Solid-state fluid thermally coupled heat pipe microreactor

Abstract

Disclosed is a passively cooled nuclear reactor comprising a heat exchanger and a reactor core disposed proximate the heat exchanger, the reactor core comprising fuel rods, heat pipes disposed proximate the fuel rods and extending from the reactor core into the heat exchanger, a moderator monolith configured to accommodate and provide space for the fuel rods and the heat pipes, and a thermal coupling disposed internally through the moderator monolith to surround the fuel rods and the heat pipes and facilitate heat transfer from the reactor core to the heat exchanger.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of and priority under 35 U.S.C. § 120 of U.S. patent application Ser. No. 18 / 057,208, filed November 19, 2022, entitled "SOLID-STATE FLUID THERMAL BONDED HEAT PIPE MICRO-REACTOR," the contents of which are incorporated herein by reference in their entirety.

[0002] The present disclosure relates to nuclear microreactors. Summary of the Invention

[0003] In one aspect, the present disclosure describes a passively cooled nuclear reactor comprising a heat exchanger and a reactor core disposed proximate to the heat exchanger, the reactor core comprising fuel rods, heat pipes disposed proximate the fuel rods and extending from the reactor core to the heat exchanger, a moderator monolith configured to accommodate and provide space for the fuel rods and heat pipes, and a thermal coupling disposed internally through the moderator monolith to surround the fuel rods and heat pipes and facilitate heat transfer from the reactor core to the heat exchanger.

[0004] In another aspect, the present disclosure describes a passively cooled nuclear reactor. The passively cooled nuclear reactor includes a heat exchanger and a reactor core disposed proximate to the heat exchanger. The reactor core includes a plurality of fuel rods, a plurality of heat pipes extending from the reactor core into the heat exchanger, and a moderator monolith having a plurality of openings. Each of the plurality of fuel rods is configured to be slidably disposed through a first set of openings defined by the moderator monolith, and each of the plurality of heat pipes is configured to be slidably disposed through a second set of openings defined by the moderator monolith. The reactor core further includes a reflector surrounding the moderator monolith and a thermal coupling material disposed internally throughout the moderator monolith to surround the plurality of fuel rods and the plurality of heat pipes and to facilitate heat transfer from the reactor core to the heat exchanger. The reactor core further includes a vessel surrounding the reflector. The passively cooled nuclear reactor further includes a plurality of control rod movement mechanisms disposed distal to the heat exchanger, each of the plurality of control rod movement mechanisms configured to move a control rod through the heat exchanger and into the reactor core, and a moderator monolith.

[0005] The novel features of the various aspects are set forth with particularity in the appended claims. Throughout the drawings, like reference characters refer to like or corresponding parts throughout the several views of the drawings. The described aspects, however, both as to organization and method of operation, will best be understood by reference to the following description taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a perspective view of a nuclear microreactor according to at least one embodiment of the present disclosure.

[0007] [Figure 2] 2 is a perspective view of the nuclear microreactor shown in FIG. 1 with the control rod movement mechanism removed, in accordance with at least one embodiment of the present disclosure.

[0008] [Figure 3] 3 is a perspective view of the nuclear microreactor shown in FIG. 2 with the control rods removed, in accordance with at least one embodiment of the present disclosure.

[0009] [Figure 4] FIG. 4 is a perspective view of the nuclear microreactor shown in FIG. 3 with the heat exchanger removed, according to at least one embodiment of the present disclosure.

[0010] [Figure 5] 5 is a perspective view of the nuclear microreactor core shown in FIG. 4 with heat pipes removed, according to at least one embodiment of the present disclosure.

[0011] [Figure 6] 2 is a side view of the nuclear microreactor shown in FIG. 1, in accordance with at least one embodiment of the present disclosure.

[0012] [Figure 7] 7 is a cross-sectional view of a nuclear microreactor core configuration taken along section line 7-7 shown in FIG. 6, in accordance with at least one embodiment of the present disclosure.

[0013] [Figure 8] FIG. 8 is a perspective view of the nuclear microreactor core configuration shown in FIG. 7, in accordance with at least one embodiment of the present disclosure.

[0014] [Figure 9A] FIG. 8 is a detailed view of the monolith structure shown in FIG. 7, according to at least one embodiment of the present disclosure.

[0015] [Figure 9B] FIG. 9B is a detailed view of the monolith structure shown in FIG. 9A, according to at least one embodiment of the present disclosure.

[0016] [Figure 10] FIG. 8 is a perspective view of a unit cell of the microreactor core nuclear microreactor core configuration shown in FIG. 7, in accordance with at least one embodiment of the present disclosure.

[0017] [Figure 11] FIG. 8 is a perspective view of a unit cell of the microreactor core nuclear microreactor core configuration shown in FIG. 7, in accordance with at least one embodiment of the present disclosure.

[0018] [Figure 12] 1 is a perspective view of a reflector configuration of a nuclear microreactor core in accordance with at least one embodiment of the present disclosure.

[0019] [Figure 13] 13 is a cross-sectional view of the entire nuclear microreactor taken along section line 13-13 shown in FIG. 7, in accordance with at least one embodiment of the present disclosure.

[0020] [Figure 14] 14 is a cross-sectional view of the entire nuclear microreactor taken along section line 14-14 shown in FIG. 7, in accordance with at least one embodiment of the present disclosure.

[0021] [Figure 15] FIG. 14 is a detailed view of a cross section of the heat exchanger shown in FIG. 13, according to at least one embodiment of the present disclosure.

[0022] [Figure 16] FIG. 16 is a detailed view of a cross section of the annular fuel rod shown in FIG. 15, according to at least one embodiment of the present disclosure.

[0023] [Figure 17] 1 is a cross-sectional view of a nuclear reactor core configuration according to at least one embodiment of the present disclosure.

[0024] [Figure 18A] 18 is a detail view of FIG. 17, according to at least one embodiment of the present disclosure.

[0025] [Figure 18B] 18B is a detail view of FIG. 18A, according to at least one embodiment of the present disclosure.

[0026] [Figure 19]FIG. 18 is a perspective view of a fuel unit cell of the nuclear microreactor core configuration shown in FIG. 17, in accordance with at least one embodiment of the present disclosure.

[0027] [Figure 20] 18 is a perspective view of a fuel unit cell of the reactor core configuration shown in FIG. 17, in accordance with at least one embodiment of the present disclosure.

[0028] [Figure 21] 18 is a perspective view of a fuel unit cell of the reactor core configuration shown in FIG. 17, in accordance with at least one embodiment of the present disclosure.

[0029] [Figure 22] 1 is a side view of a moderator rod with a transparent outer shell in accordance with at least one embodiment of the present disclosure.

[0030] [Figure 23] 1 is a side view of a moderator rod with a transparent outer shell in accordance with at least one embodiment of the present disclosure.

[0031] [Figure 24] 1 is a side view of a moderator rod with a transparent outer shell in accordance with at least one embodiment of the present disclosure.

[0032] [Figure 25] 25 is a cross-sectional view of the entire nuclear microreactor taken along section line 25-25 shown in FIG. 17, according to at least one embodiment of the present disclosure.

[0033] [Figure 26] 26 is a cross-sectional view of the entire nuclear microreactor taken along section line 26-26 shown in FIG. 17, in accordance with at least one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0034] Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described in this disclosure and illustrated in the accompanying drawings. Well-known operations, components, and elements have not been described in detail so as not to obscure the embodiments described herein. The reader can understand that the embodiments described and illustrated herein are non-limiting examples, and thus, specific structural and functional details disclosed herein may be representative and exemplary. Variations and modifications thereof can be made without departing from the scope of the claims. Furthermore, it should be understood that terms such as "top," "bottom," "front," "rear," "left," "right," "upper," "lower," etc. are terms of convenience and are not to be construed as limiting terms.

[0035] Before describing various aspects of the nuclear microreactor in detail, it should be noted that the illustrative embodiments are not limited in application or use to the details of construction and arrangement of parts illustrated in the accompanying drawings and description. The illustrative embodiments may be implemented or incorporated in other aspects, variations, and modifications, and may be practiced or carried out in various ways. Furthermore, unless otherwise indicated, the terms and phrases employed herein have been chosen for the convenience of the reader for the purpose of describing the illustrative embodiments, and not for the purpose of limiting them. It will also be understood that one or more of the aspects, aspect expressions, and / or examples described below can be combined with any one or more of the other aspects, aspect expressions, and / or examples described below.

[0036] Nuclear microreactors according to various aspects of the present disclosure may be configured to serve as an alternative power source to high-cost diesel and other fossil power sources in off-grid applications where wind and solar plus energy storage are not a viable economic option. In various aspects, nuclear microreactors according to the present disclosure may also be configured to be highly portable so that they can be transported to remote locations where power can be supplied, or can be shut down and moved within a short period of time. Aspects of nuclear microreactors according to the present disclosure may also be configured to provide a safe, hardened source of power for on-grid power applications where the possibility of extended power outages due to loss of grid integrity cannot be tolerated. Other aspects of nuclear microreactors according to the present disclosure may also be configured to operate autonomously, with a core life of at least three years, without permanent operating or maintenance staff to support normal power operations. Furthermore, additional aspects of nuclear microreactors according to the present disclosure may be configured to be air-cooled, thus eliminating the need for cooling water sources and emergency plans outside the double fences that typically define the boundaries of a nuclear reactor site. These properties enable embodiments of the microreactor according to the present disclosure to replace high-cost fossil fuel generators in near-field and off-grid applications while maintaining production reliability and eliminating fossil fuel-related pollution and carbon emissions.

[0037] An additional embodiment of a nuclear microreactor according to the present disclosure may be a solid-state, passively cooled, heat pipe reactor. In a heat pipe reactor embodiment, the fuel (both solid and annular), heat pipe, graphite moderator, and optional metal hydride moderator elements may be mechanically self-supporting and are all thermally coupled to one another by various fluids, including, but not limited to, gases such as helium, argon, or carbon dioxide; liquid metals such as molten lead, tin, or molten lead / bismuth; or molten salts such as fluorine / lithium / beryllium (FLiBe). In various embodiments, the thermal coupling occurs in a pool of liquid thermal coupling material. Liquid thermal coupling may be used in a vertical orientation with a protective vessel to eliminate the remote possibility of coolant loss due to reactor vessel failure. Liquid thermal coupling embodiments offer atmospheric pressure operation, high power conversion operating temperatures, and low fuel operating temperatures while providing a passive, failure-resistant graphite coating to prevent oxidation. Atmospheric pressure operation is compatible with pressures ranging from 30 kPa to 103 kPa. In another embodiment, a gas thermal coupling material is used. Gas thermally coupled reactors are advantageous because, although pressurized during operation, they are not restricted in terms of physical orientation.

[0038] Fuel materials for embodiments of the nuclear microreactors described herein encompass the full range of currently available materials, including, but not limited to, oxides, carbides, uranium nitride, uranium nitride silicide, and / or plutonium. Additionally, tri-structural isotropic particle fuel (TRISO) materials are also applicable to the nuclear microreactors described herein and provide particularly advantageous high-temperature fission product containment. The advantages of TRISO become attractive when considering the gas-thermal coupling aspects of the nuclear microreactors described herein. One disadvantage of TRISO is that it uses significantly higher enrichment fuel materials than oxide, carbide, silicide, or nitride ceramic fuels.

[0039] In one embodiment, a passively cooled nuclear microreactor may include a heat exchanger and a reactor core disposed proximate the heat exchanger, the reactor core may include fuel rods, heat pipes, a moderator monolith configured to accommodate and define a space for the fuel rods and heat pipes, and a thermal coupling material applied internally throughout the moderator monolith to surround the fuel rods and heat pipes and to facilitate heat transfer from the reactor core to the heat exchanger.

