Robust nuclear-propelled fission reactor having 3-pitch pattern core and drum absorber

Abstract

To provide a nuclear-propelled fission reactor structure that can constitute robust and simple nuclear thermal propulsion reactor and engine.SOLUTION: A nuclear-propelled fission reactor structure 100 has: an active core region 110 including a fuel element structure 112; a reflector 150 having a rotatable neutron absorption structure 160 (drum absorber and so on); and a core support frame 130 which conformally fits the external surface of the fuel element structure 112 to the reflector 150. The fuel element structure 112 is so located as to contact the nearest fuel element structure with 3-pitch design. A clad body 116, which defines a coolant channel, are inserted into and joined to an upper core plate and a lower core plate, thereby forming a continuous structure which is a first portion of a storage structure. The nuclear-propelled fission reactor structure can be assembled into a nuclear thermal propulsion engine for propulsion applications such as space propulsion.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates generally to nuclear fission reactors and nuclear fission reactor-related structures, particularly for propulsion. Such nuclear propulsion fission reactors have applications in a variety of non-terrestrial applications, such as space and marine environments. [Background technology]

[0002] In the following description, 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 demonstrate that such structures and / or methods are not admitted as prior art against the present invention.

[0003] A variety of propulsion systems have been developed for non-terrestrial applications such as space. These include chemical-based propulsion systems, ion-based propulsion systems, and nuclear-based propulsion systems. Each of these propulsion systems balances thrust and specific impulse to provide performance tailored to a particular mission. For example, chemical-based propulsion systems provide high thrust (e.g., 10 7 Thrust pound (4.45×10 7 Ion-based propulsion systems have low thrust (e.g., less than 10 pounds thrust (44.5 N)) but have a high specific impulse (e.g., 1,000-9,000 s) and are efficiently applied for long-duration space travel, such as interstellar travel. Nuclear-based propulsion systems combine moderate thrust (e.g., 5,000-75,000 pounds thrust (22,250-333,750 N)) and moderate specific impulse (e.g., 600-1,000 s) and are efficiently applied for short-distance space travel. Nuclear-based propulsion systems are currently being evaluated as a propulsion option for NASA's Mars Design Reference Mission Architecture 5.0 human exploration.

[0004] Previous nuclear-based propulsion systems remain complex. For example, both the Nuclear Engine for Rocket Vehicle Applications (NERVA) and Rover programs have developed nuclear thermal rocket designs. A typical design of a nuclear thermal propulsion reactor and engine 10 is shown in FIG. 1. The illustrated nuclear thermal propulsion reactor and engine 10 includes four main features: a shell 20 having a reactor 22 housed within a reflector 24, a turbomachinery 30 including a turbopump 32 and other piping and support equipment 34, a shield 40 separating the turbomachinery 30 from the shell 20, and a nozzle section 50 including a nozzle 52 and a nozzle skirt 54.

[0005] However, these conventional nuclear-based propulsion systems still have drawbacks, including utilizing complex moderator and flow technologies, and operating at minimal design margins that push the limits of the design and associated materials. Thus, there remains a need for robust and simple designs of nuclear propulsion reactors, especially for non-terrestrial applications such as space. Summary of the Invention [Problem to be solved by the invention]

[0006] In view of the above, it would be advantageous to have a robust single-pass propellant flow nuclear-based propulsion system with a simplified core pattern for ease of manufacture. Furthermore, a simplified design with a reduced number of welds during manufacture would be advantageous to reduce the risk of performance degradation. [Means for solving the problem]

[0007] In general, the present disclosure relates to a nuclear fission reactor structure suitable for use as an engine in a nuclear-based propulsion system. In an exemplary embodiment, the nuclear fission reactor structure utilizes fuel elements having hexagonal cross sections arranged in a three-pitch design and a rotatable drum neutron absorber for reactivity control. The nuclear fission reactor structure is housed within the outer shell of the nuclear thermal propulsion reactor and engine. Propulsion gases are used as a coolant for the nuclear fission reactor structure. Propulsion gases superheated within the nuclear fission reactor structure exit through a nozzle to generate thrust and impulse.

[0008] An embodiment disclosed herein includes a nuclear propulsion nuclear fission reactor structure comprising an active core region including a plurality of fuel element structures and having an axial centerline defining a longitudinal axis of the nuclear propulsion reactor, a core support frame radially outward of the active core region, a reflector radially outward of the core reformer and having a radially inner surface oriented toward the active core region, and a plurality of neutron absorbing structures disposed within a volume of the reflector. Each fuel element structure includes a cladding body having an inner surface defining a coolant channel, a fuel composition body radially outward of the cladding body, and a moderator composition body radially outward of the fuel composition body. Further, an outer surface of the moderator composition body of a first fuel element structure abuts an outer surface of a plurality of nearest neighbor fuel element structures. The core support frame has a first surface radially inward of the second surface, the first surface conforming to a radially outer surface of the active core region, and the second surface conforming to a radially inner surface of the reflector. Each of the plurality of neutron absorbing structures includes a neutron absorber body movable between a first position and a second position, the first position being radially closer to the active core region than the second position.

[0009] The embodiments disclosed herein also include a nuclear thermal propulsion engine comprising the nuclear propulsion nuclear fission reactor structure disclosed herein and an outer shell, wherein the active core region, the core support frame, the upper core plate, the lower core plate, the reflector, and the plurality of neutron absorbing structures form a nuclear reactor structure, and the nuclear reactor structure is contained within an interior volume of the outer shell. In forming the nuclear thermal propulsion engine, a shield, a storage for cryogenically storing a propulsion gas, a turbomachinery, and a nozzle are operably attached to the nuclear reactor structure and are contained within the interior volume of the outer shell such that the upper core plate is oriented toward a first end of the outer shell and the lower core plate is oriented toward a second end of the outer shell. The shield, the turbomachinery, and the storage are operably attached to the first end of the outer shell to provide a flow path from the storage to the nuclear propulsion reactor, and the nozzle is operably attached to the second end of the outer shell to provide a flow path for the superheated propulsion gas exiting the nuclear propulsion reactor.

[0010]

[0013] The embodiments disclosed herein also include a method of manufacturing a nuclear fission reactor structure. The method includes the step of bonding a first portion of each of a plurality of cladding bodies to a lower core plate, each cladding body having an inner surface defining a coolant channel, the lower core plate including a plurality of openings extending from a first side of the lower core plate to a second side of the lower core plate, and the first portion of each cladding body extending into a different one of the plurality of openings of the lower core plate. Each of the plurality of fuel composition bodies is then disposed, such as by sliding, on an outer surface of a different one of the plurality of cladding bodies, each fuel composition body having an annular cylindrical shape, and an inner surface of the annular cylinder of the fuel composition body facing the outer surface of the cladding body. Each of the moderator bodies is then disposed, such as by sliding, on an outer surface of a different one of the plurality of fuel composition bodies, where in cross section, each moderator body has a periphery and an inner opening having a regular polygon shape, and a surface of the inner opening of the moderator body facing the outer surface of the annular cylinder of the fuel composition body. A second portion of the cladding body is then bonded to the upper core plate, the upper core plate including a plurality of openings extending from a first side of the upper core plate to a second side of the upper core plate, the coolant channel of the cladding body extending into one of the plurality of openings of the upper core plate. The assembled cladding body, the fuel composition body radially outer of the cladding body, and the moderator composition body radially outer of the fuel composition body define a fuel element structure, and in each fuel element structure, the cladding body includes a first portion extending axially beyond a first axial end of the fuel composition body and a second portion extending axially beyond a second axial end of the fuel composition body. Also, an outer surface of the moderator body of the first fuel element structure abuts an outer surface of the moderator body of a plurality of nearest fuel element structures, and the portion of the upper core plate, the portion of the lower core plate, and the cladding body of each fuel element structure form a first portion of a containment structure of a nuclear propulsion reactor. [Brief description of the drawings]

[0011] The foregoing summary, as well as the following detailed description of the embodiments, can be better understood when read in conjunction with the appended drawings, in which: It is understood that the illustrated embodiments are not limited to the precise arrangements and instrumentalities shown in the drawings.

