Mutually locking fuel assembly structure for core reactivity control

Abstract

The reactor core comprises a plurality of core assemblies. The plurality of core assemblies have a cooperative structure formed on one or more load pads that mechanically connects the plurality of core assemblies to each other and restricts the relative motion between the core assemblies in a kinematically determined manner. A shear key on one core assembly is configured to fit into a tab slot on an adjacent core assembly. The motion of one core assembly is transmitted to a second core assembly, and these core assemblies move together.

Description

Detailed Description of the Invention

[0001] [Government License Rights] This invention was made with government support under DOE Cooperative Agreement No. DE-NE0009054 awarded by the United States Department of Energy. The government has certain rights in this invention.

[0002] [Background] In a sodium-cooled fast reactor ("SFR"), the main reactor components are a reactor vessel filled with a liquid sodium coolant and a reactor core. In some cases, the SFR is a once-through fast reactor that uses subcritical reload fuel that is grown and burned in situ. The reactor core is immersed in a sodium pool within the reactor vessel.

[0003] The sodium coolant is used to remove heat from the core. The sodium coolant flows into the nozzles of the core assemblies (some of which may be fuel assemblies), flows around the fuel pins within the core assemblies, and removes heat therefrom, flowing through the core assemblies. The reactor vessel is surrounded by a protective vessel so that loss of the sodium coolant is prevented even in the event of a leak from the reactor vessel. Pumps circulate the primary sodium coolant between the core and an intermediate heat exchanger disposed within the pool.

[0004] The core of a fast reactor typically includes a configuration in which core assemblies are densely arranged in a hexagonal shape. The reactivity within the core increases as the core assemblies approach each other radially, and decreases as the core assemblies move apart. The reactivity within a fast reactor core can be very sensitive to relative movement between core assemblies. The reactivity feedback depends largely on how the core assemblies are supported and constrained.

[0005] Relative movement between core assemblies arises from multiple sources, such as seismic events, deflection of core assemblies due to thermal gradients, irradiation creep, and void expansion. Core assemblies are exposed to both axial and radial temperature gradients and neutron flux gradients. Initially, the temperature gradient causes deflection of the core assemblies, which can result in contact between adjacent core assemblies through contact points above the fuel region of the core. These contact forces create bending stresses in the core assemblies. Thermal creep and irradiation creep tend to relieve these stresses, thereby reducing the contact forces over time. Simultaneously, non-uniform irradiation expansion due to fast neutron flux gradients causes inelastic deflection, which can increase the contact forces over time.

[0006] Relative movement between core assemblies can lead to inter-assembly interactions and variations in overall core reactivity. Complex inter-assembly interactions can result in reactivity insertion during startup and steady-state operation, as well as displacements that lead to off-normal states, ultimately affecting reactor stability.

[0007] Reducing inter-assembly movement would be advantageous in order to efficiently exchange and shuffle core assemblies while making core reactivity more predictable. These advantages and benefits, as well as other advantages and benefits, will be evident to those skilled in the art from the drawings and descriptions provided below.

[0008] 〔overview〕 According to some embodiments, a hexagonal load pad for a reactor core assembly includes a key extending from a first surface of the hexagonal load pad, the key having a pair of key sidewalls extending away from the first surface and a key surface substantially parallel to the first surface, and a slot formed on a second surface of the hexagonal load pad, the slot having a pair of slot sidewalls configured to cooperate with the key so as to transmit motion from a first reactor core assembly to a second reactor core assembly.

[0009] In some cases, the pair of key sidewalls extend at an obtuse angle away from the first surface. For example, the cross-section of the key as viewed from above may be trapezoidal or similar to that of a trapezoidal key. However, in some cases, the cross-section of the key as viewed from above may be rectangular or similar to that of a square.

[0010] In some embodiments, the cross-section of the load pad is hexagonal, and the load pad further includes three keys and three slots formed on alternating faces of the load pad. The second hexagonal load pad may also have one or more keys and slots, and one of the slots of the second hexagon is configured to engage with the key extending from the first face. In other words, in some cases, the key is sized and shaped to fit into the slot.

[0011] The load pad may be an upper core load pad positioned on the core assembly duct at a location above the reactor core, and additionally or alternatively, it may be a top load pad located near the upper end of the core assembly duct.

[0012] According to some embodiments, a method for restricting relative movement between fuel assemblies within the core of a reactor includes the steps of: forming a plurality of core assemblies having a polyhedral cross-section and having one or both of a top load pad and a core-upper load pad; forming a protruding key on a first surface of each of the plurality of core assemblies; forming a groove configured to receive the key on a second surface of each of the plurality of core assemblies; and positioning two or more of the plurality of core assemblies adjacent to each other such that the protruding key from the first core assembly fits into the groove on the second core assembly. The key and the groove may restrict relative horizontal movement while allowing relative vertical movement between adjacent core assemblies.

[0013] In some embodiments, the step of forming a plurality of core assemblies further includes the step of forming the protruding keys and grooves on alternating surfaces of the plurality of core assemblies, for example, the plurality of core assemblies having six surfaces, and the plurality of core assemblies being formed to have three keys and three grooves.

[0014] In some embodiments, the protruding key is formed by additive manufacturing (e.g., welding, printing, extrusion, bonding, or any other suitable process for adding material to the surface of the core assembly).

[0015] In some embodiments, the linear motion of the first core assembly causes the corresponding linear motion of one or more adjacent core assemblies. In other words, the motion of one core assembly transmits a load to surrounding core assemblies, resulting in similar motion of those surrounding core assemblies. In this way, the multiple core assemblies can be locked together to move together, while still allowing the multiple assemblies to be removed from the core and inserted into the core individually, as desired.

