposure costs, but the overall safety and integrity of the plant are improved. This last benefit comes about from gaining additional insight into the piping systems and their capabilities. Details of the LBB methodology are presented here with specific examples for two pressurized water reactor lines (one inside containment fabricated of stainless steel, and the other outside containment made from ferritic steel). The application of this approach at Beaver Valley Power Station—Unit 2 indicates that pipe rupture hardware is not necessary for stainless steel lines inside containment as small as 6-in (152 mm) nominal pipe size that have passed screening criteria designed to eliminate potential problem systems (such as the feedwater system). Similarly, some ferritic steel lines as small as 3-in (76 mm) diameter (outside containment) can qualify for pipe rupture hardware elimination.