Abstract:
Nuclear fuel rods have cladding or fuel with physical parameters that substantially change based on axial position within a rod. Parameters include inner and outer cladding and fuel diameters or widths, volume, mass, internal volume, thickness, rod width, etc. Parameters are selected and implemented based on calculated operating conditions and/or desired fuel response at an axial position across an entire rod length and/or fuel assembly position, including both fueled regions and non-fueled zones. Desired parameters can be achieved through fabrication or later alterations. Parameter variations versus axial position and fuel assembly position are intentional and achieve desired fuel properties and responses, such as optimized fuel mass, pressure drop, over-pressurization protection, etc. Fuel rods can be compatible with existing fuel types and replace conventional fuel rods therein.

Description:
BACKGROUND 
       [0001]      FIG. 1  is a sectional illustration of a conventional nuclear reactor fuel assembly  10  typically used in commercial light water nuclear 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-like 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 an interior of a related art fuel rod  14 . As shown in  FIG. 2 , fuel rod  14  includes one or more fuel elements  22 , which are pellets or other similar shapes stacked in an axial direction, within an internal volume or housing formed by cladding  22  of fuel rod  14 . Fuel elements  22  include fissile nuclear fuel and generate fission products, which are generally contained by cladding  20  surrounding and providing impermeable containment to pellets  22  and fission products generated therefrom. Fuel rod  14  may include a fueled portion  14   a,  in which fuel pellets extend, and a non-fueled zone  14   b,  where open space exists to allow fission product, which can be gasses produced through nuclear fission, accumulation and prevent over-pressurization of fuel rod  14 . A hold-down spring  23  in plenum area  14   b  may compress and generally preserve a position of fuel pellets  22  within fuel rod  14 . A thin inner liner  21  (shown in dash) may extend about an interior perimeter of fuel rod  14  to reduce effects of pellet-cladding interaction. Cladding  20  may be formed of a harder and/or stronger zirconium or other alloy, inner liner  21  may be formed of a softer material and extend inward from an inner surface of cladding  20 . 
         [0004]    As shown in  FIG. 2 , related art fuel rod  14  has a uniform and constant inner diameter, dic, through its entire axial length. Similarly, fuel pellets  22  each have a uniform width, df, throughout fuel rod  14  that is smaller than dic, so as to prevent rigid contact between pellets  22  and cladding  20 . Fuel rod  14  also has a constant outer diameter doc, such that cladding  20  has a uniform thickness throughout fuel rod  14 . Such uniformity may aid modular construction of fuel elements, rods, and assemblies, with different rods and parts thereof being useable in a variety of positions within fuel assemblies. 
       SUMMARY 
       [0005]    Example embodiments include nuclear fuel rods and assemblies containing the same with intentional variations in fuel and/or cladding. For example, fuel elements or cladding that houses a fuel element may be sized, in volume, radii, and/or thickness, based on their axial position in a fuel rod. Inner and/or outer diameters or widths of cladding may have intentional variation along an axial position of an example embodiment fuel rod, from as little as a couple to several hundred mils, even well over doubling conventional or existing sizes. Fuel sizes may also be expanded or reduced proportionally with cladding inner diameter changes at their axial position, such that two fuel elements at different axial positions may have a same axial length yet different volumes and fuel masses. Changes to cladding and/or fuel are made based on conditions at a particular axial position, which can include both fueled regions and non-fueled regions that contain accumulated fission gases. Changes in cladding thickness, fuel rod width, cladding inner/outer diameter proportions, internal volume defined by the cladding, cladding internal liner presence, fuel shape or size, etc. may be selected and implemented in any desired combination and with any other fuel changes during manufacture or through post-manufacturing modifications such as sintering, ablation, etching, reaming, polishing, etc. Variations may be used to achieve desired fuel properties and responses, such as through variations in fuel inventories, pressure drop, over-pressurization protection, etc. Example embodiment fuel rods may otherwise be compatible with existing fuel types and may be axially configured based on their radial position, including current or intended location within a fuel assembly and/or reactor core or current or intended location of a containing fuel assembly in a reactor core. For example, they may be configured to seat into and extend between upper and lower tie plates in a fuel assembly with spacers and a channel. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0006]    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. 
           [0007]      FIG. 1  is an illustration of a section of a related art nuclear fuel assembly. 
           [0008]      FIG. 2  is an illustration of an interior of a related art fuel rod. 
