HEAT PIPE FUEL ELEMENTS HAVING PARTICULAR HEAT PIPE FUEL ELEMENT PHOLLOMETRIC Spacing PATTERNS AND NUCLEAR FISSION REACTORS INCORPORATED THEREIN, AND METHODS OF MANUFACTURE

Abstract

The heat pipe fuel element includes an evaporation zone, a condensation zone, a capillary zone connecting the evaporation zone to the condensation zone, and a primary coolant. In a cross section in a plane perpendicular to the longitudinal axis of the evaporation zone, the heat pipe fuel element includes a cladding layer surrounding an interior region including a fuel body formed from a fissionable fuel composition, the fuel body having an outer surface oriented toward the cladding layer and an inner surface defining a perimeter of an evaporation space of the evaporation zone. The fuel body has a structure with a shape corresponding to a mathematically based periodic solid, such as a triply periodic minimal surface (TPMS), and the evaporation zones of multiple heat pipe fuel elements are arranged in a phyllotactic pattern (as viewed in cross section in a plane perpendicular to the longitudinal axis of the active core region).

Description

[Technical Field]

[0001] The present disclosure relates generally to nuclear fission reactors, either fast spectrum reactors or thermal reactors, in which the fuel elements are heat pipes in which a cladding wall surrounds a fuel body formed from a fissionable fuel composition, and the surface of the fuel body is in direct contact with a primary coolant. Circulation of the primary coolant through the heat pipes removes heat generated by the nuclear fission reactor and provides the heat to a heat sink, which can be used to produce work. [Background technology]

[0002] In the discussion that follows, reference is made to certain structures and / or methods. However, the following references should not be construed as an admission that these structures and / or methods constitute prior art. Applicant expressly reserves the right to disclose that such structures and / or methods do not qualify as prior art against the present invention.

[0003] A heat pipe is a passive, two-phase system that efficiently transfers heat or thermal energy from one location to another. A conventional heat pipe consists of a working fluid, a wick structure, and a vacuum-tight containment unit (enclosure). Typically, heat pipes are cylindrical in cross section with a wick on the inner diameter surface. The low-temperature working fluid moves through the wick by capillary action from the cooler side (condenser) to the hotter side (evaporator). In the evaporator section, heat input causes the liquid working fluid to evaporate at the wick surface. This vapor then travels to the condenser heat sink, carrying the thermal energy with it. In the condenser, the working fluid condenses, releasing its latent heat. This cycle then repeats to continuously remove heat from part of the system. The phase change process and two-phase flow circulation in a heat pipe continue as long as there is a sufficiently large temperature difference between the evaporator and condenser sections. The fluid stops moving when the overall temperature is uniform, but begins moving again as soon as a temperature difference exists. No power source (other than heat) is required.

[0004] Example applications for heat pipes include cooling electronics, HVAC systems, and thermal control of satellites and spacecraft. One specific example application is NASA's Safe Affordable Fission Engine (SAFE), an experimental nuclear fission reactor for electricity production in space. SAFE-400 used rhenium-coated uranium nitride fuel surrounded by molybdenum-sodium heat pipes that transferred heat to a heat pipe-to-gas heat exchanger (see Poston, David I. Nuclear Design of the SAFE-400a Space Fission Reactor. United States: N. p., 2002. Web). Another specific example application is the Special Purpose Reactor (SPR), a small 5 MWt heat pipe-cooled fast reactor (see https: / / www.osti.gov / servlets / purl / 1413987). A further illustrative example is NASA's Kilopower project, KRUSTY (Kilowatt Reactor using Stirling Technology), a prototype nuclear fission reactor connected to a Stirling engine by a heat pipe. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent Application Serial No. 16 / 835,388 [Patent Document 2] U.S. Patent Application Serial No. 16 / 835,370 [Patent Document 3] U.S. Patent Application Serial No. 16 / 951,543 [Non-patent literature]

[0006] [Non-Patent Document 1] Poston, David I. Nuclear Design of the SAFE-400a Space Fission Reactor. United States: N. p., 2002. Web [Non-patent document 2] https: / / www.osti.gov / servlets / purl / 1413987 Summary of the Invention [Problem to be solved by the invention]

[0007] Despite the existence of various heat pipes in nuclear reactor designs, there is still room for improved designs, particularly those that apply heat pipe structures and concepts to nuclear fission reactors. [Means for solving the problem]

[0008] Heat pipe applications in nuclear fission reactors connect the evaporator section of the heat pipe structure to a heat-generating reactor structure and the condenser section of the heat pipe structure to a heat sink structure, such as a heat exchanger. In contrast to current heat pipe reactors that rely on some form of thermal conductor between the fuel and the heat pipe, such as a thermal conductor in the form of a structural wall of the heat pipe, the disclosed heat pipe reactor utilizes a fuel element formed from a fissionable fuel composition inside the structural wall of the heat pipe. The structural wall of the heat pipe acts as a cladding wall surrounding the fuel element, and the inner surface of the fuel element is shaped so that the fuel element itself is a heat pipe structure in the evaporation section. In some embodiments, the fuel element is in direct contact with the primary coolant circulating inside the heat pipe, while in other embodiments, the fuel element is separated from the primary coolant circulating inside the heat pipe by an inner cladding wall. Thus, heat transfer from the fuel element to the primary coolant is more efficient, for example, by integrating the fuel and heat pipe into a single nuclear heat-generating element, eliminating thermal resistance in the heat pipe structure.

[0009] Each heat pipe associated with a fuel body forms an individual heat pipe fuel element, and additional aspects of the disclosed heat pipe nuclear fission reactor include (i) the fuel body in each heat pipe fuel element having a structure with a shape corresponding to a mathematically based periodic solid, and (ii) the evaporation zones of the multiple heat pipe fuel elements arranged in a phyllotactic pattern (when viewed in cross section in a plane perpendicular to the longitudinal axis of the active core region).

[0010] An exemplary embodiment of a heat pipe fuel element includes an evaporation zone, a condensation zone, a capillary zone connecting the evaporation zone to the condensation zone, and a primary coolant. In the evaporation zone and in cross section in a plane perpendicular to the longitudinal axis of the evaporation zone, the heat pipe fuel element includes a cladding layer surrounding an interior region including a fuel element formed from a fissionable fuel composition, the fuel element having an outer surface oriented toward the cladding layer and an inner surface defining a perimeter of an evaporation space of the evaporation zone.

[0011] In an exemplary embodiment, a plurality of heat pipe fuel elements are incorporated into a nuclear fission reactor structure where at least a portion of the vaporization area of each heat pipe fuel is contained within an active core region of the nuclear fission reactor and at least a portion of the condensation area of each heat pipe fuel is contained within a heat sink structure, and the capillary area of each heat pipe fuel element traverses the space between the active core region and the heat sink structure.

