WO2013124464A1

WO2013124464A1 - Cvd coated crucible and use

Info

Publication number: WO2013124464A1
Application number: PCT/EP2013/053634
Authority: WO
Inventors: Mike Roemmler; Steve Wendel
Original assignee: Sgl Carbon Se
Priority date: 2012-02-23
Filing date: 2013-02-22
Publication date: 2013-08-29

Abstract

A vitrification crucible (100) including a graphite shell (110) including a wall (112) and a base (115) defining an interior side portion and an exterior side portion, wherein at least the interior side portion is coated with a vapor deposited silicon carbide (120). A method including coating a vitrification crucible (100) with silicon carbide (120) by vapor deposition. A method including heating a frit material in a vitrification crucible (100) including a graphite shell (110) coated with a vapor deposited silicon carbide (120); adding a waste material to the crucible (100) such that a contents of the crucible (100) include the frit material and the waste material, and cooling the contents of the crucible (100).

Description

CVD COATED CRUCIBLE AND USE

BACKGROUND

Field

[0001] Vitrification. Background

[0002] In situ vitrification of nuclear waste, e.g. from dismantled nuclear reactors, is one solution to the long-term storage of such waste. Representatively, in situ vitrification involves combining nuclear waste with a frit material in a heated crucible and then cooling the contents of the crucible to form a glass and contain the nuclear waste in the glass.

[0003] One material used for a crucible in a nuclear waste vitrification process is graphite. Graphite tends to erode (e.g., oxidize) at higher temperatures in an oxygen atmosphere. Accordingly, efforts have been directed at coating the graphite crucible with a material to reduce oxidation or corrosion. One material suggested is alumina. Alumina coatings, however, have been observed to peel off at process temperature. Further, where alumina is coating the inside of a crucible, graphite behind the alumina can corrode due to process gases (e.g., oxygen) escaping behind the coating. Alumina liners have also been considered. Alumina is poorly thermally conductive and therefore, as a liner, alumina inhibits the heating of the contents of the crucible.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1 illustrates a cross-sectional side view of one embodiment of a crucible for use in a vitrification process.

[0005] Figure 2 shows a process of vitrifying nuclear waste.

[0006] Figure 3 shows a process of forming a crucible from a carbon fiber or filament. DETAILED DESCRIPTION

[0007] A vitrification crucible and a method of coating a vitrification crucible and storage of waste in a vitrification crucible are described. In one embodiment, the vitrification crucible includes a graphite shell including a wall and a base that define an interior side portion and an exterior side portion. At least the interior side portion and, in another embodiment, the interior side portion and the exterior side portion, and, in another embodiment, all portions of the shell are coated with a vapor-deposited or vacuum-deposited silicon carbide.

[0008] Figure 1 illustrates a cross-sectional side view of one embodiment of a crucible for use in a vitrification process. Crucible 100 includes a shell 110 of a graphite material that, in one embodiment, has a wall 112 of a cylindrical shape. In this embodiment, shell 110 also includes base 115. Wall 112 and base 115 define an interior side portion and an exterior side portion of shell 110. The interior side portion defines a volume suitable for containing a vitrification product.

[0009] In one embodiment, shell 110 may be made from an extrusion or molding process (e.g., isostatic molding process). Representatively, coke is combined with pitch and mixed. The mixture is extruded or pressed in a mold. The product is then carbonized by heating the product to a temperature on the order of 1000°C. This is followed by a graphitization treatment on the order of about 2800°C, optionally with subsequent machining to the desired final dimensions.

[0010] Representative dimensions for a graphite crucible formed by, for example, isostatic molding include, but are not limited to, crucibles having an inner diameter on the order of three inches; an outer diameter on the order of five inches; and a length or height dimension on the order of 20 inches. It is appreciated that, in another embodiment, an inner diameter is on the order of four to 12 inches; an outer diameter on the order of five to 14 inches; and a length on the order of 20-40 inches. In still another embodiment, an inner diameter is, for example, 48 inches; an outer diameter is 50 inches; and a length is on the order of up to 90 inches.

