PURPOSE: To aim at reducing the cost of a plant and as well to facilitating the periodical inspection of the plant, by eliminating a communication pipe between a cyclone separator and a pump, and by directly cooling an internal pump with the use of an external cooling water pump while purge water is filled by the external cooling water pump.

CONSTITUTION: A communication pipe, etc. between a cyclone separator and a pump are eliminated, and cooling water from a hot well 6 is filled through the suction part of an auxiliary impeller 7 by means of a cooling pump 14 to cool an internal pump 2. Further, the cooling water is thrusted toward a nuclear reactor vessel (RPV)1, by way of the inside of the pump 2, and serves as purge water. Further, the flow rate of cooling water is controlled to be optimized per temperature by means of a temperature adjusting valve 15 which is controlled by a signal from a thermometer 16 attached to the internal pump 2. With this arrangement, the cooling system may be facilitated so that the number of peripheral equipments may be reduced. Accordingly, it is possible to aim at reducing the cost of a plant and facilitating the periodical inspection of the plant, thereby the rate of radiation exposure may be reduced.