Zamak stabilization apparatus of spent sodium-cooled reactor fuel assemblies

Abstract

To provide methods and systems for stabilizing spent fuel assemblies from sodium-cooled nuclear reactors using Zamak.SOLUTION: In the method, one or more spent fuel assemblies are removed from the sodium coolant pool and placed in a protective sheath. The remaining volume of the sheath is then filled with liquid Zamak. To a certain extent Zamak will dissolve with sodium remaining on the fuel assemblies and form an alloy. Excess sodium that remains undissolved is displaced from the sheath by the Zamak fill. The Zamak is then cooled until solid and the sheath is sealed. The resulting Zamak-stabilized spent fuel assembly is calculated to have sufficient internal thermal conductivity to allow it to be stored and transported without the need for liquid cooling.SELECTED DRAWING: Figure 4

Description

Detailed Description of the Invention

[0001] [Introduction] There is a need for a method of disposing of spent nuclear fuel assemblies. Spent fuel assemblies continue to generate heat, referred to as decay heat, as the nuclear material within the assembly continues to fission. If the decay heat is not removed, it can cause components within the fuel assembly to become unacceptably hot. Potentially, this can lead to component failure and the release of nuclear material and fission products within the assembly.

[0002] To prevent spent fuel assemblies from overheating, "wet storage" is often employed, where the assemblies are immersed in large pools of water. The pools act as a coolant to remove decay heat generated by the spent fuel assemblies. However, wet storage is not considered a solution for long-term storage because the integrity of the pool must be maintained. "Dry storage," which does not require a liquid bath or other active cooling, is preferred for long-term storage. This allows spent fuel assemblies to be stored routinely at minimal cost.

[0003] One proposed method for stabilizing spent fuel assemblies for long-term dry storage is to fill the fuel assembly with a void-filling solid material. A thermally conductive metal or metal alloy (e.g., lead) could be heated above its melting point, flowed into the spent fuel assembly, filling all spaces within the fuel assembly (e.g., channels provided for coolant flow through the fuel assembly), and then allowed to cool and solidify. Theoretically, if the spent fuel assembly were completely filled with metal, the thermal conductivity of the stabilized spent fuel assembly would be sufficient to rapidly transport decay heat out of the fuel assembly and prevent the decay heat from dissipating to the external environment by natural convection and conduction, thereby preventing components from becoming unacceptably hot. Thus, in theory, a spent fuel assembly stabilized in this way could be transported and stored without the need for a liquid cooling bath or other active cooling means.

[0004] However, research has revealed one major problem with this method: It has been found that gaps typically form between the investigated void filler materials and the outer surfaces of the components within the fuel assembly upon solidification of the void filler material. This gap reduces heat transfer between the fuel assembly components and the void filler material to such an extent that this method is not viable.

[0005] Zamak stabilization of spent sodium-cooled reactor fuel assemblies Described herein are methods and systems for stabilizing spent fuel assemblies from sodium-cooled nuclear reactors using zamak. A synergistic effect has been identified between zamak and sodium, allowing the zamak to form a thermally conductive interface with the sodium-wetted surfaces of the fuel assemblies. In this method, one or more spent fuel assemblies are removed from a sodium coolant pool and placed within a protective sheath. The remaining space in the sheath is then filled with liquid zamak. To some extent, the zamak dissolves and forms an alloy with the sodium remaining on the fuel assemblies. Any excess sodium that remains undissolved is displaced from the sheath by the zamak filling. The zamak is then cooled until it solidifies, and the sheath is sealed. Calculations indicate that the resulting zamak-stabilized spent fuel assemblies have sufficient internal thermal conductivity to allow for storage and transportation without the need for liquid cooling.

[0006] One drawback of metallic stabilization (i.e., gap formation) is avoided by the methods and systems described herein. It has been determined that the primary cause of gap formation in initial stabilization efforts is the presence of an oxide layer on the outer surfaces of fuel assembly components. During normal reactor operation or subsequent wet storage, an oxide layer develops on the outer surfaces of fuel assembly components due to contact with water, air, or other oxygen-containing coolants. For example, it has been determined that at elevated temperatures, an oxide layer of several micrometers can develop after only a few seconds of exposure to air. The oxide layer prevents a good metallurgical bond between the investigated void filler material and the metal components of the fuel assembly. Without a good metallurgical bond at the interface between the fuel assembly components and the void filler material, gaps will form at the interface when the liquid void filler material solidifies.

[0007] The formation of oxide layers on the surfaces of fuel assemblies is prevented by the methods described herein. Unlike conventional pressurized water reactors, fuel assemblies in sodium-cooled reactors are not exposed to oxygen, and the formation of oxide layers on the fuel assemblies is prevented by the liquid sodium. By preventing subsequent exposure of the spent fuel assemblies to oxygen during the process of removing the assemblies from the sodium coolant pool and filling the assemblies with zamak, oxide-layer-induced gap formation is prevented, and a good metallurgical bond can be formed between the zamak and the outer surface of the fuel assemblies.

[0008] BRIEF DESCRIPTION OF THE DRAWINGS Various aspects of at least one embodiment are discussed below with reference to the accompanying drawings. The drawings are not intended to be drawn to scale. The drawings are included to provide illustration and a further understanding of various aspects and embodiments, and are incorporated into and constitute a part of this specification. However, the drawings are not intended to define limitations on particular embodiments. The drawings, together with the remainder of the specification, serve to explain the principles and operation of the described and claimed aspects and embodiments. In the drawings, each identical or nearly identical component shown in various figures is represented by a like numeral. For clarity, not every component in every figure is labeled.

[0009] FIG. 1 illustrates one embodiment of an integrated energy system having a sodium-cooled reactor.

