Patent Number: 
Section: claims

1. A method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein electrochemical corrosion potentials of said metallic component materials are decreased by injecting zirconium hydroxide and hydrogen into said reactor cooling water. 2. A method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein electrochemical corrosion potentials of said metallic component materials are decreased by injecting zirconium hydroxide and hydrogen into said reactor cooling water, and an injecting amount of said hydrogen, an injecting amount of said zirconium hydroxide and injecting timing of said zirconium hydroxide are controlled so that said electrochemical corrosion potentials may be maintained below a target value. 3. A method of operating a nuclear power plant according to  claim 1 , wherein said zirconium hydroxide is injected so that a concentration of zirconium in said reactor cooling water may be kept within a range of 0.5 to 50 ppb. claim 1 4. A method of operating a nuclear power plant according to  claim 1 , wherein said zirconium hydroxide is injected so that a concentration of zirconium in said reactor cooling water may be kept within a range of 1 to 6 ppb. claim 1 5. A method of operating a nuclear power plant according to  claim 1 , wherein said hydrogen is injected so that a concentration of hydrogen in reactor cooling water of a water supply system may be kept within a range of 0.1 to 0.6 ppm. claim 1 6. A method of operating a nuclear power plant according to  claim 1 , wherein said hydrogen is injected so that a concentration of hydrogen in reactor cooling water of a water supply system may be kept within a range of 0.3 to 0.5 ppm. claim 1 7. A method of operating a nuclear power plant according to  claim 1 , wherein said metallic component materials are materials selected from the group consisting of stainless steels, nickel based alloys, cobalt based alloys, titanium based alloys, copper based alloys and ferroalloys, non-ferrous alloys, carbon steels and low alloy steels. claim 1 8. A method of operating a nuclear power plant according to  claim 1 , wherein said injection of zirconium hydroxide is performed during operating said nuclear power plant at a full power. claim 1 9. A method of operating a nuclear power plant according to  claim 1 , wherein said injection of zirconium hydroxide is performed during shut-down operation of said nuclear power plant. claim 1 10. A method of operating a nuclear power plant according to  claim 1 , wherein said injection of hydrogen is initiated after completion of said injection of zirconium hydroxide. claim 1 11. A method of operating a nuclear power plant according to  claim 1 , wherein an electrochemical corrosion potential of said metallic component material is monitored when said hydrogen is injected, and zirconium hydroxide is additionally injected during operating said nuclear power plant at full power based on the monitored result. claim 1 12. A method of controlling water chemistry of a nuclear power plant, wherein in order to suppress occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, zirconium hydroxide is injected into said reactor cooling water before injecting hydrogen into said reactor cooling water. 13. A method of controlling water chemistry of a nuclear power plant according to  claim 12 , wherein said zirconium hydroxide is injected so that a concentration of zirconium in said reactor cooling water may be kept within a range of 0.5 to 50 ppb. claim 12 14. A method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein zirconium hydroxide and hydrogen are injected into said reactor cooling water. 15. A method of operating a nuclear power plant for suppressing occurrence of stress corrosion cracking in metallic component materials in contact with reactor cooling water of a nuclear power plant, wherein zirconium hydroxide and hydrogen are injected into said reactor cooling water, and an injecting amount of said hydrogen, an injecting amount of said zirconium hydroxide and injecting timing of said zirconium hydroxide are controlled so that said electrochemical corrosion potentials may be maintained below a target value.