Implicit Fatigue Design of Piping Systems for License Renewal of B31.1 Plants
The fatigue analysis of older vintage nuclear power plants was normally performed following the implicit method defined in the USAS B31.1, Power Piping Code. The USAS B31.1 Code applies reduction factors to allowable stresses for loadings with specific numbers of cycles. This effectively reduces stress amplitudes due to thermal bending and prevents fatigue damage from occurring. However, it does not provide sufficient detail to address the environmental impacts on fatigue when license renewal application is prepared for a specific plant. Explicit fatigue analyses were performed for many components in plants whose piping was designed by the implicit fatigue analysis methods. Over the years, modified or improved plant operating procedures may have been implemented. More sophisticated calculations became available for use on several components for which some explicit fatigue analyses is required to address the environmental effects for 60 years of operation. This paper addresses the type of information required to evaluate the effect of environmental factors on fatigue in a license renewal application for plants with reactor coolant system pressure boundary piping designed to B31.1 Code.