Comparative evaluation of plastic design methods for fatigue assessment of a nuclear class 1 piping nozzle

Design-by-analysis (DBA) procedures for Nuclear Class 1 pressure vessels such as those prescribed within ASME Boiler and Pressure Vessel Code (BPVC) Section III, provide rules to demonstrate assurance against fatigue failure. Two general assessment routes exist, linear finite element analysis (FEA) with stress categorization and elastic-plastic penalty factors, or nonlinear FEA with direct multiaxial strain evaluation. Whilst the elastic design route possesses many practical advantages, it is widely acknowledged to be very conservative, sometimes unacceptably so. At the cost of additional analysis effort, plastic design methods can provide a more appropriate evaluation of fatigue usage, potentially avoiding unnecessary design modifications and reducing the burden of in-service inspection requirements. This paper presents and compares various strain measures proposed for ASME III plastic fatigue analysis within the technical literature. A case study of a typical pressurized water reactor (PWR) main coolant line (MCL) piping nozzle subjected to pressure and thermal loads is presented. The influence of strain measure selection on the FE-derived strain concentration (Ke) factors is examined. Some important considerations for calculation of realistic Ke factors in ASME III are further discussed.