[0040] In one aspect, the thermal coupling material is configured to operate at temperatures limited by the reactor's materials of construction. A protective vessel may be provided to contain any liquid thermal coupling material that may leak during a reactor vessel failure. The clearance between the protective vessel and the reactor vessel is sized to ensure that the liquid thermal coupling material remains above the graphite to prevent oxidation and sufficiently above the fuel to prevent heat transfer degradation, while eliminating the possibility of a loss-of-coolant accident. Heat for the intended operating process is extracted from the reactor through heat pipes via a primary heat exchanger located directly above the reactor in the case of a liquid thermal coupling material or adjacent to the reactor in the case of a gas thermal coupling material. The primary heat exchanger isolates the heat pipe working fluid contaminated by exposure to the reactor neutron field during the power conversion process and / or the heat transfer working fluid (liquid or gas) on the clean side of the primary heat exchanger when the reactor is generating nuclear heat. The fluid (liquid or gas) pressure in the primary heat exchanger is such that the power conversion pressure is always higher than the heat pipe pressure, preventing leakage of contaminated primary fluid into the power conversion fluid (liquid or gas). Decay heat during shutdown is removed from the reactor through heat pipes to a final heat sink under normal conditions, or by auxiliary cooling with air and / or water via natural convection in the reactor vessel.

[0041] In an annular fuel implementation of the nuclear microreactor, the annular fuel element is positioned within the nuclear microreactor to transfer nuclear heat bidirectionally through the fuel rod outer diameter to the thermal coupler and directly to the primary heat exchanger through a heat pipe located within the annular fuel rod element's inner diameter. Heat transferred from the outer diameter of the annular fuel element and any outer diameter of the solid fuel rod element serves as a heat source for the graphite moderator to transfer heat to the non-fuel heat pipes. The optional metal hydride moderator reactor element operates in a near-isothermal state between the temperature of the fuel rod outer diameter and the temperature of the non-fuel heat pipes. In a solid fuel implementation of the nuclear microreactor, the solid fuel element transfers nuclear heat from the fuel pellets through the fuel rod outer diameter to the thermal coupler, from the thermal coupler to the graphite moderator, and then to the non-fuel heat pipes.

[0042] In various aspects, nuclear microreactors according to the present disclosure are configured to operate autonomously and inherently self-limitingly at both maximum power and operating temperature through a unique combination of fuel and feedback mechanisms. The fuel may be enriched at or below highly enriched uranium (HEU) and therefore contain significant amounts of burnable fuel materials such as 238U or 232Th, both of which have significant amounts of resonance absorption. The presence of significant amounts of resonance absorption from the fertile fuel material within the fuel ensures significant and rapid negative Doppler thermal feedback at all times. Furthermore, the combination of graphite and selective metal hydride moderators cooperate to limit the maximum steady-state operating temperature as a function of reactor power level. Burnable absorbers may be placed within the reactor to limit the available excess reactivity as a function of core life. Reactivity control allows for limiting the reactivity control system so that worst-case malfunctions of this control system are safely compensated for by the inherent negative Doppler and solid moderator power / temperature feedback. In one aspect, the reactivity control system utilizes strong absorbers inserted into the core and / or reflector regions of the reactor to provide the versatile reactivity control necessary to affect safe reactor shutdown at ambient conditions throughout the life of the core.

[0043] In one aspect, the nuclear microreactor described herein is a solid-state, fully passive reactor that utilizes heat pipes for nuclear heat transport. The heat pipes eliminate the need for active heat transport components such as pumps and valves, thereby significantly simplifying the configuration and construction of the nuclear microreactor. The only moving parts of the nuclear microreactor are the control elements of the reactivity control system. The fully passive on-stream, shutdown, and upset cooling of the nuclear microreactor allows for the complete elimination of safety systems employed in conventional nuclear reactors, reducing the cost and operational complexity of the nuclear microreactor described herein. In one aspect, the nuclear microreactor may be configured to operate without a 24 / 7 operator due to the simplicity of the nuclear microreactor, the reactor protection requirements, and the ability of the liquid thermally coupled pool and reactor containment structure to retain a large portion of the fission products that may be released from the fuel due to defects or as a result of a reactor transient. In various aspects, the nuclear microreactor according to the present disclosure is more cost-effective than conventional nuclear reactors.

[0044] (Nuclear Microreactor) Figures 1 through 26 illustrate one embodiment of a nuclear microreactor 100, with Figures 7 through 11, 13, and 15 illustrating a first inner core configuration, Figures 17 through 26 illustrating a second inner core configuration, and Figures 1 through 6, 12, 14, and 16 applying to both inner core configurations. The difference between the two inner core configurations is that the second inner core configuration illustrates an example configuration that adds additional channels for optional moderator rods.

[0045] FIG. 1 is a perspective view of a nuclear microreactor 100 according to at least one embodiment of the present disclosure. The nuclear microreactor 100 includes a nuclear microreactor core 300, a heat exchanger 200 attached to the nuclear microreactor core 300, and a control rod movement mechanism 104 attached to the heat exchanger 200. The nuclear microreactor core 300 has a vessel 302 that surrounds and houses the nuclear microreactor core 300. A proximal end of the vessel 302 at a bottom surface 306 rests on the ground. The heat exchanger 200 is attached to a distal surface 304 at a distal end ( FIG. 4 ) of the vessel 302. A proximal end 214 of the heat exchanger 200 is stationary relative to the distal end of the vessel 302. The control rod movement mechanism 104 is attached to a distal end 212 of the heat exchanger 200.

[0046] Referring to FIG. 1 in conjunction with FIG. 2, each of the control rod movement mechanisms 104 has a containment rod 102 extending distally therefrom. Each containment rod 102 surrounds and contains a reactivity control rod 106 ( FIG. 2 ). The control rod 106 is configured to slide within the containment rod 102 and be moved by the control rod movement mechanism 104. FIG. 2 is a perspective view of the nuclear microreactor 100 shown in FIG. 1 with the control rod movement mechanism removed, in accordance with at least one embodiment of the present disclosure. In FIG. 2, the nuclear microreactor 100 is shown with the control rod movement mechanism 102 removed to reveal the control rod 106. The control rod movement mechanism 104 is configured to move the control rod 106 through a channel 110 in the heat exchanger 200 and into the nuclear microreactor core 300. FIG. 3 is a perspective view of the nuclear microreactor 100 shown in FIG. 2 with the control rod removed, in accordance with at least one embodiment of the present disclosure. 3, the nuclear microreactor 100 is shown with the control rods removed to reveal the control rod channels 110. The control rods 106 are used to control the nuclear fission occurring within the nuclear microreactor core 300 and thus prevent the nuclear microreactor core 300 from reaching critical temperatures in the event of reactor and / or power supply failure or a criticality accident.

[0047] In various embodiments, the heat exchanger 200 is made from multiple heat exchanger sections 202. In one embodiment, each heat exchanger section 202 is a fused heat exchanger. Additional embodiments of the heat exchanger 200 may be made from a single piece, and may not include multiple sections attached together. In various embodiments, the heat exchanger 200 passively cools the nuclear microreactor core 300. In various other embodiments, a fluid (liquid or gas) flows through the heat exchanger 200 to cool the nuclear microreactor core 300. Each heat exchanger section 202 has a distal nozzle 204 and a proximal nozzle 208. The distal nozzle 204 defines a hole 206, and the proximal nozzle 208 defines a hole 210. The holes 206, 210 allow the working fluid (liquid or gas) to enter and exit the heat exchanger section 202. In alternative embodiments, heat exchanger 200 may be configured in any suitable shape or functional implementation to allow working fluid (liquid or gas) to enter and exit heat exchanger 200 to passively cool nuclear microreactor core 300. In some embodiments, channel 108 provides an open channel through heat exchanger 200. In at least one embodiment, heat exchanger 200 does not have channel 108.

[0048] In various embodiments, the heat exchanger 200 is thermally connected to the nuclear microreactor core 300 via fuel heat pipes 216 and non-fuel heat pipes 218. FIG. 4 is a perspective view of the nuclear microreactor 100 shown in FIG. 3 with the heat exchanger removed, in accordance with at least one embodiment of the present disclosure. In FIG. 4, the nuclear microreactor 100 is shown with the heat exchanger removed to show the fuel heat pipes 216, 218 extending from the nuclear microreactor core 300 into the heat exchanger 200. The fuel heat pipes 216 extend through channels 308 defined in the nuclear microreactor core 300. Each fuel heat pipe 216 is configured to extend through the inner diameter of annular fuel rods 330 ( FIG. 14 ) within the nuclear microreactor core 300. The non-fuel heat pipes 218 extend through channels 310 defined in the nuclear microreactor core 300. The non-fuel heat pipes 218 extend farther into the heat exchanger 200 than the fuel heat pipes 216. This configuration allows the proximal end of the heat exchanger 200 to be heated to a higher temperature than the distal end of the heat exchanger 200. In at least one embodiment, a working fluid (liquid or gas) enters the heat exchanger 200 through the distal nozzle 204 and exits through the proximal nozzle 208. This allows the working fluid (liquid or gas) to achieve a temperature close to the temperature at the proximal end. In another embodiment, the working fluid (liquid or gas) enters the heat exchanger 200 through the proximal nozzle 208 and exits through the distal nozzle 204. In at least one embodiment, the working fluid (liquid or gas) is air. In another embodiment, the working fluid is a liquid.

[0049] As the fuel heat pipes 216 and non-fuel heat pipes 218 heat up during operation of the nuclear microreactor 300, their thermal expansion causes them to thermally connect to the primary heat exchanger 200. This allows the increased temperatures of the fuel heat pipes 216 and non-fuel heat pipes 218 to be used to strengthen the thermal and mechanical connection with the primary heat exchanger 200 during operation of the nuclear microreactor 100.

[0050] 5 is a perspective view of the nuclear microreactor core 300 shown in FIG. 4 with the heat pipes removed, in accordance with at least one embodiment of the present disclosure. In FIG. 5, the nuclear microreactor core 300 is shown with the fuel heat pipe 216 and the non-fuel heat pipe 218 removed. The fuel heat pipe 216 is inserted into the vessel 302 through a channel 308. The non-fuel heat pipe 218 is inserted into the vessel 302 through a channel 310. The control rod channel 110 in the heat exchanger 200 aligns with the control rod channel 312 in the nuclear microreactor core 300 to form a single channel in the nuclear microreactor 100 that extends through the heat exchanger 200 and into the nuclear microreactor core 300. In other words, each reactivity control rod 106 can be moved through the channel 110 of the heat exchanger 200 and into the channel 312 of the nuclear microreactor core 300. The distal surface 304 of the vessel 302 defines holes for the fuel heat pipes 216, the non-fuel heat pipes 218, and channels 308, 310, and 312, which provide grid locations for the control rods 106. In some embodiments, the vessel 302 defines a channel 314 in the center of the top surface 304 to allow the gaseous medium to vent. For example, the channel 314 is aligned with the channel 108 of the heat exchanger 200, allowing the gaseous medium to vent through the two channels 314, 108. In at least one embodiment, the top surface 304 does not have the channel 314. The vessel 302 is filled with a thermal coupling material such that the thermal coupling material surrounds all components of the nuclear microreactor core 300. For example, the thermal coupling material surrounds the fuel heat pipes 216 and the non-fuel heat pipes 218 with their respective channels 308 and 310. In some embodiments, the thermal coupling material is a liquid thermal coupling material. In various other aspects, the thermal coupling material is a gas thermal coupling material.

