Common plenum fuel assembly design supporting compact vessel, long-life cores, and eased refueling in pool-type reactors

Abstract

SOLUTION: A fuel assembly for use in a nuclear reactor comprising a fuel bundle, a plenum header connection positioned on the fuel bundle, a mast extending from the fuel bundle, and a common fission gas plenum extending from the mast is disclosed. The reactor includes a vessel and coolant situated within the vessel. The fuel bundle comprises a plurality of fuel elements including nuclear fuel material positioned therein. The plenum header connection comprises a plurality of passageways defined therein that are in fluid communication with the nuclear fuel material. The elongate mast comprises an internal passage connecting the common fission gas plenum to the plurality of passageways of the plenum header connection such that the common fission gas plenum is configured to receive an amount of fission gas generated by the nuclear fuel material during operation. The common fission gas plenum is positioned in an otherwise unused portion of the vessel.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 62 / 840,775, entitled "Common Plenum Fuel assembly design supporting a compact vessel, Long-life cores, and Eased refueling in Pool-type reactors," filed April 30, 2019, the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] The challenge of refueling a liquid metal cooled (or in the future salt cooled) reactor is significantly higher than for a light water reactor. This suggests that benefits may be gained from longer intervals between refueling. Certain fast reactors can utilize unique fuel cycles, providing very high energy cores, significant breeding, and allowing for feedstocks of U:Pu, Pu+U:Pu, U:Th, and U+MinorActinides:Pu, where X:Y represents seed element:blanket bleed material. However, one of the main limitations to utilizing a high breeding ratio / high energy core is the relatively small volume of fission gas plenum per linear unit of plenum length. Current technology responds to this demand by implementing very long fuel rod plenums or cladding, potentially longer than the effective fuel length, to contain worst case fission gas in the most restrictive pin. Integrated fission gas release, and the associated high internal fuel rod pressures, occur at high fuel exposures when the cladding experiences embrittlement and swelling caused by neutron damage measured as displacements per atom (dpa). The combination of cladding swelling / embrittlement due to high internal pressures and high cladding stresses often establishes the maximum allowable fuel exposure (i.e., core residence time), thereby dictating the levelized fuel cycle costs and complicating the ability to make a positive business case for utilizing the benefits of fast reactors. Summary of the Invention

[0003] At least one embodiment of the present disclosure aims to maximize fuel exposure, thereby simplifying refueling by multiple means, thus improving simplification and reducing costs from the plant. These improvements and their implementation are described in more detail below.

[0004] The common fission gas plenum or tank is connected to and located above the collection header or top fitting located above the fuel bundles. The location of the common fission gas plenum above the top fitting is above the flow field of the core and reactor, which allows for a substantially larger plenum volume per length of plenum than is possible in the flow field of the reactor or in other shared plenum designs. This allows for a larger size common fission gas plenum without penalty because the common fission gas plenum is located in previously unused reactor vessel space, and most of the bundle flow area is available for plenum volume (compared to conventional fuel rod plenums). Additionally, the amount of structural material for the common fission gas plenum is much less than is required for current technology for individual fuel rod plenums.

[0005] The top fitting or collection header includes channels or passages that collect the fission gases released from the fuel bundles and direct them to a reduced diameter mast located above the top fitting. The fission gases travel through passages in the reduced diameter mast into a common fission gas plenum located above the reduced diameter mast. The passages between the top fitting and the common fission gas plenum to which the fission gases travel after leaving the fuel bundles can be considered to define a fission gas collection volume. The common plenum equalizes the individual fuel fission gas releases so that the limiting conditions of the fuel assembly do not have to be defined by peak pins operating at maximum power with the most restrictive set of manufacturing and design uncertainties. Rather, they are defined by the average value of the fuel bundles, which has a much lower uncertainty in the fuel rod internal pressure and the ability of the fuel rods to store volatile fission products. (Note: Subsequent references to fission products or fission gases refer only to volatile fission products.

[0006] Each fuel rod is connected to a common fission gas plenum through connections in the top fittings, and each fuel rod may have a one-way valve or fluid diode to prevent backflow from the plenum in the event of a fuel rod leak.

[0007] The common fission gas plenum eliminates the need for plenum space within the fuel rods, thus minimizing fuel rod lengths, potentially by up to six times or more, which allows for shorter bundle lengths with longer fuel stacks, allowing for higher fuel loadings and resulting longer fuel cycles compared to designs that employ traditional fission gas plenums within the fuel rods.

[0008] In addition to the above, Figure 1A illustrates a graphical representation showing the ratio of common fission gas plenum volume to plenum volume with fuel rods at different pitch to diameter ratios according to at least one embodiment of the present disclosure. The comparison is drawn for the Westinghouse LFR and historical (operating) liquid metal fast reactors.

[0009] Further to the above, the fuel mast and common fission gas plenum penetrate the surface of the coolant, thus allowing for direct handling of the fuel, and further providing easy retention through a downward vertical retention force (see arrows DF in FIG. 6 ) exerted on the structure of the fuel assembly, most notably retaining the majority of the fuel assembly, including the active fuel portion, which is located within the coolant and resists buoyancy, frictional forces and coolant drag on the fuel assembly. In at least one embodiment, the fuel mast and common fission gas plenum may approach the surface of the coolant (e.g., without penetrating the surface). In such an arrangement, easier handling of the fuel may be achieved without the fuel mast and / or common fission gas plenum penetrating the surface of the coolant.

