WO2006080948A2

WO2006080948A2 - Corrosion resistance of storage containers for nuclear waste

Info

Publication number: WO2006080948A2
Application number: PCT/US2005/021145
Authority: WO
Original assignee: HARRISON STERLING T
Current assignee: HARRISON STERLING T
Priority date: 2004-06-16
Filing date: 2005-06-15
Publication date: 2006-08-03
Anticipated expiration: 2006-12-16
Also published as: WO2006080948A3

Abstract

The present invention includes methods for improving the corrosion resistan ce of storage containers, storage devices, waste packages and/or engineered barriers of nuclear waste that are fabricated from corrosion resistant metallic alloys. The methods described herein render a corrosion resistant metal surface into a lower state of chemical reactivity. This improvement is achieved by an electro-chemical treatment and/or a chemical treatment.

Description

APPLICATION FOR PATENT

TITLE: CORROSION RESISTANCE OF STORAGE CONTAINERS FOR

NUCLEAR WASTE

SPECIFICATION CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of and claims the benefit of U.S. application number 10/869,247 filed June 16, 2004 which claims the benefit of U.S. provisional application number 60/478,987 filed June 16, 2003.

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

[0003] Not applicable.

BACKGROUND OF THE INVENTION

Description of the Related Art

[0004] Nuclear wastes result from the production and use of nuclear materials. Nuclear wastes are byproducts generated from spent nuclear fuel, dismantled nuclear weapons, and products such as radiopharmaceuticals. Nuclear waste poses a long-term threat to the environment and to public health. The half-life of many of these wastes is measured in thousands of years. The most critical issue for the safe storage of nuclear waste is effective shielding and containment of radiation. The containers that are utilized for this task must contain the radioactive waste for thousands of years.

[0005] The feasibility of a permanent national nuclear waste storage site is being evaluated at Yucca Mountain, Nevada. A critical factor in choosing Yucca Mountain was the minimal presence of moisture. The presence of moisture can promote corrosion. Corrosion is a long-term concern for nuclear waste storage containers. Many current and proposed designs of nuclear waste storage containers utilize corrosion resistant materials. The integrity and continued corrosion resistance of these construction materials will be of utmost importance over the entire life of the containers.

SUMMARY OF THE INVENTION

[0006] The present disclosure is directed to the passivation of the surfaces of storage containers, waste packages, engineered barriers, drums and/or devices of nuclear waste (all of which may be referred to as storage containers). The surface passivation will be accomplished by an electro-chemical treatment process and/or a chemical treatment process. Passivation is defined herein as the rendering of a corrosion resistant metal surface into a lower state of chemical reactivity. These containers may include but are not limited to vats, casks, drums, tanks, vessels, hoppers, bins, drip shields and pipe. These storage containers may be constructed of stainless steel alloys, precipitation hardened alloys, HASTALLOY (a trademark of Haynes International for a nickle alloy composition developed by Union Carbide), INCONELl (a trademarked product available from Special Metals Corporation), duplex alloys, nickel alloys, titanium alloys, or other corrosion resistant alloys. The improved resistance to corrosion that is a result of the passivation process, will add to the overall performance, safety and life of the storage container. [0007] Passivation of storage containers and/or storage devices of nuclear waste can be accomplished by an electro-chemical treatment process known as electropolishing, electrolytic polishing and/or electrochemical polishing, and/or a chemical treatment process known as chemical passivation. These processes can be used individually or in combination with each other.

[0008] Electropolishing is an electrochemical process by which surface material is removed by anodic dissolution. The principles of electrochemical surfaces treatments are known and do not require extensive review. Electropolishing is accomplished by applying a direct current to a metal alloy within a heated electrolyte bath. A cathode is assembled to mirror the surface of the material that is to be electropolished (the work piece). When the cathode and the work piece are submerged into an appropriate electrolyte, a direct current is applied. The work piece is charged anodic, and the cathode is made cathodic. The flow of the current through the system forces metal ions to be dissolved from the surface of the metal alloy. [0009] Surface contaminants including, but not limited to, grease, dirt, and iron, are inherent to the metal fabrication process. Mechanical cutting and polishing will leave abrasives and iron particles embedded in the metal. The presence of surface contaminates will disrupt the formation or structure of a material's naturally corrosion resistant surface layer. As can be seen in Figures 3 & 4, electropolishing removes surface material and leaves a microscopically smooth surface. Electropolishing removes the outermost surface material including it's contaminates, and embedded foreign metallic particles. Electropolishing promotes the formation of a thick, uniform and uninterrupted protective passive oxide layer.

