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(11) | EP 0 176 379 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Vanadium alloy having improved oxidation resistance |
| (57) A vanadium alloy including chromium and titanium constituents is employed as a substrate
for a structural member. To improve the oxidation resistance of the member, chromium
or aluminum is diffused into a surface of the substrate thereby forming a gradient
of the diffused metal through the substrate and defining an enriched surface layer.
The resulting member performs well as a first wall and/or blanket in a fusion reactor
in the presence of an oxidizing coolant. |
FIELD OF THE INVENTION
[0001] The present invention relates to a method and composition for treating vanadium, and more particularly to improving the oxidation resistance of the metal thereby enhancing its utilization in fusion reactors.
BRIEF DESCRIPTION OF THE PRIOR ART
[0002] Fusion reactors typically include a number of concentric structural members for containing a deuterium-tritium plasma within the center of the reactor. The first structural member enclosing the plasma is referred to as the first wall which is backed up by a thicker structure referred to as the blanket. The purpose of the blanket is to capture neutrons which have permeated the first wall. Cooling pipes are embedded within the blanket for maintaining an acceptable temperature within the reactor structure as deuterium and tritium fuels are burned in the plasma.
[0003] Critical material problem areas for fusion reactors include two major considerations. The first is the necessary compatibility of the first wall and blanket with coolants such as helium, water or liquid metals. The second consideration is the minimization of tritium fuel permeation through the first wall which would contaminate the coolant. In order to satisfy these two major critical problem areas, the leading metals used for the first wall and blanket are austenitic stainless steels, commercially known as the Series 300 steels. These steels include a high chromium-iron-nickel alloy to stabilize the high temperature phase of iron. The addition of chromium content in the austenitic stainless steels is necessary to stabilize the non-magnetic phase of the steel so that the iron component of the steel does not become magnetic, a condition which would cause great stress on the structural material due to the presence of the strong magnetic fields in the reactor which are employed to contain the plasma.
[0004] The most important advantage of the austenitic steels as reactor structural materials resides in the fact that these steels offer good oxidation resistance and high temperature strength. Unfortunately, these steels are not resistant to swelling at elevated temperatures when exposed to a high flux of neutrons. Further, these steels have inferior thermal physical properties due to their low thermal conductivity and high expansion coefficients which accentuates thermal stress when installed in a high temperature reactor.
[0005] A further problem with the austenitic stainless steels is their propensity for induced radioactivity which will result in these stainless steels becoming radioactive with time.
[0006] The austenitic steels also suffer from poor corrosion resistance when exposed to high temperature liquid metals that may be employed as a reactor coolant. This necessarily restricts the choice of coolants available.
[0007] It has been recognized that vanadium alloys would offer major advantages when compared to the austenitic stainless steels for fusion reactor applications. The major advantage with respect to vanadium alloys includes greater strength and creep resistance at elevated temperatures. In addition, vanadium alloys are not incompatible with liquid metal coolants. They appear to further offer superior neutron irradiation damage resistance, principally to swelling.
[0008] The vanadium alloys also provide superior thermophysical properties including lower thermal expansion, greater thermal conductivity, and lower modulus.
[0009] It has also been determined that vanadium alloys offer significantly lower induced radioactivity as compared with the steels.
[0010] Although these advantages present vanadium alloys as a highly desirable material for fusion reactors, these alloys suffer from two major problems which have hitherto prevented their use for first wall and blanket structures in a fusion reactor. These major problem areas include greater reactivity with coolants containing oxygen which results in oxygen embrittlement of the alloys and high tritium permeability, which is unacceptable for the first wall which must contain the hydrogen isotope plasma in the center of the reactor.
[0011] As a result of these severe problems, vanadium alloys (including chromium and titanium constituents) have not found any significant application in commercial power reactors.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
[0012] The primary purpose of the present invention is to render prior art vanadium alloys compatible with gaseous and/or liquid coolants containing oxygen and/or water vapor impurities for application to fusion reactor first wall or blanket structures at elevated temperatures, customarily reaching up to 650°C.
[0013] Oxidation resistance of the vanadium alloys is achieved by diffusing a chromium and/or aluminum constituent into a vanadium alloy substrate whereby the diffused chromium or aluminum will be concentrated at the surface of the substrate while forming a rapidly diminishing diffusion gradient through the remainder of the substrate.