[0010] At least one embodiment of the present disclosure provides a reduction in fuel power density for a given reactor vessel size and thermal power rating. In addition, at least one embodiment of the present disclosure reduces the load on the refueling period (e.g., to lengthen the fuel cycle and reduce the overall capacity factor impact from the fuel cycle period). Reducing the load on the refueling period may facilitate dry lift refueling. Dry lift refueling is typically constrained by the need to ensure the integrity of the fuel rods during transport from the primary coolant pool to the spent fuel storage area or cask. The reduced fuel power density and much larger available fission gas storage volume allow for a significant increase in fuel burnup and breeding rate by reducing the rate of fission gas pressure with increasing fuel integrated fission before structural limitations of fuel rod pressure occur. The reduced fuel power density and much larger available fission gas storage volume may result in longer fuel cycles.

[0011] At least one embodiment of the present disclosure contemplates lowering stresses on fuel rod cladding to facilitate implementation of very long fuel cycles, thereby reducing the need for infrastructure for spent fuel handling within the plant and thus reducing concerns over the potential diversion of spent fuel during refueling operations. These benefits are particularly applicable to reactors in countries that lack mature fuel cycle infrastructure / safeguards.

[0012] At least one embodiment of the present disclosure contemplates lowering neutron parasitic absorption of reactor fission products by enabling relocation of gaseous and non-volatile absorbing fission products away from critical high neutron flux reactor regions (e.g., relocation to a common fission gas plenum).

[0013] Furthermore, the option for in-vessel spent fuel storage, which would have more severely exacerbated the reactor vessel height if implemented with the conventional longer fuel rod design, would include a point where the mast where the "cut and pinch" methodology is implemented would involve a smaller diameter gas transport passage. This is to separate the common fission gas plenum from the fuel assembly, thereby sealing it within both ends of the fission gas transport tube within the smaller diameter mast. This would allow each elongated fuel assembly to be converted into a pair of relatively short elongated structures that would be relatively easier to stow than a single long structure. Also, in such a configuration, the two parts of the elongated fuel assembly could be stored or processed in a manner appropriate to their requirements for forced storage in one container or cask, designed for spent fuel but perhaps overdesigned for the common fission gas plenum. Common plenums can also have better disposal means beyond just storage. This technology is used in the oil and gas industry (e.g. blowout preventers), among other things. [Brief description of the drawings]

[0014] The various features of the embodiments described herein, together with their advantages, may be understood in accordance with the following description taken in conjunction with the accompanying drawings, in which:

[0015] [Figure 1A] FIG. 1 is a graph illustrating the ratio of common fission gas plenum volume to fuel rod plenum volume at different pitch to diameter ratios in accordance with at least one embodiment of the present disclosure.

[0016] [Figure 1] FIG. 2 is a partial cutaway view of a top end fitting disposed on a fuel bundle in accordance with at least one embodiment of the present disclosure.

[0017] [Diagram 2] FIG. 2 is a plan view of the upper end fitting and fuel bundle of FIG. 1.

[0018] [Diagram 3] FIG. 2 is a perspective view of the upper end fitting and fuel bundle of FIG. 1.

[0019] [Figure 4] FIG. 2 is a plan view of the upper end fixture of FIG. 1.

[0020] [Diagram 5] FIG. 2 is a perspective cross-sectional view of fuel assemblies, each fuel assembly including the top fitting and fuel bundle of FIG. 1, in accordance with at least one embodiment of the present disclosure.

[0021] [Figure 6] FIG. 6 is a partial perspective view of a nuclear reactor having a vessel, a coolant within the vessel, and a representative quantity of fuel assemblies from FIG. 5 within the coolant, further illustrating a downward vertical retention force applied to one exemplary fuel assembly.

[0022] [Figure 7] FIG. 6 is a cross-sectional perspective view of one of the fuel assemblies of FIG. 5 showing grooves in the top fitting that are in fluid communication with the fuel elements of the fuel bundle.

[0023] [Figure 8] FIG. 6 is another perspective cross-sectional view of the fuel assembly of FIG. 5 , showing a groove in the top fitting that is in fluid communication with a central passage in a mast that extends above the fuel assembly.

[0024] [Figure 9] FIG. 6 is another cross-sectional view of the fuel assembly of FIG. 5 showing a common fission gas plenum or tank extending from the mast section.

[0025] [Figure 10] FIG. 6 is another cross-sectional view of the fuel assembly of FIG. 5 showing the fuel bundle and the bottom extension configured to receive coolant. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] 1 and 3 show a plenum header connection, or top fitting 100, located on top of an active fuel bundle 200 that includes a flow area for coolant and internal passages for fission gases. The active fuel bundle 200 and top fitting 100 are part of a fuel assembly 500 (see FIG. 5). It should be noted that a nuclear reactor includes multiple fuel assemblies 500, and that a nuclear reactor is not limited to the number of fuel assemblies 500 shown in FIG. 5, but may utilize any suitable number of fuel assemblies 500 without departing from the intended scope of the present disclosure. Specifically, while six fuel assemblies 500 are depicted in FIG. 5, these are merely a subset of the fuel assemblies that will be received within a given nuclear reactor during operation.

