Core Assembly Sodium Flow Control System

Abstract

A masking element having an opening is disposed on a side of the core support structure. A wall of the flow stack defines a plurality of inlets. At least one inlet aligns with an opening in the masking element when the flow stack is mated with the masking element. A flow control assembly in the flow stack is configured to restrict fluid flow through the flow stack.

Description

Detailed Description of the Invention

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

[0002] 〔background〕 In a sodium-cooled fast reactor (SFR), the primary reactor components are the reactor vessel, filled with liquid sodium coolant, and the core. In some cases, SFRs are once-through fast reactors operated with subcritical reload fuel bred and burned in situ. The core is immersed in a pool of sodium within the reactor vessel. One design has several enriched uranium (U-235) rods in the center of the core, surrounded by depleted uranium (U-238) rods. The U-235 acts as an initiator, starting a traveling wave reaction (a chain reaction in which parallel waves of fission travel slowly through the uranium rods). These parallel waves start in the center of the core and slowly consume fuel, generating heat within the core. This mode of operation is sometimes visualized as a nuclear reactor in which waves travel through the fuel, breeding and then burning fissile material. However, in many cases, fission plants also include so-called "standing wave" designs, in which, instead of propagating the reaction radially outward through static rods, spent rods near the center of the core are replaced with unconsumed uranium rods from the periphery of the core.

[0003] Sodium coolant is used to remove heat from the reactor core. It enters nozzles in the core assembly (some of which may be fuel assemblies) and flows through the core assembly, flowing around and removing heat from the fuel pins in the core assembly. The reactor vessel is surrounded by a containment vessel to prevent loss of sodium coolant in the event of a reactor vessel leak. Pumps circulate the primary sodium coolant between the core and intermediate heat exchangers located in the pool. These heat exchangers have a non-radioactive intermediate sodium coolant on the other side of the heat exchanger. The heated intermediate sodium coolant is circulated to steam generators, which produce steam to drive the turbines of electrical generators.

[0004] In theory, some SFR fission plants would not require fuel reprocessing, would use depleted or natural uranium as their primary fuel, and would require only a small amount of enriched uranium for start-up, without the need for refueling. The lifetime of this core would depend on the amount of uranium initially charged and the fuel burnup achieved during reactor operation.

[0005] A nuclear reactor core may include multiple types of core assemblies, including those containing fissile fuel, fertile fuel, reflectors, neutron absorbers, etc. It would be advantageous if these core assemblies could utilize a universal nozzle to allow for efficient sharing of manufacturing and handling equipment while tailoring the hydrodynamic flow parameters of each core assembly to provide the desired pressure drop and flow control for each fuel region within the core.

[0006] 〔overview〕 In one aspect, the present technology provides a reactor core support structure, a masking element defining at least one masking element opening disposed at a first height of the masking element, and a flow stack configured to mate with the masking element; The present invention relates to an apparatus comprising: a masking element disposed on a first side of the core support structure; a flow stack including a wall defining a plurality of flow stack inlets; and a flow control assembly disposed within the flow stack, at least one flow stack inlet of the plurality of flow stack inlets configured to align with at least one masking element opening when the flow stack is mated with the masking element; and the flow control assembly configured to restrict fluid flow within the flow stack. In one embodiment, the flow control assembly includes a plurality of orifice plates, each of the plurality of orifice plates defining at least one orifice therethrough. In another embodiment, the flow stack has a height, the plurality of orifice plates are disposed at predetermined locations along the height, and at least one flow stack inlet of the plurality of flow stack inlets is disposed between each of the plurality of orifice plates. In yet another embodiment, a first flow stack inlet of the plurality of flow stack inlets has a diameter greater than a diameter of a second flow stack inlet of the plurality of flow stack inlets. In yet another embodiment, the flow control assembly includes at least one labyrinth element.

[0007] In another embodiment of the above aspect, at least one of the labyrinth elements is in fluid communication with at least one flow stack inlet of the plurality of flow stack inlets. In one example, the flow control assembly includes a filter. In another embodiment, the filter has a density that varies along a height of the flow stack. In yet another embodiment, the filter includes a plurality of discrete filter elements. In yet another embodiment, the masking element is fixed to the core support structure.

[0008] In another embodiment of the above aspect, the core support structure is configured to receive an inlet guide pin secured to a core assembly in a position substantially aligned with the masking element. In one embodiment, the flow stack is connected to a fuel assembly, and the flow stack is detachable from the masking element upon lifting of the fuel assembly. In another embodiment, the masking element includes a plurality of masking elements, and the flow stack includes a plurality of flow stacks. In yet another embodiment, a first masking element defines a first masking element opening disposed at a first height of the first masking element, and a second masking element defines a second masking element opening disposed at a second height of the second masking element, the first height being greater than the first height. In some cases, the masking element openings are disposed along a longitudinal axis of the masking element, and the openings may be spaced circumferentially in any suitable configuration. In yet another embodiment, the flow stacks are identical.

[0009] In another embodiment of the above aspect, the outer housing defines an outlet at a first end of the outer housing. In one embodiment, the flow stack is disposed within the masking element when the flow stack is mated with the masking element. In another embodiment, the masking element is disposed within the flow stack when the flow stack is mated with the masking element.

[0010] In another aspect, the present technology relates to an apparatus having an inlet nozzle defining an inlet opening and an outlet in fluid communication with the inlet opening, a duct connected to the inlet nozzle proximate the outlet, and a filter element positioned proximate the outlet. Flow entering the inlet opening and exiting the outlet passes through the filter element before entering the duct. In one embodiment, the filter element defines a plurality of openings. In another embodiment, the plurality of openings comprises a plurality of staggered openings.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are part of this disclosure and are incorporated herein. The drawings illustrate example embodiments of the present disclosure and, together with the specification and claims, serve to explain, at least in part, various principles, features, or aspects of the present disclosure. Specific embodiments of the present disclosure are described more fully below with reference to the accompanying drawings. However, various aspects of the present disclosure may be embodied in many different forms and should not be construed as limited to the implementations set forth herein. Like numbers refer to similar, but not necessarily identical, elements throughout.

[0012] The following drawings, which form part of this application, are illustrative of the described technology and are not intended in any way to limit the scope of the technology as claimed, which scope shall be based on the claims appended hereto.

[0013] FIG. 1 is a block diagram illustrating some of the basic components of a sodium-cooled fast reactor, according to some embodiments.

