Patent Description:
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 demonstrate that such structures and/or methods do not qualify as prior art against the present invention.

Various propulsion systems for non-terrestrial applications, such as in space, have been developed. 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 that is tailored to specific missions. For example, chemical-based propulsion systems have high thrust (e.g., > <NUM><NUM> lbs thrust (> <NUM> × <NUM><NUM> N)), but modest specific impulse (e.g., ≤ <NUM> sec) and are efficiently applied to heavy lift operations, such as placing payloads into earth orbit. Ion-based propulsion systems have low thrust (e.g., < <NUM> lbs thrust (< <NUM> N)), but high specific impulse (e.g., <NUM>,<NUM> - <NUM>,<NUM> sec) and are efficiently applied for long term space travel, such as inter-stellar travel. Nuclear-based propulsion systems combines modest thrust (e.g., <NUM>,<NUM> - <NUM>,<NUM> lbs thrust (<NUM>,<NUM> - <NUM>,<NUM> N)) and modest specific impulse (e.g., <NUM> - <NUM>,<NUM> sec) and are efficiently applied to near-space travel. Nuclear-based propulsion systems are currently being evaluated as a propulsion option for NASA's Human Exploration of Mars Design Reference. Architecture <NUM>.

Previous nuclear-based propulsion systems are still complex. For example, both Nuclear Engine for Rocket Vehicle Application (NERVA) and Project Rover have developed nuclear thermal rocket designs. A typical design for a nuclear thermal propulsion reactor and engine <NUM> is shown in <FIG>. The illustrated nuclear thermal propulsion reactor and engine <NUM> includes four main features: a hull <NUM> having a reactor <NUM> contained within a reflector <NUM>, turbomachinery <NUM> including turbo pumps <NUM> and other piping and support equipment <NUM>, shielding <NUM> separating the turbomachinery <NUM> from the hull <NUM>, and a nozzle section <NUM> including a nozzle <NUM> and a nozzle skirt <NUM>.

However, these prior nuclear-based propulsion systems still have drawbacks, including utilizing complex moderators and flow techniques, operating with minimal design margins that push the limits of the design and associated materials. Accordingly, there is still a need for robust and simple designs for nuclear propulsion reactors, particular for non-terrestrial applications, such as in space.

<CIT>) describes passive reactivity control technologies that enable reactivity control of a nuclear thermal propulsion system with little to no active mechanical movement of circumferential control drums. <CIT>) describes a nuclear propulsion reactor in which a pressure vessel is provided with a reactor core that is surrounded by a radial reflector. Nuclear fuel elements in the core are formed from a hexagonal housing. A stack of nuclear fuel compacts having axial bores for coolant flow is received in the central axial bore of the housing. <CIT>) describes a nuclear reactor comprising: fuel; a neutron moderator; and a neutron absorbing element with strong neutron absorption around <NUM> eV added to one or more components of a reactor core of the nuclear reactor. <NPL> [A] <NUM>-<NUM> describes aspects of nuclear reactor design. <CIT>) describes a nuclear propulsion reactor in which a reactor vessel is provided with an annular first core and a cylindrical second core that is radially encompassed by the first core. <CIT>) describes a space vehicle with a nuclear rocket engine assembly. <CIT>) describes a nuclear thermal rocket engine with an integrated and compact construction that facilitates vehicle size and weight reduction. The engine includes a nuclear reactor core having multiple fuel assemblies and moderator rods disposed therebetween.

Considering 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 manufacturing. Additionally, a simplified design with reduced number of weld points in manufacturing is advantageous to reduce the risk of performance degradation.

In general, the disclosure is directed to a nuclear fission reactor structure suitable for use as an engine in a nuclear-based propulsion system. In exemplary embodiments, the nuclear fission reactor structure utilizes a fuel element with a hexagonal cross-section arranged in a tri-pitch design and rotatable drum neutron absorbers for reactivity control. The nuclear fission reactor structure is housed in a hull of a nuclear thermal propulsion reactor and engine. A propulsion gas is used as a coolant for the nuclear fission reactor structure. Propulsion gas superheated in the nuclear fission reactor structure exits through a nozzle and generates thrust and impulse.

The invention according to a first aspect provides 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 former 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 absorber structures located 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. Additionally, an outer surface of a moderator composition body of a first fuel element structure abuts an outer surface of a moderator composition body of a plurality of nearest neighbor fuel element structures. The core former has a first surface radially inward of a second surface and the first surface is conformal to a radially outer surface of the active core region and the second surface is conformal to the radially inner surface of the reflector. Each of the plurality of neutron absorber 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.

According to a second aspect of the invention the nuclear propulsion fission reactor structure further comprises: an upper core plate; and a lower core plate, wherein the cladding body of each fuel element structure includes a first portion that extends axially past a first axial end of the fuel composition body and a second portion that extends axially past a second axial end of the fuel composition body, and wherein the first portion of each fuel element structure is joined to the upper core plate and the second portion of each fuel element structure is joined to the lower core plate.

The invention according to a third aspect provides a nuclear thermal propulsion engine comprising the nuclear propulsion fission reactor structure according to the second aspect and a hull, wherein the active core region, the core former, the upper core plate, the lower core plate, the reflector, and the plurality of neutron absorber structures form a reactor structure, and the reactor structure is housed within an interior volume of the hull.