[0027] 5, the fuel assembly 500 includes an exterior wrapper or elongated duct 510 that functions to direct coolant from the cold end of the reactor through the fuel elements or fuel rods 210 where nuclear heat is transferred. The coolant exits the duct 510 below the tank, i.e., common fission gas plenum 400. The fuel assembly 500 further includes a lower end fitting 520 extending from the bottom of the duct 510. The duct 510 is configured to receive a fuel bundle 200 comprising a plurality of fuel elements or fuel rods 210. Each of the fuel assemblies 500 includes a first portion and a second portion in fluid communication with each other. The first portion includes a lower end fitting 520, the fuel bundle 200, and an upper end fitting 100. The second portion includes a common fission gas plenum 400. The first and second portions of the fuel assembly 500 are held in fluid communication via a mast 300 (i.e., a third portion of the fuel assembly 500) located intermediate the first and second portions.

[0028] Referring primarily to Figures 1-4, the fuel bundle 200 includes a plurality of fuel rods 210 as described above. A tapered or necked fuel rod portion 220 of each fuel rod 210 is received within an opening or plenum flow connection 110 in the top end fitting 100 . In at least one embodiment, the necked down fuel rod section 220 may include, for example, a one-way valve or a fluidic diode. In any event, the fuel rods 210 are in one-way flow communication with the plenum flow connections 110 in the top end fitting 100 . The cladding of each fuel rod 210 is seal welded or otherwise secured to the plenum flow connection 110 of the top end fitting 100 .

[0029] During use, fission gases released from the fuel rods 210 escape from the plenum flow connections 110 into channels, capillaries, or flow paths 130 defined in the top fitting 100. The flow paths 130 are defined in the top fitting 100 to interconnect (i.e., fluidly connect) each of the plenum flow connections 110 disposed above the fuel rods 210. The flow paths 130 are defined in the top fitting 100 in parallel rows intermediate the plenum flow connections 110 for the fuel rods 210. The flow paths 130 are interconnected with peripheral flow paths, i.e., peripheral flow paths 135, defined in the top fitting 100 around the periphery of the plenum flow connections 110. The peripheral flow paths 135 form a hexagonal shape as shown in FIG. 2. It should be understood that different arrays and / or patterns of flow path paths positioned intermediate and around the plenum flow connections 110 are contemplated. For example, the flow paths 130 may include a crisscross pattern. In either case, the fission gas travels through the flow paths 130, 135 and out of the top fitting 100 through a plenum header connection 140 (see FIG. 7) in the center of the top fitting 100. In other words, the top fitting 100 connects each of the fuel rods 210 to a common central connection point (i.e., the plenum header connection 140). The fission gas flows from the plenum header connection 140 through a mast 300 located above the top fitting 100 to a tank or a common fission gas plenum 400, as shown in FIGS. 5 and 9. Other embodiments contemplate that the top fitting 100 links each of the fuel rods 210 to a common collection area in the top fitting 100 that is not in the center of the fuel bundle 200.

[0030] 5, the mast 300 includes an outer diameter smaller than the outer diameter encompassing the outermost surface of the fuel bundles 200 and a majority surface defined within the outer diameter encompassing the outermost surface of the common plenum 400. In other words, the mast 300 is smaller in width (e.g., transverse to a longitudinal axis LA defined by the mast 300) compared to the width of the common fission gas plenum 400 and the fuel bundles 200. In this respect, the mast 300 may be considered, for example, a reduced diameter mast. Additionally, the mast 300 includes a pipeline, or passageway 320, defined therein that connects the plenum header connection 140 of the top fitting 100 to the common plenum 400. A longitudinal axis LA extends along the passageway 320 of the mast 300 and defines a central axis of the mast 300. In the illustrated embodiment, only one passageway 320 is shown, but other embodiments having two or more passageways are contemplated. The passages 130, 135 of the top fitting 100, which are in fluid communication with the nuclear fuel material in the fuel rods 210, are also in fluid communication with a common plenum 400 via a passageway 320 in the mast 300. In at least one embodiment, the common plenum 400 is located in an area of ​​the reactor with reduced and / or stagnant coolant flow and therefore may occupy a larger volume compared to a conventional nuclear reactor, as illustrated in FIG.

[0031] Additionally, the mast 300 permits radial (i.e., transverse to the longitudinal axis LA) reactor coolant flow to the heat exchange equipment and reactor coolant pumps of the reactor. Specifically, the mast 300 includes a flow region above the fuel (i.e., within the nozzle 310) through which the coolant is free to move out of the fuel bundles 200 and into other portions of the reactor, such as the primary heat exchangers and / or reactor coolant pumps. More specifically, the mast 300 includes a nozzle 310 at its bottom end (i.e., the end of the mast 300 closest to the fuel bundles 200). The nozzle 310 is located intermediate the mast 300 and the fuel bundles 200, as shown in FIG. 7.