[0016] According to some implementations, the reactor core restraint system includes a first core assembly having a first mechanical key, which is formed as an elongated structure having a longitudinal axis and having a radius, and a second core assembly having a second mechanical key, which is configured such that the second mechanical key engages with the first mechanical key so as to suppress relative motion between the first core assembly and the second core assembly, wherein radial movement of the first core assembly causes radial movement of the second core assembly.

[0017] The reactor core restraint system may be configured such that the first and second mechanical keys do not restrict the relative vertical movement of the first and second core assemblies. That is, the core assemblies can be freely pulled out or inserted vertically along their longitudinal axis.

[0018] In some embodiments, the first mechanical key is a projection, and the second mechanical key is a slot configured to capture the projection. Of course, as described in multiple embodiments throughout this disclosure, the first and second mechanical keys can also have other geometric structures.

[0019] [Brief explanation of the drawing] The accompanying drawings are part of this disclosure and are incorporated herein by reference. The drawings illustrate examples of embodiments of this disclosure and, together with this specification and the claims, help to illustrate, at least partially, various principles, features, or aspects of this disclosure. Specific embodiments of this disclosure are described more fully below with reference to the accompanying drawings. However, various aspects of this disclosure can be implemented in many different forms and should not be construed as being limited to the implementations described herein. The same numbers indicate similar elements throughout, but are not necessarily identical or identical elements.

[0020] The following drawings, which constitute part of this application, are illustrative of the described technology and are not intended to limit in any way the scope of the claimed technology, which shall be based on the claims attached to this application.

[0021] Figure 1 shows some of the basic components of a sodium-cooled fast reactor in the form of a block diagram, according to one embodiment.

[0022] Figure 2 is a schematic cross-sectional view of the core of a sodium-cooled fast reactor according to some embodiments.

[0023] Figure 3 is a top cross-sectional view of the reactor core of a nuclear fission reactor according to one embodiment.

[0024] Figure 4 is an enlarged cross-sectional view of a fuel assembly and core support structure according to some embodiments.

[0025] FIG. 5 is a perspective view of a core assembly duct according to some embodiments.

[0026] FIGS. 6A and 6B are diagrams of a core support structure and a deformation of a predicted core assembly according to some embodiments, respectively.

[0027] FIG. 7 is a block diagram showing the kinematics (nature or characteristics of motion) of a core assembly according to some embodiments.

[0028] FIG. 8 shows a load pad of a core assembly having shear keys and slot tabs according to some embodiments.

[0029] FIG. 9 shows an interface between a shear key and a shear tab of adjacent core assemblies according to some embodiments.

[0030] FIG. 10 shows load pads of a plurality of core assemblies positioned adjacent to each other, for example, within a core, according to some embodiments.

[0031] FIG. 11 is a block diagram showing the kinematics of a core assembly when a shear key and a slot tab are integrated according to some embodiments.

[0032] FIGS. 12A and 12B show alternative geometric configurations for shear keys and shear tabs according to some embodiments, respectively.

[0033] FIG. 13 shows load pads of a plurality of core assemblies positioned adjacent to each other, for example, within a core, according to some embodiments.

[0034] FIG. 14 is a plan view showing the geometric configuration of a shear key and a slot tab according to some embodiments.

[0035] FIG. 15 is a plan view showing a geometric configuration for suppressing relative movement between adjacent fuel assemblies according to some embodiments.

[0036] Figure 16 is a plan view showing a geometric configuration for suppressing relative motion between adjacent fuel assemblies, according to some embodiments.

[0037] [Detailed explanation] This disclosure describes exemplary embodiments and is therefore not intended to limit in any way the embodiments and the scope of the appended claims. Embodiments have been described above with the help of functional configuration blocks illustrating the implementation of specified functions and their relationships. The boundaries of these functional configuration blocks are arbitrarily defined herein for the convenience of explanation. Alternative boundaries may be defined insofar as the specified functions and their relationships are adequately performed.

[0038] Figure 1 shows some of the basic components of a sodium-cooled fast reactor (SFR) 100 in block diagram form. While SFRs may be used throughout this specification as an exemplary type of reactor technology, it should be understood that the concepts presented herein may be equally applicable to other types of reactors. In some cases, the concepts presented in the following description are directly applicable to other forms of sodium-cooled fast reactors (SFRs) (e.g., traveling wave reactors, modular reactors, microreactors, etc.). Furthermore, the concepts presented herein may be applicable to other reactor types, such as thermal neutron reactors, water-cooled reactors, chloride-cooled reactors, or gas-cooled reactors, as well as alternative fuel reactors, and the scope of this disclosure and the accompanying claims should not be limited to any particular reactor, fuel source, coolant type, or reactor architecture.

[0039] Generally, an SFR fission plant 100 includes a core 102 containing multiple fuel assemblies (not shown). The core 102 is located within a pool 104 that holds a certain amount of liquid sodium coolant 106. Pool 104 is referred to as the high-temperature pool and has a higher sodium temperature than the surrounding low-temperature pool 108, which also contains liquid sodium coolant 106 (this is due to the energy generated by the fuel assemblies in the core 102). The high-temperature pool 104 is separated from the low-temperature pool 108 by a redan 110. The headspace 112 above the liquid surface of the sodium coolant 106 is filled with an inert cover gas (e.g., argon). The reactor vessel 114 surrounds the core 102, the high-temperature pool 104, and the low-temperature pool 108 and is sealed by a reactor head 116. The reactor head 116 provides various access points to the interior of the reactor vessel 114.

[0040] The size of the reactor core 102 is selected based on many factors, including fuel characteristics, desired power output, and available space for the reactor 100. Various embodiments of the SFR fission plant may be used for low-power (approximately 300 MWe to approximately 500 MWe), medium-power (approximately 500 MWe to approximately 1000 MWe), and high-power (approximately 1000 MWe and above) applications, as required or desired. The performance of the reactor 100 may be improved by providing one or more reflectors (not shown) around the reactor core 102 so that neutrons are bounced back into the reactor core 102. Furthermore, the fertile nuclear assembly and the fissile nuclear assembly may be moved (or "shuffled") within and around the reactor core 102 to control the nuclear reactions that occur therein.