The invention further provides according to a fourth aspect a nuclear thermal propulsion engine comprising the nuclear propulsion fission reactor structure of the third aspect, shielding, a reservoir for cryogenically storing a propulsion gas, turbomachinery, and a nozzle, are operatively attached to the reactor structure is housed within an interior volume of the hull such that the upper core plate is oriented toward a first end of the hull and the lower core plate is oriented toward a second end of the hull; the shielding, turbomachinery, and the reservoir are operatively mounted to the first end of the hull to provide a flow path from the reservoir to the nuclear propulsion reactor; and the nozzle is operatively mounted to the second end of the hull to provide a flow path for superheated propulsion gas exiting the nuclear propulsion reactor.

The invention further provides according to a fifth aspect a method of fabricating a nuclear fission reactor structure. The method comprises joining a first portion of each of a plurality of cladding bodies to a lower core plate, wherein each cladding body has an inner surface defining a coolant channel, wherein the lower core plate includes a plurality of openings extending from a first side of the lower core plate to a second side of the lower core plate, and wherein the first portion of each cladding body extends into a different one of the plurality of openings in the lower core plate; sliding each of a plurality of fuel composition bodies are placed over an outer surface of a different one of the plurality of cladding bodies, wherein each fuel composition body has the shape of an annular cylinder, and wherein an inner surface of the annular cylinder of the fuel composition body is oriented toward the outer surface of the cladding body; sliding each of the moderator bodies over an outer surface of a different one of a plurality of fuel composition bodies, wherein, in a cross-section, each moderator body has a periphery having a regular polygonal shape and an inner opening, and wherein a surface of the inner opening of the moderator body is oriented toward an outer surface of the annular cylinder of the fuel composition body; and joining a second portion of the cladding body to an upper core plate, wherein the upper core plate includes a plurality of openings extending from a first side of the upper core plate to a second side of the upper core plate and wherein the coolant channel of the cladding body extends into one of the plurality of openings in the upper core plate, wherein the assembled cladding body, fuel composition body that is radially outward of the cladding body, and moderator composition body that is radially outward of the fuel composition body define a fuel element structure wherein, in each fuel element structure, the cladding body includes a first portion that extends axially past a first axial end of the fuel composition body and a second portion that extends axially past a second axial end of the fuel composition body, wherein an outer surface of a moderator body of a first fuel element structure abuts an outer surface of a moderator body of a plurality of nearest neighbor fuel element structures and wherein a portion of the upper core plate, a portion of the lower core plate, and the cladding body of each fuel element structure form a first portion of a containment structure for the nuclear propulsion reactor.

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. It should be understood that the embodiments depicted are not limited to the precise arrangements and instrumentalities shown.

For ease of viewing, in some instances only some of the named features in the figures are labeled with reference numerals.

<FIG> schematically illustrates an embodiment of a nuclear propulsion fission reactor structure. The nuclear propulsion fission reactor structure <NUM> includes an active core region <NUM>, a core former <NUM>, a reflector <NUM>, and a plurality of neutron absorber structures <NUM>.

The active core region <NUM> includes a plurality of fuel element structures <NUM> and has an axial centerline defining a longitudinal axis <NUM> of the nuclear propulsion fission reactor structure <NUM>. Each fuel element structure <NUM> includes a cladding body <NUM> having an inner surface <NUM> defining a coolant channel, a fuel composition body <NUM> radially outward of the cladding body <NUM>, and a moderator composition body <NUM> radially outward of the fuel composition body <NUM>. When the plurality of fuel element structures <NUM> are arranged within the active core region <NUM>, an outer surface <NUM> of the moderator composition body <NUM> of a first fuel element structure <NUM> abuts an outer surface <NUM> of a moderator composition body <NUM> of a plurality of nearest neighbor fuel element structures <NUM>. This has been illustrated in <FIG> for first fuel element structure 112a and plurality of nearest neighbor fuel element structures 112b-<NUM>. For example, in exemplary embodiments, the side surfaces of the fuel element structures are in direct contact with side faces of adjacent fuel element structures. The collection of outer surfaces <NUM> of the moderator composition bodies <NUM> of the fuel element structures <NUM> that are at the periphery of the active core region <NUM> define the radially outer surface <NUM> of the active core region <NUM>.

A core former <NUM> is radially outward of the active core region <NUM> and a reflector <NUM> is radially outward of the core former <NUM>. A first surface <NUM> of the core former <NUM> is radially inward of a second surface <NUM> of the core former <NUM>. The first surface <NUM> of the core former <NUM> is conformal to the radially outer surface <NUM> of the active core region <NUM> and the second surface <NUM> of the core former <NUM> is conformal to a radially inner surface <NUM> of the reflector <NUM>. The radially inner surface <NUM> of the reflector <NUM> is oriented toward the active core region <NUM>, and the core former <NUM> functions to mate the geometry of the radially outer surface <NUM> of the active core region <NUM> to the geometry of the radially inner surface <NUM> of the reflector <NUM>.

A plurality of neutron absorber structures <NUM> is located within a volume of the reflector <NUM>. The neutron absorber structures <NUM> include a neutron absorber body <NUM> 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 exemplary embodiments, the first position is radially closest to the active core region and the second position is radially farthest from the active core region. In the embodiment shown in <FIG>, the neutron absorber body <NUM> is shown in the first, radially closest position. For illustration, the second, radially distal position is shown using phantom lines (see second position illustrated as <NUM>'). The neutron absorber body <NUM> is movable between the first position and the second position to control the reactivity of the active core region <NUM>. In the illustrated example, the neutron absorber body <NUM> is rotatable from the first, radially closer position (corresponding to the location shown for neutron absorber body <NUM> in <FIG>) to the second position <NUM>' by rotation (R) around axis <NUM> of the neutron absorber structure <NUM>. However, other radial positions and/or movement directions can be implemented as long as the various positions to which the neutron absorber body <NUM> can be moved provides control of the reactivity of the active core region <NUM>. In some embodiments, when the plurality of neutron absorber bodies <NUM> are each at the first, radially closer position, each of the plurality of neutron absorber bodies <NUM> are radially equidistant from the axial centerline of the active core region <NUM>.