[0010] Mechanical fabrication may create local galvanic differences. Both contaminates and residual stresses within the cold worked surface layer may produce localized galvanic corrosion cells. As can be seen in Figures 5 & 6, electropolishing removes the outermost surface material, and thus minimizes the possibility of local corrosion cells and leaves the surface with an even electrode potential.

[001 IJ Improvements of the near surface chemistry that result from electropolishing, will improve the corrosion resistance of nuclear waste storage containers. The improvements to the near surface chemistry, after electropolishing, of 316L stainless steel can be characterized by the data in Graph 1 (see Fig. 8) and Table 1. Graph 1 displays the results of an Auger Electron Spectroscopy (AES) analysis of electropolished 316L stainless steel. The AES reveals the elemental surface composition and the oxide layer thickness of the electropolished stainless steel. Graph 1 exhibits the atomic concentration of the element components of the 316L stainless steel. It can be seen that the electropolishing process preferentially dissolved the iron, leaving the surface with a greater relative concentration of Chromium and Nickel. The greater concentration of these alloys improves the corrosion resistance of the surface.

[0012] Passivation by electropolishing can be achieved by processing the material in an electrolyte bath or by "selective" or "brush" techniques.

Table 1

<~ι metallic /p metallic >i c

Cr°7Fe^ox >2.0

Oxide Layer Thickness >2.0 nm and >5.0 nm

Fe Concentration <6.9 μg/cm²

Cr Concentration ≤l .8 μg/cm²

Ni Concentration < 1.1 μg/cm²

Near Surface Chemistry Characteristics of Electropolished 316L Stainless Steel

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic view of an electropolishing system.

Figure 2 is a simple depiction of a selective or brush electropolishing system.

Figure 3 is a magnified (300Ox) view of cold-rolled annealed, 2B sheet 304 stainless steel material.

Figure 4 is a magnified (300Ox) view of electropolished cold-rolled annealed, 2B sheet 304 stainless steel material.

Figure 5 is a magnified (3000x) view of mechanically polished (320 grit) 304 stainless steel material. Figure 6 is a magnified (300Ox) view of mechanically polished (320 grit) 304 stainless steel material that has been electropolished.

Figure 7 is a schematic view of one embodiment or process for achieving same.

Figure 8 is a graph (Graph 1) of an Auger Electron Spectroscopy analysis of electropolished 316L stainless steel plotting "atomic composition" percent versus "depth".

DETAILED DESCRIPTION OF THE INVENTION

[0014] Electropolishing utilizing an electrolytic bath for submersion (or partial submersion) is described in very basic terms by Figure 1. The storage container part or material 10 to be electropolished is placed on a rack or tooling 30 which is attached to a positive DC power supply extension 33. The rack 30 is placed in a tank 34 filled with a proper electrolytic solution 36. The material 10 submerged in the electrolyte 36 is subjected to a direct current. The material 10 is maintained anodic while a cathode 38 is maintained cathodic. Anodic dissolution occurs as current flows through the electrolyte 36.

[0015] Selective and/or brush techniques of electropolishing are described in very basic terms by Figure 2. The part or material 40 to be electropolished is connected 55 to the positive terminal of a direct power source. An electropolishing brush or wand 45 is connected 56 to the negative terminal of a direct power source. A porous insulating pad 50 is used to prevent contact between the material 40 and the wand 45 and to hold a proper electrolyte 66 between the material 40 and the wand 45. The electrolyte 66 is pumped through the wand 45 and into the pad 50. Direct current is applied. The material 40 is maintained anodic and the wand 45 is maintained cathodic. Anodic dissolution occurs as current flows from the material 40 through the electrolyte 66 to the wand 45. [0016] One reason for the improved resistance to corrosion after electropolishing is that the electropolishing process enhances the properties of the materials' oxide Iayer42. This passive oxide layer after electropolishing is generally thicker, more uniform and with fewer interruptions.