[0041] The sodium coolant 106 is circulated within the vessel 114 via the primary sodium coolant pump 118. The primary coolant pump 118 draws the sodium coolant 106 from the cryogenic pool 108 and injects it into the plenum below the core 102. The coolant 106 is forced upward through the core and heated by the reactions occurring within the core 102. The heated coolant 106 enters the intermediate heat exchanger 120 from the high-temperature pool 104, exits the intermediate heat exchanger 120 and enters the cryogenic pool 108 again. In this way, the sodium coolant 106 circulates throughout the entire reactor vessel 114 via this primary coolant loop 122.

[0042] The intermediate heat exchanger 120 incorporates a segment of a closed liquid sodium loop. This segment of the closed liquid sodium loop may always be physically separated from the primary sodium pool 104 and the primary sodium pool 108 (i.e., the intermediate sodium and primary sodium never mix). The intermediate heat exchanger 120 transfers heat from the primary coolant loop 122 (which is fully contained within the vessel 114) to the intermediate coolant loop 124 (which is only partially contained within the vessel 114). The intermediate heat exchanger 120 passes through a redan 110, which bridges the high-temperature pool 104 and the low-temperature pool 108 (as a result, sodium 106 in the primary coolant loop 122 can flow between the high-temperature pool 104 and the low-temperature pool 108). In one example, four intermediate heat exchangers 120 are distributed within the vessel 114. Alternatively, two or six intermediate heat exchangers 120 are distributed within the vessel 114. Of course, any appropriate number of heat exchangers 120 may be placed inside the container 114.

[0043] The intercoolant loop 124 circulates the sodium coolant 126. The sodium coolant 126 enters and leaves the vessel 114 through piping via the reactor head 116. An intermediate sodium pump 128 located outside the reactor vessel 114 circulates the sodium coolant 126 to the power generation system 123. In the intermediate heat exchanger 120, heat is transferred from the sodium coolant 106 of the primary coolant loop 122 to the sodium coolant 126 of the intercoolant loop 124. The sodium coolant 126 of the intercoolant loop 124 passes through several tubes 130 within the intermediate heat exchanger 120. These tubes 130 transfer thermal energy between them while keeping the sodium coolant 106 of the primary coolant loop 122 separate from the sodium coolant 126 of the intercoolant loop 124.

[0044] The direct heat exchanger 132 extends into the high-temperature pool 104. The direct heat exchanger 132 normally provides cooling to the sodium coolant 106 in the primary coolant loop 122, but only in emergencies. The direct heat exchanger 132 is configured to allow the sodium coolant 106 to move in and out of the heat exchanger 132 from the high-temperature pool 104. The direct heat exchanger 132 has a structure similar to that of the intermediate heat exchanger 120. A tube 134 keeps the NaK (sodium-potassium) of the primary coolant loop 122 separated from the direct heat exchanger coolant (NaK) 136 of the direct reactor coolant loop 138, while thermal energy is transferred between them.

[0045] Other auxiliary reactor components (both inside and outside the reactor vessel 114) include, but are not limited to, pumps, check valves, shut-off valves, flanges, drain tanks, etc. (these are not shown, but will be obvious to those skilled in the art). Further penetrations through the reactor head 116 (e.g., ports for the primary coolant pump 118, inert cover gas inspection ports, sodium processing area, and cover gas ports, etc.) are not shown. A control system 140 may be used to control and monitor the various components and systems that make up the reactor 100.

[0046] Broadly speaking, this disclosure describes configurations that improve the performance of the reactor 100 shown in Figure 1. Specifically, embodiments, configurations, and arrangements of core assembly supports provide a more consistent and stable reactivity by reducing or eliminating relative movement between core assemblies. That is, the embodiments described herein are configured to limit relative horizontal movement between adjacent core assemblies while allowing individual core assemblies to be inserted into or removed from the reactor core through vertical displacement.

[0047] Figure 2 is a schematic cross-sectional view of the core 200 of the SFR. The core 200 is schematically shown and includes a central core region 202 having a plurality of core assemblies 204. The core assemblies 204 may include fissile fuel assemblies, parent fuel assemblies, shielding assemblies, reflector assemblies, control assemblies, and standby shutdown assemblies, or material test assemblies. Generally, a particular assembly is identified by its contents (e.g., fissile material, control material, etc.). The components of an assembly holding such material may be identical and interchangeable at multiple locations within the reactor core. The peripheral core region 206 may include in-vessel storage pods 208. Throughout the lifetime of the core 200, fissile fuel assemblies and parent fuel assemblies (as well as other specific assemblies) may be shuffled between the central core region 202 and the peripheral core region 206. This is performed as required or desired at various stages of the core life to initiate, maintain, accelerate, or terminate a nuclear reaction or power generation, and / or for safety reasons.

[0048] The assembly 204 is received by the upper plate 210 of the core support structure 212 in an aligned position with the masking element 216. The sodium coolant is pumped into the plenum 214 located below the upper plate 210, flows upward into the core assembly 204, where it is heated by the nuclear reaction occurring in the core 200.

[0049] Figures 3 and 4 show a fission reactor core 200, including multiple nuclear fuel assemblies (e.g., fissile fuel assembly 302, parent fuel assembly 304, mobile reactivity control assembly 306, etc.), illustrated as core assembly 204. As used throughout this specification, the terms core assembly and fuel assembly may be used interchangeably to refer to any assembly that is located within or can be located within the core, regardless of the type of fuel, presence or absence of reactivity control material, etc. Similarly, the terms “core assembly duct” and “fuel assembly duct” may be used interchangeably to describe ducts that may be used with a fuel assembly or core assembly. In many cases, fuel assembly ducts and core assembly ducts are identical in size and shape and can be inserted into any suitable location within the core.