[0014] FIG. 2 is a schematic cross-sectional view of a core of a sodium-cooled fast reactor, according to some embodiments.

[0015] FIG. 3 is an exploded view of a core assembly according to some embodiments.

[0016] FIG. 4 is an enlarged cross-sectional view of the interface between the inlet nozzle and the core support structure according to some embodiments.

[0017] 5A and 5B are perspective views of components of a sodium flow control system, according to some embodiments.

[0018] FIG. 5C is a perspective cross-sectional view of a flow stack of a sodium flow control system according to some embodiments.

[0019] 6A and 6B are schematic cross-sectional views of other example flow stacks, according to some embodiments.

[0020] FIG. 7 is an exploded view of another example core assembly, according to some embodiments.

[0021] 8A and 8B are side and cross-sectional views of a flow stack of another example sodium flow control system, according to some embodiments.

[0022] FIG. 8C is a side view of a masking element of another example sodium flow control system, according to some embodiments.

[0023] FIG. 8D is a cross-sectional view of the sodium flow control system of FIGS. 8A-8C, according to some embodiments.

[0024] 9A and 9B are enlarged partial perspective views of a filtering element for an inlet nozzle of a core assembly according to some embodiments.

[0025] 10A and 10B are cutaway views of a segmented sodium flow control system according to some embodiments.

[0026] FIG. 10C is a cutaway view showing a tip plug inserted into an inlet nozzle according to some embodiments.

[0027] FIG. 11A is a perspective partial cutaway view of a nozzle having a flow control system according to some embodiments.

[0028] FIG. 11B is a close-up perspective view of a series of orifice sockets or cups disposed within a nozzle, according to some embodiments.

[0029] FIG. 11C is a close-up perspective view of a long top orifice socket or cup disposed within a nozzle, according to some embodiments.

[0030] FIG. 11D is a perspective cutaway view showing components of a sodium flow control system, according to some embodiments.

[0031] 12A and 12B are perspective, partial cutaway views of a nozzle having a flow control assembly according to some embodiments.

[0032] FIG. 12C shows a perspective cutaway view of a tip plug secured to an inlet nozzle, according to some embodiments.

[0033] Detailed Description This disclosure describes exemplary embodiments, and therefore is not intended to limit the scope of the embodiments of this disclosure and the appended claims in any way. The embodiments have been described above with the help of functional building blocks that illustrate implementations of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for convenience of description. Alternative boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed.

[0034] 1 illustrates, in block diagram form, some of the basic components of a sodium-cooled fast reactor (SFR) nuclear fission plant 100. It should be understood that while SFRs may be used throughout this specification as an exemplary type of reactor technology, the concepts presented herein may be applicable to other types of reactors as well. In some cases, the concepts presented in the following description are readily applicable to other types of sodium-cooled fast reactors (SFRs) (e.g., traveling wave reactors, modular reactors, micro-reactors, etc.), and the present disclosure and the appended claims should not be limited to any particular reactor, fuel source, coolant type, or reactor architecture.

[0035] Generally, an SFR nuclear fission plant 100 includes a reactor core 102 containing multiple fuel assemblies (not shown). The reactor core 102 is disposed within a pool 104 that holds a quantity of liquid sodium coolant 106. The pool 104, referred to as the hot pool, has a higher sodium temperature (due to the energy generated by the fuel assemblies within the reactor core 102) than the sodium temperature of a surrounding cold pool 108, which also contains the liquid sodium coolant 106. The hot pool 104 is separated from the cold pool 108 by redan 110. A headspace 112 above the level of the sodium coolant 106 is filled with an inert cover gas (e.g., argon). A reactor vessel 114 surrounds the reactor core 102, the hot pool 104, and the cold 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.

[0036] The size of the core 102 is selected based on many factors, including fuel characteristics, desired power generation, available reactor 100 space, etc. Various embodiments of SFR fission plants may be used in low-power (about 300 MWe to about 500 MWe), medium-power (about 500 MWe to about 1000 MWe), and high-power (about 1000 MWe and above) applications, as needed or desired. Reactor 100 performance may be improved by providing one or more reflectors (not shown) around the core 102 to reflect neutrons into the core 102. Additionally, fertile and fissile nuclear assemblies may be moved (or "shuffled") within and around the core 102 to control the nuclear reactions occurring therein.

[0037] Sodium coolant 106 is circulated within vessel 114 via primary sodium coolant pumps 118. The primary coolant pumps 118 draw sodium coolant 106 from the cold pool 108 and inject it into a plenum below the reactor core 102. The coolant 106 is forced upward through the core and is heated by reactions occurring within the core 102. The heated coolant 106 passes from the hot pool 104 into an intermediate heat exchanger 120, exits the intermediate heat exchanger 120, and re-enters the cold pool 108. This primary coolant loop 122 thus circulates sodium coolant 106 throughout the reactor vessel 114.

[0038] The intermediate heat exchanger 120 incorporates a segment of the closed liquid sodium loop. This segment of the closed liquid sodium loop is always physically separated from the primary sodium pool 104 and the primary sodium pool 108 (i.e., the intermediate sodium and the primary sodium never intermix). The intermediate heat exchanger 120 transfers heat from the primary coolant loop 122 (fully contained within the vessel 114) to the intermediate coolant loop 124 (only partially disposed within the vessel 114). The intermediate heat exchanger 120 passes through Redan 110, thereby bridging the hot and cold pools 104 and 108 (allowing the sodium 106 in the primary coolant loop 122 to flow between the hot and cold pools 104 and 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.

[0039] The intermediate coolant loop 124 circulates a sodium coolant 126. The sodium coolant 126 enters and exits 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 in the primary coolant loop 122 to the sodium coolant 126 in the intermediate coolant loop 124. The sodium coolant 126 in the intermediate coolant loop 124 passes through a number of tubes 130 within the intermediate heat exchanger 120. These tubes 130 keep the sodium coolant 106 in the primary coolant loop 122 separated from the sodium coolant 126 in the intermediate coolant loop 124 while transferring thermal energy therebetween.

[0040] A direct heat exchanger 132 extends into the hot pool 104. The direct heat exchanger 132 provides cooling to the sodium coolant 106 in the primary coolant loop 122, typically in an emergency. The direct heat exchanger 132 is configured to allow the sodium coolant 106 to enter and exit the heat exchanger 132 from the hot pool 104. The direct heat exchanger 132 has a similar structure to the intermediate heat exchanger 120. Tubes 134 keep the NaK (sodium-potassium) coolant of the primary coolant loop 122 separated from the direct heat exchanger coolant (NaK) 136 of the direct reactor coolant loop 138 while transferring thermal energy therebetween.