<FIG> is a partial, top side view of a portion of a nuclear propulsion fission reactor structure. In <FIG>, a portion of the active core region <NUM> is shown, as are the core former <NUM>, the reflector <NUM>, and a portion of neutron absorber body <NUM> in a neutron absorber structure <NUM>. Several of the features already shown and described in connection with <FIG> are also shown in <FIG>.

For example, <FIG> illustrates the plurality of fuel element structures <NUM> (arranged with a plurality of nearest neighbor fuel element structures) each with a cladding body <NUM>, a coolant channel <NUM>, a fuel composition body <NUM>, and a moderator composition body <NUM>. The plurality of fuel element structures <NUM> are collectively with translational symmetry. An example of translation symmetry is the tri-pitch design illustrated in <FIG>. In the tri-pitch design, there is translational symmetry between the fuel element structures <NUM> by which a feature on one fuel element structure <NUM> is repeated on other fuel element structures <NUM> at a constant distance (or constant pitch). Based on whether one is comparing, for example, between nearest neighbors, or next nearest neighbors, the repeated structure can be at a multiple of the pitches. An example suitable pitch is <NUM> to <NUM>, alternatively <NUM> to <NUM> or, further alternatively, is <NUM>, depending on material selection and performance. <FIG> illustrates an example of the tri-pitch design. The center axis of the coolant channel <NUM> is at distance P1 from each of the center axis of the coolant channel <NUM> of a nearest neighbor fuel element structure <NUM>, and at a distance P2 (where P2 = <NUM> × P1) from a next nearest neighbor. The plurality of fuel element structures <NUM> each have a cross-section with a polygonal shape. In the illustrated example, the plurality of fuel element structures <NUM> each have a hexagonal cross-sectional shape. Other regular polygonal shaped cross-sections can also be implemented; however, other shapes may require multiple shapes in one active core region design, e.g., a combination of octagons and squares.

Also for example, <FIG> illustrates the first surface <NUM> of the core former <NUM> being conformal to the radially outer surface <NUM> of the active core region <NUM>. Similarly, <FIG> illustrates the second surface <NUM> of the core former <NUM> being conformal to a radially inner surface <NUM> of the reflector <NUM> and the second surface <NUM> of the core former <NUM> is conformal to a radially inner surface <NUM> of the reflector <NUM>.

Additional structure illustrated in <FIG> is the neutron absorber body <NUM> of the neutron absorber structure <NUM>. <FIG> also show embodiments of the neutron absorber structure <NUM> in the reflector <NUM> and related features. As noted herein, the neutron absorber structures <NUM> are located within a volume of the reflector <NUM>, where the volume is defined by the radially inner surface <NUM>, the radially outer surface <NUM>, and the top surface <NUM> and bottom surface <NUM> (not shown) of the reflector <NUM>. As shown in <FIG> and <FIG>, the neutron absorber structure <NUM> includes a cylindrical drum <NUM> encased in a tube <NUM>. The neutron absorber body <NUM> occupies a first portion of the cylindrical drum <NUM>. This first portion of the cylindrical drum <NUM> is a volume of the cylindrical drum <NUM> that includes a portion of an exterior surface of the cylindrical drum <NUM> (such as, for example, a <NUM> degree arc of a circumference of the cylindrical drum). When the tube <NUM> and the cylindrical drum <NUM> move as a unit, such as rotating (R) relative to an inner diameter surface of the reflector absorber housing, the cylindrical drum <NUM>, the neutron absorber body <NUM>, and the tube <NUM> slide along the inner surface of the reflector absorber housing. A motor (not shown in <FIG> and <FIG>) can be operatively attached to the tube <NUM> by a drum shaft to rotate the neutron absorber structure <NUM>. <FIG> illustrate examples of a first position of the neutron absorber body <NUM> being radially closer to the active core region than the second position (compare <FIG> showing the neutron absorber body <NUM> in a first position to <FIG> showing the neutron absorber body <NUM> in a second position), in this case with the first position in <FIG> being radially closest to the active core region <NUM> and the second position in <FIG> being radially farthest from the active core region <NUM>.

The cylindrical drum <NUM> other than the portion occupied by the neutron absorber body <NUM>, i.e., a second portion of the cylindrical drum, functions as a secondary reflector. In some embodiments, the secondary reflector can be manufactured of the same material as the reflector <NUM> so that the reflector <NUM> has a substantially uniform neutronics characteristic across the radial cross-section of the reflector <NUM> (whether that radial cross-section includes a neutron absorber structure <NUM> or not). In specific embodiments, the reflector <NUM> and the cylindrical drum <NUM> 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 <NUM> are made of different materials. Materials suitable for neutron absorber body <NUM> include B<NUM>C, europium(III) oxide and dysprosium(III) oxide. Materials suitable for the tube <NUM> include most forms of steel, molybdenum, tungsten and other exotic alloy combinations. However, other materials can be used as long as they do not materially interfere with the neutron absorbing function of the neutron absorber body <NUM>. In specific embodiments, the tube <NUM> is a stainless steel tube.

