Patent Description:
One type of HTGR is known as a "pebble bed" reactor. In this HTGR design, fissionable fuel is encapsulated within multilayered ceramic particles that are themselves encapsulated within multi-layered graphite spheres, referred to as "pebbles. " The spherical nature of the fuel pebbles enables gas to flow between the spheres for extracting heat from the reactor, while providing a core structure that is self assembling. Fuel pebbles are simply loaded into a cylindrically shaped core region that is formed by loosely packed graphite blocks that provide the structural support for the pebbles to remain in a randomly packed cylindrical shape. During refueling spent fuel can be removed simply by pebble unloading device which removes a single pebble at a time before feeding the spent fuel pebble to a spent fuel container using the force of gravity only. Fresh fuel pebbles are loaded into the top of the core barrel also using a gravity feed system.

<CIT> discloses TRISO fuel and pebbles, wherein a carbon bearing reticulated skeleton is chemical vapour infiltrated by nuclear fuel.

Exemplary embodiments provide methods of mass production manufacturing of fuel elements configured for use in a high-temperature gas cooled reactor (HTGR) core. The embodiment manufacturing methods may include forming fuel elements using base portion base portion additive manufacturing methods akin to three-dimensional (3D) printing methods that enable precision placement of fuel particles within a fuel zone of the fuel element structure. The embodiment methods enable efficient and high quality manufacturing of graphite-based fuel elements having a wide variety of shapes and sizes. One particular application of the embodiment methods is manufacturing spherical fuel elements, referred to as "pebbles," for use in pebble bed HTGR cores.

In a particular embodiment, a manufacturing method may include: forming a graphite base portion of the fuel element using 3D printing/additive manufacturing methods; forming a first graphite layer of one of graphite powder, graphite spheres, or a combination of graphite powder and graphite spheres on the graphite base portion using 3D printing/additive manufacturing methods; depositing a first layer of fuel particles on the graphite base portion using a first positioning chuck; forming a second graphite layer of one of graphite powder, graphite spheres, or a combination of graphite powder and graphite spheres on the graphite base portion using 3D printing/additive manufacturing methods; depositing a second layer of fuel particles on the second graphite layer using a second positioning chuck; and forming a graphite cap portion of the fuel element and/or a final graphite layer using 3D printing/additive manufacturing methods, wherein the first positioning chuck places fuel particles in particular locations on the first layer spaced apart by substantially the same distance, and the second positioning chuck places fuel particles in particular locations of the second layer spaced apart by substantially the same distance and vertically offset from the positions of fuel particles in the first layer.

In some embodiments, the graphite base portion of a fuel pebble may be formed by sequentially forming layers of graphite with increasing radii using 3D printing/additive manufacturing techniques to form a portion of a sphere. In some embodiments, a fuel zone of a fuel pebble may be formed by repeatedly laying down layers including fuel particles in the manner of the first and second layers using 3D printing/additive manufacturing methods and position chucks of different geometries to form an approximately spherical fuel zone. In some embodiments, forming a graphite cap portion of the fuel pebble on the second graphite layer may be accomplished by forming a plurality of layers of incrementally smaller radii using 3D printing/additive manufacturing methods.

The various embodiments enable the use of tri-structural-isotropic (TRISO) fuel particles that do not have an overcoat in fuel elements. Exemplary embodiments include fuel elements (e.g., fuel pebbles) formed by the method summarized above.

Exemplary embodiments of the present disclosure provide a fuel pebble configured for use in a pebble bed high-temperature gas cooled reactor core, the fuel pebble comprising: layers of fuel particles; graphite layers disposed between the layers of fuel particles, wherein adjacent fuel particles of at least one of the layers of fuel particles are spaced apart from one another by substantially the same distance.

The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.

The various embodiments provide methods of mass production manufacturing of fuel elements for use in a high-temperature gas cooled reactor (HTGR) core that enable higher production rates, lower cost and higher quality than available in conventional manufacturing methods. Embodiment methods include forming fuel elements (e.g., fuel pebbles) using additive manufacturing methods, such as three-dimensional (3D) printing methods, and precision placement of particles of fuel, burnable poison and/or breeder materials within a fuel zone of the fuel element structure.

The particles placed in the fuel zone of fuel elements may include fuel (i.e., fissile) elements (e.g., U233, U235, Th231, and/or Pu239), breeder elements (e.g., U238 and Th232), burnable poisons (e.g., B, Hf, etc.), and combinations of fuel, breeder and burnable poison elements. As is well known, uranium based fuel includes a mixture of the fissile isotope (U235) and breeder isotope (U238) of uranium. Further, during the life of a reactor, breeder elements/isotopes (e.g., U238 and Th232) transmute to fissile (i.e., fuel) elements (e.g., Pu239 and U233), and thus breeder particles become fuel particles during operation. Also, some reactor designs may include burnable poisons (e.g., B, Th) mixed in with fuel elements. Therefore, for ease of reference the term "fuel particle" is used herein to refer generally to particles that include fuel (i.e., fissionable) elements, breeder elements (i.e., elements that transmute into fissionable elements upon absorbing a neutron), burnable poison elements, and any mixture thereof.