[0032] Referring primarily to FIG. 7 , the exterior of the nozzle 310 is cone-shaped, tapering from the exterior surface of the fuel bundle 200 toward the mast 300. The nozzle 310 includes an internal cavity 305 that tapers from a wider portion closest to the fuel bundle 200 to a narrower portion closest to the mast 300. The internal cavity 305 provides an area through which the coolant may flow after it exits the fuel bundle 200 and the top fitting 100, for example. The internal cavity 305 is disposed within the nozzle 310 such that a central conical portion 340 is defined by the internal cavity 305. The central conical portion 340 extends upwardly from the top fitting 100 and includes a portion of the passageway 320 defined therein. The nozzle 310 includes openings, or coolant flow passages 330, that are radially spaced about the circumference of the nozzle 310 of the mast 300. In the illustrated embodiment, six coolant channels 330 are defined in the nozzle 310, although any suitable number of coolant channels 330 may be utilized. The coolant channels 330 allow the coolant to escape the nozzle 310 after passing through openings in the top end fixture 100, i.e., the coolant channels 120, as described in more detail below.

[0033] The top fitting 100 may be manufactured in two pieces, with the fission gas channels 130, 135 formed in one of the pieces, using means such as machining, milling, etching, and / or any other suitable machining technique. The second piece is then secured using diffusion bonding or any suitable method to form an integral, seamless top fitting or plenum header connection, such as the top fitting 100. Other means for manufacturing the top fitting 100 include, but are not limited to, additive manufacturing or investment casting. In at least one embodiment, the top fitting 100 is created by combining together radial sections of equal size. The radial sections may be combined by welding, bonding, or any suitable method.

[0034] 3 and 4, the top end fixture 100 includes coolant channels 120 that allow coolant to flow therethrough from the area surrounding the fuel bundles 200 (i.e., below the top end fixture 100) to the area above the top end fixture 100. In the illustrated embodiment, the coolant channels 120 are sized and shaped to occupy the spaces between the fuel rods 210. However, other embodiments are contemplated having different patterns, arrays, shapes, and sizes of coolant channels within the top end fixture 100. In at least one embodiment, the coolant channels 120 are large enough to eliminate clogging concerns or significantly increase pressure loss relative to conventional fuel assembly top nozzles. Design studies to date have shown >80% of the flow area for the fuel rod channels; a value that is competitive with many current fuel nozzles or mixing grids.

[0035] As mentioned above, defined within the reduced diameter portion of the mast 300 are one or more fission gas pipelines or passages 320 that connect the interior regions of the fuel rods 210 to the common plenum 400. The passages 130, 135 in the top fitting 100, the plenum header connections 140 in the top fitting 100, the passages 320 in the mast 300, and the common plenum 400 form a gas collection volume sized to achieve a significant reduction in plenum pressure through achieving a 200-300% volumetric increase over the combined plenum area in traditional fuel assemblies. The reduced plenum pressure alleviates the challenge of high exposure of fuel rod cladding and also alleviates pressurization during fuel heating transients and fuel movement (e.g., dry lift, etc.). The passages, passages, and connections to the common plenum 400 may also have one or more one-way valves to prevent backflow into the transport pipe (e.g., passage 320) if damaged. The one-way valve may be located, for example, at any point along the fission gas collection volume and / or as part of the fuel rod 210.

[0036] In conventional fuel cycles, power density, higher rod pressures, and the need for short outages make dry-lift refueling (e.g., refueling where spent fuel assemblies are lifted in the absence of their typical coolant and cooled only by air or some other gas) more difficult. For example, conventional fuel cycles require long in-vessel storage times for the spent fuel and / or lifting the fuel along with a volume of coolant to enhance cooling, complicating reactor design and fuel handling overall. Specifically, the fuel must be moved to a peripheral location within the reactor vessel and then moved into a lifting container filled with coolant. From here, it may be lifted and transported to a temporary holding location or to a highly attenuated heat-dissipating fuel cask. Movement to this peripheral location requires either short assemblies that can be lifted together within the reactor's coolant pool (thus requiring expensive ballast or latching due to the unique challenges of hold-down in lead or other fast reactors), or full-height assemblies that must be shuffled within the vessel to make room for moving the fuel assemblies to be evacuated and to return the partially burned assemblies to their home position. This shuffling requires multiple in-vessel storage locations, driving a significant increase in vessel size and complexity of the internals. The many movements required in this shuffling also increase the time and opportunity for a fuel handling accident.

[0037] At least one aspect of the present disclosure promises to reduce power density, internal rod pressure, and downtime pressure so that a direct, dry lift from the fuel location inside the core can go into a shielded refueling mast (which then goes into a dry cask (e.g., multi-month cooldowns may be acceptable if one or two refuelings are performed during the plant's life). This not only simplifies the refueling equipment, but also allows the vessel to be reduced and other refueling infrastructure in the plant to be eliminated.

[0038] Referring primarily to FIG. 9, an enlarged cutaway portion of multiple fuel assemblies 500 from FIG. 5 arranged side by side is shown. The top fitting 100 and fuel bundles 200 are located in a first portion of the fuel assemblies 500 in a duct 510 of each fuel assembly 500. The fuel assemblies 500 use a common or shared fission gas plenum 400 and relocate the gas plenum from the fuel rods 210 to a common plenum 400 located above the mast 300 in a second portion of the fuel assemblies 500, which is otherwise unused volume of the vessel above the active fuel and core exit flow region. Submergence of the fuel well below the coolant surface is necessary to position the primary heat exchanger in a manner compatible with natural convection. The addition of the common plenum 400 has little to no effect on the overall fuel length while allowing the active core region to be much taller. This allows for an increase in effective fuel mass of over 200% compared to conventional fuel designs. Additionally, the common plenum 400 away from the active core region reduces the negative impact of non-volatile neutron absorbing fission products such as xenon, samarium, and gadolinium on core reactivity.

