Abstract:
Example embodiments are directed to tie plate attachments having irradiation targets and/or fuel assemblies having example embodiment tie plate attachments with irradiation targets and methods of using the same to generate radioisotopes. Example embodiment tie plate attachments may include a plurality of retention bores that permit irradiation targets to be contained in the retention bores. Irradiation targets may be irradiated in an operating nuclear core including the fuel assemblies, generating radioisotopes that may be harvested from the spent nuclear fuel assembly by removing example embodiment tie plate attachments.

Description:
PRIORITY STATEMENT 
       [0001]    This application is a continuation of and claims priority under 35 U.S.C. §120 to co-pending application Ser. No. 12/078,705 filed Apr. 3, 2008, the entirety of which is incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Example embodiments generally relate to fuel structures and radioisotopes produced therein in nuclear power plants. 
         [0004]    2. Description of Related Art 
         [0005]    Generally, nuclear power plants include a reactor core having fuel arranged therein to produce power by nuclear fission. A common design in U.S. nuclear power plants is to arrange fuel in a plurality of fuel rods bound together as a fuel assembly, or fuel assembly, placed within the reactor core. These fuel rods typically include several elements joining the fuel rods to assembly components at various axial locations throughout the assembly. 
         [0006]    As shown in  FIG. 1 , a conventional fuel assembly  10  of a nuclear reactor, such as a BWR, may include an outer channel  12  surrounding an upper tie plate  14  and a lower tie plate  16 . A plurality of full-length fuel rods  18  and/or part length fuel rods  19  may be arranged in a matrix within the fuel assembly  10  and pass through a plurality of spacers  20 . Fuel rods  18  and  19  generally originate and terminate at upper and lower tie plates  14  and  16 , continuously running the length of the fuel assembly  10 , with the exception of part length rods  19 , which all terminate at a lower vertical position from the full length rods  18 . An upper end plug  15  and/or lower end plug  17  may join the fuel rods  18  and  19  to the upper and lower tie plates  14  and  16 , with only the lower end plug  17  being used in the case of part length rods  19 . Tie rods  28  may be full length rods placed at corner positions in fuel assembly  10  that securely join to upper and lower tie plates  14  and  16  and provide handling points for fuel assembly  10 . The end plugs  15  and  17  may mate with, and in the case of tie rods  28 , pass through, the upper and lower tie plates  14  and  16 , respectively, and may secure fuel rods  18  or  19  axially in the fuel assembly  10 . 
       SUMMARY 
       [0007]    Example embodiments are directed to tie plate attachments having irradiation targets and fuel assemblies that use example embodiment tie plate attachments and methods of using the same to generate radioisotopes. Example embodiment tie plate attachments may include a plurality of retention bores that permit irradiation targets to be inserted and contained in the retention bores. The irradiation targets may be irradiated in an operating nuclear core including the fuel assemblies, generating useful radioisotopes that may be harvested from the spent nuclear fuel assembly by removing example embodiment tie plate attachments. 
         [0008]    Example embodiment tie plate attachments may be connected to fuel assemblies via the upper tie plate, fuel rods, and/or channel surrounding the fuel assembly. Example embodiment tie plates may be held at a fixed axial position within fuel assemblies so as to expose irradiation targets therein to constant, lower-level neutron flux, thereby converting a substantial amount of the irradiation targets into useable radioisotopes. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0009]    Example embodiments will become more apparent by describing, in detail, the attached drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus do not limit the example embodiments herein. 
           [0010]      FIG. 1  is an illustration of a related art fuel assembly having no tie plate attachment. 
           [0011]      FIG. 2  is an illustration of an example embodiment fuel assembly having an example embodiment tie plate attachment. 
           [0012]      FIG. 3  is a detailed illustration of an example embodiment tie plate attachment. 
           [0013]      FIG. 4  is a detailed illustration of an example embodiment tie plate showing irradiation targets therein. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Detailed illustrative embodiments of example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only example embodiments set forth herein. 
         [0015]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0016]    It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” or “fixed” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
         [0017]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the language explicitly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0018]    It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
         [0019]      FIG. 2  illustrates an example embodiment fuel assembly  100  including upper tie plate  114  and an example embodiment tie plate attachment  150  that individually or together may function as a radioisotope production structure. Example embodiment fuel assembly  100  may be similar to conventional fuel assemblies with the exception of including example embodiment tie plate attachment  150 . Although example embodiment fuel assembly  100  is shown as similar to a conventional BWR type fuel assembly, other example embodiments, including PWR type fuel assemblies and unfinished fuel bundles, may be useable with tie plate attachments according to the present invention. 