[0014] A vanadium alloy treated in accordance with the present invention allows it to be employed in a fusion reactor first wall and/or blanket structure so that the attendant advantages of the vanadium alloy, as compared with the austenitic steels may be realized.
[0015] In utilizing this invention an oxide film is formed on the vanadium alloy which then protects the alloy from further oxidation and embrittlement. The oxide film is also useful as a tritium barrier.
BRIEF DESCRIPTION OF THE FIGURES
[0016] The above-mentioned objects and advantages of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, in which:
- figure 1 is a schematic view of a fusion reactor;
- figure 2 is a schematic representation of the present invention and illustrates the enrichment of a vanadium alloy with a diffused chromium or aluminum surface layer;
- figure 3 is a schematic representation of means for treating a vanadium alloy panel in accordance with the present invention to achieve the surface-enriched diffused layer integral with a vanadium alloy, as shown in figure 2;
- figure 4 is a plot of weight grain by various vanadium materials, illustrating the resistance to oxidation by an alloy of the present invention;
- figure 5 is a plot similar to that shown in figure 4 but with the ordinates normalized as a function of weight per unit area squared and illustrates the oxidation kinetics of-the present vanadium alloy with an enriched diffused layer.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Figure 1 schematically illustrates a fusion reactor for which the present invention has application. The reactor generally indicated in cross section by reference numeral 10 includes a central area where deuterium and tritium plasma are burned. A first wall 13 confines plasma 12 and neutrons which are generated in the plasma will penetrate the relatively thin first wall 13. In order to capture these neutrons, a relatively thick blanket 14 concentrically backs up the first wall. The present improved vanadium alloy has its greatest application as a structural material for the first wall 13 and blanket 14.
[0018] Radially outwardly from blanket 14 is shield 16, conventionally made of lead and having as its primary purpose the confinement of neutrons which have passed through the first wall and blanket. The electromagnets 18 are located around the shield 16 and are intended to generate a strong magnetic field which interacts with the plasma and confines it to the center of the reactor. Deuterium and tritium fuels are provided at inlets 20 and 22. Auxiliary heating means 24 and 26 preheat the fuel. At the outlet 30 of the reactor is a vacuum pump 28 which is employed to remove unburned fuel from the reactor as well as to remove the helium ash from the burned fuel. Helium cryopumps 32 are provided for processing the helium and recovering fuel from the evacuated gas. The recovered fuel is recycled at 36, and newly supplied fuel is introduced at 34. The fuel is introduced at 38 and 40 to the auxiliary heaters 24 and 26.
[0019] The blanket 14 is provided with embedded coolant pipes 42 for maintaining the reactor at an acceptable temperature level. Conventionally, the coolant undergoes tritium extraction at 44. Primary and secondary heat exchangers 46, 48 generate steam for the turbine 50, which is used to generate electricity.
[0020] Figure 2 schematically illustrates the surface-enriched vanadium alloy of the present invention. Structurally speaking, a substrate 54 of a conventional vanadium alloy is employed. Typically, such an alloy may include 15 percent chromium and 5 percent titanium. Then, an enriching metal, preferably chromium and/or aluminum, is diffused into the substrate 54 so as to form an enriched diffused layer 56. The diffused metal will actually form a gradient through the entire thickness of the enriched alloy 52. However, the greatest concentration of diffused metal atoms will reside immediately adjacent the surface 57 of the enriched alloy 52.
[0021] It is to be stressed that the diffused layer 56 is not a coating but an actual diffusion zone of chromium and/or aluminum into the substrate 54. In fact, a coating of the material would be unsatisfactory because the high temperature environment would probably cause delamination, flaking or the development of fissures in the coating leading to loss of protection.
[0022] Figure 3 illustrates a simplified view of a setup for performing the method of the present invention, namely, forming the diffused layer 56 (figure 2). As will be seen, an evacuated vessel 58 has a vanadium alloy panel 60 suspended therein, the panel being comprised totally of the vanadium alloy substrate 54 (figure 2). A chromium and/or aluminum charge 62 is also included within the vessel 58 and the charge is heated. As a result, chromium or aluminum vapors 64 evaporated from the charge and diffuses into the substrate material of panel 60 forming a corresponding chromium and/or aluminum diffused layer, such as 56 (figure 2). It is to be emphasized that other conventional methods may be used for diffusing the chromium or aluminum into the vanadium alloy substrate. For example, the chromium or aluminum might be deposited onto the surface of the substrate via conventional means (i.e., sputtering) and subsequently heated to effect diffusion of the chromium or aluminum into the substrate metal.