[0051] Referring back to FIG. 4 , the fuel can provide nuclear heat to a liquid thermal coupler to create a near-isothermal heat transfer boundary condition for the fuel heat pipes 216, non-fuel heat pipes 218, graphite moderator, and optional metal hydride moderator reactor element outer diameters. The liquid thermal coupler defines large gaps between components, allowing for generous tolerances due to the high thermal conductivity within the liquid coupler. In various embodiments, the liquid thermal coupler is positioned vertically with a protective vessel to prevent loss of coolant, such as in an accident. Examples of liquid thermal couplers include, but are not limited to, molten salts such as molten lead, lead-bismuth eutectic salt, sodium, potassium, sodium-potassium eutectic salt, and FLiBe.

[0052] Still referring to FIG. 4 , an alternative embodiment utilizes a gas thermal coupling material. Gas thermal coupling allows the nuclear microreactor core 300 to be positioned in any orientation. To limit the temperature gradient between the fuel rod heat source and the heat pipe heat sink, the size and variation of the gaps defined between the fuel, graphite moderator, and the fuel and non-fuel heat pipes 216 and 218 (and, to a lesser extent, solid moderator rods, if used) should be minimized. In the presence of hot graphite moderator, the gas thermal coupling material does not react with the graphite and is not a source of harmful radiation or chemical hazards. Examples of gas thermal coupling materials include, but are not limited to, helium and carbon dioxide.

[0053] 1-3, the use of a thermal coupling material minimizes thermal stresses induced in the components of the nuclear microreactor 100 through the use of a fluid (liquid or gas) to provide a thermal coupling between the components of the nuclear microreactor 100. This thermal coupling eliminates the complexities of mechanically interacting individual components, as in conventional heat pipe reactors, where the reactor components are thermally coupled to a solid structural monolith, typically made of metal.

[0054] 1-4 in conjunction with FIG. 14 , in one embodiment, the solid-state heat pipe nuclear microreactor 100 may be configured to use multiple annular fuel rods 330 ( FIG. 14 ) rather than just solid fuel rods. The temperature at the inner diameter of the annular fuel rods 330 is higher than the temperature at the outer diameter of the annular fuel rods 330. In various embodiments, the inner diameter of the annular fuel rods 330 is thermally bonded to a fuel heat pipe 216 to transfer the nuclear heat at the inner diameter of the annular fuel rods 330 to the outer diameter of the annular fuel rods 330. This higher temperature at the inner diameter of the annular fuel rods 330 is utilized in the primary heat exchanger 200, allowing for higher working fluid temperatures without requiring all structural materials and components of the nuclear microreactor 100 to withstand such high temperatures. The thermal coupling, heated by the annular fuel elements and the outer diameter of the solid fuel elements, is temperature limited by the materials of construction of the reactor vessel. Conventional structural materials have been found to be usable at temperatures of approximately 700°C. The protective vessel is used to capture any liquid thermal coupling that may leak from the reactor vessel due to a failure and to maintain a liquid level above the fuel, thereby eliminating or minimizing the possibility of loss of coolant while protecting the hot graphite moderator from potential oxidation due to air ingress. The gas thermal coupling fluid employs technology to inert the hot graphite moderator from potential oxidation due to air ingress in the unlikely event of a vessel failure or gas thermal coupling leak.

[0055] The reactor fuel cycle is initially designed for a uranium (U):plutonium (Pu) once-through fuel cycle. However, to maximize flexibility in response to future changes in fuel cycle cost and availability, U:Pu, Pu:U, and thorium (Th):U recycled fuel cycles are also possible. In various embodiments, any reactor fuel cycle can be used.

[0056] 1-4 in conjunction with FIG. 14, heat for the intended operating process is extracted from the nuclear microreactor core 300 via fuel and non-fuel heat pipes 216 and 218 via the primary heat exchanger 200, which transfers the reactor heat from the reactor fuel and non-fuel heat pipes 216 and 218 to the power conversion system working fluid. The primary heat exchanger 200 transfers the nuclear heat from the nuclear microreactor core 300 via the fuel and non-fuel heat pipes 216 and 218 and isolates the heat pipe working fluid, e.g., Na or NaK (contaminated by exposure to the reactor neutron field), from the power conversion and / or heat transfer working fluid on the clean side of the primary heat exchanger 200. The nuclear microreactor 100 utilizes the fuel and non-fuel heat pipes 216 and 218 that are mechanically and thermally coupled to the primary heat exchanger 200. The nuclear microreactor 100 utilizes a primary heat exchanger 200 mounted inside the reactor vessel head located at the vessel distal face 304, through which fuel and non-fuel heat pipes 218 and 216 extend and are inserted into the inner diameter of the core structure 378 and the annular fuel rods 330, respectively, when the nuclear microreactor 100 is assembled. This arrangement allows for replacement of the primary heat exchanger 200 and the fuel and non-fuel heat pipes 216 and 218, and for disassembly of the heat transport system from the nuclear microreactor core 300. Disassembly of the heat transport system from the nuclear microreactor core 300 allows access to the annular fuel rods 330 for repair, replacement, or removal. This process can be used for consolidation and permanent disposal or reprocessing. The nuclear microreactor 100 utilizes a primary heat exchanger 200 in which the inner diameters of the non-fuel and fuel heat pipes 218 and 216 are used for counterflow heat transfer. In this case, the non-fuel heat pipes 218 transfer heat at thermal binder temperature to the power conversion working fluid within the heat exchanger 200, and the inner diameter of the fuel heat pipes 216 transfers high temperature heat available from the inner diameter of the fuel rods 330 to the power conversion working fluid within the heat exchanger 200.This configuration allows the power conversion working fluid to approach higher fuel bore temperatures without utilizing high temperature compatible materials throughout the reactor vessel, except for the inner diameter of the fuel heat pipes 216. In at least one embodiment, the power conversion working fluid is air.

[0057] The annular fuel heat pipe 216 supplies a higher temperature to the primary heat exchanger 200 than is available from the liquid thermally coupled pool non-fuel heat pipe 218, thereby limiting the temperatures that the reactor vessel and components must withstand. The liquid thermally coupled pool non-fuel heat pipe 218 acts as an economizer within the primary heat exchanger 200, where the power conversion working fluid first contacts the low temperature pool non-fuel heat pipe 218 in a countercurrent manner, and then contacts the high temperature fuel heat pipe 216. As a result, the power conversion working fluid is provided at a high temperature available from the inner diameter of the fuel heat pipe 216, without requiring expensive high temperature materials for large amounts of reactor structure that would contact the liquid thermally coupled pool. In one embodiment, the fuel heat pipes 216 and the non-fuel heat pipes 218 are phase change heat pipes, where the heat pipe working fluid, typically Na or NaK, is evaporated in the reactor region at sub-atmospheric pressure and transported by convection to a condenser region within the primary heat exchanger 200 where it is condensed and passively returned to the reactor region by gravity and capillary wick action.

[0058] 14 , in various embodiments, the core structural monolith 378 is made of graphite. In some alternative embodiments, alternative materials can function as the core structural monolith 378. The primary purpose of the core structural monolith 378 is to provide alignment for the fuel, non-fuel heat pipes, and optional moderator rods. The core structural monolith 378 is mechanically isolated but thermally coupled to the other reactor components through the use of fluid thermal coupling material 326.

[0059] Reactor dismantling and refurbishment are expected to be carried out in purpose-designated facilities with specific equipment allowing for reactor dismantling, repair, refurbishment, fuel handling and consolidation, reactor reassembly, or disposal of end-of-life reactor components. Decay heat during shutdown is removed from the reactor through heat pipes to an ultimate heat sink under normal conditions, and through supplementary cooling by air and / or water via natural convection in the reactor vessel when heat transfer through a primary heat exchanger is not available.

[0060] Referring to Figures 1-4, along with Figure 14, the nuclear microreactor 100 utilizes control rod assemblies inserted into grid points within the graphite core structural monolith 378 and / or reflector region and extending upward through the primary heat exchanger 200 to a transfer mechanism 104 located at the top 212 of the reactor vessel head. The control rod assemblies are configured to limit reactivity if the control rod assemblies are inadvertently positioned. The nuclear microreactor 100 utilizes fixed burnable absorbers designed to allow full-power operation throughout the core's life while limiting core excess reactivity, which must be controlled by the control rod assemblies. The nuclear microreactor 100 utilizes an annular fuel / heat pipe configuration of fuel, with the fuel heat pipe 216 located at the inner diameter of the annular fuel rods 330 and potentially additional solid fuel rods. Each annular fuel pellet is thermally bonded to the inner and outer tubes using a liquid thermal coupling pool to eliminate heat transfer through the gas gap and the associated high fuel temperatures, thermal stresses, and fission gas release. The outer diameter of the annularly bonded fuel / heat pipe combination is immersed in a liquid thermal bonding pool to thermally bond graphite and optional metal hydride solid moderators, as well as additional non-fuel heat pipes 218. Alternative embodiments using separate fuel rods without annular fuel and heat pipes are also applicable. Liquid thermal bonding of the fuel pellets to the cladding, whether annular or solid, greatly simplifies the fuel rod design and allows for the use of conventional uranium dioxide fuel, thus eliminating the need to develop new fuel material infrastructure. The bonded fuel and heat pipe combination offsets the fuel heat pipes 216 above the active core level to filter out neutron / gamma ray beams that would otherwise irradiate the primary heat exchanger at the top of the reactor.

[0061] Liquid thermal coupling of the fuel to the fuel heat pipes 216 and reactor vessel provides the following technical advantages. First, conductive heat transfer through the liquid thermal coupling pool eliminates or minimizes thermal stresses within the reactor components. Liquid thermal coupling provides near-isothermal boundary conditions for each reactor component. Reducing component stress and fatigue can extend component life and potentially extend the plant's life beyond the currently anticipated eight to ten years. Second, thermally coupling the fuel to the liquid thermal coupling pool and highly conductive graphite monolith eliminates or minimizes the potential for localized fuel hot spots due to failure of adjacent heat pipes 216, 218, as well as the reactor structure stresses that would otherwise occur if the fuel were not thermally coupled to the liquid thermal coupling pool and graphite monolith. Third, eliminating or minimizing concerns about failure of individual heat pipes 216, 218 also eliminates the need for complex, safety-related, radiation-hardened heat pipe systems to protect the reactor from inadequate fuel cooling. Simply measuring the temperature of the liquid thermal coupling pool allows for accurate measurement of the thermal boundary conditions of all fuel within the reactor. Fourth, the vertical orientation of the nuclear microreactor 100 enables the use of thermosiphon heat pipes, improving the energy transport capacity of the heat pipes 216, 218 and thereby reducing the size of the nuclear microreactor 100. Fifth, liquid thermal coupling provides thermal inertia that limits the safety impact of temperature changes due to power mismatch transients, thereby enabling a simple control system design for stable operation. Sixth, the liquid thermal coupling pool eliminates or minimizes graphite oxidation and runaway Wigner energy heat transfer. Finally, the use of high-A-number liquid thermal coupling materials (e.g., lead) provides a low-cost, highly effective reactor biological shield.

[0062] 6-8, FIG. 6 is a side view of the nuclear microreactor 100 shown in FIG. 1, in accordance with at least one embodiment of the present disclosure. The nuclear microreactor core 300 extends along an axial direction that defines the length L of the nuclear microreactor core 300. Turning to FIG. 7, a cross-sectional view of the nuclear microreactor core 300 configuration taken along section line 7-7 as shown in FIG. 6 is shown, in accordance with at least one embodiment of the present disclosure. In FIG. 8, a perspective view of the nuclear microreactor core 300 configuration shown in FIG. 7 is shown, in accordance with at least one embodiment of the present disclosure. FIGS. 7 and 8 illustrate internal components of the nuclear microreactor core 300, including a plurality of unit cells 322 and reactivity control cells 324. The unit cells 322 are configured to house fuel heat pipes 216 and non-fuel heat pipes 218 and fuel in any configuration (e.g., stacks and / or rods) that collectively can generate nuclear power and manage thermal energy throughout the nuclear microreactor core 300. The reactivity control cell 324 is configured to house the heat pipes 216, 218, fuel, and control rods 106. In various embodiments, one or more of the unit cells 322, 324 can further include a moderator arrangement capable of slowing down neutrons emitted from the fuel rod arrangement. As depicted in the non-limiting embodiment shown in FIG. 7 , the unit cells 322, 324 are arranged such that the core structure 378 comprises a generally hexagonal geometric shape, although in other non-limiting embodiments, the unit cells 322, 324 can be arranged such that the core structure 378 comprises any one of several different geometric configurations depending on the intended application and / or user preference.