[0041] 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. (While not shown, these would be apparent to one skilled in the art.) Additional penetrations through the reactor head 116 are not shown (e.g., ports for the primary coolant pumps 118, inert cover gas and inspection ports, sodium treatment sections, and cover gas ports, etc.). A control system 140 is utilized to control and monitor the various components and systems that comprise the reactor 100.

[0042] Broadly speaking, this disclosure describes configurations that improve the performance of the nuclear reactor 100 depicted in Figure 1. Specifically, examples, configurations, and arrangements of flow control systems utilized to direct sodium into the core assembly are illustrated and described in further detail below with reference to the accompanying drawings.

[0043] FIG. 2 is a schematic cross-sectional view of an SFR core 200. The core 200 is shown schematically and includes a central core region 202 having multiple core assemblies 204. The core assemblies 204 may include fissile fuel assemblies, parent fuel assemblies, shielding assemblies, reflector assemblies, control assemblies, and standby shutdown or material testing assemblies. A particular assembly is generally identified by its contents (e.g., fissile material, control material, etc.). However, the components of the assemblies that hold such materials are identical. The peripheral core region 206 includes in-vessel storage pots 208. Throughout the life of the core 200, the fissile and parent fuel assemblies (as well as certain other assemblies) are shuffled between the central core region 202 and the peripheral core region 206 as needed or desired at various stages of the core's life to initiate, maintain, accelerate, or terminate nuclear reactions or power generation, and / or for safety reasons.

[0044] The assemblies 204 are received by a top plate 210 of a core support structure 212 in an aligned position with masking elements 216. Sodium coolant is pumped into a plenum 214 located below the top plate 210 and flows upward into the core assemblies 204 where it is heated by the nuclear reactions occurring within the core 202. The structures that direct the flow of sodium through the core 202 and into the various assemblies are described below.

[0045] 3 is an exploded view of a core assembly 300. The assembly 300 includes an elongated duct 302 having a longitudinal axis A. The duct 302 has a hexagonal cross-section. A handling socket 304 having an internal flow path is secured to a first end 306 of the duct 302. The handling socket 304 has internal or external features that allow mechanisms within the reactor vessel to grasp the handling socket 304 and move (e.g., raise and lower) the assembly 300 in, out of, or within the core.

[0046] An inlet nozzle 308 is fixed to a second end 310 of the duct 302. A plurality of bearing rings 312 and retaining rings 314 are used to attach the handling socket 304 and inlet nozzle 308 to the duct 302. A plurality of locking plates 316 (two in this example) and a plurality of pin strip rails 318 are included proximate the end of the inlet nozzle 308. The locking plates 316 and pin strip rails 318 cooperate to connect a pin bundle 320 to the inlet nozzle 308. An alternative configuration utilizing filtering elements in place of the locking plates 316 and pin strip rails 318 is shown in FIGS. 9A and 9B. A seal ring 322 and a flow restrictor 324 are also shown. The nozzle 308 defines a plurality of coolant inlet windows 326. The plurality of coolant inlet windows 326 are in fluid communication with an internal flow chamber (not shown) extending through the nozzle 308. The plurality of windows 326 thus provide a path for sodium to enter the nozzle 308, into the duct 302, and around the pin bundle 320 located within the duct 302. The sodium flow continues out the handling socket 304.

[0047] FIG. 4 is a cross-sectional view of the interface between the inlet nozzle 308 and the core support structure 400. The inlet nozzle 308 is seated within and engages a receptacle 402 in the core support structure 400. A base 404 of the receptacle 402 defines a passageway 406, which provides a flow path for sodium to the static reactor sodium pool. The receptacle 402 may extend above the core support structure 400 or may be flush with the core support structure 400. Beneath the core support structure 400 is a coolant flow control system 500. The coolant flow control system 500 includes a masking element 502 and a flow stack 504 disposed therein. In this example, the masking element 502 is in the form of a sleeve. The flow stack 504 includes an outer housing 506 and at least one flow control assembly 516 disposed therein. The flow control system 500 is described in further detail below.

[0048] The flow systems described herein utilize standardized flow stacks with various masking sleeves at various locations below the core support structure. The masking features are affixed to or integral with the core support structure. Meanwhile, the flow stacks may be integral with the core assembly or separate from the core assembly. Utilizing standardized flow stacks reduces manufacturing costs, assembly variations, and the risk of assembly errors because the components are standardized. The masking sleeves allow each flow stack to be used in any location over a wide range of flow conditions, such as those encountered in an SFR. The flow stacks may be integral with the inlet nozzle of the core assembly or may be fixed within the masking sleeve. The flow stack may include multiple pressure stages and an inlet for each stage. The masking sleeves are positioned around the flow stack to provide multiple alternative inlets to the flow stages. This configuration allows for variable pressure drop depending on the exposed alternative inlets, which defines the number of pressure drop stages the flow encounters. This allows for standardization of fuel assemblies while creating unique flow conditions for various core locations.

[0049] For SFRs, this can be advantageous because it allows the core assembly to be installed in a different core location or relocated (e.g., shuffled) to a different core location at any time while still receiving the appropriate metered flow rates (which may vary from location to location). In embodiments where the flow stack is integral with the inlet nozzle, the flow stack is connected to a removable component (the core assembly). This allows for lifetime effects (e.g., erosion damage) to be examined and mitigated as needed. In "re-core" operations, where the entire core assembly is replaced with one of a different design, the replacement assembly does not need to match the flow zones of the original core. This allows for more flexibility in the design of future cores, as needed or desired.

[0050] 5A and 5B are perspective views of components of sodium flow control system 500. FIG. 5C is a perspective cross-sectional view of flow stack 504 of sodium flow control system 500. FIGS. 5A-5C will be described simultaneously. Sodium flow control system 500 includes a masking element or masking sleeve 502. Masking element or masking sleeve 502 may be fixed to or integral with the core support system (not shown). Masking sleeve 502 has an overall height H M 5A , the masking sleeve 502 includes a generally cylindrical housing 506 having a height H 1 . One or more openings 508 may be disposed around the circumference of the housing 506 at a generally predetermined height h 1 . In other examples, the openings 508 in the masking sleeve may be distributed at various heights of the masking sleeve 502. Depending on the location of a particular masking sleeve opening 508 below the core structure, the sodium flow may enter a flow stack 504 disposed within the masking sleeve 502 at a particular flow stage defined by the location of a matching flow stack inlet 510 defined by an outer housing 512 of the flow stack 504. As depicted in FIG. 5A , the flow stack 504 also extends to a total height H F The total height H F may be substantially equal to the overall height of the masking sleeve 502. The predetermined height h2 of a particular flow stack inlet 510 may be such that a particular masking sleeve opening 508 is aligned with that particular flow stack inlet 510. Thus, the stage at which sodium enters the flow stack 504 is defined by the location of the masking sleeve opening 508 of the particular masking sleeve 502 where the flow stack 504 is positioned within that particular masking sleeve 502.

