A PWR start-up feedwater heating study

A domestic nuclear power plant has experienced problems with low steam generator final feedwater temperatures during plant start-up. The low feedwater temperatures have led to instabilities in steam generator level control. Also, long term thermal fatigue of the steam generator feedwater nozzles is a concern. A solution to the problem was identified and a proposed fix has been confirmed by the integration of a design review with a detailed computer simulation of the plant start-up. A detailed heat balance model for the PWR plant using the PEPSE computer program was used to evaluate the feedwater temperature for existing and proposed start-up procedures. The results of the investigation has been summarized as a plot of feedwater temperature versus plant thermal power. This plot shows that an additional source of feedwater heating is required at low power levels to maintain the final feedwater temperature above the NSSS supplier requirements of 320 F. To provide the additional feedwater heating, installation of a connection from main steam to the last feedwater heater shell has been proposed. The amount of steam flow necessary to maintain the recommended final feedwater temperature during plant start-up has also been determined using the model.