[0063] 6-8 in conjunction with FIG. 2, each reactivity control cell 324 is configured to house a reactivity control rod 106 (FIG. 2), which collectively act to control nuclear fission occurring within the nuclear microreactor core 300 in the event of a reactor and / or power failure or criticality accident, thus preventing the nuclear microreactor core 300 from achieving a critical temperature. According to various non-limiting aspects, the amount of nuclear fission can be reduced or entirely eliminated within the nuclear microreactor core 300, the latter allowing the core to be shut down. Reactivity control rods 106 contemplated by the present disclosure include neutron absorbing material and can be configured to be inserted into the reactivity control cells 324 to slow and / or stop a nuclear reaction in an emergency.

[0064] The nuclear microreactor core 300 further includes a reflector 316 shield. For example, the reflector 316 includes one or more plates constructed of a thick neutron shielding material and configured to substantially surround the nuclear microreactor core 300. The reflector 316 further includes a plurality of control drums 318 configured to house neutron absorbing material. Each control drum 318 is inserted into the reflector 316 through a channel 320. In the event of a reactor and / or power supply failure, the control drums 318 face inward toward the nuclear microreactor core 300 such that the absorbing material mitigates radiation and controls the temperature of the nuclear microreactor core 300. According to some non-limiting embodiments, the reflector 316 can additionally and / or alternatively include a gamma shield configured to further mitigate radiation in the event of a failure.

[0065] The plurality of unit cells 322 and the plurality of reactivity control cells 324 can be arranged to establish a hexagonal configuration, in particular, a non-limiting embodiment of the nuclear microreactor core 300. It is apparent that each unit cell 322 and each reactivity control cell 324 can also include a hexagonal configuration. However, those skilled in the art will understand that the hexagonal configuration is depicted solely for illustrative purposes. Accordingly, the present disclosure contemplates other non-limiting embodiments in which the unit cells 322, 324 include any number of geometric configurations (e.g., square, circular, triangular, rectangular, pentagonal, octagonal, etc.) and are arranged such that the nuclear microreactor core 300 can include any number of geometric configurations.

[0066] 7 , the plurality of unit cells 322 and the plurality of reactivity control cells 324 are arranged along a radial direction. Specifically, the non-limiting embodiment shown in FIG. 7 depicts a nuclear microreactor core 300 having 48 fuel unit cells 322 and 12 control unit cells 324. However, the present disclosure contemplates other non-limiting embodiments in which the nuclear microreactor core 300 includes any number of fuel unit cells 322 and control unit cells 324. Indeed, the nuclear microreactor core 300 can be easily scaled depending on the intended application and / or user preference, due to the fact that the number of unit cells 322, 324 can be easily increased or decreased in the nuclear microreactor core 300 without dramatically changing its design. In this manner, the power output of the nuclear microreactor core 300 design can also be tailored to numerous applications and requirements. The unit cells 322, 324 are configured to contain fuel containing radioisotopes, so that the power output of the nuclear microreactor core 300 can be changed by increasing or decreasing the number of unit cells 322, 324.

[0067] It should be understood that the term "radial" as used in this disclosure describes any direction extending from the center of the nuclear microreactor core 300 when viewed from above. Thus, use of the term "radial" is not limited to, and should not be interpreted to mean that, the nuclear microreactor core 300 is limited to, a circular or circular-like configuration. For example, the present disclosure contemplates non-limiting embodiments in which the nuclear microreactor core 300 includes a rectangular configuration. According to such embodiments, the nuclear microreactor core 300 may include one or more radial dimensions of varying lengths.

[0068] FIG. 7 is a cross-sectional view of the nuclear microreactor core 300 configuration taken along section line 7-7 shown in FIG. 6 , in accordance with at least one embodiment of the present disclosure, FIG. 8 is a perspective view of the nuclear microreactor core 300 configuration shown in FIG. 7 , FIG. 9A is a detailed view of the monolith structure shown in FIG. 7 , and FIG. 9B is a detailed view of the monolith structure shown in FIG. 9A . Referring now to FIGS. 7 through 9B , a plurality of unit cells 322 and a plurality of reactivity control cells 324 can be integrally formed from a solid block of material (e.g., graphite). Accordingly, internal features of each of the cells 322, 324, such as heat pipe channels, fuel rod channels, moderator channels, and / or the like, can be drilled and integrally formed from the solid block of material. However, according to other non-limiting aspects, each unit cell 322 of the plurality of unit cells 322 and each reactivity control unit cell 324 of the plurality of reactivity control unit cells 324 can be modularly formed and integrated into a core block to facilitate tunability of the core design. Nevertheless, the nuclear microreactor core 300 can be manufactured to include any number of fuel unit cells 322 and / or reactivity control unit cells 324. This allows the configuration of the nuclear microreactor core 300 to be scalable. For example, by changing the number of unit cells 322 and reactivity control cells 324, users can change the radial dimensions and length of the nuclear microreactor core 300, thereby altering its power output and flexibility for applications with unique power and / or space constraints. However, the configuration of the nuclear microreactor core 300 remains essentially the same, thereby enabling predictability in production and performance despite differences in power output and size. These features also reduce the amount of non-routine industrial work required to design new applications and facilitate manufacturing consistency and component standardization. The nuclear microreactor core 300 can be scaled as a means of adjusting its power output, although scaling requires additional consideration of the power rating of the implemented heat pipes, the appropriate number of reactivity control rods required for the adjusted power output, and the availability of the control drum.

[0069] 7-9, each of the cells 322 and 324 is configured to be self-sufficient. It should be understood that the term "self-sufficient" as used in this disclosure refers to the ability of each unit cell 322 or control cell 324 to independently dissipate heat generated by the fuel directed within the unit cell 322 or control cell 324 via a heat rod. The unit cells 322 and control cells 324 are positioned such that a thermal coupler 326 is present between them. Thus, in the event of a failure of one or more heat pipes within any cell 322, 324, the adjacent cells 322, 324 direct excess heat away from the nuclear microreactor core 300. In this manner, the cells 322, 324 are configured to allow the nuclear microreactor core 300 to operate at an acceptable temperature even if a heat pipe failure renders the cell non-self-sufficient.

[0070] 9A is a detailed view of the monolith structure 378 shown in FIG. 7 , and FIG. 9B is a detailed view of the monolith structure 378 shown in FIG. 9A , according to at least one embodiment of the present disclosure. The fuel rod unit cell 322 and the control rod unit cell 324 include a plurality of fuel channels 308 configured to accommodate the fuel rods of the nuclear microreactor core 300 and a plurality of heat pipe channels 310 configured to accommodate the non-fuel heat pipes 218 of the nuclear microreactor core 300. In some embodiments, each fuel rod 330 is configured such that the fuel heat pipes 216 are inserted therethrough. Specifically, each fuel rod unit cell 322 includes 24 fuel channels 308 and 7 heat pipe channels 310. However, it will be understood that the fuel rod unit cells 322 and / or control rod unit cells 324 can include any number of fuel channels 308 and heat pipe channels 310 to optimize the production of nuclear energy and increase the efficiency with which thermal energy is removed from the nuclear microreactor core 300. In various embodiments, the size of the fuel rod channels 308 is the same size as the non-fuel heat pipe channels 310. In some alternative embodiments, the size of the fuel rod channels 308 is different from the size of the non-fuel heat pipe channels 310.

[0071] The fuel rods 330 are configured to be inserted into the first set of channels 308 of the monolith structure 378, and the non-fuel heat pipes 218 are configured to be inserted into the second set of channels 310 of the monolith structure 378. In various embodiments, the monolith structure 378 is made up of fuel rod unit cells 322 and control rod unit cells 324. In some embodiments, the fuel rods 330 are annular fuel rods, with the fuel emerging from pellets 328 inserted into the outer cladding 333 of the fuel rods 330. The fuel heat pipes 216 are inserted into channels 380 that run through the fuel rods 330. The detailed view of FIG. 9B shows the pellets 328 surrounding the fuel heat pipes 216, both inserted into the first set of channels 308. In an alternative embodiment, the fuel rods have solid fuel pellets, and the fuel heat pipes 216 are not inserted through the fuel rods. A thermal coupling material 326 surrounds everything within the vessel 302. For example, thermal coupling material 326 is found between the fuel heat pipes 216 and the fuel rods 330, as well as surrounding the non-fuel heat pipes 218 in the second set of channels 310 and the fuel rods 330 in the first set of channels 308. Thermal coupling material 326 is also disposed between the unit cells 322, 324. Additionally, the control rod channel 312, which allows the control rod 106 to be inserted into the monolith structure 378, includes a thermal coupling material 326 located within the control rod channel 312. When a reactivity control rod 106 ( FIG. 2 ) is inserted into the control rod channel 312, the thermal coupling material 326 displaces, allowing the reactivity control rod 106 to enter the channel. When the reactivity control rod 106 is inserted into the control rod channel 312, the thermal coupling material 326 surrounds the reactivity control rod 106.

[0072] 9A and 9B in conjunction with FIGS. 1-3, in various embodiments, the thermal coupling material 326 may change states during operation of the nuclear microreactor 100 (FIGS. 1-3). The thermal coupling material 326 may be in a first state during initial operation of the nuclear microreactor 100, in a second state during operation of the nuclear microreactor 100, and in a third state after the nuclear microreactor 100 is shut down. For example, the thermal coupling material 326 could be lead, which changes from a solid state prior to operation of the nuclear microreactor 100 to a liquid state during operation. For example, the heat generated by the nuclear microreactor 100 in the lead thermal coupling material 326 could melt the solid lead into liquid lead. In various embodiments, the liquid lead thermal coupling material 326 solidifies once the nuclear microreactor 100 is shut down and cooled. This process, in which the thermal coupling material 326 is solid when the microreactor is not operating, is advantageous for holding in place the various internal components of the nuclear microreactor core 300. For example, this process is advantageous for transporting the microreactor.

[0073] As previously mentioned, each cell 322, 324 is configured to be self-sufficient. Thus, each heat pipe channel 310 is surrounded by a plurality of fuel channels 308 of the nuclear microreactor core 300 such that thermal energy generated by fuel inserted within the fuel channels 308 is effectively transferred away from the nuclear microreactor core 300. For example, the fuel may include a neutron-emitting material (e.g., a tri-structural isotropic particle fuel having a uranium oxide, uranium nitride, or uranium oxycarbide kernel).

[0074] In various embodiments, the unit cells 322, 324 can further include a moderator channel configured to accommodate a moderator for the nuclear microreactor core 300 (e.g., a hydride-based moderator, BeO, etc.), the moderator configured to slow and inhibit the propagation of neutrons emitted by the fuel inserted in the plurality of fuel channels 308. Figures 17 through 26 illustrate an example of adding a moderator channel to the nuclear microreactor core 300.

[0075] 9A-9B along with Figures 1-3, in some embodiments, the unit cells 322, 324 may also include features configured to accommodate neutron absorbing materials that moderate the nuclear reactions occurring in the fuel rod channels 308 of the unit cells 322, 324. Thus, the power distribution and radial power peaking of the unit cells 322, 324, and thus the nuclear microreactor core 300 itself, may be further tuned through the influence of the neutron absorbing materials.