[0010] FIG. 2 is an exploded view of a fuel assembly for use in a sodium-cooled reactor.

[0011] FIG. 3 shows a side view of a different type of fuel assembly.

[0012] FIG. 4 illustrates one embodiment of a zamak stabilization system suitable for use in the integrated energy system shown in FIG.

[0013] FIG. 5 illustrates an alternative embodiment of the sheath that includes additional features that could be implemented in any of the sheath embodiments.

[0014] FIG. 6 illustrates one embodiment of a method for stabilizing spent nuclear fuel assemblies from sodium-cooled reactors using zamak.

[0015] Detailed Description Before disclosing and describing the zamak stabilization method and system, it is to be understood that the present disclosure is not limited to the specific structures, process steps, components, or materials disclosed herein, but extends to equivalents thereof as would be recognized by one of ordinary skill in the art. It is also to be understood that the terminology used herein is solely for the purpose of describing particular embodiments for stabilizing sodium-wet fuel assemblies and is not intended to be limiting. It should be noted that, as used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context dictates otherwise. Thus, for example, a reference to “a lithium hydroxide” should not be construed as being quantitative or source-limiting, a reference to “a step” may include multiple steps, a reference to “producing” or the “products” of a reaction should not be construed as all of the products of the reaction, and a reference to “reacting” may include a reference to one or more such reaction steps. Thus, the reacting step may involve multiple or repeated reactions of similar materials to produce a specified reaction product.

[0016] A method and system for stabilizing spent fuel assemblies from sodium-cooled nuclear reactors using zamak is described below. As discussed above, it has been determined that a synergistic effect exists between zamak and sodium that allows zamak to form a thermally conductive interface with the sodium-wetted surfaces of the fuel assemblies. To a certain extent, zamak dissolves with sodium to form an alloy, but this does not reduce the effectiveness of the resulting sodium-enriched zamak as a stabilizing material. Furthermore, because the density of liquid zamak is greater than that of liquid sodium, any excess liquid sodium can be easily displaced and collected from the fuel assemblies and sheaths during the zamak filling process.

[0017] Zamak refers to a family of alloys containing zinc as the base metal and aluminum, magnesium, and copper as alloying elements. Zamak alloys are part of the zinc-aluminum alloy family and are distinguished from other zinc-aluminum alloys by their constant nominal aluminum composition of 4% (substantially 3.5-4.3% by weight Al). Generally, zamak contains 3.5-4.3% Al, 0.0-0.25% Cu, and 0.01-0.02% Mg, with the remainder being Zn. For purposes of this document, zamak alloys are defined as alloys containing 1-10% Al; 0-1% Cu; 0.01-1% Mg; less than 0.5% impurities (where impurities are any element other than Al, Cu, Mg, and Zn); and the remainder being Zn. Specifically, ASTM B240 defines formulas for ingots of various members of the zamak family. Some of the members of the zamak family defined in ASTM B240 include Zamak 2, KS, Zamak 3, Zamak 4, Zamak 5, and Zamak 7. Any particular member of the zamak family may be used herein. In one embodiment, Zamak 3 is used. The composition and properties of Zamak 3 are set forth below. The compositions of other members of the zamak family can be found in the ASTM B240 standard. The ASTM B240 standard is incorporated herein by reference.

[0018] [Table 1]

[0019] [Table 2]

[0020] In this method, one or more spent fuel assemblies are removed from the sodium coolant pool and placed in a protective sheath. The remaining space in the sheath is then filled with liquid zamak. To a certain extent, the zamak dissolves and forms an alloy with the sodium remaining on the fuel assemblies. Excess sodium that remains undissolved is displaced (displaced) from the sheath by the zamak fill. The zamak is then cooled until it solidifies, and the sheath is sealed. Calculations indicate that the resulting zamak-stabilized spent fuel assemblies have sufficient internal thermal conductivity to be stored and transported without the need for liquid cooling.

[0021] FIG. 1 illustrates one embodiment of an integrated energy system 100 having a sodium-cooled reactor 102. The sodium-cooled reactor 102 includes a reactor vessel 104 containing a pool of sodium 106. One or more fuel assemblies 108 (three shown) containing nuclear fuel are immersed in the pool of sodium 106. Once immersed in the sodium 106, the interior chambers of the fuel assemblies 108 are filled with sodium. The sodium 106 acts as a primary coolant and transfers heat from the immersed fuel assemblies 108 to a secondary coolant during operation. The fuel assemblies are described in more detail with reference to FIGS. 3 and 4.

[0022] 1, the sodium in the pool circulates between the fuel assemblies 108 and a submerged in-pool heat exchanger called primary heat exchanger 110. Heated sodium flows from the fuel assemblies 108 to the primary heat exchanger 110, and cooled sodium flows back to the fuel assemblies 108, creating a circulation loop (indicated by liquid flow direction arrows 111).

[0023] The primary heat exchanger 110 cools the sodium by transferring heat into a secondary coolant, which may be sodium or some other fluid coolant, which circulates between the primary heat exchanger 110 and the secondary heat exchanger 112. In the illustrated embodiment, the secondary heat exchanger 112 transfers heat from the secondary coolant to a thermal storage medium. A supply of cryogenic medium is provided in a cryostat 114. The secondary heat exchanger 112 transfers heat to the cryogenic medium, which is then heated and stored in a hot tank 116.

[0024] Heat storage is well known, and any suitable heat storage medium now known or later developed may be used. In the illustrated embodiment, the heat storage medium is a salt. Examples of suitable heat storage media include eutectic solutions, phase change materials, miscibility alloys, and the like. gap alloys), mixtures of metals (e.g., AlSi 12 ), cementitious materials, molten salts (e.g., one or more chloride salts of sodium, potassium, and calcium; one or more nitrate salts of sodium, potassium, and calcium, particularly NaKMg, or NaKMgCl), solid or molten silicon, or combinations of these or other materials.