[0012] The disclosed design allows for the elimination of large amounts of heat-conducting structure (typically included in conventional designs), reducing the size of the void space in the reactor and resulting in a lighter, more transportable core. Additionally, integrating heat pipe fuel elements into the design of the active core region of a nuclear fission reactor system allows the heat removal zone to be implemented as a separate structure, which isolates the inherent design challenges faced by most conventional reactors when designing an optimal system. Additionally, the elimination of heat-conducting structure can be neutronically advantageous since the fuel replaces metal in the core.

[0013] Other aspects of the disclosed nuclear fission reactor system with heat pipe fuel elements include (i) the use of much larger heat exchangers in the overall reactor design, (ii) the use of alternative working fluids that are not suitable or desirable for use in nuclear reactors due to material concerns, and (iii) the use of larger segmented heat pipe heat exchangers that allow the application of flow optimization techniques (turbulent-to-turbulent features, spiral flow paths, etc.) for significant heat transfer that would otherwise be neutronically undesirable or unachievable in conventional reactor designs. Also, for example, the condenser area of the heat pipe section can be larger compared to the evaporation area of a direct heat pipe in the reactor, and the larger evaporation space allowed by the heat pipe design allows for intermediate heat exchanger (IHX) designs to exist that would otherwise not be critical due to adding too much gas volume directly in the reactor core. Certain material concerns, such as activation, fuel element corrosion, and working fluid shortage, are overcome by the use of the disclosed heat pipe fuel elements. For example, N2 is much worse thermally than He, but securing a supply of N2 is easier, and designing a gas-cooled N2 reactor is much more difficult than a reactor using He. However, using a heat pipe reactor allows N2 reactors to replace the poor thermal conductivity to the IHX outside the reactor itself, which can be designed for optimal N2 heat transfer, negating reactivity concerns.

[0014] Still further aspects of the disclosed nuclear fission reactor system with heat pipe fuel elements include: (a) the use of continuous tube cladding at least throughout the active core region, which can eliminate irradiation welding; (b) tighter packing provided by phyllotactic designs, which allow for smaller active core regions with less waste of fissile fuel, such as uranium, and enhance high purity, low enriched uranium (HALEU) design capabilities; (c) local, heat pipe fuel element-by-element control of fissile fuel density, which can limit peaking factors throughout the active core region (such as by varying parameters of a triple periodic minimal surface (TPMS) that defines a functionally graded lattice fuel structure, as disclosed in U.S. patent application Ser. No. 16 / 835,388, the entire contents of which are incorporated herein by reference); and (d) the ability to integrate moderators into the design of heat pipe fuel elements, which enables thermal reactor designs (for fast spectrum reactors).

[0015] The disclosed heat pipe fuel elements can also be manufactured using additive manufacturing processes. Examples of suitable additive manufacturing processes are disclosed in ISO / ASTM 52900-15, which defines classifications of additive manufacturing processes, including binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion bonding, sheet lamination, and photopolymerization. The contents of ISO / ASTM 52900-15 are incorporated herein by reference. Additionally, compositions for additive manufacturing processes and methods are disclosed in U.S. patent application Ser. No. 16 / 835,370, the entire contents of which are incorporated herein by reference, and additive manufacturing methods for in-situ monitoring of the fabrication of additively manufactured products are disclosed in U.S. patent application Ser. No. 16 / 951,543, the entire contents of which are incorporated herein by reference.

[0016] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the present disclosure as claimed. Additional features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the present disclosure. The objectives and other advantages disclosed herein will be realized and attained by the structure particularly pointed out in the written description and claims thereof, as well as in the appended drawings.

[0017] The foregoing summary, as well as the following detailed description of the embodiments, can be better understood when read in conjunction with the accompanying drawings. It should be understood that the depicted embodiments are not limited to the precise arrangements and mediums shown. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a simplified schematic perspective view of a nuclear fission reactor system showing a nuclear fission reactor connected to a heat exchanger by a plurality of heat pipes containing heat pipe fuel elements. [Figure 2A] FIG. 1 is a perspective view of an embodiment of a nuclear fission reactor system in which the nuclear fission reactor is coupled to a heat exchanger by a plurality of heat pipes that include heat pipe fuel elements. [Figure 2B] FIG. 1 is an end view of an embodiment of a nuclear fission reactor system in which the nuclear fission reactor is coupled to a heat exchanger by a plurality of heat pipes that include heat pipe fuel elements. [Figure 3A] FIG. 1 is a simplified schematic cross-sectional view of an individual heat pipe fuel element, also showing the location of an example section of the heat pipe fuel element relative to components of a nuclear fission reactor system, including the nuclear fission reactor and a heat exchanger. [Figure 3B] FIG. 1 illustrates an individual heat pipe fuel element in the context of the phyllotactic pattern of heat pipe fuel elements in the active core region of a nuclear fission reactor. [Figure 3C]FIG. 1 illustrates an individual heat pipe fuel element in relation to the phyllotactic pattern of the heat pipe fuel element in a heat sink structure. [Figure 4A] FIG. 10 is a schematic diagram detailing the placement of individual heat pipe fuel elements. [Figure 4B] FIG. 1 illustrates an example arrangement of evaporation zones of multiple heat pipe fuel elements in a nuclear fission reactor. [Figure 4C] 3B is a schematic diagram of a different embodiment of a cross section of a heat pipe fuel element as viewed along section A-A' in FIG. 3A showing internal features. [Figure 4D] 3B is a schematic diagram of a different embodiment of a cross section of a heat pipe fuel element as viewed along section A-A' in FIG. 3A showing internal features. [Figure 4E] 3B is a schematic diagram of a different embodiment of a cross section of a heat pipe fuel element as viewed along section A-A' in FIG. 3A showing internal features. [Figure 5] FIG. 1 shows (at a magnified view) an embodiment of a cross section of a heat pipe fuel element, where the fuel body has a gyroid structure (shown in Cartesian coordinates). [Figure 6] FIG. 1 shows (at a magnified view) an embodiment of a cross section of a heat pipe fuel element, where the fuel body has a gyroid geometry (shown in cylindrical coordinates). [Figure 7] FIG. 1 illustrates (in enlarged view) an embodiment of the evaporation area of a heat pipe fuel element. [Figure 8] FIG. 10 shows (in enlarged view) another embodiment of the evaporation area of a heat pipe fuel element. [Figure 9] 1 is a schematic cross-sectional view of an embodiment of a nuclear fission reactor structure showing a plurality of heat pipe fuel elements arranged in a phyllotactic pattern in the active core region of the nuclear fission reactor and surrounded by a reflector. [Figure 10A] FIG. 10 is a diagram of another example of the layout of evaporation zones of multiple heat pipe fuel elements in a nuclear fission reactor. [Figure 10B] FIG. 10 is a schematic diagram of another embodiment of a cross section of a heat pipe fuel element showing internal features. [Figure 11]FIG. 10 is a schematic diagram showing details of the arrangement of individual double-sided heat pipe fuel elements. [Figure 12A] FIG. 1 shows a schematic diagram of two simulation structures used in Monte-Carlo N-Particle (MCNP) simulations. [Figure 12B] FIG. 1 shows a schematic diagram of two simulation structures used in Monte-Carlo N-Particle (MCNP) simulations. [Figure 12C] 1 is a table detailing the variables used in seven example simulations. [Figure 13] 12A-12C are graphs showing reactor neutron energy spectra for the simulations from FIGS. 12A-12C. [Figure 14] 12B is a graph showing radial flux intensity with and without an inner reflector for two of the simulations of FIGS. 12A-12C. [Figure 15] FIG. 1 shows the heat pipe thermal performance equation for the disclosed heat pipe fuel element. DETAILED DESCRIPTION OF THE INVENTION