[0076] Alternatively and / or additionally, the unit cells 322 , 324 may include additional features configured to accommodate other equipment of the nuclear microreactor core 300 .

[0077] 7-9, the reflectors 316 are arranged in a circular configuration surrounding the hexagonally arranged monolith structure 378. However, in other non-limiting embodiments, the reflectors 316 can be arranged to form any of several different geometric configurations for the plurality of unit cells 322, 324 depending on the intended application and / or user preference.

[0078] FIG. 10 is a perspective view of unit cells 322, 324 of the configuration of the nuclear microreactor core 300 shown in FIG. 7 , FIG. 11 is a perspective view of the unit cells 322, 324 of the configuration of the nuclear microreactor core 300 shown in FIG. 7 , and FIG. 12 is a perspective view of a reflector 316 configuration of the nuclear microreactor core 300, according to at least one embodiment of the present disclosure. Referring now to FIGS. 10-12 , in one embodiment, the reflector 316, the fuel unit cells 322, and the control rod unit cells 324 are configured to span at least a portion of the length L of the nuclear microreactor core 300. Depending on the configuration of the unit cells 322, 324 for containing fuel, the length L of the nuclear microreactor core 300 can correspond to the desired power output of the nuclear reactor. For example, each unit cell 322, 324 can be modularly formed and assembled into a core block. 12 includes, by way of example, a plurality of reflectors 316 including a control drum channel 320, where the reflectors 316 are configured to extend along at least a portion of the length L of the nuclear microreactor core 300. In some embodiments, the reflectors 316 can be integrally formed.

[0079] In one aspect, the reflector 316 includes a plurality of control drums 318 configured to contain neutron absorbing and reflective materials. Each control drum 318 is disposed within a channel 320 on the reflector 316. In the event of a reactor and / or power supply failure or reactor shutdown, the control drums 318 face inward toward the nuclear microreactor core 300 to allow the absorbing material to shut down the nuclear microreactor core 300. Additionally, the reflector 316 can further include a gamma shield configured to substantially surround the reflector 316, the nuclear microreactor core 300, and its internal components to further mitigate radiation. In various aspects, the gamma shield is disposed between the reflector 316 and the interior of the vessel 302.

[0080] In various aspects, the nuclear microreactor 100 can be configured for a wide variety of applications, many of which may have size and / or weight constraints. Accordingly, the configuration of the nuclear microreactor core 300 allows for the length L to be specifically configurable to accommodate the power, size, and / or weight requirements of the reactor.

[0081] FIG. 13 is a cross-sectional view of the entire nuclear microreactor 100 taken along section line 13-13 shown in FIG. 7, and FIG. 14 is a cross-sectional view of the entire nuclear microreactor 100 taken along section line 14-14 shown in FIG. 7, in accordance with at least one embodiment of the present disclosure. Referring now to FIGS. 13 and 14, rods 340 extend into the vessel 302 from the bottom 306 of the vessel 302. Each non-fuel heat pipe 218 defines a slot 222 that mates with one of the rods 340. Each fuel heat pipe 216 also defines a slot 220 that mates with one of the rods 340. The rods 340 serve to hold the fuel and non-fuel heat pipes 216 and 218 in place. The fuel heat pipes 216 and non-fuel heat pipes 218 extend through the bottom reflector 336, the core structural monolith 378, the top reflector 338, and the top 304 of the vessel 302 into the heat exchanger 200. Each non-fuel heat pipe 218 extends through a channel 310 in the nuclear microreactor core 300. The channel 310 extends through the bottom reflector 336, the core structural monolith 378, the top reflector 338, and the top 304 of the vessel 302. When the heat exchanger 200 is placed on the top 304 of the nuclear microreactor core 300, the channel 226 of the heat exchanger 200 aligns with the channel 310 of the nuclear microreactor core 300. This causes the non-fuel heat pipes 218 to extend through the channel 310 of the nuclear microreactor core 300 and into the channel 226 of the heat exchanger 200.

[0082] Similarly, fuel rods 330 are disposed within channels 308 of the nuclear microreactor core 300, and the fuel rods 330 extend through the core structural monolith 378. Each fuel heat pipe 216 extends through the entire channel 308 of the nuclear microreactor core 300 and also through a channel 380 ( FIG. 16 ) of the fuel rod 330. Channels 308 are similar to channels 310 and penetrate the bottom reflector 336, the core structural monolith 378, the top reflector 338, and the top 304 of the vessel 302. When the heat exchanger 200 is placed on the top 304 of the nuclear microreactor core 300, the channels 224 of the heat exchanger 200 are aligned with the channels 308 of the nuclear microreactor core 300. This allows the fuel heat pipes 216 to extend through the channels 308 of the nuclear microreactor core 300 and into the channels 224 of the heat exchanger 200. As can be seen in FIG. 14, the non-fuel heat pipes 218 extend further into the heat exchanger 200 than the fuel heat pipes 216 .

[0083] Similar to channels 308 and 310, control rod channel 312 extends through bottom reflector 336, core structural monolith 378, top reflector 338, and top 304 of vessel 302. When heat exchanger 200 is placed on top 304 of nuclear microreactor core 300, channels 110 of heat exchanger 200 align with channels 312 of nuclear microreactor core 300. Each reactivity control rod 106 is configured to be positioned through reactivity control rod channel 312 and through one or more reactivity control cells 324. As previously mentioned, each reactivity control rod 106 includes a neutron-absorbing material configured to slow and / or stop a nuclear reaction in nuclear microreactor core 300 in the event of an emergency. Collectively, reactivity control rods 106 act to prevent nuclear microreactor core 300 from reaching a critical temperature in the event of reactor and / or power failure.

[0084] 15 shows a detailed view of a cross section of the heat exchanger 200 shown in FIG. 13 , according to at least one embodiment of the present disclosure. In various embodiments, the top nozzle 204 is connected to the bottom nozzle 208 by a microchannel 228 that extends within the heat exchanger 200 between the nozzles 204, 208. The channels 110, 224, 226 within the heat exchanger 200 extend from the bottom 214 of the heat exchanger 200 toward the top 212 of the heat exchanger. The channel 110 extends the entire length from the bottom 214 to the top 212. In one embodiment, the microchannel 228 wraps around the larger channels 110, 224, 226 such that the microchannel 228 forms a continuous channel between the nozzles 204, 208. In another embodiment, the microchannel 228 merges with the larger channels 110, 224, 226. Microchannels 228 allow a power conversion working fluid to flow through nozzles 204, 208. In at least one embodiment, the power conversion working fluid is air. In one embodiment, the power conversion working fluid enters heat exchanger 200 at nozzle 208 and exits through nozzle 204. In another embodiment, the power conversion working fluid enters heat exchanger 200 at nozzle 204 and exits through nozzle 208.

[0085] FIG. 16 is a detailed view of a cross section of the annular fuel rod 330 shown in FIG. 14 , according to at least one embodiment of the present disclosure. The fuel heat pipe 216 defines a slot 220 that is inserted over a rod 340 extending from the bottom of the vessel 302. The fuel heat pipe 216 extends through the fuel rod 330 through a channel 380 in the fuel rod 330. The fuel rod 330 has an outer cladding tube 333 that extends from a bottom plug 334 to a top plug 332. Fuel is disposed within the outer cladding tube 333. In various embodiments, the fuel is in the form of annular pellets 328. A spring 339 is used to compress the annular pellets 328. In some alternative embodiments, the fuel could be in the form of solid pellets or rods, for example. In some embodiments, the fuel rod 330 is configured to attach to the fuel heat pipe 216 to maintain the position of the fuel rod 330 above the bottom reflector 336 within the moderator monolith structure 378. In an alternative embodiment, the diameter of the channels 308 in the bottom reflector 336 is smaller so that the fuel rods 330 are located on top of the bottom reflector 336. The smaller diameter may be selected to allow the heat pipes 216 to extend through the bottom reflector 336. As shown in FIG. 16 , the fuel rods 330 and fuel heat pipes 216 extend through the channels 308.

[0086] 13-16, it will be apparent to the reader that removing the heat exchanger 200 allows an operator to access the fuel rods 330, fuel heat pipes 216, and non-fuel heat pipes 218 for inspection, replacement, or repair. This is made possible by the ability of the fuel rods 330, fuel heat pipes 216, and non-fuel heat pipes 218 to slide through the channels 308, 310 into the nuclear microreactor core 300.

[0087] The nuclear microreactor 100 can be configured to accommodate a moderator in the nuclear microreactor core 300. Examples of monolith structures 382 for adding moderators to the nuclear microreactor core 300 are shown in FIGS. 17 through 26. The nuclear microreactor core 300 functions substantially similarly using either the monolith structure 378 shown in FIG. 7 or the monolith structure 382 shown in FIG. 17. There are differences between the monolith structures 378 and 382. The monolith structure 382 shown in FIG. 17 is one example of how moderator rods 352 can be added to the nuclear microreactor core 300. The moderator rods 352 can be added to the nuclear microreactor core 300 in a variety of different ways. For example, some of the fuel rods 330 could simply be replaced with moderator rods. For the sake of brevity, not all similarities between the two monolith structures 378 and 382 of the nuclear microreactor core 300 will be described in detail.

[0088] 17 is a cross-sectional view of a configuration of a nuclear microreactor core 300 in accordance with at least one embodiment of the present disclosure. Referring to FIG. 17 , similar to the first monolith structure 378, the second monolith structure 382 of the nuclear microreactor core 300 includes a plurality of fuel unit cells 342, 344, 346, 348 and a reactivity control unit cell 350. Furthermore, the second monolith structure 382 is surrounded by a reflector 316. The fuel unit cells 342, 344, 346, 348 are configured to house the fuel and non-fuel heat pipes 216, 218 and fuel in any suitable configuration (e.g., stacks and / or rods), which collectively can generate nuclear power and manage thermal energy throughout the nuclear microreactor core 300. The reactivity control cell 350 is configured to house the fuel and non-fuel heat pipes 216, 218, fuel, and reactivity control rods 106.

[0089] FIG. 18A is a detailed view of a monolith structure 382, ​​and FIG. 18B is a detailed view of the monolith structure 382 shown in FIG. 18A, according to at least one embodiment of the present disclosure. Referring to FIGS. 17, 18A, and 18B, moderator rod channels 354 are formed between the fuel unit cells 342, 344, 346, 348 and / or the reactivity control unit cells 350. The moderator rods 352 are inserted into the moderator channels 354. With respect to the fuel rods 330 and non-fuel heat pipes 218, the monolith structure 382 shown in FIG. 17 is similar to the monolith structure 378 shown in FIG. 7. The fuel rods 330 are inserted into the channels 308 of the monolith structure 382, ​​and the non-fuel heat pipes 218 are inserted into the channels 310 of the monolith structure 382. Each fuel heat pipe 216 is inserted into a channel 380 extending through a fuel rod 330. The detailed view of FIG. 18B shows pellets 328 surrounding fuel heat pipes 216 both inserted into channel 308.

[0090] Thermal coupling material 326 surrounds everything within vessel 302. For example, thermal coupling material 326 is found between fuel heat pipes 216 and fuel rods 330, as well as surrounding non-fuel heat pipes 218 within channel 310. Thermal coupling material 326 is also found between fuel unit cells 342, 344, 346, 348 and / or control rod unit cells 350. This includes thermal coupling material 326 surrounding moderator rod 352 within moderator rod channel 354. Similar to monolith structure 378, control rod channel 312 allows control rod 106 to be inserted into monolith structure 382. When reactivity control rod 106 ( FIG. 2 ) is inserted into control rod channel 312, thermal coupling material 326 displaces, allowing reactivity control rod 106 to enter the channel. When reactivity control rod 106 is inserted into control rod channel 312, thermal coupling material 326 surrounds reactivity control rod 106.

