Abstract:
A fuel assembly for use in a core of a nuclear power reactor. The assembly includes a frame shaped and configured to fit within the nuclear reactor internal core structure; and a plurality of helically twisted fuel elements supported by the frame in a fuel rod bundle. Each of the fuel elements includes fissile material. When viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle define a substantially circular perimeter. Other features, and a nuclear reactor, are also described.

Description:
CROSS-REFERENCE 
       [0001]    This application is a non-provisional of U.S. Application No. 62/050,985, filed on Sep. 16, 2014. This application is also a continuation-in-part of applicant&#39;s co-pending U.S. application Ser. No. 14/081,056, filed on Nov. 15, 2013, which claims priority to U.S. Provisional Application No. 61/821,918, filed on May 10, 2013. This application is also a continuation-in-part of applicant&#39;s co-pending U.S. application Ser. No. 13/695,792, filed on Jun. 3, 2013, which is the U.S. National Stage of PCT/US2011/036034, filed on May 11, 2011, which in turn claims priority to U.S. Application No. 61/444,990, filed Feb. 21, 2011, U.S. Application No. 61/393,499, filed Oct. 15, 2010, and U.S. Application No. 61/333,467, filed May 11, 2010. The entire content of all of the foregoing applications is expressly incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates generally to nuclear reactors and nuclear fuel assemblies used in the core of nuclear reactors. More specifically, the present invention relates to Canadian Deuterium-Uranium (CANDU) heavy-water reactors, and fuel assemblies for use in the same. 
         [0004]    2. Related Art 
         [0005]      FIGS. 1A and 1B  depict simplified cross-sectional views of examples of conventional fuel assemblies  10 .  FIG. 1A  depicts a fuel assembly  10  of the PWR type, and  FIG. 1B  depicts a fuel assembly  10  of the water-cooled water-moderated power rector (VVER) type. In  FIG. 1A , the fuel rod assembly  10  comprises fuel rods assembled into a square grid. The PWR fuel assembly  10  of  FIG. 1A  has fuel rod bundle self-spacing that can be described as having a square cross-sectional shape. In  FIG. 1B , the fuel assembly  10  comprises fuel rods arranged into a triangular grid. The VVER fuel assembly  10  of  FIG. 1B  has fuel rod bundle self-spacing that can be described as having a regular hexagonal cross-section shape. 
         [0006]    When these assemblies are fitted into a tube  12 , empty segments not used by the fuel rod assembly are formed, as shown by the shaded area  14  located between the tube  12  and the square  14  in  FIG. 1A , and between the tube  12  and the hexagon  16  in  FIG. 1B . According to embodiments, an assembly in a square grid occupies approximately 63.7% of the area of the circumscribed circle (e.g., tube  12 ), while an assembly in a triangular grid occupies approximately 82.7% of the area of the circumscribed circle (e.g., tube  12 ). 
         [0007]    It is known to use the empty space to address concerns of fuel rod and assembly swelling during burnup. It is also known to fill these areas with a burnable absorber, etc. 
       SUMMARY 
       [0008]    According to an embodiment, a fuel assembly for use in a core of a nuclear power reactor can include a frame shaped and configured fit within the nuclear reactor internal core structure; and a plurality of helically twisted fuel elements supported by the frame in a fuel rod bundle, with each of the fuel elements comprises fissile material. As viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle can define a substantially circular perimeter (e.g., dodecagon). According to embodiments, the frame can be shaped and configured to fit within a pressure tube of a CANDU reactor. 
         [0009]    According to embodiments, each of the plurality of fuel elements can have substantially the same circumscribed diameter. The plurality of fuel elements can be arranged in concentric circles. Additionally or alternatively, the plurality of fuel elements can be arranged into a mixed grid pattern that includes a first, rectangular grid pattern and a second, triangular grid pattern. 
         [0010]    According to embodiments, the first, rectangular grid pattern and the second, triangular grid pattern can at least partially alternate with one another. Some of the plurality of fuel elements can be separated from adjacent fuel elements by a common centerline-to-centerline distance, and a circumscribed diameter of some of the plurality of fuel elements can equal the centerline-to-centerline distance. 
