1. Field of the Invention
This invention relates to a chemical decontamination method and a treatment method and apparatus of chemical decontamination solution, and more particularly to a chemical decontamination method of dissolving an oxide film of a surface of a contaminated component, such as piping, instruments and components and a treatment method and apparatus of chemical decontamination solution in the decontamination process of dissolving the oxide film during or after the decontamination.
2. Description of the Related Art
In operation of a nuclear power plant, as an example of a radiation handling institutions, oxide film adheres or is generated in the inside of the piping, instruments, components, and the like, which are in contact with the fluid. If the fluid contains a radioactive material, for example, generated oxide film contains radionuclide. Therefore, a radiological dosage rises in the circumference of the piping or the instruments, which causes an increase of worker's dose of radioactivity at the time of the scheduled inspection work or the demolition work of the decommission of a nuclear reactor.
Several methods of removing the oxide film are known by now. In such methods, a method combining a process of oxidizing and dissolving chromium oxide in the oxide film by permanganic acid and a process of reducing and dissolving iron oxide which is a main component of the oxide film by oxalic acid is learned. The chemical decontamination method of dissolving and removing an oxide film chemically is enforced in a part of lately systems, which is much effective in reduction of radioactive material.
In order to remove such an oxide film, for example, the method of dissolving the oxide film or a metal base is used, in which method the oxide firm is made dissolved or exfoliated in solution.
In these decontamination methods, iron ions elute in the case of the reduction dissolution by oxalic acid. Since oxalic acid corrodes a metal base of carbon steel and stainless steel, a method of adjusting the valence and concentration of the iron ions (Fe2+, Fe3+) is learned in order to keep corrosion potential of the stainless steel in a passivation and suppress the corrosion.
The valance adjustment of the iron ion depends on a reaction shown in the following formulas that occurs by irradiating ultraviolet radiation into the oxalic acid, in which Fe3+ is reduced to Fe2+.H2O->e−+O2+H+->HO2(radical)  (i)Fe3++HO2(radical)->H++O2+Fe2+  (ii)
Dissociating reduced Fe2+ by a cation resin adjusts the concentration of the iron ion in the oxalic acid aqueous solution.
Moreover, as a decomposition method of the oxalic acid after decontamination of the oxalic acid, a decomposition method combining ultraviolet rays and hydrogen peroxide is learned.
Generation of Fe2+: the formulas (i) and (ii) mentioned above Decomposition of oxalic acid:H2O2+Fe2+->Fe3++OH−+OH(radical)  (iii)H2C2O4+2OH(radical)->2CO2+2H2O  (iv)
As the other decomposition method of oxalic acid, oxidation decomposition method by using the oxidization power of ozone is learned, and anodic oxidation decomposition method by electrolysis is also learned.
Moreover, a method of using ozone water as a decontamination solution that oxidizes and dissolves chromium oxide is also learned.
For example, Japanese Patent Disclosure (Kokai) No. S55-135800, which is equivalent to U.S. Pat. No. 4,287,002, shows a decontamination method combining an aqueous solution in which ozone gas was dissolved as an oxidizing agent, an organic acid, and decontamination solution of the oxidizing material. And Japanese Patent Disclosure (Kokai) No. H9-159798 shows a decontamination method sending decontamination solution with air bubbles generated by blowing ozone gas into a solution containing cellular material into a contaminated component.
Moreover, Japanese Patent Publication (Kokoku) No. H3-10919, which is equivalent to U.S. Pat. No. 4,756,768, indicates a chemical decontamination method using a permanganic acid as an oxidizing agent and using a dicarboxylic acid as a reducing agent. By using both the permanganic acid having high oxidization effect with low concentration and the dicarboxylic acid that can be decomposed into CO2 and H2O, it is possible to reduce the amount of secondary waste generated in this method compared with the chemistry decontamination method used till then.
Although the reduction of Fe2+ by ultraviolet rays has abundant results of applying to actual systems as a treatment method of oxalic acid decontamination solution, there is a possibility that glass covered an ultraviolet ray lamp may be damaged by a foreign substance, and there is an awaiting solution of the fall of reduction efficiency caused by extraction of sludge, such as ferrous oxalate, deposited on the glass surface in the case treating aqueous solution with high salt concentration or prolonged use.
And the ultraviolet rays used in the oxalic acid decomposition also has the same subject as mentioned above, and there is a possibility of ignition when combustibles to which hydrogen peroxide adhered are left in the state as it is, so sufficient cautions for their handling are needed.
Moreover, by using the aqueous solution in which ozone gas is dissolved as an oxidizing agent, not only chromium oxide in the oxide film but also metal base of the contaminated component are oxidized and dissolved, which cannot secure the material soundness for re-use of the instruments and causes an awaiting solution.
Furthermore, the decomposition reaction of oxalic acid by using ozone independently is slow, and there is a subject in the decomposition by using electrolysis independently that the electric conductivity of the aqueous solution falls and the decomposition reaction suspends.
Moreover, by using the dicarboxylic acid as a reducing agent, the contaminated metal component for decontamination other than the oxide film is dissolved by acid, which cannot secure the material soundness for re-use of an instrument and causes an awaiting solution.