PURPOSE:To omit flushing operation and reduce a load on an operator by providing a circulation line in the emergency reactor core cooling system and circulating cleaning water at all times, and maintaining cleanness in the system. CONSTITUTION:Connections with positions A and B which are above the suction piping of an RHR pump 5 and as closest to a water source, i.e. a nuclear reactor pressure vessel 2 and a suppression pool 3 as possible in consideration of the system constitution are made. Further, return piping 33 is arranged from a position close to a system terminal on the opposite side from the side where supply piping 31 for cleaning water is connected to a condensed water storage tank 30 so as to circulate the water in the tank 30 into the RHR system. Connection positions of the piping 33 are a position C nearby a reactor water return outside isolation valve 20, a position D nearby a water injection valve 21, and a position E nearby the spray outside isolation valve 22 of the reactor top part of the pressure vessel 2. In this system constitution, the part required for the system flushing of the RHR system is supplied with a cleaning water flow from the tank 30, so impurities such as corrosion matter, etc., which is easily produced in pipes and equipments in pollutant water and stagnant water, i.e. are prevented from being produced and mixed into the system water.
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