         [0011]    According to embodiments, each of the fuel elements can have a multi-lobed profile that includes ribs, for example, spiral ribs. The ribs of adjacent fuel elements can periodically contact one another over the axial length of the fuel elements to at least partially maintain the spacing of the fuel elements relative to each other. According to embodiments, the fuel elements can comprise extruded fuel elements. 
         [0012]    According to embodiments, the plurality of fuel elements can consist of  61  fuel elements. 
         [0013]    According to embodiments, the frame can include a structure circumscribing the fuel rod bundle, such that all of the fuel elements are located inside the structure. The structure can comprise a shroud. When viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the shroud can define a cross-section substantially defining a circle or dodecagon. When viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the fuel assembly can occupy greater than about 64%, more specifically greater than about 83% of the internal cross-sectional area of a tube circumscribing the fuel assembly. According to an embodiment, the fuel assembly can occupy between about 83% and about 95% of the internal cross-sectional area of the tube circumscribing the fuel assembly. 
         [0014]    According to embodiments, the fuel assembly is thermodynamically designed and physically shaped for operation in a conventional land-based nuclear power reactor of a conventional nuclear power plant having a reactor design that was in actual use before 2014, and the frame is shaped and configured to fit into the land-based nuclear power reactor in place of a conventional fuel assembly for said reactor. For example, the conventional land-based nuclear power reactor can be a CANDU reactor. 
         [0015]    According to another aspect of the present invention, a nuclear reactor includes a core and one or more fuel assemblies disposed within the core. The fuel assembly can include: a frame shaped and configured to fit within the core; and a plurality of helically twisted fuel elements supported by the frame in a fuel rod bundle, with each of the fuel elements comprising fissile material. As viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle can define a substantially circular perimeter. According to embodiments, the nuclear reactor is a CANDU reactor comprising pressure tubes, and the frame is shaped and configured to fit within the pressure tubes. 
         [0016]    According to embodiments, each of the plurality of fuel elements can have substantially the same circumscribed diameter. The plurality of fuel elements can be arranged in concentric circles, and/or the plurality of fuel elements can be arranged into a mixed grid pattern that includes a first, rectangular grid pattern and a second, triangular grid pattern. The first, rectangular grid pattern and the second, triangular grid pattern can at least partially alternate with one another 
         [0017]    According to embodiments, the nuclear reactor was in actual use before 2014. 
         [0018]    According to embodiments, each of the fuel elements has a multi-lobed profile that includes spiral ribs. The ribs of adjacent fuel elements can periodically contact one another over the axial length of the fuel elements to at least partially maintain the spacing of the fuel elements relative to each other. According to embodiments, the fuel elements can comprise extruded fuel elements. 
         [0019]    According to embodiments, the frame of the fuel element comprises a structure circumscribing the fuel rod bundle, such that all of the fuel elements are located inside the structure. The structure can comprise a shroud that when viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, defines a cross-section substantially defining a circle or dodecagon. 
         [0020]    These and other aspects of various embodiments of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. In one embodiment of the invention, the structural components illustrated herein are drawn to scale. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. In addition, it should be appreciated that structural features shown or described in any one embodiment herein can be used in other embodiments as well. As used in the specification and in the claims, the singular form of “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a better understanding of embodiments of the present invention, as well as other features thereof, reference is made to the following description which is to be used in conjunction with the following drawings, wherein: 
           [0022]      FIG. 1A  is a simplified cross-sectional view of a conventional fuel assembly having fuel rods assembled in a square grid; 
           [0023]      FIG. 1B  is a simplified cross-sectional view of a conventional fuel assembly having fuel rods assembled in a triangular grid; 
           [0024]      FIG. 2  is a simplified cross-sectional view of a layout of a self-spaced fuel assembly made up of 61 fuel rods in a square-triangular grid, according to an embodiment; 
           [0025]      FIG. 3  is a simplified cross-sectional view of a layout of a self-spaced fuel assembly made up of 19 fuel rods in a square-triangular grid, according to an embodiment; 
           [0026]      FIG. 4  depicts a cross-sectional view of an embodiment of a fuel assembly at an initial reference position along the fuel assembly, referred to herein as the initial 0° position; 
           [0027]      FIG. 5  depicts a cross-sectional view of the fuel assembly of  FIG. 4  at a 30° fuel rod rotation, or at a lengthwise displacement of 1/12 of the fuel rod swirl pitch, with respect to the initial 0° position of  FIG. 4 ; and 
           [0028]      FIG. 6  depicts a cross-sectional view of the fuel assembly of  FIG. 4  at a 60° fuel rod rotation, or at a lengthwise displacement of 1/6 of the fuel rod swirl pitch, with respect to the initial 0° position of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Embodiments described herein can increase the fuel burnup power and/or level (operating time until unloading) of a CANDU fuel assembly and/or reactor as a whole, while maintaining or increasing the level of safety. According to embodiments, this can be achieved through the use of fuel assemblies made from twisted, self-spaced, monolithic fuel rods, for example, the extruded uranium-zirconium (U-Zr) fuel rods disclosed in applicant&#39;s co-pending U.S. application Ser. Nos. 14/081,056 and 13/695,792, the entire contents of which are expressly incorporated herein by reference. 
