Abstract:
A fuel rod for a nuclear fission reactor is disclosed and claimed. The fuel rod includes an elongate hollow cladding configured to retain a nuclear fuel therein. The cladding includes an elongate hollow tube. Fiber layers are positioned around the outside surface of the tube or within the tube forming an integral part thereof. Both the tube and the fibers are formed of a ceramic material. A fuel assembly including a plurality of such fuel rods is also disclosed and claimed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/810,618 filed on Apr. 10, 2013, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to nuclear fuel assemblies for use in a nuclear fission reactor, and, more particularly, the present invention relates to nuclear fuel assemblies with nuclear fuel rods formed with a cladding of a composite ceramic material. 
         [0004]    2. Description of the Related Art 
         [0005]    During operation of a nuclear fission reactor, the function of the fuel rods within a fuel assembly is to allow transmission of the heat resulting from the fission reaction inside the fuel pellets mounted within the fuel rod while separating the radioactive material from the streaming cooling fluid, which in light water reactors is water. A nuclear fuel rod typically includes a cladding tube that houses a. stack of fuel pellets formed of uranium oxide, plutonium oxide, or a mixture thereof, and end plugs that seal both the upper and lower ends of the tube. During operation, the fuel rod claddings are subjected to heat, irradiation from the fuel pellets, and a chemical reactive environment from the streaming medium. 
         [0006]    The fuel rod cladding in light water reactors is usually manufactured from a zirconium alloy, Zirconium alloys are used in fuel rod cladding due to their good mechanical properties, low neutron cross-section, and relatively high corrosion resistance. Different types of zirconium alloys are available for different types of light water reactors. 
         [0007]    In spite of the favorable properties of the zirconium alloys, fuel rod claddings manufactured from a zirconium alloy are affected by the environment in the reactor (heat, radiation, chemistry environment, amount of deposition, and location of deposition on fuel rods) such that the material expands differently, or in a non-uniform manner. The expansion of the zirconium alloy creates a permanent deformation of fuel rods, for example an elongation, such that the fuel rod dimensions in relation to its original dimensions change along the life of the fuel assembly and are different from fuel rod to fuel rod. The expansion of the zirconium alloy arises anisotropically, which results in an originally straight fuel assembly becoming bent during its life in a number of directions away from its original longitudinal axis. 
         [0008]    The permanent deformation of the zirconium alloy in the fuel rod claddings is induced by heat, irradiation from the fuel rods, and by corrosion and hydrogen pick up. The corrosion is a function of the type of chemical environment, the type of deposition, and the quantity of deposition on each rod cladding at each location. The hydrogen pick up is a function of the cooling fluid chemical environment and the deposition resulting on fuel rod claddings. Hydrogen pick up is concentrated in the form of hydrides in the zirconium alloy, which, in addition to the permanent deformation, also results in a weakening of the mechanical properties of the fuel rod cladding. It is to be noted that the quantities of hydrides inside the fuel rod cladding resulting from the hydrogen pick up varies azimuthally and along the longitudinal axis of the fuel rod, creating different mechanical properties of the fuel rod cladding at every location along the fuel rod. 
         [0009]    In light water reactors, the water is guided along the fuel rods from the bottom to the top of the reactor. Light water reactors are controlled by means of control elements, typically control blades that are displaced into and displaced out of positions between the fuel assemblies mounted in fuel channels for boiling water reactors (BWRs) and control rods that are displaced into and displaced out of the guide thimbles of the fuel assemblies for pressurized water reactors (PWRs). Due to the fuel assembly&#39;s great length, even a small inhomogeneous permanent deformation of the fuel rods may create a large bending of the fuel assembly. Any permanent deformation of a fuel assembly results in difficulties in movement of the control elements since there could be, for example in BWRs, frictional contact between the control blades and fuel channels resulting in “slow to settle” or totally inactive control blades. 
         [0010]    The melting temperature of zirconium alloys is around 1750° C., substantially below the maximum temperatures reached in a dry core (2400° C.) during a beyond conceivable limits accident resulting in a total dry-out core (that is, no water available to cool the nuclear reactor). This condition would allow dissipation of radioactive materials in the melt resulting during a beyond conceivable limits accident inside the reactor vessel, and create higher impacts to the environment and to the cost and duration of recovery after such an accident. While the probability of such an accident is extremely small, it stresses the importance of introducing better materials than zirconium alloys for the fuel rod cladding as the ultimate barrier against the dissipation of large quantities of radioactive materials in the environment. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention relates to a fuel assembly, part of a nuclear fission reactor, comprising a bottom nozzle, an upper nozzle, and a plurality of elongate fuel rods mounted between the two nozzles. The fuel rods pass through a number of spacer grids mounted between the two nozzles, which fuel rods each comprises nuclear fuel organized in fuel pellets mounted inside fuel rod claddings and are adapted to transfer energy to a streaming medium during operation of the nuclear reactor. 