Other features of the example embodiment of the nuclear propulsion fission reactor structure <NUM> include an upper core plate and a lower core plate. <FIG> schematically illustrates in cross-section an upper section of the nuclear propulsion fission reactor structure <NUM> including an example of an upper core plate <NUM>. The upper core plate <NUM> includes a first side <NUM> and a second side <NUM> and a plurality of openings <NUM>. The openings <NUM> extend from the first side <NUM> to the second side <NUM> of the upper core plate <NUM>. Each fuel element structure <NUM> (the outer surfaces <NUM> of the moderator body <NUM> of each fuel element structure <NUM> is denoted by dashed lines in <FIG>) is associated with one opening <NUM> such that a first potion of each of the cladding bodies <NUM>, such as first end <NUM>, extends into a different one of the plurality of openings <NUM> and is joined to the upper core plate <NUM>. For example, the first end <NUM> of the cladding body <NUM> can extend at least partially (relative to the length) into the opening <NUM>. Alternatively, the first end <NUM> of the cladding body <NUM> extends into the opening <NUM> a distance that is coextensive with the thickness (T) of the upper core plate <NUM>. After extending the first end <NUM> into the opening <NUM>, the first potion of each of the cladding bodies <NUM> is joined to the upper core plate <NUM> by any suitable means, such as be welding, including resistance welding, full-penetration welding, or by suitable epoxy systems, such as J-B-weld®. As can be seen in <FIG>, the coolant channel <NUM> defined by the inner surface <NUM> of the cladding body <NUM> concentrically mates to one of the plurality of openings <NUM> in the upper core plate <NUM>.

Although not shown in <FIG>, the lower core plate has corresponding features to the upper core plate <NUM>, including a first side and a second side and a plurality of openings that extend from the first side to the second side of the lower core plate <NUM>. The lower core plate <NUM> is attached to a second end of the fuel element structures <NUM> in the same way as upper core plate <NUM> is attached to the first end of the fuel element structures <NUM>. Namely, a second end of the cladding body <NUM> extends into a different one of the plurality of openings in the lower core plate <NUM>. For example, the second end of the cladding body <NUM> can extend at least partially (relative to the length) into the opening in the lower core plate. Alternatively, the second end of the cladding body <NUM> extends into the opening in the lower core plate <NUM> a distance that is coextensive with the thickness of the lower core plate <NUM>. After extending the second end of the cladding body <NUM> into the opening in the lower core plate <NUM>, the second potion of each of the cladding bodies <NUM> is joined to the lower core plate <NUM> by any suitable means, such as be welding, including resistance welding and full-penetration welding, or by suitable epoxy systems, such as J-B-weld®. Also, similar to the upper core plate <NUM>, the coolant channel <NUM> defined by the inner surface <NUM> of the cladding body <NUM> mates to one of the plurality of openings in the lower core plate <NUM>.

As illustrated, in part, in <FIG> and discussed above, at least a portion of the upper core plate <NUM>, at least a portion of the lower core plate <NUM>, and the cladding body <NUM> of each fuel element structure <NUM> form a first portion of a containment structure for the nuclear propulsion fission reactor structure <NUM> (i.e., propellant to fission product separation).

<FIG> show components of an example fuel element structure in unassembled, schematic, perspective view (<FIG>) and in an assembled, cross-sectional view (<FIG> illustrates an embodiment of the cladding body <NUM>. The cladding body <NUM> is substantially in the shape of a tube with an outer surface <NUM> and an inner surface <NUM>, which defines a coolant channel. When the tube is a cylindrical tube, the outer surface <NUM> is an outer diameter surface and the inner surface <NUM> is an inner diameter surface. In some embodiments, the cladding body <NUM> is a continuous, extruded tube that spans the height of the active core region <NUM>. Embodying the cladding body <NUM> as an extruded tube reduces or eliminates seaming and the need for weld joints in the cladding body <NUM>, which reduces the risk of failure of the component during operation as well as reduces manufacturing complexity.

<FIG> illustrates an embodiment of the fuel composition body <NUM>. The fuel composition body <NUM> is substantially in the shape of an annular cylinder with an tube with an outer surface <NUM> and an inner surface <NUM>. The inner surface <NUM> defines a space into which the cladding body <NUM> fits. For example, when the cladding body <NUM> is a cylindrical tube, the inner surface <NUM> is sized to complement the outer diameter surface of the cladding body <NUM>. The composition body <NUM> and the cladding body <NUM> can be joined by, for example, sliding the cladding body <NUM> into the space defined by the inner surfaces <NUM> of the fuel composition body <NUM> and press fitting the two components or joining the two components in a hot isostatic pressing (HIP) operation.

<FIG> illustrates an embodiment of the moderator composition body <NUM>. The moderator composition body <NUM> is substantially in the shape of a polygon-shaped sleeve with a central opening <NUM> and planar outer surfaces <NUM>. In in the illustrated embodiment, the moderator composition body <NUM> is an annular hexagon, although the moderator composition body <NUM> can take other shapes, including other polygon and regular polygon shapes. The annular hexagon shape of the moderator composition body <NUM> includes an outer surface <NUM> and a central opening <NUM> with an inner surface <NUM>. The inner surface <NUM> defines a space into which the fuel composition body <NUM> (or the fuel composition body <NUM> joined with the cladding body <NUM>) fits. The composition body <NUM> and the cladding body <NUM> can be assembled by, for example, sliding the fuel composition body <NUM> (or the fuel composition body <NUM> joined with the cladding body <NUM>) into the space defined by the inner surfaces <NUM> of the moderator composition body <NUM>. No intimate attachment is required between the moderator composition body <NUM> and the outer surface of the fuel composition body <NUM>, although the two components can optionally be joined by, for example, press fitting or hot isostatic pressing.