[0025] When power is needed from the integrated energy system 100, the hot salt from the hot reservoir 116 is passed through a steam generator 118. In the steam generator 118, heat is transferred from the hot salt to a stream of pressurized water, producing superheated steam. The (now cooler) salt is sent to the cold reservoir 114 where it is stored until more heat is needed from the secondary coolant.

[0026] The thermal energy in the superheated steam is converted to mechanical energy by passing the superheated steam through a conventional steam turbine and condenser system 120. In the illustrated embodiment, the steam turbine 122 drives a generator 124 to produce electricity.

[0027] In the illustrated embodiment, the reactor vessel 104 is capped by a vessel head 126. The vessel head 126 includes access ports that allow for the insertion and removal of fuel assemblies from the reactor vessel 104. A fuel assembly handling system 130 is provided for retrieving new fuel assemblies from a fuel assembly storage device 132, inserting and removing fuel assemblies 108 from the reactor 102, and transferring spent fuel assemblies 108 to a Zamak stabilization system 140. The fuel assembly handling system 130 maintains the fuel assemblies in an inert environment during handling to prevent exposure of the fuel assemblies to water, air, or any other undesirable environment, thereby inhibiting, if not preventing, the formation of an oxide layer on the surfaces of the fuel assemblies during transfer between the reactor vessel 104 and the Zamak stabilization system 140. In one embodiment, the fuel assembly handling system 130 maintains the fuel assemblies in a sodium environment during transfer. In an alternative embodiment, the fuel assembly is maintained in an inert atmosphere, such as argon or nitrogen, during handling. In yet another embodiment, the fuel assembly handling system 130 maintains the fuel assembly in a low-oxygen environment having less than 0.1% oxygen, less than 0.01% oxygen, or less than 0.001% oxygen.

[0028] The zamak stabilization system 140 receives spent fuel assemblies from the handling system 130. The system 140 may be implemented to receive one or more spent fuel assemblies at a time. Furthermore, the system 140 may be implemented to process the fuel assemblies one at a time, or batches or two or more simultaneously. After receiving the spent fuel assemblies, the zamak stabilization system 140 places them in a protective sheath. As described in more detail below, any vacant space within the fuel assemblies 108 and the remaining space in the sheath surrounding the fuel assemblies 108 are filled with liquid zamak. Any excess sodium that remains undissolved is displaced from the sheath by the zamak filling and collected by the zamak stabilization system 140. The excess sodium may be reused within the nuclear reactor 102 or elsewhere in the integrated energy system 100. The zamak stabilization system 140 may then actively cool the sheathed spent fuel assembly until the zamak becomes solid. The zamak stabilization system 140 then seals the sheath (either before, during, or after cooling). The resulting sheathed zamak-stabilized spent fuel assembly is calculated to have sufficient internal thermal conductivity to allow for storage and transportation without the need for liquid cooling. In one embodiment, the zamak stabilization system 140 maintains the spent fuel assembly in an inert environment during the stabilization process.

[0029] FIG. 2 is an exploded view of an embodiment of a fuel assembly 200 for use in a traveling wave reactor or other sodium-cooled reactor. The assembly 200 includes an elongated coolant channel 202 having an axis A. The channel 202 has a hexagonal cross-section. A handling socket 204 having an internal flow path is secured to a first end 206 of the channel 202 and has internal or external features that allow it to be grasped by mechanisms within the reactor vessel to raise, lower, and otherwise move the assembly 200 into, out of, or within the core. An inlet nozzle 208 is secured to a second end 210 of the channel 202. A plurality of bearing rings 212 and a plurality of retaining rings 214 are used to attach the handling socket 204 and the inlet nozzle 208 to the channel 202. A plurality of locking plates 216 (two in this example) and a plurality of rod strip rails 218 are included proximate the end of the inlet nozzle 208. The plurality of locking plates 216 and the plurality of rod strip rails 218 cooperate to connect a fuel rod bundle 220 to the inlet nozzle 208. In one embodiment, all of the fuel rods in the fuel rod bundle 220 are annular metallic fuel rods as described above. In an alternative embodiment, only a portion of the fuel rods may be annular metallic fuel rods, with the remaining fuel rods having a different type or configuration. A seal ring 222 and a flow restrictor 224 are also shown.

[0030] FIG. 3 shows a side view of an alternative design of a fuel assembly 300. The assembly includes a set of fuel rods 320 that pass through and are held in place by multiple (six shown) spacer grids 330. A bottom nozzle assembly 340 supports the fuel assembly 300 within the reactor core. A top nozzle assembly 310 is located at the top of the assembly 300, which includes multiple guide thimble tubes 302. The guide thimble tubes 302 extend from the top nozzle assembly 310 to the bottom nozzle assembly 340. The spacer grids 330 may be attached to the guide thimble tubes 302 for stability. A hold-down spring 312 is located above the top nozzle assembly 310 at the top of the assembly 300 to ensure the appropriate amount of hold-down force against the fuel assembly components.

[0031] It should be noted that the fuel rods described above need not be uniform along their length. For example, regions of greater or lesser enrichment could be provided along the length of the fuel rod. This could be achieved by providing different annular slugs of fuel or different particulate fuel in different regions during assembly. Similarly, burnable poisons, other additives, or different types of metallic fuel could be provided in specific regions. In addition to different materials, the different regions could have different attributes (e.g., different porosity, bulk density, or different annular slug sizes) even if the metallic fuel material remains the same.