The use of precision placement of fuel particles and 3D printing/additive manufacturing methods to form fuel elements enables more precise controls of the composition the graphite throughout the fuel element and controllable separation of fuel/poison/breeder particles within the fuel zone (i.e., the "packing fraction" of fuel within the fuel zone) of the fuel pebble. Consequently, the embodiment methods enable higher quality fuel elements to be manufactured than feasible with conventional methods, while lowering the costs of manufacturing, increasing yields, and improving the quality control process. The use of 3D printing/additive manufacturing methods enable efficient and high-yield manufacture of fuel elements having complex geometries, such as spherical fuel pebbles. Further, the embodiment methods are suitable for scaling up into production lines capable of manufacturing large volumes of fuel elements cost effectively and with tight quality control.

The various embodiment methods are particularly suitable for manufacturing spherical fuel pebbles for use within pebble bed HTRGs. An example of a pebble bed HTGR reactor is illustrated <FIG>. This illustration shows the placement of fuel pebbles within the fuel zone. The illustrated example of a fuel pebble is a sphere with a diameter of about <NUM> (about the size of a baseball) although fuel pebbles may be larger or smaller, such as about <NUM> to about <NUM> in diameter. Fuel pebbles are loaded within the core barrel in order to create the reactor core. In this embodiment design, a steel pressure vessel includes within it a core barrel that supports graphite reflector blocks, which define an interior space in which the fuel pebbles are positioned to form the cylindrical reactor core. Control rods enter the pressure vessel and pass into the graphite reflector blocks. The coolant gas flows into the pressure vessel through a gas inlet through the graphite reflector blocks and the core formed by the fuel pebbles and out through a gas outlet (not shown in <FIG>). In the example pebble bed reactor illustrated in <FIG>, there may be approximately <NUM>,<NUM> fuel pebbles in the core, each of which may include approximately <NUM> grams of fuel or any heavy metal loading desired.

Fuel pebbles are primarily made of graphite, which provides the neutron moderator for the reactor as well as the structural support for individual fuel particles. Heat from fission is conducted through the fuel pebble to its surface where heat is removed by the cooling gas (e.g., helium or a helium/argon mixture), which flows around the fuel pebbles and out of the core to the energy conversion system (not shown).

<FIG> illustrates a conventional pebble bed reactor fuel pebble <NUM> in cross-section. A fuel pebble <NUM> is made up of a graphite matrix <NUM> that encapsulates a large number of small fuel particles <NUM>, which are visible as the small dots towards the center <NUM> of the fuel pebble <NUM> illustrated in <FIG>. An exterior surface of the fuel pebble may be formed with a ceramic fuel free shell <NUM> or coating that provides structural rigidity and protects graphite from erosion and exposure to oxygen. The outer <NUM> of the fuel pebble <NUM> may be a fuel free zone that does not contain fuel particles <NUM> particles and is made up of only the graphite matrix <NUM> material.

Conventional fuel pebbles <NUM> are manufactured by blending fuel particles <NUM> in graphite that forms the matrix <NUM>. As illustrated in <FIG>, such an uncontrolled process may result in an uneven distribution of fuel particles <NUM> within the matrix <NUM>. Accommodating the inevitable clumping of fuel particles requires reducing limits on the power-density and/or the burn up that fuel pebbles are permitted to experience. This process is also a manual batch process, which restricts the production rate of the fuel pebbles <NUM> and could introduce inconsistent quality of the fuel pebble <NUM>.

As illustrated in <FIG>, a fuel particle <NUM> has a coated multilayer structure with a fuel kernel <NUM> surrounded by multiple layers of ceramic and graphite materials. In particular, the fuel kernel <NUM> may include a fissile oxide (e.g., UO<NUM> or ThO<NUM>/UO<NUM>) or carbide, which is surrounded by a buffer layer <NUM>. The buffer layer <NUM> may include a porous carbon material such as graphite. The buffer layer <NUM> accommodates expansion of the fuel kernel <NUM> and serves as a reservoir for fission gases. The buffer layer <NUM> is surrounded by a dense inner carbon layer <NUM>, e.g., a layer of pyrolytic carbon. The inner carbon layer <NUM> seals the buffer layer <NUM> and attenuates migration of radionuclides. The inner carbon layer <NUM> is surrounded by a ceramic layer <NUM>, e.g., a layer of silicon carbide or zircon carbide. The ceramic layer <NUM> constrains fission products (i.e., retaining fission products within the kernel), thereby preventing fission products from migrating out of the kernel, and improves structural rigidity. The ceramic layer <NUM> is covered by an outer carbon layer <NUM> that may also contain pyrolytic carbon. The outer carbon layer <NUM> acts as a further barrier to fission gas release. Such fuel particles <NUM> may be referred to as Tri-structural-ISOtropic (TRISO) fuel particles. The multilayer structure of fuel particles has been well tested and characterized in earlier HTGR designs, and exhibit very good performance for retaining fission products under extreme temperature conditions.

Further details of an example fuel pebble and its included fuel particles are listed in the table shown in <FIG>. This table lists example diameters of the fuels user, thicknesses of the outer graphite fuel free shell, diameters and densities of the fuel kernels, example materials making up the coatings of the fuel particles, and example fuel loadings.

