Effects of Simulated Seismic Loading on LBB Assessment of High Energy Piping

Abstract

The current Leak Before Break (LBB) assessment is based primarily on the monotonic fracture tearing instability. In it the maximum design accident load is compared with the fracture-tearing resistance load. The effect of cyclic loading has not been generally considered in the fracture assessment of nuclear power plant piping. It is a well-known fact that reversible cyclic loading decreases the fracture resistance of the material, which leads to increased crack growth. Indian nuclear power reactors consider Operational-Basis-Earthquake (OBE) and Safe-Shutdown-Earthquake (SSE) events in the design of various structures, systems, and components. Keeping this in view a series of cyclic tearing tests have been conducted on straight pipes, made of ASTM SA333 Gr.6 carbon steel. This is the material of primary heat transport (PHT) piping material of Indian Pressurized Heavy Water Reactors (PHWR). In this series 13 tests have been carried out circumferentially through wall cracked seamless and circumferential seam welded straight pipes under reversible cyclic bending loading. All the tests have been conducted under quasistatic, i.e., slow loading rates and dynamic inertia effects are not considered. The cyclic test results have been compared with the corresponding monotonic pipe fracture test results. These test results and its comparison with corresponding monotonic tearing clearly illustrate the need of addressing the reduction in apparent fracture toughness of material under reversible cyclic loading and the safe number of load cycles in the LBB assessment.