Abstract:
A first-wall component for a fusion reactor contains at least one heat shield having a first region inclined toward the plasma and a second region lying opposite the first region and formed of a graphitic material. The heat shield has one or more slots that end in the first or second regions and are oriented generally in the direction of an axis of the cooling tube. The components suitably cope with the mechanical stresses resulting both from production and from thermal cycling.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation, under 35 U.S.C. §120, of copending international application No. PCT/AT2006/000113, filed Mar. 17, 2006, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of Austrian patent application GM 179/2005, filed Mar. 22, 2005; the prior applications are herewith incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The invention relates to a first-wall component of a fusion reactor. The first-wall component contains at least one heat shield with a closed or open lead-through formed of a graphitic material and a cooling tube through which a coolant flows and which is at least partially material-bonded to the heat shield and is formed of a material having a thermal conductivity&gt;200 W/m·K. 
     A typical example of the use of first-wall components of this type is diverters and limiters that are exposed to the high thermal loads of more than 10 mW/m 2 . First-wall components normally contain a heat shield and a heat-dissipating region. The material of the heat shield must be compatible with plasma, have high resistance with respect to physical and chemical sputtering, possess a high melting point/sublimation point and be as resistant as possible to thermal shock. In addition, they must also have high thermal conductivity, low neutron activatability and sufficient strength/fracture toughness, along with good availability and acceptable costs. In addition to refractory metals, such as, for example, tungsten, and graphitic materials (for example, fiber-reinforced graphite) best fulfill this diverse and sometimes contradictory requirement profile. Since the energy flows from the plasma act on these components over a lengthy period of time, first-wall components of this type are typically cooled actively. The heat discharge is assisted by heat sinks that contain, for example, copper or copper alloys and which are usually mechanically connected with the heat shield. 
     First-wall components may be implemented in a varying configuration. A customary design is in this context is what is known as a monobloc design. In the monobloc design, the first-wall component contains a heat shield with a concentric bore. The heat shield is connected to the cooling tube via this concentric bore. 
     First-wall components have to tolerate not only thermally induced, but also additionally occurring mechanical stresses. Such additional mechanical loads may be generated via electromagnetically induced currents that flow in the components and interact with the magnetic field of the surroundings. In this case, high-frequency acceleration forces may arise, which have to be transmitted by the heat shield, that is to say, for example, by the graphitic material. However, graphitic materials have low mechanical strength and fracture toughness. In addition, during use, neutron embrittlement occurs, thus resulting in a further increase in the sensitivity of these materials with respect to crack introduction. Fiber-reinforced graphite (CFC) is usually employed as the graphitic material. The fiber reinforcement is in this case disposed three-dimensionally and linearly. The architecture of the fibers gives the material different properties, depending on the orientation. CFC is usually reinforced in one orientation by Ex-pitch fibers that have both the highest strength and also thermal conductivity. The other two orientations are reinforced by Ex-PAN fibers, one direction typically only being needled. 
     Thus, whereas CFC has a linear material architecture, the heat shield/cooling tube connection geometry is circular. On account of the different coefficients of thermal expansion of the materials used, during the production process a stress build-up occurs which may lead to cracks in the CFC. These cracks can be detected, if at all, only by highly complicated methods because of the geometric conditions and the material combination used. This presents corresponding problems against the background of a nuclear environment for such components, above all also because cracks/peelings are seen as a possible trigger for a major incident. Despite complicated year-long development activity in the field of first-wall components, the components available hitherto do not optimally fulfill the requirement profile. 
     BRIEF SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a first-wall component for a fusion reactor which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which suitably satisfies the requirements resulting from mechanical stresses. 
     The first-wall component contains at least one heat shield formed of a graphitic material with a first face inclined toward the plasma and with a second face lying opposite the first face. The heat shield has one or more slots that end in the first or second face and, as seen in the direction of the axis of the cooling tube, run generally over a length of the heat shield. It is advantageous, further, that the maximum slot width in the region of the slot bottom does not overshoot D/2, where D is the outside diameter of the cooling tube. Tests, set out in more detail in the examples, have shown that the components according to the invention suitably cope with the mechanical stresses resulting both from production and from thermal cycling. The slot advantageously runs approximately perpendicularly with respect to the first or second face. The slot depth, in turn, is advantageously greater than half the distance between the first and second faces and the nearest surface of the cooling tube. A particularly favorable range for the slot depth x is u/2≦x ≦9u/10, u being the spacing, measured in the vertical direction, between the first and second faces and the nearest cooling tube surface. The slot may, however, even extend as far as the cooling tube or as far as a ductile layer enveloping the cooling tube. In this case, the heat shield does not have a closed lead-through, but an open one. Since cooling tubes with a circular cross section are normally used, the lead-through also has a circular cross section. 
