Normally, nuclear power plants operate at 100% full power level. However, in some special cases, nuclear power plants need to decrease power and operate at lower power level. For example, because of the typhoon, test and equipment maintenance requirements etc., nuclear power plants decrease power to specific power level and stay for several hours at this power level. Because there is inserted time limit of the power compensation rod (G rod) in the technical specification, inserting control rod to decrease power cannot always be chosen, and the strategy that decreases power by boron injection should be taken. In this paper, powers down strategies are introduced. And the similarities and differences between decreasing power by inserting control rod and decreasing power by injecting boron are also introduced. Before reducing power, reactivity equilibrium calculation should be performed and the predicted amount of boron injection can be calculated. Then according to the power down rate, Boron injection rate can be estimated. Finally, Boron injection operation can be performed. During the power reducing, R rod and some important equipment associated with boron injection should be monitored closely, especially the drain tank water level of condensate extraction system (CEX) and the response of feedwater flow control system (ARE). A key control of power down is to control the axial power deviation (delta I) that is one of the most difficult technical problems. Delta I must be kept in the functional area and it is inhibited that the operating point goes outside limits (left limit and right limit) of region II in any situations. It is performed on CPR1000 full scope simulator (FSS) to decrease power by inserting control rod and boron injection in this paper. Several curves obtained during power reducing are analyzed in detail. According to analysis results, the key procedure, operation risks and operation control during power decreasing are proposed as reference. The purpose of them is to meet the requirements of nuclear safety during power down operation.

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