Quantitative Comparison of Environmental Fatigue Methods

Abstract

For many nuclear power plants worldwide the operation period will be extended from 40 to 60 years in the coming years. As the operation period increases the importance of knowledge of ageing mechanisms like fatigue increases. Knowledge of the influence of the environment is crucial, since environmental fatigue is a relatively new development which is a modification to the existing assessment method and has to be projected to 60 years as well. This paper is a follow up of the ASME PVP2013-97695 paper: overview of international implementation of environmental fatigue. A quantitative comparison of the resulting cumulative usage factors including environmental fatigue is made for the most commonly used and well defined methods. The comparison of the environmental fatigue codes is made on a spray nozzle of the pressurizer. This is a known fatigue relevant location with high stresses due to thermal loading. The high thermal loading is due to the spraying of relative cold water into the warm pressurizer. The comparison is made for 11 methods, sets of fatigue curves and environmental fatigue correction factors (Fen factor), and 4 types of material. The 4 materials are: low alloy, carbon, nickel alloy and austenitic stainless steel. The fatigue curves of ASME 2007, ASME 2010, KTA 1996, KTA 2013, NUREG/CR-6909 and Code Case N-792 are compared. The Fen factors are compared for the following methods: NUREG/CR-6583, NUREG/CR-5704, NUREG/CR-6909, Code Case N-792, JNES SS-0503, JNES SS-1005 and NUREG/CR-6909 rev1. Code Case N-761 is included for the final comparison of the cumulative usage factors including environmental fatigue. The differences in percentages are considerable between the different methods. For this specific case, the difference in cumulative usage factor including environmental fatigue for austenitic steels is 70 %. For nickel alloy materials the difference is 115%. For low alloy materials the difference is the highest: 267%. For carbon steels the difference in cumulative usage factor is 146%. The most conservative cumulative usage factors including environmental fatigue are ASME 2007 or KTA 1996 fatigue curves combined with the NUREG/CR-5704 (austenitic steel and nickel alloy) or NUREG/CR-6583 (low alloy and carbon steel). The next highest results are found by the Japanese methods (JNES-SS-0503 and JNES-SS-1005). The common factor for these methods, is the fatigue curve for austenitic steels as used before 2010. The lowest cumulative usage factors are obtained by implementing NUREG/CR-6909. Using the latest revision of NUREG/CR-6909 the cumulative usage factors increase slightly (about 7%). The paper shows the considerable differences of usage factors when different codes are applied to the same problem.