Conventionally, graphite fuel pebble forming techniques involve labor intensive processes that require a number of interim quality checks in order to ensure that human error does not result in poor quality pebbles. Referring again to <FIG>, one of the key challenges in manufacturing a fuel pebble is insuring the fuel particles <NUM> are homogenously distributed throughout the interior portion <NUM> graphite matrix of the fuel pebble <NUM>. Conventionally, homogenization performed by randomized mixing of the graphite matrix before the fuel pebble is formed.

However, this process relies on achieving a statistical homogeneous mixture of the coated particles and the graphite matrix material. A further limitation of this process is the fact that it is not well suited for mass production of fuel pebbles. In addition, such techniques are not capable of precisely locating fuel particles within a pebble.

Conventional pebble forming techniques involve a core pressing step and a fuel free zone pressing step. Due to these pressing steps, the fuel particles used in such techniques require an additional overcoat layer to withstand the high pressures applied during the pressing steps and to insure sufficient fuel particle spacing in a randomized fuel particle/graphite matrix.

To address these limitations of conventional methods, the various embodiments include a method of forming a fuel pebble that enables the precise placement of fuel particles while simplifying the fabrication of the matrix that holds the fuel particles and/or burnable poison particles within the core and the graphite fuel free shell that surrounds the core. In particular, the various embodiments provide a method of using 3D printing/additive manufacturing methods to form fuel pebbles from a fine graphite powder and/or graphite particles or spheres about the same size as fuel particles. Fuel elements of various shapes and sizes, such as approximately spherical fuel pebbles, are formed by using 3D printing/additive manufacturing techniques to bind thin layers of graphite into discs of varying diameters. The formed spherical fuel pebbles are then compressed using an isostatic press according to conventional manufacturing methods to form the finished fuel pebble.

The method various embodiment methods enable the controlled positioning of fuel particles and/or burnable poison particles within a fuel zone formed in a layer-by-layer format that can control the separation distance between fuel particles within layers and between layers. This method also reduces the waste as the supporting powder is removed and reused, in contrast to conventional fuel pebble manufacturing processes that require the pebble to be cut to size by a lathe after pressing resulting in wasted graphite shavings.

Additionally, by controlling the placement of fuel, poison and breeder particles within fuel elements (e.g., fuel pebbles), the various embodiment manufacturing methods do not need to account for the stresses applied to fuel particles when two or more particles are touching during the application of high pressures involved in the pebble forming processes. Eliminating the potential for high local stresses due to particles pressing against one another eliminates the need for the additional overcoat on fuel particles conventionally applied to give fuel particles sufficient strength to resist such stresses. Therefore, in contrast to conventional pebble forming techniques, the various embodiments may utilize fuel particles that do not include an overcoat, such as the non-overcoated TRISO fuel particles described herein. This reduces a step in the manufacturing of TRISO fuel particles, thereby decreasing costs and increasing process yields.

<FIG> illustrates side sectional view of a fuel pebble <NUM> according to various embodiments of the present disclosure. <FIG> illustrates a top sectional view of a fuel pebble <NUM>, taken along a fuel particle layer 532D of <FIG> and showing underlying fuel particle layers 532C-A. Again, the various embodiments may be used to manufacture fuel elements of a variety of shapes and sizes and are not limited to the manufacture of spherical fuel elements.

Referring to <FIG> and <FIG>, a fuel pebble <NUM> includes a fuel zone <NUM> and a fuel free shell <NUM> disposed around the fuel zone <NUM>. The fuel zone <NUM> and the fuel free shell <NUM> are both formed by depositing graphite layers <NUM> with a binder applied to hold the graphite together into a shape until final processing (e.g., sintering). For example, the same graphite layer may form a layer of the fuel zone <NUM> and a layer of the fuel free shell <NUM>. As such, in contrast to conventional fuel pebbles, the same processes are used to form the fuel zone <NUM> and the fuel free shell <NUM>, enabling fuel pebbles to be formed (prior to compression and sintering) in a single continuous process. By using 3D printing/additive manufacturing methods to apply the binder to each layer, the shape of fuel pebble <NUM> may be controlled. In some embodiments as illustrated in the figures, the fuel pebble <NUM> may be spherical. Again, fuel elements manufactured according to various embodiments are not limited to any particular shape or size. For example, fuel elements manufactured according to the various embodiments may be ovoid, pill-shaped, prismatic, columnar, or conical. Some fuel elements may be manufactured using embodiment methods with interior passages for flowing coolant through the fuel element by not depositing graphite at the locations of the interior passages. In some embodiments, the shape of the fuel pebble <NUM> may be selected in accordance with corresponding characteristics thereof, such as fission rate and/or heat convention characteristics.

