Evaluation of LBB in Piping Considering Multiple Fatigue Crack Growth.

Through the recent development of fracture mechanics methodology in piping analysis, the LBB concept has been recognized as applicable to the safety design of LWR primary circuit piping. An important subject still remaining in the LBB analysis is the consideration of multiple fatigue crack growth and development in crack geometry in piping. In the evaluation procedure currently employed, a simple crack geometry is assumed to judge whether the crack is leak-detectable or not. The paper describes a LBB evaluation procedure in which the growth of multiple fatigue cracks is considered. Two criteria, i.e., critical condition of crack coalescence preceding the penetration of wall thickness and evaluation of crack configuration at the penetration, are introduced to account for cracks that coalesce and develop to the largest possible through-wall single crack at the onset of penetration. The paper also presents the results of a case study of PLR piping of BWR using the above procedure. It is shown that LBB can be justified in PLR piping of 4 inches or greater in diameter.