BACKGROUND

Field

This invention relates to methods for the passivation of metallic surfaces, more particularly, metallic surfaces, such as carbon steel, which have been exposed to a decontamination process in a water-containing system of a nuclear reactor.

Background Information

Most metallic surfaces which are exposed to hot water become corroded and develop a layer of metal oxide film. If this oxide film is formed in water containing radioactive species, or is otherwise exposed to water containing radioactive species, the radioactive species can become trapped in the corrosion layer. The accumulation of radioactive material on the internal surfaces of piping, tubing and components of a water coolant system in a nuclear reactor can produce undesirable radiation fields, which present a serious hazard to maintenance workers. The radioactive material can be removed by decontamination. Decontamination is generally defined as the removal of contamination from surfaces by washing, heating, chemical or electrochemical action, and mechanical action. Decontamination can be effective to remove the contamination, e.g., radioactive material, from components to reduce dose level. A usual method of removing this hazardous material is to employ an aqueous decontaminating solution, i.e., a chemical solution which will dissolve and/or loosen the contaminated oxide layer, and flush the dissolved or loosened materials from the system. This process is commonly referred to as chemical decontamination.

The type of oxide formed on a metallic surface by the corrosion process depends upon the particular alloy of the metal component, and the chemical conditions in the water. With the proper selection of chemicals, almost all metal oxide films containing radionuclides may be removed from metallic surfaces using a decontamination process. A further advantage is that chemical decontamination allows the treatment of complex surface geometry. The main disadvantage of chemical decontamination, however, is the generation of secondary liquid waste which requires appropriate processing for final treatment and conditioning.

Following decontamination, the decontaminated metallic surfaces are then re-exposed and in contact with cooling water, which originally caused the surfaces to become corroded and develop a metal oxide film. In the absence of a mitigating process, the decontaminated metallic surfaces are re-contaminated and re-develop the metal oxide film with radioactive materials contained therein. Thus, there is a need in the art to provide methods for mitigating the re-contamination of decontaminated metallic surfaces. The invention provides a method of mitigation that includes forming a passivation film on the decontaminated metallic surface directly following completion of the decontamination process. The mitigation method is performed while the decontaminated system or component is out of service. This passivation process can be conducted using a chemical solvent that was previously utilized in the decontamination process. The re-use of solvent is advantageous to reduce or minimize secondary waste that can be produced by the decontamination and passivation processes. Generally, decontamination and passivation techniques produce secondary waste, such as abrasive particles, liquid effluents, fumes and aerosols. Methods to control the production of this waste material is important since it can influence worker safety (e.g., production of aerosols, handling of this waste, and the like), and impact waste management (more waste that requires disposal). Further, in decontamination and passivation processes the final wastes are concentrated and therefore, represent a significant radiation source.

SUMMARY

In one aspect, the invention provides a method for mitigating recontamination of a decontaminated carbon steel surface in a water-containing system or component of a nuclear reactor. The method includes performing a decontamination of an oxide-containing carbon steel surface in the water-containing system or component, which includes removing the water-containing system or component from operation, adding a decontamination solvent to the water-containing system or component to contact the oxide-containing carbon steel surface, wherein the decontamination solvent includes a chelating agent, and removing oxide from the oxide-containing carbon steel surface to produce the decontaminated carbon steel surface. The method further includes, following performing of the decontamination and prior to returning the water-containing system or component to operation, providing a passivation solvent in the water-containing system or component; adding caustic and oxidant to the passivation solvent to form a passivation solution; inducing passivation and forming a passivation film on the decontaminated carbon steel surface; returning the water-containing system or component to operation; and as a result of forming the passivation film, reducing the re-growth of oxide on the decontaminated carbon steel surface. The decontamination solvent and the passivation solvent are the same or different and when the same, the decontamination solvent optionally remains in the water-containing system or component following decontamination for re-use in subsequent passivation.

The chelating agent can include citric acid. In certain embodiments, the chelating agent is selected from the group consisting of citric acid, ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), ascorbic acid, picolinic acid, ethylenediamine (EDA), and mixtures thereof. Further, in certain embodiments, the chelating agent is present in an amount that constitutes from about 0.5 g/L to about 2.0 g/L of the passivation solution, or about 1.75 g/L of the passivation solution.

The solvent can include oxalic acid and citric acid. In certain embodiments, the oxalic acid is present in an amount that constitutes from about 0.2 g/L to about 0.75 g/L of the passivation solution.

The caustic can include ammonium hydroxide. In certain embodiments, the caustic is selected from the group consisting of ammonium hydroxide, sodium hydroxide, sodium bicarbonate, hydrazine, ethylenediamine (EDA), and mixtures thereof. Further, in certain embodiments, the caustic is present in an amount that is sufficient to increase pH of the passivation solution, such as, to a range from about 9 to about 9.5.

The oxidant can include hydrogen peroxide. In certain embodiments, the chelating agent is selected from the group consisting of hydrogen peroxide, ozone, oxygen, potassium permanganate, sodium nitrite, and mixtures thereof. Further, in certain embodiments, the oxidant is present in an amount that is sufficient to achieve an oxidation reduction potential (ORP) of greater than 0 mV SCE in the passivation solution.

