Heat pipe networks for heat removal, such as heat removal from nuclear reactors, and associated systems and methods

Nuclear reactor systems and associated devices and methods are described herein. A representative nuclear reactor system includes a heat pipe network having an evaporator region, an adiabatic region, and a condenser region. The heat pipe network can define a plurality of flow paths having an increasing cross-sectional flow area in a direction from the evaporator region toward the condenser region. The system can further include nuclear fuel thermally coupled to at least a portion of the evaporator region. The heat pipe network is positioned to transfer heat received from the fuel at the evaporator region, to the condenser region. The system can further include one or more heat exchangers thermally coupled to the evaporator region for transporting the heat out of the system for use in one or more processes, such as generating electricity.

TECHNICAL FIELD

The present technology relates generally to nuclear reactors and associated systems and methods and, more particularly, to nuclear reactors having heat pipes for removing heat generated by a nuclear fuel.

BACKGROUND

Power plants come in many different shapes and sizes. Large power plants can be used to provide electricity to a geographic area, whereas relatively small power plants can be used to power, for example, local areas, submarines, space craft, etc. In addition to providing electricity, power plants can be used for a myriad of additional or different purposes, from desalinating seawater to creating nuclear isotopes for medical purposes. Similarly, the types of power plants that are available cover a wide spectrum of technologies including gas-powered, coal-fired, and nuclear-powered, to name a few.

To date, nuclear reactor designs that use heat pipes as a means for heat removal from a core of nuclear material maintain constant heat pipe flow area in discreet unconnected heat pipes. For a given reactor core geometry, heat removal is limited by the total effective heat pipe flow area and the fraction of the core volume occupied by the heat pipes. For example, a larger number of heat pipes will increase the total effective flow area and heat removal capacity but will also displace fuel and other core material, which will reduce the heat production potential of the core.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed generally toward nuclear reactor systems. In several of the embodiments described below, a representative nuclear reactor system includes a network of interconnected heat pipes having an evaporator region, an adiabatic region, and a condenser region. The heat pipes are configured to contain a working fluid, and the network of heat pipes can define an increasing cross-sectional flow area for the working fluid in a direction from the evaporator region toward the condenser region. The system can further include a nuclear fuel, such as a fissile material, configured to generate heat and thermally coupled to at least a portion of the evaporator region. The network of heat pipes is configured to transfer the heat from the evaporator region to the condenser region. The system can further include one or more heat exchangers thermally coupled to the condenser region for transporting the heat out of the system for use in one or more processes, including, but not limited to, generating electricity.

In one aspect of the present technology, the increasing flow area of the heat pipes can increase the heat removal capacity of the heat pipes as compared to, for example, conventional heat pipes having a single pipe of constant flow area. In some embodiments, the heat pipes can branch or bifurcate in the direction from the evaporator region toward the condenser region to increase the flow area of the network of heat pipes.

In some embodiments, the network of heat pipes can be provided in a heat pipe layer, and the nuclear reactor system can include a plurality of stacked heat pipe layers. In one aspect of the present technology, the number of heat pipe layers can be varied in accordance with, for example, the amount of fuel in the system, and thus the power/heat output of the system. In another aspect of the present technology, the heat pipe layers can be loosely coupled to a common frame and/or other ones of the heat pipe layers. This can allow the heat pipe layers to expand/contract independently due to heat and/or irradiation—thereby reducing mechanical stress on the heat pipe layers, the frame, and/or other components of the system. In some embodiments, to refuel the system, one or more of the heat pipe layers—and the fuel attached to or otherwise associated therewith—can be removed and replaced and/or shifted to another location within the stack of heat pipe layers, for example, without requiring the entire network of heat pipes to be removed.

In some embodiments, the fuel can be directly attached to the evaporator region of the heat pipes, which can provide a high thermal coupling between the fuel and the heat pipes. For example, the fuel can be formed on the heat pipes using a hot isostatic pressing (HIP) process.

Certain details are set forth in the following description and inFIGS.1-5Cto provide a thorough understanding of various embodiments of the present technology. In other instances, well-known structures, materials, operations, and/or systems often associated with nuclear reactors, heat pipes, heat exchangers, etc., are not shown or described in detail in the following disclosure to avoid unnecessarily obscuring the description of the various embodiments of the technology. Those of ordinary skill in the art will recognize, however, that the present technology can be practiced without one or more of the details set forth herein, and/or with other structures, methods, components, and so forth.

