Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C21/00—Apparatus or processes specially adapted to the manufacture of reactors or parts thereof
  • G21C21/02—Manufacture of fuel elements or breeder elements contained in non-active casings
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E30/00—Energy generation of nuclear origin
  • Y02E30/30—Nuclear fission reactors

Definitions

• the invention relates to a method for producing a fuel element of a nuclear reactor comprising the method steps of: forming a fuel core containing a fissible material and providing the fuel core with a cladding.
• Such a method is known from CLARK, C. R., et al.: Monolithic fuel plate development at Argonne national laboratory, in: RETR 2003, Chicago, USA.
• uranium and molybdenum are massed and melted.
• an ingot is formed which is rolled by a rolling mill to produce a foil with the uniform thickness of for example 0.3 mm.
• the foil is heated to at least 600 0 C for producing the so called ⁇ -phase.
• the exact temperature of the heat treatment depends on the molybdenum concentration and may reach a maximum of 1100 0 C. Only the ⁇ -phase is resistant to the radiation in pile.
• the foil If the foil is quenched after the heat treatment the ⁇ -phase is even metastable at room temperature.
• the foil For use in pile the foil must be provided with a so called cladding to prevent the fission product from leaking out of the fuel element and to enhance the mechanical stability of the fuel element.
• the cladding consists of an aluminium layer with a thickness of 0.38 mm. The bonding between the foil and the cladding is performed by friction-stir-welding.
• the known method was developed in the context of an interna- tional effort to replace the currently used highly enriched fuel elements by low enriched fuel elements or at least medium enriched fuel elements.
• most research and test reactors depend on the use of highly enriched uranium since most research and test reactors are no longer used for research on reactor technology but are operated as neutron sources for fundamental and applied research.
• the most important property of such a reactor is the neutron flux which is basically proportional to the power density in the reactor core.
• a high power density requires a high fission rate. Since the power density in the reactor core depends on the fission rate, a high fuel density is necessary for obtaining a high power density in the core.
• the fuel with the highest fuel density for research reactors and test reactors is currently U 3 Si 2 .
• U 3 Si 2 is qualified up to a density of 4.8 gU/cm 3 .
• fuel elements based on monolithic U/Mo would reach a fuel density of more than 15 gU/cm 3 .
• the increase in fuel density can be used for a gain in performance or for a lower enrichment of the fuel.
• the known method for producing a fuel element has a number of disadvantages. If large foils in the metastable ⁇ -phase are quenched, an inhomogeneous and fast cooling of the foils may induce fissures and cracks caused by stress. Therefore, only small round foils with a diameter of up to 12 mm have been produced so far. For comparison reasons it should be noted, that a foil with an extension of 700 x 59 mm 2 is necessary for FRM-II.
• the known method for producing fuel elements may only be used for small specimens on a laboratory scale, but is not suitable for the production of whole fuel elements on an industrial scale.
• Ki Hwan Kim et.al. Characterization of U-2wt%Mo and U-10wt%Mo alloy powders prepared by centrifugal atomisa- tion. in: Journal of Nuclear Materials, Volume 245, Issues 2- 3, June 1997, p. 179-184 discloses an atomized powder method for the production of U/Mo alloy powder.
• the present invention is based on the object of providing an improved method for producing fuel elements for a nuclear reactor. This object is achieved by a method having the features of the independent claim. Advantageous embodiments and refinements are specified in the appended claims.
• the fuel core is formed by depositing the fissible material from the gaseous phase on a substrate.
• gaseous phase means a state of aggregation in which atoms, molecules or atomic or molecular clusters can move freely in contrast to a liquid or a solid phase.
• the fuel core formed in the process is the fuel core of a fuel plate.
• a number of these fuel plates can be assembled to a complete fuel element.
• a stress relieved fuel core can be produced.
• the surface of the fuel core can be prepared for further bonding between the fuel core and the cladding.
• the structure of the fuel core can be adapted to the requirements on fuel density and mechanical stability.
