Perspectives on Safety Margin Associated With Internal Repair of Buried Class 2 and 3 Safety Related Piping Using Carbon Fiber Reinforced Polymer Composites

Carbon fiber reinforced polymer (CFRP) composites have been used for decades in various industries such as aerospace, oil and gas, and transportation mainly due to their high strength-to-weight-ratio and excellent corrosion resistance. However, the use of CFRP in nuclear industry applications has been very limited. Recently, a new ASME Boiler and Pressure Vessel (BPV) Code Case N-871 has been proposed for internal repair of buried Class 2 and 3 nuclear safety related piping using CFRP for Service Levels A, B, C and D at temperatures not exceeding 200F. This is a first-of-a-kind CFRP application for nuclear safety related piping. It is known that CFRP materials are subject to property degradation due to environmental exposure as well as creep behavior under sustained loading. These factors should be considered when designing the CFRP repair for any nuclear safety related piping application to ensure an adequate safety margin is maintained. In the proposed Code Case, there are provisions for using two different design (analysis method) methods — Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) methods. The LRFD method has been widely used in civil engineering applications but has never been used in ASME Section III Code for nuclear piping applications. This paper presents a comparison of these two methods from a safety margin point of view. As CFRP is a new material for ASME BPV Code for nuclear safety related applications, several areas have been identified in the design concepts to ensure an adequate safety margin for Service Levels A, B, C and D. An effort is also made to provide guidelines on the required safety margin for CFRP repair of safety related piping. Finally, Code Case N-871 is reviewed to evaluate the minimum safety margin available for both ASD and LRFD methods.