           [0009]      FIG. 3  is an illustration of an example embodiment fuel rod 
           [0010]      FIG. 4  is an illustration of an example embodiment fuel rod. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    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. 
         [0012]    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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    Applicants have recognized that nuclear fuel rods are exposed to neutronic and thermo-hydraulic conditions that can vary greatly with axial position in an operating nuclear reactor. Uniform fuel rod characteristics may not take advantage of, and/or may reduce fuel performance at, certain axial conditions that deviate from average or overall conditions across the entire rod length. Cladding thickness, fuel element shape, and/or fuel rod shape may not require uniformity, and can be individually adjusted to optimize fuel performance based on anticipated conditions at different axial positions. Applicants have recognized that any of fuel, cladding, and rod characteristics can be varied at fine axial lengths, based on radial positioning within the assembly, core, or other parameters to improve fuel rod performance, including safety margins, fuel mass and lifetime expectancy, and/or energy production efficiency, for example. Example embodiments described below address these and other problems recognized by Applicants with unique solutions enabled by example embodiments. 
         [0017]    The present invention is a fuel rod that is useable to generate nuclear power with nuclear fuel contained in a cladding and/or fuel assemblies using such fuel rods. The present invention includes fuel rods with cladding that is intentionally varied at different axial positions and/or fuel elements that are intentionally varied at different axial positions. As used herein, “intentionally varied” is defined to exclude defects that inevitably occur as part of a manufacturing process or though damage as well as incidental changes that occur through operation, and terminations required to form an internal volume. In this way “intentionally varied” includes variations made during manufacture or as alterations thereafter of such a purposeful and substantial character to intentionally achieve different fuel rod responses. As used herein, “axial” is defined as the longest dimension of a whole fuel rod or assembly, often a vertical direction in operation. 
         [0018]      FIG. 3  is an illustration of a cross-section of an example embodiment fuel rod  114 . As shown in  FIG. 3 , fuel rod  114  may include several similar features to conventional fuel rods and be useable in several different types of fuel assemblies in place of conventional fuel rods. Example embodiment fuel rod  114  includes a cladding  120  that houses and contains fuel elements  122 , which may be, for example, cylindrical pellets, powders, prismatic solids, etc., providing fissile material for nuclear power generation. Fuel rod  114  may further include a hold down spring  123  or other stabilization device at any other position to secure fuel elements  122  within cladding  120 . 
         [0019]    As shown in  FIG. 3 , example embodiment fuel rod  114  has varying axial configurations. For example, a first zone  114   a  may be identified as an axial zone that will be exposed to operating conditions, such as changing moderator phase or varying control element exposure, within a nuclear reactor that are more likely to cause adverse interactions between fuel elements  122  and cladding  120  or more likely to be subject to damage or wear such that through spacer-induced fretting or puncture. First zone  114   a  may be, for example, an upper axial two-thirds of a fueled portion of example embodiment fuel rod  114 . Based on the identification of expected conditions in first zone  114   a,  cladding  120  and/or fuel elements  122   a  may be configured to best accommodate these conditions. For example, cladding  120  may include a maximum or conventional thickness between outer diameter doc 1  and inner diameter dic 1  to reduce effects of fuel pellet/cladding interactions and failure. Fuel elements  122   a  arranged in first axial zone  114   a  may have a minimum or conventional width df 1  in order to accommodate cladding inner diameter dic 1  and increased cladding thickness. 
         [0020]    A second axial zone,  114   c  may be identified as an axial zone that will be exposed to different operating conditions based on its position, such as one with less risk of cladding damage, fuel-cladding interaction, and/or benefiting from higher fuel inventories, for example. Second zone  114   c  may be, for example, a lower axial third of a fueled portion of example embodiment fuel rod  114 . Based on the identification of expected conditions in second zone  114   c,  cladding  120  and/or fuel elements  122   b  may be configured to best accommodate these conditions. For example, cladding  120  may include a smaller thickness between outer diameter doc 1  and inner diameter dic 3  by removing inner liner  121  if present and/or thinning cladding in second axial zone  114   c  during manufacture or through later internal ablating, for example. Fuel elements  122   b  arranged in second axial zone  114   c  may have an increased width df 2  in order to take advantage of the larger internal volume provided by a larger cladding inner diameter dic 3  and decreased cladding thickness. For example, compared to some types of conventional light water fuel rods, dic 3  may be increased by about 7 to 14 mils (thousandths of an inch) over dic 1 , with proportional increases to df 2 . Of course, other increases are useable in example embodiments. 