[0051] As can be seen in FIGS. 5A and 5C, a plurality of flow stack inlets 510 are provided across the height H of the flow stack 504. F The diameter of these flow stack inlets 510 is equal to the height H of the flow stack 504. FThe diameters of the flow stack inlets 510 may vary based on their position along the flow stack 504 direction, which affects the pressure of the sodium coolant as it enters the flow stack 504 at a particular flow stage. In the illustrated example, the diameter of the flow stack inlets 510 increases as the distance above the bottom 514 of the flow stack 504 increases. The flow stack 504 includes an internal flow control assembly 516, an example of which is depicted in FIG. 5C. In this example, the flow control assembly 516 includes multiple flow stages, each of which is defined, at least in part, by an orifice plate 518 disposed substantially perpendicular to the axis A of the flow stack 504. These orifice plates 518 also extend beyond the height H of the flow stack 504. F The orifice plates 518 are positioned at predetermined locations along the flow control system 500 direction. Each orifice plate 518 defines one or more orifices 520 through which sodium flows. The diameter of the openings 520 may vary from one orifice plate 518 to another, thereby further controlling the associated pressure drop. In another embodiment, each orifice plate 518 may instead be formed from discrete filters that restrict or impede (to varying degrees, if desired) the flow of sodium therethrough. These filters may be fabricated from materials similar to those used for other components of the flow control system 500.

[0052] Flow stack inlets 510 are positioned around the housing 512 of the flow stack 504. In the illustrated configuration, the flow stack inlets 510 are positioned between adjacent orifice plates 518. In addition, multiple flow stack inlets 510 are positioned between each orifice plate 518. When the flow stack 504 is positioned within the masking sleeve 502, the flow stack inlets 510 of the flow stack 504 are aligned with the associated masking sleeve openings 508. Thus, when flow stacks 504 having the same configuration are used at various locations below the core support structure, the masking sleeve 502 defines the flow stage into which the sodium enters based on the location of the masking sleeve openings 508. This allows a single flow stack 504 configuration to be used throughout the reactor, with flow control as needed at various locations within the reactor based on the particular masking sleeve 502 utilized at each location. Of course, multiple types of flow stacks 504 may be used throughout the core, and these flow stacks 504 may differ based on their location within the core, the purpose of the core assembly, or both. As an example, a first flow stack configuration may be used for fuel assemblies, a second flow stack may be used for reflector assemblies, and either the first flow stack, the second flow stack, or a third flow stack may be used for shielding assemblies. In some cases, a first flow stack configuration may be used for one core assembly and a second flow stack configuration may be used for a second core assembly. These core assemblies may be the same type of core assembly or different types of core assemblies. For example, a first flow stack configuration may be used for fuel assemblies located at a first location within the core, and a second flow stack configuration may be used for fuel assemblies located at a second location within the core.

[0053] 6A and 6B are schematic cross-sectional views of alternative example flow stacks 600a and 600b, respectively. Each of the flow stacks 600a and 600b includes an outer housing 602a and 602b. The outer housings 602a and 602b may define multiple flow stack inlets 604a and 604b. The diameters, shapes, or other aspects of the multiple flow stack inlets 604a and 604b may vary relative to the height H of the flow stack 600a and 600b, as depicted schematically in the figures. F The flow stack inlets 600a, 600b may vary along the length of the stack. Additionally, some or all of the flow stack inlets 604a, 604b may be covered with mesh or screen 606a, 606b to further restrict flow, retain the flow control assemblies 608a, 608b within the outer housings 602a, 602b, or further reinforce the outer housings 602a, 602b. The illustrated flow stacks 600a, 600b define open top ends 610a, 610b and closed bottom ends 612a, 612b. However, in other embodiments, the flow stacks may include open bottom ends to increase sodium flow therethrough.

[0054] In particular, a masking sleeve used with such an open-bottomed flow stack may have one or more openings of various sizes defined therein so that flow into the flow stack through the bottom of the masking sleeve is controlled.

[0055] In FIG. 6A, flow control assembly 608a is a flow resistance structure (e.g., a packed bed). The packed bed may be formed from a plurality of discrete packing particles 614a. The plurality of discrete packing particles 614a may be round, oblong, or have some other shape. The packed bed resists the flow of sodium as it flows upward through flow stack 600a. The resistance to flow can be increased over the height H of flow stack 600a by utilizing packing particles 614a of varying size (e.g., smaller particle size near bottom end 612a and increasing particle size higher in flow stack 600a), surface texture, or both. F The flow rate may vary along the flow stack direction. It is known that smaller particles are more densely packed and therefore offer more resistance to flow than spaces containing larger particles. The resistance to flow through the packed bed may be controlled and optimized via known processes. Additionally, the flow of sodium into the flow stack 600a may be controlled by aligning the desired flow stack inlet 604a with an appropriately positioned opening in a masking sleeve (not shown).

[0056] 6B, flow control assembly 608b is a flow resistance structure (e.g., a labyrinth element). The labyrinth element may be formed from a plurality of discrete fins or plates 614b. The plurality of discrete fins or plates 614b may be oriented parallel, perpendicular, or diagonally relative to housing 602b. The labyrinth element is in fluid communication with flow stack inlet 604b. The labyrinth element resists the flow of sodium as it flows upward through flow stack 600b. The resistance to flow is increased over the height H of flow stack 600b by utilizing a plurality of discrete fins or plates 614b spaced at various distances (e.g., closer spacing near bottom end 612b and increasing spacing higher within flow stack 600b). FThe density of the metal wool or filter may vary along the length of the stack. It is known that smaller spacing between elements creates more resistance to flow than larger spacing between elements. The resistance to flow through the packed bed may be controlled and optimized via known processes. Furthermore, by aligning the desired flow stack inlet 604b with an appropriately positioned opening in a masking sleeve (not shown), the flow of sodium into the flow stack 600b may be controlled. In another embodiment, the flow control assembly 608b may be metal wool or a filter or other material packed within the flow stack 600b. In some embodiments, the density of the metal wool or filter (and subsequent resistance to flow) varies with the height H of the flow stack 600b. F It may vary along the direction.