[0011] Referring again to Figure 1, overlying shell 110 in one embodiment is coating or film 120 of a silicon carbide material. Silicon carbide is introduced through a vapor-deposited or vacuum-deposited process, including chemical vapor deposition, physical vapor deposition and pulsed laser deposition. It has been found that silicon carbide, when deposited by a vapor or vacuum deposition process, is resistant to removal and inhibits corrosion under process conditions associated with the vitrification of nuclear waste. Silicon carbide as a protective coating or film also does not significantly increase the electrical and thermal resistance of the crucible, which makes it suitable for use in vitrification processes that heat the crucible through induction heating.

[0012] In one embodiment, a silicon carbide coating or film has a thickness on the order of five microns to 70 microns. In another embodiment, a representative thickness is on the order of five microns to 50 microns. In another embodiment, a thickness of coating or film is in the range of 20 microns to 30 microns, such as 25 microns. Coating or film 120 of silicon carbide is represented in Figure 1 as a distinct layer. It is appreciated that, as described below, a vapor- or vacuum-deposited silicon carbide film may be formed, at least in part, by reacting with the graphite of shell 110. In that sense, a portion of the coating or film may be formed in shell 110 and coating or film 120 as a distinct layer may not be present.

[0013] In an embodiment where silicon carbide coating or film 120 is formed by a vapor- deposited or vacuum-deposited process that is chemical vapor deposition (CVD), the crucible is placed in a chamber under vacuum and exposed to a silicon-containing source gas. One silicon- containing source gas is silicon oxide (SiO). An SiO gas reacts with a surface of a graphite shell (graphite shell 110) and the surface of shell 110 is converted into silicon carbide, forming a silicon carbide film or layer (layer or film 120). Without wishing to be bound by theory, it is thought that the reaction method is as follows:

SiO(g) + C(s)→SiC(s) + CO(g) (1)

SiO(g) + 3CO(g)→ SiC(s) + 2C0₂(g) (2)

[0014] A representative temperature for the above film-forming process is on the order of 1400-1600°C. A representative pressure is 20 pascals (about 0.003 atmospheres).

[0015] In another embodiment where silicon carbide coating or film 120 is formed by CVD, methyltrichlorosilane (MTS) is used for the conversion gas. In this process, a crucible is placed in a chamber and exposed to a MTS and a hydrogen gas mixture. A representative temperature for such process is 1000-1300°C and a representative pressure is on the order of 2-10 kilopascals (about 0.02-0.1 atmospheres) such as 4-5 kilopascals (0.04-0.05 atmospheres).

[0016] For a physical vapor deposition process, in one embodiment, a silicon oxide source may be utilized as a solid material and the film formed in a physical process such as a plasma sputter bombardment.

[0017] As shown in Figure 1, silicon carbide coating or film 120 is formed on an interior side portion and an exterior side portion of shell 110, and covers each portion of shell 110 including base 115 and top side portions. In another embodiment, only an interior side portion of shell 110 includes silicon carbide coating or film 120. This may be accomplished by protecting the exterior surface portion and other portions with a sacrificial protective material prior to the vapor- or vacuum-deposition, and then removing the sacrificial material after the deposition.

[0018] Figure 2 shows a process of vitrifying nuclear waste. In a first step of process 200, a frit material is added to a crucible and heated to a melting point such as on the order of 1100°C to 1400°C, representatively, 1200°C to 1300°C (process 210). One way to heat the contents of a crucible is through inductive heating. Inductive heating energizes, for example, multi-turn coil 215 external to the crucible. Heat is generated in the crucible to heat the contents. Coil 215 may be controlled to move up and down during the heating process.

[0019] Once heated, a waste, such as a nuclear waste, is added to the heated (e.g., melted) frit material (process 220). The waste may be added as a solid or liquid. Once added, the contents of the crucible are then allowed to cool (process 230). The cooling process allows the frit to form a glass (e.g., borosilicate glass) and contain the nuclear waste.

