INTRODUCTION I

摘要

The criteria for setting the allowable stress intensity values for Class 1 Nuclear components (Class A in ASME Ⅲ, Division 5) were established in the 1970s. At that time, the candidate materials and the understanding of their high-temperature, time-dependent behavior was such that tertiary creep was considered to be a valid indicator of the loss of integrity of the material. One of the criteria for setting St was 80% of the minimum stress to initiate tertiary creep in a specified time. In recent years, new materials are of interest for the construction of high-temperature nuclear components. Also, there is a need to expand coverage of currently-approved materials to higher temperatures and longer times. The greatly expanded database has shown that the classical creep behavior on which the tertiary creep criterion was based is not characteristic of several of the materials of interest. The implications of the tertiary creep criterion on the St need to be re-examined in light of these needs.