Identification of Error Sources in Fatigue Analyses for Thermal Loadings

To identify thermal loadings (thermal shocks and thermal stratification), in German NPPs special fatigue monitoring systems have been installed. The detailed temperature measurement uses sheathed thermocouples, which are located on the external component surface. Tightening straps are used for the widespread method of locking the thermocouples into position. The basic objective of the paper is to identify several sources of error in fatigue analyses and to evaluate their specific importance. In fatigue analyses based on the outer-surface temperature, the thermal situation at the inner-surface has to be determined. This leading analysis step is not regulated in the ASME III code. In the paper, the variance in the calculated fatigue is quantified for different inner surface temperature profiles with a common good compliance to the outer surface temperature. Another focus of the paper is the presentation of findings concerning the exactness of fatigue analyses based on FEA. The effect of geometrical truncation (mesh density) and the time-increments on the calculated fatigue level is shown. Furthermore, regarding discontinuities with thermal shock loading, the differences in the calculated fatigue between applying ASME NB-3200 on the one hand and NB-3600 on the other are quantified. Finally, the uncertainty in the calculated fatigue associated with the known variance of the physical material property values (E-Modulus, thermal conductivity, thermal expansion) is highlighted.