[0012] [Figure 1] 1 illustrates the structure and arrangement of features in a typical design of a nuclear thermal propulsion reactor and engine.

[0013] [Diagram 2] 1 illustrates a schematic diagram of an embodiment of a nuclear propulsion nuclear fission reactor structure.

[0014] [Diagram 3] 1 is a partial top view of a portion of a nuclear propulsion nuclear fission reactor structure showing the coupling between the active core region and the reflector;

[0015] [Figure 4A] 1 illustrates an embodiment of a neutron absorbing structure and related features within a reflector. [Figure 4B] 1 illustrates an embodiment of a neutron absorbing structure and related features within a reflector. [Figure 4C] 1 illustrates an embodiment of a neutron absorbing structure and related features within a reflector.

[0016] [Diagram 5] 1 illustrates a schematic cross-section of an upper portion of a nuclear propulsion nuclear fission reactor structure including an example upper core plate;

[0017] [Figure 6-1] FIG. 1 illustrates components of an exemplary fuel element structure in a schematic, unassembled perspective view. [Figure 6-2] FIG. 1 illustrates components of an exemplary fuel element structure in an assembled cross-sectional view.

[0018] [Figure 7] FIG. 2 is a schematic perspective view of a portion of the top surface of the active core region, showing one end of a plurality of assembled fuel element structures.

[0019] [Figure 8] 2 is a schematic cross-sectional view of an embodiment of a reflector;

[0020] [Figure 9] 1 illustrates a schematic diagram of an embodiment of a nuclear propulsion nuclear fission reactor structure having an outer hull.

[0021] [Figure 10A] 1 illustrates one embodiment of an outer shell. [Figure 10B] FIG. 10B is an enlarged view of portion P1 of FIG. 10A showing one embodiment of the interface between the reactor structure and the shell.

[0022] [Figure 11] 1 is a flow chart illustrating the basic steps in one embodiment of a method for manufacturing a nuclear propulsion nuclear fission reactor structure.

[0023] For clarity, in some instances, only some of the named features in the figures are labeled with reference numbers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] 2 is a schematic diagram of one embodiment of a nuclear propulsion nuclear fission reactor structure 100. The nuclear propulsion nuclear fission reactor structure 100 includes an active core region 110, a core support framework 130, a reflector 150, and a number of neutron absorbing structures 160.

[0025] The active core region 110 includes a plurality of fuel element structures 112 and has an axial centerline that defines a longitudinal axis 114 of the nuclear propulsion nuclear fission reactor structure 100. Each fuel element structure 112 includes a cladding body 116 having an inner surface 118 that defines a coolant channel, a fuel composition body 120 radially outward of the cladding body 116, and a moderator composition body 122 radially outward of the fuel composition body 120. When the plurality of fuel element structures 112 are positioned within the active core region 110, an outer surface 124 of the moderator composition body 122 of a first fuel element structure 112 abuts an outer surface 124 of the moderator composition body 122 of a plurality of nearest fuel element structures 112. This is illustrated in FIG. 2 for a first fuel element structure 112a and a plurality of nearest fuel element structures 112b-112g. For example, in an exemplary embodiment, a side of a fuel element structure is in direct contact with a side of an adjacent fuel element structure. The assembly of outer surfaces 124 of the moderator assemblies 122 of the fuel element structures 112 around the active core region 110 defines a radial outer surface 126 of the active core region 110 .

[0026] The core support frame 130 is radially outward of the active core region 110, and the reflector 150 is radially outward of the core support frame 130. A first surface 132 of the core support frame 130 is radially inward of a second surface 134 of the core support frame 130. The first surface 132 of the core support frame 130 conforms to a radially outer surface 126 of the active core region 110, and the second surface 134 of the core support frame 130 conforms to a radially inner surface 152 of the reflector 150. The radially inner surface 152 of the reflector 150 faces the active core region 110, and the core support frame 130 functions to fit the shape of the radially outer surface 126 of the active core region 110 to the shape of the radially inner surface 152 of the reflector 150.

[0027] A plurality of neutron absorbing structures 160 are disposed within the volume of the reflector 150. The neutron absorbing structures 160 include a neutron absorber body 162 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 an exemplary embodiment, the first position is radially closest to the active core region and the second position is radially furthest from the active core region. In the embodiment shown in FIG. 2, the neutron absorber body 162 is shown in a radially closest first position. For purposes of illustration, a second radially distal position is shown using phantom lines (see second position shown as 162′). The neutron absorber body 162 is movable between the first position and the second position to control the reactivity of the active core region 110. In the illustrated example, the neutron absorber body 162 is rotatable from a radially closer first position (corresponding to the position shown for the neutron absorber body 162 in FIG. 2) to a second position 162′ by rotation (R) about the axis 164 of the neutron absorber structure 160. However, other radial positions and / or directions of movement can be implemented so long as the various positions to which the neutron absorber body 162 can be moved provide control of the reactivity of the active core region 110. In some embodiments, when the multiple neutron absorber bodies 162 are each in the radially closer first position, each of the multiple neutron absorber bodies 162 is radially equidistant from the axial centerline of the active core region 110.

[0028] Figure 3 is a partial top view of a portion of a nuclear propulsion nuclear fission reactor structure. In Figure 3, a portion of the active core region 110 is shown, along with the core support frame 130, the reflector 150, and a portion of the neutron absorber body 162 within the neutron absorbing structure 160. Some of the features already shown and described in connection with Figure 2 are also shown in Figure 3.

[0029] For example, FIG. 3 shows a plurality of fuel element structures 112 (disposed with a plurality of nearest neighbor fuel element structures), each having a cladding body 116, a coolant channel 128, a fuel composition body 120, and a moderator composition body 122. The plurality of fuel element structures 112 collectively have translational symmetry. An example of translational symmetry is the three-pitch design shown in FIG. 3. In a three-pitch design, there is translational symmetry between the fuel element structures 112, whereby features on one fuel element structure 112 are repeated at a fixed distance (or fixed pitch) on the other fuel element structures 112. For example, based on whether nearest neighbors or next nearest neighbors are being compared, the repeating structure may be a multiple of the pitch. Exemplary suitable pitches are 3 cm to 10 cm, or 3 cm to 6 cm, or even alternatively 4 cm, depending on material selection and performance. FIG. 3 shows an example of a three-pitch design. The central axis of the coolant channel 128 is a distance P1 from each of the central axes of the coolant channels 128 of the nearest fuel element structures 112 and a distance P2 (P2=2×P1) from the next nearest neighbor. Each of the fuel element structures 112 has a polygonal cross-section. In the illustrated example, each of the fuel element structures 112 has a hexagonal cross-sectional shape. Other regular polygonal cross-sections may also be implemented. However, other shapes may require multiple shapes in one active core region design, such as a combination of an octagon and a square.