The accompanying Figures depict embodiments of the present technology and are not intended to be limiting of its scope unless expressly indicated. The sizes of various depicted elements are not necessarily drawn to scale, and these various elements may be enlarged to improve legibility. Component details may be abstracted in the Figures to exclude details such as position of components and certain precise connections between such components when such details are unnecessary for a complete understanding of how to make and use the present technology. Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular embodiments of the disclosure. Accordingly, other embodiments can have other details, dimensions, angles and features without departing from the present technology. In addition, those of ordinary skill in the art will appreciate that further embodiments of the present technology can be practiced without several of the details described below.

FIG.1is a partially schematic side cross-sectional view of a nuclear reactor system100(“system100”) configured in accordance with representative embodiments of the present technology. In the illustrated embodiment, the system100includes a reactor container102and a radiation shield container104surrounding/enclosing the reactor container102. In some embodiments, the reactor container102and the radiation shield container104can be roughly cylinder-shaped or capsule-shaped. The system100further includes a plurality of heat pipe layers106within the reactor container102. In the illustrated embodiment, the heat pipe layers106are spaced apart from and stacked over one another. In some embodiments, the heat pipe layers106can be mounted/secured to a common frame109, a portion of the reactor container102(e.g., a wall thereof), and/or other suitable structures within the reactor container102. In other embodiments, the heat pipe layers106can be directly stacked on top of one another such that each of the heat pipe layers106supports and/or is supported by one or more of the other ones of the heat pipe layers106.

In the illustrated embodiment, the system100further includes a shield or reflector region114at least partially surrounding a core region116. The heat pipe layers106can be circular, rectilinear, polygonal, and/or can have other shapes, such that the core region116has a corresponding three-dimensional shape (e.g., cylindrical, spherical, etc.). In some embodiments, the core region116is separated from the reflector region114by a core barrier115, such as a metal wall. The core region116can include one or more fuel sources, such as fissile material, for heating the heat pipe layers106. The reflector region114can include one or more materials configured to contain/reflect products generated by burning the fuel in the core region116during operation of the system100. For example, the reflector region114can include a liquid or solid material configured to reflect neutrons and/or other fission products radially inward toward the core region116. In some embodiments, the reflector region114can entirely surround the core region116. In other embodiments, the reflector region114may only partially surround the core region116. In some embodiments, the core region116can include a control material117, such as a moderator and/or coolant. The control material117can at least partially surround the heat pipe layers106in the core region116and can transfer heat therebetween. In some embodiments, as described in greater detail below, the control material117can be a liquid moderator (e.g., a liquid metal alloy, a liquid metal hydride) configured to control the reactivity of the system100.

In the illustrated embodiment, the system100further includes at least one heat exchanger108positioned around the heat pipe layers106. The heat pipe layers106can extend from the core region116and at least partially into the reflector region114, and are thermally coupled to the heat exchanger108. As described in greater detail below with reference toFIGS.4A and4B, the heat exchanger108can be positioned outside of or partially within the reflector region114. As described in greater detail below with reference toFIGS.2A-5C, the heat pipe layers106can each include an array of heat pipes that provide a heat transfer path from the core region116to the heat exchanger108. During operation of the system100, the fuel in the core region116can heat and vaporize a fluid within the heat pipes in the heat pipe layers106, and the fluid can carry the heat to the heat exchanger108.

In some embodiments, the heat exchanger108can include one or more helically-coiled tubes that wrap around the heat pipe layers106. The tubes of the heat exchanger108can include or carry a working fluid (e.g., a coolant such as water or another fluid) that carries the heat from the heat pipe layers106out of the reactor container102and the radiation shield container104for use in generating electricity, steam, etc. For example, in the illustrated embodiment the heat exchanger108is operably coupled to a turbine110, a generator111, a condenser112, and a pump113. As the working fluid within the heat exchanger108increases in temperature, the working fluid may begin to boil and vaporize. The vaporized working fluid (e.g., steam) may be used to drive the turbine110to convert the thermal potential energy of the working fluid into electrical energy via the generator111. The condenser112can condense the working fluid after it passes through the turbine110, and the pump113can direct the working fluid back to the heat exchanger108, where it can begin another thermal cycle.