[0050] In some embodiments, the fuel assembly 204 may be partially supported by a core support grid plate 210. The core support grid plate 210 may engage with the nozzles of the fuel assembly 204 to provide support to the lower end of the fuel assembly 204. According to some embodiments, a primary sodium coolant flows through the fuel assembly 204 and as a result absorbs the heat generated by the fuel in the fuel assembly undergoing a nuclear fission reaction.

[0051] In some embodiments, the fuel assembly 204 includes a plurality of nuclear fuel pins (e.g., fuel rods, fuel elements, etc.) arranged within a duct containing a tubular body. In some cases, the tubular body has a hexagonal cross-sectional shape, as shown in Figures 3 and 4. During use, the primary sodium coolant flows upward into the fuel assembly 204, flows around the fuel elements within the fuel assembly, extracts heat from the fuel assembly, and carries it to a heat exchanger.

[0052] According to some embodiments, the fuel assembly 204 is cantilevered in that it is fixed to an opening in the grid plate 210 by a nozzle 402 that forms part of the fuel assembly 204. The remaining length of the fuel assembly may be largely unsupported. However, in some cases, the core upper ring and / or top ring may provide an opening that provides lateral support to the fuel assembly once it has deformed by a distance sufficient to contact the upper support plate. In other words, the fuel assembly may not be constrained at a position above the nozzle so as to allow deformation.

[0053] Figure 5 shows an example of a duct 502 that may be used with respect to a fuel assembly 204. In some embodiments, the duct 502 is a hollow tube whose cross-section may be hexagonal. As shown in Figure 3, the hexagonal cross-section allows for the filling of multiple ducts into the core using efficient filling techniques such as hexagonal arrangement. The duct 502 may be filled with bundles of fissile fuel elements or parent fuel elements, neutron reflectors, neutron absorbers, or other materials. The duct 502 has a lower end 504 that can be coupled to a nozzle and an upper end 506. An above-core load pad 508 and / or a top load pad 510 may be positioned along the length of the duct. The above-core load pad 508 may be positioned adjacent to an above-core load pad (ACLP) ring in the core. The ACLP ring defines the lateral boundary around the fuel assembly in the core. Similarly, the top load pad 510 may be positioned along the duct 502 so as to be adjacent to the top load pad (TLP) ring in the core. The TLP ring defines a second lateral boundary around the fuel assemblies in the core. In some cases, the ACLP ring and / or TLP ring may be positioned around the core and fuel assembly configuration so as to provide annular restraint to the bundle of fuel assemblies, but not necessarily to restrain each fuel assembly individually. During use, as the fuel assemblies begin to deform, for example by deflection, the outermost fuel assemblies may eventually come into contact with one or both of the ACLP rings or TLP rings.

[0054] Figures 6A and 6B illustrate the mechanical core design elements and hypothetical deformations. The deformation of the core assemblies is driven by various phenomena that result in inter-assembly interactions (e.g., contact between adjacent assemblies) due to the design of the gaps between adjacent ACLP508 and TLP510. Relative movement of fuel assemblies 204 is largely driven by thermal and bundle gradients, inter-assembly contact, contact with core support structures, and seismic excitation. Furthermore, fuel assemblies 204 undergo inelastic deformation due to thermal creep, irradiation creep, and void expansion during their lifetime. As a result of the complex interactions between assemblies, displacements can occur that lead to reactivity insertion during startup, steady-state operation, and transient operation, thereby affecting reactor stability.

[0055] To achieve a Limited Free Bow (LFB) configuration designed to limit reactivity insertion (e.g., changes in reactivity due to relative movement between fuel assemblies), many SFRs rely on ACLP508 and TLP510. Figures 6A and 6B show a series of hexagonal fuel assemblies within the reactor core. The bottom of the fuel assemblies 204 are constrained at the nozzle. Above the core region is a portion of the fuel duct with ACLP508 located at ACLP height. The ACLP is configured to maintain the ACLP gap 602 between adjacent fuel assemblies as adjacent fuel assemblies begin to deform, providing a preferential contact point between the fuel assemblies 204.

[0056] The illustrated core restraint system further includes ACLP ring 604 and TLP ring 606. The core restraint system is configured to serve several important functions: to control the radial positioning of the core and maintain alignment between core components; to limit the motion of fuel assemblies during earthquakes; and to limit the deflection of fuel assemblies so that negative reactivity feedback occurs during overpower transient events.

[0057] As a result of creep strain caused by heat and irradiation, fuel assemblies initially deform into an arc shape and can eventually undergo secondary and tertiary deformations. This inelastic deflection creates residual contact forces between fuel assemblies, and due to frictional effects and additional loads, difficulties can arise during fuel replacement. In a typical shuffle or fuel replacement operation, individual core assemblies can be removed vertically from the core and replaced, rearranged, or reinserted. Therefore, core assemblies need to be able to move vertically, even though they are constrained radially (e.g., horizontally).

[0058] During power increase and power decrease events, core temperature changes tend to cause non-uniform thermal gradients throughout the core in both axial and radial directions. As further shown in Figure 6B, these thermal gradients can result in bending effects on the fuel assemblies. The core restraint system provides protection against over-power events by utilizing and limiting thermally induced bending of the fuel assemblies. As shown in (i), a row of three fuel assemblies is arranged radially away from the center of the reactor core. As shown in (ii), as the thermal gradient increases and the temperature becomes higher closer to the center of the reactor core due to the thermal gradient, the fuel assemblies begin to bend outward away from the center of the reactor core, thereby reducing their reactivity. As shown in (iii), once the outermost fuel assembly comes into contact with the TLP ring 606, the temperature gradient increases, causing the center of fuel assembly 204 to bend inward, thereby increasing its reactivity. As the temperature gradient increases due to the increased reactivity, the fuel assemblies continue to flex inward until they come into contact with adjacent fuel assemblies in each ACLP508. Once the fuel assemblies 204 come into contact with each other, no further compression can occur, and the reactor is considered locked up. While an exemplary core-restrained system is shown, the description herein is equally applicable to other core-restrained systems. For example, the solutions for relative motion between core assemblies described herein can be applied to any type of core-restrained system for any type of reactor.