[0030] 3 illustrates a first surface 132 of the core support frame 130 that is conformal to the radially outer surface 126 of the active core region 110. Similarly, FIG. 3 illustrates a second surface 134 of the core support frame 130 that is conformal to the radially inner surface 152 of the reflector 150;

[0031] An additional structure shown in FIG. 3 is the neutron absorber body 162 of the neutron absorbing structure 160. FIGS. 4A-4C also show an embodiment of the neutron absorbing structure 160 and associated features within the reflector 150. As described herein, the neutron absorbing structure 160 is disposed within a volume of the reflector 150, the volume being defined by the radially inner surface 152, the radially outer surface 154, and the top surface 156 and bottom surface 158 (not shown) of the reflector 150. As shown in FIGS. 3 and 4A-4C, the neutron absorbing structure 160 includes a cylindrical drum 166 housed within a tube 168. The neutron absorber body 162 occupies a first portion of the cylindrical drum 166. This first portion of the cylindrical drum 166 is a volume of the cylindrical drum 166 that includes a portion of the outer surface of the cylindrical drum 166 (e.g., a 120 degree arc of the circumference of the cylindrical drum, etc.). When the tube 168 and cylindrical drum 166 move as a unit, such as rotating (R) relative to the inner diameter surface of the reflector-absorber housing, the cylindrical drum 166, neutron absorber body 162, and tube 168 slide along the inner surface of the reflector-absorber housing. A motor (not shown in FIGS. 3 and 4A-4C) can be operatively attached to the tube 168 by a drum shaft to rotate the neutron absorber structure 160. FIGS. 4B and 4C show an example in which a first position of the neutron absorber body 162 is radially closer to the active core region than a second position (compare FIG. 4C, which shows the neutron absorber body 162 in a first position, with FIG. 4B, which shows the neutron absorber body 162 in a second position), where the first position in FIG. 4C is radially closest to the active core region 110 and the second position in FIG. 4B is radially furthest from the active core region 110.

[0032] The cylindrical drum 166 other than the portion occupied by the neutron absorber body 162, i.e., the second portion of the cylindrical drum, functions as a secondary reflector. In some embodiments, the secondary reflector can be made of the same material as the reflector 150, such that the reflector 150 has substantially uniform neutron properties across a radial cross-section of the reflector 150 (whether or not that radial cross-section includes the neutron absorbing structure 160). In certain embodiments, the reflector 150 and the cylindrical drum 166 are formed of suitable neutron thermalizing materials, such as beryllium, beryllium oxide, and graphite, as well as combinations of such materials. However, in other embodiments, the secondary reflector and the reflector 150 are made of different materials. Suitable materials for the neutron absorber body 162 include B4C, europium (III) oxide, and dysprosium (III) oxide. Suitable materials for the tube 168 include most forms of steel, molybdenum, tungsten, and combinations of other exotic alloys. However, other materials may be used so long as they do not substantially interfere with the neutron absorbing function of the neutron absorber body 162. In a particular embodiment, the tube 168 is a stainless steel tube.

[0033] Other features of the exemplary embodiment of the nuclear propulsion nuclear fission reactor structure 100 include an upper core plate and a lower core plate. FIG. 5 illustrates a schematic top cross section of the nuclear propulsion nuclear fission reactor structure 100, including an example of an upper core plate 200. The upper core plate 200 has a first side 202 and a second side 204, and a number of openings 206. The openings 206 extend from the first side 202 to the second side 204 of the upper core plate 200. Each fuel element structure 112 (the outer surface 124 of the moderator body 122 of each fuel element structure 112 is shown in FIG. 5 with dashed lines) is associated with one opening 206 such that a first portion of each cladding body 116, such as a first end 210, extends into a different one of the multiple openings 206 and is joined to the upper core plate 200. For example, the first ends 210 of the cladding bodies 116 can extend at least partially (relative to length) into the openings 206. Alternatively, the first ends 210 of the cladding bodies 116 extend into the openings 206 a distance equal to the thickness (T) of the upper core plate 200. After the first ends 210 extend into the openings 206, the first portion of each cladding body 116 is joined to the upper core plate 200 by any suitable means, such as resistance welding, welding including full penetration welding, or by a suitable epoxy system, such as JB Weld®. As seen in FIG. 5, the coolant channel 128 defined by the inner surface 118 of the cladding body 116 fits concentrically into one of the multiple openings 206 in the upper core plate 200.

[0034] Although not shown in FIG. 5 , the lower core plate has features corresponding to the upper core plate 200, including a first side and a second side, and a number of openings extending from the first side to the second side of the lower core plate 250. The lower core plate 250 is attached to the second end of the fuel element structure 112 in a manner similar to the way that the upper core plate 200 is attached to the first end of the fuel element structure 112. That is, the second end of the cladding body 116 extends into a different one of the number of openings in the lower core plate 250. For example, the second end of the cladding body 116 can extend at least partially (relative to its length) into the opening in the lower core plate. Alternatively, the second end of the cladding body 116 extends into the opening in the lower core plate 250 a distance equal to the thickness of the lower core plate 250. After the second ends of the cladding bodies 116 extend into the openings in the lower core plate 250, the second portion of each cladding body 116 is joined to the lower core plate 250 by any suitable means such as welding, including resistance welding and full penetration welding, or by a suitable epoxy system such as JB Weld®. Also, similar to the upper core plate 200, the coolant channel 128 defined by the inner surface 118 of the cladding body 116 fits into one of a plurality of openings in the lower core plate 250.

[0035] As shown, in part, in FIG. 5 and as described above, at least a portion of the upper core plate 200, at least a portion of the lower core plate 250, and the cladding body 116 of each fuel element structure 112 form a first portion of a containment structure (i.e., a fission product separation propellant) of the nuclear propulsion nuclear fission reactor structure 100.

[0036] 6-1-6-2 show components of an exemplary fuel element structure in a schematic perspective unassembled view (FIG. 6-1) and in an assembled cross-sectional view (FIG. 6-2). FIG. 6-1 (A) shows one embodiment of the cladding body 116. The cladding body 116 is substantially tubular in shape having an outer surface 260 and an inner surface 118 that defines a coolant channel. When the tube is a cylindrical tube, the outer surface 260 is an outer diameter surface and the inner surface 118 is an inner diameter surface. In some embodiments, the cladding body 116 is a continuous extruded tube that spans the height of the active core region 110. By embodying the cladding body 116 as an extruded tube, the need for seaming and welded joints in the cladding body 116 is reduced or eliminated, reducing the risk of component failure during operation and reducing manufacturing complexity.