In addition to fissionable and transmutable heavy metal fuel, the fuel zone <NUM> may be loaded with burnable poisons to control reactivity as is well known. Burnable poisons may be included in fuel pebbles in a variety of manners, one of which is in the form of particles included in the fuel zone with fuel particles, and another of which is in the form of mixtures within fuel particles. In such embodiments, burnable poison particles may be placed within layers of the fuel zone <NUM> in the same manner as fuel particles. For example, one or more layers of the fuel zone <NUM> may include both fuel particles (e.g., fissile particles) and burnable poison particles. A pattern in which the fuel and burnable poison particles are arranged in a particular layer may be substantially regular, or may vary according to location within the fuel zone <NUM>. In other embodiments, the fuel zone <NUM> may also include breeder particles, such as thorium fertile particles. Therefore, for ease of description, all particles placed in layers of the fuel zone <NUM> are referred to herein as "fuel particles," regardless of whether the particles include or are substantially only burnable poisons or breeder particles. Thus, references to fuel particles in the embodiment descriptions and the claims are not intended to exclude the inclusion of burnable poisons within particles. In other words, references to fuel particles in the descriptions and the claims are intended to encompass particles containing only fuel, particles containing some fuel and some burnable poisons, and particles containing only burnable poisons.

The fuel zone <NUM> includes fuel particles <NUM> disposed between and/or embedded in the graphite layers <NUM>. The fuel particles <NUM> may be similar to the fuel particles <NUM> described above. In some embodiments, the fuel pebble <NUM> will have a diameter of from about <NUM> to about <NUM>, for example, about <NUM> once completed i.e. pressed and sintered. The fuel free shell may have a thickness of about <NUM> to about <NUM>, for example, about <NUM>. However, the various embodiments are not limited to any particular fuel pebble dimensions. The process allows for the manufacturing of any diameter of the fuel zone <NUM> and final fuel pebble <NUM>.

The fuel particles <NUM> are arranged in fuel particle layers <NUM>, which are separated by, and/or imbedded in, the graphite layers <NUM>. For purposes of illustration, only four fuel particle layers <NUM> (layers 532A-532D) are shown in <FIG> and <FIG>. However, the fuel pebble <NUM> may comprise any number of fuel particle layers <NUM> sufficient to distribute the fuel particles <NUM> throughout the fuel zone <NUM>. For example, the fuel pebble <NUM> fuel zone <NUM> may include from <NUM> to <NUM> fuel particle layers <NUM>, from <NUM> to <NUM> fuel particle layers <NUM>, from <NUM>-<NUM> fuel particle layers <NUM>, i.e., about <NUM> fuel particle layers <NUM>. It is also possible to vary the particle patterns according to a given fuel power density and thus, various particle patterns are within the scope of the present disclosure.

While the fuel pebble <NUM> is described as being fabricated in distinct graphite layers <NUM>, once the fuel pebble is finished with all manufacturing steps these individual graphite layers <NUM> may not be distinguishable from one another in the fuel pebble <NUM>. Specifically, the processes of compressing and sintering will cause the layers to fuse. In other words, the graphite layers <NUM> may be configured and processed so that they effectively form of a single graphite body in which fuel particle layers <NUM> are disposed, particularly after the fuel pebbles <NUM> are compressed and sintered.

For convenience of explanation, the manufacture of a fuel pebble <NUM> is divided into a base portion <NUM>, a central portion <NUM> that includes the fuel zone, and a cap portion <NUM>, although the processes forming the three portions may be performed in a single continuous process. The base portion <NUM> and the cap portion <NUM> encompass the applied layers that do not include fuel particles, while the central portion <NUM> encompasses layers made up of both a ring of graphite that will form the fuel free shell and a central circular layer including fuel particles that forms the fuel zone. The base portion <NUM> may include a portion of the fuel pebble <NUM> disposed below a first fuel particle layer 532A. In other words, the base portion <NUM> may include portions of the fuel free shell <NUM> and fuel zone <NUM> disposed below a lowermost fuel particle layer 532A. The base portion <NUM> may also include a portion of the fuel zone <NUM> in which the first fuel particle layer 532A is at least partially imbedded. The cap portion <NUM> includes a corresponding portion of the fuel particle <NUM> above an uppermost fuel particle layer <NUM> (not shown). The central portion <NUM> includes a remaining portion of the fuel pebble <NUM>. The base portion <NUM> and the cap portion <NUM> may have a cord dimension equal to the fuel free shell thickness in the central portion <NUM>.

The fuel particles <NUM> in each fuel particle layer <NUM> are disposed in a pattern, such as by using a placement chuck that positions individual fuel particles in a graphite layer such that adjacent fuel particles <NUM> are regularly spaced apart from one another by a first distance. In addition, the layers <NUM> may also be regularly spaced apart from one another by the layer thickness with particles in each layer offset from one another to provide a second separation distance. The first and second distances may be the same or different. The first and second distances may depend upon the total heavy metal loading of the fuel pebble <NUM>. Fuel particles <NUM> may be placed in layers in a regular pattern such as hexagonal or square to form three-dimensional packing patterns, with the separation distances and packing patterns selected to achieve design objectives. In some embodiments, the fuel particles <NUM> may be disposed, such that a minimum distance between adjacent fuel particles <NUM> in the same layer <NUM>, and between adjacent fuel particles <NUM> in different adjacent layers <NUM>, is the same.

