Recontamination mitigation method by carbon steel passivation of nuclear systems and components
The invention relates to methods for mitigating the recontamination of carbon steel surfaces in a nuclear reactor or related water-containing systems and components, which have undergone a decontamination process. The methods include conducting a passivation process of the carbon steel surfaces directly following completion of the decontamination process, prior to the system or component being returned to service. In certain embodiments, a chelating agent is used in the decontamination process and is retained following completion of the process, for use in the subsequent passivation process. The passivation process forms a passivation film that is effective to reduce recontamination of the decontaminated carbon steel surfaces.
1. A method for mitigating recontamination of a decontaminated carbon steel surface in a water-containing system or component of a nuclear reactor, comprising:
performing decontamination of a radionuclide-containing oxide film deposited on a carbon steel surface in the water-containing system or component, comprising:
removing the water-containing system or component from operation;
adding to the water-containing system or component a decontamination solvent to contact the radionuclide-containing oxide film on the carbon steel surface, the decontamination solvent comprising a chelating agent; and
removing the radionuclide-containing oxide film on the carbon steel surface to produce the decontaminated carbon steel surface;
following the decontamination and prior to returning the water-containing system or component to operation, using the decontamination solvent that is remaining from the decontamination in a subsequent passivation process;
conducting the passivation process, comprising:
cooling the water-containing system or component and decontamination solvent to a passivation temperature of about 140° F. to 160° F.;
adding caustic and oxidant to the decontamination solvent that is remaining to form a passivation solution;
inducing passivation of the water-containing system or component with the passivation solution; 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 the passivation film, reducing the re-growth of a radionuclide-containing oxide film on the decontaminated carbon steel surface when the water-containing system or component is returned to operation.
2. The method of claim 1 , wherein the chelating agent is citric acid.
3. The method of claim 2 , wherein the decontamination solvent further comprises oxalic acid.
4. The method of claim 3 , wherein the oxalic acid is present in an amount that constitutes from about 0.2 g/L to about 0.5 g/L of the passivation solution.
5. The method of claim 1 , wherein 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.
6. The method of claim 1 , wherein the caustic comprises ammonium hydroxide.
7. The method of claim 1 , wherein the caustic is selected from the group consisting of ammonium hydroxide, sodium hydroxide, sodium bicarbonate, hydrazine, ethylenediamine (EDA), and mixtures thereof.
8. The method of claim 1 , wherein the oxidant comprises hydrogen peroxide.
9. The method of claim 1 , wherein the oxidant is selected from the group consisting of hydrogen peroxide, ozone, oxygen, potassium permanganate, sodium nitrite, and mixtures thereof.
10. The method of claim 1 , wherein 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.
11. The method of claim 1 , wherein the chelating agent is present in an amount that constitutes about 1.75 g/L of the passivation solution.
12. The method of claim 1 , wherein the amount of caustic added to the decontamination solvent is sufficient to increase pH of the passivation solution resulting therefrom.
13. The method of claim 12 , wherein the pH is in a range from about 9 to about 9.5.
14. The method of claim 1 , wherein the amount of oxidant added to the solvent is sufficient to achieve an oxidation reduction potential (ORP) of greater than 0 mV SCE in the passivation solution.
15. A method of employing a decontamination solvent to decontaminate and passivate a carbon steel surface in a water-containing system or component of a nuclear reactor, comprising:
decontaminating the carbon steel surface containing a radionuclide-containing oxide deposit, comprising:
removing the water-containing system or component from operation;
adding to the water-containing system or component a decontamination solvent comprising a chelating agent, to contact the radionuclide-containing oxide deposit on the carbon steel surface; and
removing the radionuclide-containing oxide deposit from the carbon steel surface to produce a decontaminated carbon steel surface;
following decontamination, prior to removing the decontamination solvent from the water-containing system or component and prior to returning the water-containing system or component to operation, conducting a passivation process that comprises:
cooling the water-containing system or component and decontamination solvent to a passivation temperature of about 140° F. to 160° F.;
adding caustic and oxidant to the decontamination solvent retained in the water-containing system or component to form a passivation solution; and
inducing passivation and forming a passivation film on the decontaminated carbon steel surface; and
returning the water-containing system or component to operation; and
as a result of the passivation film, reducing the re-growth of a radionuclide-containing oxide film on the decontaminated carbon steel surface when the water-containing system or component is returned to operation.