[0037] FIG. 6-1B illustrates one embodiment of the fuel composition body 120. The fuel composition body 120 has substantially the shape of an annular cylinder with a tube having an outer surface 270 and an inner surface 272. The inner surface 272 defines a space into which the clad body 116 fits. For example, if the clad body 116 is a cylindrical tube, the inner surface 272 is sized to complement the outer diameter surface of the clad body 116. The composition body 120 and the clad body 116 can be joined together, for example, by sliding the clad body 116 into the space defined by the inner surface 272 of the fuel composition body 120 and pressing or joining the two components together in a hot isostatic pressing (HIP) operation.

[0038] FIG. 6-1C illustrates one embodiment of the moderator composition body 122. The moderator composition body 122 is substantially in the shape of a polygonal sleeve having a central opening 280 and a flat outer surface 124. In the illustrated embodiment, the moderator composition body 122 is an annular hexagon, but the moderator composition body 122 can have other shapes, including other polygons and regular polygons. The annular hexagon of the moderator composition body 122 includes the outer surface 124 and a central opening 280 having an inner surface 282. The inner surface 282 defines a space into which the fuel composition body 120 (or the fuel composition body 120 joined with the clad body 116) fits. The composition body 120 and the clad body 116 can be assembled, for example, by sliding the fuel composition body 120 (or the fuel composition body 120 joined with the clad body 116) into the space defined by the inner surface 282 of the moderator composition body 122. Although a tight fit between the outer surfaces of the moderator assembly body 122 and the fuel assembly body 120 is not required, the two components may optionally be joined by, for example, press fitting or hot isostatic pressing.

[0039] FIG. 6-2 is a cross-sectional view of one embodiment of an assembled fuel element structure 112 illustrating the relative positions within the assembled fuel element structure 112 of the tubular cladding body 116, the annular cylindrical fuel composition body 120, and the moderator composition body 122 in the form of a polygonal sleeve.

[0040] FIG. 7 is a schematic perspective view of a portion of the top surface of the active core region, showing one end of a plurality of assembled fuel element structures. In the view of FIG. 7, the assembled fuel element structures 112 are assembled in the active core region 110 with the outer surfaces 124 of the moderator composition bodies 122 of the fuel element structure 112 abutting the outer surfaces 124 of the moderator composition bodies 122 of a plurality of nearest fuel element structures 112, resulting in an essentially continuous moderator body. In some embodiments, a plurality of fuel elements 112 can be manufactured together as a single component, i.e., an active core section larger than one fuel element 112. The axial end faces of the moderator composition body 122 and the fuel composition body 120 are in (or substantially in) the same plane (relative to each other) and form a substantially flat top surface 190 of the active core region 110. In each fuel element structure 112, a portion 220 of the cladding body 116 extends axially beyond both the axial end of the moderator composition body 122 and the fuel composition body 120. FIG. 7 corresponds to the end of the assembled fuel element structure 112 where the upper core plate 200 is joined.

[0041] From FIG. 7 it can be seen that the opposite end of the assembled fuel element structure 112 has a similar substantially flat surface with a protruding portion of the cladding body 116 that extends axially beyond both the axial end of the moderator assembly body 122 and the fuel assembly body 120 and corresponds to the end of the assembled fuel element structure 112 where the lower core plate 250 is joined.

[0042] FIG. 8 is a schematic cross-sectional view of one embodiment of a reflector. The reflector 150 surrounds the active core region 110 and mates with the core support frame 130 that spans the shape of the outer surface 126 of the active core region 110 (e.g., formed by the hexagonal surfaces 124 of the fuel element structures 112) with a uniform inner annular reflector surface 152. A neutron absorbing structure 160 is contained within the volume of the reflector 150. In the embodiment of FIG. 8, the neutron absorber body 162 is shown with the drum-shaped cylindrical neutron absorbing structure 160 facing inward in a position corresponding to the shutdown configuration of the nuclear propulsion nuclear fission reactor structure 100. The drum-shaped cylindrical neutron absorbing structure 160 is configured such that the neutron absorber body 162 can be positioned radially equidistant from the axial centerline of the active core region 110 to smooth fission hot spots.

[0043] The reflector 150 functions to thermalize the "reflected" neutrons returning to the active core region 110 to increase criticality and reduce the "leakage" of neutrons, which have no chance to generate a fission reaction, thus reducing the criticality potential of the nuclear propulsion fission reactor structure. Secondly, the reflector houses the neutron absorbing structure 160, which is the primary system for reactivity control. In FIG. 8, an embodiment of the neutron absorbing structure 160 is in the form of a rotatable control drum. In order to house a neutron absorbing structure 160 of sufficient size in the form of a rotatable control drum to control reactivity, the reflector cannot be made too thin (width (W) between the inner surface 152 and the outer surface 154). In an exemplary embodiment, the beryllium-based reflector has a width (W) of 15 cm to 30 cm. The width can vary based on the reflector material and weight requirements for non-terrestrial applications of the nuclear propulsion fission reactor structure, with materials having lower neutron reflection properties requiring a thicker reflector, i.e., a wider width (W).

[0044] The nuclear propulsion nuclear fission reactor structure may further include an outer shell. FIG. 9 shows a schematic diagram of an embodiment of the nuclear propulsion nuclear fission reactor structure 100 having an outer shell 300. The reactor structure, including the active core region 110, the core support frame 130, the upper core plate 200, the lower core plate 250, the reflector 130, and the plurality of neutron absorbing structures 160, is contained within an internal volume 302 of the outer shell 300. The upper reactor plate 304 is disposed above (or outside) the first side 202 of the upper core plate 200 and includes a plurality of holes 306 for passing the propulsion gas. The plurality of holes are in fluid communication with the coolant channel 128 in the active core region 110 so that the propulsion gas passes through the plurality of holes 306 and then passes through the coolant channel 128 to serve as a coolant for the nuclear propulsion nuclear fission reactor structure 100. Similarly, a lower reactor plate 310 is disposed below (or outside) the lower core plate 250 and includes a number of holes 306 for passing the propellant gases exiting the coolant channels 128 .

[0045] 9 also shows a motor 320 operably attached to the cylindrical drum 166 of the neutron absorbing structure 160 by a drum shaft 322 for rotating the cylindrical drum 166. In the illustrated embodiment, the motor 320 is external to the outer shell 300, and the drum shaft 322 penetrates the outer shell 300, for example, by a port or other opening 324 in the outer shell 300.

[0046] An embodiment of the shell 300 is formed from a sheet of material such as stainless steel and may include ribs or other reinforcing structures to provide additional structural support. As seen in FIG. 10A, the shell 300 may be a single continuous component. However, in other embodiments, the shell 300 may be multiple components that are then assembled with fasteners. An inner ledge 330 of the shell 300 supports the reflector 130 and the active core region 110. The inner ledge 330 may be attached to the inner surface of the shell 300 or may be formed by a portion of the inner surface of the shell 300. FIG. 10B (an enlarged view of portion P1 of FIG. 10A) illustrates an exemplary interface between the reactor structure and the shell 300. The lower core plate 250 of the active core region 110 rests on the lower core plate ledge portion 332 of the inner ledge 330, forming a mechanical interface.