FIGS.2A and2Bare a top cross-sectional view and an enlarged top cross-sectional view, respectively, of the reflector region114and the core region116configured in accordance with representative embodiments of the present technology. More specifically,FIG.2Aillustrates one of the heat pipe layers106. In the illustrated embodiment, the heat pipe layer106includes a network of at least partially interconnected heat pipes220. The heat pipes220can be separate tubes, channels formed in/between one or more solid members, and/or other structures configured (e.g., positioned and shaped) to provide one or more flow paths (e.g., for a working fluid contained therein).

The heat pipes220can have a branching/tapering arrangement in which the heat pipes220bifurcate as they extend radially outward in a direction from the reactor container102toward the reflector region114(e.g., in a direction away from a central axis C of the heat pipe layer106as indicated by arrow R inFIG.2A). More specifically, referring toFIG.2B, the heat pipes220can comprise first portions221that are positioned radially inward of and bifurcate into (e.g., divide into, connect to, etc.) second portions222. Likewise, the second portions222can be positioned radially inward of and bifurcate into third portions223, the third portions223can be positioned radially inward of and bifurcate into fourth portions224, and so on. In some embodiments, the heat pipes220include more or fewer bifurcations. The heat pipes220can all be fluidly connected together, or two or more different subsets of the heat pipes220can be fluidly connected together. For example, a subset or branch of the heat pipes220(e.g., a wedge-shaped branch shown inFIG.2B) including one of the first portions221and its progeny—for example, two of the second portions222, four of the third portions223, eight of the fourth portions224, and so on—can be connected together. Adjacent wedge-shaped branches can be independent of each other, or individual wedge-shaped branches can be connected together (e.g., at the entrances of the adjacent first portions221).

FIG.2Cis a cross-sectional isometric view of one of the heat pipes220configured in accordance with embodiments of the present technology. In the illustrated embodiment, the heat pipe220includes an outer wall230having an outer surface232aand an inner surface232b, and defining a channel234. The heat pipe220includes a working fluid (not shown) that is contained within the channel234. The working fluid can be a two-phase metal (e.g., liquid and vapor phase) such as, for example, sodium or potassium. The wall230can be formed from any suitably strong, thermally conductive, and neutronic resistant material such as, for example, one or more metal or ceramic materials. In a particular embodiment, the wall230comprises molybdenum alloy. In the illustrated embodiment, the wall230has a generally square cross-sectional shape while, in other embodiments, the wall230can have a circular, rectangular, polygonal, irregular, or other cross-sectional shape.

The heat pipe220further includes a first mesh or wick236extending along/over a portion of the inner surface232b, such as a lower/floor portion of the inner surface232b(e.g., relative to gravity). The heat pipe220can further include a second mesh or wick238extending along/over all or a portion of the rest of the inner surface232band the first wick236. In some embodiments, the first wick236is a coarse wick capable of relatively high throughput of the working fluid compared to the second wick238. The second wick238can be a fine wick configured to pump the working fluid against a larger pressure gradient than the first wick236, but for shorter distances that the first wick236. Accordingly, the first and second wicks236,238can together form a compound wick in which (i) the first wick236allows for long distance flow of the working fluid and (ii) the second wick238allows for localized flow of the working fluid. In other embodiments, the heat pipe220can include other wick arrangements (e.g., compound or single wick arrangements) for promoting the flow of the working fluid through the channel234of the heat pipe220.

Referring toFIGS.2A-2Ctogether, the heat pipes220extend radially outward from the central axis C at least partially through (i) the core region116and the control material117therein and (ii) the reflector region114. More specifically, in some embodiments the first, second, and third portions221-223of the heat pipes220are positioned within the core region116, while the fourth portions224extend through the reflector region114. In the illustrated embodiment, the fourth portions224of the heat pipes220extend entirely through the reflector region114such that an outward terminus225of the heat pipes220is positioned radially outside of the reflector region114.

Fuel226(FIG.2B) is positioned around/proximate to the heat pipes220in the core region116(e.g., on the first, second, and third portions221-223of the heat pipes220). In some embodiments, the fuel226can be a solid metallic fuel including fissile material, such as a uranium molybdenum alloy having high thermal conductivity and a high density of fissile material. In some embodiments, the fuel226can be inserted into slots (not shown) along the heat pipes220or the structure supporting the heat pipes220(e.g., the frame109shown inFIG.1) such that there is no gap or only a small gap between the walls330of the heat pipes220and the fuel226. In such embodiments, the slots can cover the fuel226and act as a cladding that inhibits or even prevents growth of the fuel226and/or the escape of fission products. In other embodiments, the fuel226can be directly formed on/attached to the heat pipes220using, for example, a hot isostatic pressing (HIP) process, sintering, additive manufacturing, and/or other suitable process.

