To control reactor water with high accuracy by calculating the relation between the corrosion current density and crack developing speed of a required region by simulation calculation.

An operation condition is hourly inputted to an operation apparatus 7 and water quality is calculated at every time and place by radiolysis analysis. Reactor water from the bottom of a pressure container 1 turned toward a reactor water purifying system 9 or a recirculating system 2 is guided to a sampling line 15 and diissolved oxygen, dissolved hydrogen, conductivity and pH are measured by a water quality measuring part 8 to be taken in the operation apparatus 7 and the calculated result is approximately corrected. The electrochemical reaction of stainless steel and a nickel base alloy is performed in high temp. pure water by using the result of radiolysis analysis and, from the data base of current density and a crack advancing speed, the crack developing speed of the structure at each region in the pressure container 1 is evaluated. When the calculated crack advance speed does not satisfy a control reference, a water quality control device 6 is operated by the operation apparatys 7. For example, hydrogen may be injected from the water quality control apparatus 6.