Abstract:
Fuel spacers include a perimeter piece and alignment strips extending within the perimeter piece. Alignment strips may have directional variation while still extending in an overall straight line between two contact points on the perimeter piece. Two alignment strips, by their relative positioning and shape, create distinct openings for fuel rods, through which rods may pass and be supported by the spacer on all sides. Alignment strips can be parallel or skew but need not physically intersect or extend at overall right angles to form such surrounding and supporting openings. Shape may be variable, such as a waveform, zig-zag, or saw-tooth. Several layers of alignment strips at any desired angle are useable in spacers, and alignment strips may be varied in length, shape, and number to account for different fuel assembly sizes and features, such as water rods.

Description:
BACKGROUND 
       [0001]      FIG. 1  is an illustration of a conventional nuclear reactor fuel assembly  10  typically used in commercial nuclear power reactors for electricity generation throughout the world. Several fuel assemblies  10  are placed in a reactor in close proximity to sustain a nuclear chain reaction. A fluid moderator and/or coolant conventionally passes through fuel assembly  10  in a length-wise (axial) direction, enhancing the chain reaction and/or transporting heat away from the assembly  10 . 
         [0002]    As shown in  FIG. 1 , fuel assembly  10  includes multiple fuel rods  14  containing fissile material and extending in the axial direction within the assembly  10 . Although not shown in  FIG. 1 , fuel rods  14  are often seated into a lower tie plate  16  and extend upward into an upper tie plate  17  at ends of fuel assembly  10 . Fuel rods  14  are bounded by a channel  12  that forms an exterior of the assembly  10 , maintaining fluid flow within assembly  10  throughout the axial length of assembly  10 . Conventional fuel assembly  10  also includes one or more conventional fuel spacers  18  at various axial positions. Fuel spacer  18  permits fuel rods  14  to pass through grid openings in spacer  18 , thereby aligning and spacing fuel rods  14 . One or more water rods  19  may also be present to provide a desired level of moderator or coolant through-flow to assembly  12 . 
         [0003]      FIG. 2  is an illustration of a related art fuel spacer  18  from an axial direction. As shown in  FIG. 2 , conventional spacer  18  includes several grid openings  41 , which may be formed by several ferrules  40 . Each ferrule  40  conventionally forms a full circle and is joined with other ferrules  40  to form a grid-like pattern of openings  41  where fuel rods  14  ( FIG. 1 ) should pass and be stabilized through spacer  18 . That is, several fuel rods  14  ( FIG. 1 ) may pass through spacer  18  through corresponding ferrules  40  and grid openings  41 , when used in an assembly. Ferrules  40  may be joined together by welding among each touching ferrule  40 , with perimeter ferrules  40  being welded to perimeter band  49 . In this way, each ferrule  40  is rigidly joined to and stabilized with several adjacent ferrules. 
         [0004]    Ferrules  40  and thus grid openings  41  may be sized substantially similar to perimeter sizes of fuel rods intended to pass therethrough, permitting a frictional sliding relationship between spacer  18  and a fuel rod. Ferrules  40  in  FIG. 2  may further each include two rod-contact stops  42  and one rod-contact spring  43  to control frictional forces when a fuel rod passes therethrough. Grid openings  41  may all be of a substantially similar size and positioned in rectilinear fashion as shown in  FIG. 2 , or may be positioned and sized differently to accommodate other fuel designs. For example, grid openings  41  for water rods  16  may be larger than grid openings  41  for smaller fuel rods  14 . Alternatively, all grid openings  41  may be a same size. Perimeter band  49  may enclose spacer  18  and contact channel  12  ( FIG. 1 ). 
       SUMMARY 
       [0005]    Example embodiments include nuclear fuel spacers that sit along axial positions of a fuel assembly and contact/align fuel rods that pass therethrough. Example embodiment spacers include a perimeter piece of a variety of shapes and formed of a number of different pieces, such as a belly band or an annular fitting, with alignment strips inside of the perimeter piece. Two alignment strips alone can define one or more holes or “quasi-ferrules” that will at least partially surround and allow axial passage of a fuel rod through the spacer. Example embodiments can use a variety of structures for alignment strips, including simple waveform internal spans that can be directly welded to the perimeter piece, more complex stamped or machined pieces with flow tabs, swirl vanes, trippers, and/or any other feature, or composite meshes, for example. Alignment strips can be parallel, such as in evenly-spaced diagonal rows, or skew, but alignment strips do not have to touch or be at the same elevation to form openings for fuel rods. Example embodiments using simple alignment strips, such as continuous internal spans that are formed of a single, non-interrupted material, may be welded only to the perimeter piece, such that manufacture is greatly simplified and welds and other often failing connections are reduced in some example embodiments. Example embodiment spacers are useable with a variety of fuel assembly configurations, including parallelepiped assemblies with fuel rods arranged in grids of rows and columns. 