[0023] Although a particular vanadium alloy (15 percent chromium, 5 percent titanium) has been disclosed, this is not to be construed as a limitation of the invention. Other vanadium alloys may be employed as long as the alloy constituents enhance the oxidation resistance property of the base metal and are soluble in the base metal so as to be metallurgically compatible therewith. For example, it is conceivable that a nickel constituent would be acceptable for a vanadium alloy substrate.
[0024] Further, metals other than chromium or aluminum could be used for enriching the surface of the vanadium alloy. The depth and operating parameters of the diffusing material may be determined by experimentation in accordance with the basic concepts of the present invention, namely, enriching the surface of the vanadium alloy by diffusing an appropriate metal onto the surface which will provide oxidation resistance in the form of a protective oxide barrier in the presence of an oxidizing coolant as employed in a fusion reactor operating at temperatures up to 650°C.
[0025] In order to visually compare the oxidation resistance of the surface-enriched vanadium alloy with pure vanadium and the non-enriched vanadium alloy, reference is made to figure 4. The left ordinate represents the oxygen weight gain (vs. time) of pure vanadium and the known vanadium-chromium-titanium alloy. The right ordinate corresponds to the oxygen pickup (vs. time) for the diffusion-enriched vanadium alloy of the present invention, and stainless steel, the latter being the predominant structural material for first walls and blankets, as previously discussed. As will be seen, the oxygen pickup of stainless steel is the lowest over time. However, of the remaining vanadium metals, the lowest plot, representing the chromium version of the present invention, offers superior results.
[0026] Figure 5 compares the oxidation kinetics for pure vanadium, the non-enriched vanadium alloy, and the chromium diffused surface-enriched alloy of the invention. Again, the left ordinate corresponds to oxygen weight gain of the upper and middle plots, while the right ordinate represents the weight gain for the chromium version of the present invention. Inasmuch as the ordinate for these plots is normalized as the square of micrograms per unit area, all of the plots illustrate that the oxidation rates in fact follow parabolic curves which, in accordance with accepted metallurgical principles, indicates a protective oxide layer. Again, it will be observed that the lowest weight gain on a parabolic basis is the chromium surface diffused vanadium alloy. It is to be understood that similar results would follow if the surface- diffused enrichment was done with aluminum instead of chromium.
1.- A method for improving the oxidation resistance of a structural member fabricated
from a vanadium alloy, the method comprising the steps:
- positioning the structural member in confronting relation to a charge having a metal selected from the group including chromium and aluminum;
- heating the charge to form vapors;
- subjecting the member to the vapors for a period of time sufficient for the charge metal to diffuse into a confronting surface of the member thus creating a gradient of the charge metal through the member and defining an oxidation resistant enriched surface layer.
2.- The method set forth in claim 1 wherein the vanadium alloy includes chromium and
titanium constituents.
3.- A method for improving the oxidation resistance of a structural member fabricated
from a vanadium alloy, the method comprising the steps of:
- coating at least one preselected surface of the member with a metal selected from the group including chromium and aluminum;
- heating the coated metal until the coating diffuses into the member surface thus creating a gradient of the metal through the member and defining an oxidation resistant enriched surface layer.
. 4.- The method set forth in claim 3 wherein the vanadium alloy includes chromium
and titanium constituents.
5.- An oxidation resistant structural member comprising:
- a vanadium alloy substrate; and
- a metal, selected from the group including chromium and aluminum, diffused into a surface of the substrate and forming anlenriched layer in which the concentration of diffused metal decreases inwardly along a gradient.
6.- The member set forth in claim 5 wherein the alloy includes chromium and titanium
constituents.
7.- In a fusion reactor, at least one structural member pretreated for high temperature
oxidation resistance, said member comprising:
- a vanadium alloy substrate; and
- a metal selected from the group including chromium and aluminum diffused into a surface of the substrate and forming an enriched layer in which the concentration of the diffused metal decreases inwardly along a gradient.
8.- The structure set forth in claim 7 wherein the alloy includes chromium and titanium
constituents.