[0059] As further illustrated in Figure 7, which shows the kinematics of a conventional core-constrained system, a series of hexagonal fuel assemblies 204 are shown arranged within the core in a densely packed hexagonal configuration. Note that load pads are shown in the figure, and each of these load pads is associated with a core assembly. Therefore, the motion of the load pads reflects the motion of the core assemblies. In some cases, recessed surfaces are provided on each of the six faces of the ACLP and TLP to provide preferential contact points between adjacent fuel assemblies and to maintain appropriate gaps between them. Contact between adjacent fuel assemblies occurs at the ACLP and / or TLP (collectively, the “load pads”), and this contact imparts a force normal to the surface. For example, when the middle fuel assembly 704 moves in the direction indicated by arrow 706, the gap between adjacent fuel assemblies closes, and contact occurs at the faces of the ACLP and TLP. As a result, the adjacent assemblies move normal to the contact interface, as indicated by the small arrow 708. Fuel assembly 204, marked with a circle, remains stationary, and the overall gap between the stationary fuel assemblies and the moving fuel assemblies increases. The moving fuel assemblies close contact across multiple rows, resulting in compression, which detaches the remaining fuel assemblies located in the opposite direction of their movement, increasing the gap and reducing their reactivity.

[0060] Figure 8 shows an exemplary embodiment illustrating a hexagonal shear key that may be formed within the plane of one or both of the ACLP508 or TLP load pads. For efficiency, the figure shows a perspective view of a single load pad. However, it should be understood that the illustrated load pads form part of a core assembly, and in many cases, there are two load pads associated with each core assembly. The hexagonal shear key may include shear keys 802 and cooperative slot tabs 804 formed within the alternating planes of the ACLP. In embodiments utilizing hexagonal fuel ducts, the planes of the ACLP508 and TLP510 may include three shear keys 802 and three slot tabs 804. Referring further to Figure 9, the interface between load pads associated with adjacent fuel assemblies 204 is shown. The fuel assemblies are positioned within the core such that a shear key 802 on the first fuel assembly 204(1) contacts and fits into a slot tab 804 on the adjacent fuel assembly 204(2). In some embodiments, a single tab on every other surface of the TLP and ACLP is a shear key 802, and double tabs on every other surface of the TLP and ACLP form a slot tab 804. The interface between adjacent fuel assemblies may have an interface tolerance designed such that the width of the shear key 802 relative to the slot tab 804 is much smaller than the distance between adjacent fuel assemblies 204. Load transfer between adjacent fuel assemblies is tangential to each surface of the fuel assembly. This is in contrast to the applied force being normal to the surface of the fuel assembly (as shown in Figure 7). As a result, when the first fuel assembly 204(1) moves in a given direction, the interface with the adjacent fuel assembly causes the adjacent fuel assembly to move in the same direction as the first fuel assembly 204(1). The moving fuel assembly is configured to push or pull the adjacent fuel assembly, causing it to move in the same direction.

[0061] Figure 10 shows multiple load pads associated with a fuel assembly 204, each with a shear key positioned to engage with a cooperating slot tab of an adjacent fuel assembly. As can be seen, in the hexagonal fuel duct, three faces are configured to have shear keys 802, each of which engages with three slot tabs 804 of an adjacent fuel duct. In this way, the fuel ducts (and fuel assemblies) are mechanically key-coupled to one another. In some cases, the fuel assemblies are mechanically coupled to one another such that the relative motion of a first fuel duct facilitates the transmission of forces to the adjacent fuel duct tangentially to the interface between the adjacent fuel assemblies. In other words, if a first fuel duct has a first face that interacts with a second fuel duct having a second face, and the second and first faces are parallel, then the force transmitted from the first fuel duct to the second fuel duct is parallel to the first and second faces. Force transmission may be achieved through any suitable structure. In some cases, this structure is a mechanical key structure on one or both of the first and second fuel ducts, for example, in a load pad. The mechanical key structure on the first surface may be different from the mechanical key structure on the second surface. For example, the mechanical key structure on the first surface may be a projection, boss, ridge, key, spline, tooth, gear, or any other structure, and the mechanical key structure on the second surface may be a pocket, slot, hole, ridge, projection, groove, gear, or any other structure configured to cooperate with the mechanical key structure on the first surface.

[0062] In some cases, the fuel assembly is formed to have a first mechanical key structure and a second mechanical key structure, which are formed on alternating faces of a multifaceted fuel duct. In some embodiments, the fuel duct has a hexagonal cross-section, with three faces containing the first mechanical key structure and the other three faces containing the second mechanical key structure. The cooperating mechanical key structures allow load transfer from the first fuel duct to the second fuel duct to be tangential to the faces of the fuel assembly.

