Control device for nuclear power plant

This invention concerns FBR type reactors. In a case if a power setting signal from a power setter is lower than a predetermined set value for switching to feedwater flow rate, a comparator reduces the number of rotation for the driving motor of a secondary coolant pump to one-half. In this case, an aimed control value for the primary coolant temperature at the exit of a nuclear reactor generated from a function generator is stepped-down at the switching point for the number of rotation of the secondary coolant pump. Further, for the water-system, an aimed control value for the secondary coolant temperature at the exit of a steam generator generated from the function generator is stepped-down at the switching point for the number of rotation of the secondary coolant pump in the same manner as in the primary coolant circuit. According to the this invention, instable flow of 2-phase stream to water-steam system can be prevented in low power state, without continuous control for the coolant flow rate. (K.M.)