Loading conditions in horizontal feedwater pipes of LWRs influenced by thermal shock and thermal stratification effects

In recent years, a particular form of crack formation has occurred in a number of pressurized and boiling water reactors on the internal surfaces of horizontal lengths of feedwater piping upstream of the steam generators and reactor pressure vessels.The fractographic evaluation and the orientation of the cracks show that these are to be attributed to cyclic stressing in the axial direction. Comparison of the stresses due to thermal shock and thermal stratification reveals that, on account of the associated load cycles, the cracks were in essence caused by thermal stratification. This is also indicated by the orientation of the cracks.The present results of the corrosion tests show that with high oxygen content (450 ppb) and temperature level (210°C) the strain rate at thermal stratification exerts an essential influence on the number of cycles to crack initiation. With the conservative test conditions described, the values may fall below those given in the ASME fatigue design curve. In the case of strain rates that apply to thermal shock, the cycles to crack initiation are on the safe side of the curve.The remedial measures taken by KWU by the installation of the siphon reduce the frequency of stress amplitudes. It can be concluded from corrosion tests simulating these conditions that with the strain rates occurring in this case, the number of cycles to crack initiation are on the safe side according to the ASME fatigue design curve.