[en] In the present invention, a dose rate of turbine system during plant operation due to moving behavior of ¹⁶N generated by nuclear reaction in a pressure vessel of a nuclear reactor is reduced. Concentrations of dissolved oxygen and dissolved hydrogen in the reactor are monitored to recognize the oxidation/reduction atmosphere of a coolant cleanup system. That is, a hydrogen ion concentration control device is disposed to a feedwater pipeline for optimizing hydrogen ion concentration in the reactor water by feeding water. It is monitored as to whether the inside of the reactor is excessively oxidative atmosphere or excessively reducing atmosphere in each of the cases when hydrogen is injected or not injected. Since excessive injection of hydrogen leads to elevation of the dose rate, a valve of the hydrogen injection device is controlled properly. Further, a control device for hydrogen ion concentration of the reactor water is operated for suppressing the dose rate of the turbine system. In this case, since the optimum value for the hydrogen ion concentration is different depending on whether hydrogen is injected or not, a monitored value for the dose rate of the main steam pipe is fedback to a control device. (I.S.)