[0032] The fuel assemblies of Figures 2 and 3 are just two examples of fuel assemblies that may be stabilized using the methods and systems described herein. Many other fuel assembly designs exist for use in other types of reactors. The configuration (arrangement) of fuel rods and other types of rods (e.g., control rods, reflectors, and instrumentation rods) within a particular assembly for a particular reactor does not affect the Zamak stabilization technique. The shape and configuration (arrangement) of the rods within the assembly, and the shape, orientation, and configuration (arrangement) of the assembly in the reactor core may vary accordingly for a particular reactor design and depending on the number, type, and performance of the annular metallic fuel rods used, but does not affect the stabilization process.

[0033] FIG. 4 illustrates one embodiment of a zamak stabilization system suitable for use in the integrated energy system shown in FIG. 1 . In the illustrated embodiment, a spent fuel assembly 402, upon receipt by the zamak stabilization system 400, is placed into a sheath 404, which is then capped by an end cap 406. The sheath 404 is provided with a receptacle 410 that engages with a bottom nozzle 408 of the fuel assembly 402. Additional supports (not shown) may be provided within the sheath to guide fuel assembly placement or provide additional support within the sheath prior to filling with zamak. In one embodiment, the receptacle 410 supports the fuel assembly 402 and allows for injection of zamak into the fuel assembly 402 via the bottom nozzle 408 prior to the sheath 404 being entirely filled with zamak. The receptacle 410 is connected to a lower access port 415 at the bottom of the sheath through which the zamak is delivered by a pipe 416 .

[0034] The sheath 404 may be made from any suitable structural material. The sheath material must have a melting point higher than that of zamak and must maintain an adequate amount of strength at the zamak operating temperature. The melting point of zamak is approximately 380-390°C. In one embodiment, the zamak is delivered at a temperature of 400-450°C when filling the fuel assembly and / or sheath chamber. In an alternative embodiment, the zamak temperature at delivery is 380-2,000°C.

[0035] Suitable materials for the sheath 404 and other components in the zamak flow loop include any suitable steel (e.g., stainless steels such as 304 steel, 316 steel, ferritic-martensitic steels such as T-91, among others), or other non-corrosive materials with suitable melting points and strengths. Further examples of suitable steels include martensitic steels, ferritic steels, austenitic steels, stainless steels including aluminum-containing stainless steels, FeCrAl alloys, HT9, oxide dispersion strengthened steels, T91 steel, T92 steel, HT9 steel, 316 steel, 304 steel, and advanced steels such as APMT (Fe-22% by weight Cr-5.8% by weight Al) and Alloy 33 (a mixture of iron, chromium, and nickel, nominally 32% by weight Fe-33% by weight Cr-31% by weight Ni). Unless otherwise specified, all percentages (%) herein are weight percentages (wt%). Steels may have any type of microstructure. For example, in one embodiment, substantially all of the steel in the cladding 106 has at least one phase selected from a tempered martensite phase, a ferrite phase, and an austenite phase. In one embodiment, the steel is HT9 steel or a modified version of HT9 steel.

[0036] A controllable valve 412 is provided to control whether the zamac is introduced into the bottom nozzle 408 or into the space between the sheath and the outer surface of the spent fuel assembly. This allows the interior space of the fuel assembly accessible through the bottom and top nozzles to be filled independently of the exterior space between the sheath 404 and the outer surface of the spent fuel assembly 402. Both spaces may be filled simultaneously or at staggered times. In the illustrated embodiment, the valve 412 is within the sheath 404. In an alternative embodiment, the valve 412 may be external to the sheath 404, allowing the valve 412 to be reused. In this embodiment, the sheath has at least two access ports: one for filling the exterior space between the sheath 404 and the outer surface of the spent fuel assembly 402 and one for filling the interior space of the fuel assembly 402 accessible through the bottom nozzle 408 and the top nozzle 414.

[0037] As the interior space of the fuel assembly 402 fills, excess sodium that does not alloy with the zamak is displaced by the heavier zamak and is eventually forced out of the top nozzle 414 and into the sheath chamber (i.e., the space between the sheath 404 and the exterior surface of the spent fuel assembly 402). As the sheath chamber fills, the excess sodium is further displaced and eventually forced out of the sheath through an access port 418 in the sheath cap 406. The access port 418 in the sheath cap 406 is connected to a zamak storage tank 420.

[0038] A sodium trap 422 is shown between the access port 418 and the zamak storage tank 420. In the illustrated embodiment, the sodium trap includes a sodium sensor 424, a controllable valve 426, and a sodium storage tank 428. When sodium is detected in the pipe, the valve directs the sodium into the sodium storage tank 428. When zamak is detected, the zamak is directed by the valve 426 to the zamak storage tank 420. The sodium trap 422 is just one possible method of collecting excess sodium from the sheath. Many other sodium trap designs or sodium trap strategies can be used to achieve the same result, and any suitable such designs or strategies could be utilized herein.

[0039] The flow of zamak through the zamak loop is driven by a pump 430. Additional components include sensors such as a temperature sensor 432 and a pressure sensor 434. Such sensors can be located throughout the system 400. In the illustrated embodiment, sensors are provided on both the zamak storage tank 420 and on the delivery pipe 416 just before the access port 415. A flow meter 436 is shown which allows the flow rate and volume of zamak delivered to the sheath to be monitored.

[0040] To control the processes and operations of the various components, a controller 438 is provided that is communicatively coupled to the sensors, controllable valves, pump 430, and other components to receive data and send commands to the various components based on processing of the received data.

[0041] After the sheath and spent fuel assemblies are filled with zamak, the sheath is sealed and removed from the loop. In one embodiment, the attachment pipes at the bottom and top are cut, shielded, or sealed in any suitable manner. However, this is just one technique, and any suitable alternative sealing and removal technique could be used.

[0042] Another component shown is a heater 440 for heating the zamak in the storage tank 420 and maintaining it in a liquid state during the stabilization process. In one embodiment, the heater 440 could take the form of a resistance jacket heater around the exterior of the storage tank 420.