[0019] For ease of viewing, in some instances, only some of the named features in the figures are labeled.

[0020] FIG. 1 is a simplified schematic perspective view of a nuclear fission reactor system. The nuclear fission reactor system 10 includes a nuclear fission reactor 20 coupled to a heat sink structure 30, such as a heat exchanger, gas generator, or engine (e.g., a Stirling engine), by a plurality of heat pipe fuel elements 40. Each heat pipe fuel element 40 includes a structural wall enclosing an interior volume including an evaporation zone, a condensation zone, and a capillary zone of the heat pipe. At least a portion, such as most or all, of the evaporation zone of each heat pipe fuel element 40 is located in the nuclear fission reactor 20 and includes a fuel body formed from a fissionable fuel composition that is within the structural wall of the heat pipe fuel element 40. At least a portion, such as most or all, of the condensation zone of each heat pipe fuel element 40 is located in the heat sink structure 30 and is also within the structural wall of the heat pipe fuel element 40. In each heat pipe fuel element 40, a capillary zone connects the respective evaporation zone to the condensation zone. When the nuclear fission reactor 20 and the heat sink structure 30 are spatially separated, the capillary section of each heat pipe fuel element 40 can traverse the space between the active core region of the nuclear fission reactor 20 and the heat sink structure 30. The structural wall of the heat pipe fuel element 40 serves as a cladding layer for the heat pipe fuel element 40 and is vacuum-tight. In a heat pipe embodiment, a wick structure is in contact with at least a portion of the interior surface of the structural wall and is saturated with a working fluid.

[0021] FIG. 2A shows a perspective view of an embodiment of the nuclear fission reactor system 10, showing the nuclear fission reactor 20, heat pipe fuel elements 40, and heat sink structure 30 arranged sequentially along a longitudinal axis 50 of the nuclear fission reactor system 10. The heat sink structure 30 is radially larger than the nuclear fission reactor 20. As shown in FIG. 2B, which is an end view along the longitudinal axis 50 as viewed from the end of the heat sink structure 30 of the nuclear fission reactor system 10, the radial dimension of the heat sink structure 30 is represented by a radius R extending radially from the longitudinal axis 50 to the radially outermost surface of the heat sink structure 30. The radial dimension of the heat sink structure 30 is sized to include a region 60 in the condensation zone of each of the heat pipe fuel elements 40 where the structural wall 70 has an increased surface area. The increased surface area of the structural wall 70 in the region 60 is relative to the surface area of the structural wall in the capillary zone and the evaporation zone. In an example embodiment, the increased surface area of structured wall 70 in region 60 is 2 to 5 times the surface area of the structured wall in the capillary and evaporation zones. The large surface area of structured wall 70 in region 60 promotes efficient heat exchange between the condensation zone and the medium (liquid or gas) in contact with the exterior surface of structured wall 70 in region 60.

[0022] In the embodiment shown in FIGS. 2A and 2B , a first portion of the structural wall 70 in region 60 is formed into a fin that defines the perimeter of an open space 80, and a second portion of the structural wall 70 is a coating layer positioned within the open space 80 and surrounding the condensation zone (the second portion is more easily seen in FIG. 2B ). The open space 80 provides a flow path for a cooling medium, such as a gas or liquid. An example flow path is shown in FIG. 2A by arrow F, extending longitudinally from the inlet end 72 to the outlet end 74 of region 60, although other flow paths may be implemented. With the structural configuration described, heat exchange can occur through both the first and second portions of the structural wall 70. Specifically, two surfaces of the fin, i.e., the inner and outer surfaces, function as heat exchange surfaces, and the outer surface of the coating layer in the second portion also functions as a heat exchange surface.

[0023] The region 60 of the structural wall 70 having increased surface area can be integrally formed with the covering layer surrounding the condensation zone, or the region 60 of the structural wall 70 having increased surface area can be formed separately and attached to the covering layer surrounding the condensation zone. In both instances, the region 60 of the structural wall 70 having increased surface area provides a thermal conduction path for heat removal from the condensation zone.

[0024] In some embodiments, large surface area structures, such as the fins shown, associated with any one heat pipe fuel element 40 can be arranged in a phyllotactic pattern that allows for a tightly packed pattern with similarly sized rhomboidal bodies throughout the phyllotactic pattern. In Figure 2B, the phyllotactic pattern is embodied by a plurality of counter-spiraling and intersecting radial arms 90, the intersections of which form rhomboidal structural walls 70.

[0025] 2A , for example, the capillary section of each heat pipe fuel element 40 connects a respective evaporation section (located in the nuclear fission reactor 20) to a condensation section (located in the heat sink structure 30). In this embodiment, because the heat sink structure 30 is larger in volume than the nuclear fission reactor 20, the capillary section has a transition shape to connect the volumetrically mismatched heat sink structure 30 and nuclear fission reactor 20. In one aspect, this results in the capillary section being spaced apart in a portion that connects to the heat sink structure 30 near the heat sink structure 30, and transitioning to a more closely spaced arrangement in a portion that connects to the nuclear fission reactor 20 near the nuclear fission reactor 20. The shape of the transition and the amount of change in the transition, such as with a curved shape or one or more sections of a constant slope, can correspond to a shape that promotes vapor and liquid flow within the heat pipe fuel element 40.

[0026] 3A is a simplified schematic cross-sectional view of an individual heat pipe fuel element 40. The heat pipe fuel element 40 comprises an evaporation zone 200 at a first end, a condensation zone 300 at a second end, and a capillary zone 400 connecting the evaporation zone 200 and the condensation zone 300. The heat pipe fuel element has a structural wall 500 that acts as a cladding layer that encloses the entire volume of the heat pipe fuel element 40. The evaporation zone 200 has a length L E The capillary section 400 has a length L P The coating layer in the capillary section 400 surrounds the wick structure 405 and the vapor space 410. The condensation section 300 has a length L C , where the coating layer in the condensation zone 300 surrounds the condensation space 305. In some embodiments, the wick structure 405 extends into at least a portion of the condensation zone 300 instead of the entire portion of the condensation zone 300.

