Negative pressure maintaining device in the pedestal of a reactor pressure vessel

In a BWR type nuclear power plant, a closing damper is disposed to a duct formed by penetrating the pedestal of a reactor pressure vessel. A connection nozzle is branched from the intermediate between the closing damper and the pedestal. A local exhaust duct penetrating the reactor container, a filter and a local exhaust blower are connected to the nozzle in this order. Upon periodical inspection, the closing damper is closed and the local exhaust blower is operated. Then, the pressure at the inside of the pedestal is maintained to a predetermined negative pressure due to the reduction of the pressure by the operation of the blower and the air pressure by spontaneous flow from the opening for transferring the control rod driving mechanism. Accordingly, the inside of the pedestal can be maintained at a negative pressure by a simple operation and the contamination can be prevented from prevailing to the inside of the dry well. (I.N.)