     The minimum slot width of 10 μm is obtained as a result of the cutting methods available for graphitic materials, such as the diamond saw method or wire cutting. The preferred maximum slot width is D/3. In order to avoid stress peaks in the slot bottom, it is advantageous if this has a radius which is in the region of 0.5×the slot width. It is advantageous, further, if the slot ends in the second face, since, during use, slight erosion occurs in the region of the slot on the face confronting the plasma. A further advantageous version is the single-slot variant, the slot being directed toward the cooling tube center point. The use of two or three slots, as also illustrated in detail in the examples, to a great extent also reduces the stresses occurring during production and thermal cycling. The combination of CFC for the heat shield with the slots according to the invention leads to a particularly beneficial combinational effect especially when the Ex-pitch fibers are oriented approximately perpendicularly with respect to the face A, the Ex-PAN fibers are oriented parallel to the axis of the cooling tube and the needled Ex-PAN fibers are oriented radially with respect to the cooling tube axis. For economic reasons and because of the high thermal conductivity, the use of copper alloys for the cooling tubes is to be preferred. Further, the stresses in the component may be reduced by the introduction of a very soft layer (hardness&lt;200 HV) between the cooling tube and heat shield. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a first-wall component for a fusion reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagrammatic, oblique view of a component according to the invention with a slot; 
         FIG. 2  is a diagrammatic, top plan view of the component according to  FIG. 1 ; 
         FIG. 3  is a diagrammatic, cross-sectional view of the component according to  FIG. 1  and, further, the CFC fiber direction; 
         FIG. 4  is a diagrammatic, oblique view of a component according to the invention with two slots; 
         FIG. 5  is a diagrammatic, top plan view of the component according to  FIG. 4 ; 
         FIG. 6  is a diagrammatic, cross-sectional view of the component according to  FIG. 4  and, further, the CFC fiber direction; and 
         FIG. 7  is a diagrammatic, top plan view of a component according to the invention with a V-shaped slot. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIGS. 1-3  thereof, there is shown as a first example a first-wall component  1  produced as now described. Heat shields  2  in the form of monoblocs with a lead-through formed by a bore  4  are worked out from fiber-reinforced graphite blocks (CFC), the high-strength Ex-pitch fibers lie in the direction of the highest thermal conductivity, the Ex-PAN fibers lie parallel to the axis of the cooling tube and the needled Ex-PAN fibers lie in the cooling tube axis. The dimensions of the individual monoblocs are 40 mm (Ex-pitch), 30 mm (Ex-PAN) and 20 mm (Ex-PAN needled). The diameter of the bore  4  is 14 mm and is located at a center of symmetry  9  of the heat shield  2 . Before further processing, the wall of the bore  4  is structured by a laser, with the result that a multiplicity of conical holes are introduced in the CFC. Such holes typically have a depth of about 0.5 mm and an opening on the surface of 0.2-0.3 mm. The spacing was selected such that the surface of the bore wall is maximized. On a side  6  facing away from the plasma, a slot  7  with a slot width of 0.3 mm is introduced in the heat shield  2  by wire cutting. The slot  7  lay in an axis of symmetry of the heat shield  2  and runs from the surface  6  facing away from the plasma into the centrally lying bore  4 . Subsequently, the bore  4  is filled via a casting process with oxygen-free copper in the presence of a carbide former, such as, for example, titanium. The process was conducted such that the previously introduced 0.3 mm wide slot  7  in the heat shield  2  is not wetted by copper during the casting process. After the casting process, the flanks of the slot  7  have a smaller spacing, as compared with the processing state. This fact showed that the stresses occurring were converted into deformation. This lead to a stress reduction, without the functioning capacity and the beneficial properties of the component  1  being lost due to this measure. A visual and metallographic assessment of the CFC/Cu interfaces in the backed-up state gave no indications as to possible delaminations in the CFC/copper composite. 
     The copper-filled bore  4  thus obtained was subsequently subjected to mechanical machining, so that a bore with a diameter of 12.5 mm and therefore an about 0.5-1.0 mm thick copper layer  8  remained on the CFC. 