         [0020]    Example embodiment tie plate attachment  150  may be generally rectangular and frame full-length fuel rods  118  in fuel assembly  100 . An outer perimeter of example embodiment tie plate attachment  150  may extend to about an outer perimeter of fuel assembly  100  formed by fuel rods  118  so as to form a substantially uniform axial profile within example embodiment fuel assembly  100 . 
         [0021]    Although example embodiment tie plate attachment  150  is shown as generally rectangular with a hollow center, other shapes are possible. For example, example embodiment tie plate attachments may extend along only one or two sides of example embodiment fuel assemblies instead of all four sides. Similarly, example embodiment tie plate attachments may have varied thicknesses or even extend through the entire cross-sectional profile of example embodiment fuel assemblies and have channels permitting coolant flow therethrough instead of having a hollow center. Example embodiment tie plate attachments may also have other shapes to match example embodiment fuel assemblies and tie plates therein, including hexagonal, triangular, etc. shapes. 
         [0022]    In the example embodiment tie plate of  FIG. 2 , attachment  150  may have a cross-sectional edge thickness equal to a single row of fuel rods  118  along a transverse cross section of example embodiment fuel assembly  100 . That is, example embodiment tie plate attachment  150  may surround, or be co-located with, the outer fuel rods  118  in example embodiment assembly  100 . In this way, example embodiment tie plate attachment  150  may not significantly reduce or interfere with coolant flow through interior rods in assembly  100  and may be placed at a position with typically lower neutron flux within the assembly  100 . 
         [0023]    As shown in  FIG. 2 , example embodiment tie plate attachment  150  may be positioned under upper tie plate  114  in an axial direction. Example embodiment tie plate attachment  150  may be held under upper tie plate  114  in a variety of ways. For example, example embodiment attachment  150  may be directly welded to upper tie plate  114 , forged into or be otherwise structurally continuous with upper tie plate  114 , may fit into upper tie plate  114  frictionally and/or in a lock-and-key fashion, or may be joined to upper tie plate  114  via fasteners such as bolts or screws. 
         [0024]    As shown in  FIGS. 2 and 3 , as another attachment option, example embodiment tie plate attachment  150  may permit one or more fuel rods  118  and/or upper end and tie plugs  120  to pass axially through attachment  150  via holes  155  and into upper tie plate  114 . Fuel rods  118  may thus fix example embodiment tie plate attachment  150  in a transverse position under upper tie plate  114 . Example embodiment tie plate attachment  150  may be held in a constant axial position under tie plate  114  by fuel rods  118  seating into holes  155  or by flow of coolant through assembly  100  in an axial direction, and/or fixing example embodiment tie plate attachment  150  against upper tie plate  114 . Or, for example, fuel rods  118  and/or upper end plugs  120  may be screwed into, locked into, welded onto, etc., example embodiment tie plate attachment  150  so as to hold attachment  150  in a constant axial position under upper tie plate  114 . 
         [0025]    Even further, example embodiment tie plate attachment  150  may attach to outer channel  112  by being welded and/or removably fitted into outer channel  112  surrounding example embodiment fuel assembly  100 . Lateral extensions (discussed below) may facilitate such contact between outer channel  112  and example embodiment tie plate attachment  150 . 
         [0026]    In example embodiment fuel assemblies, example embodiment tie plate attachments may thus be held near or attached under an upper tie plate in the axial direction. This position affords easy access to example embodiment tie plate attachments during assembly disassembly, as the example embodiment tie plate attachment may be accessed with removal of the upper tie plate alone. 
         [0027]      FIG. 3  is a detailed illustration of an example embodiment tie plate attachment  150 . Although example embodiment tie plate attachment  150  is shown as a hollow rectangle that matches the shape of the outer channel  112 , other shapes and orientations are possible as discussed above. Example embodiment tie plate attachment  150  is fabricated of a material that substantially maintains its physical and neutronic properties when exposed to conditions in an operating nuclear core, such that example embodiment tie plate attachment  150  does not interfere with or affect the neutron flux present in the operating reactor. Example embodiment tie plate attachments may be fabricated of, for example, stainless steel, Inconel, a nickel alloy, a zirconium alloy, aluminum, etc. 
         [0028]    As discussed above, holes  155  may penetrate entirely through example embodiment tie plate attachment  150  and permit fuel rods  118  (shown in shadow) and/or upper end plugs  120  to pass through and/or connect to example embodiment tie plate attachment  150 . As such, holes  155  may be sized with an inner diameter sufficiently greater than a fuel rod  118  and/or upper end plug  120  outer diameter. The example joining method of  FIG. 3  shows example embodiment tie plate attachment  150  “sitting” on the shoulder  117  of the fuel rod  118  and upper end plug  120  joint. It is understood and several other joining methods discussed above and below may be used, including frictional contact between rods or end plugs and example embodiment tie plate attachments, lock-and-key, slot-type, or dovetail-type joints, welding, and/or continuous connection between the parts. 