[0027] A working fluid is contained within the heat pipe fuel element 40. In embodiments in which the heat pipe fuel element 40 is incorporated into a nuclear fission reactor system, the working fluid takes the form of a primary coolant for a fuel body 205 formed from a fissionable fuel composition. An example working fluid suitable for the primary coolant is a sodium-potassium alloy, which not only has excellent heat transfer properties but is also a liquid metal at room temperature. Other example working fluids include other liquid metals, such as sodium, potassium, and their alloys. In certain embodiments, the heat pipe material is Inconel 600 / 790 or Haynes 230.

[0028] The structure of the heat pipe fuel element 40 supports closed-loop circulation of the working fluid. The working fluid contained within the heat pipe fuel element 40 forms a vapor in the evaporation zone 200, which corresponds to the heated end of the heat pipe fuel element 40. The vapor travels through the vapor space 410 of the capillary zone 400 toward the condensation zone 300 (vapor travel is represented by arrow V in FIG. 3A ), where the vapor condenses to form a liquid in the condensation space 305, storing its heat of vaporization with a small associated temperature change. The condensed working fluid is transported back to the evaporation zone 200 using a wick structure 405 that exerts capillary action on the liquid phase of the working fluid. Capillary action between the liquid phase of the working fluid and the wick structure 405 draws the condensate from the condensation zone 300 back through the capillary zone 400 toward the evaporation zone 200 (condensate travel is represented by arrow C in FIG. 3A ). In some embodiments, gravity may be combined with capillary action to effect the return of condensed working fluid to evaporation zone 200. For example, evaporation zone 200 and condensation zone 300 may be at different heights or may have a separation distance such that condensation zone 300 may be located above evaporation zone 200. Figure 3A shows an example of a height difference ΔH between the longitudinal axis of evaporation zone 200 (designated 210) and the longitudinal axis of condensation zone 300 (designated 310).

[0029] Examples of wick structures include sintered metal powder, screens, and axially grooved structures. In the illustrated embodiment of the heat pipe fuel element 40, the wick structure 405 is a sintered metal mesh, which has a triple periodic minimal surface (TPMS) geometry.

[0030] The evaporation zones 200 of multiple heat pipe fuel elements 40 can be arranged within the nuclear fission reactor 20. Figure 3B shows an example of such an arrangement. The heat pipe fuel elements 40, shown in cross-sectional side view in Figure 3A, are positioned in the phyllotactic pattern shown in end view in section (I) of Figure 3B and in an enlarged view of area P1 in section (II) of Figure 3B. An individual heat pipe fuel element 40, shown in cross-sectional side view in Figure 3A, is designated by the numeral 40a in section (II) of Figure 3B.

[0031] The condensation zones 300 of multiple heat pipe fuel elements 40 can be arranged within the heat sink structure 30. Figure 3C shows an example of such an arrangement. The heat pipe fuel elements 40 shown in cross-sectional side view in Figure 3A are positioned in the phyllotactic pattern shown in end view in Figure 3C. An individual heat pipe fuel element 40 shown in cross-sectional side view in Figure 3A is designated 40a in Figure 3C.

[0032] FIG. 4C schematically illustrates a cross-section of a heat pipe fuel element 40 in a plane perpendicular to the longitudinal axis 210 of the evaporation zone 200 taken along section A-A' in FIG. 3A. For reference, the cross-section shown in FIG. 4C is for the heat pipe fuel element designated 40a in FIG. 4B, which is an enlarged view of section P1 from FIG. 4A showing the evaporation zones 200 of multiple heat pipe fuel elements 40 arranged within the nuclear fission reactor 20 in a phyllotactic pattern. FIGS. 4A and 4B are similar to portions (I) and (II) in FIG. 3B. As seen in the cross-sectional view in FIG. 4C, the structural wall 500, i.e., cladding layer, of the heat pipe fuel element 40 encloses an interior region containing the fuel element 205. The fuel element 205 has an outer surface 215 oriented toward the cladding layer and an inner surface 220 that defines the perimeter of the evaporation space 225 of the evaporation zone 200. The working fluid is in direct contact with the surface of the evaporation space 225.

[0033] In some embodiments, as shown in FIG. 4C , the structural wall 500 of the heat pipe fuel element 40, i.e., the interior region surrounded by the cladding layer, further includes a moderator 230 between the exterior surface 215 of the fuel body 205 and the interior surface 235 of the structural wall 500. When the moderator 230 is included, the neutronics of the heat pipe fuel element 40 are such that a nuclear fission reactor system including such a heat pipe fuel element 40 is a fast spectrum reactor. In other embodiments, as shown in FIG. 4D , no moderator is present within the interior region surrounded by the structural wall 500, in which case the neutronics of the heat pipe fuel element 40 are such that a nuclear fission reactor system including such a heat pipe fuel element 40 is a thermal reactor. When no moderator is present, the exterior surface 215 of the fuel body 205 can be in contact with the interior surface 235 of the structural wall 500. In some embodiments, as shown in Figure 4D, there is no gap between the outer surface 215 of the fuel body 205 and the inner surface 235 of the structural wall 500, and the fuel body 205 occupies the entire interior area enclosed by the structural wall 500 of the heat pipe fuel element 40, i.e., the coating layer, except for the evaporation space 225. In other embodiments, as shown in Figure 4E, there is a gap 250 between at least a portion of the outer surface 215 of the fuel body 205 and the inner surface 235 of the structural wall 500. This gap 250 can function as a secondary evaporation space of the evaporation zone 200.

[0034] The cross-sectional shape of the heat pipe fuel element 40 is not particularly limited. In an example embodiment, in the evaporation zone 200 and in a cross-section in a plane perpendicular to the longitudinal axis 210 of the evaporation zone 200, the structural wall 500 of the heat pipe fuel element 40, i.e., the coating layer surrounding the interior region, has a polygonal shape. Example polygonal shapes include a square, a diamond, or a rhomboid. In some embodiments, the square is skewed, and the square is not symmetrical about the plane of symmetry. An example plane of symmetry 260 is shown in FIG. 4D and extends between opposite vertices of the structural wall 500. Generally, the cross-sectional shape of the heat pipe fuel element 40 depends on the arrangement of the heat pipe fuel element 40, such as in a phyllotactic pattern, and the location of the heat pipe fuel element 40 within that arrangement.