         [0006]    Example embodiments can be formed of any material that is resilient in nuclear reactor conditions, including stainless steels, nickel alloys, aluminum alloys, and/or zirconium alloys and several other materials. Components of example embodiment spacers may be formed with any desired stiffness such that example embodiments may not elastically deform to typical forces experienced by fuel rods or without rigidity such that alignment strips elastically bend or flex under such forces. A number of other features are also compatible with example embodiments, including rod contacts (stops and/or springs) that contact and brace fuel rods in each direction in openings, flow tabs that mix or direct fluid coolant/moderator in a desired manner through openings and against fuel rods, trippers, filters, etc. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0007]    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 terms which they depict. 
           [0008]      FIG. 1  is an illustration of a section of a conventional nuclear fuel assembly. 
           [0009]      FIG. 2  is an illustration of a related art fuel spacer from an axial direction. 
           [0010]      FIG. 3  is an illustration of an example embodiment fuel spacer. 
           [0011]      FIG. 4  is an illustration of a detailed section of the example embodiment fuel spacer of  FIG. 3  in use with fuel rods. 
           [0012]      FIG. 5  is an illustration of another detailed section of the example embodiment fuel spacer of  FIG. 3  in use with fuel rods. 
           [0013]      FIG. 6  is an illustration of another example embodiment fuel spacer. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    This is a patent document, and general broad rules of construction should be applied when reading and understanding it. Everything described and shown in this document is an example of subject matter falling within the scope of the appended claims. Any specific structural and functional details disclosed herein are merely for purposes of describing how to make and use example embodiments. Several different embodiments not specifically disclosed herein fall within the claim scope; as such, the claims may 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.). Similarly, a term such as “communicatively connected” includes all variations of information exchange routes between two devices, including intermediary devices, networks, etc., connected wirelessly or not. 
         [0017]    As used herein, the singular forms “a”, “an” and “the” are intended to include both the singular and plural forms, unless the language explicitly indicates otherwise with words like “only,” “single,” and/or “one.” It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, steps, operations, elements, ideas, and/or components, but do not themselves preclude the presence or addition of one or more other features, steps, operations, elements, components, ideas, and/or groups thereof. 
         [0018]    It should also be noted that the structures and operations discussed below may occur out of the order described and/or noted in the figures. For example, two operations and/or figures shown in succession may in fact be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Similarly, individual operations within example methods described below may be executed repetitively, individually or sequentially, so as to provide looping or other series of operations aside from the single operations described below. It should be presumed that any embodiment having features and functionality described below, in any workable combination, falls within the scope of example embodiments. 
         [0019]    Applicants have recognized that fuel assemblies are subjected to a variety of shocks and strains over their lifetime, including shipping, installation, handling, seismic events, and power generation, that cover a wide array of force profiles on the assembly. As such, it is desirable to minimize vibration and maintain fuel rods in particular positions in a fuel assembly for fluid flow, neutronics, and handling purposes, while minimizing risk of damage from structures that provide desired positioning. Such damage may be caused by failures at particularly weak and/or intersection points, such as welds or other connections within a spacer. Further, Applicants have recognized a need for fuel rod spacing and securing with minimal flow blockage and simplified manufacturing of components to so space and secure each and every fuel rod in an assembly, of which there may be dozens. These and other problems recognized by Applicants are addressed below with unique solutions enabled by example embodiments. 
         [0020]    The present invention is fuel spacers, fuel assemblies having spacers, and methods of forming and using the same. Spacers of the present invention include plural alignment strips within a perimeter piece that are shaped, aligned, and extend in such a manner so as to form distinct axial openings for fuel rods without a need to directly or proximally contact each other, although direct contact between alignment strips may be used to provide other functions. Example embodiments discussed below illustrate just a few of the variety of different configurations and structures that can be used in connection with the present invention. 
         [0021]      FIG. 3  is an illustration of an example embodiment fuel spacer  118 . As shown in  FIG. 3 , example embodiment fuel spacer  118  may include several features of, and be useable with or in place of, related fuel spacers, in nuclear fuel assemblies, such as those shown in  FIGS. 1 and 2 . For alignment strips, example embodiment fuel spacer  118  includes several internal spans  140  positioned within a perimeter piece, here, outer perimeter band  149 . Internal spans  140  are positioned and configured to create openings for and support fuel rods that may pass through example embodiment spacer  118 , providing any desired vibration and/or movement dampening, ensuring proper rod placement, and/or maintaining fuel rods in such positions. 