         [0012]    A fuel rod of the present invention includes an elongate hollow cladding, configured to retain a nuclear fuel therein. The nuclear fuel is typically provided in fuel pellets made of fissile material including uranium, plutonium, or a mixture thereof. The fuel cladding is formed of a tube made of a ceramic material, with silicon carbide being a preferred ceramic. The ceramic material is chosen such that it can withstand temperatures and radiation typical of an operating nuclear reactor without deformation. 
         [0013]    The fuel cladding further includes one or more fiber layers positioned or spun about the tube. Each fiber layer is also formed of at least one fiber of a ceramic material, and preferably of the same material as the tube. The fiber layers provide added strength and enhance the structural integrity of the fuel cladding. The fuel cladding may further include a residual substance such as boron or graphite to reduce friction between the fibers. This lubricating residual substance can be an integral part of the fibers themselves, or an added substance separate from the fiber layers. The mass of the residual substance preferably does not exceed 8% of a total mass of the fibers. 
         [0014]    The fibers may be positioned about the tube in a number of ways. For example, the fibers may be wrapped around the tube such that the fiber layers are not parallel to the longitudinal axis of the tube. The fibers preferably are wrapped around the tube at an angle of approximately 30° to 70° relative the tube longitudinal axis. The overall strength of the fuel cladding is increased if the orientation of the fiber layers varies, and thus adjacent fiber layers preferably are positioned at different angles. other words, adjacent fiber layers are not parallel. A relative angle of 75° to 105° between adjacent fiber layers is preferred. The fibers may be grouped together as a mesh. The fibers may be provided as an integral part of the tube rather than being wrapped about the outer surface of the tube. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention is described with reference to the accompanying drawings, which illustrate exemplary embodiments and in which like reference characters reference like elements. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
           [0016]      FIG. 1  shows a cross-sectional view of a preferred composite fuel rod of the present invention. 
           [0017]      FIG. 2  shows the fuel rod of  FIG. 1 , with only two fiber layers for clarity of explanation. 
           [0018]      FIG. 3  shows a cross-sectional view of a nuclear fuel assembly of the present invention. 
           [0019]      FIG. 4  shows a perspective view of a preferred composite fuel rod of the present invention. 
           [0020]      FIG. 5  illustrates a fiber mesh of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The present invention provides nuclear fuel rods  1  formed of a composite ceramic cladding  6  configured to retain nuclear fuel pellets  20  therein in a known manner. Silicon carbide (SiC) is a preferred ceramic material. As illustrated in  FIG. 1 , the composite ceramic cladding  6  comprises a tube  10  covered by a number of layers  12  of fibers  8  spun around the tube  10 . Preferably, the fiber layers  12  are placed about the tube  10  in varying directions, enhancing the strength of the fuel rod  1 . For example, as shown in  FIG. 2 , the fiber layers  12  may be arranged iii a direction non-parallel with the fuel rod longitudinal axis  15  at an angle α that is between approximately 30° to 70° relative the longitudinal axis  15 . (It should be noted that for the sake of clarity, only a single fiber  8  per fiber layer  12  is illustrated in  FIG. 2 .) Subsequent fiber layers  12  preferably are placed atop the previously placed fiber layer(s)  12  such that the additional fiber layer  12  is non-parallel to both the fiber layer  12 . onto which it is placed and the longitudinal axis  15 . Adjacent fiber layers  12  preferably are positioned at a relative angle of approximately 75° to 105°, with substantially perpendicularly being more preferred. The density of each fiber layer  12  can be different. The number of fiber layers  12  added on top of the tube  10  to form the cladding  6  is determined by the fuel designer to obtain a cladding  6  with a specific ductility or ultimate strength design value. 
         [0022]    The composite ceramic cladding  6  may contain a balance of possible residual substances to reduce the friction coefficient between the layers  12  of ceramic fibers  8  or between the ceramic fibers  8 . The anti-friction materials preferably have low neutron absorption cross-sections. Boron nitrates, boron carbide (B4C), and graphite are examples of such residual substances and may be used with the present invention. The anti-friction materials can be sprayed on one or more of the fibers  8  and/or of the fiber layers  12  rather than, or in conjunction with, the anti-friction materials being incorporated in the ceramic fiber material. Preferably, the mass of the anti-friction residual substances existing in the fibers  8  and/or sprayed on the fibers  8  and/or on the fiber layers  12  does not exceed 8% of the total mass of the fibers. 
         [0023]    Preferably, the ceramic material of the composite fuel rod  1  consists essentially of silicon carbide. Silicon carbide has properties that reduce the problem of the permanent deformation of the claddings  6  of the fuel rods  1 . Silicon carbide exhibits irradiation induced expansion in an amount that is approximately one-third that of zirconium alloys. Furthermore, the irradiation induced expansion of silicon carbide is predictable, in that the expansion is homogeneous in all directions; that is, the expansion is isotropic. Thus, use of silicon carbide to form the tubes  10  reduces the bending problem associated with known fuel rod claddings while increasing the dimensional stability of the fuel assembly  2 , an example of which is illustrated in  FIG. 3 . 