[0035] In some embodiments, the coating layer forms the exterior wall of the heat pipe fuel element 40 along its entire length. In other embodiments, the coating layer forms at least a portion of the exterior wall of the heat pipe fuel element 40, such as the portion corresponding to the evaporation zone 200. In some embodiments, the coating layer is a seamless, continuous tube. This is particularly preferred in the evaporation zone 200 of the heat pipe fuel element 40. In at least the evaporation zone 200, and alternatively along the entire length of the heat pipe fuel element 40, the composition of an example structural wall 500 of the heat pipe fuel element 40 includes an aluminum alloy or a zirconium alloy, as appropriate for the expected reactor temperatures. In some embodiments, the structural wall 500 is the same material, but the cooling grid structure in the condenser zone, which transfers heat to the heat exchanger gases, can be a different material, such as Al, to reduce weight.

[0036] In various embodiments, the fuel element 205 is formed from a fissile fuel composition, typically including a uranium-containing material, preferably uranium nitride, uranium oxide, uranium carbide, or cermets thereof. Specific examples of fissile fuel compositions include high-purity, low-enriched uranium (HALEU) with a U-235 content of 5 percent or more and 20 percent or less, or highly enriched uranium (HEU) with 20% or more U-235. Other examples include U10Mo (uranium with 10 weight percent molybdenum) and UN.

[0037] The fuel body 205 can have any suitable structure. In one embodiment, the fuel body 205 is extruded from a powder containing the fuel composition to form a cylindrical body with an annular-shaped cross-section, then sintered and inserted into the evaporation zone of a heat pipe fuel element. In another embodiment, the powder containing the fuel composition is fed into an additive manufacturing machine, for example, as a powder or as a composition in a slurry, and a fuel body 205 having a structure with a shape corresponding to a mathematically based periodic solid is manufactured using an additive manufacturing process. Examples of core slurries and additive manufacturing of core compositions using core slurries are disclosed in U.S. Patent Application No. 16 / 835,370, the entire contents of which are incorporated herein by reference. Examples of mathematically based periodic solids include triply periodic minimal surfaces (TPMS), Schwartzian minimal surfaces, gyroid structures, and lattice structures, examples of which are disclosed in U.S. Patent Application No. 16 / 835,388, the entire contents of which are incorporated herein by reference.

[0038] In some embodiments, as shown in Figure 4D, there is no gap between the outer surface 215 of the fuel body 205 and the inner surface 235 of the structural wall 500, and the fuel body 205 occupies the entire interior area enclosed by the structural wall 500 of the heat pipe fuel element 40, i.e., the coating layer, except for the evaporation space 225. In other embodiments, as shown in Figure 4E, there is a gap 250 between at least a portion of the outer surface 215 of the fuel body 205 and the inner surface 235 of the structural wall 500. This gap 250 can function as a secondary evaporation space of the evaporation zone 200.

[0039] FIG. 5 depicts (at a magnified view) an embodiment of a cross section of a heat pipe fuel element 40 in which the fuel body 205 has a gyroid structure (shown in Cartesian coordinates), and FIG. 6 depicts (at a magnified view) an embodiment of a cross section of a heat pipe fuel element 40 in which the fuel body 205 has a gyroid structure (shown in cylindrical coordinates). In both FIGS. 5 and 6, the magnified view is looking down the general direction of the axis 210 of the evaporation zone 200. Both FIGS. 5 and 6 show the surface 260 of the gyroid structure defining a plurality of passages 265 in the fuel body 205. While the surface 260 follows a morphology as defined by a mathematically based periodic solid or a deformation thereof, at least a portion of the passages 265 formed by the surface 260 extend from a first location on the outer surface 215 of the fuel body 205 to a second location on the outer surface 215 of the fuel body 205. At least a portion of passages 265 , alternatively a majority of passages 265 , or even alternatively, all of passages 265 provide a path for working fluid through fuel body 205 .

[0040] In the mathematically based periodic solid form of fuel body 205, the composition of the structure of fuel body 205 includes fissile fuel, such that the structure of fuel body 205 has a volumetric density of 35% to 85%. For example, the fissile fuel composition may include fissile fuel having an enrichment of up to 20%, where the specific enrichment of the fuel body (% enrichment per unit volume) is a constant ±2%.

[0041] In various alternative embodiments in which the fuel body 205 is in the form of a mathematically-based periodic solid, the volumetric density of the fuel body 205 is greater than or equal to 40%, 45%, 50%, or 55% and less than or equal to 80%, 75%, 70%, or 65%, or the volumetric density is 60±10%. The volumetric density is determined by considering the amount of solids in a unit volume of the fuel body 205 relative to the total volume of the unit volume of the fuel body 205, including both the solids and the open space (i.e., the passages 265). Furthermore, in these embodiments, the open space, i.e., the passages 265, forms part of the evaporation space 225, and the working fluid is in direct contact with the surfaces of the evaporation space 225.

[0042] In an optional embodiment, the inner surface 220 of the fuel body 205 can have a coating to protect it from corrosion and wear by the working fluid. If the fuel body 205 has a structure with a shape corresponding to a mathematically periodic solid, the surfaces of the passages defined by the surface of the mathematically periodic solid can also have a coating. Such a coating can be formed, for example, by deposition techniques, electroplating, etc. In one exemplary embodiment, a thin layer of Mo, W, or NbC can be applied by physical vapor deposition (PVD) to form a layer to prevent damage to the fuel over its lifetime.

[0043] 7 and 8 each depict (in a magnified view) an embodiment of the evaporation zone 200 of a heat pipe fuel element 40. FIG. 7 is a magnified view looking down the general direction of the axis 210 of the evaporation zone 200, and FIG. 8 is a magnified view showing the wicking zone of the heat pipe fuel element in the evaporation zone 200. FIGS. 7 and 8 show schematic depictions of wicking structures 405 in the form of mathematically based periodic solids, including triply periodic minimal surfaces (TPMS), Schwartzian minimal surfaces, gyroid structures, and lattice structures, examples of which are disclosed in U.S. Patent Application No. 16 / 835,388, the entire contents of which are incorporated herein by reference. The surface 310 of the wicking structure 405 has a large surface area that promotes efficient heat exchange from the working fluid within the heat sink structure 30.

[0044] 9 is a schematic cross-sectional view (in a plane perpendicular to longitudinal axis 50) of an embodiment of a nuclear fission reactor structure showing a plurality of heat pipe fuel elements 40 arranged in a phyllotactic pattern in the active core region of nuclear fission reactor 20 and surrounded by a reflector 330. In FIG. 9, core formation 350 is radially outward of the active core region, and reflector 330 is radially outward of core formation 350 (the radial direction is relative to longitudinal axis 50). A first surface of core formation 350 conforms to the outer surface of the active core region, and a second surface of core formation 350 conforms to an inner surface 332 of reflector 330. Inner surface 332 of reflector 330 is oriented toward the active core region, and core formation 350 functions to match the shape of the outer surface of the active core region to the shape of inner surface 332 of reflector 330.