[0091] 19-21 are perspective views of fuel unit cells 344, 346 and control rod unit cell 350 of the nuclear microreactor core 300 configuration shown in FIG. 17 , in accordance with at least one embodiment of the present disclosure. Referring to FIGS. 19-21 , similar to the fuel unit cell 322 and control rod unit cell 324 shown in FIG. 7 , the fuel unit cells 342, 344, 346, 348 and control rod unit cell 350 shown in FIG. 17 are configured to extend at least a portion of the length L of the nuclear microreactor core 300. FIG. 19 shows a stack of fuel unit cells 342, and FIG. 20 shows a stack of fuel unit cells 346. As shown in FIGS. 17-20 , the difference between the fuel unit cells 342, 344, 346, 348 is the number of moderator rod channels 354. In other words, the moderator rod channels 354 are disposed on the sides of the fuel unit cells 342, 344, 346, and 348, and the number of moderator rod channels 354 is the difference between the fuel unit cells 342, 344, 346, and 348. For example, the fuel unit cell 342 has five moderator rod channels 354, the fuel unit cell 344 has seven moderator rod channels 354, the fuel unit cell 346 has twelve moderator rod channels 354, and the fuel unit cell 348 has nine moderator rod channels 354. The control rod unit cell 350 shown in FIG. 21 has twelve moderator rod channels 354. The moderator rod channels 354 of the fuel unit cells 342, 344, 346, and 348 and the control rod unit cell 350 form a circular channel when the moderator monolith 382 is constructed by arranging the unit cells 342, 344, 346, 348, and 350 together. However, in various alternative embodiments, the moderator bar channel 354 may have any shape or means that allows a moderator bar to be inserted into the channel.

[0092] 22-24 show three different examples of moderator rods 352. FIG. 22 is a side view of a moderator rod 352 with a transparent outer shell, FIG. 23 is a side view of a moderator rod 352 with a transparent outer shell, and FIG. 24 is a side view of a moderator rod 352 with a transparent outer shell, according to at least one embodiment of the present disclosure. The difference between the moderator rods 352 shown in FIGS. 22-24 is what is located inside the outer cladding tube 356. The moderator rod 352 has a bottom plug 372 that is inserted into the outer cladding tube 356. In one embodiment, a rod 374 is inserted through the outer cladding tube 356 and the bottom plug 372 to lock the bottom plug 372 in place. The bottom plug 372 defines a slot 373 that extends therethrough. Outer jacketing tube 356 extends distally from bottom plug 372 until it reaches top plug 360, which is inserted into the distal end of outer jacketing tube 356. In some embodiments, a rod 362 is inserted through channel 361 in outer jacketing tube 356 and a hole in top plug 360, allowing top plug 360 to slide relative to outer jacketing tube 356 the length of channel 361. Top plug 360 is provided with a rod 359 extending distally from top plug 360. A spring 358 is disposed within outer jacketing tube 356 and on rod 359 such that spring 358 extends distally from top plug 360 and outside of outer jacketing tube 356.

[0093] The distance between the bottom plug 372 and the top plug 360 can be divided into three sections, for example, the bottom stack 368, the middle stack 366, and the top stack 364. In various alternative embodiments, the distance can be divided into any number of sections. The type of material inserted into the outer cladding tube 356 of the moderator rod 352 between the top plug 360 and the bottom plug 372 can vary. For example, the type of material can vary based on the section in which the material is located. Some possible material types inserted into the moderator rod 352 can be pellets, reflectors, and / or shields. FIG. 22 shows the bottom section 368, middle section 366, and top section 364 all containing pellets 370. FIG. 23 shows the bottom section 368, middle section 366, and top section 364 all containing moderator 376. In Figure 24, the bottom section 368 and top section 364 include moderator 376, and the middle section 366 includes pellets. There are many possible combinations that can be achieved by including pellets, reflectors, and shields in the moderator rod 352. For the sake of brevity, not all combinations are shown. However, the reader will readily understand that any portion of the moderator 376 shown in Figure 23 or Figure 24 could be replaced with a shield or pellets.

[0094] FIG. 25 is a cross-sectional view of the entire nuclear microreactor 100 taken along section line 25-25 shown in FIG. 17 , and FIG. 26 is a cross-sectional view of the entire nuclear microreactor 100 taken along section line 26-26 shown in FIG. 17 , in accordance with at least one embodiment of the present disclosure. Referring to FIGS. 25 and 26 , rods 340 extend into the vessel 302 from the bottom 306 of the vessel 302. Each one of the non-fuel heat pipes 218 defines a slot 222 that mates with one of the rods 340, each fuel heat pipe 216 defines a slot 220 that mates with one of the rods 340, and each moderator rod bottom plug 372 defines a slot 373 that mates with one of the rods 340. The rods 340 help hold the fuel heat pipes 216, non-fuel heat pipes 218, and moderator rods 352 in place. Fuel heat pipes 216, non-fuel heat pipes 218, and moderator rods 352 extend through bottom reflector 336, core structural monolith 382, ​​and top reflector 338. Fuel heat pipes 216 and non-fuel heat pipes 218 extend through top 304 of vessel 302 and into heat exchanger 200. Moderator rods 352 are positioned within nuclear microreactor core 300 such that springs 358 are positioned on rods 341 that extend from top 304 into vessel 302. Springs 358 exert a compressive force on the material within moderator rods 352. Each one of the moderator rods 352 extends through a channel 354 defined in the nuclear microreactor core 300, each one of the non-fuel heat pipes 218 extends through a channel 310 defined in the nuclear microreactor core 300, and each one of the fuel rods 330 and each one of the heat pipes 216 extends through a channel 308 defined in the nuclear microreactor core 300.

[0095] (alternative configuration) Two configurations have been described in detail, however, there are many different configurations that can be used for the nuclear microreactor 100. Some of the different configurations are described below.

[0096] An alternative configuration of the nuclear microreactor 100 utilizes an annular fuel / heat pipe configuration with the heat pipe as the inner diameter of the annular fuel rod, and potentially additional solid fuel rods. In this configuration, the thermal coupling material 326 is a gas, which would replace the liquid. Candidate gases include, but are not limited to, carbon dioxide or helium. The purpose of the gas is two-fold: first, to exclude oxygen, which could oxidize the graphite moderator, and second, to provide thermal coupling between the various reactor components. The use of a gas thermal coupling allows for a horizontal reactor orientation, thereby enabling the use of dual condenser heat pipes, which allow practical heat removal from both sides of the reactor, rather than from one side as required with a liquid thermal coupling. For example, if the heat exchanger is oriented horizontally, the heat exchanger 200 could be positioned against both the bottom surface 306 and the top surface 304. In one embodiment, the annular fuel pellets are thermally bonded to the inner and outer tubes with a pressurized helium backfill. In this case, the fuel temperature may be higher than with liquid-coupled fuel, but the higher fuel temperature is tolerable due to the low thermal power density of the heat pipe reactor. Pressurized operation of the microreactor may be required to maximize heat transfer from the fuel to the heat pipe. The outer diameter of the annular fuel and heat pipe combination transfers heat through a gas gap to the graphite and metal hydride solid moderator and to an additional non-fuel heat pipe. Alternative configurations using separate fuel rods without annular fuel and heat pipes are also applicable. In this embodiment, gas thermal coupling is effective due to the low thermal power density typically associated with heat pipe reactors.

[0097] Yet another configuration uses solid fuel rods, heat pipes, and optional solid moderator rods in addition to or to the annular fuel elements in a graphite reactor configuration, with the reactor components thermally coupled to each other by either the liquid thermal coupler or the gas thermal coupler. This configuration simplifies the primary heat exchanger configuration at the expense of more efficient heat transfer than would be possible without the annular fuel elements. Alternatively, adding solid fuel elements in addition to the annular fuel elements could increase the proportion of low-temperature heat transfer power to the graphite moderator. For example, solid fuel elements could be added similarly to how moderator rods were added to the moderator monolith 382. Additionally or alternatively, some of the annular fuel rods could be replaced with solid fuel rods, which would require their fuel rod channels to be the same as the moderator channels. Therefore, those fuel rod channels would not have fuel heat pipes 216 exiting the nuclear microreactor core 300. Increasing the low temperature constant power heat transfer ratio allows for optimization of low temperature heat transfer from the outer diameter of the annular and solid fuel rods and high temperature heat transfer from the inner diameter of the annular fuel rods, thereby optimizing fuel rod heat transfer and primary heat exchanger performance.

[0098] Another heat pipe configuration is contemplated: a thermosiphon device, again typically using Na or NaK as the working fluid, but this time at atmospheric or slightly higher pressure within the melting column. This configuration is most advantageous for liquid thermal coupling techniques, as it requires a vertical orientation to achieve the thermosiphon effect. The difference between these configurations is that the mass transport mechanism within the heat pipe is primarily vapor convection for classical heat pipes, e.g., fuel heat pipe 216 and non-fuel heat pipe 218, and primarily liquid convection with vapor capture for thermosiphons. In some embodiments, thermosiphons are less susceptible to failure and less susceptible to contamination due to a much larger working fluid inventory.

[0099] (stability) The reactor physics are designed to be autonomous and inherently self-limiting in both maximum power and maximum operating temperature through a unique combination of Doppler feedback, a graphite reactor architecture, and selective metal hydride moderators, which work together to limit the maximum steady-state operating temperature versus reactor power level. The presence of significant amounts of 238U or 232Th ensures that significant and rapid negative temperature feedback due to Doppler broadening is always present in these fertile resonance absorbers, limiting short-term (<1 to 60 seconds) reactivity imbalances to maintain safe and inherently limited operation. This rapid negative fuel temperature reactivity feedback provides an inherent limit on achievable peak power under conditions where rapid reactivity insertion from reactivity control or power conversion systems is highly unlikely.

[0100] The medium-term (minutes to days) reactivity balance and the resulting steady-state reactor power and steady-state upper operating temperature are limited by controlling the proportion of graphite moderation relative to the proportion of selective solid metal hydride moderation, ensuring that the reactor is substantially under moderation even without additional effective moderation from the solid metal hydride moderator. The graphite reactor structure provides the majority of the moderation to enable criticality. However, the proportion of graphite to metal hydride moderator is determined so that, although the reactor is near-optimally moderated up to operating temperatures, the nonlinear reactivity feedback of the metal hydride moderator, including reactivity control system failures, always leaves the core insufficient to sustain additional nuclear heat above the temperature at which the reactor's structural materials are capable. This reactivity control means allows the reactivity control system to be limited so that its worst-case malfunction is safely compensated for by the inherent negative Doppler feedback and solid moderator power / temperature feedback.

[0101] Long-term steady-state reactivity control is achieved through the appropriate use of burnable absorber materials to limit the excess reactivity available throughout the operating life. In addition to being generally good engineering practice, limiting the available excess core reactivity has the beneficial effect of limiting the reactor heat generation that would occur in the event of a loss of heat sink. This ensures that reactor materials remain within their qualified operating temperature range without the credit of reactor control measures, and that pressures within the solid moderator elements remain below those that could result in cladding failure. This allows the reactor to safely withstand a loss-of-heat-sink event indefinitely while passive shutdown heat removal systems provide decay heat removal.

[0102] Finally, the reactivity control system provides negative reactivity to allow reactor cooling to ambient temperature and provides a reactivity balance that optimizes reactor operating temperatures for power and lifetime. The reactivity control system utilizes strong absorbers inserted into the core and / or reflector regions of the reactor. These absorbers provide the variable reactivity control necessary to safely shut down the reactor at ambient conditions throughout the life of the core. The maximum available reactivity worth of the reactivity control system is designed so that any failure or breakdown of the reactivity control system will not result in positive reactivity that would threaten the fuel safety limits or allow the reactor to exceed its maximum safe operating temperature.