More specifically,FIG.3is a flow diagram of a process or method360for forming the fuel226on the heat pipes220(e.g., forming a nuclear reactor component) using a HIP process in accordance with embodiments of the present technology. At block362, the method360includes positioning one or more of the heat pipes220in a pressure vessel. At block364, the method360includes providing/depositing a powdered fuel material (e.g., a fissile material) on a portion of the heat pipes220. For example, the fuel226can be provided as a metal powder (e.g., a powder of uranium molybdenum alloy) on the walls230of the heat pipes220to be located in the core region116. The fuel226can be provided on the heat pipes220before or after the heat pipes220are positioned in the pressure vessel. At block366, the method360includes increasing a pressure and a temperature in the pressure vessel to solidify/compress the fuel material and directly attach the fuel material to the heat pipes220. For example, the increased temperature and pressure can compress the powdered fuel226into a compact solid on the walls330of the heat pipes220. In some embodiments, increasing the pressure in the pressure vessel includes increasing an isostatic gas pressure in the pressure vessel by pumping an inert gas (e.g., argon) into the pressure vessel that does not react with the material of the heat pipes220or the fuel226. In some embodiments, the temperature can be greater than about 450° C., greater than about 1000° C., greater than about 1300° C., or hotter. In some embodiments, the pressure can be greater than about 20 MPa, greater than about 50 MPa, greater than about 100 MPa, or higher.

Accordingly, referring toFIGS.1-3together, in one aspect of the present technology the fuel226can be physically attached to/integrated with the walls330of the heat pipes220, which can provide a high thermal coupling between the fuel226and the heat pipes220. In some embodiments, a cladding material can be formed around the fuel226using the same or a different process for attaching the fuel226to the heat pipes220. For example, a HIP process can also be used to form a cladding layer around the fuel226.

During system operation (e.g., after the fuel226and heat pipes220have been installed in the system100), the fuel226generates heat that is transferred to the heat pipes220to evaporate/vaporize the working fluid therein. The first, second, and third portions221-223of the heat pipes220in the core region116can define an evaporator region of the heat pipes220in which the working fluid is heated and evaporated/vaporized. The evaporated working fluid then flows radially outward through the channels234along the fourth portions224of the heat pipes220(e.g., an adiabatic region of the heat pipes220) through the reflector region114toward the outward termini225of the heat pipes220(e.g., a condenser region of the heat pipes220) where the working fluid cools and condenses. The first and second wicks236,238are configured to transport the condensed/cooled working fluid against the pressure gradient in the heat pipes220to the evaporator region of the heat pipes220where the working fluid can be heated and vaporized once again. Accordingly, in some embodiments heat is deposited into the evaporator region of the heat pipes220, removed from the condenser region of the heat pipes220, and neither removed from nor added to the heat pipes220in the adiabatic region. The vapor mass flow rate of the working fluid can increase over the length of the evaporator region and decrease over the length of the condenser region.

In one aspect of the present technology, the branching (e.g., fractionalizing, bifurcating) arrangement of the heat pipes220provides an increasing flow area (e.g., cross-sectional flow area) in the radial direction (e.g., in the direction indicated by the arrow R inFIG.2A). That is, the heat pipes220originate from near a common point (e.g., the central axis C) and grow in flow area by branching from the first portions221to the second portions222, from the second portions222to the third portions223, and so on. This arrangement can increase the heat removal capacity of the heat pipes220as compared to conventional heat pipes having a single pipe of constant flow area.

More specifically, there are two at least two properties that affect the heat removal capacity of a heat pipe—the capillary limit and the sonic limit. The capillary limit occurs when the forces from the pressure difference across the length of the heat pipe exceed the capillary forces within the wick of the heat pipe, preventing the coolant liquid from reaching the end of the heat pipe. This limit can be addressed by improving the wick to increase the capillary force, or by improvements to the vapor flow path to reduce the pressure difference. The sonic limit represents the velocity when choked flow will occur, at which point more fluid cannot be transported by the heat pipe. This limit can be addressed by larger flow areas or lower power. The arrangement of the heat pipes220increases the vapor and liquid flow area along the length of the heat pipes220compared to conventional heat pipes having a constant flow area, without exceeding the capillary and sonic limits, and therefore provides improved heat transfer. Specifically, the increase in flow area limits the increase in vapor velocity that typically occurs in the evaporator section of a heat pipe. This allows the branching heat pipes220to draw more heat before encountering an operational limit.