[0063] Figure 11 shows multiple fuel assemblies with a cooperating mechanical key structure, and further illustrates the kinematics of load transfer and the motion of the fuel assemblies. As shown in the figure, the fuel assemblies are key-coupled to one another, and their relative motion is constrained. When the central fuel assembly 204(1) moves in the direction indicated by arrow 1102, the gap in the shear key 802 closes, and the force acting on the adjacent fuel assemblies develops tangentially to the surface of the fuel assemblies, as indicated by arrow 1104. Due to the mutual locking (interlocking) of the shear key and slot tab, the motion of the entire core is linked to one another. In other words, according to some embodiments, no fuel assembly in the core can move independently. In some cases, the constrained kinematics compel all fuel assemblies to move in the same direction as the drive assembly, as indicated by arrow 1106. According to some embodiments, this configuration provides a mechanically locked core regardless of the nature of the motion (e.g., thermal, seismic, expansion, etc.). Therefore, the motion of one fuel assembly results in the motion of all fuel assemblies, not just those directly adjacent to the moving assembly. The maximum displacement between any two assemblies in the core is limited by the gap between the shear key and the slot tab. This can be much smaller than in conventional designs. In some cases, the shear key and slot tab may engage with each other, like teeth on meshing gears. For example, the rotation of the first fuel assembly tends to cause the second fuel assembly to rotate in the opposite direction. However, when multiple fuel assemblies are all engaged with each other, the rotational force is hindered, and linear motion becomes more likely instead. In effect, these multiple fuel assemblies behave as if they were locked together and mechanically connected, allowing them to move linearly. However, relative vertical movement is largely unrestricted, and a single fuel assembly can be removed vertically from the core or inserted vertically into the core.

[0064] The inventors in this application demonstrated these techniques through advanced dynamic modeling and simulation, observing that the fuel assemblies behave as if they were locked together, minimizing relative motion.

[0065] According to some embodiments, the shear keys and slot tabs are configured to restrict, or at least suppress, the relative horizontal motion while allowing the relative vertical motion to facilitate the loading and unloading of fuel assemblies into the core. Where used herein, suppression of relative horizontal motion is used to mean that not all relative motion is necessarily eliminated. For example, a small amount of relative motion may exist before the mechanical key structure of one fuel assembly engages with the mechanical key structure of an adjacent assembly. Therefore, tolerance between cooperative structures allows for some relative movement. However, relative movement between adjacent fuel assemblies is significantly reduced compared to conventional configurations, and as described, all fuel assemblies in the core behave as if they were locked together to move together. This significantly reduces relative motion between adjacent assemblies. As a result, each fuel assembly maintains a certain distance from adjacent assemblies even during motion caused by thermal gradients, seismic events, or expansion.

[0066] The specific geometric shapes of the shear key and slot tab can be modified indefinitely while still producing the advantages described herein. Figures 12A, 12B, and 13 show several other such variations of the shear key 1202 and slot tab 1204 according to some embodiments.

[0067] In some cases, the upper edge 1208 of the shear key 1202 and / or slot tab 1204 may be chamfered to facilitate the entry of the shear key 1202 into the slot tab 1204. Similarly, the lower edge 1210 of the shear key 1202 may also be chamfered to facilitate the fitting of the shear key 1202 into the slot tab 1204. The shear key 1202 has a pair of sidewalls 1212, which extend away from the fuel assembly by a distance that determines how far the shear key extends away from the fuel assembly. In some cases, the sidewalls 1212 are perpendicular and parallel to each other. Similarly, the slot tab 1204 may have a corresponding sidewall 1214 which is also perpendicular, and which cooperates with the shear key 1202 to transmit motion in a direction perpendicular to the fuel assembly surface 1214.

[0068] Figure 14 is a plan view of another geometric configuration of the shear key 1402 and slot tab 1404. In some cases, the shear key 1402 may have a side wall 1406 that forms an angle with respect to the ACLP and / or TLP plane. As shown, the shear key 1402 may have a first surface having a first dimension and contacting the fuel assembly 204, and a second surface on the opposite side having a second dimension and being a certain distance away from the fuel assembly. The second dimension is smaller than the first dimension. As shown, the shear key 1402 may be narrower in width as it extends away from the fuel assembly 204. Similarly, the slot tab 1404 may have a shape that cooperates to receive the shear key 1402, in which case the slot tab 1404 may be wider as it extends radially away from the fuel assembly 204. In this configuration, the side walls 1406 and 1408 of the shear key 1402 and slot tab 1404 may be substantially vertical when the fuel assembly is inside the reactor core. The verticality of the side walls 1406 and 1408 facilitates the insertion or removal of one or more fuel assemblies 204 in a vertical (longitudinal, vertical) direction within the reactor core.

[0069] In some cases, the clearance between the shear key sidewall 1406 and the tab slot sidewall 1408 is smaller than the clearance between opposing surfaces of multiple fuel assemblies 204. In some cases, this allows contact between adjacent fuel assemblies 204 to be located at the sidewalls of the shear key 1402 and tab slot 1404, resulting in load transfer from one fuel assembly to an adjacent fuel assembly being normal to the surfaces of the shear key sidewall 1406 and slot tab sidewall 1408. Thus, the motion of a moving fuel assembly pulls adjacent fuel assemblies in the same direction as the moving fuel assembly. This concept is further illustrated in Figure 11, and it holds similarly for other shear key and slot tab geometric shapes.

[0070] The surfaces of the shear key sidewalls 1406 and slot tab sidewalls 1408 of adjacent fuel assemblies 204 may be parallel to each other, and surface contact between the shear key sidewalls 1406 and slot tab sidewalls 1408 may be facilitated when one or both of the fuel assemblies 204 move. The contact between the shear key sidewalls 1406 and slot tab sidewalls 1408 of adjacent fuel assemblies may transmit a load from the shear key 1402 to the slot tab 1404, or from the slot tab 1404 to the shear key 1402. The load vector 1410 may have a direction that forms an acute angle α1414 with respect to the surface 1412 of the fuel assembly 204. As a result, the load transmission from one fuel assembly to an adjacent fuel assembly may not have a tendency to push the adjacent fuel assembly away from the moving fuel assembly; rather, the load transmission vector will pull the adjacent fuel assembly in the direction of movement of the moving fuel assembly.

