Thermally activated nuclear core restraint system

Abstract

A nuclear reactor core includes a plurality of core assemblies. At least some of the core assemblies are formed with bi-metallic assembly (BiMA) devices having two or more metallic materials with different coefficients of thermal expansion such that the BiMA devices are configured to preferentially deform in response to an increase in temperature. The preferential deformation is a bowing deformation and is directed toward the center of the core with a predetermined stroke such that the BiMA devices impart forces on adjacent core assemblies sufficient to achieve mechanical core lock-up during a startup routine and before the reactor reaches full power.

Description

THERMALLY ACTIVATED NUCLEAR CORERESTRAINT SYSTEMCROSS REFERENCE

[0001] The present application claim benefit of priority to U.S. Provisional Patent Application No. 63 / 661,012, filed June 17, 2024, titled “THERMALLY ACTIVATED NUCLEAR CORE RESTRAINT SYSTEM,” the entire contents of which is hereby incorporated by reference.BACKGROUND

[0002] 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, an SFR is a once-through fast reactor that runs on subcritical reload fuel that is bred up and burned in situ. The reactor core is immersed in the sodium pool in the reactor vessel.

[0003] The sodium coolant is used to remove the heat from the core. The sodium coolant flows through the core assemblies, some of which may be fuel assemblies, by entering a nozzle of the core assembly and flowing about the fuel pins within the core assemblies to remove heat therefrom. A guard vessel surrounds the reactor vessel to prevent loss of sodium coolant in case of an unlikely leak from the reactor vessel. The pumps circulate primary sodium coolant between the reactor core and intermediate heat exchangers located in the pool.

[0004] The core of a fast reactor typically includes a closely packed hexagonal arrangement of core assemblies. As the core assemblies move closer together in a radial direction, the reactivity within the core increases and as the core assemblies move farther apart, the reactivity decreases. Reactivity within a fast reactor core can be very sensitive to relative movement between core assemblies. The reactivity feedback depends, in large part, on how the core assemblies are supported, restrained, and constrained.

[0005] Relative movement between core assemblies comes from several sources, such as seismic events, core assembly bowing due to thermal gradients, irradiation creep, and void swelling. The core assemblies are exposed to both axial and radial temperature and neutron flux gradients. The temperature gradients initially cause the core assemblies to bow whichcan cause contact between adjacent core assemblies through contact points above the fuel region of the core. The contact forces create bending stresses in core assemblies. Thermal and irradiation creep tend to relax those stresses, thereby reducing the contact forces over time. At the same time, differential irradiation swelling due to the fast neutron flux gradient causes inelastic bowing that can increase the contact forces over time.

[0006] The relative movement between core assemblies can lead to inter-assembly interaction and variations in overall core reactivity. The complex interactions between assemblies may cause displacements that result in reactivity insertion during start-up and steady-state operations and off-normal conditions that impact reactor stability.

[0007] It would be advantageous to reduce the inter-assembly movement to encourage a more predictable core reactivity while allowing for efficient refueling and shuffling of core assemblies, especially during startup, shutdown, and transients. These, and other advantages and benefits, will become apparent to those of ordinary skill in the art by the figures and description that follow.SUMMARY

[0008] According to some embodiments, a thermally activated nuclear core restraint device includes an elongate bar having a first bar end and a second bar end, the elongate bar having a bar axis and formed of a first metallic material; and an elongate strut having a first strut end and a second strut end, the elongate strut having a strut axis, the elongate strut coupled to the elongate bar such that a bar axis and a strut axis are parallel, the elongate strut formed of a second metallic material; wherein the first metallic material has a first coefficient of thermal expansion, and the second metallic material has a second coefficient of thermal expansion that is greater than the first coefficient of thermal expansion; and wherein the thermally active nuclear core restraint device is configured to bow in a first direction in response to an increase in thermal energy.

[0009] The elongate bar may be a first elongate bar and the device may include a second elongate bar and wherein the elongate strut is disposed between the first elongate bar and the second elongate bar.

[0010] The first elongate bar, the elongate strut, and the second elongate bar may be coupled together with a first coupling at the first bar end and the first strut end. In some cases, the first coupling is a pin connection.

[0011] A second coupling may couple the first elongate bar, the elongate strut, and the second elongate bar at the second bar end and the second strut end. In this way, the bars and strut are coupled together at their ends. In some examples, the elongate strut further comprises a first mounting location at the first strut end and a second mounting location at the second strut end, and wherein the first mounting location and the second mounting location are not colinear with the strut axis. In other words, a compression force on the strut at the mounting locations will impart an eccentric compression force (e.g., an off axis compressive force) on the strut causing it to deform in a predetermined direction. The first mounting location and the second mounting location may comprise a hole configured to receive a pin.

[0012] In some cases, a hexagonal duct surrounds the elongate bar and the elongate strut. The thermally activated nuclear core restraint device may further include a nozzle coupled to a first duct end and a handling socket coupled to a second duct end. The hexagonal duct may thus be sized and shaped to be interchangeable with one or more core assemblies within a nuclear reactor core. This provides a size and shape of the thermally activated nuclear core restraint assembly to be identical to, and interchangeable with, other core assemblies, such as fuel assemblies (e.g., assemblies containing fertile and / or fissile fuel), shield assemblies, reflector assemblies, neutron absorbing assemblies, among others.

[0013] In some cases, the thermally activated nuclear core restraint device is located within an outer ring of core assemblies within a nuclear reactor core. It may also, or alternatively, be located within a penultimate ring of core assemblies within a nuclear reactor core, or at other locations within the core.

[0014] According to some embodiments, a method of restraining core assemblies within a nuclear reactor core includes the steps of placing a plurality of fuel assemblies within the nuclear reactor core, the nuclear reactor core defining a plurality of core assembly locations formed in concentric rings about the core; placing a thermally activated nuclear core restraint device in an outermost ring of the nuclear reactor core, the thermally activated nuclear core restraint device configured to preferentially bow in a desired direction in response to thermal energy; and increasing the temperature within the nuclear reactor core such that the thermally activated core restraint device bows toward the center of the core and contacts one or more adjacent core assemblies.

[0015] The method may further include the step of placing additional thermally activated nuclear core restraint devices in other locations within the outermost ring of the nuclear reactor core.

[0016] The method may also include placing additional thermally activated nuclear core restraint devices in other locations within a penultimate ring of the nuclear reactor core. This step may be performed in lieu of the previous step, that is, in some embodiments, thermally activated nuclear core restraint devices are placed in the penultimate ring without placing them in the outermost ring of the nuclear reactor core.

[0017] In some instances, the thermally activated nuclear core restraint device is configured to achieve mechanical core lock-up at a power to flow ratio (P / F) below 0.8.

[0018] In examples, the thermally activated nuclear core restraint device is formed of metals having different coefficients of thermal expansion that cause the thermally activated nuclear core restraint device to bow in a desired direction with a predetermined stroke.

[0019] The thermally activated nuclear core restraint device may be configured to impart a force on an adjacent core assembly of greater than 4kN and may be configured to deflect with a stroke of 2mm or greater.

[0020] In some example embodiments, a nuclear core restraint system includes a plurality of thermally activated core restraint devices configured to be located within core assembly locations within a nuclear reactor core, the plurality of thermally activated core restraint devices configured for preferential deformation toward a center of the nuclear reactor core in response to an increase in thermal energy, the preferential deformation configured to impart forces on adjacent core assemblies to achieve mechanical core lock-up during a startup procedure.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings are part of the disclosure and are incorporated into the present specification. The drawings illustrate examples of embodiments of the disclosure and, in conjunction with the description and claims, serve to explain, at least in part, various principles, features, or aspects of the disclosure. Certain embodiments of the disclosure are described more fully below with reference to the accompanying drawings. However, various aspects of the disclosure may be implemented in many different forms and should not beconstrued as being limited to the implementations set forth herein. Like numbers refer to like, but not necessarily the same or identical, elements throughout.