[0043] Another component shown is a sheath cooler 442 for cooling the sheath 404 after filling with zamak. In one embodiment, the cooler 442 could take any suitable form, including a cooling jacket or a fan directing room temperature air over the exterior of the sheath. In yet another alternative, a temperature-controlled room could be provided as the cooler 442. After the sheath is filled, sealed, and removed from the loop, it could be placed in the temperature-controlled room. Cooling of the zamak could then be precisely controlled. In one embodiment, as shown, the cooler is positioned to cool the bottom of the sheath 404, causing the zamak to solidify from the bottom up.

[0044] Another component of system 400 is cover gas control system 450. Cover gas system 450 maintains a cover gas environment within the loop when zamak does not completely fill the loop. For example, when fuel assembly 402 is initially placed into sheath 404, cover gas system 450 controls the environment around the fuel assembly and reduces the fuel assembly's exposure to oxygen to inhibit, if not prevent, the formation of a surface oxide layer on the fuel assembly. Cover gas systems are known in the art. Any suitable configuration for system 450 may be used to reduce the fuel assembly's exposure to oxygen before and during the zamak filling process.

[0045] FIG. 5 illustrates an alternative embodiment of a sheath 500, showing additional features that could be implemented in any sheath embodiment.

[0046] In the illustrated embodiment, the sheath 504 and cap 506 are designed such that the sheath 504 is a unitary container having only a top opening that is covered by the cap 506. This simplifies the fabrication of the sheath and reduces its complexity. The sheath 504 is provided with a receptacle 510 that mates with a bottom nozzle 508 of the fuel assembly 502.

[0047] Two dip tubes 550 and 552 are provided so that zamak filling still occurs from the bottom. The fuel assembly dip tube 550 is fluidly connected to the receptacle 510 so that zamak flowing through the fuel assembly dip tube 550 enters the fuel assembly 502 through the bottom nozzle 508. A second dip tube 552 is provided to fill the annular region between the fuel assembly and the sheath. In an alternative embodiment, this second dip tube is omitted, and the annular region is filled by overflowing zamak from the top nozzle 514 of the fuel assembly 502. An additional control valve 512 may be provided on the zamak inlet line to control / distribute the flow between the two dip tubes 550, 552. The dip tubes 550, 552 may be integrated into the cap 506 so that the dip tubes are properly positioned when the cap 506 and sheath 504 are connected. Alternatively, the dip tubes 550 , 552 may be incorporated into the sheath 504 , with appropriate corresponding openings provided in the cap 506 .

[0048] The cap 506 is further provided with an outlet 554. When the sheath 504 is filled, excess sodium and zamak are removed through the outlet 554. Each dip tube 550, 552 and outlet 554 may be provided with a connecting valve 556 that can be closed after filling.

[0049] The sheath 504 can be easily cooled from the bottom using a cooler 542 connected to the bottom exterior of the sheath 504. The sheath 504 no longer has any fittings or connections to work with the surroundings.

[0050] In the illustrated embodiment, the exterior of the sheath 504 is provided with two eyes 558 for handling and lifting the sheath 504 before, during and after filling.

[0051] 6 illustrates one embodiment of a method for stabilizing spent nuclear fuel assemblies from a sodium-cooled reactor using zamak. In the illustrated embodiment, the method begins with removing a spent fuel assembly from the sodium-cooled reactor in a removal operation 602. The removal operation 602 may include accessing (reaching) the spent fuel assembly in a sodium pool within the reactor vessel and removing the spent fuel assembly using a fuel assembly handling system. As described in more detail below, during transfer, the fuel assembly is maintained in a controlled environment with less than 0.1% oxygen, thereby reducing the formation of an oxide layer on the surface of the fuel assembly between removal of the fuel assembly from the pool in this removal operation 602 and completion of the zamak loading operation in the second loading operation 612.

[0052] The spent fuel assemblies are then delivered to a Zamak stabilization system in a delivery operation 604 .

[0053] Once the spent fuel assembly is received, it is placed into a sheath during sheathing operation 606. In one embodiment of sheathing operation 606, the bottom nozzle of the fuel assembly is engaged with a receptacle at the bottom of the sheath, capping the sheath. The now sealed sheath is then connected to the Zamak piping.

[0054] In a zamak production operation 608, liquid zamak is produced by heating a quantity of zamak above its melting point, if the zamak has not already been heated to its liquid operating temperature.

[0055] A first filling operation 610 is performed to fill the internal chamber(s) of the spent nuclear fuel assembly with liquid zamak by flowing the liquid zamak through a receptacle in the sheath and through the bottom nozzle of the fuel assembly. Excess sodium is displaced (displaced, diverted) from the top nozzle of the fuel assembly. The sodium may be collected from the top nozzle or may be allowed to accumulate in the annular space between the exterior of the fuel assembly and the sheath if zamak has not yet filled that area.

[0056] A second filling operation 612 is performed to fill the annular space between the outer surface of the fuel assembly and the sheath with zamak. Again, excess sodium on the outer surface of the fuel assembly is displaced (dislodged) as the space is filled, floating on the surface of the zamak. In one embodiment, the annular space between the outer surface of the fuel assembly and the sheath may be filled through a sheath access port at the bottom of the sheath. Alternatively, the annular space between the outer surface of the fuel assembly and the sheath may be filled by passing additional zamak through the fuel assembly and allowing the excess zamak to overflow from the top nozzle of the fuel assembly until the annular space is filled. The second filling operation 612 may be performed until all of the excess sodium has been displaced from the sheath and the zamak has flowed out of the sheath. Any excess sodium displaced from the sheath may be collected and stored for later cleaning and / or reuse in the reactor pool.