[0047] During operation and before the nozzle throat reaches Mach 1, the incoming flow must overcome the increasing pressure in the convergent nozzle section. This can cause a potential backflow condition if a crack exists in the space below the active core region 110, for example in the shell 300 below the reactor structure. Therefore, the seal of the mechanical interface between the lower core plate ledge section 332 and the lower core plate 250 should be as stable as possible during start-up to prevent leakage into the reflector 130 and the active core region 110. At steady state conditions during operation (i.e., for the nozzle throat Mach 1 and after the shock has exited the divergent section), the acceleration of the flow through the divergent nozzle section "pulls" the flow and causes a seal to form between the lower core plate ledge section 332 and the lower core plate 250 due to the negative dynamic pressure difference.

[0048] The present disclosure also relates to a nuclear thermal propulsion engine including a nuclear propulsion nuclear fission reactor structure 110 within an outer hull 300. The nuclear thermal propulsion engine further includes a shield, turbomachinery, and a nozzle section attached to or supported by the outer hull 300, for example, in accordance with that shown and described in connection with FIG. 1. In an exemplary embodiment, the active core region 110 rests on a lower core plate ledge portion 332 of an inner ledge 330, a reflector 130 is attached to a reflector ledge portion 334 of the inner ledge 330 by fasteners such as bolts or pins, and a drum shaft 322 operably attaches a motor 320 (e.g., secured to the outer hull via a motor support plate) to a neutron absorbing structure 160 and passes through an opening 324 in the top of the outer hull 300. The nozzle section is bolted to the bottom of the outer hull 300, and the shield and turbomachinery are secured to the top of the outer hull 300. A storage section for cryogenically storing the propulsion gas is operably connected, along with the shielding and the turbomachinery, to provide a flow path from the storage section to the nuclear propulsion reactor, and the nozzle section is operably connected to provide a flow path for the superheated propulsion gas exiting the nuclear propulsion reactor.

[0049] Nuclear propulsion fission reactor structures (as well as nuclear thermal propulsion engines including nuclear propulsion fission reactor structures) can be manufactured using suitable means. In general, nuclear propulsion fission reactor structures are manufactured by a method that includes the steps of bonding a cladding body to a lower core plate, sliding a fuel composition body and a moderator body into position over the radially inner features, e.g., sliding the fuel composition body over the cladding body and the moderator body over the assembled fuel composition body-cladding body to form a fuel element structure, and bonding an upper core plate to a portion of each cladding body that extends axially beyond the fuel composition body and the moderator body. A reflector is then placed around the outer surface of the assembled fuel element structure, and the inner surface of the reflector is fitted to the outer surface of the assembled fuel element structure by a core support frame.

[0050] 11 is a flow diagram illustrating basic steps in one embodiment of a method for manufacturing a nuclear propulsion nuclear fission reactor structure. The method 400 includes a step 410 of joining a first portion of each of a plurality of cladding bodies to a lower core plate. Each cladding body 116 has an inner surface 118 that defines a coolant channel 128, and the lower core plate 250 includes a plurality of openings that extend from a first side of the lower core plate to a second side of the lower core plate. When joining the cladding bodies 116 to the lower core plate 250, the first portion of each cladding body 116 extends into a different one of the plurality of openings in the lower core plate 250. When joining the cladding bodies 116 to the lower core plate 250, the entire interface between the first portion and the lower core plate is preferably joined together, such as by welding, to form a continuous metal body including the cladding body 116 and the lower core plate 250.

[0051] The method 400 also includes sliding 420 each of the plurality of fuel composition bodies over an outer surface of a different one of the plurality of cladding bodies. Each fuel composition body 120 has an annular cylindrical shape. When the fuel composition body 120 is placed on the outer surface 260 of the cladding body 116, an annular cylindrical inner surface 272 of the fuel composition body 120 is oriented toward the outer surface 260 of the cladding body 116. Because a first portion of the cladding body 116 extends into the opening of the lower core plate 250, the fuel composition body 120 is prevented by the lower core plate 250 from extending to the same axial position as the end of the cladding body 116. Thus, the first portion of the cladding body 116 extends axially beyond the first axial end of the fuel composition body 120. Similarly, a second portion of the cladding body 116 extends axially beyond the second axial end of the fuel composition body 120 to provide a portion of the cladding body 116 for mating with the opening 206 of the upper core plate 200. After the fuel composition 120 is disposed on the outer surface 260 of the clad body 116, the fuel composition 120 and the clad body 116 may be secured or otherwise joined to one another, for example, by pressing or hot isostatic pressing (HIP).

[0052] The method 400 also includes a step 430 of sliding each of the moderator bodies over an outer surface of a different one of the plurality of fuel composition bodies. Each moderator body 122 has a perimeter having a regular polygon (in a particular embodiment, a hexagon) in cross section and an inner opening 280. When the moderator body 122 is placed on the outer surface 270 of the fuel composition body 120, a surface 282 of the inner opening 280 of the moderator body 122 is oriented toward the annular cylindrical outer surface 270 of the fuel composition body 120. Although a tight fit between the moderator composition body 122 and the outer surface 270 of the fuel composition body 120 is not required, the two components can optionally be joined, for example, by press fitting or hot isostatic pressing.

[0053] The method 400 also includes a step 440 of joining the second portion of the cladding body to the upper core plate. The upper core plate 200 includes a plurality of openings 206 extending from a first side 202 of the upper core plate 200 to a second side 204 of the upper core plate 200. The second portion of the cladding body 116 (extending axially beyond the axial end of the fuel composition body 120) is inserted into the openings 206 and joined to the upper core plate 200. When joining the cladding body 116 to the upper core plate 200, the entire interface between the second portion and the upper core plate is preferably joined together, such as by welding, to form a continuous metal body including the cladding body 116 and the upper core plate 200. It should be noted that by joining the cladding body 116 of each fuel element structure to the upper core plate 200 and the lower core plate 250, a portion of the upper core plate forms a first portion of the containment structure of the nuclear propulsion nuclear fission reactor structure 100. Additionally, the inner surface 118 of the cladding body 116 defines a coolant channel 128 having a first portion and a second portion inserted into the openings in the lower and upper core plates, respectively, such that the coolant channel 128 of the cladding body 116 similarly extends into the openings in the lower and upper core plates.

[0054] Each fuel element structure 112, assembled to include a cladding body 116, a fuel composition body 120 radially outward of the cladding body 116, and a moderator composition body 122 radially outward of the fuel composition body 120, is positioned within the active core region 110 such that an outer surface 124 of the moderator body 122 of a first fuel element structure abuts an outer surface 124 of the moderator body 122 of a plurality of nearest fuel element structures, for example, in a three-pitch relationship.

[0055] After assembly of the fuel element structures 112 into the active core region 110, a reflector 150 is disposed about the outer surface 126 of the assembled fuel element structures 112. The core support frame 130 helps the inner surface 152 of the reflector 150 mate with the outer surface 126 of the assembled fuel element structures 112. The core support frame 130 also mates with the upper core plate 200 and the lower core plate 250, such that the reflector 150 forms a second portion of the containment structure of the nuclear propulsion nuclear fission reactor structure 100.