[0039] As with some other fast reactor designs, the fuel assembly structure in this concept may penetrate the coolant surface, thus making handling much easier. In at least one embodiment, the location of the common fission gas plenum 400 takes advantage of the height required to exit the coolant surface, thus advantageously taking advantage of this extra length and assembly to facilitate identification and capture during refueling, as well as retention functions.

[0040] FIG. 10 shows an enlarged cutaway portion of the bottom end of the fuel assembly 500 of FIG. 5. The fuel bundle 200 comprising the fuel rods 210 is disposed in a duct 510 (i.e., a first portion of the fuel assembly 500) of the fuel assembly 500. The lower end fitting 520 of the fuel assembly 500 includes a bottom extension 530 including a number of inlet holes 540 for the entry of coolant. The coolant flows into the inlet holes 540, into the lower end fitting 520, around the fuel rods 210 in the fuel bundle 200, through the coolant passages 120 in the upper end fitting 100, and out of the coolant passages 330 in, for example, the nozzle 310 of the mast 300.

[0041] For heavy liquid metal coolants such as lead or lead-bismuth, a long fuel assembly such as fuel assembly 500 that rises above the liquid metal allows for easier retention of the fuel without complicated internal structures, latches, or expensive ballast (note, with these coolants, if the coolant is denser than the fuel, the fuel will tend to be positively buoyant and float). Referring primarily to FIG. 6, arrow DF indicates a downward vertical retention force applied to a fuel assembly such as fuel assembly 500. Due to their location above the core and core exit flow, the fission gases are primarily in a relatively low temperature region (i.e., relative to the in-rod temperature), thus reducing the pressure of the fission gas plenum common for a given released fission gas molar content and fuel exposure.

[0042] The typical fission gas plenum 400 has a larger volume than would typically be practical for a fuel rod or other concept because it is located within a large, unused portion of the vessel. Furthermore, it is located away from the highest fluence region, i.e., the core, as compared to conventional in-fuel rod fission gas plenums, thus reducing fission gas pressure and radiation damage to the plenum walls; this increases the mechanical margin against failure.

[0043] Locating the common fission gas plenum 400 away from the flowing coolant streams can mitigate material selection challenges associated with flow induced corrosion / erosion.

[0044] If the fuel rod 210 were to develop a leak, fission gas release from the leak would be limited to fission gas generated after the leak. The check valves (or fluidic diodes) in the fuel rods 210 and the inlet of the common plenum 400 would preclude previously generated (and stored) fission gas in the common plenum 400 from leaking into the reactor coolant system. Additionally, the check valves (or fluidic diodes) can prevent previously generated (and stored) fission gas in the passages 320, plenum header connections 140, and / or the flow passages 130, 135 of the top fitting 100 from leaking into the reactor coolant system.

[0045] At least one aspect of the present disclosure allows for monitoring of plenum pressure, which is not practical in conventional designs employing individual fuel rod plenums. By monitoring the common plenum pressure, fuel assemblies containing leaking fuel rods can be identified.

[0046] The reduced pressure due to the large plenum tanks may alleviate concerns about leaking fuel assemblies. Additionally, perhaps the ability to do controlled venting / collection of the common plenum may allow other means to address / mitigate leaking assemblies.

[0047] The low power density within a given vessel size, low rod pressure, infrequent refueling, and ease of fuel handling by being located on or near the coolant surface allow for dry lift refueling at each location, greatly easing the cost of these systems and plant layout / size. Direct extraction with no in-coolant shuffling is a notable simplification compared to many other refueling schemes.

[0048] A "cut and pinch" method (e.g., similar to that used in oil drilling rig blowout preventers) of separating the common plenum 400 from the active fuel area (e.g., fuel bundle 200) including the fuel elements 210 and top fitting 100 may be utilized in the reduced diameter mast section 300, i.e., the third portion of the fuel assembly 500. The "cut and pinch" methodology may facilitate long-term storage of spent fuel or damaged assemblies (both in vessels and in casks).

[0049] Lower stresses on fuel rod cladding make it easier to implement ultra-long fuel cycles, thereby reducing the need for spent fuel infrastructure and reducing concerns about the potential diversion of spent fuel during refueling operations. These benefits apply especially to reactors in countries without mature fuel cycle infrastructure / safeguards.

[0050] At least one embodiment of the present disclosure allows for a substantial increase in the overall fuel load that can be placed in a pool reactor, reduces fission gas pressures, and eases the refueling challenges in pool plants that use liquid metal or salt coolants. It does so while offering the possibility of extending refueling intervals to 20 years or more. This allows customers to avoid purchasing refueling equipment that is intended for frequent use and is an integral part of the nuclear island. It simplifies the overall plant layout, provides guaranteed fuel costs over the entire capitalization period (and for longer periods), reduces the volume of spent fuel generated, increases proliferation resistance, and eases access to markets that would otherwise be challenged by the lack of a mature fuel cycle infrastructure / safeguards. Cost savings are derived. Other advantages will be apparent.

