Temperature and wall strain fluctations during the TEMR thermal stratification tests at HDR

A form of crack formation has been observed in a number of PWRs and BWRs on the internal surfaces of horizontal feedwater piping upstream of the steam generators and reactor pressure vessels. These cracks are caused by special loading conditions associated with certain operating transients under reduced feedwater mass flow. Under such conditions stratified flow develops and leads to the formation of a rather then, separating mixing layer with large temperature gradients between the cold water at the bottom and the hot water in the upper part of the pipe. To examine these stratification phenomena the HDR-test series TEMR, T33 /1/, has been performed. Under certain flow conditions the separating mixing layer between cold and hot fluid regimes performs wave-type oscillations subjecting the pipe inside surface in its vicinty to fluctuating thermal loads. The impact of this dynamic loading on pipe fatigue is the subject of this paper