<FIG> is a cross-sectional view of an embodiment of an assembled fuel element structure <NUM> and showing the relative locations within the assembled fuel element structure <NUM> of the tube-shaped cladding body <NUM>, the annular cylindricalshaped fuel composition body <NUM>, and the moderator composition body <NUM> in the form of a polygon-shaped sleeve.

<FIG> is a schematic, perspective view of a portion of the top side of an active core region showing one end of a plurality of assembled fuel element structure. In the <FIG> view, the assembled fuel element structures <NUM> are assembled in the active core region <NUM> with outer surfaces <NUM> of the moderator composition body <NUM> of the fuel element structure <NUM> abutting outer surfaces <NUM> of the moderator composition bodies <NUM> of a plurality of nearest neighbor fuel element structures <NUM>, which results in an essentially continuous moderator body. In some embodiments, more than one fuel element <NUM> can be manufactured together in a single component, i.e., as active core sections larger than one fuel element <NUM>. The axial end surfaces of the moderator composition body <NUM> and the fuel composition body <NUM> are (relative to each other) in the same plane (or substantially so) and forms a substantially planar upper surface <NUM> of the active core region <NUM>. In each fuel element structure <NUM>, a portion <NUM> of the cladding body <NUM> extends axially past both the axial end of the moderator composition body <NUM> and the fuel composition body <NUM>. <FIG> corresponds to an end of the assembled fuel element structures <NUM> on which the upper core plate <NUM> will be joined.

From <FIG>, one can understand that an opposite end of the assembled fuel element structures <NUM> has a similar substantially planar surface with protruding portions of the cladding body <NUM> extending axially past both the axial end of the moderator composition body <NUM> and the fuel composition body <NUM> and corresponding to an end of the assembled fuel element structures <NUM> on which the lower core plate <NUM> will be joined.

<FIG> is a schematic, cross-sectional view of an embodiment of a reflector. The reflector <NUM> surrounds the active core region <NUM>, mating with the core former <NUM> that bridges the geometry of the outer surface <NUM> of the active core region <NUM> (such as formed by hexagonal surfaces <NUM> of the fuel element structure <NUM>) with the uniform inside annular reflector surface <NUM>. Neutron absorber structures <NUM> are contained within the volume of the reflector <NUM>. In the <FIG> embodiment, the neutron absorber bodies <NUM> are shown with the drum-like cylindrical neutron absorber structures <NUM> being turned inward, in a position corresponding to a shutdown configuration for the nuclear propulsion fission reactor structure <NUM>. The drum-like cylindrical neutron absorber structures <NUM> are constructed so the that neutron absorber bodies <NUM> can be located radially equidistant from the axial centerline of the active core region <NUM> so as to smooth out fission hotspots.

The reflector <NUM> functions to thermalize "reflected" neutrons travelling back into the active core region <NUM> to increase criticality and reduces "leakage" of neutrons, which would have no chance to generate fission reactions and thus lowers the criticality potential of the nuclear propulsion fission reactor structure. Secondarily, the reflector houses the neutron absorber structures <NUM>, which are the primary system for reactivity control. In <FIG>, the embodiment of neutron absorber structures <NUM> are in the form of rotatable control drums. In order to house sufficiently sized neutron absorber structures <NUM> in the form of rotatable control drums to control reactivity, the reflector cannot be overly thin (in width (W) between inner surface <NUM> and outer surface <NUM>). In exemplary embodiments, the width (W) is <NUM> to <NUM> for a beryllium-based reflector. The width may vary based on the materials of the reflector and the weight requirements for non-terrestrial applications of the nuclear propulsion fission reactor structure, with materials with lower neutron reflecting properties requiring a thicker reflector, i.e., a large width (W).

The nuclear propulsion fission reactor structure can further comprise a hull. <FIG> schematically illustrates an embodiment of a nuclear propulsion fission reactor structure <NUM> with a hull <NUM>. The reactor structure, which includes the active core region <NUM>, the core former <NUM>, the upper core plate <NUM>, the lower core plate <NUM>, the reflector <NUM>, and the plurality of neutron absorber structures <NUM>, is housed within an interior volume <NUM> of the hull <NUM>. An upper reactor plate <NUM> is positioned above (or outward) the first side <NUM> of the upper core plate <NUM> and includes a plurality of holes <NUM> for passage of a propulsion gas. The plurality of holes are in fluid communication with the coolant channels <NUM> in the active core region <NUM> such that, after passage of the propulsion gas through the plurality of holes <NUM>, the propulsion gas passes through the coolant channels <NUM> and functions as a coolant for the nuclear propulsion fission reactor structure <NUM>. Similarly, a lower reactor plate <NUM> is positioned below (or outward) the lower core plate <NUM> and includes a plurality of holes <NUM> for passage of the propulsion gas exiting the coolant channels <NUM>.

Also shown in <FIG> are motors <NUM> operatively attached to the cylindrical drum <NUM> of the neutron absorber structures <NUM> by a drum shaft <NUM> to rotate the cylindrical drum <NUM>. In the illustrated embodiment, the motors <NUM> are external to the hull <NUM> and the drum shaft <NUM> penetrates the hull <NUM>, for example by ports or other openings <NUM> in the hull <NUM>.