[0071] Figure 15 shows another embodiment of a method for suppressing relative motion between adjacent fuel assemblies 204 while enabling vertical removal and vertical insertion of fuel assemblies into the reactor core. As shown, the load pad 1502 may be configured such that each of its six faces is identical, and each of the six faces may have a step 1504 formed thereon. The step 1504 may define a radially extending surface 1506 that interferes with the step 1504 on adjacent fuel assembly load pads 1502. In some cases, the load pad 1502 has a substantially hexagonal cross-section, and its six faces are formed identically so as to suppress relative movement between adjacent fuel assemblies.

[0072] As with any embodiment described herein, the cooperative structure may be formed on the load pad, for example, by a material removal process. In some cases, the load pad may be machined to form the cooperative structure described herein. In some cases, the load pad may undergo additive manufacturing to add surface features for forming the cooperative structure. For example, additive manufacturing may include welding, bonding, printing, swaging, or other processes for adding material to the load pad. In some cases, the cooperative structure may be formed when manufacturing the load pad, for example by extrusion, molding, casting, etc. According to any embodiment described herein, the same structural components may be used in the cooperative structure, but the number of structural components may be changed. For example, a single shear key may be added to the surface of the load pad, and the cooperative load pad may have two shear keys spaced apart on the surface of the load pad such that a tab slot is formed into which a single shear key is fitted.

[0073] Figure 16 shows a plan view of a plurality of load pads and illustrates another embodiment for suppressing relative motion between adjacent fuel assemblies. As shown, a load pad 1502 may be configured to have keys 1602 attached to one or more of the corners of the load pad 1502. In some cases, the keys 1602 are attached to three corners of a hexagonal load pad 1502. As shown, the keys 1602 may be attached to every other corner of the load pad so that the load pad 1502 can carry three keys 1602. The keys 1602 may have a shape having three legs, two of which 1604 and 1606 are attached to the load pad, and the third leg 1608 extends radially away from the load pad 1502. These keys 1602 provide spacers between the load pad and adjacent load pads. Furthermore, the keys 1602, due to their geometric shape and direction of extension, may transmit loads caused by the motion of one fuel assembly to adjacent fuel assemblies, which in turn pull the adjacent fuel assemblies in the same direction as the motion of the moving fuel assembly. Thus, when fuel assemblies are subjected to motion, they may behave as if they are locked together, but individual fuel assemblies are free to move vertically, for example, for insertion into or removal from the core.

[0074] The foregoing description relating to specific embodiments sufficiently reveals the general nature of the embodiments of the Disclosure so that others can readily modify and / or adapt such specific embodiments for various uses without excessive experimentation and without departing from the general concept of the embodiments of the Disclosure by applying the knowledge of those skilled in the art. Such adaptations and modifications are therefore intended to be within the spirit and equivalents of the disclosed embodiments, based on the teachings and guidance presented herein. The terms and phrases herein are for illustrative purposes only and not intended to be limiting, so as to be interpreted by those ordinary in the art in the relevant field in light of the teachings and guidance presented herein.

[0075] The breadth and scope of the embodiments of this disclosure should not be limited by any of the exemplary embodiments described above, but should be defined solely in accordance with the following claims and their equivalents.

[0076] Conditional language, particularly "can," "maybe," "might," or "might," is generally intended to convey that a particular implementation may include certain features, elements, and / or behaviors, but other implementations may not, unless otherwise specifically stated or understood within the context in which they are used. Therefore, such conditional expressions are generally not intended to imply that features, elements, and / or operations are required in some way by one or more implementations, or that one or more implementations necessarily include logic for determining, with or without user input or prompting, whether these features, elements, and / or operations are included in or performed by any particular implementation.

[0077] Unless otherwise specified, the terms “connected to” and “joined to” (and their derivatives) as used herein shall be interpreted to allow both direct and indirect (i.e., through other elements or components) connections. Furthermore, the terms “a” or “an” as used herein shall be interpreted to mean “at least one.” Finally, for ease of use, the terms “contain” and “have” (and their derivatives) as used herein shall be interpreted to mean “contain” and “have.”

[0078] This specification and accompanying drawings disclose examples of systems, apparatus, devices, and techniques that can provide control and optimization of coolant flow through the core assembly. Of course, it is impossible to describe all possible combinations of elements and / or methods for the purpose of illustrating the various features of this disclosure, but those skilled in the art will recognize that many further combinations and permutations of the disclosed features are possible. Thus, various modifications can be made to this disclosure without departing from its scope or spirit. Furthermore, other embodiments of this disclosure may become apparent by considering this specification and accompanying drawings, as well as the practices of the disclosed embodiments presented herein. The embodiments presented herein and in accompanying drawings should be considered in all respects as illustrative and not restrictive. Certain terms are used herein, but they are used in a general and descriptive sense only and not for limiting purposes.

[0079] From the foregoing, it will be understood that although specific embodiments have been described herein for illustrative purposes, various modifications can be made without departing from the spirit and scope of the appended claims and the elements described herein. Furthermore, although specific embodiments are shown below in the form of specific claims, the inventors intend various embodiments in the form of any available claims. For example, in some cases, only some embodiments are described as being embodied in a particular configuration, but other embodiments may be embodied in the same way. Various modifications and changes can be made as will be obvious to those skilled in the art who are interested in this disclosure. It is intended to encompass all such modifications and changes, and therefore the above description should be considered illustrative rather than restrictive. [Brief explanation of the drawing]

