In the design of a fast breeder reactor (FBR) piping system, asymptotic crack size () is traditionally estimated using the leak-before-break (LBB) master curve given in RCC MRx A16. The current approach is seen not to be sufficiently efficient in the LBB demonstration of FBR piping systems. Numerical crack growth studies have received much attention in the past decade. Deployment of fracture mechanics based numerical crack growth studies on prototype-sized pipe geometries generates considerable interest in estimating asymptotic crack size. This paper outlines the advantages of numerical finite element based crack growth studies to estimate the Cs on a prototype-sized pipe bend. The numerical procedure has resulted in an accurate and more realistic estimate of Cs than the traditional LBB Master curve as per A16. Considering the economic advantages of a relatively lower value of Cs for LBB demonstration, it is recommended to adopt the numerical method to estimate Cs for the FBR piping system.