[0051] Various aspects of the subject matter described herein are described in the following examples.

[0052] Example 1 - A fuel assembly for use in a nuclear reactor having a vessel and further disposing a coolant within the vessel. The fuel assembly comprises a first portion and a second portion. The first portion comprises an elongated duct, a plenum header connection including a plurality of flow passages formed therein, and a plurality of fuel elements disposed within the elongated duct. Each fuel element includes a cladding tube including an interior region formed therein. The interior region includes nuclear fuel material located therein. The interior region of the plurality of fuel elements is in fluid communication with the plurality of flow passages. The second portion includes a common fission gas plenum in fluid communication with the plurality of flow passages of the plenum header connection. The common fission gas plenum is disposed in a portion of the vessel that is otherwise unused. The common fission gas plenum is configured to receive an amount of fission gas produced by the nuclear fuel material during operation of the reactor.

[0053] Example 2 - A fuel assembly as described in Example 1, wherein the fuel assembly is configured to have a retention force applied to the fuel assembly to resist at least one of frictional forces, form drag, and buoyancy forces applied to the fuel assembly by the coolant and retain the plurality of fuel elements within the coolant.

[0054] Example 3 - A fuel assembly as described in example 1 or 2, wherein the fuel assembly includes a third portion disposed intermediate the first portion and the second portion, the third portion including a passage that fluidly connects the first portion and the second portion to each other.

[0055] Example 4 - The fuel assembly of example 3, wherein the first portion includes a first outermost surface defined within a first diameter, the second portion includes a second outermost surface defined within a second diameter, and the third portion includes a third outermost surface defined within a third diameter smaller than the first diameter.

[0056] Example 5 - The fuel assembly of Example 4, wherein the third diameter is smaller than the second diameter.

[0057] Example 6 - The fuel assembly of Examples 3, 4, or 5, wherein at least one of the common fission gas plenum, the passages, the flow paths, the plenum header connections, and the plurality of fuel elements includes a check valve that resists flow of fission gas from the common fission gas plenum in a direction toward the plurality of fuel elements.

[0058] Example 7 - The fuel assembly of example 6, wherein the check valve comprises a fluidic diode.

[0059] Example 8 - The fuel assembly of Examples 1, 2, 3, 4, 5, 6 or 7, wherein the plenum header connection includes a plurality of coolant flow passages defined therein.

[0060] Example 9 - The fuel assembly of Examples 3, 4, 5, 6, or 7, wherein the plenum header connection further comprises a central collection passage configured to fluidly connect the plurality of flow paths of the plenum header connection to the passages of the third portion.

[0061] Example 10 - A fuel assembly for use in a nuclear reactor having a vessel and further having a coolant located within the vessel. The fuel assembly includes a fuel bundle, a plenum header connection, an elongated mast, and a common fission gas plenum. The fuel bundle includes a plurality of fuel elements. Each fuel element includes nuclear fuel material positioned therein. The plenum header connection includes a plurality of passages defined therein. The plenum header connection is disposed on the fuel bundle. The plurality of passages are in fluid communication with the nuclear fuel material. The elongated mast extends from the fuel bundle and includes an internal passage. The common fission gas plenum extends from the elongated mast. The internal passage connects the common fission gas plenum to the plurality of passages of the plenum header connection such that the common fission gas plenum is configured to receive a quantity of fission gas produced by the nuclear fuel material during operation of the nuclear reactor. The common fission gas plenum is disposed in a portion of the vessel that is otherwise unused.

[0062] Example 11 - A fuel assembly as described in Example 10, wherein the fuel assembly is configured to have a retention force applied to the fuel assembly to resist at least one of frictional forces, form drag, and buoyancy forces applied to the fuel assembly by the coolant and retain a plurality of fuel elements positioned within the coolant.

[0063] Example 12 - The fuel assembly of example 10 or 11, wherein the fuel bundle includes a first outermost surface defined within a first diameter, the common fission gas plenum includes a second outermost surface defined within a second diameter, and the elongated mast includes a third outermost surface defined within a third diameter smaller than the first diameter.

[0064] Example 13 - The fuel assembly of Example 12, wherein the third diameter is smaller than the second diameter.

[0065] Example 14 - The fuel assembly of Examples 10, 11, 12, or 13, wherein at least one of the common fission gas plenum, the internal passageway, the passageway, the plenum header connection, and the plurality of fuel elements includes a check valve that resists flow of fission gas from the common fission gas plenum in a direction toward the plurality of fuel elements.

[0066] Example 15 - The fuel assembly of example 14, wherein the check valve includes a fluidic diode.

[0067] Example 16 - The fuel assembly of Examples 10, 11, 12, 13, 14, or 15, wherein the plenum header connection further includes a central collection passage, the central collection passage fluidly connecting the multiple passages of the plenum header connection to the internal passage of the elongated mast.

[0068] Example 17 - A method of forming a fission gas plenum header connection for use with a fuel assembly in a nuclear reactor. The fuel assembly includes a plurality of fuel elements. The method includes forming a flow passage by machining, etching, or otherwise removing material in a first portion, and diffusion bonding a second portion to the first portion to form a unitary, seamless plenum header connection including an internal flow passage configured to allow passage therethrough of fission gas released from the fuel assembly during use.

