NB-3650M fatigue calculation procedure

Fatigue is considered a significant long-term degradation mechanism for safety significant pressure equipment in nuclear power plants. The ASME Section III [1] procedure aims to prevent formation of fatigue cracks during the specified service period. Practical experience gathered during tens of years with hundreds of reactors has shown that the ASME methodology functions very well or is even conservative. Several studies performed in the USA and Japan indicated significant environmental effects on the fatigue as compared to fatigue without environmental effects. This has yielded a methodology as described in the NUREG/CR-6909 [2] and the JSME S NF1-2006 [17]. This methodology should obviously be applied on a best estimate fatigue analysis with reasonable safety factors. Due to the fact that such an analysis is not available and as the ASME methodology functions well this methodology is applied in conjunction with the ASME methodology. Obviously this is conservative, possibly even far overconservative. From an analytical point of view the NB3600 (engineering approach) and NB3200 (detailed 3D analysis) fatigue analysis methodology and the one in the NUREG/CR-6909 (full time history) are not compatible. In the work presented here a methodology was developed to combine these methods.