[0045] A plurality of neutron absorption structures 335, each including a neutron absorber 340, are positioned within the volume of the reflector 330 and are movable, such as by rotation, between a first position and a second position, the first position being radially closer to the active core region than the second position. In the illustrated embodiment, the first position is radially closest to the active core region 305 and the second position is radially farthest from the active core region 305. The neutron absorbers 340 are movable between the first and second positions to control the reactivity of the active core region. In the illustrated example, the neutron absorbers 340 are rotatable from a first radially closer position to a second position by rotation (R) about the axis of the neutron absorption structures 335. However, other radial positions and / or movement directions may be implemented as long as the various positions to which the neutron absorbers 340 can be moved provide control of the reactivity of the active core region. In some embodiments, when the multiple neutron absorbers 340 are closer to each other in the first radial direction, each of the multiple neutron absorbers 340 is equidistant radially from the axial centerline of the active core region 305.

[0046] The reflector 330 functions to thermalize "reflected" neutrons traveling back toward the active core region to increase criticality and reduce neutron "leakage," which would eliminate the opportunity for a fission reaction to occur and thereby reduce the criticality capability of the nuclear fission reactor structure. Second, the reflector 330 houses the neutron absorber 340 of the neutron absorbing structure 335, which is the primary system for reactivity control. In FIG. 9, an embodiment of the reflector 330 is in the form of an annulus, with the neutron absorber 340 in the form of a rotatable control drum. To accommodate the neutron absorber 340 in the form of a rotatable control drum that is sized sufficiently to control reactivity, the annulus of the reflector 330 cannot be excessively thin (in width (W) between the inner surface 332 and the outer surface 334). In an exemplary embodiment, the width (W) is 15 cm to 30 cm for a beryllium-based reflector. The width may vary based on the material of the reflector 330, with materials with lower neutron reflectivity requiring thicker reflectors, i.e., larger widths (W), and, if applicable, weight requirements for extraterrestrial applications of nuclear fission reactor structures.

[0047] In some embodiments, the active core region design is also annular. That is, the evaporation zones of multiple heat pipe fuel elements 40 are contained within an annular region (e.g., as seen in cross section in FIG. 9 ). An annular inner surface 360 defines an opening 362 that can be configured to receive additional features. For example, in some optional embodiments, secondary systems for reactivity control, such as control rods or safety shutdown rods (SCRAM rods), can be inserted into and retracted from opening 362 to provide reactor control. In other optional embodiments, an inner reflector can be positioned in opening 362. In yet other optional embodiments, a target delivery system for isotopes can be inserted into and retracted from opening 362.

[0048] Although the active core region is shown in a phyllotactic pattern (see, e.g., FIG. 9 ), alternative arrangements may be used, such as a concentric ring pattern resulting in a circular interference with the core formation 325, a rectangular packing pattern, a circular packing pattern, and various packing patterns, each with the evaporator sections 200 of the fuel element heat pipes in a close-packed arrangement. FIG. 10A shows an example of a rectangular packing pattern, as well as the fuel elements 205 and evaporation spaces 225 in each fuel element heat pipe. In the illustrated embodiment, the cross-sectional shapes in the active core region pattern are arranged in a close-packed relationship, resulting in the tightest or most space-efficient packing that maximizes packing efficiency and minimizes unfilled volume, whether the cross-sectional shape is circular, polygonal, or the like. Shown in FIG. 10B is an enlarged schematic depiction of a cross section of one heat pipe fuel element 40 from the rectangular packing pattern in FIG. 10A , showing internal features including the fuel elements 205 and evaporation spaces 225.

[0049] In some embodiments, adjacent heat pipe fuel elements 40, whether arranged in a phyllotactic or a close-packed pattern, may optionally be spaced apart from one another by a separation distance. Such separation distances define void spaces 520 and may contain a moderator, such as graphite, or a non-moderator. The presence of separation distances, their size and location, and the inclusion of a moderator or non-moderator depend on the design and neutronics of the active core region of the nuclear fission reactor. In alternative embodiments, the separation distances (and thus the void spaces) are absent or nominal to accommodate manufacturing tolerances.

[0050] In exemplary embodiments, the nuclear fission reactor structure (including the active core region) is positioned within the interior volume of the pressure vessel. In such embodiments, supports may be attached to the interior surface of the pressure vessel, with the supports at a first location connected to a first end plate of the nuclear fission reactor structure and the supports at a second location connected to a second end plate of the nuclear fission reactor structure. The pressure vessel is typically fabricated from stainless steel and may include sealable openings positioned to allow for the insertion and removal of accessory equipment, such as instruments, controls, and targeted delivery systems for isotopes. The heat sink structure may be external to the pressure vessel and heat pipe fuel elements 40; specifically, a capillary section may extend through the pressure vessel to operatively connect the active core region with the heat sink structure.

[0051] The heat pipe fuel element 40 can be single-sided, as shown and described with respect to, for example, FIG. 3A, or the heat pipe fuel element 40 can be double-sided, as shown and described with respect to, for example, FIG. 11. FIG. 11 schematically illustrates the details of an individual double-sided heat pipe fuel element arrangement. In a double-sided embodiment, the heat pipe fuel element 40 includes two capillary sections 400a, 400b, two condensation sections 300a, 300b, and one evaporation section 200. The first condensation section 300a is at a first end of the heat pipe fuel element 40, and the second condensation section 300b is at a second end of the heat pipe fuel element 40. The first capillary section 400a connects the first condensation section 300a to the evaporation section 200, and the second capillary section 400b connects the second condensation section 300b to the evaporation section 200. In the double-sided embodiment, the heat pipe fuel element 40 has similar features and functions similarly to the single-sided heat pipe fuel element 40, with working fluid evaporated in the evaporation zone 200 flowing to one of the condensation zones 300a, 300b via the corresponding capillary zone 400a, 400b, and working fluid condensed in the respective condensation zone 300a, 300b returning to the evaporation zone 200 via the corresponding capillary zone 400a, 400b.

[0052] The disclosed heat pipe fuel elements can be manufactured by any suitable manufacturing method. In one embodiment, the heat pipe fuel element includes enclosing a fuel element within the structural wall 500, i.e., cladding layer, of at least the evaporation zone 300 of the heat pipe fuel element 40. In one example, the fuel element 205 is manufactured as a cylinder or rod, and one or more fuel elements 205 are inserted into an extruded tube that will form the structural wall. In another embodiment, the tube that will form the structural wall is crimped into shape around one or more fuel elements 205. With the fuel element retained in the evaporation zone, additional tubes that will form the structural walls for the condensation zone 300 and capillary zone 400 are either bonded to the evaporation zone 200, or, if the tube is longer than the evaporation zone 200, the portions of the tube that will form the structural walls for the condensation zone 300 and capillary zone 400 are shaped, such as by bending, to give the final shape for the heat pipe fuel element 40. Following the formation of the shaped heat pipe fuel element 40 with the fuel body 205 and wick structure 405 in place, with the evaporation, condensation, and capillary zones, a working fluid, i.e., primary coolant, is added to the interior volume of the heat pipe fuel element. The structural walls are then sealed at one or both tube ends, such as with resistance welded end caps.