[0069] Example 18 - The method of example 17, further comprising machining, etching, or otherwise removing material from the unitary, seamless plenum header connection to form a plurality of plenum flow connections therein, each plenum flow connection configured to receive an end of one of the fuel elements of the fuel assembly.

[0070] Example 19 - The method of example 18, wherein the internal flow passage interconnects with the plurality of plenum flow connections such that the internal flow passage and the plurality of plenum flow connections are in fluid communication with each other.

[0071] Example 20 - The method of example 18 or 19, further comprising machining, etching, or otherwise removing material from the plenum header connection to form a flow path for the reactor coolant.

[0072] Although specific embodiments have been described in detail, it will be understood that those skilled in the art, in light of all the teachings of this disclosure, may develop various modifications and alternatives to these details, and may combine one or more selected elements of the exemplary embodiments with one or more elements from other embodiments without departing from the scope of the disclosed concepts. Accordingly, the specific embodiments disclosed are intended to be illustrative only and not limiting, the scope of the disclosure, which is to be given the full scope of the appended claims and any and all equivalents thereof.

[0073] Those skilled in the art will understand that, generally, the terms used herein, and in particular the terms used in the appended claims (e.g., the body of the appended claims), should generally be construed as "open" terms (e.g., including but not limited to "comprising"), the term "having" should be construed as "having at least," the term "including" should be construed as "including but not limited to," and the like, and further, that where a particular number of introduced claim recitations are intended, such intention will be expressly set forth in the claim, and in the absence of such a recitation, for example, to aid in understanding, the appended claims may include the use of the introductory terms "at least one" and "one or more" to introduce claim recitations, but the use of such terms should not be construed as The introduction of a claim recitation with an indefinite article "a" or "an" limits the particular claim that contains the introduced claim recitation to a claim that contains only one article, even if that same claim includes an indefinite article such as "one or more" or "at least one or more" and "a" or "an" (e.g., "a" and / or "an" should typically be construed to mean "at least one or more" or "one or more"), and is used to introduce the claim recitation.

[0074] Furthermore, even if a particular number of introduced claim recitations is explicitly recited, one of ordinary skill in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., a bare recitation of "two recitations" without other qualifiers typically means at least two recitations, or more than two recitations). Furthermore, when a convention similar to "at least one of A, B, and C, etc." is used, such a construction is generally intended to be understood by one of ordinary skill in the art (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, a system having A alone, B alone, C alone, A and B alone, A and B alone, A and C alone, A and C alone, B and C alone, and / or A, B, and C alone, etc.). When a convention similar to "at least one of A, B, or C, etc." is used, such a configuration is generally intended to be understood by one of ordinary skill in the art (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, a system having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B and C together, etc.). Furthermore, one of ordinary skill in the art will appreciate that typically, in either the description, claims, or drawings, disjunctions and / or phrases presenting two or more alternative terms should be understood to contemplate the possibility of including one of the terms, either of the terms, or both terms, unless the context dictates otherwise. For example, the phrase "A or B" is typically understood to include the possibilities of "A" or "B" or "A and B."

[0075] With respect to the appended claims, those skilled in the art will appreciate that the operations recited therein may generally be performed in any order. Also, while various operational flow diagrams are presented in a sequence, it should be understood that various operations may be performed in orders other than those depicted, or may be performed simultaneously. Examples of such alternative orderings may include overlapping, interleaving, interrupting, reordering, incremental, preparatory, supplementary, simultaneous, reverse, or other variant orderings, unless the context dictates otherwise. Moreover, terms such as "responsive to," "related to," or other past adjectives are generally not intended to exclude such variants, unless the context dictates otherwise.

[0076] It should be noted that references to "one embodiment," "one aspect," "example," "one example," etc. mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment, and thus the appearances of the phrases "in one embodiment," "in one embodiment," "in one example," and "in one example" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0077] Any patent application, patent, non-patent publication, or other disclosure material mentioned herein and / or listed in any Application Data Sheet is incorporated herein by reference to the extent that the incorporated material is not inconsistent with this specification. Thus, to the extent necessary, the disclosure expressly set forth herein supersedes any conflicting material incorporated herein by reference. Any material or portion thereof that is said to be incorporated herein by reference but that conflicts with existing definitions, statements, or other disclosure material set forth herein is incorporated only to the extent that no conflict arises between the incorporated material and the existing disclosure material.

[0078] The terms "comprise" (and any formations such as "comprise" and "comprises"), "have" (and any formations such as "have" and "having"), "include" (and any formations such as "include" and "includes"), and "include" (and any formations such as "includes") are open ended verb binding. As a result, "comprise", "have", "have" or "include" or "includes" one or more elements include only those one or more elements, but are not limited to only having those one or more elements. Similarly, "comprise", "have", "have", "include" or "include" system, device, or "include" apparatus elements include only those one or more features, but are not limited to only having those one or more features.

[0079] In summary, many advantages are described that accrue from employing the concepts described herein. The foregoing description of one or more embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to be limited to the precise form disclosed. Modifications or variations are possible in light of the above teachings. One or more embodiments have been selected and described in order to illustrate the principles and practical applications, thereby enabling those skilled in the art to utilize various embodiments and various modifications suited to the particular use intended. The claims submitted herein are intended to define the overall scope.

