Thermal stratification monitoring of Angra 2 steam generator main feedwater nozzles

In former times several utilities reported a large temperature excursion in the Steam Generator Main Feedwater nozzles during heat up and cool down of the nuclear plants. In Angra 2 this behavior was also observed and, besides that, thermal stratification during the hot standby mode of operation has also been measured. This behavior was detected thru a temperature monitoring program, in which a set of thermocouples were installed on the external surface of the pipe with the aim to monitor thermal transient. Because the stratification loading has not been defined in the design phase of the system, it is necessary to check the effect of this phenomenon on the fatigue analysis of the piping. This stratification leads to undesired temperature gradients which can conduct to strain and consequently stress in the nozzle material. This behavior can be part of the cause of accelerated fatigue of the component. The objective of this paper is to present the thermal stratification monitoring program implemented in Angra 2 and determined the usage factor of the components due to the phenomenon. (author)