[0022] The following drawing figures, which form a part of this application, are illustrative of described technology and are not meant to limit the scope of the technology as claimed in any manner, which scope shall be based on the claims appended hereto.

[0023] FIG. 1 illustrates, in a block diagram form, some of the basic components of a sodium-cooled fast reactor, in accordance with some embodiments.

[0024] FIG. 2 is a schematic sectional view of a core of a sodium-cooled fast reactor, in accordance with some embodiments.

[0025] FIG. 3 is a top sectional view of a reactor core of a nuclear fission reactor, in accordance with some embodiments.

[0026] FIG. 4 is an enlarged sectional view of fuel assemblies and a core support structure, in accordance with some embodiments.

[0027] FIG. 5 is a perspective view of a core assembly duct, in accordance with some embodiments.

[0028] FIGS. 6A and 6B are views of a core support structure and expected core assembly deformations, respectively, in accordance with some embodiments.

[0029] FIGs. 7A and 7B are schematic representations of a bi-metallic assembly (BiMA) device, showing the bars and strut, in accordance with some embodiments.

[0030] FIGs. 8A and 8B illustrate a top view and a perspective view of a BiMA device, respectively, in accordance with some embodiments.

[0031] FIG. 9 is a perspective view of an example core assembly duct that may be used with the BiMA device, in accordance with some embodiments.

[0032] FIGs. 10A and 10B illustrate an example bar and strut, respectively of a BiMA device, in accordance with some embodiments.

[0033] FIG. 11 illustrates a method for achieving mechanical core lock-up of a nuclear reactor, in accordance with some embodiments.

[0034] FIGs. 12A and 12B illustrate an example bar of a BiMA device, in accordance with some embodiments.

[0035] FIGs. 13 A and 13B illustrate an example strut of a BiMA device, in accordance with some embodiments.

[0036] FIG. 14 illustrates a pin layout within a core assembly envelope, in accordance with some embodiments.

[0037] FIG. 15 illustrates a top view pin layout within a core assembly showing the BiMA device, in accordance with some embodiments.

[0038] FIGs. 16A and 16B illustrate isometric views of an example BiMA assembly having three bars, two struts, and two other pins, in accordance with some embodiments.DETAILED DESCRIPTION

[0039] The disclosure sets forth example embodiments and, as such, is not intended to limit the scope of embodiments of the disclosure and the appended claims in any way. Embodiments have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined to the extent that the specified functions and relationships thereof are appropriately performed.

[0040] FIG. 1 illustrates, in a block diagram form, some of the basic components of a sodium-cooled fast reactor (SFR) 100. While an SFR may be used through the description as an example type of reactor technology, it should be appreciated 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 appliable to other forms of sodium-cooled fast reactors (SFRs), such as, for example, traveling wave reactors, modular reactors, micro reactors, among others. Furthermore, the concepts presented herein may also be applicable to other reactor types, such as, without limitation, thermal reactors, water, chloride, or gas cooled reactors, as well as alternative fueled reactors, and the disclosure and appended claims should not be limited to any specific nuclear reactor, fuel source, coolant type, or reactor architecture.

[0041] In general, an SFR fission plant 100 includes a reactor core 102 containing a plurality of fuel assemblies (not shown) and other types of core assemblies. The core 102 is disposed within a pool 104 holding a volume of liquid sodium coolant 106. The pool 104 is referred to as a hot pool and has a sodium temperature higher than that of a surrounding cold pool 108 (due to the energy generated by the fuel assemblies in the reactor core 102), whichalso contains liquid sodium coolant 106. The hot pool 104 is separated from the cold pool 108 by the redan 110. A headspace 112 above the level of the sodium coolant 106 is filled with an inert cover gas, such as argon. The reactor vessel 114 surrounds the reactor core 102, hot pool 104, and cold pool 108, and is sealed with a reactor head 116. The reactor head 116 provides various access points into the interior of the reactor vessel 114.

[0042] The size of the reactor core 102 is selected based on a number of factors, including the characteristics of the fuel, desired power generation, available reactor 100 space, and so on. Various examples of an SFR fission plant may be used in low power (around 300 MWe - around 500 MWe), medium power (around 500 MWe - around 1000 MWe), and high power (around 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 core 102 to reflect neutrons back into the core 102. Additionally, fertile and fissile nuclear assemblies may be moved (or “shuffled”) within and about the core 102 to control the nuclear reaction occurring therein.

[0043] The sodium coolant 106 is circulated within the vessel 114 via a primary sodium coolant pump 118. The primary coolant pump 118 draws sodium coolant 106 from the cold pool 108 and injects it into a plenum below the reactor core 102. The coolant 106 is forced upward through the core and is heated due to the reactions taking place within the reactor core 102. Heated coolant 106 enters an intermediate heat exchanger(s) 120 from the hot pool 104 and exits the intermediate heat exchanger 120 and re-enters the cold pool 108. This primary coolant loop 122 thus circulates sodium coolant 106 entirely within the reactor vessel 114.

[0044] The intermediate heat exchanger 120 incorporates a segment of a closed liquid sodium loop that may be physically separated from the primary sodium pools 104 and 108 at all times (i.e., intermediate and primary sodium are never co-mingled). The intermediate heat exchanger 120 transfers heat from the primary coolant loop 122 (fully contained within the vessel 114) to an intermediate coolant loop 124 (that is only partially located within the vessel 114). The intermediate heat exchanger 120 passes through the redan 1 10, thus bridging the hot pool 104 and the cold pool 108 (so as to allow flow of sodium 106 in the primary coolant loop 122 therebetween). In an example, four intermediate heat exchangers 120 are distributed within the vessel 114. Alternatively, two or six intermediate heat exchangers 120are distributed within the vessel 114. Of course, any suitable number of heat exchangers 120 may be located within the vessel 114.

[0045] The intermediate coolant loop 124 circulates sodium coolant 126 that passes through pipes into and out of the vessel 114, via the reactor head 116. An intermediate sodium pump 128 located outside of the reactor vessel 114 may be used to circulate the sodium coolant 126 to a power generation system 123. Heat is transferred from the sodium coolant 106 of the primary coolant loop 122 to the sodium coolant 126 of the intermediate coolant loop 124 in the intermediate heat exchanger 120. The sodium coolant 126 of the intermediate coolant loop 124 passes through a plurality of tubes 130 within the intermediate heat exchanger 120. These tubes 130 keep separate the sodium coolant 106 of the primary coolant loop 122 from the sodium coolant 126 of the intermediate coolant loop 124, while transferring thermal energy therebetween.

[0046] A direct heat exchanger 132 extends into the hot pool 104 and provides cooling to the sodium coolant 106 within the primary coolant loop 122, usually in case of emergency. The direct heat exchanger 132 is configured to allow sodium coolant 106 to enter and exit the heat exchanger 132 from the hot pool 104. The direct heat exchanger 132 may have a similar constmction to the intermediate heat exchanger 120, where tubes 134 keep separate the NaK (Sodium -Potassium) of the primary coolant loop 122 from the direct heat exchanger coolant (NaK) 136 of a direct reactor coolant loop 138, while transferring heat energy therebetween.

[0047] Other ancillary reactor components (both within and outside of the reactor vessel 114) include, but are not limited to, pumps, check valves, shutoff valves, flanges, drain tanks, etc., that are not depicted but would be apparent to a person of skill in the art. Additional penetrations through the reactor head 116 (e.g., a port for the primary coolant pump 118, inert cover gas and inspection ports, sodium processing, and cover gas ports, etc.) are not depicted. A control system 140 may be utilized to control and monitor the various components and systems which make up the reactor 100.

