Operation method for nuclear power plant

An aqueous solution containing at least one kind of metal or metal compound is injected to a reactor upon initial starting, or re-starting or before the starting of operation of a nuclear power plant, to conduct metal-injection prefilming operation and keep the metal oxide films satisfactory. This suppresses the amount of radioactive nuclides deposited on primary system pipelines or equipments in the nuclear power plant. Namely, exposure dose can be reduced and integrity of materials can be improved by the injection of at least one kind of metal selected from the group of metals such as Ni contained in the structural materials, corrosion resistant metals forming metal acid, such as V, or metals having a possibility of lowering corrosion potential such as platinum in order to stabilize membranes upon start-up of the operation. In addition, the effect of the injection techniques is further enhanced also after decontamination in which formation of films on the surface of the materials tends to undergo the effect of water quality. (T.M.)