Fatigue Analysis of Nuclear Class-1 Small-Bore Piping Connections in CANDU Reactors

Class 1 nuclear piping systems, small-bore piping branch connections are susceptible to high thermal fatigue stresses, particularly in cases where harsh thermal transients are present. Calculating the Cumulative Usage Factor (CUF) using the procedure defined in the ASME Boiler and Pressure Vessel Code in Section III NB-3600 can limit the permissible cycle life (i.e., fatigue life) of the piping component. As per NB-3630(c), when a design does not satisfy the requirements of NB-3640 and NB-3650, a more detailed alternative analysis defined in NB-3200 can be used. This work compares the code requirements, analysis methodology, and results of a typical small bore branch connection connected to a piping header that is assessed against the requirements of NB-3600 and NB-3200. Piping is modeled using beam elements by utilizing PIPESTRESS piping analysis software for the NB-3600 based analysis. In comparison, a finite element model in Ansys Workbench is developed for the NB-3200 transient thermal and structural analysis. Representative pressure and thermal transients applicable to the heat transport system of a typical CANDU reactor are utilized in the analysis. The analysis results show that a significant drop in the Cumulative Usage Factor is achieved with the NB-3200 approach when compared with NB-3600.