In another aspect of the present technology, the branching heat pipes220also have an increasing surface area in the radial direction (e.g., in the direction indicated by the arrow R inFIG.2A). This allows for a relatively high effective contact surface area between the heat pipes220and the fuel226—providing more surface area to attach the fuel226to—and thereby allowing for the fuel226to be made thinner and to be spread out across the larger surface area. Further, the branching heat pipes220can provide additional surface area, which in turn provides more radially uniform cooling in the condenser region near the outward termini225.

Branching also reduces the number of separate heat pipes220needed in the core region116. For example, a single one of the branching heat pipes220can cover a section of the core region116with multiple flow exits from the evaporator region. Accordingly, the branching heat pipes220reduce the volume of pipes within the core region116for the same heat removal rate (e.g. the same evaporator exit flow area). Because fewer pipes can be used, the heat exchanger108(FIG.1) can be made more compact and/or more of the fuel226and/or the control material117(e.g., a moderator) can be included in the core region116. In sum, the present technology provides a means for increasing the heat removal capacity of the system100while reducing a fraction of the volume of the core region116needed for heat removal.

FIGS.4A and4Bare enlarged top cross-sectional views of the portion of the system100shown inFIG.2Band illustrating the heat exchanger108in accordance with embodiments of the present technology. Referring first toFIG.4A, in some embodiments the heat exchanger108can be positioned within the reflector region114proximate to the outward termini225of the heat pipes220(e.g., proximate to the condenser region of the heat pipes220). In some such embodiments, the reflector region114comprises a liquid440, such as a liquid metal material (e.g., a liquid metal alloy, a liquid metal hydride) configured to reflect neutrons inward toward the core region116. In some embodiments, the heat exchanger108can be directly coupled/attached to the heat pipes220and/or their supporting structure while, in other embodiments, the heat exchanger108can be spaced apart from the heat pipes220in the reflector region114. For example, the liquid440can act as a heat transfer medium for transferring heat from the heat pipes220to the heat exchanger108. In some embodiments, the core barrier115can separate the liquid440in the reflector region114from the control material117in the core region116, and the heat pipes220can penetrate through the core barrier115. In one aspect of the present technology, the thermal properties of the liquid440in the reflector region114can help distribute heat around the heat exchanger108, and thereby inhibit failure of the heat exchanger108from a failure of any of the heat pipes220.

Referring next toFIG.4B, in some embodiments the heat exchanger108can be positioned radially outside of the reflector region114proximate to the outward termini225of the heat pipes220(e.g., proximate to the condenser region of the heat pipes220). In some such embodiments, the reflector region114comprises a solid material442configured to reflect neutrons inward toward the core region116. Accordingly, the heat pipes220can penetrate through the solid material442.

Referring toFIGS.1-2C,4A, and4Btogether, in some embodiments the core region116can include the control material117at least partially surrounding the heat pipes220and the fuel226in the core region116. In the illustrated embodiment, the control material117terminates at the edge of the core region116and thus entirely surrounds the fuel226. In other embodiments, the control material117can extend at least partially into the reflector region114and can act as part of the reflector. In yet other embodiments, the control material117can be omitted.

The control material117can be a moderator configured to control the reactivity of the system100and/or a coolant configured to distribute heat between the heat pipe layers106. In some embodiments, the control material117can be a liquid moderator including, for example, a metal hydride that can be used to control the reactivity of the fuel226by adding or removing hydrogen from the system100. More particularly, the control material117can be a mixture of calcium hydride (e.g., CaH2), calcium bismuth (Bi—Ca), and/or other suitable compounds. Such mixtures can undergo reversible conversions between metal and metal hydride based on the hydrogen content of the system100and, accordingly, can be used as a moderator to control a reactivity of the fuel226based on the state of the control material117. For example, in some embodiments the system100can include a hydrogen control system119(FIG.1) configured to vary a hydrogen content level of the core region116to thereby control the state of the control material117and thus its moderating properties. The control material117can also act as a coolant to evenly distribute heat between the heat pipe layers106. For example, if one or more of the heat pipes220fail, the control material117can help transfer heat from the fuel226around the failed one of the heat pipes220to other, functioning heat pipes220for removal from the system100. In one aspect of the present technology, the control material117therefore reduces thermal-gradient induced stresses arising from heat pipe failures and simplifies the structural design of the core region116.