Embodiments of the hull <NUM> are formed from a sheet of material, such as stainless steel, and can include ribs or other reinforcement structures to provide additional structural support. As seen in <FIG>, the hull <NUM> can be one contiguous component. However, in other embodiments, the hull <NUM> can be multiple components that are then assembled together with fasteners. An inner ledge <NUM> of the hull <NUM> supports the reflector <NUM> and active core region <NUM>. The inner ledge <NUM> can be attached to an interior surface of the hull <NUM> or can be formed by a portion of the interior surface of the hull <NUM>. <FIG> (which is a magnified view of portion P1 in <FIG>) illustrates an example interface between the reactor structure and the hull <NUM>. The lower core plate <NUM> of the active core region <NUM> rests on a lower core plate ledge portion <NUM> of the inner ledge <NUM> and forms a mechanical interface.

During operation and before the throat of the nozzle reaches Mach <NUM>, the incoming flow must overcome the increasing pressure in the converging nozzle section. This can cause potential back-flow conditions if a crack is present below the active core region <NUM>, e.g., in the space within the hull <NUM> below the reactor structure. Therefore, the seal of the mechanical interface between the lower core plate ledge portion <NUM> and the lower core plate <NUM> should be as stable as possible during start-up to prevent leakage into the reflector <NUM> and active core region <NUM>. At operational, steady-state conditions (i.e., when nozzle throat Mach <NUM>) and after shocks have left the diverging section), the acceleration of the flow through the diverging nozzle section will "pull" the flow, causing a seal the to be formed between the lower core plate ledge portion <NUM> and the lower core plate <NUM> due to negative dynamic pressure differential.

The disclosure is also directed to a nuclear thermal propulsion engine that includes the nuclear propulsion fission reactor structure <NUM> within a hull <NUM>. The nuclear thermal propulsion engine further includes shielding, turbo machinery, and a nozzle section attached to or supported by the hull <NUM>, for example, as consistent with that shown and described in connection with <FIG>. In exemplary embodiments, the active core region <NUM> rests on the lower core plate ledge portion <NUM> of the inner ledge <NUM>, the reflector <NUM> is attached by fasteners, such as bolts or pins, to the reflector ledge portion <NUM> of the inner ledge <NUM>, drum shafts <NUM> operatively attach a motor <NUM> (affixed to the hull via, for example, a motor support plate) to the neutron absorber structures <NUM> and pass through openings <NUM> in the top of the hull <NUM>. The nozzle section bolts to the bottom of the hull <NUM> and shielding and turbomachinery is affixed to the top of the hull <NUM>. A reservoir for cryogenically storing a propulsion gas is operatively connected, along with the shielding and turbo machinery, to provide a flow path from the reservoir to the nuclear propulsion reactor and the nozzle section is operatively connected to provide a flow path for superheated propulsion gas exiting the nuclear propulsion reactor.

The nuclear propulsion fission reactor structure (as well as a nuclear thermal propulsion engine including the nuclear propulsion fission reactor structure) can be manufactured using suitable means. In general, the nuclear propulsion fission reactor structure is manufactured by a method that comprises joining cladding bodies to the lower core plate, sliding fuel composition bodies and moderator bodies into place over the radially inner feature, e.g., the fuel composition bodies over the cladding bodies and the moderator bodies over the assembled fuel composition bodies - cladding bodies to form a fuel element structure, and joining the upper core plate to a portion of each cladding body that axially extends past the fuel composition bodies and moderator bodies. Subsequently, a reflector is positioned about an outer surface of the assembled fuel element structures and an inner surface of the reflector is mated to an outer surface of the assembled fuel element structures with a core former.

<FIG> is a flow diagram setting forth foundational steps in an embodiment of a method of manufacturing a nuclear propulsion fission reactor structure. The method <NUM> comprises <NUM> joining a first portion of each of a plurality of cladding bodies to a lower core plate. Each cladding body <NUM> has an inner surface <NUM> defining a coolant channel <NUM> and the lower core plate <NUM> includes a plurality of openings extending from a first side of the lower core plate to a second side of the lower core plate. When joining the cladding body <NUM> to the lower core plate <NUM>, a first portion of each cladding body <NUM> extends into a different one of the plurality of openings in the lower core plate <NUM>. When joining the cladding bodies <NUM> to the lower core plate <NUM>, it is preferable that the whole interface between the first portion and the lower core plate be joined together, for example by welding, to form a continuous metal body including the cladding bodies <NUM> and the lower core plate <NUM>.

The method <NUM> also comprises <NUM> sliding each of a plurality of fuel composition bodies over an outer surface of a different one of the plurality of cladding bodies. Each fuel composition body <NUM> has the shape of an annular cylinder. When the fuel composition body <NUM> has been positioned over an outer surface <NUM> of the cladding body <NUM>, an inner surface <NUM> of the annular cylinder of the fuel composition body <NUM> is oriented toward the outer surface <NUM> of the cladding body <NUM>. Because the first portion of the cladding body <NUM> extends into an opening in the lower core plate <NUM>, the fuel composition body <NUM> is prevented by the lower core plate <NUM> from extending to the same axial position as the end of the cladding body <NUM>. Therefore, the first portion of the cladding body <NUM> extends axially past a first axial end of the fuel composition body <NUM>. To similarly provide a portion of the cladding body <NUM> for joining to an opening <NUM> in an upper core plate <NUM>, a second portion of the cladding body <NUM> extends axially past a second axial end of the fuel composition body <NUM>. After positioning the fuel composition body <NUM> over an outer surface <NUM> of the cladding body <NUM>, the fuel composition body <NUM> and cladding body <NUM> can be affixed or otherwise joined together by, for example, press fitting or hot isostatic pressing (HIP).

