CRITICAL REVIEW OF ASME III PLASTICITY CORRECTION FACTORS FOR FATIGUE DESIGN-BY-ANALYSIS OF NUCLEAR POWER PLANT COMPONENTS

Despite significant technological progress in recent years, elastic-plastic fatigue analysis of pressure-retaining components remains a time-consuming venture. Accordingly, nuclear pressure vessel design codes such as ASME Section III provide simplified elastic-plastic analysis procedures as a practical alternative. This approach can be excessively conservative under certain conditions due to the bounding nature of the applied plasticity correction factor, K-e. Whilst this overconservatism was tolerable in the past, recent technical challenges arising due to consideration of environmentally-assisted fatigue (EAF) and design for long-term operation have posed difficulty in achieving acceptable fatigue usage based on extant Code assessment procedures for certain components. The incorporation of more accurate K-e factors has since been identified as a nuclear industry priority. This paper presents a critical review of K-e factors within ASME Section III, with particular attention given to a recently proposed approach by Ranganath, which is currently being considered for inclusion as an ASME Section III Code Case. Correction factors adopted within other nuclear and non-nuclear codes and standards (C&S) were also considered. The code-based K-e factors were comparied with K-e factors obtained directly from various elastic-plastic finite element (FE) models of representative plant components. The results revealed a considerable difference in conservatism between the code-based methods. Based on the elastic-plastic FE results, an alternative improved plasticity correction method was proposed. The need for a harmonized approach to determining K-e based on elastic-plastic FE analysis is discussed and identified as a desirable industry objective.