[0020] In the above embodiment, a graphite crucible is described that is made from an extrusion or molding process. In another embodiment, a graphite crucible may be made from a carbon fiber-reinforced carbon (CFRC). Such process may allow for larger crucibles and/or lighter-weight crucibles. Figure 3 illustrates one process for forming such a crucible. Referring to Figure 3, process 300 includes winding a carbon fiber or filament on a mandrel to a desired shape (block 310). The carbon filament or fiber is a combination of carbon and an epoxy, such as a phenolic epoxy. Representative diameters of suitable carbon fibers are on the order of two inches to 60 inches. The fiber is wound on the mandrel to a desired size and shape of a crucible. Limits on the size and shape of any such crucible will depend, in part, on the dimensions of the mandrel. Such winding of a carbon fiber or filament may be a single wind or may be an overlap consisting of multiple winds of a fiber or filament.

[0021] Once a carbon fiber is wound on the mandrel to a desired shape for a crucible, the wound fiber is removed from the mandrel (block 320) and cured at, for example, 180 to 200°C (block 330). The cured, wound fiber is then carbonized and graphitized (block 340). A suitable temperature for carbonization is on the order of 800 to 1000°C, and a suitable temperature for a graphitization treatment is on the order of about 1800 to 2800°C. Following carbonization and graphitization, the crucible structure is inspected for pores (block 345). As noted above, carbon fiber-reinforced carbon is a combination of carbon fiber and an epoxy. The carbonization and graphitization process will remove through, for example, vaporization, the epoxy, potentially leaving pores in the crucible structure. In one embodiment, to fill any such pores, pitch is added to the graphitized crucible (block 350) via an impregnation process. Following the addition of pitch to the crucible, the crucible is again carbonized and graphitized to form an impermeable crucible. Following this second carbonization and graphitization, the crucible is again inspected for pores. If any pores are found, the filling, carbonization and graphitization is repeated until all the pores are filled. Once the crucible is found to be free of pores, the crucible is subjected to a vapor or vacuum deposition process, such as described above, to add a silicon carbide coating or film (block 360).

[0022] In the preceding detailed description, specific embodiments are described. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the claims. The specification and drawings are, accordingly, is to be regarded in an illustrative rather than restrictive sense.

Claims

CLAIMS What is claimed is:

1. A vitrification crucible comprising:

a graphite shell comprising a wall and a base defining an interior side portion and an exterior side portion, wherein at least the interior side portion is coated with a vapor deposited silicon carbide.

2. The vitrification crucible of claim 1, wherein the vapor deposited silicon carbide is a chemical vapor deposited silicon carbide.

3. The vitrification crucible of claim 1, wherein the vapor deposited silicon carbide has a thickness of at least 25 micrometers.

4. The vitrification crucible of claim 1, wherein the vapor deposited silicon carbide is selected from physical vapor deposited silicon carbide and pulsed laser deposited silicon carbide.

5. A method comprising :

coating a vitrification crucible with silicon carbide by vapor deposition.

6. The method of claim 5, wherein the vapor deposition is chemical vapor deposition.

7. The method of claim 5, wherein the vapor deposition is selected from physical vapor deposition and pulsed laser deposition.

8. The method of claim 5, wherein the crucible comprises a graphite shell comprising a wall and a base defining an interior side portion and an exterior side portion and coating the crucible with a vapor deposited silicon carbide comprises coating at least the interior portion.

9. The method of claim 5, further comprising:

forming a vitrification crucible comprising a graphite shell comprising a wall and a base defining an interior side portion and an exterior side portion.

10. The method of claim 9, wherein forming comprises machining the crucible from a graphite block.

11. The method of claim 9, wherein forming comprises forming the crucible of carbon reinforced fiber.

12. The method of claim 11, wherein coating comprises coating the interior side portion of the vitrification crucible with silicon carbide.

13. The method of claim 11, wherein coating comprises coating at least the interior side portion and the exterior side portion of the vitrification crucible.

14. A method comprising :

heating a frit material in a vitrification crucible comprising a graphite shell coated with a vapor deposited silicon carbide;

adding a waste material to the crucible such that a contents of the crucible comprise the frit material and the waste material; and

cooling the contents of the crucible.

15. The method of claim 14, wherein the waste is nuclear waste.