[0057] In one embodiment, after filling, an excess amount of zamak may be flowed through either the sheath or the fuel assembly to further flush the filled components. The amount of excess zamak flowed into a space may be determined based on the size of the space, as would be known. Alternatively, the zamak may be flushed based on time after the initial filling (e.g., flowing zamak through the sheath for 10 minutes after the sheath is filled to reduce unfilled areas or to mobilize residual sodium).

[0058] In yet another embodiment, no excess sodium or zamak is removed from the sheath. Instead, the sheath is only filled to the point where the fuel assemblies are partially or completely submerged in zamak, and then the sheath is sealed. In this embodiment, all sodium from the reactor remains in the sheath and is disposed of with the stabilized final fuel assemblies. Although sodium may be wasted in this embodiment, the amount of irradiated sodium handled is reduced.

[0059] In one embodiment, the flow of zamak into the bottom nozzle of the fuel assembly and into the sheath can be controlled independently, allowing the first filling operation 610 and the second filling operation 612 to occur in any order. For example, in one embodiment, the first filling operation 610 occurs first, and the second filling operation 612 occurs only after the fuel assembly is filled. The first filling operation 610 and the second filling operation 612 may be assisted by vibrating the sheath during the filling operations.

[0060] A cooling operation 614 is then performed on the zamak-filled sheath. As described above, the cooling process may be active or passive. In one embodiment, a cooler is used to cool the sheath from below so that the bottom portion of the zamak solidifies first. The cooling rate may be controlled. The zamak is cooled below its melting point. The zamak may be further cooled to room temperature.

[0061] Once cooled, the packaged zamak stabilized fuel assembly is ready for dry storage, as indicated by dry storage operation 616. This may involve placing the packaged zamak stabilized fuel assembly outdoors, in a building, or in an underground borehole or other chamber. The packaged zamak stabilized fuel assembly may further be placed in a cask, frame, or other container to facilitate transportation or storage.

[0062] Either or both of the removal operation 602 and the delivery operation 604 may include a draining operation in which liquid sodium is allowed to drain from the spent nuclear fuel assembly, thereby reducing or even eliminating the amount of excess sodium recovered when filling the sheath.

[0063] In one embodiment of the stabilization method 600, all of the method operations are performed without exposing the spent fuel assembly to an environment having more than 0.1% oxygen, so as to prevent the formation of oxides on the surface of the fuel assembly. This may be achieved by maintaining the spent fuel assembly in an inert atmosphere (i.e., an atmosphere of one or more inert gases, such as argon, nitrogen, and helium, having less than 0.1% oxygen by weight), a low-oxygen atmosphere (e.g., less than 0.1% oxygen by weight, less than 0.01% oxygen by weight), or a sodium environment during handling of the spent fuel assembly. In an alternative embodiment, the removal operation 604 and the delivery operation 606 may be performed rapidly, relying on sodium wetting of the spent fuel to prevent any substantial formation of oxides. If the sheath interior filling operation is performed in an inert or sodium environment, the displaced inert material or sodium may be collected for reuse.

[0064] In an alternative embodiment of method 600, the sheath is omitted. In this embodiment, the spent fuel assembly is filled with zamak, the nozzle is sealed, and the zamak is cooled, resulting in a zamak-stabilized but unwrapped spent fuel assembly. In one embodiment, this simpler approach may be sufficient if decay heat generation is minimal.

[0065] Although described in the context of disposing of spent fuel assemblies, the above-described systems and methods could be used to stabilize new fuel assemblies, or any other components exposed to sodium, regardless of whether they contain decay heat-producing nuclear material. For example, in one embodiment, the above-described systems and methods could be used to dispose of metal components, such as tubes that have held sodium or been in a sodium environment. A zamak stabilization system could be used to fill the tubes with zamak to displace (displace) any residual sodium within the tubes, and then cool the stabilized tubes to create zamak-stabilized tubes ready for storage and / or disposal.

[0066] Notwithstanding the appended claims, the present disclosure is also defined by the following numbered clauses:

[0067] 1. 1. A method of preparing spent nuclear fuel assemblies for storage, comprising: the spent nuclear fuel assembly having an exterior and one or more interior chambers accessible through a bottom nozzle and a top nozzle; providing a quantity of zamak having a first melting point; providing a sheath made from a material having a second melting point higher than the first melting point, the sheath further including a receptacle configured to engage the bottom nozzle of the spent nuclear fuel assembly; removing the spent nuclear fuel assembly from the liquid sodium environment; placing the spent nuclear fuel assembly within a sheath and engaging the receptacle with the bottom nozzle; forming liquid zamak by heating the quantity of zamak to a temperature above the first melting point but below the second melting point; filling one or more of the internal chambers of the spent nuclear fuel assembly with liquid zamak by flowing liquid zamak through the receptacle and the bottom nozzle; filling the sheath with liquid zamak by flowing liquid zamak into a space between the sheath and the exterior of the spent nuclear fuel assembly until the sheath is filled with liquid zamak; A method comprising:

[0068] 2. 2. The method according to clause 1, wherein said zamak is selected from zamak 2, KS, zamak 3, zamak 4, zamak 5 and zamak 7.

[0069] 3. 2. The method of clause 1, wherein the zamak is zamak 3.

[0070] 4. 4. The method of any one of clauses 1 to 3, further comprising, after filling the sheath with liquid zamak, cooling the zamak in the sheath to a temperature below the first melting point.

[0071] 5. placing the spent nuclear fuel assembly within the sheath in an inert environment; filling the sheath with liquid zamak, thereby displacing the inert environment; 5. The method of any one of clauses 1 to 4, further comprising:

[0072] 6. 6. The method of any one of clauses 1-5, wherein the material having a second melting point higher than the first melting point is selected from 304 stainless steel, 316 stainless steel, and T91 steel.

