This paper presents a reliability-based low-cycle fatigue design approach for class 2 and 3 nuclear pipes. Initially, the methodology and background for the design of the pipes according to the ASME Boiler and Pressure Vessel (B&PV) Code is presented. Then a probabilistic design methodology based on the ASME B&PV Code and focused on straight pipes with butt-welds is presented. The method ensures an acceptable reliability level for piping by keeping the failure probability within some upper threshold and accounting for the uncertainties of the design variables. In opposition, the current design may result in diverse and often unknown probabilities of failure for piping. A computational example illustrates the developed method.

Nuclear pipes are designed to withstand primary membrane stresses generated by internal pressure according to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code, Section III, Parts NB-3641, NC-3641, and ND-3641, which uses the allowable stress design (ASD) method. This paper presents limit states and equations for the design of nuclear pipes for internal pressure based on the load and resistance factor design (LRFD) method. The LRFD method is shown and explained to be more consistent than the ASD method. The paper presents the procedure for the derivation of the partial safety factors. Moreover, these factors are evaluated, example calculations are provided, and comparisons with the present design are made.

Class 2 and 3 nuclear piping is designed according to the allowable stress design (ASD) method used in the ASME Boiler and Pressure Vessel (B&PV) code, Sec. III, Division 1, NC and ND-3600 according to which safety factors applied to the strength of steel (resistance) provide acceptable safety margins for the piping design. This paper describes the development of design equations according to the load and resistance factor design (LRFD) method for loads that cause primary stress such as sustained weight, internal pressure, and earthquake for different levels of piping operation. The LRFD method differs from the ASD since multiple factors, applied separately to each load and the strength of steel, provide safety margins that correspond to a known and acceptable probability of failure for the piping. Load combinations are provided, statistical properties for the variables under consideration are presented and the partial safety factors are moreover illustrated for different values of the target reliability index.

Nuclear safety-piping is designed according to the ASME Boiler and Pressure Vessel Code, Sections III, NB-, NC-, and ND-3600 that use the allowable stress design method (ASD). The potential use instead of reliability-based design equations for nuclear piping could benefit the structural design by providing, among others, consistent reliability levels for piping. For the development of such equations, not only the probabilistic characteristics of the design variables are needed, but also the quantification of the uncertainties introduced by the strength models that are used in order to estimate the resistance of pipes subjected to different loadings. This paper evaluates strength models, and therefore provides necessary information for the reliability-based design of pipes for burst or yielding due to internal pressure and for excessive bending.

Previous research provided a reliability-based approach for the low-cycle fatigue design of Class 2 and 3 nuclear pipes. Reliability-based equations incorporate a threshold for the piping reliability and yield a safe and uniform design that moreover can be associated with the design risk. This brief discusses a direct reliability method to be used for Eq. (11) in Subparagraphs NC and ND-3653.2 of the ASME B&PV Code for Class 2 and 3 pipes, respectively. A computational example illustrates the developed methodology.