The method <NUM> also comprises <NUM> sliding each of the moderator bodies over an outer surface of a different one of a plurality of fuel composition bodies. Each moderator body <NUM>, in a cross-section, has a periphery having a regular polygonal shape (in particular embodiments, a hexagonal shape) and an inner opening <NUM>. When the moderator body <NUM> has been positioned over an outer surface <NUM> of the fuel composition body <NUM>, a surface <NUM> of the inner opening <NUM> of the moderator body <NUM> is oriented toward the outer surface <NUM> of the annular cylinder of the fuel composition body <NUM>. No intimate attachment is required between the moderator composition body <NUM> and the outer surface <NUM> of the fuel composition body <NUM>, although the two components can optionally be joined by, for example, press fitting or hot isostatic pressing.

The method <NUM> also comprises <NUM> joining a second portion of the cladding body to an upper core plate. The upper core plate <NUM> includes a plurality of openings <NUM> extending from a first side <NUM> of the upper core plate <NUM> to a second side <NUM> of the upper core plate <NUM>. The second portion of the cladding body <NUM> (which extends axially past an axial end of the fuel composition body <NUM>) is inserted into the opening <NUM> and joined to the upper core plate <NUM>. When joining the cladding bodies <NUM> to the upper core plate <NUM>, it is preferable that the whole interface between the second portion and the upper core plate be joined together, for example by welding, to form a continuous metal body including the cladding bodies <NUM> and the upper core plate <NUM>. It should be noted that the joining of the cladding bodies <NUM> of each fuel element structure to the upper core plate <NUM> and lower core plate <NUM> a portion of the upper core plate forms a first portion of the containment structure for the nuclear propulsion fission reactor structure <NUM>. Also, because inner surfaces <NUM> of the cladding bodies <NUM> define coolant channels <NUM>, having the first and second portions inserted into openings in the lower and upper core plates, respectively, the coolant channels <NUM> of the cladding bodies <NUM> similarly extend into the openings in the lower and upper core plates.

Each fuel element structure <NUM>, which includes the assembled cladding body <NUM>, fuel composition body <NUM> that is radially outward of the cladding body <NUM>, and moderator composition body <NUM> that is radially outward of the fuel composition body <NUM>, is arranged in the active core region <NUM> such that an outer surface <NUM> of a moderator body <NUM> of a first fuel element structure abuts an outer surface <NUM> of a moderator body <NUM> of a plurality of nearest neighbor fuel element structures, for example, in a tri-pitch relationship.

After assembling the fuel element structures <NUM> in the active core region <NUM>, a reflector <NUM> is positioned about an outer surface <NUM> of assembled fuel element structures <NUM>. The core former <NUM> assists in mating an inner surface <NUM> of the reflector <NUM> to the outer surface <NUM> of the assembled fuel element structures <NUM>. The reflector <NUM> forms a second portion of the containment structure for the nuclear propulsion fission reactor structure <NUM> as the core former <NUM> will also mate with the upper core plate <NUM> and the lower core plate <NUM>.

It should be noted that other features and structures of the nuclear propulsion fission reactor structure <NUM> can be manufactured as part of the method or supplied for use in the method. Thus, the method <NUM> can optionally include one or more of any of the following: forming the plurality of cladding bodies <NUM>, forming the plurality of fuel composition bodies <NUM>; and forming the plurality of moderator bodies <NUM>. Forming the plurality of cladding bodies <NUM> is by any suitable technique, including metal working techniques such as extrusion. Forming the plurality of fuel composition bodies <NUM> can be by any suitable technique, including a fuel compaction technique or an additive manufacturing technique. Forming the plurality of moderator bodies <NUM> can be by any suitable technique, including powder compaction or an additive manufacturing technique.

In some manufacturing methods or steps in manufacturing methods, features and structures (or portions thereof) of the nuclear propulsion fission reactor structure <NUM> are manufactured as an integral, unitary structure using, for example, an additive manufacturing process. As used herein, additive manufacturing processes include any technologies that build 3D objects by adding material on a layer-upon-layer basis. An example of a suitable additive manufacturing process utilizes 3D printing of metal alloys, such as molybdenum-containing metal alloy, Zircaloy-<NUM> or Hastelloy X, or 3D printing of ceramics, such as uranium or beryllium oxide, to form the noted structural features such as the cladding or fuel. In other embodiments, the fissionable nuclear fuel composition and/or the thermal transfer agent and/or the moderator materials and/or poisons used as part of the nuclear propulsion fission reactor structure <NUM> can be included within the integral, unitary structure when suitable multi-material, additive manufacturing processes with multiple metals and ceramics within the feedstock are employed. If the molten metal is not included in the additive manufacturing process, the additive manufacturing process can be paused, a volume of molten metal placed into the fuel cavity (either in liquid or solid form) and the additive manufacturing process continued to complete the structure of the closed chamber. Other alloys that can be used when suitable multi-material, additive manufacturing processes with multiple metals within the feedstock are employed include: steel alloys, zirconium alloys, and molybdenumtungsten alloys (for the cladding and/or for the containment structure); beryllium alloys (for the reflector); and stainless steel (for the containment structure). Even when not manufactured by an additive manufacturing process, the above materials can be used in manufacturing the various features and structures disclosed herein.

Additionally, although the disclosed reactor and core have complex mechanical geometries, integral and iterative manufacturing on a layer-by-layer basis using additive manufacturing techniques, such as 3D printing, of elemental metal or metal alloys enables the structure and features disclosed herein to be more easily manufactured.