[0057] FIG. 7 is an exploded view of another embodiment of a core assembly 700. The assembly 700 includes an elongated duct 702 having an axis A. The duct 702 has a hexagonal cross-section. A handling socket 704 having an internal flow passage is secured to a first end 706 of the duct 702. The handling socket 704 has internal or external features that allow mechanisms within the reactor vessel to grasp the handling socket 704 and move (e.g., raise and lower) the assembly 700 into, out of, or within the core. An inlet nozzle 708 is secured to a second end 710 of the duct 702. A plurality of bearing rings 712 and retaining rings 714 are used to attach the handling socket 704 and flow stack nozzles 728 to the duct 702. A plurality of locking plates 716 (two in this example) and a plurality of pin strip rails 718 are included proximate the end of the flow stack nozzle 728. The locking plates 716 and pin strip rails 718 cooperate to connect the pin bundle 720 to the flow stack nozzle 728. A seal ring 722 and a flow restrictor 724 are also shown.

[0058] The illustrated core assembly 700 differs from the core assembly shown in FIG. 3 by including a flow stack nozzle 728 instead of the inlet nozzle 308. Thus, the flow stack nozzle 728 integrates a flow stack 730 into the core assembly 700. The flow stack nozzle 728 defines a plurality of flow stack inlets 732. The plurality of flow stack inlets 732 are in fluid communication with a flow control assembly (not shown) disposed within the flow stack 730. The flow control assembly may have one of the configurations shown herein. Thus, the flow stack inlet 732 provides a path for sodium to flow into the flow stack nozzle 728, into the duct 702, and around the pin bundle 720 disposed within the duct 702. The sodium flow then continues out the handling socket 704. Unlike the embodiment shown in FIGS. 3-5C, the flow stack nozzle 728 is movable with the core assembly 700 as the core assembly 700 is moved within the core. In one such configuration, flow through the flow stack nozzles 728 and the core assembly 700 is controlled based on the position of the core assembly 700 within the core by a masking sleeve (not shown) into which the flow stack nozzles 728 are inserted.

[0059] 8A and 8B are side and cross-sectional views of a flow stack 804, and FIG. 8C is a side view of a masking element 802 of another embodiment of a sodium flow control system 800. FIG. 8D is a cross-sectional view of the sodium flow control system 800 of FIGS. 8A-8C. FIGS. 8A-8D will be described simultaneously. As an alternative to the flow control system of FIGS. 5A-5C, the illustrated flow control system 800 is configured such that the masking element 802 fits within the flow stack 804. As such, the masking element 802 acts as a flow distribution boss. The sodium flow control system 800 includes a masking element 802. The masking element 802 may be fixed to or integral with a core support system (not shown). The masking element 802 has an overall height H M8A and 8B , the flow stack 804 also includes a generally cylindrical housing 806 having a height H 1 . One or more openings 808 may be disposed around the circumference of the housing 806 at a generally predetermined height h 1 . In other examples, the openings 808 in the masking element 802 may be distributed at various heights of the masking element 802. The sodium flow enters the masking element 802 through a bottom inlet 807. Depending on the location of a particular masking element opening 808 below the core structure, the sodium flow flows upward through the masking element 802 and enters a flow stack 804 disposed around the masking element 802 at a particular flow stage defined by the location of a matching flow stack inlet 810 defined by an inner wall 812 of the flow stack 804. In this example, the outer housing 813 of the flow stack 804 does not include any openings. As depicted in FIGS. 8A and 8B , the flow stack 804 also has a total height H 1 . F The total height H F is generally greater than the overall height of the masking element 802 received within the space 815 defined by the interior wall 812. The predetermined height h2 of a particular flow stack inlet 810 may be such that a particular masking element opening 808 is aligned with the particular flow stack inlet 810. Thus, the stage at which the sodium enters the flow stack 804 is defined by the location of the masking element opening 808 of the particular masking element 802 around which the flow stack 804 is positioned.

[0060] As can be seen in FIG. 8B, multiple flow stack inlets 810 extend across the height H of the flow stack 804. F The diameter of these flow stack inlets 810 is equal to the height H of the flow stack 804. FThe diameters of the flow stack inlets 810 may vary based on their position along the flow stack 804 direction, which affects the pressure of the sodium coolant as it enters the flow stack 804 at a particular flow stage. In the illustrated example, the diameter of the flow stack inlets 810 increases with increasing distance above the bottom 814 of the flow stack 804. The flow stack 804 includes an internal flow control assembly 816, an example of which is depicted in FIGS. 8B and 8D. In this example, the flow control assembly 816 includes multiple flow stages, each of which is defined, at least in part, by an orifice plate 818 disposed substantially perpendicular to the axis A of the flow stack 804. These orifice plates 818 also extend beyond the height H of the flow stack 804. F , and are positioned at predetermined locations along the flow control system 800. Each orifice plate 818 defines one or more orifices (not shown) through which sodium flows. The diameter of the openings may vary from one orifice plate 818 to the next, thereby further controlling the associated pressure drop. In another embodiment, each orifice plate 818 may instead be formed from discrete filters that restrict or impede (to varying degrees, if desired) the flow of sodium therethrough. These filters may be fabricated from materials similar to those used for other components of the flow control system 800.

[0061] Flow stack inlets 810 are positioned along the inner wall 812 of the flow stack 804. In the illustrated configuration, the flow stack inlets 810 are positioned between adjacent orifice plates 818. In addition, multiple flow stack inlets 810 are positioned between each orifice plate 818. As depicted in FIG. 8D , when the flow stack 804 is positioned around the masking element 802, the flow stack inlets 810 of the flow stack 804 align with the associated masking element openings 808 (flow stack inlets 810 that do not align with the masking element openings 808 are not shown in FIG. 8D for clarity). Thus, when flow stacks 804 having the same configuration are used in various locations below the core support structure, the masking element 802 defines the flow stages into which the sodium enters based on the location of the masking element openings 808. This allows a single flow stack 804 configuration to be used throughout the reactor, while controlling flow as needed at various locations within the reactor based on the particular masking element 802 utilized at each location.