In addition to or alternatively to the control material117, the system100can include one or more control rods (not shown), such as a boron control rod, configured to absorb (e.g., poison) neutrons in the core region116that may otherwise induce fission of the fuel226. Such control rods can help facilitate startup and shutdown of the system100.

FIGS.5A-5Care a top cross-sectional view, an enlarged top cross-sectional view, and a further enlarged top cross-sectional view, respectively, of a portion of one of the heat pipe layers106positioned at the core region116in accordance with additional embodiments of the present technology. Some of the features and associated functionality of the heat pipe layer illustrated inFIGS.5A and5Bcan be generally similar to or identical to the features and/or functionality of the heat pipe layer described in detail with respect toFIGS.2A-4B. For example, referring toFIGS.5A-5Ctogether, the heat pipe layer106includes a network of interconnected heat pipes. In the illustrated embodiment, however, the heat pipe layer106includes (i) arterial heat pipes520(e.g., arterial flow paths; identified individually as first through third arterial heat pipes520a-520c) extending radially outward in a direction indicated by the arrow R away from the central axis C of the heat pipe layer106(FIG.5A) and (ii) off-branching heat pipes552(e.g., off-branching flow paths) extending circumferentially relative to the central axis C and connecting two or more of the arterial heat pipes520.

In the illustrated embodiment, the first arterial heat pipes520aextend farther radially inward than the second and third arterial heat pipes520b, c, and the second arterial heat pipes520bextend farther radially inward than the third arterial heat pipes520c. Moreover, the arterial heat pipes520can be generally equally spaced circumferentially about the central axis C, and the number of the third arterial heat pipes520ccan be greater (e.g., double) the number of the second arterial heat pipes520b, the number of second arterial heat pipes520bcan be greater (e.g., double) the number of the first arterial heat pipes520a, and so on. Accordingly, the arterial heat pipes520can form an alternating/interleaved arrangement in which each of the first arterial heat pipes520ais directly adjacent to two (e.g., a pair) of the second arterial heat pipes520b, each of the second arterial heat pipes520bis directly adjacent two of the third arterial heat pipes520c, and so on. In other embodiments, the core region116can have more or fewer of the arterial heat pipes520, and/or the arterial heat pipes520can be arranged differently (e.g., asymmetrically). The arterial heat pipes520contain a working fluid and provide a heat removal path at least partially through the reflector region114(FIG.1) to the heat exchanger108(FIG.1).

The off-branching heat pipes552each extend between and connect an adjacent pair of the arterial heat pipes520(e.g., a circumferentially adjacent pair). The arterial heat pipes520and the off-branching heat pipes552(collectively “heat pipes520,552”) can all be fluidly connected together, or two or more different subsets of the heat pipes520,552can be fluidly connected together. For example, a subset or branch of the heat pipes520,552can be connected together, such as a wedge-shaped branch shown inFIG.5Bincluding one of the first arterial heat pipes520a, an adjacent two of the second arterial heat pipes520b, an adjacent four of the third arterial heat pipes520c, and so on.

In the illustrated embodiment, the off-branching heat pipes552each include one or more (e.g., two or more) evaporator heads554. Referring toFIG.5C, the fuel226can be positioned around/proximate to the evaporator heads554. For example, in the illustrated embodiment the fuel226is directly attached to the evaporator heads554(e.g., via a HIP process) such that there is no gap or a small gap between the evaporator heads554and the fuel226. In other embodiments, the fuel226can be inserted into slots on the evaporator heads554. In some embodiments, the evaporator heads554can be evenly spaced apart from another along the off-branching heat pipes552such that the number of evaporator heads554increases in the radial direction as the length and/or number of the off-branching heat pipes552increases.