         [0022]    For example, internal spans  140  may be waveform with multiple peaks and valleys/crests and troughs having curvature approximating outer perimeters of fuel rods. Such waveform internal spans  140  may be aligned in diagonal rows at a consistent angle and pitch to accommodate a grid of fuel rods, so as to mimic portions of diagonally-proceeding sinusoids or curves formed by conventional ferrule-type spacers when viewed along certain sight lines. That is, waveform internal spans  140  may each generally extend about a straight line while evenly deviating from the line to create desired openings with other internal spans  140 . Of course, other configurations, arrangements, and numbers of internal spans  140  may be used with other fuel designs; for example, internal spans  140  may be relatively straight or zig-zag, or placed at other non-uniform and non-orthogonal angles aside from the roughly consistent 45-degree diagonal to accommodate fuel rods laid out in other arrangements. 
         [0023]    Internal spans  140  may be uniform or non-uniform; they may extend only partially throughout example embodiment spacer  118  or may be varied in particular positions, allowing for conventional ferrules or grid openings to be used in other portions of spacer  118  or for other components to be present. For example, as shown in  FIG. 3 , a central void may be formed for a water rod  16  by varying two central-most internal spans  140  with larger central crests and troughs sized to permit a water rod to pass therethrough. 
         [0024]    Internal spans  140  connect to perimeter band  149  at junction points  142 . Each internal span may use two junction points  142 , one at each end, in order to secure to perimeter band  149 . Junction points  142  may use any form of material joining, including welding, fastening, tang-and-auger type receiving, pass-through and crimping, etc. Internal spans  140  may be continuous pieces or formed from several joined segments. If internal pans  140  are continuous and each use only one or two junction points  142  with welding, example embodiment fuel spacer  118  may have considerably simplified manufacture and construction, requiring only a number of welds equal to or less than twice the number of internal spans  140  used, which may be limited to less than a number of rows or columns of fuel rods to be used with example embodiments. 
         [0025]    Internal spans  140  may be fabricated of any materials that substantially maintain their mechanical properties in an operating nuclear reactor environment. For example, a metallic material may be used, such as a zirconium, aluminum, nickel and/or iron alloy like Zircaloy, X750 or stainless steel. Internal spans  140  may be formed of a thickness or other rigidity-determinative feature to achieve a desired stiffness and/or flexibility based on a selected fabrication material. For example, internal spans  140  may be formed relatively thin so as to be somewhat elastic and minimize cross-sectional flow blockage. In such a case, the individual fuel rods loaded through an example embodiment spacer and contacting internal spans  140  may impart actual stiffness to an example embodiment spacer  118 . Load stiffeners, such as rigid and removable braces extending across a spacer or fuel assembly, may be used in this example to provide stiffness and shock support during shipping. Or, for example, internal spans  140  may be formed of a material and thickness to remain relatively rigid and provide relatively static positioning of fuel rods passing therethrough, without the need for additional load stiffeners during shipping. 
         [0026]    Other known features, including swirl vanes, rod contacts, trippers, etc. are useable in example embodiment fuel spacer  118 .  FIG. 4  is an illustration of a section of example embodiment fuel spacer  118  from  FIG. 3  in use with different types of rod contacts useable in example embodiment fuel spacers. For example,  FIG. 4  shows indents  143  in internal spans  140  being used as rod contacts. Indents  143  may be materially continuous with the reminder of internal span  140  and sized and shaped to make a small and direct contact with fuel rods  14 . As shown in  FIG. 4 , if adjacent internal spans  140  use opposing and adequately spaced indents  143 , fuel rods  14  may be contacted at three or more positions spaced about their perimeter by indents  143  from different internal spans  140 . Also as shown in  FIG. 4 , conventional or other types of non-indent rod contacts  141  are useable at perimeter positions where three surrounding indents  143  are not available to provide a consistent force profile in each dimension, or any other desired position. 
         [0027]    One or more perimeter springs  144  may be used in example embodiment fuel spacers to provide a desired level of elastic movement of fuel rods  14  within perimeter band  149 . For example, if contacts between internal span  140  and fuel rods  14  are relatively rigid, such as with indents  143 , then fully populating an example embodiment spacer with fuel rods may result in a generally static spacer with little relative movement between individual fuel rods  14  and between rods  14  and internal spans  140 . Perimeter springs  144  about perimeter band  149  and/or substituted for indents  142  at desired positions in such an example may add a desired degree of elasticity and control friction forces when inserting rods during fabrication and dampened relative movement. 