[0053] Multiple heat pipe fuel elements can be assembled to form a nuclear fission reactor system by a method that includes arranging and joining the evaporation zones of multiple heat pipe fuel elements to form a reactor bundle and incorporating the condensation zones of multiple heat pipe fuel elements to form a heat sink structure. For example, the condensation zones of the heat pipe fuel elements, or at least a portion thereof, can be formed into a portion of the heat sink structure, such as the region 60 in the condensation zone of each of the heat pipe fuel elements 40, where the structural wall 70 has an increased surface area, as shown in Figures 2A-2B.

[0054] A nuclear fission reactor formed from multiple heat pipe fuel elements 40 as disclosed herein was simulated as a homogeneous cylindrical core using Monte-Carlo N-Particle (MCNP) simulation of all necessary atoms. The first simulation included an outer reflector, while the second simulation included both an outer and inner reflector. Figures 12A and 12B show these two simulation configurations in schematic top and side cross-sectional views, designated (I) and (II), respectively. In Figures 12A-12B, the outer reflector 600 is 15 cm in diameter, the core 605 has an outer diameter of 100 cm, and the inner reflector 610 (if present) has an outer diameter of 15 cm. Figure 12C is a table showing materials for the noted features in seven examples (Examples 1-7) and reporting critical values from the MCNP simulation. All seven examples (Examples 1-7) show viable criticality with fast spectra, but some produced slightly more thermal spectra, as can be seen from the reactor neutron energy spectra in Figure 13. It should be noted that although the cladding is not visible to scale in Figures 12A-12B, the cladding in volume percentage is considered in the data in Figure 12C to ensure that effects were included in the MCNP simulations.

[0055] Figure 13 is a graph showing reactor neutron energy spectra for the simulations from Figures 12A-12C. Figure 13 is a log-log plot in which the neutron flux in neutrons per square centimeter per second (neutrons / cm s) is plotted as a function of energy in MeV for Examples 1-7 (Ex. 1 = 700, Ex. 2 = 705, Ex. 3 = 710, Ex. 4 = 715, Ex. 5 = 720, Ex. 6 = 725, Ex. 7 = 730). In Figure 13, simulations in the higher left "tail" contain more thermal neutrons, which means a higher probability of a fission event per neutron generated (i.e., a more efficient reactor core).

[0056] FIG. 14 is a graph showing radial flux intensity in neutrons per square centimeter per second (neutrons / cm s) as a function of radial position (in cm from the axial centerline) for the two simulated configurations in FIGS. 12A-12B, i.e., with and without the inner reflector. Plot 800 is the radial flux intensity with the inner reflector (corresponding to the simulation for FIG. 12A), and plot 805 is the radial flux intensity without the inner reflector (corresponding to the simulation for FIG. 12B). FIG. 14 shows that the flux intensity is not constant in the core in the region from 15 cm to 50 cm for either plot 800 or plot 805. However, it is contemplated that the fuel loading or volume ratio in the fuel elements can be changed to attenuate the flux intensity in the core, and that a constant radial flux and fission intensity in the core can be achieved, mitigating material hot spots.

[0057] In summary, the simulations reveal that for the disclosed fuel element heat pipes and nuclear fission reactor systems incorporating such fuel element heat pipes, the material allocation and fuel loading can be varied to produce desired nuclear characteristics without adversely affecting core criticality and radioactivity control.

[0058] The heat pipe fuel elements and nuclear fission reactor systems disclosed herein may alternatively be embodied in a loop heat pipe design. In a heat loop embodiment, heat pipe features are arranged in a loop system with vapor piping providing vapor transport from the evaporation zone to the condensation zone and liquid piping providing liquid transport from the condensation zone to the evaporation zone.

[0059] The heat pipe fuel elements and nuclear fission reactor systems disclosed herein have several advantages over previous reactor designs. For example, numerous welds are eliminated by the heat pipe fuel elements, particularly in the active core region where the structural wall, or cladding, of the heat pipe fuel element can be embodied in seamless tubing. Direct cooling of the fuel element, such as by a working fluid directly contacting the surface of the fuel element, can reduce thermal gradients. Direct cooling can also be more resilient to failure of individual heat pipe fuel elements. Additionally, the phyllotactic arrangement of the heat pipe fuel elements about the evaporation zone allows for more efficient placement of the fissionable fuel in a compact form, allowing for less absorbent or non-functional materials, particularly in the active core region.

[0060] Finally, the heat pipe fuel elements and nuclear fission reactor systems disclosed herein eliminate thermal resistances preferred in conventional systems. Specifically, FIG. 15 shows a heat pipe thermal performance equation that includes terms for various factors that contribute to heat pipe performance. By using a fuel element with a fissionable fuel composition as the heat pipe inner material, the first three terms in the heat pipe thermal performance equation can be eliminated (as represented by the arrows in FIG. 15). Furthermore, direct evaporation at the surface of the fuel element enhances heat transfer and reduces the thermal resistance due to temperature drop that would be present in heat pipe designs that do not incorporate a fuel element inside the structural wall of the heat pipe.

[0061] Each arrangement shown and described herein is a single example, and the basic dimensions disclosed herein can be varied to optimize different reactor characteristics based on material proportions (e.g., fuel enrichment or U-235 mass minimization).

[0062] While the present invention has been described in connection with embodiments thereof, it will be understood by those skilled in the art that additions, deletions, modifications, and substitutions not expressly described may be made without departing from the spirit and scope of the invention as defined in the appended claims. For example, while described in connection with fissionable fuel materials, nuclear reactors, and related components, the principles, compositions, structures, features, arrangements, and processes described herein may be applied to other materials, other compositions, other structures, other features, other arrangements, and other processes, as well as their manufacture and to other types of nuclear reactors.

[0063] Those skilled in the art will appreciate that the specific example processes, devices, and / or techniques described above are representative of more general processes, devices, and / or techniques taught elsewhere herein, including in the claims filed herewith, and / or elsewhere in this application.

[0064] The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting, as other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein.

[0065] Those skilled in the art will recognize that the components (e.g., operations), devices, objects, and accompanying discussion described herein are used as examples for conceptual clarity and that various configuration variations are contemplated. Consequently, as used herein, the specific examples set forth and accompanying discussion are intended to be representative of their more general fields. In general, the use of specific examples is intended to be representative of the field, and the inclusion of specific components (e.g., operations), devices, and objects should not be construed as limiting.