Said another way, the fuel particle layers <NUM> may be spaced apart and patterned such that the fuel particles <NUM> are separated from adjacent fuel particles <NUM> in three dimensions by a minimum distance that depends upon the total heavy metal loading of the fuel pebble <NUM>. For example, when viewed from perspective in <FIG>, the fuel particles <NUM> of layer 532C may be disposed between the fuel particles <NUM> of layer 532D. In other words, the fuel particles of adjacent layers, such as layers 532D and 532C, may be disposed so as not to directly overlap one another in a vertical direction. The distribution and spacing of fuel particles within fuel elements are not limited to the examples illustrated in the figures. In various embodiments, the fuel particles may be positioned according a variety of different layer patterns, particularly patterns that enable the fuel particles <NUM> to be regularly spaced apart from adjacent fuel particles <NUM>. For example, fuel particles may be positioned in fuel elements using the embodiment methods in one of two regular lattices that achieve highest average density; face-centered cubic (fcc) (also called cubic close-packed), or hexagonal close-packed (hcp). Both lattices are based upon sheets of spheres (i.e., fuel particles in this case) arranged at the vertices of a triangular tiling, differing in how the sheets are stacked upon one another. The fcc lattice is also known to mathematicians as that generated by the As root system.

However, in some embodiments, the fuel particles <NUM> may be disposed within fuel pebbles <NUM> in other patterns. In particular, the fuel particles may be deposited at different densities in different regions of the fuel zone <NUM> of the fuel pebble. For example, the manufacturing methods of the various embodiments may be used to place fuel particles in specific locations within fuel pebbles so that as distance from the center of the fuel zone <NUM> increase, the density of the fuel particles <NUM> may also increase. Manufacturing fuel pebbles in this manner may provide power density benefits, such as by helping to control the peak temperature at the center of fuel pebbles while enabling a higher fuel particle loading in fuel pebbles. Such as a change in density of fuel particles with radial position may be consistent (e.g., may change linearly or exponentially with distance from the center of the fuel zone <NUM>). For example, the fuel particle density of the fuel zone <NUM> may increase from the center to the outer surface thereof. In other embodiments, the particle density of the fuel zone <NUM> may be stepped as a function of distance from the center of the fuel pebble. For example, the manufacturing methods of the various embodiments may be used to form a central region having a relatively low fuel particle density within the fuel zone <NUM>, and a peripheral region that surrounds the central region that has a relatively high fuel particle density. In other embodiments, the manufacturing methods of the various embodiments may be used to form the fuel zone <NUM> with multiple concentric peripheral regions with different fuel particle densities.

Variations in the fuel particle density within fuel pebbles may be accomplished by omitting a number of fuel particles <NUM> from a particular region of the fuel zone <NUM>, such as the central region of the fuel zone <NUM>, but otherwise maintaining the spacing of the fuel particles <NUM> in the fuel particle layers. In the alternative, fuel particle density may be adjusted by varying the spacing between adjacent fuel particles <NUM> in different regions of the fuel zone <NUM>. For example, the distance between fuel particles <NUM> in the central region may be greater than that of one or more peripheral regions of the fuel zone <NUM>. The ability of the manufacturing methods of the various embodiments to place fuel particles in specific locations within fuel pebbles enables a wide variety of fuel particle loading configurations to be used.

<FIG> illustrates a perspective sectional view of a fuel pebble including a composite particle layer <NUM> manufactured according to various embodiments. Referring to <FIG>, the composite particle layer <NUM> may include first particles <NUM> and second particles <NUM> disposed in a pattern. The first and second particles <NUM>, <NUM> may be selected from fissile particles, burnable poison particles, and breeder particles, for example.

The relative density of first and second particles <NUM>, <NUM> within the fuel pebble <NUM> may be selected according to particle type. For example, a composite particle layer may include a relatively large number of fissile particles and a relatively small number of burnable poison particles. In the alternative, a composite particle layer may include a relatively large number of breeder particles and a relatively small number of fissile particles. The primary particles <NUM> may be fissile particles and the second particles <NUM> may be.

Fuel pebbles manufactured according to various embodiments may include composite particle layers including different types of particles, including fissile particles, breeder particles, and poison particles. For example, each fuel particle layer of the fuel pebble may include fissile and poison particles disposed in a pattern. The distribution of fuel particles (e.g., fissile particles) and poison particles may also vary in the fuel zone <NUM>. For example, the central region of the fuel zone <NUM> may include a higher density of burnable poison particles (e.g., a higher ratio of burnable poison particles to fuel particles) than one or more peripheral regions of the fuel zone <NUM>. In other words, the one or more of the peripheral regions may have a higher density of fissile particles than the central region. Having a higher density of fuel particles in the peripheral region, and/or having a higher ratio of poison to fuel particles in the central region, may provide improved heat transfer characteristics enabling higher fuel loading in each pebble, and/or may improve fuel utilization of a fuel pebble.

In some embodiments, the first particles <NUM> may be fissile particles and the second particles <NUM> may be burnable poison particles or breeder particles.

As an example, <FIG> illustrates how the manufacturing methods of the various embodiments may be used to form fuel pebbles with a central volume V that has little or no fuel/poison particles. Such a configuration may result in fuel pebbles that exhibit flatter temperature profiles during reactor operations. The manufacturing methods of the various embodiments may also be used to control the density of graphite within the central void V, such as to provide a region of lower strength at the center to enable inward expansion to relieve pressures within fuel pebbles due to fission gasses.