[0048] Broadly speaking, this disclosure describes configurations that improve the performance of the reactor 100 described in FIG. 1 . Specifically, examples, configurations, and arrangements of core assembly supports and thermally activated core restraint system components provide for a more consistent and stable reactivity by reducing or eliminating relative movement between core assemblies. That is, embodiments described herein areconfigured to limit relative horizontal movement between adjacent core assemblies by promoting core lockup, while allowing individual core assemblies to be inserted or removed from the reactor core through vertical displacement during a refueling or shuffling operation.

[0049] FIG. 2 is a schematic sectional view of a core 200 of an 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 one or more of fissile nuclear fuel assemblies, fertile nuclear fuel assemblies, shield assemblies, reflector assemblies, control assemblies, core restraint assemblies, material testing assemblies, and standby shutdown assemblies. In general, the contents of the assemblies (e.g., fissile material, control material, etc.) identifies the particular assembly. The components of the assemblies that hold such material may be identical and interchangeable at locations within the reactor core. A peripheral core region 206 may include in-vessel storage pots 208. Throughout the life of the core 200, the fissile nuclear fuel assemblies and fertile nuclear fuel assemblies (as well as certain other assemblies) may be shuffled between the central core region 202 and the peripheral core region 206. This is performed at various stages of core life as required or desired to initiate, maintain, accelerate, or terminate nuclear reactions or power generation and / or for safety reasons.

[0050] The core assemblies 204 are received by an upper plate 210 of a core support structure 212 at locations aligned with a masking element 216. Sodium coolant is pumped into a plenum 214 disposed below the upper plate 210 and flows upward into the core assemblies 204 where it is heated by the nuclear reactions taking place within the core 200.

[0051] FIG. 3 and 4 illustrate a nuclear fission reactor core 200 that includes a plurality of nuclear core assemblies (e.g., fissile nuclear fuel assemblies 302, fertile nuclear fuel assemblies 304, movable reactivity control assemblies 306, etc.), shown as core assemblies 204. As used throughout this description, the terms core assemblies includes fuel assemblies and in some cases, may be used interchangeably, and relate to any assembly that is or may be positioned in the core whether having a type of fuel, reactivity control material, shield material, or otherwise. The term core assembly may also include other assemblies that may not have nuclear fuel, and may include neutron reflectors, neutron shields, neutron poisons, and core restraint assemblies. Similarly, the terms “core assembly duct” and “fuel assembly duct” may be used interchangeably to describe a duct that may be used with a fuel assemblyor a core assembly, depending on the material housed within the duct. In many cases, a fuel assembly duct and a core assembly duct are identical in size and shape and may be inserted at any suitable location with the core. Similarly, fuel assemblies and core assemblies may be sized and shaped to be interchangeable within the core locations within the nuclear reactor core.

[0052] In some embodiments, fuel assemblies 204 may be supported in part by a core support grid plate 210. The core support grid plate 210 may engage with a nozzle 402 of the fuel assembly 204 to provide support at the lower end of the fuel assembly 204. Primary sodium coolant flows through fuel assemblies 204, according to some embodiments to absorb heat generated by fuel within the fuel assemblies undergoing fission reactions. The core support grid plate 210 may define a plurality of apertures through which core assemblies may be inserted. In this way, the core support grid plate 210 defines core assembly locations that are configured to receive and locate core assemblies.

[0053] In some embodiments, a fuel assembly 204 includes a plurality of nuclear fuel pins (e.g., fuel rods, fuel elements, etc.), disposed within a duct that includes a tubular body. In some cases, the tubular body has a hexagonal cross-sectional shape as shown in FIGS 3 and 4. In use, the primary sodium coolant flows upwardly into the fuel assemblies 204 and around the fuel elements therein and draws heat away from the fuel assemblies and to the heat exchangers.

[0054] According to some embodiments, the fuel assemblies 204 are cantilevered in that they are secured in an aperture in the grid plate 210 by a nozzle 402 that forms a part of the fuel assembly 204. The remaining length of the fuel assembly may be largely unsupported. However, in some cases, an above core ring and / or a top ring may provide apertures that provide lateral support to the fuel assemblies once the fuel assemblies deform a sufficient distance to contact the upper support plate. In other words, the fuel assemblies may not be restrained at a location above the nozzle so that they are allowed to deform within restrained limits.

[0055] FIG. 5 illustrates an example of a duct 502 that may be used with a fuel assembly 204. In some embodiments, the duct 502 is a hollow tube that may be hexagonal in cross section. The hexagonal cross section allows a plurality of ducts to be packed into the core in an efficient packing technique, as illustrated in FIG. 3, such as a hexagonal array. The duct502 may be filled with a bundle of fissile or fertile fuel elements, neutron reflectors, neutron absorbers, core restraint devices, or other materials. The duct 502 has a lower end 504 that may be coupled to a nozzle and an upper end 506 that may be coupled to a handling socket. Disposed along the length of the duct may be an above core load pad 508 and / or a top load pad 510. The above core load pad 508 may be positioned such that the above core load pad 508 is adjacent to an above core load pad (ACLP) ring within the core that defines a lateral boundary around the fuel assemblies within the core. Similarly, the top load pad 510 may be positioned along the duct 502 such that the top load pad (TLP) 510 is adjacent to a top load pad ring within the core that defines a second lateral boundary around the fuel assemblies within the core. In some cases, an ACLP ring and / or a TLP ring may be positioned around the core and the arrangement of fuel assemblies to provide an annular constraint to the bundle of core assemblies, but may not necessarily constrain each core assembly individually. In use, as the core assemblies begin to deform, such as by bowing, the outermost fuel assemblies may eventually contact one or more of the ACLP ring or the TLP ring.

[0056] FIGS. 6A and 6B illustrate mechanical core design elements and notional deformation. Core assembly deformations are driven by various phenomena causing interassembly interactions (e.g., contact between adjacent assemblies) due to the design of the gaps between adjacent ACLPs 508 and TLPs 510. The movement of the fuel assemblies 204 relative to each other is largely driven by thermal and flux gradients, inter-assembly contact, contact with core support structures, and seismic excitation. In addition, over its lifetime fuel assemblies 204 undergo inelastic deformations due to the thermal and irradiation creep and void swelling. The complex interactions between assemblies may cause displacements that result in reactivity insertion during start-up, steady-state and transient operations which impact reactor stability.

[0057] Many SFRs rely on ACLPs 508 and TLPs 510 to achieve a Limited Free Bow (LFB) configuration that is designed to limit reactivity insertion (e.g., the changing of reactivity due to relative movement between fuel assemblies). FIGS. 6A and 6B illustrate a series of hexagonal fuel assemblies in a reactor core. The bottom portion of the fuel assemblies 204 is constrained at the nozzle. Above the core region is the portion of the fuel duct having the ACLP 508 located at an ACLP elevation. The ACLP is configured tomaintain an ACLP gap 602 between adjacent fuel assemblies as they start to deform and provides preferential contact points between fuel assemblies 204.

[0058] The core restraint system shown further includes an ACLP ring 604 and a TLP ring 606. The core restraint system is configured for several important functions: to control the radial position of the core and maintain alignment between core components; limit motion of the fuel assemblies during seismic events; and provide a limit on fuel assembly bowing such that a negative reactivity feedback occurs in an over-power transient event.

[0059] The thermal and irradiation creep strain causes the fuel assemblies to deform, initially in a bowing direction, and may ultimately cause the fuel assemblies to take on 2ndand 3rdorder deformations. This inelastic bowing results in residual contact forces between assemblies and can cause difficulties during refueling due to friction effects and additional loading forces. In a typical shuffling or refueling operation, individual core assemblies may be removed vertically from the core and either replaced, repositioned, or reinserted. Thus, the core assemblies should be able to move vertically, despite being constrained radially (e.g., horizontally).