[0062] 9A and 9B are enlarged, partial perspective views of a filtering element 900 for an inlet nozzle 902 of a core assembly. The core assemblies shown above in FIGS. 3, 4, and 7 depict multiple locking plates and multiple pin strip rails connecting the pin bundles in the core assembly to the inlet nozzle. While this configuration presents certain advantages, FIGS. 9A and 9B illustrate a filtering element 900 that can replace the locking plates and pin strips. The filtering element 900 may be utilized to capture debris that may flow through the inlet nozzle 902 to prevent damage to individual pins in the fuel bundle. Such debris may include small, wire-type debris that can pass through the inlet nozzle and have sufficient mass to damage the fuel pins by fretting wear. The filter element 900 is formed of a solid plate having multiple serpentine openings 904 therethrough. Each opening 904 defines a substantially semicircular profile. This staggered configuration of the multiple openings 904 eliminates line-of-sight through each opening 904 from the lower extent 906 of each filter element 900 to its upper extent 908. This configuration allows debris to be trapped without causing a significant increase in pressure drop, which could impair cooling of the fuel pins. The complex shape of the individual openings 904 in the filtering element 900 may be fabricated using additive manufacturing techniques (e.g., 3D printing). Thus, sodium flow entering the inlet nozzle 902 (e.g., through the inlet opening) flows through the filtering element 900 and out the outlet of the inlet nozzle. The sodium flow then enters a duct and travels around the individual pins of the pin bundle therein.

[0063] 10A, 10B, and 10C are cutaway views of a segmented sodium flow control system 500. In some cases, the sodium flow control device is subjected to neutron irradiation. Neutron irradiation has been shown to affect weld strength. In some cases, the components may be formed with threaded ends and then threadedly engaged together for assembly. As shown, the sodium flow control system 500 may have a first segment 1002 and a second segment 1004. The segments 1002, 1004 may be manufactured as separate components and then assembled together. In some cases, the first segment may have a female end with an internal thread 1006. The second segment may have a male end with an external thread 1008 that cooperates with the internal thread 1006 to facilitate coupling of the first segment 1002 to the second segment 1004. Additional segments may be attached in a similar manner to form a group of segments of any desired length.

[0064] During the manufacture of these segments, in some cases, an orifice plate 1010 may be placed in the segment. The orifice plate may be formed and coupled to the segment via any suitable process. For example, one or more orifice plates may be integrally formed within the segment, for example, via additive manufacturing or suitable material removal manufacturing methods. In some cases, the orifice plate 1010 may be formed separately and then attached to the segment, for example, by welding. In some cases, to properly position the orifice plate 1010 within the segment 1002, the segment 1002 may be formed with a ledge 1012 on which the orifice plate 1010 rests. The segment 1002 may include one or more orifice plates 1010, as desired. In some embodiments, the segment 1002 may have a first orifice plate 1010 inserted into a first side 1014 of the segment and a second orifice plate 1016 inserted into a second side 1018 of the segment.

[0065] In some embodiments, when multiple segments 1002 are coupled together, for example by threaded engagement, the multiple segments may be further secured to each other by another securing means (e.g., welding, pinning, brazing, adhesive, etc.).

[0066] 10C, in some cases, a tip plug 1020 may be inserted into the bottommost segment and secured thereto. In some cases, the tip plug 1020 may be coupled to the segment 1004 through any suitable mechanism (e.g., threaded engagement, welding, pinning, friction, adhesive, or a combination of multiple coupling techniques). In some cases, the tip plug is designed to be in a low fluence region of the reactor, and welding may be a suitable technique for securing the tip plug 1020 to the segment 1004.

[0067] 11A, 11B, 11C, and 11D illustrate alternative embodiments and components of the flow control system 1100. According to some embodiments, the orifice socket 1102 is a cup-shaped device having a circular bottom 1104 and a sidewall 1106 extending upwardly from the bottom 1104. The bottom 1104 may be formed with one or more holes to allow the primary coolant to flow therethrough. As with other embodiments described herein, the holes in the bottom 1104 of the orifice socket 1102 may be sized and distributed to allow a predetermined pressure drop across each orifice socket 1102 or across the entire flow stack 504. The sidewall 1106 may also be formed with holes that may align with one or more holes in the inlet nozzle 1108 to allow sodium coolant to enter the flow stage defined by each respective orifice socket 1102. A series of orifice sockets 1102 may be stacked within the inlet nozzle 1108 to control the pressure drop across the flow stack 504 and provide one or more flow stages. Coolant may enter the flow stack 504 in the one or more flow stages. The height of the sidewalls 1106 may be configured to control the spacing of the stacked orifice sockets 1102 so that each orifice socket 1102 defines a flow stage and each orifice socket bottom 1104 is spaced a desired distance from an adjacent orifice socket bottom 1104. In some embodiments, orifice sockets 1102 having different heights may be used to form flow stages with different volumes. For example, one or more first orifice sockets 1110, each having a first height, may be stacked to form one or more flow stages. A second orifice socket 1112 having a second height greater than the first height may be similarly stacked and arranged to form a flow stage having a volume greater than the volume of the flow stage formed by the first orifice socket.The orifice sockets 1102 may be arranged in any suitable configuration such that the desired flow characteristics are produced by the flow control system. In some cases, the inlet nozzle 1108 is configured to receive the orifice socket 1102, and these structures cooperate to form the flow control system 1100.

[0068] In some embodiments, the inner surface 1114 of the inlet nozzle 1108 includes a structure 1116 that engages one or more of the multiple orifice sockets 1102, such as to precisely position the first orifice socket 1102. Subsequent orifice sockets 1102 may be stacked on top of the first orifice socket 1102, with the geometry of each orifice socket 1102 being precise to facilitate a desired pressure drop along the flow control system 1100. The structure 1116 may be, for example, a ledge as shown in FIG. 11B. Additional interference structures may include a protrusion, boss, pin, groove, reduced diameter, or any other structure that may cooperate with the first orifice socket 1102 to position the first orifice socket 1102 within the inlet nozzle 1108. In some cases, the outer surface of the orifice socket is in intimate contact with the inner surface of the inlet nozzle. In some cases, the tolerance between the orifice socket and the inlet nozzle is within 0.01", 0.005", or even less. In some cases, this construction prevents primary coolant from leaking between the stacks.