[0017] Chemical passivation will improve corrosion resistance. During a chemical passivation treatment, the surface of a part is chemically treated by an acidic solution or bath, and/or a mild chemical oxidant. The process will remove free iron, imbedded particles and other foreign contaminants and will promote the formation of a thick uniform corrosion resistant layer. Chemical passivation can be accomplished by, for example, submersing, swabbing and/or spraying all surfaces with the proper passivating chemicals.

[0018] An example of the spray technique of chemical passivation is depicted in Figure

7. The passivating chemical 75 is pumped and sprayed onto the surface 72 of the part, material or container 70 to be chemically passivated. The passivating chemical 75 is sprayed onto the external surfaces of a container 70 through nozzles 75 connected to a distribution manifold 77. The passivating chemicals 75 are shown being applied to the internal surfaces of the container 70 by a spray ball nozzle 79.

[0019] The metal material or part 10, 40 or 70 to be passivated may be only one layer of a multilayer storage container (such other layers could, for example, be concrete, ceramic, etc.). Such material may be, for example, a 200 series (UNS 2XXXX) stainless steel, 300 series (UNS 3XXXX) stainless steel, 400 series (UNS 4XXXX) stainless steel, precipitation-hardened stainless steel, Duplex stainless steel, a grade of HASTALLOY, or a grade of INCONEL, nickel alloy, or titanium alloy material.

[0020] The metal material or part 10, 40 or 70 can be treated when new or used; internally and/or externally; and on or off site. [0021] The foregoing description of the preferred embodiments has been presented and described. It is not intended to limit the scope of the invention. Many modifications and variations of the invention as shown and described can be made without departing from the spirit of the invention.

Claims

What is claimed is:

1. A method for improving the corrosion resistance of at least one metallic part utilized in constructing a nuclear waste storage container, comprising the steps of: passivating a surface of the part.

2. The method according to claim 1, wherein said passivating step includes treating the part by an electro-chemical process.

3. The method according to claim 2, further including applying an acid wash to the surface of the part.

4. The method according to claim 2, wherein said passivating step including said treating the part by the electro-chemical process comprises electro-polishing the surface of the part.

5. The method according to claim 4, wherein said electropolishing step includes covering the surface of the part with an electrolyte, and rubbing an cathodic wand over the surface of the part.

6. The method according to claim 1 , wherein said passivating step includes treating the part by a chemical passivation process.

7. The method according to claim 6, wherein the step of treating the part by a chemical passivation process includes applying an acid wash to the surface of the part.

8. The method according to claim 1 , wherein the surface of the part to be passivated is a 200 series (UNS 2XXXX) stainless steel material.

9. The method according to claim 1, wherein the surface of the part to be passivated is a 300 series (UNS 3XXXX) stainless steel material.

10. The method according to claim 1 , wherein the surface of the part to be passivated is a 400 series (UNS 4XXXX) stainless steel material.

1 1. The method according to claim 1 , wherein the surface of the part to be passivated is a precipitation-hardened stainless steel material.

12. The method according to claim 1, wherein the surface of the part to be passivated is a Duplex stainless steel material.

13. The method according to claim 1, wherein the surface of the part to be passivated is a grade of HASTALLOY material.

14. The method according to claim 1 , wherein the surface of the part to be passivated is a nickel alloy material.

15. The method according to claim 1 , wherein the surface of the part to be passivated is a titanium alloy material.

16. In a method for storing a nuclear waste by placing the nuclear waste in a corrosion resistant metal container, the improvement comprising: passivating a surface of the corrosion resistant metal container.

17. The method according to claim 16 wherein said step of passivating the surface of the corrosion resistant metal container consists of treating only an external surface of the corrosion resistant metal container.

18. The method according to claim 1 , wherein the surface of the part to be passivated is a grade of INCONEL material.

19. An improved apparatus for storing a volume of nuclear waste wherein the apparatus includes a storage container having at least one layer made of a metal material and wherein a surface of the metal material has a passive oxide layer thereon, the improvement comprising: the passive oxide layer has improved properties accomplished by applying a means for passivating the surface of the metal material.