Patent Application
20140169516
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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A second axial zone, 114c 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 114c 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 114c, cladding 120 and/or fuel elements 122b may be configured to best accommodate these conditions. For example, cladding 120 may include a smaller thickness between outer diameter doc1 and inner diameter dic3 by removing inner liner 121 if present and/or thinning cladding in second axial zone 114c during manufacture or through later internal ablating, for example. Fuel elements 122b arranged in second axial zone 114c may have an increased width df2 in order to take advantage of the larger internal volume provided by a larger cladding inner diameter dic3 and decreased cladding thickness. For example, compared to some types of conventional light water fuel rods, dic3 may be increased by about 7 to 14 mils (thousandths of an inch) over dic1, with proportional increases to df2. Of course, other increases are useable in example embodiments.
By varying cladding and/or fuel parameters between axial zones 114a and 114c 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 114c in a lower third of example embodiment fuel rod 114, and cladding inner diameter dic3 in second axial zone 114c 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.
Example embodiment fuel rod 114 may include additional axial variations. For example, A third axial zone, 114b 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 114b 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 114b, cladding 120 may be configured to best accommodate these conditions. For example, cladding 120 may include a smaller thickness between outer diameter doc2 and inner diameter dic2 by removing inner liner 121 if present, thinning cladding in third axial zone 114b during manufacture or through later shaping, for example. Cladding outer diameter doc2 may, for example, increase with axial height, and cladding inner diameter dic2 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 dic2 and doc2 may be decreased by about 3.5 to 7 mils (thousandths of an inch) in third axial zone 114b. Of course, several other decreases are useable in example embodiments.
By varying cladding parameters between axial zones 114a and 114b 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 114b 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.
Although example embodiment fuel rod 114 has been described in three distinct axial zones 114a, 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.
Other axial zones, 214b and 214c 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 214c may be, for example, a portion of a fueled portion of example embodiment fuel rod 214 while zone 214b may be an unfueled axial portion. Based on the identification of expected conditions in zones 214c and 214b, cladding 220 may be configured to best accommodate these conditions. For example, cladding 220 may be thinned in axial zones 214b. Outer diameter doc2 may be reduced in 214c while inner diameter dic1 and fuel element width df1 are held uniform, by thinning cladding in axial zone 214c during manufacture or through later external etching, for example. Similarly, cladding outer diameter doc3 may decrease with axial height in 214b, and inner diameter dic2 may increase. For example, doc3 and/or doc2 may be decreased by about 7 to 14 mils over doc1. Of course, other decreases are useable in example embodiments.
By varying cladding sizing and fuel rod outer diameter between axial zones 214a, 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 214b and 214c 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.
Although example embodiment fuel rods 114 and 214 in
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 dic2/doc3 from zone 214b of example embodiment fuel rod 214 may be used in an unfueled lower plenum position, wider inner diameter and fuel width dic3/df2 from zone 114c 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 doc2 from zone 214c 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 dic2/doc2 from zone 114b 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.
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
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.