[0069] By varying the geometry of the orifice sockets 1102, including the number, arrangement, and size of the holes in the orifice sockets 1102, the thermal-hydraulic properties of each flow control system 1100 may be varied, resulting in a set of components that are easy to manufacture and assemble and that can be mixed and matched to produce any desired flow control parameters. In use, the orifice sockets 1102 may be located at the inlet nozzle of any core assembly (including, but not limited to, fissile fuel assemblies, parent fuel assemblies, reflectors, neutron absorbers, neutron poisons, neutron shields, control rod assemblies, etc.).

[0070] 12A, 12B, and 12C illustrate components and embodiments of a sodium flow control device 1200. According to some embodiments, the orifice socket 1202 is a cup-shaped device having a circular top 1204 and a sidewall 1206 extending downwardly from the top 1204. The top 1204 may be formed with one or more holes to allow the primary coolant to flow therethrough. As with other embodiments described herein, the holes in the top 1204 of the orifice socket 1202 may be sized and distributed to allow a predetermined pressure drop across each orifice socket 1202 or across the entire flow stack 504. The sidewall 1206 may also be formed with holes that may align with one or more holes in the inlet nozzle 1208 to allow the sodium coolant to enter the flow stage defined by each respective orifice socket 1202. A series of orifice sockets 1202 may be stacked within the inlet nozzle 1208, for example, by inserting a plurality of such orifice sockets 1202 into the bottom of the inlet nozzle 1208, to control the pressure drop across the flow stack 504 and provide one or more flow stages. Coolant may enter the flow stack 504 in the one or more flow stages. The height of the sidewall 1206 may be configured to control the spacing of the stacked orifice sockets 1202, such that each orifice socket 1202 defines a flow stage and each orifice socket apex 1204 is spaced a desired distance from an adjacent orifice socket apex 1204. In some embodiments, orifice sockets 1202 having different heights may be used to form flow stages having different volumes. For example, one or more flow stages may be formed by stacking one or more first orifice sockets 1210, each having a first height.A second orifice socket 1212 having a second height greater than the first height may be similarly stacked to form a flow stage having a volume greater than the volume of the flow stage formed by the first orifice socket. The orifice sockets 1202 may be arranged in any suitable configuration such that the desired flow characteristics are produced by the flow control system 1200. In some cases, the inlet nozzle 1208 is configured to receive the orifice socket 1202, and these structures cooperate to form the flow control system 1100.

[0071] In some embodiments, the inlet nozzle 1208 may have a step 1216 that engages with a first orifice socket inserted into the inlet nozzle 1208, thereby precisely positioning the first orifice socket 1202. Subsequent orifice sockets 1202 may be stacked adjacent to the first orifice socket 1202, and the geometry of each orifice socket 1202 may position each orifice socket in a precise location to promote a desired pressure drop along the flow control system 1200. Additional interference structures may be used in place of the ledge 1216. The additional interference structures may include protrusions, bosses, pins, grooves, reduced diameter portions, or any other structure that may cooperate with the first orifice socket 1202 to position the first orifice socket 1202 within the inlet nozzle 1208. In some cases, the outer surface of the orifice socket is in intimate contact with the inner surface of the inlet nozzle. In some cases, the tolerance between the orifice socket and the inlet nozzle is within 0.01", 0.005", or even less. In some cases, this construction prevents primary coolant from leaking between the stacks.

[0072] Subsequent orifice sockets may be inserted into the inlet nozzle until the desired number of orifice sockets are positioned within the inlet nozzle. A tip plug 1220 may be inserted into the inlet nozzle and secured via any suitable technique. In some cases, the tip plug 1220 is threadably engaged with the inlet nozzle. The tip plug 1220 may provide a compressive force against the multiple orifice sockets, compressing them together and securing them in place. In some cases, the tip plug 1220 may alternatively or additionally be welded in place.

[0073] By varying the geometry of the orifice sockets 1202, including the number, arrangement, and size of holes in the orifice sockets 1202, the thermal-hydraulic characteristics of each flow control system 1200 may be varied, resulting in a set of components that are easy to manufacture and assemble and that can be mixed and matched to produce any desired flow control parameters. In use, the orifice sockets 1202 can be placed at the inlet nozzle of any core assembly (including, but not limited to, fissile fuel assemblies, parent fuel assemblies, reflectors, neutron absorbers, neutron poisons, neutron shields, control rod assemblies, etc.). In some embodiments, multiple orifice sockets having different dimensional volumes, hole sizes, and / or number of holes are used in different core assemblies to vary the thermal-hydraulic characteristics of the selected core assembly. In other words, the same configuration of orifice sockets need not be used in each core assembly; different orifice sockets or orifice socket configurations can be used in different core assemblies.

[0074] The foregoing description of specific embodiments sufficiently clarifies the general nature of the embodiments of the present disclosure so that others can, by applying the knowledge of those skilled in the art, readily modify and / or adapt such specific embodiments for various uses without undue experimentation and without departing from the general concepts of the embodiments of the present disclosure. Accordingly, such adaptations and modifications are intended to be within the spirit and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. The terms or phrases used herein are for the purpose of description and not of limitation, as would be interpreted by one of ordinary skill in the relevant art in light of the teaching and guidance presented herein.

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

[0076] In particular, conditional language such as "can," "could," "might," or "might" is intended to generally convey that certain implementations may include certain features, elements, and / or operations, while other implementations do not, unless specifically stated otherwise or understood otherwise within the context in which it is used. Thus, such conditional language is not generally intended to imply that features, elements, and / or operations are somehow required in one or more implementations, or that one or more implementations necessarily include logic for determining whether those features, elements, and / or operations are included in or performed in any particular implementation, with or without user input or prompting.

[0077] Unless otherwise noted, the terms "connected to" and "coupled to" (and their derivatives) as used herein shall be interpreted to allow both direct and indirect (i.e., via other elements or components) connections. Additionally, the terms "a" or "an" as used herein shall be interpreted to mean "at least one." Finally, for ease of use, the terms "comprise" and "have" (and their derivatives) as used herein shall be interpreted to mean "comprise" and "have."

