Load and Resistance Factor Design for Nuclear Pipes: Benefits and Challenges

The Board on Nuclear Codes and Standards (BNCS) recently decided to promote standards that use risk-informed design methods. In civil engineering practice a risk informed method, namely the Load and Resistance Factor Design (LRFD), has been in usage for quite some time. It is possible to extend such methods to the design of safety-related piping as well. This paper provides a brief overview of the LRFD method. Discussion is included for load factors to be used to account for the uncertainties in piping loads (e.g., internal pressure, sustained weight, etc.) and resistance factors to be used for addressing the uncertainties in strength of piping and analysis methods. Different load factors and resistance factors can be suggested for each load type and resistance type (e.g., hoop stress, bending stress, etc.). A design example for a feed water Class 2 piping system is provided to demonstrate the benefits of LRFD. This way, benefits such as the achievement of consistent reliability levels and the facilitation of a detailed risk analysis of mechanical systems are illustrated. Finally, the challenges associated with development of the LRFD method for nuclear piping are discussed. Such challenges pertain to the selection of the appropriate target reliability indices for piping, the development of equations for components such as tees, elbows, etc.