<FIG> illustrates a perspective sectional view of a fuel pebble including a patterned composite particle layer <NUM> that mixes fuel particles 530a with poison or breeder particles 530b manufactured according to various embodiments.

<FIG> is a process flow diagram illustrating a method <NUM> of forming a fuel pebble, according to various embodiments. <FIG> graphically illustrates operations of the method <NUM>. <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> include enlarged views of the operations illustrated in <FIG>.

Referring to <FIG>, <FIG>, and <FIG>, in operation <NUM>, the method <NUM> includes forming a graphite base portion <NUM> of a fuel pebble <NUM>, as shown in <FIG>. In particular, operation <NUM> includes depositing graphite powder on a substrate <NUM> to form a graphite layer <NUM> (step <NUM>). The graphite powder may be deposited using any suitable method, such as, for example, by a slot coating apparatus <NUM>. The graphite layer <NUM> is then pressed (step <NUM>). The graphite layer <NUM> may be pressed using a roller <NUM>. However, any suitable pressing device, such as a vertical press, or the like, may be used. The pressing ensures that the graphite layer <NUM> is evenly packed.

A binder <NUM> may be applied to the graphite layer <NUM> (step <NUM>) such as by using 3D printing/additive manufacturing methods. The binder <NUM> may be liquid binder that can be printed on the graphite layer <NUM> using, for example, an inkjet printer <NUM>. In particular, the binder <NUM> may be an alcohol-based binder or a water-based binder. The binder <NUM> is applied in a particular pattern on the graphite layer <NUM>. For example, the binder <NUM> may be applied in a circular pattern corresponding to a cross-sectional portion of the fuel pebble <NUM>.

Operation <NUM> (i.e., steps <NUM>-<NUM>) may be repeated a number of times to increase the thickness of the graphite layer <NUM> with each layer having a larger diameter (in the case of a spherical fuel element), such that the graphite layer <NUM> forms a base portion <NUM> in form of a portion of a sphere having a cord dimension approximately equal to a thickness of the pebble fuel free shell. For example, each time operation <NUM> is repeated, the thickness of the graphite layer <NUM> may be increased by about <NUM> to about <NUM>, for example, about <NUM>. However, the thickness of the graphite layer <NUM> may be dependent upon the minimum diameter of graphite particles of the graphite layer <NUM>.

This layer forming operation <NUM> may be repeated about <NUM> to about <NUM> times to form the base portion <NUM>, which in spherical fuel elements is a portion of a sphere. For example, operation <NUM> may be repeated, such that the base portion <NUM> has a thickness of from about <NUM> to about <NUM> (i.e., about <NUM>), such that the thickness of the base portion <NUM> corresponds to the thickness of the fuel free shell <NUM>. As described above, the base portion <NUM> may also include a portion of the fuel zone <NUM> disposed below the first fuel particle layer 532A. As such, the thickness of the base portion <NUM> may be increased by an additional amount (or the base portion may include a first layer of graphite to form the central portion <NUM>), such as from about <NUM> to about <NUM> (i.e., about <NUM>), such that the base portion <NUM> includes a portion of the fuel zone <NUM>, in which fuel particles <NUM> may be imbedded, as discussed below. In other words, the base portion <NUM> may have a thickness of from about <NUM> to about <NUM>, i.e., about <NUM> for typical fuel pebble types of fuel elements; more or less for other shapes and sizes of fuel elements.

The amount of graphite powder <NUM> deposited may be varied according to processing conditions and fuel pebble design characteristics. As such, the number of times that the operation <NUM> is repeated may be increased or decreased accordingly.

In some embodiments, instead of or in addition to using graphite powder to form each graphite layers, the graphite may be in the form of graphite spheres about the same size as fuel particles, which may be mixed with graphite powder. Spherical graphite may be particularly beneficial in forming the fuel zone of a fuel element when the graphite spheres are close in size (e.g., approximately the same size) as fuel particles. An example of a suitable form of graphite spheres is disclosed in <CIT>, which discloses compositions and methods of making dustless graphite spheres. Such graphite spheres may be formed of graphite powder and a resin. An advantage of using graphite spheres is that spheres about the same size as fuel particles may better support fuel particles in a layer than a layer of fine graphite powder due to their similar size and density. In some embodiments, the graphite layer <NUM> formed in the various layer-forming steps may be a combination of graphite powder and graphite spheres. In some embodiments, graphite layers <NUM> that do not include fuel particles (i.e., layers forming the base portion <NUM> and top portion of a the fuel pebble) may be formed using fine graphite powder, while the graphite layers that include fuel particles (i.e., the fuel zone of the fuel element) may be formed using graphite spheres or a mixture of graphite spheres and graphite powder. Further, the graphite layers including fuel particles (the fuel zone) may be manufactured by depositing graphite spheres in the central fuel zone portion where fuel and depositing graphite powder in a surrounding portion <NUM> (<FIG>) or <NUM> (<FIG>).

In operation <NUM> illustrated in <FIG>, <FIG>, a deposition apparatus <NUM>, such as a positioning chuck, is used to deposit a layer of fuel particles <NUM> on the base portion <NUM> (step <NUM>) or on a layer of graphite (e.g., graphite powder, graphite spheres, or a combination of graphite powder and graphite spheres) formed on the base portion <NUM>. In particular, the deposition apparatus <NUM> may include at least one deposition head <NUM> configured to pick up and control the placement of fuel particles <NUM> on graphite layers.