         [0024]    The use of silicon carbide, however, presents some challenges due to its low ductility and thermal conductivity. The fibers  8  provided about the tube  10  increase the ductility of the fuel rod cladding  6 . The fibers  8  also strengthen the fuel rod cladding  6  by, for example, dissipating any energy driving crack formation and generating a pseudo-ductile failure mode of the composite. 
         [0025]    Silicon carbide has much lower reaction rates with water in comparison with zirconium alloys used in known fuel cladding materials. The lower reaction rate beneficially eliminates corrosion as a concern during normal plant operation and allows for reaching higher maximum center fuel pellet temperature both during normal operations and during accident conditions. Silicon carbide has a neutron absorption cross section that is 30% smaller than known fuel cladding materials. This, together with higher allowable maximum center fuel pellet temperature, can compensate for its lower conductivity. Because the contribution of corrosion to the expansion and/or deformation of a silicon carbide composite material is small or negligible, the changing of the total fuel rod dimensions due to cladding corrosion is greatly reduced compared with zirconium alloy fuel claddings. 
         [0026]    The use of silicon carbide as a cladding material also offers significant advantages for accident conditions. Chief among the advantages are the higher melting point (2700° C. versus 1750° C. for known fuel rods). The use of silicon carbide also avoids exothermic reactions with steam under severe accident conditions (at temperatures higher than 1400° C.). Up to 1500° C., the rate of reactions with steam is down by two orders of magnitude with respect to zirconium alloys. Therefore, a silicon carbide composite fuel rod cladding  6  would have better behavior than the known zirconium alloy fuel rod claddings, substantially reducing the probability of spreading its contents (fuel pellets  20 ) around the reactor vessel in case of a beyond design accident in a nuclear reactor. 
         [0027]    The present invention provides a fuel rod cladding  6  with improved dimensional stability, and, in particular, reduced potential of inhomogeneous plastic deformation of the fuel rods  1  and the fuel assemblies  2 . The present invention further provides a fuel rod cladding  6  with improved corrosion resistance relative to known fuel rod claddings. The present invention further provides a fuel rod  1  with greater integrity and resistance to beyond design accidents. 
         [0028]      FIG. 3  shows a cross-sectional view of a nuclear fuel assembly  2  of the present invention. In the illustrated embodiment, the fuel assembly  2  is a fuel assembly for a boiling water reactor. The fuel assembly  2  includes a housing  30  inside of which several fuel rods  1  are arranged. The fuel rods  1  extend and are retained by upper and lower plates or nozzles  32 ,  33  respectively. One or more spacer grids  35  may be provided to support the fuel rods  1  along their length. Preferably, several spacer grids  35  are provided at regular intervals along the length of the fuel assembly  2 . In use, the reactor coolant flows through the fuel assembly  2 , contacting the outside surfaces of the individual fuel rods  1 . Heat generated within the fuel pellets  20  is transmitted through the cladding  6  to the coolant. In this manner, heat is removed from the fuel assembly  2  and it can ultimately be converted into electricity. 
         [0029]      FIG. 4  shows a perspective view of a fuel rod  1  according to a (preferred embodiment of the present invention. In this embodiment, the fiber layers  12  are provided within the tube  10  and form an integral part thereof. The tube  10  may include multiple fiber layers  12  separated by layers of solid ceramic material. A preferred spacing between subsequent fiber layers  12  is 1-2 mm radially. Each fiber layer  12  may actually be multiple fiber layers atop each other, with groupings of 3-10 fiber layers being preferred. The fiber layers  12  may be biased toward the tube inner diameter, in which case fiber layers  12  would be positioned at location  12 A and not locations  12 B or  12 C. Similarly, the fiber layers  12  may be biased toward the tube outer diameter, in which case fiber layers  12  would be positioned at location  12 C and not locations  12 A or  12 B. If the fiber layers  12  are to be distributed substantially equally spaced (radially) throughout the tube  10 , they would be positioned at each of locations  12 A,  12 B, and  12 C. In this embodiment, the fiber layer(s)  12  are positioned as desired during fabrication of the tube  10 , with one or more layers of solid ceramic material placed atop the fiber layers  12 . 
         [0030]    In lieu of individual fibers  8 , a mesh can be used.  FIG. 5  illustrates a fiber mesh  14  of the present invention. The fiber mesh  14  includes a plurality of fibers  8  arranged in a grid pattern. An aspect ratio of the grid, defined by first grid diagonal D1 divided by a second grid diagonal D2, preferably ranges from 1 to 5. 
         [0031]    While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Furthermore, while certain advantages of the invention have been described herein, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.