[0056] It should be noted that other features and structures of the nuclear propulsion nuclear fission reactor structure 100 may be manufactured as part of the method or may be provided for use in the method. Thus, the method 400 may optionally include one or more of forming a plurality of cladding bodies 116, forming a plurality of fuel composition bodies 120, and forming a plurality of moderator bodies 122. Forming the plurality of cladding bodies 116 may be performed by any suitable technique, including metal processing techniques such as extrusion. Forming the plurality of fuel composition bodies 120 may be performed by any suitable technique, including fuel compaction techniques or additive manufacturing techniques. Forming the plurality of moderator bodies 122 may be performed by any suitable technique, including powder compaction or additive manufacturing techniques.

[0057] In some manufacturing methods or steps of the manufacturing methods, the features and structures of the nuclear propulsion fission reactor structure 100 (or portions thereof) are manufactured as a monolithic, unitary structure, for example, using an additive manufacturing process. As used herein, an additive manufacturing process includes any technique that builds a 3D object by adding material layer by layer. Examples of suitable additive manufacturing processes utilize 3D printing of metal alloys, such as molybdenum-containing metal alloys, Zircaloy-4 or Hastelloy X, or 3D printing of ceramics, such as uranium or beryllium oxide, to form the structural features described above, such as cladding or fuel. In other embodiments, fissile nuclear fuel compositions and / or heat transfer agents and / or moderator materials and / or poisons used as part of the nuclear propulsion fission reactor structure 100 can be included in the monolithic structure if a suitable multi-material additive manufacturing process is used with multiple metals and ceramics in the feedstock. If molten metal is not included in the additive manufacturing process, the additive manufacturing process can be paused and a volume of molten metal (either in liquid or solid form) is placed into the fuel cavity, and the additive manufacturing process is continued to complete the construction of the closed chamber. Other alloys that may be used when a suitable multi-material additive manufacturing process having multiple metals in the feedstock is used include steel alloys, zirconium alloys, and molybdenum-tungsten alloys (for the cladding and / or containment structure), beryllium alloys (for the reflector), and stainless steel (for the containment structure). Even if not produced by an additive manufacturing process, the above materials may be used in the manufacture of the various features and structures disclosed herein.

[0058] Additionally, while the disclosed reactors and cores have complex mechanical geometries, the structures and features disclosed herein can be more easily manufactured using additive manufacturing techniques, such as 3D printing of elemental metals or metal alloys, in an integrated and repeatable manner, layer by layer.

[0059] Additive manufacturing techniques for producing monolithic and unitary structures can include additional steps of (a) predictive and causal analytics, (b) in situ monitoring combined with machine vision and accelerated processing during layer-by-layer manufacturing of the structure, (c) automated analytics combined with machine learning components, and (d) virtual inspection of a digital representation of the as-built structure. Additionally, additive manufacturing techniques can create complex geometries and, when coupled with in situ sensors, machine vision images, and artificial intelligence, allow for tuning of manufacturing quality as components are built on a layer-by-layer additive basis (often these layers are on the scale of 50 microns), providing predictive quality assurance of the manufacture of such reactors and structures.

[0060] As used herein, cladding is a layer of fuel that includes features located between the coolant and the nuclear fuel. The cladding acts as a safety barrier that prevents radioactive fission fragments from escaping from the fuel into the coolant and contaminating it. Some design constraints of the cladding include neutron absorption, radiation resistance, and temperature behavior. The cladding is typically made of corrosion-resistant materials that have a low absorption cross section for thermal neutrons. Exemplary materials include Zircaloy or steel, but other materials may be used, such as compositions including metal and ceramic-based (Be, C, Mg, Zr, O, Si), as well as molybdenum, tungsten, rhenium, tantalum, hafnium, and their alloys, including carbides, if appropriate for the conditions of the reactor. In some embodiments, the cladding material can be isotopically enriched to increase reactivity by reduction of isotopes with higher neutron absorption cross sections, for example, molybdenum-enriched Mo-92 has less parasitic neutron absorption cross section than elemental molybdenum. In an embodiment of the disclosed nuclear propulsion fission reactor structure, the upper and lower core plates are made of cladding material, preferably having the same composition as the cladding body.

[0061] The fissile nuclear fuel composition contains more than 5% and less than 20% U. 235 It can be high-enriched low-enriched uranium (HALEU) with assay, or it can be 20% or more U 235The fuel may be highly enriched uranium (HEU) having 100% uranium by weight. Suitable fissile nuclear fuel compositions applicable to the disclosed fuel element structures include uranium oxide (UO2) enriched less than 20%, uranium with 10% by weight molybdenum (U-10Mo), uranium nitride (UN), and other stable fissile fuel compounds. Burnable poisons may also be included. Typically, the fissile nuclear fuel composition is in the form of a ceramic metal (cermet), such as UO2 with W or Mo and UN with W or Mo. In some embodiments, molten metal can also function as the "metal" portion of the cermet.

[0062] When used, a heat transfer agent, such as a salt or metal that melts at operating temperatures, can be included in the fuel element structure to improve thermal bonding between the fuel composition body and the cladding body. Additionally, the heat transfer agent can occupy cracks or other defects in the fuel element structure (whether present initially or developed during operation of the reactor) to facilitate thermal bonding. Molten metals suitable for inclusion in the disclosed nuclear propulsion fission reactor structures and for inclusion in the fuel element structure to provide heat transfer contact include sodium (Na), sodium-potassium (NaK), potassium (K), iron (Fe), copper (Cu), silver (Ag), lead (Pb), and bismuth (Bi), or alloy compositions thereof.

[0063] It is contemplated that the disclosed nuclear propulsion fission reactor structures and nuclear thermal propulsion engines may incorporate various supporting auxiliary equipment, such as moderators (e.g., zirconium hydride (ZrH), beryllium (Be), beryllium oxide (BeO), water, graphite, etc.), control rods for launch safety (e.g., iridium control rods), and scientific equipment (e.g., temperature sensors or radiation detectors).

[0064] The disclosed configurations relate to any configuration in which a heat source comprising a fissile nuclear fuel composition, whether a fuel element or the fissile nuclear fuel composition itself, is surrounded by a cladding. Although generally described herein in the context of a gas-cooled nuclear thermal propulsion reactor (NTP reactor), the structures and methods disclosed herein may be applicable to other nuclear fission reactor systems.

[0065] The nuclear propulsion fission reactor structures disclosed herein may be used in any suitable application, including, but not limited to, non-terrestrial power applications, space power, space propulsion, and maritime applications, including submarines.