[0103] (transfer) Reactivity control systems are used as an integral part of safely transporting nuclear reactors with liquid metal / molten salt thermal coupling from their manufacturing site to their destination. Reactors are assembled in a manufacturing environment so that criticality is strictly avoided by having the control elements inserted in a fully mechanically locked state in the reactor area at all times when fuel is being loaded into the reactor. Locking the control elements to the reactor is sufficient to avoid criticality in the extremely unlikely event that the reactor is fully loaded with fuel and the two most reactive control elements (N-2 criteria) are unlocked and fully withdrawn. One of the final steps in reactor manufacturing is to fill the assembled reactor with liquid thermal coupling and solidify or freeze the liquid, thereby locking the reactor in a known safe shutdown state during transport and until the reactor is safely installed in its final location.

[0104] Shipping a reactor frozen in an active liquid thermal binder is a substantial and passive safety item because it provides the following safety-related functions during transport: First, it prevents the reactor and control elements from moving from their respective positions within the reflector, ensuring that the reactor remains critical with a substantial safety margin at all times. Second, it prevents the reactor from being flooded with water for moderation and reactivity. Third, it allows completed reactors to be manufactured in factories, and the associated improvements in quality and productivity allow for faster, lower cost and shorter delivery times. Fourth, it distributes normal shipping accelerations throughout the reactor structure, rather than allowing localized loads caused by accelerations across gaps and gap elements to cause stress concentrations that could damage delicate fuel structures. Fifth, it essentially prevents fuel material from diversion due to the weight of a frozen reactor and the time and energy required to melt and remove the liquid from the reactor. Sixth, it protects the environment from potential scattering due to shock loads during transport. Seventh and finally, it provides environmental shielding of fuel materials allowing for the use of recycled fuels.

[0105] Alternatively, gas-thermal coupled configurations also rely on the reactivity control system as an integral part of safely transporting the reactor from its manufacturing site to its destination. The reactor is assembled in a manufacturing environment so that criticality is strictly avoided by ensuring that the control elements are always fully mechanically locked into the reactor zone when fuel is being loaded into the reactor. Locking the control elements into the reactor is sufficient to avoid criticality even in the extremely unlikely event that the reactor is fully loaded with fuel and the two most reactive control elements are unlocked and fully withdrawn. Transporting the reactor with the control elements mechanically locked in their fully inserted position is a substantial safety item because it provides the following safety-related functions during transport: First, it prevents the control elements from moving from their respective positions within the reactor or reflector, ensuring a safety margin that the reactor remains subcritical at all times. Second, the ability to manufacture the complete reactor in a factory allows for faster, lower cost and shorter delivery times due to the associated improvements in quality and productivity. Finally, thirdly, the weight of the reactor for transportation must be minimized.

[0106] (advantage) The nuclear microreactor 100 offers numerous advantages. For example, the annular fuel rods transfer heat from the outer diameter of the fuel at a lower temperature and the inner diameter of the fuel at a higher temperature to the thermal coupling material, and the temperature difference increases the source temperature and improves the power conversion cycle efficiency. Additionally, the nuclear microreactor 100 has the flexibility to utilize non-annular clad fuel rods to increase fuel loading and transfer heat directly to the thermal coupling material. Thermal coupling of the fuel rods to both the solid and annular fuel pellets with a liquid thermal coupling material can minimize thermal stresses and fission gas release in the fuel pellets.

[0107] The nuclear microreactor 100 has a non-structural, low-parasitic graphite moderator monolith to provide lattice spacing for reactor components and natural circulation coolant channels, thereby reducing reactor fissile load requirements and enabling thermal coupling of all reactor components. All reactor components float radially within the graphite moderator structure and can be thermally coupled to a pool of liquid thermal coupling material to eliminate gas-gap heat transfer in favor of liquid heat conduction. Liquid thermal coupling has many advantages, including, but not limited to, minimizing thermal stresses, ensuring adequate fuel cooling even in the event of multiple heat pipe failures, increasing the reactor's thermal inertia, thereby slowing the reactor's power / temperature transient response to anticipated operational events, eliminating damage from graphite moderator burnout and Wigner energy release, providing a low-cost, highly efficient reactor vessel and biological shield, enabling reactor transportation with control elements frozen in the shutdown position, and preventing flooding by replacing the reactor void volume with frozen liquid thermal coupling material.

[0108] Alternatively, gas thermal coupling can be utilized with free-floating reactor components within a graphite moderator monolith structure. The use of gas thermal coupling allows for a horizontal orientation, allowing for the use of dual condenser heat pipes with dual primary heat exchangers to maximize power output. Gas thermal coupling requires operation above atmospheric pressure to obtain acceptable heat transfer, which necessitates compensation for coolant loss due to leaks or thermal diffusion to maintain acceptable shutdown heat transfer, and an inert atmosphere to prevent oxidation of the graphite moderator monolith.

[0109] The nuclear microreactor 100 emergency planning area is as large as the site boundary due to the use of a high-A liquid thermal coupling material, such as lead, which is an effective mechanism for limiting radioactive release in the event of a fuel failure. Additionally, there is an integral reflector and vessel shield housed within the reactor vessel and thermally bonded to the reactor thermal coupling material. The protective vessel prevents loss of liquid thermal coupling. The protective vessel is designed to maintain the fuel enveloped in the thermal coupling material even in the event of an beyond-design-basis vessel failure. If necessary, the reactor 100 can be designed so that passive shutdown cooling by air is augmented by water heat transfer from the protective vessel for shutdown decay heat removal.

[0110] The nuclear microreactor 100 passively limits the maximum reactor temperature by utilizing selective metal hydride solid moderator negative reactivity feedback. Additionally, reactor safety equipment is limited to maximum reactor thermal coupler temperature and nuclear flux monitoring. No in-reactor equipment for nuclear or heat transfer parameters is required. This allows for autonomous operation to maintain constant high-temperature criticality. Operator intervention is only required if criticality is approached due to strong negative feedback from the metal hydride solid moderator.

[0111] The thermal / mechanical connection of the heat pipes connected to the primary heat exchanger allows access to the reactor by lifting the primary heat exchanger, with the heat pipe extensions from the bottom of the primary heat exchanger, out of the reactor area. This allows for reactor repair, refueling, and replacement of the primary heat exchanger and heat pipes. The thermal connection of the heat pipes to the primary heat exchanger is enhanced by the use of bimetallic, differential thermal expansion of the heat pipes (e.g., steel / copper) to the primary heat exchanger. This takes advantage of the higher temperature of the heat pipes to create a strong thermal and mechanical connection with the primary heat exchanger.

[0112] The nuclear microreactor 100 can use metal hydrides for supplemental moderation and additional negative reactivity feedback, taking advantage of the negative reactivity feedback resulting from high temperature moderation degradation of metal hydrides.

[0113] The nuclear microreactor core 300 has grid positions for non-burning heat pipes for nuclear and shutdown heat transfer. It also has grid positions for fuel rods. In some embodiments, the fuel rods have annular pellet fuel rod components. Additionally or alternatively, the fuel rods can have solid pellet fuel rod components. The microreactor can accommodate the use of burnable absorbers in both solid and annular fuel elements. Both non-fuel and fuel heat pipes can accommodate the use of both alkali metal phase change heat pipes and thermosiphon heat pipes for passive nuclear heat transfer from the reactor fuel to the primary heat exchanger. The microreactor also has grid positions for control elements for reactor shutdown and fine power / temperature adjustment.

[0114] (example) Various aspects of the subject matter described herein are set forth in the following numbered examples.

[0115] Example 1 - A passively cooled nuclear reactor comprising a heat exchanger and a reactor core disposed proximate to the heat exchanger, the reactor core comprising fuel rods, heat pipes disposed proximate the fuel rods and extending from the reactor core into the heat exchanger, a moderator monolith configured to accommodate and provide space for the fuel rods and heat pipes, and a thermal coupling material disposed internally through the moderator monolith to surround the fuel rods and heat pipes and facilitate heat transfer from the reactor core to the heat exchanger.

[0116] Example 2 - The nuclear reactor of Example 1, wherein the moderator monolith further defines a plurality of openings, the fuel rods configured to be slidably disposed through first openings defined by the moderator monolith, and the heat pipes configured to be slidably disposed through second openings defined by the moderator monolith.

[0117] Example 3 - The nuclear reactor of Example 1 or 2, wherein the reactor core further comprises a moderator rod configured to be slidably disposed through a third opening defined by the moderator monolith.

[0118] Example 4 - The reactor of Examples 1, 2 or 3, wherein the reactor core is in contact with the heat exchanger through a heat pipe.

[0119] Example 5 - The nuclear reactor of Examples 1, 2, 3, or 4, wherein the thermal coupling material comprises a two-state material.

[0120] Example 6 - The nuclear reactor of Example 1, 2, 3, 4, or 5, wherein the thermal coupling material is in a solid state in a first state of the reactor and locks the fuel rods and heat pipes in place, and the thermal coupling material is in a liquid state in a second state of the reactor.

[0121] Example 7 - The reactor of Example 5, wherein the two-state material is lead.

[0122] Example 8 - The nuclear reactor of Example 1, 2, 3, or 4, wherein the thermal coupling material is a gas.

[0123] Example 9 - The nuclear reactor of Example 1, 2, 3, 4, 5, 6, 7, or 8, wherein the moderator monolith comprises a unit cell.

[0124] Example 10 - The nuclear reactor of Examples 1, 2, 3, 4, 5, 6, 7, 8, or 9, wherein the fuel rods are annular fuel rods defining openings, the heat pipes are first heat pipes, and the reactor core further comprises a second heat pipe configured to be slidably disposed through the openings defined by the annular fuel rods.

[0125] Example 11 - The reactor of example 10, wherein the first heat pipe extends further into the heat exchanger than the second heat pipe.

[0126] Example 12 - The reactor of Example 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11, wherein the reactor is configured to operate at an atmospheric pressure in the range of 30 kPa to 103 kPa.

[0127] Example 13 - The nuclear reactor of Examples 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, further comprising a control rod movement mechanism disposed distal to the heat exchanger, the control rod movement mechanism configured to move the control rod through the heat exchanger and into the reactor core.

[0128] Example 14 - A passively cooled nuclear reactor comprising: a heat exchanger; and a reactor core disposed proximate to the heat exchanger. The reactor core comprises a plurality of fuel rods, a plurality of heat pipes extending from the reactor core into the heat exchanger, and a moderator monolith having a plurality of openings. Each of the plurality of fuel rods is configured to be slidably disposed through a first set of openings defined by the moderator monolith, and each of the plurality of heat pipes is configured to be slidably disposed through a second set of openings defined by the moderator monolith. The reactor core further comprises a reflector surrounding the moderator monolith; a thermal coupling material disposed internally through the moderator monolith to surround the plurality of fuel rods and the plurality of heat pipes and to facilitate heat transfer from the reactor core to the heat exchanger; and a container surrounding the reflector. The passively cooled nuclear reactor further comprises a plurality of control rod movement mechanisms disposed distal to the heat exchanger, each of the plurality of control rod movement mechanisms configured to move a control rod through the heat exchanger and into the reactor core.

[0129] Example 15 - The nuclear reactor of Example 14, wherein the reactor core further comprises a plurality of moderator rods configured to be slidably disposed through a third set of openings in the moderator monolith.

[0130] Example 16 - The reactor of example 14 or 15, wherein the thermal coupling material is a two-state material.

[0131] Example 17 - The nuclear reactor of Example 14, 15, or 16, wherein the thermal coupling material is in a solid state in a first state of the reactor and locks the plurality of fuel rods and the plurality of heat pipes in place, and the thermal coupling material is in a liquid state in a second state of the reactor.