The deposition head <NUM> may use a variety or combination of techniques for picking up and holding fuel particles <NUM> include vacuum and electrostatic forces.

In an embodiment, the deposition head <NUM> may include a plurality of vacuum tubes <NUM> arranged in a particular pattern corresponding to a deposition pattern. In such an embodiment a vacuum is applied to the vacuum tubes <NUM> to pickup and hold the fuel particles <NUM> to the deposition head <NUM>. Fuel particles may be released when pressed against a graphite layer by releasing the vacuum or applying pressure through the vacuum tubes <NUM>. Such an embodiment deposition head <NUM> may be referred to as a "vacuum deposition head.

In some embodiments, one or more of the deposition heads <NUM> may use electrostatic forces to pick up and hold fuel particles, which may be referred to as an "electrostatic deposition head. " Fuel particles <NUM> may be picked up by an electrostatic deposition head <NUM> by applying a voltage (positive or negative) to the head or particle-holding portions to attract fuel particles <NUM>, such as into positioning pores as illustrated. The deposition heads <NUM> may be disposed over or pressed onto the base portion <NUM>, and the charge applied to the deposition heads <NUM> may be neutralized or the polarity reversed to release the fuel particles <NUM> onto the graphite.

The deposition head <NUM> is pressed into the base portion <NUM> to imbed the fuel particles <NUM> into a layer of graphite formed on the base portion. As discussed above, this layer of graphite may be a layer of graphite powder, graphite spheres, or a combination of graphite powder and spheres that is applied over the base portion. After pressing the fuel particles <NUM> into the graphite layer, the vacuum is removed to release the fuel particles <NUM>. After pressing, the fuel particles <NUM> are pressed into the base portion <NUM> and/or the graphite layer is compressed, using a roller <NUM>, for example (step <NUM>). In this manner, the lower fuel particle layer 532A is deposited.

The deposition apparatus <NUM> may include any number of the deposition heads <NUM>. For example, the deposition apparatus <NUM> may include one deposition head <NUM> for each distinct pattern of fuel particles <NUM> in a fuel particle layer <NUM>. In the alternative, the deposition apparatus <NUM> may include one or more arrays <NUM> of deposition heads <NUM>, with the deposition heads <NUM> of each array having the same pattern, and different arrays having different patterns. Additionally or alternatively, the individual vacuum tubes may be actuated in order to vary the pattern of fuel particles picked up and applied in each layer in order to form a pattern. The deposition apparatus <NUM> may include actuators <NUM> to raise and lower the array <NUM>.

Once the layer of fuel particles <NUM> is deposited, the method proceeds to operation <NUM>, as illustrated in <FIG>, <FIG>, and <FIG>. In operation <NUM>, a graphite layer <NUM> is formed by depositing graphite particles on the base portion <NUM>. In particular, graphite particles are deposited to form a graphite layer <NUM> covering the fuel particles <NUM> (step <NUM>). In particular, the graphite layer <NUM> may operate to fill holes formed by pressing the fuel particles <NUM> in step <NUM>. The graphite layer <NUM> may be pressed (step <NUM>). The binder <NUM> is then applied (step <NUM>).

Operation <NUM> (steps <NUM>-<NUM>) may be repeated multiple times in order to increase the thickness of the graphite layer <NUM>, thereby forming a graphite layer <NUM> on the base portion <NUM>. For example, operation <NUM> may be repeated from about <NUM> to <NUM> times, such as about <NUM> times. Therefore, the graphite layer <NUM> may have a thickness ranging from about <NUM> to <NUM>. As such, the graphite layer <NUM> may be thinner than the base portion <NUM>.

In operation <NUM>, the process may determine whether the fuel particle layer deposition is complete (i.e., whether additional fuel particle layers <NUM> remain to be deposited). If additional fuel particle layers <NUM> remain to be deposited, the method returns to operation <NUM> and additional fuel particle layers <NUM> and graphite layers <NUM> may be deposited. If no additional fuel particle layers <NUM> remain to be deposited, the method proceeds to operation <NUM>. Operation <NUM> is optional as the process may be implemented such that the layers are performed in a defined manner obviating the need for a determination.

In operation <NUM>, as shown in <FIG>, the cap portion <NUM> is formed. In particular, the cap portion <NUM> is formed by depositing a layer of graphite particles (step <NUM>), pressing the graphite layer (step <NUM>), and then depositing a binder (step <NUM>), in a manner similar to operation <NUM> described above. Operation <NUM> (steps <NUM>, <NUM>, and <NUM>) may also be repeated multiple times, as described for operation <NUM>. As a result, the cap portion is formed.

In operation <NUM> illustrated in <FIG>, <FIG>, loose graphite powder is removed (step <NUM>), thereby exposing the fuel elements, e.g., fuel pebbles <NUM>. The fuel elements are then processed in an isostatic press <NUM> (step <NUM>) that applies pressure to the fuel pellets <NUM>. The isostatic pressing may result in a reduction in size of the fuel elements. For example, the diameter of the fuel pebbles <NUM> may be reduced from <NUM> to <NUM> by the isostatic pressing.