• the fuel core may be provided with a multilayer structure or with a varying thickness across the fuel core, or barriers against interdiffusion processes may be integrated in the structure of the fuel core .
• melt of uranium and molybdenum is reduced to a desired degree according to a top-down approach.
• a bottom-up approach is chosen in which the thickness of the fuel core is increased by a deposition process.
• the fuel core is cladded by depos- iting cladding material from the gaseous phase on the fuel core.
• the fuel core and the cladding can be produced in a single production apparatus.
• the risk for inserting pollutions into the fuel core or degrading the surface of the fuel core before cladding is minimized. Therefore, the cladding can be firmly bonded to the fuel core.
• the substrate, on which the fissible material is deposited can be made from the cladding material which is also used for cladding the fuel core after the deposition of the fuel core.
• the cladding material which is also used for cladding the fuel core after the deposition of the fuel core.
• the substrate can also be provided with a deposition surface which deviates from a flat surface.
• the complete fuel element may comprise stress relieved fuel plates with a complex structure.
• the fuel plates may for example have a cross section which follows a segment of a involute.
• the fuel core may be heat treated during or after the deposition of the fissible material on the substrate .
• the fissible material will be deposited together with a stabilizer material which stabilizes the mechanical and thermal properties of the fuel element.
• a stabilizer material which stabilizes the mechanical and thermal properties of the fuel element.
• the fissible material contains mainly uranium, and molybdenum is used for improving the thermal and mechanical stability of the fuel element.
• Molybdenum is an especially suited material for stabilizing purposes since its presence does not affect the fission process, but suppresses the formation of extended gas bubbles and thus contributes to the thermal and mechanical stability of the fuel element.
• a sputtering process is used as an effective process for depositing the material for the fuel core.
• sputtering By sputtering, thick layers up to 10 mm can be produced for the fuel core.
• sputtering is also an appropriate process for covering an extended area of the substrate with fissible material.
• argon is used as a sputter gas wherein the argon concentration in the deposited material is chosen such that the stress within the fuel core is minimized.
• the pressure of the gas atmosphere is brought to a pressure below 10 ⁇ 5 mbar before the beginning of the sputter process.
• the morphology of the crystallites on the surface of the substrate can be influenced.
• the stress within the fuel core can be diminished.
• the material used for the fuel core can be deposited in a single uniform layer in order to gain a maximum fuel density and accordingly high macroscopic cross sections for fission resulting in high neutron fluxes within the fuel element.
• the material used for the fuel core may also be deposited in a multilayer structure with alternating layers of fissible material and stabilizer material or cladding material. By such a structure the thermal and mechanical properties of the fuel element may be improved.
• the structure of the fuel core is varied along a cross-sectional profile of the fuel core.
• the effective area density of the fissible material can be varied across the fuel plate manufactured in the sputter process.
• These variations can be provided by varying the thickness of the fuel core or by varying the density or concentration of the fissible material deposited on the substrate.
• the fuel concentration can be adapted to optimized concentration profiles obtained for instance from numerical simulations or calculations .
• Figure 1 shows a cross section of a fuel plate for the fuel element of a nuclear reactor
• Figure 2 shows a modified embodiment of the fuel plate
• Figure 3 is a phase diagram of U/Mo
• Figure 4 depicts an apparatus for producing the fuel core of the fuel element by sputtering
• FIG. 5 illustrates the deposition process
• Figure 6 is a diagram, which shows the sputter yield for various elements
• Figure 7 is a diagram showing the argon pressure or argon concentration, necessary for depositing stress re- lieved layers of sputter material, while using argon as a sputter gas;
• Figure 8 is a cross-sectional view of a structure with three layers composed of an aluminium substrate covered with a copper layer and sealed with another layer of aluminium;
• Figure 9 is a cross-sectional view of fuel element assembled from a number of fuel plates.
• Figure 1 shows a cross section of a fuel plate 1.