Referring again toFIGS.5A-5Ctogether, during operation of the system100, the fuel226generates heat that is transferred to the evaporator heads554to evaporate/vaporize the working fluid therein and/or in the off-branching heat pipes552. The evaporated working fluid then flows circumferentially toward one or both of the connected arterial heat pipes520, and then radially outward along the arterial heat pipes520through the reflector region114(FIG.1) toward the heat exchanger108(FIG.1) where the working fluid cools, condenses, and transfers heat to the heat exchanger108. The heat pipes520,552can include wicks (e.g., as described in detail with reference toFIG.2C) configured to transport the condensed/cooled working fluid against the pressure gradient in the heat pipes520,552to the evaporator heads554where the working fluid can be heated and vaporized once again. Accordingly, the heat pipes520,552remove heat from the fuel226and transport the heat radially outward toward the heat exchanger108(FIG.1).

Similar to the embodiments described above with reference toFIGS.2A-4B, the arrangement of the heat pipes520,552provides an increasing flow area in the radial direction (e.g., in the direction indicated by the arrow R). That is, the network of heat pipes520,552originates from near a common point (e.g., the central axis C) and grows in flow area as the number and/or length of heat pipes520,552increases in the radial direction. As described in detail above, this arrangement can increase the heat removal capacity of the heat pipes520,552as compared to conventional heat pipes having a single pipe of constant flow area. Likewise, in some embodiments the control material117can at least partially surround the heat pipes520,552and the fuel226in the core region116.

Referring again toFIG.1, in other embodiments the heat pipe layers106can each comprise one or more disk-shaped (e.g., circular) planar heat pipes. In some embodiments, fuel elements having a corresponding flat disk shape can be attached to the heat pipes in the core region116. That is, the heat pipe layers106can be positioned between corresponding flat fuel elements. Such embodiments can provide a relatively simple arrangement that still provides a relatively high vapor flow area and high surface area for contact with the fuel.

Referring toFIGS.1-5Ctogether, the heat pipe layers106can each be identical. In other embodiments, the configurations of the heat pipe layers106can be varied. For example, some of the heat pipe layers106can include the network of heat pipes220shown inFIGS.2A-2C, while some of the heat pipe layers106include the network of heat pipes520,552shown inFIGS.5A-5C, and/or while some of the heat pipe layers106include flat disk-shaped heat pipes.

In one aspect of the present technology, the number of heat pipe layers106can be varied to vary the amount of fuel226in the system100, and thus the power/heat output of the system100. In another aspect of the present technology, the heat pipe layers106can be loosely coupled (e.g., via a plurality of flexible joints) to the frame109and/or the other ones of the heat pipe layers106. This can allow the heat pipe layers106to expand/contract independently due to heat and/or irradiation—thereby reducing mechanical stress on the heat pipe layers106, the frame109, and/or other components of the system100. Similarly, all or a subset of the heat pipes in each of the heat pipe layers106can expand/contract independently due to heat and/or irradiation. For example, each of the wedge-shaped branches of the heat pipes220(e.g., the branch shown inFIG.2B) or the heat pipes520,552(e.g., the branch shown inFIG.5B) can move independently of the other wedge-shaped branches in the heat pipe layer106.

In some embodiments, to refuel the system100, one or more of the heat pipe layers106—and the fuel226attached to or otherwise associated therewith—can be removed and replaced and/or shifted to another location within the stack of heat pipe layers106. Similarly, in some embodiments one or more of the independent branches of the heat pipes can be shifted/moved during a refueling process. In another aspect of the present technology, the modular nature of the heat pipe layers106can enable the heat pipe layers106to be manufactured/fabricated in parallel and subsequently assembled.