[0066] Although particular embodiments have been referenced, it will be apparent that other embodiments and variations can be devised by those skilled in the art without departing from the spirit and scope thereof, and it is intended that the appended claims be construed to include all such embodiments and equivalent variations. The following (item 1) to (item 45) are within the scope of the disclosure of this application. (Item 1) 1. A nuclear propulsion fission reactor structure, comprising: an active core region including a plurality of fuel element structures and having an axial centerline defining a longitudinal axis of the nuclear propulsion reactor; a core support frame radially outward of the active core region; a reflector radially outward of the core reformer and having an inner radial surface oriented toward the active core region; a plurality of neutron absorbing structures disposed within the reflector volume; an upper core plate; a lower core plate; Each fuel element structure includes a cladding body having an inner surface defining a coolant channel, a fuel composition radially outward of the cladding body, and a moderator composition radially outward of the fuel composition; an outer surface of the moderator assembly of the first fuel element structure abuts outer surfaces of the moderator assemblies of the nearest fuel element structures; the core support frame has a first surface radially inward of a second surface, the first surface conforming to a radially outer surface of the active core region and the second surface conforming to the radially inner surface of the reflector; each of the plurality of neutron absorbing structures includes a neutron absorber body movable between a first position and a second position, the first position being radially closer to the active core region than the second position; the cladding body of each fuel element structure includes a first portion extending axially beyond a first axial end of the fuel composition body and a second portion extending axially beyond a second axial end of the fuel composition body; the first portion of each fuel element assembly is bonded to the upper core plate and the second portion of each fuel element assembly is bonded to the lower core plate. (Item 2) 2. The nuclear propulsion nuclear fission reactor structure of claim 1, wherein the fuel composition has an annular cylindrical shape and the fuel element structure has a polygonal cross section. (Item 3) 3. The nuclear propulsion nuclear fission reactor structure of claim 2, wherein a side of the fuel element structure is in direct contact with a side of an adjacent fuel element structure, and an arrangement of the plurality of fuel element structures in the active core region has translational symmetry. (Item 4) 4. The nuclear propulsion nuclear fission reactor structure of claim 3, wherein the fuel composition of the fuel composition comprises enriched uranium oxide less than 20%, uranium containing 10% by weight molybdenum (U-10Mo), uranium nitride (UN), or a cermet thereof. (Item 5) 1. A nuclear propulsion fission reactor structure, comprising: an active core region including a plurality of fuel element structures and having an axial centerline defining a longitudinal axis of the nuclear propulsion reactor; a core support frame radially outward of the active core region; a reflector radially outward of the core reformer and having an inner radial surface oriented toward the active core region; a plurality of neutron absorbing structures disposed within the reflector volume; Each fuel element structure includes a cladding body having an inner surface defining a coolant channel, a fuel composition radially outward of the cladding body, and a moderator composition radially outward of the fuel composition; an outer surface of the moderator assembly of the first fuel element structure abuts outer surfaces of the moderator assemblies of the nearest fuel element structures; the core support frame has a first surface radially inward of a second surface, the first surface conforming to a radially outer surface of the active core region and the second surface conforming to the radially inner surface of the reflector; 1. A nuclear propulsion nuclear fission reactor structure, wherein each of the plurality of neutron absorbing structures includes a neutron absorber body movable between a first position and a second position, the first position being radially closer to the active core region than the second position. (Item 6) an upper core plate; a lower core plate; the cladding body of each fuel element structure includes a first portion extending axially beyond a first axial end of the fuel composition body and a second portion extending axially beyond a second axial end of the fuel composition body; the first portion of each fuel element structure is bonded to the upper core plate and the second portion of each fuel element structure is bonded to the lower core plate. Item 5. A nuclear propulsion fission reactor structure as described in item 5. (Item 7) 7. The nuclear propulsion nuclear fission reactor structure according to claim 5 or 6, wherein the fuel composition has the shape of an annular cylinder. (Item 8) 7. The nuclear propulsion nuclear fission reactor structure according to claim 5 or 6, wherein the fuel element structure has a polygonal cross-section. (Item 9) 9. The nuclear propulsion nuclear fission reactor structure of claim 8, wherein the fuel element structure has a cross section in the shape of a regular polygon. (Item 10) 9. The nuclear propulsion nuclear fission reactor structure of claim 8, wherein the regular polygon shape is a hexagon. (Item 11) 7. The nuclear propulsion nuclear fission reactor structure of claim 5 or 6, wherein a side of the fuel element structure is in direct contact with a side of an adjacent fuel element structure. (Item 12) 7. The nuclear propulsion nuclear fission reactor structure of claim 5 or 6, wherein the arrangement of the plurality of fuel element structures in the active core region has translational symmetry. (Item 13) 13. The nuclear propulsion fission reactor structure of item 12, wherein the distance between repeating elements within the structure is constant. (Item 14) 7. The nuclear propulsion nuclear fission reactor structure according to claim 5 or 6, wherein the fuel composition of the fuel composition comprises enriched uranium oxide less than 20%, uranium with 10% by weight molybdenum (U-10Mo), uranium nitride (UN), and / or cermets thereof. (Item 15) 7. The nuclear propulsion nuclear fission reactor structure according to claim 5 or 6, wherein the neutron absorber is movable between the first position and the second position to control the reactivity of the active core region. (Item 16) 7. The nuclear propulsion fission reactor structure according to item 5 or 6, wherein each cladding body is a continuous extruded tube. (Item 17) 7. The nuclear propulsion nuclear fission reactor structure according to claim 5 or 6, wherein the cladding body has a composition including molybdenum, tungsten, rhenium, tantalum, hafnium, and alloys thereof including carbides. (Item 18) 7. The nuclear propulsion nuclear fission reactor structure of claim 5 or 6, wherein the first portion of the cladding body extends axially beyond a first axial end of the moderator assembly and the second portion of the cladding body extends axially beyond a second axial end of the moderator assembly. (Item 19) 7. The nuclear propulsion nuclear fission reactor structure of claim 5 or 6, wherein the portion of the upper core plate, the portion of the lower core plate, and the cladding body of each fuel element assembly form part of a containment structure for the nuclear propulsion reactor. (Item 20) 7. The nuclear propulsion nuclear fission reactor structure according to claim 5 or 6, wherein the fuel composition body has a shape of an annular cylinder, and the moderator composition body is a polygonal sleeve having a central opening, the inner diameter of the central opening defining a space in which the fuel composition is disposed. (Item 21) 7. The nuclear propulsion nuclear fission reactor structure of claim 5 or 6, wherein when the plurality of neutron absorbers are each in the first position, each of the plurality of neutron absorbers is radially equidistant from the axial centerline of the active core region. (Item 22) 7. The nuclear propulsion fission reactor structure of claim 5 or 6, wherein the neutron absorber has a composition comprising beryllium, beryllium oxide, graphite, or a combination thereof. (Item 23) 7. The nuclear propulsion nuclear fission reactor structure of claim 5 or 6, wherein each of the plurality of neutron absorbing structures includes a cylindrical drum housed within a tube, the neutron absorber body occupies a first portion of the cylindrical drum, a second portion of the cylindrical drum is a secondary reflector, and the first portion of the cylindrical drum is a volume of the cylindrical drum including a portion of an outer surface of the cylindrical drum. (Item 24) 24. The nuclear propulsion nuclear fission reactor structure of claim 23, wherein said portion of said outer surface of said cylindrical drum corresponds to a 120 degree arc of the circumference of said cylindrical drum. (Item 25) 24. The nuclear propulsion fission reactor structure of claim 23, wherein the tubes are stainless steel. (Item 26) 24. The nuclear propulsion nuclear fission reactor structure of claim 23, wherein the cylindrical drum is rotatable relative to the inner diameter surface of the tube. (Item 27) 24. The nuclear propulsion nuclear fission reactor structure of claim 23, further comprising a motor operably attached to the cylindrical drum by a drum shaft for rotating the cylindrical drum. (Item 28) 7. The nuclear propulsion fission reactor structure according to item 5 or 6, wherein the composition of the reflector comprises beryllium, beryllium oxide or graphite. (Item 29) 24. The nuclear propulsion fission reactor structure of claim 23, wherein the reflector composition comprises beryllium, beryllium oxide, or graphite, and the secondary reflector composition comprises beryllium, beryllium oxide, or graphite. (Item 30) 30. The nuclear propulsion nuclear fission reactor structure of claim 29, wherein the composition of the secondary reflector is the same as the composition of the reflector. (Item 31) 7. The nuclear propulsion nuclear fission reactor structure according to item 5 or 6, wherein the coolant is a propulsion gas, and the nuclear propulsion reactor further comprises an upper reactor plate and a lower reactor plate, each reactor plate including a plurality of holes for passing the propulsion gas. (Item 32) Further comprising an outer shell; the active core region, the core support frame, the upper core plate, the lower core plate, the reflector, and the plurality of neutron absorbing structures form a nuclear reactor structure; 7. The nuclear propulsion nuclear fission reactor structure of claim 6, wherein the reactor structure is contained within an interior volume of the shell. (Item 33) 33. The nuclear propulsion nuclear fission reactor structure of item 32, wherein the coolant is a propulsion gas, and the nuclear propulsion reactor further comprises an upper reactor plate and a lower reactor plate, each reactor plate including a plurality of holes for passing the propulsion gas. (Item 34) 33. The nuclear propulsion nuclear fission reactor structure of claim 32, wherein the reactor structure is supported by a ledge attached to or formed by a portion of the inner surface of the outer shell. (Item 35) 1. A nuclear thermal propulsion engine, comprising: A nuclear propulsion fission reactor structure according to item 32, A shield; a storage section for cryogenically storing the propulsion gas; A turbomachine; A nozzle, the upper core plate is oriented toward a first end of the shell and the lower core plate is oriented toward a second end of the shell; a shield, a turbomachine, and the reservoir are operably attached to the first end of the shell to provide a flow path from the reservoir to the nuclear propulsion reactor; The nozzle is operably mounted to the second end of the shell to provide a flow path for superheated propulsion gases exiting the nuclear propulsion reactor. (Item 36) 1. A method for manufacturing a nuclear propulsion fission reactor structure, comprising: bonding a first portion of each of a plurality of cladding bodies to a lower core plate, each cladding body having an inner surface defining a coolant channel, the lower core plate including a plurality of openings extending from a first side of the lower core plate to a second side of the lower core plate, the first portion of each cladding body extending into a different one of the plurality of openings in the lower core plate; sliding each of a plurality of fuel composition bodies over an outer surface of a different one of the plurality of clad bodies, each fuel composition body having an annular cylindrical shape, an inner surface of the annular cylinder of the fuel composition body being oriented toward the outer surface of the clad body; sliding each of the moderator bodies over an outer surface of a different one of the fuel composition bodies, wherein in cross section, each moderator body has a periphery having a regular polygon shape and an inner opening, the surface of the inner opening of the moderator body being oriented toward the annular cylindrical outer surface of the fuel composition body; bonding a second portion of the cladding body to an upper core plate, the upper core plate including a plurality of openings extending from a first side of the upper core plate to a second side of the upper core plate, the coolant channel of the cladding body extending into one of the plurality of openings of the upper core plate; the assembled cladding body, a fuel composition radially outward of the cladding body, and a moderator composition radially outward of the fuel composition define a fuel element structure; In each fuel element structure, the cladding body includes a first portion extending axially beyond a first axial end of the fuel composition body and a second portion extending axially beyond a second axial end of the fuel composition body; an outer surface of the moderator body of the first fuel element structure abuts outer surfaces of the moderator bodies of the nearest fuel element structures; 11. A method for manufacturing a nuclear propulsion nuclear fission reactor structure, wherein the portion of the upper core plate, the portion of the lower core plate, and the cladding body of each fuel element assembly form a first portion of a containment structure for the nuclear propulsion reactor. (Item 37) 37. The method of claim 36, wherein forming the plurality of clad bodies comprises extruding the clad bodies into a seamless tube. (Item 38) Item 37. The method of claim 36, wherein joining the first portion of each of the cladding bodies to the lower core plate comprises welding. (Item 39) Item 37. The method of claim 36, wherein forming the plurality of fuel compositions comprises a fuel compression technique or an additive manufacturing technique. (Item 40) 37. The method of claim 36, further comprising the step of fastening the fuel composition to the cladding. (Item 41) 41. The method of claim 40, wherein the step of fixing the fuel composition to the cladding comprises pressing or hot isostatic pressing. (Item 42) Item 37. The method of claim 36, wherein joining the second portion of each of the cladding bodies to the upper core plate comprises welding. (Item 43) Item 37. The method of manufacturing a nuclear propulsion nuclear fission reactor structure of item 36, further comprising the steps of disposing a reflector around an outer surface of the assembled fuel element structures and mating an inner surface of the reflector to the outer surface of the assembled fuel element structures with a core support frame. (Item 44) Item 44. The method of claim 43, wherein the inner surface of the reflector forms a second portion of a containment structure for the nuclear propulsion reactor. (Item 45) forming the plurality of clad bodies; forming the plurality of fuel compositions; forming the plurality of decelerator bodies; Item 37. A method for producing a nuclear propulsion nuclear fission reactor structure according to item 36. [Explanation of symbols]