[0132] Example 18 - The nuclear reactor of Example 14, 15, 16, or 17, wherein the plurality of fuel rods are annular fuel rods, the plurality of heat pipes are a first plurality of heat pipes, and the nuclear reactor core further comprises a second plurality of heat pipes, each heat pipe of the second plurality of heat pipes configured to be slidably positioned through an opening defined by a respective annular fuel rod of the plurality of fuel rods.

[0133] Example 19 - The nuclear reactor of example 18, wherein the first plurality of heat pipes extend further into the heat exchanger than the second plurality of heat pipes.

[0134] Example 20 - The nuclear reactor of example 14, 15, 16, 17, 18, or 19, wherein the reflector comprises a plurality of control drums.

[0135] All patents, patent applications, publications, or other disclosure materials mentioned herein are incorporated by reference in their entirety, as if each individual reference were expressly incorporated by reference. All references and their materials, or portions thereof, that are incorporated herein by reference are incorporated herein only to the extent that the incorporated material does not contradict existing definitions, descriptions, or other disclosure material set forth in this disclosure. Therefore, to the extent necessary, the disclosure set forth herein will supersede any conflicting material incorporated herein by reference, and the disclosure expressly set forth in this application will control.

[0136] Various embodiments of solid-state fluid thermally coupled heat pipe microreactors have been described with reference to various exemplary and illustrative embodiments. It is understood that the embodiments described herein provide exemplary features of various details of various embodiments of the present disclosure; thus, unless otherwise specified, it is understood that, to the extent possible, one or more features, elements, components, ingredients, structures, modules, and / or aspects of the disclosed embodiments can be combined, separated, interchanged, and / or rearranged with one or more other features, elements, components, ingredients, structures, modules, and / or aspects of the disclosed embodiments without departing from the scope of the present disclosure. Accordingly, those skilled in the art will recognize that various substitutions, modifications, or combinations of any of the exemplary embodiments can be made without departing from the scope of the present disclosure. Additionally, those skilled in the art will recognize, or be able to ascertain with no more than routine experimentation, many equivalents to the various embodiments of the present disclosure described herein upon review of this specification. Accordingly, the present disclosure is not limited by the description of the various embodiments, but rather by the scope of the claims.

[0137] Those skilled in the art will generally recognize that terms used in this specification, and particularly in the appended claims (e.g., the body of the appended claims), are generally intended as "open" terms (e.g., the term "comprises" should be interpreted as "including, but not limited to," the term "having" should be interpreted as "having at least," the term "comprises" should be interpreted as "including, but not limited to," etc.). Those skilled in the art will further understand that if a specific number of introduced claim recitations are intended, such intention will be explicitly stated in the claim; otherwise, no such intention exists. For example, as an aid to understanding, the appended claims below may use the introductory phrases "at least one" and "one or more" to introduce the claim recitations. However, the use of such phrases should not be construed as meaning that the introduction of a claim recitation by the indefinite article "a" or "an" limits a particular claim that includes such introduced claim recitation to claims that include only one such introduced claim recitation, even if that same claim includes the introductory phrases "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should ordinarily be interpreted to mean "at least one" or "one or more"); the same applies when a definite article is used to introduce a claim recitation.

[0138] Furthermore, even when a specific number in an introduced claim is explicitly recited, a person skilled in the art will recognize that such recitation should generally be interpreted to mean at least the recited number (e.g., the recitation "two items" without other modifiers generally means at least two items, or more than two items). Furthermore, when a conventional description similar to "at least one of A, B, and C, etc." is used, such a configuration is generally intended in the sense that a person skilled in the art would understand the conventional description (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, a system having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). When a conventional description similar to "at least one of A, B, or C, etc." is used, such a configuration is generally intended in the sense that one of ordinary skill in the art would understand the conventional description (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, a system having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). As will be further understood by one of ordinary skill in the art, whether in the specification, claims, or drawings, disjunctive words and / or phrases that typically present two or more alternative terms should be understood to contemplate the possibility of including one of the terms, either of the terms, or both terms, unless the context dictates otherwise. For example, the phrase "A or B" is typically understood to include the possibilities of "A" or "B" or "A and B."

[0139] With respect to the appended claims, those skilled in the art will understand that the actions recited therein may generally be performed in any order. Also, while the claims are presented in a sequential order, it should be understood that various actions may be performed in orders other than the order shown, or may be performed simultaneously. Examples of such alternative orders may include overlapping, alternating, interrupted, reordered, progressive, preparatory, supplemental, simultaneous, reversed, or other variations, unless the context dictates otherwise. Furthermore, terms such as past tense adjectives, such as "responsive to" and "related to," are generally not intended to exclude such variations, unless the context dictates otherwise.

[0140] It should be noted that references to "one embodiment," "an embodiment," "an example," "one example," etc. mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment," "in an embodiment," "in an example," and "in one example" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0141] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0142] As used herein, directional terms such as, by way of example and not limitation, top, bottom, left, right, below, over, front, rear, and variations thereof, refer to the orientation of the elements as shown in the accompanying drawings and do not limit the scope of the claims, unless expressly stated otherwise.

[0143] As used in this disclosure, the terms "substantially," "about," or "generally," unless otherwise specified, refer to an acceptable error for a particular value as determined by one of ordinary skill in the art, which depends in part on how the value is measured or determined. In certain embodiments, the term "about" or "generally" means within 1, 2, 3, or 4 standard deviations. In certain embodiments, the term "about" or "generally" means within 50%, 200%, 105%, 100%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range.

[0144] As used herein, unless otherwise indicated, all numerical parameters are understood to be prefaced and modified in all respects by the term "about." Such numerical parameters have the inherent variability characteristic of the underlying measurement techniques employed to determine the numerical value of such parameters. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter set forth herein should, at the very least, be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0145] Numerical ranges recited herein include all subranges subsumed within the stated range. For example, a range of "1 to 100" includes all subranges between (and including) the recited minimum of 1 and the recited maximum of 100, i.e., having a minimum equal to or greater than 1 and a maximum equal to or less than 100. Also, all ranges recited herein include the endpoints of the recited range. For example, the range "1 to 100" includes the endpoints 1 and 100. Every maximum numerical limitation recited herein is intended to include every subnumerical limitation subsumed therein, and every minimum numerical limitation recited herein is intended to include every upper numerical limitation subsumed therein. Accordingly, applicants reserve the right to amend this specification, including the claims, to expressly describe any subranges encompassed within the expressly recited ranges. All such ranges are inherently described herein.

[0146] Any patent applications, patents, non-patent publications, or other disclosure materials referred to herein and / or set forth in an Application Data Sheet are incorporated herein by reference to the extent that the incorporated material does not contradict this specification. Therefore, to the extent necessary, the disclosure expressly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, purportedly incorporated herein by reference that contradicts any existing definitions, statements, or other disclosure material set forth herein is incorporated only to the extent that there is no conflict between the incorporated material and the existing disclosure material.

[0147] "Comprise" (and any form of comprise, such as "comprises" or "comprising"), "have" (and any form of have, such as "has" or "having"), "include" (and any form of include, such as "includes" or "including"), and "contain" (and any form of contain, such as "contains" or "containing") are open-ended linking verbs. Consequently, a system that "comprises," "has," "includes," or "contains" one or more elements possesses those one or more elements, but is not limited to possessing only those one or more elements. Similarly, a system, device, or equipment element that "comprises," "has," "includes," or "contains" one or more features possesses those one or more features, but is not limited to possessing only those one or more features.

[0148] While particular examples of the present disclosure have been described above for purposes of illustration, it will be apparent to those skilled in the art that many changes can be made to the details of the present disclosure without departing from the disclosure as defined in the appended claims.

Claims

1. 1. A passively cooled nuclear reactor, comprising: A heat exchanger; a reactor core disposed proximate to the heat exchanger; The reactor core comprises: Fuel rods and a heat pipe disposed adjacent to the fuel rod and extending from the reactor core into the heat exchanger; a moderator monolith configured to receive and provide space for the fuel rods and the heat pipes; a thermal coupling material disposed internally through the moderator monolith to surround the fuel rods and the heat pipes and to facilitate heat transfer from the reactor core to the heat exchanger.

2. the moderator monolith further defines a plurality of openings; the fuel rod is configured to be slidably disposed through a first opening defined by the moderator monolith; 10. The nuclear reactor of claim 1, wherein the heat pipe is configured to be slidably disposed through a second opening defined by the moderator monolith.

3. 10. The nuclear reactor of claim 1, wherein the reactor core further comprises a moderator rod configured to be slidably disposed through a third opening defined by the moderator monolith.

4. 10. The nuclear reactor of claim 1, wherein the reactor core contacts the heat exchanger through the heat pipe.

5. The nuclear reactor of claim 1 , wherein the thermal coupling material comprises a two-state material.

6. the thermal coupling material is solid in the first reactor state and locks the fuel rods and the heat pipes in place; 6. The nuclear reactor of claim 5, wherein the thermal coupling material is in a liquid state in the second state of the reactor.

7. 6. The nuclear reactor of claim 5, wherein the two-state material is lead.

8. 10. The nuclear reactor of claim 1, wherein the thermal coupling material is a gas.

9. The nuclear reactor of claim 1 , wherein the moderator monolith comprises a unit cell.

10. the fuel rod is an annular fuel rod defining an opening; the heat pipe is a first heat pipe, 10. The nuclear reactor of claim 1, wherein the reactor core further comprises a second heat pipe configured to be slidably disposed through the opening defined by the annular fuel rod.

11. 11. The nuclear reactor of claim 10, wherein the first heat pipe extends further into the heat exchanger than the second heat pipe.

12. 10. The nuclear reactor of claim 1, wherein the reactor is configured to operate at atmospheric pressure in the range of 30 kPa to 103 kPa.

13. 10. The nuclear reactor of claim 1, further comprising a control rod movement mechanism disposed distal to the heat exchanger, the control rod movement mechanism configured to move control rods through the heat exchanger and into the reactor core.

14. 1. A passively cooled nuclear reactor, comprising: A heat exchanger; a nuclear reactor core disposed proximate to the heat exchanger, a plurality of fuel rods; a plurality of heat pipes extending from the reactor core into the heat exchanger; a moderator monolith having a plurality of openings, wherein each of the plurality of fuel rods is configured to be slidably disposed through a first set of openings defined by the moderator monolith and each of the plurality of heat pipes is configured to be slidably disposed through a second set of openings defined by the moderator monolith; a reflector surrounding the moderator monolith; a thermal coupling material disposed internally through the moderator monolith to surround the plurality of fuel rods and the plurality of heat pipes and to facilitate heat transfer from the reactor core to the heat exchanger; the reactor core comprising a vessel surrounding the reflector; a plurality of control rod movement mechanisms disposed distal to the heat exchanger, each of the plurality of control rod movement mechanisms configured to move a control rod through the heat exchanger to the reactor core.

15. 15. The nuclear reactor of claim 14, wherein the reactor core further comprises a plurality of moderator rods configured to be slidably disposed through a third set of openings in the moderator monolith.

16. 15. The nuclear reactor of claim 14, wherein the thermal coupling material is a two-state material.

17. the thermal coupling material is solid in a first state of the reactor and locks the plurality of fuel rods and the plurality of heat pipes in place; 15. The nuclear reactor of claim 14, wherein the thermal coupling material is in a liquid state in the second state of the reactor.

18. the plurality of fuel rods are annular fuel rods; the plurality of heat pipes is a first plurality of heat pipes; 15. The nuclear reactor of claim 14, wherein the reactor core further comprises a second plurality of heat pipes, each heat pipe of the second plurality of heat pipes configured to be slidably positioned through an opening defined by a respective annular fuel rod of the plurality of fuel rods.

19. 20. The nuclear reactor of claim 18, wherein the first plurality of heat pipes extend further into the heat exchanger than the second plurality of heat pipes.

20. 15. The nuclear reactor of claim 14, wherein the reflector comprises a plurality of control drums.

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