[0060] Temperature changes in the core during power up and power down events tend to cause nonuniform thermal gradients in both axial and radial directions throughout the core. As used herein, the axial direction refers generally to a vertical direction of the core. Notably, the core assemblies are typically elongate members having a hexagonal cross section, and the axial direction of the core assemblies is along the length of the core assemblies, which is a vertical direction when the core assemblies are positioned within the core. Similarly, the term radial refers to a generally horizontal direction that is orthogonal to the axial direction. The thermal gradients can introduce bending effects to the core assemblies, as further shown in FIG. 6B. The core restraint system provides protection against overpower events by taking advantage of, and limiting, thermally induced bending of the fuel assemblies. As shown in (i), a row of three fuel assembles are located radially away from the center of a reactor core. As a thermal gradient increases, as shown in (ii .), and the thermal gradient introduces higher temperatures nearer to the center of the reactor core, the fuel assemblies begin to bow outward away from the center of the reactor core, which therefore reduces the reactivity. As shown in (iii.), once the outermost fuel assemblies contact the TLP ring 606, the temperature gradient increases and the center of the fuelassembly 204 bows inward which increases reactivity. As the temperature gradient increases due to the increased reactivity, the inward bowing continues until the fuel assemblies contact adjacent fuel assemblies at their respective ACLPs 508. When the fuel assemblies 204 contact one another, no further compaction can occur, and the reactor is considered locked up. While an example core restraint system is shown, the descriptions herein apply equally to other core restraint systems. For example, the solutions to relative motion between core assemblies described herein may be applied to any type of core restraint system and with any type of nuclear reactor.

[0061] According to some embodiments, one or more core assemblies may include thermally activated core restraint system components that provide preferential bowing to encourage core lockup. In some cases, the core restraint components may encourage core lockup sooner than the core would naturally achieve core lockup without the core restraint components.

[0062] A robust understanding of the performance characteristics of the core restraint system is helpful for safe, reliable, and economic operation of the nuclear reactor. In the design of the fast reactor core, the positions of the core assemblies and their mechanical interactions with other core assemblies and core support structures should be controlled in order to ensure adequate core safety and performance.

[0063] The reactivity feedback from core radial expansion is an important part of maintaining a net negative power and temperature coefficients of reactivity. That is, power and temperature are inextricably linked, such that an increase in either power or temperature increases the other. A net negative coefficient of reactivity provides that as power and / or temperature are increased, the net reactivity goes down, thus inhibiting a runaway reactor and avoiding core damage. In a fast reactor core, small motions of the fuel region give rise to changes in the core reactivity. Positive reactivity is added when the core assemblies displace radially inward. Core assembly motions are most likely during changes of power to flow ratio (P / F), which occur during reactor startup or during reactor transients. As PZF changes from lower to higher values, core assembly temperatures and temperature gradients build up. Transverse duct temperature gradients cause the assemblies to bow and the resultant deformations change the core reactivity level.

[0064] In some reactors, a limited free bow design concept is employed to cause positive reactivity during reactor startup until the core locks up. The core is defined as mechanically locked-up when a solid load path exists from the core former ring through the top load pads of the outer rings of assemblies, to the above core load pads of the inner ring assemblies. In this way, the bowing of the core assemblies due to transverse temperature differences and radiation induced swelling and creep is limited by the core support structure. The direction and magnitude of bowing may be controlled by restraining lateral motion at three elevations along the core assemblies: such as at the TLP, the ACLP, and the inlet nozzle.

[0065] According to some embodiments, components of a core restraint system may be thermally activated, that is, a system that is characterized by introducing artificially larger rates of thermal distortion within selected core assemblies, and / or the peripheral boundary core assemblies in order to increase refueling and / or improve core restraint response during startup. In some cases, a bi-metallic core assembly (BiMA) may be provided that is designed to experience preferential deformation in a predetermined direction in response to temperature increases within the core. In some cases, the bi-metallic core assemblies may be provided around the periphery of the core to encourage core lock-up. The bi-metallic core assemblies may be provided in every peripheral location in the outer ring of core assemblies, or in every other position, in every third position, in every fourth position, or otherwise within the outer ring of core assemblies. In some examples, the bi-metallic core assemblies are located in the penultimate ring of core assemblies in addition to, or in the alternative to, the outer ring and may be located in each location in the penultimate ring, in every other location, in every third location, and so on.

[0066] FIGs. 7A and 7B illustrate an embodiment of a bi-metallic core assembly (BiMA) 700 configured to respond to a thermal energy increase by preferentially deforming in a predictable direction and magnitude to encourage core lock-up. In some cases, the BiMA configuration, number, and location can be selected to achieve core lock-up at a specific P / F ratio, or below a specific P / F ratio, such as below 1.0, or below 0.8, or below 0.7, or below 0.5.

[0067] In particular, FIG. 7A is an exploded top view of the BiMA device 700 showing two bars 702 and one strut 704 in between the bars 702. In some cases, the struts 704 and bars 702 are formed of dissimilar metals. For example, the bars may be made of a materialhaving a first coefficient of thermal expansion, while the struts may be made of a material having a second coefficient of thermal expansion, and the second coefficient of thermal expansion may be higher than the first coefficient of thermal expansion. Because of the dissimilarity in materials, as the BiMA device experiences a temperature rise, such as the transition from a refueling state to a hot shutdown state (e.g., from about 180° C to about 305° C in some cases), the struts 704 and bars 702 will experience compression and tension loading, respectively. The struts 704 center axis is offset by a certain distance with respect to the longitudinal axis of the bars 702 in order to promote and control bowing. In some cases, the struts 704 and bars 702 are pinned together, such as at one or more of the TLP, ACLP, and above nozzle positions. In some cases, the bars 702 and struts 704 are pinned together at all three identified locations. The top and bottom pins are thus constrained against horizontal displacement, while the ACLP joint is free. Because of this, the ACLP will move in the core radial direction and be able to impart load to the adjacent core assembly. Of course, the bars 702 and struts 704 may be coupled together by any suitable coupling, and need not use pins at all the coupling points. For example, the coupling could be a bearing, a ring coupling, a weld, a hinge, or some other type of coupling.

[0068] In a refueling state, the core is cooled to a temperature that alleviates the elastic thermal deformation of the core assemblies, such that one or more core assemblies can be removed from the core and shuffled to new core locations or replaced with fresh core assemblies. In a hot shutdown state, the reactor remains subcritical, but has an increased temperature as compared with the refueling state. Therefore, in some examples, the BiMA devices 700 are configured to return to their original shape when the nuclear reactor transitions from an operating condition to a hot shutdown state. Similarly, the BiMA devices 700 may be configured to return to their original shape whe the nuclear reactor transitions from an operating condition to a refueling state. As the temperature in the nuclear reactor reduces, the BiMA devices 700 may be configured to return to their original shape, which alleviates the assembly to assembly contact forces, to allow removal and insertion of other core assemblies. As used herein, where the BiMA devices return to their original shape, it should be appreciated that minor residual variations in the shape of the BiMA devices may occur due to temperature affects, radiation, creep, and swelling. Therefore, where the description teaches that the BiMA devices return to their original shape as the reactor corecools, it is used to mean that the BiMA devices substantially return to their original shape, such as within 90% or 95%, or 97%, or 98%, or 99% of their original shape. For instance, where the designed stroke of the BiMA device is a 2mm deflection, the BiMA device may return to its original shape by returning with a stroke of 1 ,8mm, which is within 90% of its original shape. Thus, while the deformation of the BiMA devices 700 is mostly elastic deformation, it should be appreciate that there may be a small amount of inelastic deformation due to the environmental conditions within the operating nuclear reactor core.