• Several of these fuel plates 1 can be assembled to a fuel element for a nuclear reactor.
• the fuel elements are supplied to nuclear reactors. For each reactor cycle new fuel elements must be loaded into the reactor core.
• the fuel plate 1 comprises an inner fuel core 2, which is surrounded by a cladding 3.
• the fuel core 2 represents the so called fuel meat of the fuel plate 1.
• the fuel core 2 may be made from a monolithic alloy of uranium and molybdenum, whereas the cladding 3 is usually made from a material based on aluminium.
• the thickness of the fuel core 2 amounts to a few tenth of a millimetre.
• a typical value for the thickness of the fuel core 2 ranges between 0.3 mm and 0.6 mm.
• the layer of the outer cladding shows similar thicknesses within the range of a few tenth of a millimetre.
• a typical value for the thickness of one cladding layer 3 ranges also between 0.3 mm and 0.6 mm.
• Figure 2 shows a modified embodiment of the fuel plate 1, in which the fuel core 2 shows a multilayer structure composed of a number of fuel layers 4 separated by intermediate layers 5.
• the fuel layers 4 may be made from a material based on uranium.
• the material of the fuel layer 4 may be an alloy of uranium with molybdenum.
• the intermediate layers 5 may consist of the same material, which is used for the cladding 3 for improving the thermal conductivity or mechanical stability of the fuel plate 1. It is also possible that the fuel layer 4 is made from a material based on ura- nium, whereas the intermediate layers 5 contain a third material .
• the thickness of the fuel layers 4 and the intermediate layers 5 is considerably smaller than the overall thickness of the fuel core shown in figure 1. Typically the thickness of the fuel layer 4 may be well below a tenth of a millimetre.
• Figure 3 shows a phase diagram of U/Mo, which seems to be a promising combination of the fissible material uranium and the mechanically and thermally stabilizing molybdenum.
• One important requirement on an alloy of uranium and molybdenum is the resistance of the alloy against a high number of radiation damages during irradiation. It is supposed that among the various phases of the U/Mo-system only the metasta- ble ⁇ -phase 6 is sufficiently stable under the high flux conditions of a high flux reactor.
• the metastable ⁇ -phase 6 requires a concentration of molybdenum between 0 and about 42 percent of atomic weight, wherein the temperature for the formation process ranges from about 600 0 C up to nearly
• the temperature may range between 600 0 C and 1100 0 C depending on the molybdenum concentration. Therefore, it is advantageous to keep the temperature at least during a section of the production process of the fuel core above 600 0 C and to quench the fuel core 2 after the end of the production process in order to get a fuel core in the metastable ⁇ -phase 6 at room temperature.
• FIG 4 depicts an apparatus 7 for producing the fuel core 2 of the fuel plate 1 by sputtering.
• the apparatus 7 comprises a vacuum chamber 8 with an internal space 9, which is connected via valves 10 and 11 with a turbo pump 12 and a fore pump 13.
• the fore pump 13 may be connected via bypass 14 directly with the internal space 9 of vacuum chamber 8.
• Bypass 14 is controlled by valve 15.
• the vacuum chamber 8 is provided with gas inlets 16, each controlled by a valve 17.
• the vacuum chamber 8 is connected via gas inlets 16 to gas reservoirs for gases, which may be used for the sputtering process.
• magnetrons 18, 19 and 20 are disposed, which act as cathodes for the sputtering process, whereas the rest of the vacuum chamber has the function of an anode. Furthermore the vacuum chamber 8 is equipped with a substrate table 21, which might be moved across the magnetrons 18, 19 and 20 by means of a transport system 22. Ac- cordingly the substrate table either performs a lateral oscillating movement in front of the magnetrons or rests in front of the magnetrons during the sputtering process.