[0078] This specification and the accompanying drawings disclose examples of systems, apparatus, devices, and techniques that can provide for control and optimization of coolant flow through a reactor core assembly. It is, of course, impossible to describe every conceivable combination of elements and / or methodologies for purposes of describing various features of the present disclosure, but those skilled in the art will recognize that many additional combinations and permutations of the disclosed features are possible. Accordingly, various modifications can be made to the present disclosure without departing from its scope or spirit. Moreover, other embodiments of the present disclosure will become apparent from consideration of this specification and the accompanying drawings, and the practice of the disclosed embodiments presented herein. The examples presented in this specification and the accompanying drawings are to be considered in all respects as illustrative and not restrictive. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[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 therein. Moreover, while certain aspects are set forth below in certain claim forms, the inventors contemplate the various aspects in any available claim form. For example, while only some aspects may currently be described as embodied in a particular configuration, other aspects may likewise be so embodied. Various modifications and changes can be made, as would be apparent to one skilled in the art having the benefit of this disclosure. All such modifications and changes are intended to be encompassed, and therefore the foregoing description is to be regarded in an illustrative and not a limiting sense. [Brief explanation of the drawings]

[0080] [Figure 1] 1 is a block diagram illustrating some of the basic components of a sodium-cooled fast reactor, according to some embodiments. [Figure 2] 1 is a schematic cross-sectional view of a sodium-cooled fast reactor core, according to some embodiments. [Figure 3] FIG. 2 is an exploded view of a core assembly according to some embodiments. [Figure 4] FIG. 2 is an enlarged cross-sectional view of the interface between the inlet nozzle and the core support structure according to some embodiments. [Figure 5A] FIG. 1 is a perspective view of components of a sodium flow control system, according to some embodiments. [Figure 5B] FIG. 1 is a perspective view of components of a sodium flow control system, according to some embodiments. [Figure 5C] FIG. 1 is a perspective cross-sectional view of a flow stack of a sodium flow control system according to some embodiments. [Figure 6A] 10 is a schematic cross-sectional view of another example flow stack, according to some embodiments. [Figure 6B] 10 is a schematic cross-sectional view of another example flow stack, according to some embodiments. [Figure 7]FIG. 10 is an exploded view of another example core assembly, according to some embodiments. [Figure 8A] 10A-10C illustrate side and cross-sectional views of a flow stack of another example sodium flow control system, according to some embodiments. [Figure 8B] 10A-10C illustrate side and cross-sectional views of a flow stack of another example sodium flow control system, according to some embodiments. [Figure 8C] FIG. 10 is a side view of a masking element of another example sodium flow control system, according to some embodiments. [Figure 8D] FIG. 8C is a cross-sectional view of the sodium flow control system of FIGS. 8A-8C, according to some embodiments. [Figure 9A] FIG. 2 is an enlarged partial perspective view of a filtering element for an inlet nozzle of a core assembly according to some embodiments. [Figure 9B] FIG. 2 is an enlarged partial perspective view of a filtering element for an inlet nozzle of a core assembly according to some embodiments. [Figure 10A] FIG. 1 is a cutaway view of a segmented sodium flow control system according to some embodiments. [Figure 10B] FIG. 1 is a cutaway view of a segmented sodium flow control system according to some embodiments. [Figure 10C] FIG. 10 is a cutaway view showing a tip plug inserted into an inlet nozzle, according to some embodiments. [Figure 11A] FIG. 1 is a perspective partial cutaway view of a nozzle having a flow control system according to some embodiments. [Figure 11B] FIG. 10 is a close-up perspective view of a series of orifice sockets or cups disposed within a nozzle, according to some embodiments. [Figure 11C] FIG. 13 is a close-up perspective view of a long top orifice socket or cup positioned within a nozzle, according to some embodiments. [Figure 11D] FIG. 1 is a perspective cutaway view illustrating components of a sodium flow control system, according to some embodiments. [Figure 12A]FIG. 1 is a perspective partial cutaway view of a nozzle having a flow control assembly according to some embodiments. [Figure 12B] FIG. 1 is a perspective partial cutaway view of a nozzle having a flow control assembly according to some embodiments. [Figure 12C] 10 shows a perspective cutaway view of a tip plug secured to an inlet nozzle according to some embodiments.

Claims

1. a core support structure; a masking element defining at least one masking element opening disposed at a first height of the masking element; a flow stack configured to mate with the masking element; An apparatus comprising: the masking element is disposed on a first side of the core support structure; The flow stack comprises: a wall defining a plurality of flow stack inlets; a flow control assembly disposed within the flow stack; Including, at least one flow stack inlet of the plurality of flow stack inlets is configured to align with at least one masking element opening when the flow stack is mated with the masking element; The apparatus, wherein the flow control assembly includes a plurality of spaced apart orifice plates configured to restrict fluid flow within the flow stack.

2. 10. The apparatus of claim 1, wherein each plate of the plurality of orifice plates defines at least one orifice therethrough.

3. the flow stack has a height; The orifice plates are arranged at predetermined positions along the height direction, The apparatus of claim 2 , wherein at least one flow stack inlet of the plurality of flow stack inlets is disposed between each of the plurality of orifice plates.

4. The apparatus of claim 1 , wherein a first flow stack inlet of the plurality of flow stack inlets has a diameter greater than a diameter of a second flow stack inlet of the plurality of flow stack inlets.

5. The apparatus of claim 1 , wherein the flow control assembly includes at least one labyrinth element.

6. The apparatus of claim 5 , wherein at least one of the labyrinth elements is in fluid communication with at least one flow stack inlet of the plurality of flow stack inlets.

7. The device of claim 1 , wherein the flow control assembly includes a filter.

8. The apparatus of claim 7 , wherein the filter has a density that varies along the height of the flow stack.

9. The apparatus of claim 7 , wherein the filter comprises a plurality of discrete filter elements.

10. The apparatus of claim 1 , wherein the masking element is fixed to the core support structure.

11. The apparatus of claim 1 , wherein the core support structure is configured to receive an inlet guide pin secured to a core assembly in a position substantially aligned with the masking element.

12. the flow stack is connected to a fuel assembly; The apparatus of claim 1 , wherein the flow stack is removable from the masking element upon lifting of the fuel assembly.

13. the masking element includes a plurality of masking elements; The apparatus of claim 1 , wherein the flow stack comprises a plurality of flow stacks.

14. the first masking element defines a first masking element opening disposed at a first height of the first masking element; the second masking element defines a second masking element opening disposed at a second height of the second masking element; The apparatus of claim 13 , wherein the first height is greater than the first height.

15. The apparatus of claim 14 , wherein a plurality of the flow stacks are identical.

16. the walls of the flow stack include an outer housing; The device of claim 1 , wherein the outer housing defines an outlet at a first end of the outer housing.

17. The device of claim 1 , wherein the flow stack is disposed within the masking element when the flow stack is mated with the masking element.

18. The apparatus of claim 1 , wherein the masking element is disposed within the flow stack when the flow stack is mated with the masking element.

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