[0069] FIGs. 8A and 8B illustrate a top view and a perspective view of a BiMA 800 device, respectively. In the illustrated embodiment, three bars 802 and two struts 804 are interposed between the three bars 802. The bars 802 have a longitudinal bar axis, and the struts 804 have a longitudinal strut axis. In some cases, the struts are interposed between the bars such that their respective longitudinal axes are parallel. The bars 802 each have a first end and a second end. Similarly, the struts have a first end and a second end. The bars 802 and struts 804 may share one or more pin connections 806, and as best shown in FIG. 8A at their respective first ends, second ends, or both. The struts 804 may be pinned at a location that is off-axis with respect to a central longitudinal axis of the strut 804. The bars may be pinned at a location that aligns with a longitudinal bar axis. The bars are formed to have a coefficient of thermal expansion that is less than the coefficient of thermal expansion of the strut. Consequently, upon experiencing an increase in temperature, the bars experience tension as the struts attempt to expand while the strut experiences an eccentric compression load because it is constrained by the bars. The eccentric compression load on the strut causes the strut to deform laterally, as shown by arrow 810. The BiMA device 800 may have 3 bars 802 and two struts 804, as illustrated, but may have other numbers of bars and struts, such as 4 bars and 3 struts, 5 bars and 4 struts, etc. In some cases, different BiMA devices 800 used in the core may have a different number of bars and struts when compared with other BiMA devices 800 disposed in the core.

[0070] In some examples, two or more struts 804 may be linked together, such as by a web 812 that inhibits lateral distortion of the strut 804 and promotes the struts 804 to move together in response to temperature changes.

[0071] The differing materials of the BiMA may be selected to achieve the required lateral forces to achieve mechanical lock-up of the core at a desired point during reactorstartup. In some cases, the BiMA includes the bars 802 and struts 804, as illustrated, and are disposed within an assembly duct (502 of FIG. 5), and may additionally have a nozzle attached to a first end of the duct and a handling socket coupled to a second end of the duct. Consequently, the BiMA may have the same size and shape of other core assemblies, and are therefore insertable into any core location within the nuclear core and are interchangeable with other core assemblies.

[0072] In some instances, the BiMA materials are selected to have a high coefficient of thermal expansion for the struts and a relatively lower coefficient of thermal expansion for the bars. These materials may be purposefully selected and the components sized to result in a bowing of the BiMA to impart compressive forces to the core. By compressing the core, the core assemblies are moved closer together, which increases reactivity, and achieves mechanical lock-up. Subsequently, any relative motion between core assemblies will have a tendency to reduce reactivity, thus providing negative reactivity in response to additional core movement. For instance, once the core achieves mechanical lockup, any power surges will introduce negative reactivity. In some cases, the BiMA are positioned at each position around the periphery of the core, in every other position around the core, or some other placement schema in order to achieve the desired limited free bow mechanism. The selected arrangement schema also allows the timing of the core lockup to be predetermined. For instance, the arrangement of the BiMA devices determines power ramp up modes, with more BiMA devices achieving core lock-up sooner which allows the reactor to ramp up from standby to full power much quicker than without the BiMA devices.

[0073] According to some embodiments, the struts may be formed of any suitable steel, and in some embodiments, is 316SS. The bars may be formed of any suitable steel having a lower coefficient of thermal expansion than the struts, and in some cases is HT9 steel, T91 / T92 steel or a variation of HT9 steel. In some examples, any other materials having different coefficients of thermal expansion may be used for the struts and bars, including, without limitation, 316 stainless steel, D9 steel, HT9 steel, T91 / T92 steel, SS304 stainless steel, Inconel alloys, carbon steel, and vanadium alloys. The BiMA assemblies may be oriented within the core such that the preferential deformation is always toward the center of the core, thus promoting core lock-up. The BiMA assemblies may additionally be positioned such that the preferential deformation has a component of the force vector that is toward thecenter of the core even though the direction of deformation may not be directly toward the center of the core.

[0074] FIG. 9 is a perspective view of an example core assembly duct 900 that may be used with the embodiments of systems, components, and methods described herein. In particular, the duct 900 is an elongate member having a hexagonal cross section and further having a first end 902 and second end 904. The duct may have one or more load pads 906, as described in reference to other ducts herein, that presents a surface for adjacent core assemblies to achieve mechanical contact. The duct 900 may be hollow and further have a window 910 formed in a sidewall of the duct 900. In some cases, the window 910 is sized and configured to allow at least a portion of the BiMA device (not shown) to extend therethrough at maximum deformation. In other words, as the BiMA device heats up, the thermal response of the BiMA device and its different materials is to bow such that at least a portion of the BiMA device protrudes through the window 910. In some cases, as the BiMA device protrudes through window 910, it will contact an adjacent core assembly and impart a radial force on the adjacent core assembly. In other cases, the BiMA device achieves its maximum stroke adjacent the above core load pad such that the above core load pad is displaced to contact load pads of adjacent core assemblies.

[0075] The BiMA device may contact an adjacent core assembly at its load pad in order to transmit a load from a first core assembly to a second core assembly. In many cases, the radial force is in a direction generally toward a central core region and will compact the core toward the central core region.

[0076] According to some examples, the window 910 may be provided so that the BiMA device does not need to deform the duct 900 in order to exert a lateral force on an adjacent core assembly. As an alternative to the window, which may impact coolant flow through the periphery of the core, a predefined region of weakness of the duct could be employed so as to maintain performance of the core. The predefined region of weakness may include, for example, a thinner duct wall, slots that leave a reduced amount of duct material, a different duct material at a preferred location along the duct, a region of the duct having more elasticity, or some other feature or structure that reduces at least a portion of the duct’s ability to withstand deformation of the BiMA device within the duct.

[0077] As with the various embodiments described herein, the BiMA assembly may be located toward the periphery of the core and may be located in every peripheral location of the outermost ring of the core, or in every other location in the outermost ring of the core, or every third location in the outermost ring of the core, or some other location schema and number of BiMA assemblies. In some cases, one or more BiMA assemblies may be located in the penultimate ring of the core in any suitable configuration and number in order to compact the core, as desired, to tune the core for mechanical lock-up at a certain power level and at a certain temperature of the core. In some embodiments, the BiMA assemblies have been shown to achieve mechanical core lock-up up at a P / F equal to about 0.5, or 0.6, or 0.7, or 0.8. The reactivity response past the lockup (e g., P / F =0.5-0.8, in some cases) appears linear with a negative constant slope. By adding additional BiMA’s into the core before startup, the slope following core lock-up increases negatively, which may be favorable to stabilize the core during transients.

[0078] As described herein, BiMA devices may be added to any suitable core location but will typically be added in the last ring or the penultimate ring of the core. In some cases, similar behavior is observed between the configurations in which the BiMA devices are included at the last ring within the core and the penultimate ring within the core. In some cases, it may be preferable to include BiMA assemblies in the penultimate ring in the core, as the BiMAs would replace reflector assemblies located in the penultimate ring rather than shield assemblies located in the outermost ring. According to some embodiments, a nuclear reactor core may utilize rings of shield assemblies in the outermost rings of core locations, which may include one, two, three, four, or more rings of shield assemblies. The BiMA devices described herein may be used to replace one or more shield assemblies at any suitable location in the nuclear reactor core. Similarly, a nuclear reactor core may include one or more rings of reflector assemblies, which are usually disposed nearer to the center of the core than the shield assemblies. The nuclear reactor core may include one, two, three, or more rings of shield assemblies and the BiMA devices described herein may be used to replace one or more shield assemblies at any suitable location in the nuclear reactor core.

[0079] In some cases, the BiMA devices are configured to exert a lateral force on adjacent core assemblies with a force sufficient to deform, at least elastically, adjacent core assemblies once the nuclear core temperature reaches a certain temperature. In some cases,the BiMA devices are configured to deliver an initial force to an adjacent core assembly of greater than 3kN, or 4kN, and in some cases, greater than 5kN, or 7kN, or 8kN, or 9kN, or lOkN, or more to achieve core lock-up. The strut(s) of the BiMA devices may be sized to deliver the desired force and are generally longer in an axial direction than they are wide or thick.