         [0021]    By varying cladding and/or fuel parameters between axial zones  114   a  and  114   c  based on anticipated operating conditions at their respective positions, both axial and radial, safety margins and/or operating limits may be preserved, while fuel volume, neutronic response, and thermodynamic parameters may be optimized. For example, if second axial zone  114   c  in a lower third of example embodiment fuel rod  114 , and cladding inner diameter dic 3  in second axial zone  114   c  is increased with proportional fuel volume increase, Applicants have calculated that more kilograms of fissile uranium can be included in a typical BWR fuel assembly using example embodiment fuel rods  114 , while preserving other safety and operating limits, over a rod using a single configuration over all axial positions. 
         [0022]    Example embodiment fuel rod  114  may include additional axial variations. For example, A third axial zone,  114   b  may be identified as an axial zone that will be exposed to different operating conditions based on its position, such as one with less risk of cladding damage, fuel-cladding interaction, and/or benefiting from increased volume, for example. Third zone  114   b  may be, for example, an unfueled portion of example embodiment fuel rod  114  where fission products, such as gasses, accumulate. Based on the identification of expected conditions in third zone  114   b,  cladding  120  may be configured to best accommodate these conditions. For example, cladding  120  may include a smaller thickness between outer diameter doc 2  and inner diameter dic 2  by removing inner liner  121  if present, thinning cladding in third axial zone  114   b  during manufacture or through later shaping, for example. Cladding outer diameter doc 2  may, for example, increase with axial height, and cladding inner diameter dic 2  may increase at an even greater rate with axial height, resulting in a thinning cladding  120  with axial height. For example, compared to some types of conventional light water fuel rods, a thickness of cladding  120  between dic 2  and doc 2  may be decreased by about 3.5 to 7 mils (thousandths of an inch) in third axial zone  114   b.  Of course, several other decreases are useable in example embodiments. 
         [0023]    By varying cladding parameters between axial zones  114   a  and  114   b  based on anticipated operating conditions at their respective positions, both axial and radial, safety margins and/or operating limits may be optimized. For example, if third axial zone  114   b  in an unfueled upper plenum position of example embodiment fuel rod  114  includes thinned cladding, plenum volume will be increased, which will allow for increased accommodation of fission gas and/or reduce rod internal pressures. Applicants have calculated that this permits an increase in thermal-mechanical operating limits and energy production efficiency, while preserving safety and operating limits, over a rod using a single configuration at all axial positions. 
         [0024]    Although example embodiment fuel rod  114  has been described in three distinct axial zones  114   a, b , and  c  with different cladding and/or fuel configurations in each based on anticipated operating conditions in those zones, it is understood that any number of different zones and cladding and/or fuel variances are useable in example embodiment fuel rod  114 . Example embodiment fuel rods may include different unfueled areas and positions, different fuel enrichments, and/or different cladding thermo-mechanical and/or neutronic properties at different axial positions, for example. Such changes may be made or accounted for based on anticipated axial reactor conditions throughout the lifecycle of a fuel assembly containing example embodiment fuel rods. 
         [0025]      FIG. 4  is an illustration of a different example embodiment fuel rod  214  useable in nuclear reactors; fuel rod  214  may include several conventional features like a hold-down spring  223 , cladding  220 , inner liner  221  if present, and/or fuel elements  222 . Example embodiment fuel rod  214  may include a first zone  214   a  that will be exposed to operating conditions where fuel inventories should not be increased and/or where cladding  220  should have a maximum or conventional thickness for safety or operational concerns. First zone  214   a  may be, for example, an upper axial two-thirds of a fueled portion of example embodiment fuel rod  214 . Based on the identification of expected conditions in first zone  214   a,  cladding  220  and/or fuel elements  222  may be configured to best accommodate these conditions. For example, cladding  220  may include a maximum or conventional thickness between outer diameter doc 1  and inner diameter dic 1  to reduce effects of fuel pellet/cladding interactions and failure. Fuel elements  222  throughout example embodiment fuel rod  214  may have a standardized width df 1 . 