         [0030]    CANDU fuel assemblies typically utilize very short (e.g., on the order of 50 cm) fuel rods. Embodiments of the present invention provide partially or fully self-spaced assemblies of CANDU fuel rods. For example, some fuel assemblies disclosed herein provide for self-spacing of all the fuel rods among themselves (e.g., rib by rib). However, alternative embodiments can include non-self-spaced arrangements. Embodiments can include a frame having a shroud, or other channel or device surrounding all or a part of the fuel rod bundle (referred to generally herein as a “shroud”), and better utilize the space available inside the shroud than is possible with the prior art. For example, as will be described in more detail below, embodiments use a “square-triangular” fuel rod grid in an array. 
         [0031]      FIG. 2  is a simplified cross-sectional view of an embodiment of a self-spaced fuel assembly  100 . The fuel assembly can include 61 fuel rods  102  in a square-triangular grid, however, other configurations may be possible. The fuel assembly shown in  FIG. 2  can have the same or similar envelope as an Advanced CANDU Reactor (ACR) CANDU Flexible (CANFLEX) 43-element assembly. Whereas a typical CANFLEX assembly has 43 fuel elements each with an outer diameter of about 13.5 mm, the fuel assembly  100  shown in  FIG. 2  can have 61 fuel elements  102  each with an outer diameter of about 11.5 mm, however, other quantities and sizes of fuel elements are contemplated. 
         [0032]    The fuel assembly of  FIG. 2  can be fitted into a shroud  104 . For example, the shroud  104  can have a cross-section in the shape of a dodecagon, however, other shapes are envisioned. According to embodiments, the radius R of a circle circumscribing the fuel elements  102  can be less than or equal to 51 mm. According to embodiments, the inner radius of the shroud  104  can be about 51.7 mm, however, other embodiments are possible. Shroud  104  can have a dodecagon shape, and can define a width h across the flats of about 100 mm (≦99.99 mm) According to embodiments, the square-triangular grid of  61  fuel elements defines an outer perimeter that occupies approximately 95.5% of the area of the circumscribed circle (e.g., the shroud  104  or pressure tube). With reference to  FIG. 3 , the central area of 19 fuel rods  102  can fit nearly perfectly into a tube. According to embodiments, the radius R 19  of a circle circumscribing the central 19 fuel rods can have a diameter of 3.922 mm, however, other dimensions are possible. 
         [0033]    Referring to  FIGS. 2 and 3 , the fuel elements can be located in first and second grid patterns intermixed with one another to form what is referred to herein as a “square-triangular grid.” The first grid pattern includes squarely arranged rows and columns of fuel elements having a centerline-to-centerline distance between the rows and columns that equals the common circumscribed diameter “d” of the fuel elements (see reference  106  in  FIG. 3  for an example of the first “square” grid). The second grid pattern includes equilateral triangles in which a length of each side of each triangle (i.e., the centerline-to-centerline distance between adjacent fuel elements defining the corners of each triangle) is the common circumscribed diameter “d” of the fuel elements (see reference  108  in  FIG. 3  for an example of a second “triangular” grid). Thus, the second/triangular grid pattern  108  is different from the first/square grid pattern  106 . According to alternative embodiments, additional and/or alternative grid patterns could also be used (e.g., rectangular grid patterns, isometric grid patterns, parallelogram patterns, other regular repeating patterns) without deviating from the scope of the present invention. According to embodiments, a given fuel element  102  may be located in a square grid pattern with one set of surrounding fuel elements, and simultaneously be located in a triangular grid pattern with another set of surrounding fuel elements, however, other configurations are possible. 