Additive manufacturing techniques for the manufacture of integral and unitary structures can include the additional steps of: (a) predictive and causal analytics, (b) in-situ monitoring combined with machine vision and accelerated processing during the layer-by-layer fabrication of the structure, (c) automated analysis combined with a machine learning component, and (d) virtual inspection of a digital representation of the as-built structure. In addition, additive manufacturing technology can create complex geometries and, when coupled with in-situ sensors, machine vision imagery, and artificial intelligence, allows for tuning of the manufacturing quality as the components are built on a layer-by-layer additive basis (often, these layers are on the scale of <NUM> microns) and provides predictive quality assurance for the manufacture of such reactors and structures.

As used herein, cladding is the layer of fuel containing features that is located between the coolant and the nuclear fuel. The cladding functions as a safety barrier that prevents radioactive fission fragments from escaping the fuel into the coolant and contaminating it. Some design constraints of cladding include neutron absorption, radiation resistance and temperature behavior. The cladding is typically made of a corrosion-resistant material with low absorption cross section for thermal neutrons. Example materials include Zircaloy or steel, although other materials may be used if suitable to the reactor conditions, such as metallic and ceramic systems (Be, C, Mg, Zr, O, and Si), as well as compositions including molybdenum, tungsten, rhenium, tantalum, hafnium and alloys thereof, including carbides. In some embodiments, the cladding material can be isotope enriched to enhance reactive through reduction of isotopes with higher neutron absorption cross-sections, e.g., molybdenum enriched Mo-<NUM> will have a less parasitic neutron absorption cross-section than elemental molybdenum. In embodiments of the disclosed nuclear propulsion fission reactor structure, the upper and lower core plates are made of cladding material and, preferably have the same composition as the cladding bodies.

The fissionable nuclear fuel composition can be high-assay low-enriched uranium (HALEU) with has a U<NUM> assay above <NUM> percent but below <NUM> percent or can be highly enriched uranium (HEU) with uranium that is <NUM>% or more U<NUM>. A suitable fissionable nuclear fuel composition applicable to the disclosed fuel element structure includes uranium oxide (UO<NUM>) that is less than <NUM>% enriched, uranium with <NUM> wt. % molybdenum (U-10Mo), uranium nitride (UN), and other stable fissionable fuel compounds. Burnable poisons may also be included. Typically, the fissionable nuclear fuel composition is in the form of a ceramic-metal (cermet), such as UO<NUM> with W or Mo and UN with W or Mo. In some embodiments, a molten metal can also function as the "metal" portion of a cermet.

When used, a thermal transfer agent, such as a salt or metal that will be molten at operating temperatures, can be included in the fuel element structure to improve thermal coupling between the fuel composition body and the cladding body. Additionally, a thermal transfer agent can occupy cracks or other defects in the fuel element structure (whether originally present or developing during reactor operation) to promote thermal coupling. Suitable molten metals for inclusion in the disclosed nuclear propulsion fission reactor structure and to be included in the fuel element structure to provide thermal transfer contact includes sodium (Na), sodium-potassium (NaK), potassium (K), iron (Fe), copper (Cu), silver (Ag), lead (Pb), and bismuth (Bi), or alloy compositions thereof.

It is contemplated that various supporting and ancillary equipment can be incorporated into the disclosed nuclear propulsion fission reactor structure and nuclear thermal propulsion engine. For example, at least one of a moderator (such as a zirconium hydride (ZrH), beryllium (Be), beryllium oxide (BeO), water and graphite), a control rod (such as iridium control rod) for launch safety, and a scientific instrument (such as a temperature sensor or radiation detector) can be incorporated into the nuclear propulsion fission reactor structure.

The disclosed arrangements pertain to any configuration in which a heat generating source including a fissionable nuclear fuel composition, whether a fuel element or the fissionable nuclear fuel composition per se, is surrounded by cladding. Although generally described herein in connection with a gas-cooled nuclear thermal propulsion reactors (NTP reactors), the structures and methods disclosed herein can also be applicable to other fission reactor systems.

Nuclear propulsion fission reactor structure disclosed herein can be used in suitable applications including, but not limited to, non-terrestrial power applications, space power, space propulsion, and naval applications, including submersibles.

Claim 1:
A nuclear propulsion fission reactor structure (<NUM>), comprising:
an active core region (<NUM>) including a plurality of fuel element structures (<NUM>) and having an axial centerline defining a longitudinal axis of the nuclear propulsion reactor;
a core former (<NUM>) radially outward of the active core region (<NUM>);
a reflector (<NUM>) radially outward of the core former (<NUM>) and having a radially inner surface oriented toward the active core region (<NUM>); and
a plurality of neutron absorber structures (<NUM>) located within a volume of the reflector (<NUM>),
wherein each fuel element structure (<NUM>) includes a cladding body (<NUM>) having an inner surface defining a coolant channel (<NUM>), a fuel composition body (<NUM>) radially outward of the cladding body (<NUM>), and a moderator composition body (<NUM>) radially outward of the fuel composition body (<NUM>),
wherein an outer surface of a moderator composition body (<NUM>) of a first fuel element structure (<NUM>) abuts an outer surface of a moderator composition body (<NUM>) of a plurality of nearest neighbor fuel element structures (<NUM>),
wherein the core former (<NUM>) has a first surface radially inward of a second surface and the first surface is conformal to a radially outer surface of the active core region (<NUM>) and the second surface is conformal to the radially inner surface of the reflector (<NUM>), and
wherein each of the plurality of neutron absorber structures (<NUM>) includes a neutron absorber body (<NUM>) movable between a first position and a second position, the first position being radially closer to the active core region (<NUM>) than the second position.