Moderator level elevation suppression method and device in control rod guide tube

In a reactor having a system of returning moderators to a reactor core tank by utilizing a control rod guide tube, cover gases are supplied to the guide tube in order to suppress the moderator level in the guide tube. However, since the pressure in the guide tube is relatively negative upon stopping a moderator recycling device, the cover gases are supplied by utilizing a pressure difference. Further, a back-flow checking device is disposed, and a recovering channel is disposed for returning the back-flown moderators to a recycling path. Since it is not necessary to open the upper end of the guide tube, the cover gases can be supplied easily and the elevation of the moderator level due to repeating startup and stopping of the recycling device is suppressed. Accordingly, there is no worry to wet a driving device even if a long control rod guide tube is not used, and the fluctuation range of moderators in the guide tube is reduced to decrease the pressure change in the reactor core tank. Accordingly, the length of the guide tube can be shortened, to lower the height of buildings and facilitate earthquake-proof designs. (N.H.)