In a light water reactor (LWR) using water as a coolant, measures for the reduction of a radiation exposure dose for workers in periodic inspection works (periodic inspection), preventive maintenance works and the like have become important. As a part of such measures, a chemical decontamination operation has been frequently applied to structural materials, pipes, pumps and the like of the reactor. The chemical decontamination can remove an oxide film on the metallic material surface (hereafter referred to as structural material surface) with which a coolant containing a radioactive substance comes in contact, such as a structural material, and as a result, can remove radioactive substances such as cobalt-60 and cobalt-58 in a crud or in the oxide film on the metallic material surface, by combining reductive dissolution, oxidative dissolution and the like with the use of chemicals.
However, when the reactor is restarted after the decontamination operation, the radioactive substance will again deposit on the structural material surface. The deposition of the radioactive substance occurs together with the formation of the oxide film. Because the film grows at a high speed on a bared surface of metal after the decontamination operation, a radioactive substance is quickly deposited on that surface, and as a result, a dosage rate of the structural material surface rises again in a short time after the decontamination operation.
In order to solve this problem, there are proposed a method of forming an iron oxide film by bringing high-temperature water, steam, oxygen or ozone into contact with the surface of the material after the decontamination operation (Patent Documents 1 and 2), and a method of forming an iron oxide film by bringing a chemical agent containing an iron ion into contact with the surface of the material after the decontamination operation (Patent Documents 2 and 3).
In addition, as a general way to suppress the deposition of radioactivity, there is proposed a method of forming a film that resists capturing the radioactivity by injecting zinc or the like during the operation of the reactor (Patent Document 4).