[0080] As the nuclear reactor core cools, the contact force imparted by the BiMA devices reduces as the BiMA devices return to their original shape. For example, as the nuclear reactor core cools, such as to a refueling state (e.g., around 180° C in some cases), the BiMA devices relax and the core is no longer mechanically locked up, which allows core assemblies to be removed and inserted into locations throughout the core.

[0081] In some cases, the contact forces developed between adjacent core assemblies may be selectively configured by selecting the configuration of the BiMA, such as, by changing the geometry of the strut, and / or the bars, which will impact the thermal response of the BiMA device. Where the BiMA devices are stronger as compared to a baseline BiMA configuration, fewer of them are required to achieve full mechanical lock-up. In some cases, stronger BiMA devices may be used to achieve full mechanical lock-up at P / F=0.0 during startup. The BiMA implementation scheme can be selected to achieve full core mechanical lock-up at any desirable P / F ratio, such as 0.5, 0.6, 0.7, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0 or other desirable P / F ratio.

[0082] FIGs. 10A and 10B illustrate an example bar 1002 and strut 1004, respectively.In general, a strut 1004 is sandwiched between two bars 1002, and therefore, in an assembled BiMA device, the number of bars 1002 is generally one more than the number of struts 1004. In some embodiments, a BiMA device includes 2 bars and 1 strut, or 3 bars and 2 struts, or 4 bars and 3 struts, or 5 bars and 4 struts, etc.

[0083] The strut 1004 has a longitudinal axis 1006, and as is illustrated, a mounting hole 1008 may be located off-axis, such that a compression loading on the strut 1004 at the mounting hole 1008 is eccentric, which will cause the strut to bow in a preferential direction in response to a compression load. Moreover, the selection of the strut material, and its physical properties (e.g., length, thickness, width, etc.) will determine the amount of bowing of the strut at a particular temperature.

[0084] The bars 1002 have a similar mounting hole 1010 that may align with the longitudinal axis of the bar 1002. The bars 1002 and struts 1004 may be coupled together at their respective mounting holes, such as by a pin. As the BiMA device heats up, the materials forming the bars 1002 and struts 1004 will thermally expand according to their respective coefficients of thermal expansion (CTE). As described, the bar 1002 has a CTE that is lower than the CTE of the strut 1004. Consequently, as the materials heat up, the strut 1004 will attempt to expand more than the bar 1002, which places the bar in tension and the strut in compression. In response to the forces, and because the strut 1004 is constrained eccentrically by pins at its ends, the stmt will experience a bowing mode of distortion in a lateral direction. The pin may restrain longitudinal expansion of the stmt 1004, while allowing the stmt to bow in response to the compressive force.

[0085] In some embodiments, the BiMA device may be installed into an assembly duct, thereby forming a BiMA core assembly, or just BiMA assembly. The BiMA core assembly has the same size and shape as other core assemblies and may therefore be installed at any location within the core, and is interchangeable with any other core assembly. The BiMA core assembly may be formed of different materials having different coefficients of thermal expansion that causes preferential bowing of the BiMA assembly. Moreover, the BiMA device within the BiMA assembly may have a portion that is connected eccentrically, thus causing the eccentric loading to result in preferential bowing in a desired direction. It should be appreciated that the terms BiMA device, BiMA assembly, and BiMA core assembly may be used interchangeably throughout this disclosure.

[0086] In some instances, the BiMA devices may be formed to have multiple functions. For example, one or more of the BiMA devices, or the BiMA core assemblies, may be configured to have radiation shielding properties. For instance, the BiMA device, or the duct surrounding the BiMA device may be coated with shielding materials, or the duct may be filled with shielding materials, or the duct may be formed of a material that acts as a shield.

[0087] Similarly, one or more BiMA devices and / or a BiMA assemblies may be configured to function as a neutron reflector. A coating may be applied to the BiMA device, or the duct, or the duct may be formed of, or contain, reflector materials in order to reflect neutrons back into the core. While a multi-functional BiMA assembly may not operate as efficiently as a dedicated shield or reflector, it may nevertheless be configured to have adesired function to influence core neutronics. In this way, one or more BiMA devices may replace reflector assemblies or shield assemblies within the core and may therefore assist with the function provided by the replaced core assembly.

[0088] The impacts of such as core restraint system has been investigated and modeled by the inventors hereof. For instances, core distortions, core radial expansion reactivity feedback have been calculated with proprietary codes bases designed to perform all mechanical calculations for the reactor core and using finite element theory code bases which allows for analysis of complicated geometries and mixed analysis conditions. In addition, further analysis has been performed to solve for the change in reactivity of the core based on changes in core assembly positions or displacement using virtual density theory of neutronics to model non-uniform and localized geometry distortions via perturbation theory.

[0089] FIG. 11 illustrates a method 1100 for achieving mechanical core lock-up of a nuclear reactor, in accordance with some embodiments. At block 1102, a plurality of fuel assemblies are located in a nuclear reactor core. The nuclear reactor core defines locations for core assemblies, such as by providing apertures for receiving nozzles of the fuel assemblies, along with an upper plate of the core support structure that also may define holes for receiving core assemblies.

[0090] At block 1104, a thermally activated nuclear core restraint device is located in a peripheral ring (e.g., outermost ring or penultimate ring) of the nuclear reactor core in a core assembly location. It should be appreciated that more than one thermally activated nuclear core restraint devices may be located in the core, and any number and location of the devices may be placed within core assembly locations within the core as desired. As discussed above, in some cases, the thermally activated nuclear core restraint devices may be located in each location in the outermost ring of the nuclear reactor core, or in every other location, or in every third location, in every fourth location, or some other arrangement schema as desired. As further described, the thermally activated nuclear core restraint devices are not limited to the outermost ring of the core, but may likewise be located in the penultimate ring, the antepenultimate ring, or the preantepenultimate ring, as desired. In some cases, the thermally activated nuclear core restraint devices do not contain nuclear fuel, that is they do not contain either fissile or fertile fuel, but rather, contain a BiMA device that is formed of at least two different metallic materials having different coefficients of thermal expansion thatwill cause the thermally activated nuclear core restraint device to deform in a predetermined direction a distance sufficient to contact adjacent core assemblies and impart a force thereon.

[0091] At block 1106, the temperature within the nuclear reactor core is increased which causes the thermally activated nuclear core restraint devices to bow toward the center of the core and contact adjacent core assemblies. The contact imparts a force on the adjacent core assemblies. The contact further causes the adjacent core assemblies to deform toward the center of the core. As the core assemblies are compacted toward the center of the core, the reactivity within the core increases, which also increases the temperature within the core. In many cases, this initial deformation and core compaction happens during reactor startup. Once the core is compacted and the core assemblies are in physical contact with adjacent core assemblies, the core is mechanically locked-up and it cannot be compacted further. Therefore, as the reactor has power and / or temperature fluctuations, these fluctuations do not cause further deformation that would cause increased reactivity. On the contrary, any further deformation of the core assemblies will tend to separate the core assemblies from one another and therefore has a negative reactivity effect on the core.

[0092] FIGs. 12A and 12B illustrate an embodiment of a bar in accordance with some embodiments. Similar with other embodiments of bars 1200 described herein, the bar 1200 is a generally elongate structure formed of a suitable material having a coefficient of thermal expansion. As illustrated, the cross section of the bar may be generally circular. For instance, the bar may have a circular cross section, an ovoid cross section, a rounded rectangular cross section, or some other suitable shape. At a first and second end of the bar 1200, a flat 1202 may be formed. The flat 1202 may sized and shaped to facilitate engagement with one or more struts, as further described below. In some cases, the flat 1202 may have an aperture 1204 formed therein to allow a connection with adjacent bars or struts, such as by a pin. In generally, the flats 1202 at the first end and the second end will be coplanar to one another. By making the flats 1202 coplanar, the bar will tend to bow in a direction that is parallel to the plane of the flat. As illustrated, the aperture 1204 may be formed in the center of the flat 1202, which is along a longitudinal axis of the bar.