[0066] While various aspects and embodiments are disclosed herein, other aspects and embodiments will become apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. [Explanation of symbols]

[0067] 10 Nuclear Fission Reactor Systems 20 Nuclear fission reactor 30 Heat sink structure 40, 40a Heat Pipe Fuel Element 50 Longitudinal axis 60 Area in the condensation zone 70 Structural Wall 72 Inlet end 74 Outlet end 80 open space 90 Radial Arm 200 evaporation area 205 Fuel body 210 Longitudinal axis of evaporation zone 200 215 Exterior 220 Inside 225 Evaporation Space 230 Moderator 235 Internal surface 250 gap 260 Plane of Symmetry 265 Passage 300 Condensation Area 300a First Condensation Zone 300b Second condensation zone 305 Condensed Space 305 Active core region 310 longitudinal axis of condensation section 300 310 Wicking structure 405 surface 325 Core Assembly 330 Reflector 332 Inside 334 External surface 335 Neutron Absorption Structure 340 Neutron Absorber 350 Core Assembly 360 Inside 362 Aperture 400 capillary area 400a First capillary section 400b second capillary section 405 Wick structure, wicking structure 410 Steam Space 500 structural wall 520 Void Space 600 outer reflector 605 Reactor Core 610 Inner reflector 800, 805 plots AA' area C Condensate migration F flow path L C Length of condensation area 300 L E Length of evaporation area 200 L P Length of capillary section 400 P1 area R Radius of heat sink structure 30 R Rotation of neutron absorbing structure 335 W: width between the inner surface 332 and the outer surface 334 ΔH Height difference

Claims

1. A heat pipe fuel element (40), Evaporation area (200), Condensation zone (300), A capillary section (400) connecting the evaporation section to the condensation section, Primary coolant and Equipped with, In the evaporation region (200) and in a cross-section in a plane perpendicular to the longitudinal axis (50) of the evaporation region, the heat pipe fuel element includes a covering layer (500) that surrounds the internal region including the fuel body (205) and the evaporation space. The fuel body (205) is positioned between the coating layer (500) and the evaporation space, and the fuel body (205) has an outer surface oriented toward the coating layer (500) and an inner surface that defines the periphery of the evaporation space of the evaporation area (200). The fuel assembly (205) is formed from a uranium-based fissile fuel composition, The fuel body (205) has a structure with a shape corresponding to a mathematically based periodic solid, the surface of the mathematically based periodic solid defines a plurality of passages (265) in the fuel body, and the structure has a volume density of 35% to 85%, wherein the fuel body is a heat pipe fuel element.

2. The heat pipe fuel element according to claim 1, wherein the internal region surrounded by the coating layer further includes a moderator (230) between the outer surface of the fuel body (205) and the inner surface of the coating layer (500).

3. The heat pipe fuel element according to claim 1, wherein the evaporation region is located at the first end of the heat pipe fuel element, the condensation region is located at the second end of the heat pipe fuel element, and the condensation region is elevated relative to the evaporation region.

4. The heat pipe fuel element includes two capillary regions (400a, 400b) and two condensation regions (300a, 300b), A first condensation region is located at the first end of the heat pipe fuel element, and a second condensation region is located at the second end of the heat pipe fuel element. The heat pipe fuel element according to claim 1, wherein a first capillary region connects the first condensation region to the evaporation region, and a second capillary region connects the second condensation region to the evaporation region.

5. The heat pipe fuel element according to any one of claims 1 to 4, wherein the capillary region includes a wick structure in contact with the inner surface of the heat pipe fuel element.

6. The heat pipe fuel element according to claim 5, wherein the wick structure is a sintered metal mesh.

7. The heat pipe fuel element according to any one of claims 1 to 4, wherein the primary coolant is in direct contact with the inner surface of the fuel body.

8. The heat pipe fuel element according to claim 7, wherein the primary coolant is a liquid metal.

9. The heat pipe fuel element according to claim 8, wherein the liquid metal is sodium or a sodium-containing alloy, preferably a sodium-potassium alloy.

10. The heat pipe fuel element according to any one of claims 1 to 4, wherein the coating layer forms at least a portion of the outer wall of the heat pipe fuel element.

11. The heat pipe fuel element according to any one of claims 1 to 4, wherein the coating layer in the evaporation region is a seamless continuous tube.

12. The heat pipe fuel element according to any one of claims 1 to 4, wherein in the evaporation region and in the cross-section in the plane perpendicular to the longitudinal axis of the evaporation region, the covering layer surrounding the internal region has a polygonal shape.

13. The heat pipe fuel element according to claim 12, wherein the polygon is a quadrilateral, preferably a rhombus or a rhombic.

14. The heat pipe fuel element according to claim 13, wherein the rectangle is angled.

15. The heat pipe fuel element according to any one of claims 1 to 4, wherein the periodic solid based on the aforementioned mathematics is a triple periodic minimum surface (TPMS).

16. The heat pipe fuel element according to claim 15, wherein the triple periodic minimum surface (TPMS) is a Schwarz minimum surface.

17. The heat pipe fuel element according to claim 15, wherein the triple periodic minimum surface (TPMS) has a gyroid structure.

18. The heat pipe fuel element according to any one of claims 1 to 4, wherein the periodic solid based on the aforementioned mathematics has a lattice structure.

19. The heat pipe fuel element according to any one of claims 1 to 4, wherein the fissile fuel composition based on uranium contains uranium enriched to a maximum of 20%, and the specific enrichment (enrichment in %) of the fuel assembly is a constant ±2%.

20. The heat pipe fuel element according to any one of claims 1 to 4, wherein the uranium-based fissile fuel composition comprises a uranium-containing material, preferably uranium nitride, uranium oxide, U10Mo, or cermets thereof.

21. The heat pipe fuel element according to any one of claims 1 to 4, wherein the fissile fuel composition based on uranium is (a) high-purity low-enriched uranium (HALEU) with a U-235 content of 5 percent or more and 20 percent or less, or (b) highly enriched uranium (HEU) with a U-235 content of 20 percent or more.

22. A plurality of heat pipe fuel elements according to any one of claims 1 to 4, Heat sink structure and Equipped with, A nuclear fission reactor system in which at least a portion of the evaporation area is included within the active core area of ​​the fission reactor, and at least a portion of the condensation area is included within the heat sink structure.

23. The nuclear fission reactor system according to claim 22, wherein in a cross-section in a plane perpendicular to the longitudinal axis of the active core region, the evaporation regions of the plurality of heat pipe fuel elements are arranged in a phyllotaxy pattern.

24. The nuclear fission reactor system according to claim 22, wherein in a cross-section in a plane perpendicular to the longitudinal axis of the active core region, the evaporation regions of the plurality of heat pipe fuel elements are arranged in a dense relationship.

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