[0073] 7. 7. The method of any one of clauses 1-6, further comprising draining liquid sodium from the spent nuclear fuel assembly.

[0074] 8. 8. The method of any one of clauses 1-7, further comprising collecting sodium displaced from the sheath and one or more of the internal chambers by a plurality of the filling operations.

[0075] 9. The placement operation is capping the sheath after placing the spent nuclear fuel assembly within the sheath. 9. The method of any one of clauses 1 to 8, further comprising:

[0076] 10. 9. The method of any one of clauses 1-8, further comprising the step of cooling the sheath after the step of filling the sheath and the one or more internal chambers with zamak.

[0077] 11. The cooling operation is cooling the sheath from the bottom 11. The method of clause 10, further comprising:

[0078] 12. 12. The method of any one of clauses 1-11, further comprising, after the step of filling the sheath and one or more of the internal chambers with zamak, placing the sheath in a dry storage device.

[0079] 13. 12. The method of any one of clauses 1-11, further comprising, after filling the sheath and the one or more internal chambers with zamak, placing the sheath in a dry underground storage location.

[0080] 14. 14. The method of any one of clauses 1-13, further comprising preventing oxide formation on at least one surface of the spent nuclear fuel assembly between removing the spent nuclear fuel assembly from the sodium environment and filling the spent nuclear fuel assembly with liquid zamak.

[0081] 15. inhibiting the formation of an oxide layer on the surface of the spent nuclear fuel assembly by maintaining the spent nuclear fuel assembly in a low-oxygen environment. 14. The method of any one of clauses 1 to 13, further comprising:

[0082] 16. 14. The method of any one of clauses 1 to 13, wherein one or more operations of the method are carried out in one of an inert atmosphere or a low-oxygen atmosphere to inhibit the formation of an oxide layer on the surface of the spent nuclear fuel assembly.

[0083] 17. 1. A method of preparing spent nuclear fuel assemblies for storage, comprising: providing a quantity of zamak having a first melting point; removing the spent nuclear fuel assembly from the liquid sodium environment while controlling exposure of the spent nuclear fuel assembly to oxygen, thereby inhibiting the formation of an oxide layer on the spent nuclear fuel assembly; filling the spent nuclear fuel assembly with the zamak in a liquid state to obtain a zamak-filled spent nuclear fuel assembly, thereby dissolving at least a portion of the liquid sodium in the spent nuclear fuel assembly into the zamak and displacing remaining liquid sodium from the spent nuclear fuel assembly; cooling the zamak filled spent nuclear fuel assembly until the zamak has a temperature below the first melting point to obtain a zamak stabilized spent nuclear fuel assembly; dry storing the zamak stabilized spent nuclear fuel assemblies; A method comprising:

[0084] 18. The filling operation is placing the spent nuclear fuel assembly within a sheath made from a material having a second melting point higher than the first melting point; filling both the sheath and the spent nuclear fuel assembly with liquid zamak; 18. The method of clause 17, further comprising:

[0085] 19. 19. The method of clause 17 or 18, wherein the zamak is selected from zamak 2, KS, zamak 3, zamak 4, zamak 5, and zamak 7.

[0086] 20. 19. The method according to clause 17 or 18, wherein the zamak is zamak 3.

[0087] twenty one. 21. The method of any one of clauses 17 to 20, further comprising, after filling the sheath with liquid zamak, cooling the zamak in the sheath to a temperature below the first melting point.

[0088] twenty two. placing the spent nuclear fuel assembly within the sheath in an inert environment; filling the sheath with liquid zamak, thereby replacing the inert environment; 22. The method of any one of clauses 17 to 21, further comprising:

[0089] twenty three. 23. The method of any one of clauses 17-22, wherein the material having a second melting point higher than the first melting point is selected from 304 stainless steel, 316 stainless steel, and T91 steel.

[0090] twenty four. 24. The method of any one of clauses 17-23, further comprising draining liquid sodium from the spent nuclear fuel assembly.

[0091] twenty five. 25. The method of any one of clauses 17-24, further comprising collecting sodium displaced from the sheath and the spent nuclear fuel assembly by a plurality of said filling operations.

[0092] 26. The placement operation is capping the sheath after placing the spent nuclear fuel assembly within the sheath. 26. The method of any one of clauses 17 to 25, further comprising:

[0093] 27. 27. The method of any one of clauses 17-26, further comprising cooling the sheath after filling the sheath and the spent nuclear fuel assembly with zamak.

[0094] 28. The cooling operation is cooling the sheath from the bottom 28. The method of clause 27, further comprising:

[0095] 29. 29. The method of any one of clauses 17-28, further comprising, after filling the sheath and the spent nuclear fuel assembly with zamak, placing the sheath in a dry storage device.

[0096] 30. 29. The method of any one of clauses 17-28, further comprising, after filling the sheath and the spent nuclear fuel assembly with zamak, placing the sheath in a dry underground repository.

[0097] 31. 31. The method of any one of clauses 17 to 30, wherein the method is carried out without exposing the spent nuclear fuel assembly to oxygen.

[0098] 32. 31. The method of any one of clauses 17 to 30, wherein one or more operations of the method are performed without exposing the spent nuclear fuel assembly to oxygen.

[0099] 33. 33. The method of any one of clauses 17 to 32, wherein one or more steps of the method are carried out in an inert atmosphere.

[0100] 34. An apparatus comprising a fuel assembly filled with solid zamak.

[0101] 35. 35. The apparatus of clause 34, wherein the fuel assembly is sealed within a container.

[0102] 36. 36. The apparatus of clause 35, wherein a space between the fuel assembly and the vessel is filled with solid zamak.