[0080] [Figure 1] Some of the basic components of a sodium-cooled fast reactor, according to certain embodiments, are shown in the form of a block diagram. [Figure 2]This is a schematic cross-sectional view of the core of a sodium-cooled fast reactor according to one embodiment. [Figure 3] This is a top cross-sectional view of the reactor core of a nuclear fission reactor according to one embodiment. [Figure 4] This is an enlarged cross-sectional view of a fuel assembly and core support structure according to some embodiments. [Figure 5] This is a perspective view of a core assembly duct according to one embodiment. [Figure 6A] This figure shows the deformation of the core support structure and the expected core assembly according to some embodiments. [Figure 6B] This figure shows the deformation of the core support structure and the expected core assembly according to some embodiments. [Figure 7] This is a block diagram showing the kinematics (nature or characteristics of motion) of a core assembly according to some embodiments. [Figure 8] A load pad for a core assembly having a shear key and slot tabs, according to one embodiment, is shown. [Figure 9] This shows the interface between the shear key and shear tab of adjacent core assemblies in some embodiments. [Figure 10] This shows load pads of multiple core assemblies positioned adjacent to each other, for example, within the core, according to some embodiments. [Figure 11] This block diagram shows the kinematics of a core assembly when the shear key and slot tab are integrated, according to some embodiments. [Figure 12A] Alternative geometric configurations for the shear key and shear tab in some embodiments are shown. [Figure 12B] Alternative geometric configurations for the shear key and shear tab in some embodiments are shown. [Figure 13] This shows load pads of multiple core assemblies positioned adjacent to each other, for example, within the core, according to some embodiments. [Figure 14] This is a plan view showing the geometric configuration of a shear key and slot tab according to some embodiments. [Figure 15] This is a plan view showing a geometric configuration for suppressing relative motion between adjacent fuel assemblies, according to some embodiments. [Figure 16] This is a plan view showing a geometric configuration for suppressing relative motion between adjacent fuel assemblies, according to some embodiments.

Claims

1. A hexagonal load pad for a reactor core assembly, A key extending from the first surface of the hexagonal load pad, having a pair of key sidewalls extending away from the first surface, and a key surface substantially parallel to the first surface, A slot formed on the second surface of the hexagonal load pad, having a pair of slot sidewalls configured to cooperate with the key so that motion is transmitted from the first reactor core assembly to the second reactor core assembly, A hexagonal load pad, including one.

2. The hexagonal load pad according to claim 1, wherein the pair of key sidewalls extend at an obtuse angle away from the first surface.

3. The cross-section of the load pad is hexagonal, The hexagonal load pad according to claim 1, wherein the load pad further comprises three keys and three slots formed on alternating surfaces of the load pad.

4. Further comprising a second hexagonal load pad configured to have one or more keys and slots, The hexagonal load pad according to claim 1, wherein one of the one or more slots of the second hexagonal load pad is configured to engage with the key extending from the first surface.

5. The hexagonal load pad according to claim 1, wherein the load pad is an upper core load pad positioned on a fuel duct at a location above the core of the reactor.

6. The hexagonal load pad according to claim 1, wherein the load pad is a top load pad located near the upper end of the fuel duct.

7. A hexagonal load pad according to claim 1, wherein a key on the first hexagonal load pad engages with a slot on the adjacent second hexagonal load pad such that relative vertical movement between the first hexagonal load pad and the adjacent second hexagonal load pad is permitted, while relative horizontal movement between the first hexagonal load pad and the second hexagonal load pad is restricted.

8. A method for restricting the relative movement between fuel assemblies within the core of a nuclear reactor, The process of forming a plurality of core assemblies having one or both of the top load pads and the core-upper load pads, and having a cross-section with multiple faces, A step of forming a protruding key on the first surface of each of the multiple core assemblies, A step of forming grooves configured to receive the protruding keys on the second surface of each of the multiple core assemblies, A step of positioning two or more of the multiple core assemblies adjacent to each other such that the protruding key from the first core assembly is fitted into the groove on the second core assembly, Methods that include...

9. The method according to claim 8, wherein the step of forming a plurality of core assemblies further includes the step of forming the protruding keys and grooves on alternating surfaces of the plurality of core assemblies.

10. The multiple core assemblies each have six faces, The method according to claim 9, wherein the plurality of core assemblies are formed to have three keys and three grooves.

11. The method according to claim 8, wherein the protruding key is formed by additive manufacturing.

12. The method according to claim 8, wherein the linear motion of the first core assembly causes corresponding linear motion of one or more adjacent core assemblies.

13. The method according to claim 12, wherein the load from the first core assembly is transmitted to the groove of the second core assembly through the protruding key of the first core assembly.

14. The key includes a plurality of key sidewalls extending away from the core assembly, The groove includes a plurality of groove side walls that define the depth of the groove, The method according to claim 13, wherein the load is transmitted from a plurality of key side walls to a plurality of groove side walls.

15. The method according to claim 8, wherein the movement of the first core assembly causes contact between the protruding key on the first core assembly and the groove on the second core assembly.

16. The method according to claim 8, wherein the protruding key and the plurality of grooves are configured to allow vertical motion between adjacent core assemblies while restricting relative horizontal motion between adjacent core assemblies.

17. The method according to claim 8, wherein the step of positioning two or more of the plurality of core assemblies adjacent to one another further includes the step of arranging seven or more core assemblies in a hexagonal arrangement within a reactor such that each of the seven or more core assemblies engages with an adjacent core assembly through corresponding keys and grooves.

18. A reactor core containment system, A first core assembly having a first mechanical key, formed as an elongated structure having a longitudinal axis, and having a radius, A second core assembly having a second mechanical key, wherein the second mechanical key is configured to engage with the first mechanical key so as to suppress relative motion between the first core assembly and the second core assembly, Includes, A reactor core restraint system in which the radial movement of the first core assembly causes the radial movement of the second core assembly.

19. The reactor core restraint system according to claim 18, wherein the first mechanical key and the second mechanical key do not restrict the relative vertical movement of the first core assembly and the second core assembly.

20. The first mechanical key is a protrusion, The reactor core restraint system according to claim 18, wherein the second mechanical key is a slot configured to capture the protrusion.

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