• a voltage is applied between the cathodes and the wall of the vacuum chamber. Electrons emitted by the cathodes are concentrated by a magnetic field near the sputter targets. Generally the magnetic field is produced by the magnetrons. By the electrons moving in the vacuum chamber positively charged argon ions are formed which are acceler- ated to the cathodes. On hitting the cathodes the argon ions cause atoms of the target material to be sputtered into the gaseous phase. The target material in the gaseous phase will then be deposited on the inner surface of the vacuum chamber 8, especially on a substrate 23 situated on the substrate table 21.
• Figure 5 illustrates the deposition process on the substrate
• the substrate 23 is fixed upon the substrate table 21.
• the substrate 23 may be a thin foil or a massive plate made from the same material, which is used for the cladding 3 of the fuel core 2.
• DC- or RF-magnetron-sputtering a fuel core layer
• fuel core layer 24 is deposited on the substrate 23.
• fuel core layer 24 is shown to have only half of the thickness of the final fuel core 2.
• a transport arrow 25 illustrates the net material flux from the magnetrons 18, 19 and 20 to the surface of the fuel core layer 24. It is to be noted, that the fuel core layer 24 may be also composed of a number of sublayers, made from different materials.
• a cladding layer 26 may advantageously be sputtered on the fuel core layer 24 to prevent the fission material from leaking out and to prevent the fuel core layer 24 from oxidation.
• the fuel core layer 24 is formed by a U/Mo-alloy.
• the metastable ⁇ -phase 6 of the fuel core layer 24, made from uranium and molybdenum ' may be achieved either by the heating of the substrate caused by the sputtering process or by heating the layers by means of a infrared heater, similar to a heater used for the known atomized powder-method, or by an ex situ process.
• the sputtering process allows achieving each desired concentration of uranium and molybdenum by cosputtering of molybdenum and uranium.
• the composition of the fuel core layer 24 can be varied depending on position. Furthermore, it is possible to create gradients of the concentration and of the thickness of the fuel core layer 24. These variations result in varying effective area densities of the fissible material across the fuel plate. It is also possible to produce any uranium alloy by cosput- tering with at least two of the magnetrons 18, 19 or 20.
• magnetron 18 may contain pure uranium and magnetron 19 may be provided with a suitable alloy.
• the sputter process is conducted such, that the basic vacuum before the beginning of the sputter process has a pressure p 0 between 10 "12 mbar and 10 "1 mbar.
• the pressure of the basic vacuum before the beginning of the sputtering process should preferably be below 10 "5 mbar.
• the sputtering process is conducted in an inert gas atmos- phere, preferably in an argon atmosphere.
• reactive gases like nitrogen, hydrogen, oxygen or CO 2 and other gases can be added to the inert gas atmosphere.
• the pressure p s should range between 10 ⁇ 7 mbar and 1 mbar. If the pressure during sputtering is outside the range of 10 ⁇ 4 mbar up to 5 x 10 "2 mbar the plasma formed during magnetron sputtering may become instable and the sputtering process will malfunction. However, it is possible to work with very low sputter pressures if ion beam sputtering is used.
• the cathodes are operated with a power density in the range of 0.01 W/cm 2 and 100 W/cm 2 . If the power density is above 30 W/m 2 and if the sputter pressure p 0 above 5 x 10 "2 mbar the mean free path of the charge carriers will be too small to keep the sputter process running. On the other hand, if the power density is below 0.2 W/cm 2 , the ionisation probability for the formation of argon ions is generally too low and no plasma is formed.
• the substrate normally made of aluminium or molybdenum must be kept on a temperature between 100 Kelvin and 1100 Kelvin during sputtering.
• the temperature will be kept almost constant by means of a heating or cooling system in the substrate table 9 working with a fluid or by the exposure of the substrate table 21 to electromagnetic radiation, for example infrared light.
• an etch or glow process may be conducted in argon by supplying additionally oxygen or hydrogen to the vacuum chamber.
• Figure 6 shows the sputter yield for various elements depend- ing on the atomic number.
• the sputter yield depicted in figure 6 is the number of atoms, which are sputtered per incident argon ion.