     Three heat shields  2  thus obtained, with a slot  7 , were slipped onto a cooling tube  3  formed of a CuCrZr alloy with a diameter of 12 mm and introduced into a metal can. After the welding of the can, the latter was evacuated and the suction-extraction connection piece was thereafter sealed, vacuum-tight. The components canned in this way were then subjected to an HIP process at 550° C. and 1000 bar. During this process, a material bonding occurred between the CuCrZr tube  3  and the copper layer in the bore  4  of the CFC monobloc  2 . In addition, a curing of the CuCrZr material also took place, with the result that excellent mechanical properties in the cooling tube  3  could be achieved. After the connection process, the can was removed from the first-wall component  1  thus obtained. A visual assessment give no indications as to any faults, such as, for example, delaminations. An ultrasound test additionally carried out with an inner tube probe showed a perfect interface. 
     In conclusion, the first-wall component  1  was subjected to the plasma of a VPS plant. The component  1  was in this case connected to the cooling water system present in the plant and was held by the gripping arm of a robot installed in the plant. A heat flow in the range of 10-15 MW/m 2  was determined by analyzing a flow velocity, the temperature rise of the cooling medium and the surface  5  acted upon by the plasma. Overall, the component  1  was cycled by movement through the plasma about 100 times. During movement, the component  1  was in each case held in the plasma until the temperature of the cooling water did not heat up any further. After this test, the component  1  was tested to destruction. It was shown that a crack could not be detected in any of the heat shields  2  investigated, this being a fact that could not yet be achieved in components which are not according to the invention. 
     Second Example 
     The first-wall component  1  was manufactured according to the first example. In the subsequent test, the slotted surface was exposed to the plasma. The test furnished similar results to those in the first example 1, the difference being that slight erosion took place in the region of the slot  7 . 
     Third Example 
     The first-wall component  1  according to  FIGS. 1 to 3  is produced as now described. 
     The heat shields  2  in the form of monoblocs with the bore  4  are worked out from fiber-reinforced graphite blocks (CFC), once again the high-strength Ex-pitch fibers lie in the direction of the highest thermal conductivity, the Ex-PAN fibers lie parallel to the axis of the cooling tube and the needled Ex-PAN fibers lie in the cooling tube axis. The dimensions of the individual monoblocs corresponded to those of the first example. The introduction of the bore and laser structuring also took place, as described in the first example. On the side  6  facing away from the plasma, the slot  7  with a slot width of 0.3 mm is introduced in the heat shield  2  by wire cutting. The slot  7  lies on the axis of symmetry of the heat shield  2  and penetrated the bore  4 . The bore  4  was subsequently filled in a similar way to the first example with oxygen-free copper, subjected to mechanical machining and connected to a cooling tube  3  formed of a CuCrZr alloy by soldering, the soldering temperature lying in the region of the solution heat treatment temperature (970° C.) of the CuCrZr. The cooling from the soldering temperature to below 400° C. took place with a cooling rate&gt;1 K/sec, with the result that optimal strength values could be established during subsequent age hardening at 475° C./3 h. The composites thus produced also showed no cracks after thermal cycling according to the first example. 
     Fourth Example 
     The first-wall component  1  according to  FIGS. 4 to 6  is produced as now described. 
     The heat shields  2  in the form of monoblocs with the bore  4  are worked out from fiber-reinforced graphite blocks (CFC), once again the high-strength Ex-pitch fibers lie in the direction of the highest thermal conductivity, the Ex-PAN fibers lie parallel to the axis of the cooling tube and the needled Ex-PAN fibers lie in the cooling tube axis. The dimensions of the individual monoblocs correspond to those of the first example. The introduction of the bore and the laser structuring also took place, as described in the first example. On the side  6  facing away from the plasma, two slots  7  with a slot width of 0.3 mm are introduced in the heat shield  2  by wire cutting. The slots  7  lay mirror-symmetrically to the axis of symmetry of the heat shield  2 . The slots  7  each had a depth x of 0.8 u, u being the smallest spacing between the heat shield surface  5  and the cooling tube  3 . The bore  4  is subsequently filled in a similar way to the first example with oxygen-free copper, subjected to mechanical machining and material-bonded to a cooling tube  3  consisting of a CuCrZr alloy by soldering according to the sequence in the third example. The composites thus produced also showed no cracks after thermal cycling according to the first example. 
     Fifth Example 
     The first-wall component  1  according to  FIG. 7  is produced as now described. Monoblocs are produced according to the first example. On the side  6  facing away from the plasma, a V-shaped slot  7 , as illustrated in  FIG. 7 , is introduced by wire cutting. The further manufacturing steps took place, as described in the first example. The composites thus produced also showed no cracks after thermal cycling according to the first example.