The following examples are illustrative of several embodiments of the present technology:1. A nuclear reactor, comprising:a heat pipe network including an evaporator region, an adiabatic region, and a condenser region, wherein the heat pipe network defines a plurality of flow paths having an increasing cross-sectional flow area in a direction from the evaporator region toward the condenser region; andnuclear fuel thermally coupled to at least a portion of the evaporator region, wherein the heat pipe network is positioned to transfer heat received from the fuel at the evaporator region to the condenser region.2. The nuclear reactor of example 1 wherein the nuclear fuel is directly attached to the evaporator region.3. The nuclear reactor of example 2 wherein the nuclear fuel is a uranium molybdenum alloy pressed onto the evaporator region.4. The nuclear reactor of any one of examples 1-3 wherein the nuclear reactor further comprises a liquid metal moderator at least partially surrounding the evaporator region, and wherein the liquid metal moderator is positioned to control a reactivity of the nuclear fuel.5. The nuclear reactor of any one of examples 1-4 wherein the nuclear reactor further comprises a reflector positioned to reflect neutrons resulting from fission of the nuclear fuel, and wherein at least a portion of the heat pipe network extends through the reflector.6. The nuclear reactor of example 5 wherein the adiabatic region extends away from the evaporator region through the reflector, and wherein the condenser region is positioned outside the reflector.7. The nuclear reactor of any one of examples 1-6, further comprising a heat exchanger thermally coupled to the condenser region.8. The nuclear reactor of any one of examples 1-7 wherein individual ones of the flow paths branch into two or more portions in the direction from the evaporator region toward the condenser region.9. The nuclear reactor of any one of examples 1-8 wherein individual ones of the flow paths include (a) a first portion proximate a central axis of the heat pipe network, (b) a pair of second portions branching from the first portion, (c) a pair of third portions branching from each of the second portions, and (d) a pair of fourth portions branching from each of the third portions.10. The nuclear reactor of example 9 wherein the nuclear fuel is thermally coupled to the first portion, the second portions, and the third portions of the flow paths.11. The nuclear reactor of example 10 wherein the nuclear reactor further comprises a reflector positioned to reflect neutrons resulting from fission of the nuclear fuel, and wherein the fourth portions extend through the reflector.12. The nuclear reactor of any one of examples 1-11 wherein the heat pipe network includes (a) arterial flow paths extending radially away from a central axis of the heat pipe network and (b) off-branching flow paths extending circumferentially about the central axis and connecting circumferentially adjacent pairs of the arterial flow paths.13. The nuclear reactor of example 12 wherein individual ones of the off-branching flow paths include one or more evaporator heads, and wherein the nuclear fuel is directly attached to the evaporator heads.14. A nuclear reactor, comprising:a plurality of heat pipe layers, wherein individual ones of the heat pipe layers include (a) a heat pipe network extending radially outward from a central axis and (b) fissile material thermally coupled to at least a portion of the heat pipe network, and wherein the heat pipe network has an increasing cross-sectional flow area in a radially-outward direction from the central axis;a reflector positioned to reflect neutrons resulting from fission of the fissile material in a radially-inward direction toward the central axis, wherein the heat pipe networks in individual ones of the heat pipe layers extend at least partially through the reflector; anda heat exchanger thermally coupled to the heat pipe layers, wherein the heat pipe layers are positioned to transfer heat received from the fissile material to the heat exchanger.15. The nuclear reactor of example 14 wherein the heat pipe layers are stacked one over another.16. The nuclear reactor of example 14 or example 15 wherein the heat pipe layers are coupled to and supported by a frame.17. The nuclear reactor of any one of examples 14-16 wherein the reflector comprises a solid material, and wherein the heat exchanger is positioned radially outside of the reflector.18. The nuclear reactor of any one of examples 14-17 wherein the reflector comprises a fluid, wherein the heat exchanger is positioned at least partially within the fluid, and wherein the fluid is in thermal contact with the fluid and the heat pipe layers to transfer heat from the heat pipe networks to the heat exchanger.19. A method of forming a nuclear reactor component, the method comprising: positioning a heat pipe in a pressure vessel; providing a powdered nuclear fuel material on at least a portion of the heat pipe; and increasing a pressure and a temperature in the pressure vessel to solidify the nuclear fuel material and directly attach the nuclear fuel material to the heat pipe.20. The method of example 19 wherein the nuclear fuel material is a fissile material.21. The method of example 19 or example 20 wherein the nuclear fuel material is uranium molybdenum alloy.22. The method of any one of examples 19-21 wherein increasing the pressure in the pressure vessel includes pumping an inert gas into the pressure vessel.23. The method of any one of examples 19-22 wherein providing the powdered nuclear fuel material on at least the portion of the heat pipe includes providing the powdered nuclear fuel material on an evaporator region of the heat pipe, wherein the heat pipe further includes a condenser region, and wherein the heat pipe defines a flow path having an increasing cross-sectional flow area in a direction from the evaporator region toward the condenser region.

As used herein, the phrase “and/or” as in “A and/or B” refers to A alone, B alone, and A and B. To the extent any materials incorporated herein by reference conflict with the present disclosure, the present disclosure controls. Additionally, the term “comprising” is used throughout to mean including at least the recited feature(s) such that any greater number of the same feature and/or additional types of other features are not precluded. It will also be appreciated that specific embodiments have been described herein for purposes of illustration, but that various modifications may be made without deviating from the technology. Further, while advantages associated with some embodiments of the technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described herein.