         [0026]    Other axial zones,  214   b  and  214   c  may be identified as zones that will be exposed to different operating conditions due to their position, such as one with less risk of cladding damage, fuel-cladding interaction, and/or benefiting larger moderator volumes and/or decreased pressure drop, for example. Zone  214   c  may be, for example, a portion of a fueled portion of example embodiment fuel rod  214  while zone  214   b  may be an unfueled axial portion. Based on the identification of expected conditions in zones  214   c  and  214   b,  cladding  220  may be configured to best accommodate these conditions. For example, cladding  220  may be thinned in axial zones  214   b.  Outer diameter doc 2  may be reduced in  214   c  while inner diameter dic 1  and fuel element width df 1  are held uniform, by thinning cladding in axial zone  214   c  during manufacture or through later external etching, for example. Similarly, cladding outer diameter doc 3  may decrease with axial height in  214   b,  and inner diameter dic 2  may increase. For example, doc 3  and/or doc 2  may be decreased by about 7 to 14 mils over doc 1 . Of course, other decreases are useable in example embodiments. 
         [0027]    By varying cladding sizing and fuel rod outer diameter between axial zones  214   a, b , and  c  based on anticipated operating conditions at their respective positions, both axial and radial, safety margins and/or operating limits may be optimized. For example, axial zones  214   b  and  214   c  may provide a lower pressure drop to a fluid coolant/moderator flowing axially along fuel rod  214  and/or provide for better moderation, providing for improved hydrodynamic performance and plant efficiency. 
         [0028]    Although example embodiment fuel rods  114  and  214  in  FIGS. 3 and 4  have been described with particular combinations of axial properties, it is understood that any single feature may be present in example embodiments, and other combinations can be present in an example embodiment fuel rod in any number of axial zones. For example, an engineer wishing to use example embodiment fuel rods having a constant outer diameter that matches the outer cladding diameter of conventional fuel rods, so that example embodiment fuel rods can replace conventional fuel rods, may implement only the variation from zone  114   c  of  FIG. 3  for use at desired axial positions. In this way, outer diameter doc 1  may remain constant along an entire example embodiment fuel rod and mimic conventional fuel rods&#39; geometry while providing optimization advantages through increased inner diameter and fuel mass. Or, for example, a fuel fabricator wishing to use fuel elements of a single size for manufacturing compatibility and modularity may use example embodiment fuel rod  214  from  FIG. 4  with a single, uniform configuration for fuel elements  222  while providing hydrodynamic and other advantages through outer diameter decreases at particular axial positions. 
         [0029]    Still further, a nuclear fuel provider may apply any or all modifications across several different axial zones and among various fuel assembly positions to achieve desired fuel rod response. For example, a narrowed outer and inner diameter dic 2 /doc 3  from zone  214   b  of example embodiment fuel rod  214  may be used in an unfueled lower plenum position, wider inner diameter and fuel width dic 3 /df 2  from zone  114   c  of example embodiment fuel rod  114  may be used at several axial positions where larger fuel inventories are desired based on fuel assembly or reactor core parameters, narrower outer diameter doc 2  from zone  214   c  of example embodiment fuel rod  214  may be used at a higher zone where lower pressure drop and more moderator volume is desired, and widened outer and inner diameter dic 2 /doc 2  from zone  114   b  of example embodiment fuel rod  114  may be used at a terminal unfueled plenum region to provide larger fission product accommodation. Yet further, the engineer can mix features within a same zone; for example, both an cladding inner diameter may be increased and a cladding outer diameter decreased in cladding for a particular zone, combining a lower pressure drop and larger fuel volume in that zone. 
         [0030]    Example embodiment fuel rods are useable in a variety of reactor and fuel assembly types. Example embodiment fuel rods can be configured to be used in the assemblies  10  of  FIG. 1  and replace conventional rods  14  within fuel assemblies. Individual example embodiment fuel rods include axial variations based on anticipated reactor operating conditions and a favorable response thereto. Thus, several example embodiment fuel rods within an assembly may have axial features configured based on anticipated bundle placement as well as the effects of each other on operating conditions. In this way example embodiment fuel rods permit a core designer to more finely adjust core response and operating characteristics and potentially achieve improved burnup, fuel lifetime, operating and safety margins, and/or improved plant efficiency. 
         [0031]    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 unfueled areas only in a top axial position and modular fuel structures, it is understood that example embodiment fuel rods may include any combination of unfueled and fueled zones, as well as different types, shapes, and enrichments for fuel elements. Further, it is understood that example embodiments and methods can be used in connection with any type of fuel and reactor where fuel rods are used, including BWR, PWR, heavy-water, fast-spectrum, graphite-moderated, etc. reactors. All cladding and fuel size values given above are exemplary and do not in any way limit the independent claims. Such variations are not to be regarded as departure from the scope of the following claims.