[0067] 100 Nuclear propulsion fission reactor structures 110 Active Core Region 112 Fuel element structure 116 Clad Body 120 Fuel composition object 122 Moderator composition object 128 Cooling Channel 130 Core Support Frame 150 reflector 160 Neutron Absorbing Structure 200 Upper core plate 250 Upper core plate

Claims

[Claim 1] 1. A nuclear propulsion fission reactor structure, comprising: an active core region including a plurality of fuel element structures and having an axial centerline defining a longitudinal axis of the nuclear propulsion reactor; a core support frame radially outward of the active core region; a reflector radially outward of the core reformer and having an inner radial surface oriented toward the active core region; a plurality of neutron absorbing structures disposed within the reflector volume; an upper core plate; a lower core plate; Each fuel element structure includes a cladding body having an inner surface defining a coolant channel, a fuel composition radially outward of the cladding body, and a moderator composition radially outward of the fuel composition; an outer surface of the moderator assembly of the first fuel element structure abuts outer surfaces of the moderator assemblies of a plurality of nearest fuel element structures; the core support frame has a first surface radially inward of a second surface, the first surface conforming to a radially outer surface of the active core region and the second surface conforming to the radially inner surface of the reflector; each of the plurality of neutron absorbing structures includes a neutron absorber body movable between a first position and a second position, the first position being radially closer to the active core region than the second position; the cladding body of each fuel element structure includes a first portion extending axially beyond a first axial end of the fuel composition body and a second portion extending axially beyond a second axial end of the fuel composition body; the first portion of each fuel element structure is bonded to the upper core plate and the second portion of each fuel element structure is bonded to the lower core plate.

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