[0093] FIGs. 13A and 13B illustrate isometric view of an embodiment of a pair of struts 1300 in accordance with some embodiments. As can be seen in the closeup view of FIG. 12B, the pair of struts 1300 may have a circular cross section. Of course, other crosssectional geometries may be used, such as an ovoid cross section, a rounded rectangular cross section, or otherwise. By varying the geometry, the BiMA device may be tuned to have a predetermined bowing direction, bowing displacement, and applied force on adjacent core assemblies. The pair of struts 1300 may be connected together by a plate 1302. In some cases, the plate is rigid and locks the struts 1300 together so they move together as well. The struts 1300 have a first end 1304 and a second end 1306 coupled together by the elongate struts 1300. The first end 1304 and the second end 1306 may be formed with couplings 1308 that permit the struts 1300 to be coupled to one or more bars.

[0094] The couplings 1308 may be added to the struts through any suitable additive manufacturing or material removal process, and in some cases, the couplings 1308 are welded to the structs, or may be machined into the struts. The couplings 1308 may comprise two protrusions 1310 that define a channel 1312 therebetween. The channel 1312 may be sized and shaped to receive the flat 1202 (Fig. 12B) of the bar 1200. The couplings 1308 may have a hole 1314 or aperture formed therethrough to facilitate a connection with a bar 1200, such as by a pin. The pair of struts 1300 may be in surface contact with one another along the respective periphery of each of the two struts. In some cases, a gap may be left between each of the pair of struts, so that they are not in surface contact, but are only coupled by the plate 1302. In some cases, the hole 1314 is not colinear with a longitudinal axis of either strut or the entire assembly itself, but rather, is offset to provide an eccentric connection location. Due to the eccentricity of the mounting location, a compressive force on the struts 1300 will cause the struts to bow in a predetermined direction. The struts, in some cases, are formed of a different material, or a different alloy, as compared to the bars, and in some cases, the struts have a larger coefficient of thermal expansion than the material associated with the bars. As such, as the assembly is heated, the struts will bow outwardly due to the eccentric mounting with the bars.

[0095] In some examples, the protrusions 1310 have one end 1316 that is convexly curved to match the curvature of the struts and is flush therewith, and a second end 1309 that is concavely curved to receive a convexly curved bar, as is shown in FIG 15.

[0096] FIG. 14 illustrates an arrangement 1400 of cylinder pins 1402 within an envelope of a core assembly duct 1404. In many cases, core assemblies have a hexagonal duct 1404 that allows efficient packing of core assemblies within a nuclear reactor core. The illustratedarrangement shows seven pins arranged within a hexagonal duct 1404. The locations of the pins relative to one another adhere to a triangular pitch with the longitudinal axis of the respective pins forming triangles therebetween. In some cases, the 7-pin configuration allows for a BiMA assembly to have three bars, two struts, and two additional pins that may be used for a different purpose, which may be a purpose different from promoting core lock up. For example, in some cases, one, two, or more additional pins may be used for reflection, shielding, or some other purpose. In some cases, the additional pins may be formed of materials that provide neutron reflection, neutron absorption, or some other purpose to assist with reactivity control.

[0097] The cylinder pins 1402 may be wire wrapped, as is known to those of skill in the art of fuel pins. The wire wrapping may be used to maintain a specified distance between the cylinder pins 1402, such as to allow channels for coolant to flow through the hexagonal duct 1404.

[0098] FIG. 15 illustrates a top view of a 7-pin BiMA device 1500 within a hexagonal duct 1404. As illustrated, a BiMA device 1500 having three bard and two struts may be positioned within the hexagonal duct 1404, which may still leave room for additional pins 1502. The additional pins may be configured to have reactivity influence, and may include neutron absorbers, neutron reflectors, neutron poisons, neutron shields, or some other function. Furthermore, while the additional pins 1502 are illustrated to have a circular cross section, they may be formed to have a different cross sections from each other, or a different cross section from the other BiMA device pins. In some cases, the additional pins 1502 may have an ovoid cross section, which allows them to occupy more volume within the hexagonal duct as compared to cylindrical pins. Similarly, the additional pins, or even a single pin, may be shaped to occupy the space within the duct that is not required for the BiMA device 1500 components. In some cases, the additional pins 1502 may have a trapezoidal cross-sectional shape to occupy the volume within the hexagonal duct.

[0099] As can be seen, the second end of the strut protrusion may have a concave face 1309 that allows the strut to nest with the additional pin 1502. In some cases, there may be surface contact between the concave face 1309 and the additional pin. In some examples, the struts 1300 may be positioned adjacent the duct and are not located along an axis of the duct, such as an axis from corner to comer of the duct, or an axis from a center of a first face to acenter of an opposing second face. In this way, the struts may be offset from a central region of the duct and are positioned to exert a force on the duct. As described herein, the struts may be positioned to exert a force in the direction toward a face of the duct, or may be oriented to exert a force in the direction of a corner of the duct. In some cases, the orientation of the strut force, either toward a face or toward a corner, is dependent upon the location of the BiMA device within the core, such that BiMA devices located around the core may exert forces both toward a face of the duct, and toward a corner of the duct, in order to provide compaction forces in the direction of the center of the nuclear reactor core.

[0100] As illustrated, the pair of struts 1300 is coupled to the bars 1200 by a connecting pin 1504. The struts and bars, at each of their respective ends, has a hole that allows a pin to pass therethrough to connect the bars 1200 and struts 1300 together. The pin may be cylindrical to pass through circular holes in the bars and struts.

[0101] FIG. 16A illustrates a perspective view of the BiMA device 1500 without the hexagonal duct and FIG. 16B illustrates a close-up perspective view of one end of the BiMA device 1500 that further illustrates the coupling of the bars 1200 and struts 1300, which may be accomplished by a pin 1504 through holes formed near the ends of the bars 1200 and struts 1300. In this way, the bars 1200 and struts 1300 are constrained from differential longitudinal movement, and thermal expansion of the bars 1200 and struts 1300 has a tendency to encourage the struts 1300 to bow. Due to the eccentric compressive loading on the struts, they will bow outwardly, away from the center of the BiMA device 1500, which may be first order deformation (e.g., banana shaped). In some cases, the struts 1300 are coupled by a plate 1302 that causes the struts 1300 to deform together. The plate 1302 may have a thickness that is substantially less than its width, which has a tendency to constrain the struts 1300 from moving in an X direction 1602, but allows the struts 1300 to bow in a Y direction 1604.

[0102] The embodiments described herein improve the radial expansion reactivity response for a nuclear reactor. The BiMA system described herein results in a thermally activated restraint system having a pronounced and beneficial effect on the core restraint system performance. According to experimental results, the core compaction during the transition from the refueling state to the hot shutdown state is beneficial and a certain amount of force and displacement may be required to provide compaction. Moreover, compactingthe core during the transition between refueling states and the hot shutdown state can result in core lock-up before the transition to starting up the reactor on nuclear power. A BiMA device can apply upwards of 5kN, 6kN, 7kN, 8kN or more force at a stroke of about 2mm, 3mm, 4mm, 5mm, or more can provide sufficient core compaction loading to lock-up the core during the transition between refueling states and the hot shutdown state. The stroke is a linear distance measured as a distance of deformation. Based on the quantity, location and orientation of the BiMA core assemblies, the state of core compaction changes and can be tuned to achieve core lock-up at a desired P / F ratio, at a desired temperature, or at a desire time, such as a transition between a refueling state and a hot shutdown state.