In operation <NUM> illustrated <FIG> and <FIG>, the fuel elements (e.g., fuel pebbles <NUM>) are sintered in a high-temperature oven <NUM> (step <NUM>). The fuel elements may be sintered at a temperature ranging from <NUM> to <NUM>, for example <NUM>. After sintering, the manufacturing process may be complete and the fuel elements may be ready for quality checks.

In some embodiments, a sintering binder may be applied to the fuel elements prior to sintering. In particular, a sintering binder may be mixed with the graphite powder or could be included in the binder <NUM>. In other embodiments, the binder <NUM> may be a sintering binder.

In various embodiments, the binder <NUM> may be deposited via <NUM>-D printing/additive manufacturing in steps <NUM>, <NUM>, and <NUM>. In particular, the binder <NUM> may be deposited such that the fuel element may be formed in three dimensions, from the deposited graphite powder, in a layer-by-layer fashion. Again, using 3D printing/additive manufacturing to apply the binder and building up fuel elements in a layer-by-layer fashion enables the manufacturing process to form fuel elements in a variety of sizes and shapes (e.g., spherical as illustrated, ovoid, pill-shaped, prismatic, columnar, and conical) in the same manufacturing line. Some fuel elements may be manufactured using embodiment methods with interior passages for flowing coolant through the fuel element by not depositing graphite at the locations of the interior passages.

The foregoing description of the various embodiment manufacturing methods refers to fuel pebbles as an example of one form of fuel element that may be manufactured using the various embodiments. The various embodiments may be used to manufacture graphite fuel elements of any shape and configuration, including prismatic blocks, rods, pellets, etc. Thus, references to "fuel pebbles" is for ease of describing the manufacturing process and are not intended to limit the scope of the claims to the manufacture of fuel pebbles or spherical fuel elements unless specifically recited in the claims.

The number of layers and configurations of each layer may vary depending upon a variety of factors, including the density and/or amount of graphite deposited in each layer, the packing fraction of fuel within the fuel element, the shape of the fuel element being formed, etc. Thus, the description of various embodiment manufacturing operations and repetitions are not intended to limit the scope of the claims to a certain number of layers or operation cycles unless specifically recited in the claims.

In various embodiments, the manufacturing method may be implemented using robotics and 3D printing/additive manufacturing technology, and may be implemented by an automated process. Such an integrated process may provide for higher throughput, higher product quality, and better product consistency, as compared to conventional methods of manufacturing graphite-based nuclear fuel elements. Further, the method can be implemented to mass-produce fuel elements (e.g., fuel pebbles), such as by forming a production line of stages for each or a few operations and moving a plurality of fuel elements from stage to stage for parallel processing. Also, inspection operations may be added after each or selected ones of the operations described above to assess the quality of each applied layer, and control parameters of each operation may be adjusted in order to maintain a desired quality level or design tolerances for each layer and the fuel pebble as a whole.

Further, the manufacturing methods of the various embodiments enable fuel, poison and breeder particles to be placed in controlled positions within layers of the fuel elements so that the particles are spaced apart within fuel elements, which is an improvement over the randomly placed fuel particles of the conventional art. In particular, the manufacturing methods of the various embodiments enable manufacturing fuel elements with fuel particle packing structures that ensure that the fuel particles never contact one another.

Also, as noted above, the various embodiments eliminate the risk of two or more particles contacting each other during the application of high pressures involved in the final processes of manufacturing the fuel elements. As a result, the possibility of particles crushing one another during pressing is eliminated. Therefore, in contrast to conventional graphite fuel element forming techniques, the various embodiments enable the use of fuel particles that do not include an overcoat, such as the non-overcoated TRISO fuel particles described above. This eliminates a process step and cost of fuel particles, resulting in more efficient fuel element manufacturing.

Claim 1:
A manufacturing method of mass-producing fuel elements configured for use in a high-temperature gas cooled reactor core, wherein it uses additive manufacturing methods and robotic mechanisms to control spatial placement and packing density of particles within a matrix of graphite and comprises:
repeatedly performing a sequence of operations comprising forming a uniform graphite layer on a previous layer and applying a binder using additive manufacturing methods to bind each layer with controlled exterior dimensions to form a base portion of the fuel elements;
repeatedly performing a sequence of operations comprising depositing a uniform graphite layer of one of graphite powder, graphite spheres, or a combination of graphite powder and graphite spheres over a previous layer, forming a layer of particles on the uniform graphite layer within a fuel zone so that the particles are spaced apart in a predefined pattern, and applying a binder using additive manufacturing methods to bind each layer with controlled exterior dimensions to form a fuel zone portion of fuel elements; and
repeatedly performing a sequence of operations comprising forming a uniform graphite layer on a previous layer and applying a binder using additive manufacturing methods to bind each layer with controlled exterior dimensions to form a cap portion of fuel elements,
wherein the exterior dimensions of the graphite base portion, the fuel zone portion and the cap portion are controlled so as to form a fuel element of a controlled shape selected from the group consisting of spherical, ovoid, pill-shaped, prismatic, columnar, and conical,
and wherein the particles comprise one or more of a fissile material, a burnable poison material, or a breeder material.