• the sputter yield is indicated for argon ions with a energy of 500 eV.
• the accelerating voltage in the production process of the fuel plates 1 should be between 200 and 800 V, especially between 250 V and 350 V, which results to an ion energy between 250 eV and 350 eV. It is to be expected, that the yield will scale with the energy of the incident ions. In practice the sputter yield depends also on the contamination of the surface of the target.
• a regression curve 27 indicates the interrelation between atomic weight of various elements and the argon concentration resulting in a stress relieved deposited layer.
• Another regression line 28 indicates the interrelation between the atomic weight and the argon pressure resulting in stress relieved deposited layers. If the argon pressure is chosen above the regression line 28 the sputtering process will result in a deposited layer with tensile stress. In contrast, the deposited layer will be subjected to compressive stress if the argon pressure is below the regres- sion line 28.
• uranium as well as molybdenum are sputtered on a substrate it is to be expected that the pressure should be kept within a range of 6 x 10 "3 mbar up to 4 x 10 "2 mbar, depending on the relative concentration of uranium and molyb- denum in the fuel core 2.
• the U/Mo composition will be applied directly on the plasma cleaned cladding material, in most times aluminium or alloys based on aluminium, and after the desired thickness of the fuel core 2 has been achieved, the fuel core 2 is covered with the cladding 3 without any need for breaking the vacuum during the production process. Thus contaminations of the fuel plate 1 are excluded. Subsequently, the fuel composite may be brought to the desired thickness by electro-plating whereby the mechanical stability of the fuel plate 1 can be enhanced.
• FIG 8 a cross section of a foil is shown which has been produced in a test with surrogate material.
• a copper layer 30 instead of uranium was produced with a thickness of 0.3 mm.
• Figure 8 shows, that the bonding between the copper layer 30 and a lower aluminium layer 31 and a upper aluminium layer 32 is excellent and that the bonding has not been broken up even by the strong mechanical strain during the cutting of the specimen.
• Figure 9 finally shows a cross-section of a fuel element 33 which is assembled from a number of curved fuel plates 1 having the profile of an involute.
• the cross section illustrates that the fuel plates 1 are mounted between two concentric tubes 34 and 35.
• the control rod of the reactor moves in the inner tube 34.
• sputtering is not the only process which may be used for depositing a fuel core layer 24 on a substrate 23. Besides sputtering other physical vapour depo- sition (PVD) processes may be used. Also chemical vapor deposition (CVD) or other vacuum deposition processes may be suitable . Furthermore it should be noted that the cladding may be further strengthened by increasing the thickness of the cladding layer 26 and the substrate 23 by electro-plating.
• the fuel plate 1 may also be bent, rolled or subjected to any other mechanical process for shaping the fuel plate 1.
• the fuel plate 1 may also be heat treated after the fuel core 2 has been provided with cladding 3.
• the production of a fuel core 2 with a thickness of 0.3 mm will take fifteen hours with at a power density of the cathode of 5 W/cm 2 . Since several fuel plates 1 can be produced in the same vacuum chamber 8 at a time a production time of 1 to 2 months has to be taken into account for a complete fuel element 33 which comprises 113 fuel plates 1 at FRM II. This production time can be reduced by cosputtering with several cathodes.
• the market price for an apparatus 7 amounts to around 1000 € per day inclusive the operating personal. Therefore, the total costs for sputtering will amount to around 120,000 € per fuel element.
• fuel is deposited on the walls of the vacuum chamber 8.
• the fuel deposited on the walls of the vacuum chamber 8 can be recovered and can be recycled.
• the vacuum chamber is equipped with a number of glass baffles coated with silver or another anti-adhesion material and defining an inner process chamber within the vacuum chamber 8. Due to the anti-adhesion coating, the uranium deposited on the inner surfaces of these baffles can easily be stripped off from the surface of these baffles and can thus be recovered. Accordingly the material balance which is very important for nuclear fuels is always traceable and the effective yield is accordingly high.

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