Patent Number: 
Section: claims

1. A method for mitigating a stress corrosion cracking of structural material in a nuclear power plant, comprising the step of:injecting hydrogen and a reductive nitrogen compound containing nitrogen having a negative oxidation number into a reactor water of a boiling water nuclear power plant; andstopping injection of the reductive nitrogen compound when the oxygen concentration reaches a prescribed value,wherein said reductive nitrogen compound is injected into the cooling water from at least any one of a system line selected from a group consisting of a reactor water clean up system, a primary loop re-circulation system, an emergency core cooling system and a control rod drive hydraulic system, at a time of start up operation or shut down operation of the reactor, and during a part or all of a period of injecting the hydrogen into the reactor water,wherein the injection of said reductive nitrogen compound is stopped when the oxygen concentration in the cooling water has become a prescribed value or less, andwherein the injection of said reductive nitrogen compound is controlled so that the concentration of said reductive nitrogen compound in the cooling water comes to 9.4×10−6 mol/liter or less. 2. The method for mitigating a stress corrosion cracking of structural material in a nuclear power plant according to claim 1, wherein the injection of said reductive nitrogen compound is stopped when the ammonia concentration in the cooling water becomes a prescribed value or more. 3. The method for mitigating a stress corrosion cracking of structural material in a nuclear power plant according to claim 1, wherein said reductive nitrogen compound is injected into the cooling water from at least any one system line selected from the group consisting of a reactor water clean up system, a primary loop re-circulation system, an emergency core cooling system and a control rod drive hydraulic system, at the time of start up operation or shut down operation of the reactor. 4. The method for mitigating a stress corrosion cracking of structural material in a nuclear power plant according to claim 3, wherein a site of the injection of said reductive nitrogen compound from the reactor water clean up system line is at least one injection connection selected from the group consisting of an injection connection provided on a line connecting the down stream of the filter demineralizer and the reactor pressure vessel and an injection connection provided on the line connecting the down stream of the connection part in the reactor water clean up system line of the sampling line provided for the purpose of measuring water quality at the outlet of filter demineralizer and the reactor pressure vessel. 5. The method for mitigating a stress corrosion cracking of structural material in a nuclear power plant according to claim 1, wherein said reductive nitrogen compound is at least any one compound selected from the group consisting of hydroxylamine, carbohydrazide, hydrazine, ammonia and diazine.