Water quality control method and device for nuclear power plant, and nuclear power plant

A nuclear power plant comprising:

• a nuclear reactor;
• a turbine which is driven by steam generated by said nuclear reactor;
• a condenser which condenses the steam from said turbine;
• a purifying unit which purifies the condensed water from said condenser, said condensed water purifying unit includes a condensate demineralizer and one of a hollow fiber filter and a pre-coat filter having an iron removing rate which enables to maintain the iron concentration rate in the feed water below 0.1 ppb and disposed at upstream side of said condensate demineralizer;
• a feed water pump and heater which heats the purified water from said purifying unit and feeds the same into said nuclear reactor as cooling water;
• a gas injection unit having a gas injecting point at the downstream side of said condensate demineralizer and at the upstream side of said feed water pump and heater and injecting at least one of nitrogen gas, dinitrogen monoxide gas and carbon dioxide gas into the cooling water;
• a pH measuring means for determining pH of the reactor water in said nuclear reactor at a room temperature;
• and
• a control unit which controls a gas injection amount from said gas injecting unit based upon the determined value from said pH measuring means so as to decrease the pH of the reactor water below 6.8 determined at the room temperature.

An operating method of a nuclear power plant which includes a nuclear reactor, a turbine which is driven by steam generated by said nuclear reactor, a condenser which condenses the steam from said turbine, a purifying unit which purifies the condensed water from said condenser, a feed water pump and heater which heats the purified water from said purifying unit and feeds the same into said nuclear reactor as cooling water, comprising the steps of:

• either exchanging all of the fuel rod in said nuclear reactor or loading non-exchanged fuel rods after removing oxides of Co deposited on the fuel rod cladding tubes after a regular inspection and before restarting of the nuclear power plant;
• maintaining the iron concentration in the feed water below 0.1 ppb;
• and
• controlling the water quality of the cooling water so that the pH of the reactor water in said nuclear reactor is shifted into a weak acid state determined at a room temperature.