[0103] The BiMA core assemblies may be oriented such that they provide face to face contact with adjacent core assemblies. Alternatively, they may be orientated to provide corner to comer contact with adjacent core assemblies. In other words, in a hexagonal core assembly, the BiMA devices may be disposed in a hexagonal duct such that their preferential bowing is in a direction toward a face of the duct, or toward a corner of the duct to provide a desired force on adjacent core assemblies. In some cases, some BiMA devices disposed within a core may act in a face-to-face direction while some BiMA devices disposed within a core may act in a corner-to-comer direction.

[0104] By adding additional BiMA assemblies within the core map, the P / F value at which the core mechanically locks-up can be lowered. For example, with enough BiMA assemblies deployed throughout the core, the core can lock-up before the transition to start up the reactor on nuclear power. Similarly, moving the BiMA assemblies from the outer ring to the penultimate ring may have a beneficial effect proportional to the number of BiMA assemblies used and may therefore replace a reflector assembly rather than a shield assembly. As discussed above, the BiMA devices may be configured as neutron reflectors and thus may be configured to perform dual purposes.

[0105] The foregoing description of specific embodiments will so fully reveal the general nature of embodiments of the disclosure that others can, by applying knowledge of those of ordinary skill in the art, readily modify and / or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of embodiments of the disclosure. Therefore, such adaptation and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on theteaching and guidance presented herein. The phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the specification is to be interpreted by persons of ordinary skill in the relevant art in light of the teachings and guidance presented herein.

[0106] The breadth and scope of embodiments of the disclosure should not be limited by any of the above-described example embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0107] Conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain implementations could include, while other implementations do not include, certain features, elements, and / or operations. Thus, such conditional language generally is not intended to imply that features, elements, and / or operations are in any way required for one or more implementations or that one or more implementations necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and / or operations are included or are to be performed in any particular implementation.

[0108] Unless otherwise noted, the terms “connected to” and “coupled to” (and their derivatives), as used in the specification, are to be construed as permitting both direct and indirect (i.e., via other elements or components) connection. In addition, the terms “a” or “an,” as used in the specification, are to be construed as meaning “at least one of.” Finally, for ease of use, the terms “including” and “having” (and their derivatives), as used in the

[0109] The specification and annexed drawings disclose examples of systems, apparatus, devices, and techniques that may provide mechanical core lock-up at a low P / F and provide core stability during startup, shutdown, and transients. It is, of course, not possible to describe every conceivable combination of elements and / or methods for purposes of describing the various features of the disclosure, but those of ordinary skill in the art recognize that many further combinations and permutations of the disclosed features are possible. Accordingly, various modifications may be made to the disclosure without departing from the scope or spirit thereof. Further, other embodiments of the disclosure may be apparent from consideration of the specification and annexed drawings, and practice of disclosed embodiments as presented herein. Examples put forward in the specification andannexed drawings should 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 used for purposes of limitation.

[0110] From the foregoing, it will be appreciated that, although specific implementations have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the appended claims and the elements recited therein. In addition, while certain aspects are presented 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 recited as being embodied in a particular configuration, other aspects may likewise be so embodied. Various modifications and changes may be made as would be obvious to a person skilled in the art having the benefit of this disclosure. It is intended to embrace all such modifications and changes and, accordingly, the above description is to be regarded in an illustrative rather than a restrictive sense.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A thermally activated nuclear core restraint device, comprising: an elongate bar having a first bar end and a second bar end, the elongate bar having a bar axis and formed of a first metallic material; and an elongate strut having a first strut end and a second strut end, the elongate strut having a strut axis, the elongate strut first strut end coupled to the elongate bar first bar end and the second strut end coupled to the second bar end such that a bar axis and a strut axis are parallel, the elongate strut formed of a second metallic material; wherein the first metallic material has a first coefficient of thermal expansion, and the second metallic material has a second coefficient of thermal expansion that is greater than the first coefficient of thermal expansion; and wherein the thermally activated nuclear core restraint device is configured to bow in a first direction in response to an increase in thermal energy.

2. The thermally activated nuclear core restraint device of claim 1, wherein the elongate bar is a first elongate bar and further comprising a second elongate bar and wherein the elongate strut is disposed between the first elongate bar and the second elongate bar.

3. The thermally activated nuclear core restraint device of claim 2, wherein the first elongate bar, the elongate strut, and the second elongate bar are coupled together with a first coupling at the first bar end and the first strut end.

4. The thermally activated nuclear core restraint device of claim 3, wherein the first coupling is a pin connection.

5. The thermally activated nuclear core restraint device of claim 3, further comprising a second coupling that couples the first elongate bar, the elongate strut, and the second elongate bar at the second bar end and the second strut end.

6. The thermally activated nuclear core restraint device of claim 1, wherein the elongate strut further comprises a first mounting location at the first strut end and a second mounting location at the second strut end, and wherein the first mounting location and the second mounting location are not colinear with the strut axis.

7. The thermally activated nuclear core restraint device of claim 6, wherein the first mounting location and the second mounting location comprise a hole configured to receive a pin.

8. The thermally activated nuclear core restraint device of claim 1, further comprising a hexagonal duct surrounding the elongate bar and the elongate strut.

9. The thermally activated nuclear core restraint device of claim 8, further comprising a nozzle coupled to a first duct end and a handling socket coupled to a second duct end.

10. The thermally activated nuclear core restraint device of claim 9, wherein the hexagonal duct is sized and shaped to be interchangeable with one or more core assemblies within a nuclear reactor core.

11. The thermally activated nuclear core restraint device of claim 1, wherein the thermally activated nuclear core restraint device is located within an outer ring of core assemblies within a nuclear reactor core.

12. The thermally activated nuclear core restraint device of claim 1, wherein the thermally activated nuclear core restraint device is located within a penultimate ring of core assemblies within a nuclear reactor core.

13. A method of restraining core assemblies within a nuclear reactor core, comprising: locating a plurality of core assemblies within the nuclear reactor core, the nuclear reactor core defining a plurality of core assembly locations formed in concentric rings;locating a thermally activated nuclear core restraint device in an outermost ring of the nuclear reactor core, the thermally activated nuclear core restraint device configured to preferentially bow in a predetermined direction in response to thermal energy; and increasing a temperature within the nuclear reactor core such that the thermally activated core restraint device bows toward the center of the core and contacts one or more adjacent core assemblies.

14. The method of restraining core assemblies within a nuclear reactor core as in claim 13, further comprising placing additional thermally activated nuclear core restraint devices in other locations within the outermost ring of the nuclear reactor core.

15. The method of restraining core assemblies within a nuclear reactor core as in claim 13, further comprising placing additional thermally activated nuclear core restraint devices in other locations within a penultimate ring of the nuclear reactor core.

16. The method of restraining core assemblies within a nuclear reactor core as in claim 13, wherein the thermally activated nuclear core restraint device is configured to achieve mechanical core lock-up at a power to flow ratio (P / F) below 0.8.

17. The method of restraining core assemblies within a nuclear reactor core as in claim 13, wherein the thermally activated nuclear core restraint device is formed of metals having different coefficients of thermal expansion that cause the thermally activated nuclear core restraint device to bow in a predetermined direction with a predetermined stroke.

18. The method of restraining core assemblies within a nuclear reactor core as in claim 13, wherein the thermally activated nuclear core restraint device is configured to impart a force on an adjacent core assembly of greater than 3kN.

19. The method of restraining core assemblies within a nuclear reactor core as in claim 13, wherein the thermally activated nuclear core restraint device is configured to deflect with a stroke of 2mm or greater.

20. A nuclear core restraint system, comprising:a plurality of thermally activated core restraint devices configured to be located within core assembly locations within a nuclear reactor core, the plurality of thermally activated core restraint devices configured for preferential deformation toward a center of the nuclear reactor core in response to an increase in thermal energy, the preferential deformation configured to impart forces on adjacent core assemblies to achieve mechanical core lock-up during a startup procedure of the nuclear reactor core.

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