         [0028]    As shown in  FIG. 5 , another configuration may include other rod contacts  141  positioned and sized to contact each fuel rod  14  that passes between adjacent internal spans  140 . If internal spans are waveform with appropriate sizing and pitch, four rod contacts  141  may contact fuel rods  14  approximately orthogonally at every 90-degrees or so about each fuel rod  14 . Rod contacts  141  may provide relatively rigid and/or elastic contact to fuel rods  14 , depending on a desired stabilization and positioning characteristic. For example, deflection-limited rod contacts from co-owned application “SPACERS WITH DEFLECTION-LIMITED ROD CONTACTS FOR NUCLEAR FUEL ASSEMBLIES AND METHODS OF MAKING THE SAME” Ser. No. 13/603,184 filed Sep. 4, 2012, incorporated herein in its entirety, are useable as contacts  141 , as are a variety of other known contact designs. Still alternatively, continuous and substantial direct contact between internal spans  140  and fuel rods  14 , instead of smaller, discreet rod contacts  141 , is permissible in example embodiments. 
         [0029]    As seen in  FIGS. 4 and 5 , example embodiment fuel spacer  118  may use internal spans  140  with simplified construction and manufacture and less cross-sectional area, and thus flow blockage, to provide nearly equivalent surrounding support and positioning to fuel rods  14  passing therethrough. Use of internal spans  140  alone in an example embodiment spacer may reduce cross-sectional blockage by up to half, compared to conventional ferrule-type spacers, by surrounding fuel rods  14  about less of their outer perimeter while still supporting fuel rods from each direction. Similarly, continuous internal spans  140  joined to perimeter band  149  at welded connection points  142  may decrease a number of welds required by an example embodiment spacer from approximately 500 to about 50. Example embodiments with various types of alignment strips are also useable with a greatly reduced number of springs, if rod-contacting springs are limited to use on a perimeter piece and only other contacts are positioned on alignment strips. 
         [0030]    Internal spans  140  may further be compatible with other types of rod contacts from those shown in  FIGS. 4 ,  5 , or the incorporated documents, as well as flow mixers, swirl vanes, flow wings, filters, trippers or other mechanisms to provide desired fluid dynamics to fuel rods  14  passing therethrough and spaced by the same. Example embodiment fuel spacers may further include conventional features for compatibility with existing fuel designs as well as additional features to enhance fuel performance, such as bathtubs  145  in perimeter band  149 , which may be the same bathtubs as in co-owned application “SPACERS WITH DEFLECTION-LIMITED PERIPHERAL SPRINGS FOR NUCLEAR FUEL ASSEMBLIES AND METHODS OF MAKING THE SAME” Ser. No. 13/429,217 filed Mar. 23, 2012, incorporated herein in its entirety. 
         [0031]    Example embodiment fuel spacers may include internal spans at different elevations, as opposed to internal spans  114  of the example embodiment spacer  118  of  FIG. 3  at a substantially same axial position. Example embodiment fuel spacers may also include intersecting and/or non-parallel internal spans, as opposed to the substantially diagonal and parallel internal spans  114  of the example embodiment spacer  118  of  FIG. 3 . 
         [0032]      FIG. 6  is an illustration of another example embodiment spacer  218 , which may have several similar features to other example embodiments and be useable with the same, including an outer perimeter band  249 . As shown in  FIG. 6 , a first set of internal spans  240  may be waveform and parallel in a first direction and at a first elevation. A second set of internal spans  241  may be waveform and parallel in a second direction and at a second elevation. The two sets of internal spans  240  and  241  may be overlaid and positioned with appropriate pitch as shown in  FIG. 6  so as to form multiple grid “openings” where individual fuel rods can pass. Two internal spans  240  and  241  that intersect/overlap in the axial direction may be in contact, joined, or completely separate. In the example of  FIG. 6 , two internal spans  241  and  240  may use a same junction point  242  to join to spacer band  249 , so as to minimize any joining of parts and/or welds. 
         [0033]    Through an example embodiment fuel spacer  218  shown in  FIG. 6 , internal spans  240  and  241  may together nearly or fully surround each fuel rod passing through the spacer, closely matching the seating and surrounding provided by a conventional ferrule spacer, yet without the need to form and join several individual ferrules. Example embodiment fuel spacer  218  may also use a variety of other spacer features alone or in combination, including rod contacts, stops, swirl vanes, trippers, flow vanes, filters, etc. to provide a desired mechanical and fluid-dynamic response from example embodiment spacer  218 . Similarly, each internal span  240  and  241  may be chosen of a desired reactor-resilient material and formed with a desired thickness or stiffness to provide a desired mechanical and fluid-dynamic response from example embodiment spacer  218 . 
         [0034]    Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied and substituted through routine experimentation while still falling within the scope of the following claims. For example, although some example embodiments are described with rod contacts extending in opposite directions from internal spans, it is understood that example embodiment spacers may include any combination and positioning of rod contacts and internal spans. Further, it is understood that example embodiments and methods can be used in connection with any type of fuel and reactor where axial spacers are used to align fuel rods. Such variations are not to be regarded as departure from the scope of the following claims.