Claims

1. 1. A fuel assembly for use in a nuclear reactor having a vessel and further having a coolant disposed within said vessel, comprising: A first portion; and a second portion, The first portion is A long thin duct, a plenum header connection portion having a plurality of flow paths; a plurality of fuel elements positioned within the elongated duct; Each fuel element comprises a cladding tube including an interior region formed therein; the interior region including nuclear fuel material positioned therein; an interior region of the fuel elements in fluid communication with the flow passages; The second portion is a common fission gas plenum in fluid communication with the plurality of flow paths of the plenum header connection; the common fission gas plenum is positioned within an otherwise unused portion of the vessel; The common fission gas plenum is configured to receive a quantity of fission gas produced by the nuclear fuel material during operation of the nuclear reactor.

2. 10. The fuel assembly of claim 1, wherein the fuel assembly is configured to exert a retaining force to resist at least one of frictional forces, form drag, and buoyancy forces exerted on the fuel assembly by the coolant and to retain the plurality of fuel elements within the coolant.

3. the fuel assembly includes a third portion disposed intermediate the first portion and the second portion; The fuel assembly of claim 1 , wherein the third portion includes a passageway that fluidly connects the first portion and the second portion to one another.

4. the first portion includes a first outermost surface defined within a first diameter; the second portion includes a second outermost surface defined within a second diameter; The fuel assembly of claim 3 , wherein the third portion includes a third outermost surface defined within a third diameter that is smaller than the first diameter.

5. The fuel assembly of claim 4 , wherein the third diameter is smaller than the second diameter.

6. 4. The fuel assembly of claim 3, wherein at least one of the common fission gas plenum, the passages, the flow paths, the plenum header connections, and the plurality of fuel elements includes a check valve that resists flow of fission gas in a direction from the common fission gas plenum toward the plurality of fuel elements.

7. The fuel assembly of claim 6 , wherein the check valve comprises a fluidic diode.

8. The fuel assembly of claim 1 , wherein the plenum header connection comprises a plurality of coolant passages defined therein.

9. 4. The fuel assembly of claim 3, wherein the plenum header connection further comprises a central collection passage configured to fluidly connect the plurality of flow paths of the plenum header connection to the passages of the third portion.

10. 1. A fuel assembly for use in a nuclear reactor having a vessel and further having a coolant disposed within said vessel, comprising:

1. A fuel bundle including a plurality of fuel elements, each fuel element including nuclear fuel material positioned therein; and a plenum header connection including a plurality of passages defined therein, the plenum header connection being disposed on the fuel bundle, the plurality of passages being in fluid communication with the nuclear fuel material; an elongated mast extending from the fuel bundle, the elongated mast including an internal passage; a common fission gas plenum extending from the elongated mast, the internal passage connecting the common fission gas plenum to a plurality of passages of the plenum header connection such that the common fission gas plenum is configured to receive an amount of fission gas produced by the nuclear fuel material during operation of the reactor, the common fission gas plenum being disposed within an otherwise unused portion of the vessel.

11. 11. The fuel assembly of claim 10, configured to resist at least one of frictional forces, form drag, and buoyancy forces exerted on the fuel assembly by the coolant and to have a retention force exerted thereon to retain the plurality of fuel elements positioned within the coolant.

12. the fuel bundle includes a first outermost surface defined within a first diameter; the common fission gas plenum includes a second outermost surface defined within a second diameter; The fuel assembly of claim 10 , wherein the elongated mast includes a third outermost surface defined within a third diameter that is smaller than the first diameter.

13. The fuel assembly of claim 12 , wherein the third diameter is smaller than the second diameter.

14. 11. The fuel assembly of claim 10, wherein at least one of the common fission gas plenum, the internal passage, the passage, the plenum header connection, and the plurality of fuel elements includes a check valve that resists fission gas flow in a direction from the common fission gas plenum toward the plurality of fuel elements.

15. The fuel assembly of claim 14 , wherein the check valve comprises a fluidic diode.

16. the plenum header connection further includes a central collection passage; The fuel assembly of claim 10 , wherein the central collection passage fluidly connects the plurality of passages of the plenum header connection to the interior passage of the elongated mast.

17. 1. A method of forming a fission gas plenum header connection for use with a fuel assembly in a nuclear reactor, the fuel assembly including a plurality of fuel elements, comprising the steps of: forming a flow channel by machining, etching, or otherwise removing material in the first portion; and diffusion bonding a second portion to the first portion to form a unitary, seamless plenum header connection including an internal flow passage configured to permit movement therethrough of fission gases released from the fuel assembly during use.

18. machining, etching, or otherwise removing material from the unitary, seamless plenum header connection to form a plurality of plenum flow connections therein; 20. The method of claim 17, wherein each plenum flow connection is configured to receive an end of one of the fuel elements of the fuel assembly.

19. 20. The method of claim 18, wherein the internal flow passage interconnects with the plurality of plenum flow connections such that the internal flow passage and the plurality of plenum flow connections are in fluid communication with one another.

20. 20. The method of claim 19, further comprising the step of machining, etching, or otherwise removing material from the unitary, seamless plenum header connection to form a flow path for reactor coolant.

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