         [0029]    Example embodiment tie plate attachment  150  may include one or more lateral extensions  165  that facilitate positioning relative to and/or connection with channel  112 . For example, lateral extensions  165  may connect or abut channel  112  on each side of example embodiment tie plate attachment  150  in order to center and/or secure example embodiment tie plate attachment  150  within example embodiment fuel assembly  100 . Lateral extensions  165  may further match extensions and/or shape of the upper tie plate  114  in order to provide a consistent axial profile among upper tie plate  114  and example embodiment tie plate attachment  150 . 
         [0030]    Example embodiment tie plate attachment  150  includes a plurality of retaining bores  160  in its top face into which one or more irradiation targets  170  are placed and contained, as shown in  FIG. 4 , which is a blown up portion of area A in  FIG. 3 . Bores  160  do not pass through example embodiment tie plate attachment  150  but instead have a depth sufficient to allow irradiation targets  170  to fit within bores  160 . Bores  160  may be geometrically placed around or between holes  155 . Alternatively, bores  160  may be scattered in no particular pattern throughout example embodiment tie plate attachment  150 , so long as the structural integrity of attachment  150  is not compromised by the position and/or number of bores  160 . 
         [0031]    Irradiation targets  170  may be in the shape of small “seeds” or small rod shapes for insertion into retaining bores  160 . Based on the size of bores  160 , irradiation targets  170  may have a width and length to fit within bores  160  and may be, for example, on the scale of millimeters. Several irradiation targets  170  containing potentially different types of parent materials, including solids, liquids, and/or gasses, may be placed into a single retaining bore  160 . Alternatively, each bore  160  may contain homogenous irradiation targets  170 . 
         [0032]    Irradiation targets  170  may be made of a variety of materials that substantially convert into radioisotopes when exposed to a neutron flux encountered under tie plates  114  in an operating nuclear reactor. Because neutron flux may be lower at axial ends of example embodiment fuel assembly  100  ( FIG. 2 ), example embodiment tie plate attachments and irradiation targets  170  therein may be exposed to a lower flux as well. Hence, materials having high neutron cross sections and shorter half-lives may be preferable for use as irradiation targets  170 , including, for example, Iridium-191, which may convert to Iridium-192 when exposed to neutron flux encountered in an operating nuclear reactor. Similarly, other isotopes, including Cobalt-59, Selenium-74, Strontium-88, and/or Iridium-191 for example, may be used as irradiation targets  170 . 
         [0033]    Retention bores  160  may be sealed or closed by a cap  161 , shown in  FIG. 4 , that covers bores  160  and joins to example embodiment tie plate attachment  150 . For example, caps  161  may be welded onto attachment  150  or screwed into bores  160 , if the bores  160  are threaded. Other methods of securely attaching caps  161  over bores  160  in order to provide containment of irradiation targets  170  may be known and useable with example embodiments. Because cap  161  may provide containment to retention bores  160 , irradiation targets  170  may contain or produce useful gaseous, liquid, and/or solid radioisotopes when exposed to a neutron flux, and these radioisotopes may be contained in irradiation bores  160  by cap  161  even though they may be liquid, gaseous, or solid. 
         [0034]    Because of the higher axial position of example embodiment tie plate attachments, irradiation targets contained therein may be irradiated by lower amounts of neutron flux over a longer period of time, resulting in more predictable and effective generation of radioisotopes with shorter half-lives from irradiation targets having higher cross sections. Further, because upper tie plate areas, where example embodiment tie plate attachments may be placed, are associated with low fretting, example embodiment tie plate attachments may provide robust containment for irradiation targets. Lastly, upper tie plates may be easily removed from irradiated example embodiment fuel assemblies without disturbing fuel rods or irradiated fuel, permitting easier harvesting of example embodiment tie plate attachments and useful radioisotopes therein. Example embodiment tie plate attachments may further provide robust containment for retaining and containing solid, liquid, or gas radioisotopes produced from irradiation targets in example embodiment tie plate attachments. 
         [0035]    Example embodiments thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied through routine experimentation and without further inventive activity. For example, other fuel types, shapes, and configurations may be used in conjunction with example embodiment fuel assemblies and tie plate attachments. Variations are not to be regarded as departure from the spirit and scope of the exemplary embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.