On Fatigue Verification of Class 1 Nuclear Power Piping According to ASME NB-3600

In this paper, we address fatigue verification of Class 1 nuclear power piping according to ASME Boiler & Pressure Vessel Code Section III (ASME III), NB-3600, and several relevant issues that are often discussed in connection to the power uprate of several Swedish BWR reactors in recent years. We review first the basic requirements and their verifications using finite element analysis in detail. Thereafter, we clarify a so-called simplified elastic-plastic discontinuity analysis for further verification if the basic requirements found unsatisfactory, and examine necessary computational procedures for evaluating alternating stress intensities and cumulative damage factors. Our emphasis is placed on alternative verification procedures, which do not violate the general design principles upon which ASME III NB-3600 is built, when fatigue damage usages predicted by the simplified elastic-plastic discontinuity analysis are unaccepted. An alternative which employs a non-linear finite element computation and a refined numerical approach for re-evaluating the cumulative damage factors is suggested. Concluding remarks are given.