         [0034]    Still referring to  FIGS. 2 and 3 , the square  106  and triangular  108  grid patterns can alternate with one another when viewed from one or more perspectives. For example, the square  106  and triangular  108  grid patterns can alternate with one another (but not necessarily on a one-to-one basis) with movement along any given radius from the center  110  of the fuel assembly to the outer perimeter, e.g., shroud  104 . Additionally or alternatively, the fuel elements  102  can be arranged in concentric circles, and the square and triangular grid patterns can alternate with one another (but not necessarily on a one-to-one basis) with movement around any one of the concentric circles. 
         [0035]    As mentioned before, the fuel elements may be self-spacing. According to embodiments, the self-spacing can be a factor of the fuel rod circumscribed diameter, independent of the fuel rod shape selected, however, other configurations are possible. According to certain embodiments, the fuel rods  102  may be any shape with twisted ribs (e.g., a tube with ribs, squares, etc.). However, other shapes may be possible, such as circular cross-sections, regular geometric cross-sections, etc. 
         [0036]      FIGS. 4-6  depict cross-sectional views of an embodiment of a fuel assembly  200  comprising four-lobe fuel rods  202 , such as those described in applicant&#39;s co-pending United U.S. application Ser. Nos. 14/081,056 and 13/695,792, the entire contents of which are incorporated herein by reference. According to a further aspect, certain fuel rod shapes such as the four-lobe design, may be standardized for different reactors. For example, a fuel rod with a four-lobe shape, a circumscribed diameter of 12±1 mm, and slight modifications may become standard for different reactors such as the PWR and CANDU. 
         [0037]      FIG. 4  depicts the fuel assembly  200  at an initial reference position, referred to herein as the initial 0° position. The initial 0° position can occur at any point along the fuel rods  202 , and can occur at regular intervals.  FIG. 5  depicts the fuel assembly  200  of  FIG. 4  at the point of  30 ° rotation of the fuel rod&#39;s lobes  204  (e.g., lengthwise displacement of 1/12 of the fuel rod swirl pitch) with respect to  FIG. 4 .  FIG. 6  depicts the fuel assembly of  FIG. 4  at the point of 60° rotation of the fuel rods&#39; lobes  204  (e.g., lengthwise displacement of 1/6 of the fuel rod swirl pitch) with respect to  FIG. 4 . A 90° rotation of the lobes  204 , or a lengthwise displacement of 1/4 of the fuel rod swirl pitch, away from the position of  FIG. 4  replicates the tentative initial position of 0° shown in  FIG. 4 . In  FIGS. 4-6 , the eight fuel rods  202 ′ indicate the only rods within the cross-section that do not have contact with other fuel rods  202  or the shroud  206 . At axial locations between those shown in  FIGS. 4 ,  5 , and  6 , there is no lengthwise contact of the fuel rods with one another or with the shroud  206 . Accordingly, the fuel assembly is self-spacing and all the fuel rods are self-spaced along the length of the assembly. 
         [0038]    As mentioned previously, the fuel rods can comprise the four-lobe fuel rods described in applicant&#39;s co-pending U.S. application Ser. Nos. 14/081,056 and 13/695,792. However, according to alternative embodiments, any of the four-lobe fuel rods in the afore-described fuel assemblies can replaced by standard pelleted cylindrical fuel rods (uranium or thorium), or burnable poison bearing fuel rods (e.g., containing gadolinium (Gd), erbium (Er), and/or dysprosium (Dy)) 
         [0039]    As used throughout this application, the term “shroud” encompasses a variety of different designs that can surround the fuel rod bundle, either partially or completely. For example, according to embodiments, a “shroud” can be a solid dodecagonal shroud, perforated or with slits. Alternatively, the “shroud” can comprise individual bands or a shrouding strip, or riveting on cylindrical shell (e.g., solid or “openwork” with slits). Moreover, the term “shroud” can encompass other similar structures and designs apparent to one of ordinary skill in the art based on this description. 
         [0040]    The foregoing illustrated embodiments are provided to illustrate the structural and functional principals of the present invention and are not intended to be limiting. To the contrary, the principles of the present invention are intended to encompass any and all changes, alterations, and/or substitutions within the spirit and scope of the following claims.