[0103] 37. 1. A system for stabilizing spent nuclear fuel assemblies from a sodium-cooled nuclear reactor, comprising: Zamak storage tanks, an amount of zamak having a melting point; a sheath configured to receive and hermetically retain the spent nuclear fuel assembly, the sheath having an inlet port for receiving and injecting liquid zamak into the sheath; a heater adapted to heat the zamak to a temperature above the melting point; A system comprising:

[0104] 38. 38. The system of clause 37, further comprising a controller configured to control the flow of liquid zamak into the sheath.

[0105] 39. 38. The system of clause 37, further comprising a cooler adapted to cool the sheath and its contents below the melting point.

[0106] 40. 38. The system of claim 37, further comprising a sodium trap configured to collect liquid sodium from the sheath.

[0107] 41. 38. The system of claim 37, further comprising the sodium-cooled nuclear reactor including a sodium pool containing a plurality of fuel assemblies.

[0108] 42. 42. The system of clause 41, further comprising a fuel assembly handling system adapted to transfer a plurality of fuel assemblies from the sodium pool to the sheath.

[0109] 43. 43. The system of any one of clauses 37-42, further comprising a cover gas system adapted to maintain the spent nuclear fuel assembly in a low-oxygen environment.

[0110] 44. The system and method of any one of clauses 1-43, wherein sodium-exposed components are stabilized in place of spent nuclear fuel assemblies.

[0111] 45. 1. A method for preparing a component having a sodium-exposed surface for storage, comprising: providing a quantity of zamak having a first melting point; displacing sodium from said surface by liquid zamak so that a zamak coated surface is obtained on said component; cooling the zamak to a temperature below the first melting point to obtain a zamak-stabilized component; A method comprising:

[0112] 46. the component is a sodium-filled tube; The moving process is filling the tube with liquid zamak 46. ​​The method of clause 45, further comprising:

[0113] 47. 47. The system and method of any one of clauses 1-46, wherein the zamak is an alloy having 1-10% Al; 0-1% Cu; 0.01-1% Mg; less than 0.5% impurities (i.e., any elements other than Al, Cu, Mg, and Zn); and the balance Zn.

[0114] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and the like used in the specification and claims are to be understood as being modified in all instances by the word "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and appended claims are approximations that may vary depending upon the desired properties sought.

[0115] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the technology are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as possible, however, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0116] It will be apparent that the systems and methods described herein are well adapted to achieve the objects and advantages mentioned, as well as those inherent therein. Those skilled in the art will recognize that the methods and systems described herein may be implemented in many ways and are not limited by the exemplary embodiments and examples set forth above. In other words, functional elements may be performed by single or multiple components in various combinations of hardware and software, and individual functions may be distributed among software applications at either the client or server level. In this regard, any number of features of the various embodiments described herein may be combined into a single embodiment, and alternative embodiments are contemplated that have fewer or more than all of the features described herein.

[0117] While various embodiments have been described for purposes of this disclosure, various changes and modifications may be made that are well within the scope contemplated by this disclosure. For example, the reader will immediately recognize numerous alternative embodiments of the basic design shown in FIG. 1 . For example, the secondary heat exchanger 112 may be omitted, and a thermal storage medium may be used as the secondary coolant. Eliminating the intermediate thermal loop simplifies the structure, piping, and valves, reducing costs. Many other modifications may be made that will be readily suggested to those skilled in the art and are within the spirit (intent) of this disclosure. [Brief explanation of the drawings]

[0118] [Figure 1] 1 illustrates an embodiment of an integrated energy system having a sodium-cooled reactor. [Figure 2] FIG. 1 is an exploded view of a fuel assembly for use in a sodium-cooled reactor. [Figure 3] 1A-1D show side views of different types of fuel assemblies. [Figure 4] 2 illustrates one embodiment of a zamak stabilization system suitable for use in the integrated energy system shown in FIG. 1. [Figure 5] 10 illustrates an alternative embodiment of a sheath that includes additional features that could be implemented in any embodiment of the sheath. [Figure 6] 1 illustrates an embodiment of a method for stabilizing spent nuclear fuel assemblies from sodium-cooled reactors using zamak.

Claims

1. An apparatus comprising a fuel assembly filled with solid zamak.

2. The apparatus of claim 1 , wherein the fuel assembly is sealed within a container.

3. 3. The apparatus of claim 2, wherein an annular region between the fuel assembly and the vessel is filled with solid zamak.

4. the container includes a sheath and a cap; the sheath being a unitary container having an upper opening; The device of claim 2 , wherein the cap covers the top opening.

5. The device of claim 4 , wherein the sheath comprises a material having a melting point higher than the melting point of the zamak.

6. The apparatus of claim 2 , wherein the fuel assembly includes a plurality of fuel rods.

7. The apparatus of claim 1 further comprising a fuel assembly dip tube.

8. The apparatus of claim 3 further comprising an annular region dip tube.

9. The apparatus of claim 2 further comprising a cooler.

10. the fuel assembly is sealed within a container; the container includes a sheath having an upper opening; 10. The apparatus of claim 9, wherein the cooler is disposed at a lower end of the sheath opposite the upper opening.

11. The apparatus of claim 9 , wherein the cooler is configured to provide at least one of active and passive cooling.

12. The device of claim 4 , further comprising a handling structure on an exterior surface of the sheath.

13. 2. The device of claim 1, wherein the zamak is selected from zamak 2, KS, zamak 3, zamak 4, zamak 5, and zamak 7.

14. The apparatus of claim 3 , wherein the amount of zamak filling the fuel assembly and the annular region at least partially submerges the fuel assembly.

15. The apparatus of claim 14 , wherein the amount of zamak filling the fuel assembly and the annular region completely submerges the fuel assembly.

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