EP4695827A1 - Use of burnable absorbers in graphite matrix of triso-fueled compacts in nuclear cores - Google Patents

Abstract

A composite fueled compact for a nuclear reactor is disclosed. The composite fueled compact comprises particles, a graphite matrix, and a neutron absorber. The particles comprise a fissile fuel. The graphite matrix defines a continuous phase surrounding the fissile fuel. The neutron absorber and the fissile fuel are configured to remain fixed in position relative to each other during reactor operation. A fuel assembly comprising a first group of stationary fueled compacts and a second group of stationary fueled compacts is also disclosed.

Description

TITLE

USE OF BURNABLE ABSORBERS IN GRAPHITE MATRIX OF TRISO-FUELED COMPACTS IN NUCLEAR CORES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of and priority under 35 U.S.C. § 120 to U.S. Patent Application Serial No. 18/299,588 filed April 12, 2023, entitled “USE OF BURNABLE ABSORBERS IN GRAPHITE MATRIX OF TRISO-FUELED COMPACTS IN NUCLEAR CORES,” the contents of which is hereby incorporated by reference in its entirety herein.

GOVERNMENT CONTRACT

[0002] This invention was made with government support under Government Contract No. DE-NE0008853 awarded by the Department of Energy. The government has certain rights in the invention.

BACKGROUND

[0003] Advanced nuclear reactors employing smaller footprints and lower power designs, relative to conventional designs, can be transported to remote locations and deployed therein to generate power onsite. In order to operate nuclear reactors in a safe manner, an adequate management of reactor conditions is necessary to reduce the likelihood of potentially catastrophic reactor runaway states. Conventional reactivity management devices and methods can increase the size and/or complexity of an overall footprint of a reactor, thereby compromising portability and ease of deployment. Therefore, a need exists to develop alternative reactivity management devices and systems to optimize the efficiency and safety of operating advanced nuclear reactors while maintaining the form factor thereof.

SUMMARY

[0004] The following summary is provided to facilitate an understanding of some of the innovative features unique to the aspects disclosed herein and is not intended to be a full description. A full appreciation of the various aspects disclosed herein can be gained by taking the entire specification, claims, and abstract as a whole.

[0005] In various aspects, a composite fueled compact for a nuclear reactor is disclosed. In some aspects, the composite fueled compact includes particles, a graphite matrix, and a neutron absorber. In some aspects, the particles include a fissile fuel. In some aspects, the graphite matrix defines a continuous phase surrounding the fissile fuel. In some aspects, the neutron absorber and the fissile fuel are configured to remain fixed in position relative to each other during reactor operation.

[0006] In various aspects, a fuel assembly for a nuclear reactor core is disclosed. In some aspects, the fuel assembly includes a first group of stationary fueled compacts and a second group of stationary fueled compacts. In some aspects, each of the stationary fueled compacts includes TRISO fuel dispersed in a graphite matrix phase and a burnable absorber. In some aspects, each of the first group and second group of stationary fueled compacts are configured to manage an excess reactivity for the duration of a planned fuel cycle length of the fuel assembly.

[0007] These and other objects, features, and characteristics of the present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of any of the aspects disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The various aspects described herein, together with objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings as follows.

[0009] FIG. 1 illustrates a cross-sectional elevation view of a fuel assembly, according to at least one non-limiting aspect of the present disclosure.

[0010] FIG. 2 illustrates a cross-sectional view of a microreactor core, according to at least one non-limiting aspect of the present disclosure.

[0011] FIG. 3 illustrates a perspective view of a fuel compact according to at least one nonlimiting aspect of the present disclosure.

[0012] FIG. 4 is a radial cross-section of the fuel compact of FIG. 3, in accordance with at least one non-limiting aspect of the present disclosure.

[0013] FIG. 5 is an axial cross-section of the fuel compact of FIG. 3, in accordance with at least one non-limiting aspect of the present disclosure. [0014] FIG. 6 is a cross-sectional schematic representation of a fuel assembly for a nuclear reactor core, in accordance with at least one non-limiting aspect of the present disclosure.

[0015] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate various aspects of the present disclosure, in one form, and such exemplifications are not to be construed as limiting the scope of any of the aspects disclosed herein.

DETAILED DESCRIPTION

[0016] Certain exemplary aspects of the present disclosure will now be described to provide an overall understanding of the principles of the composition, function, manufacture, and use of the compositions and methods disclosed herein. An example or examples of these aspects are illustrated in the accompanying drawing. Those of ordinary skill in the art will understand that the compositions, articles, and methods specifically described herein and illustrated in the accompanying drawing are non-limiting exemplary aspects and that the scope of the various examples of the present disclosure is defined solely by the claims. The features illustrated or described in connection with one exemplary aspect may be combined with the features of other aspects. Such modifications and variations are intended to be included within the scope of the present disclosure.

[0017] Reference throughout the specification to “various examples,” “some examples,” “one example,” “an example,” or the like, means that a particular feature, structure, or characteristic described in connection with the example is included in an example. Thus, appearances of the phrases “in various examples,” “in some examples,” “in one example,” “in an example,” or the like, in places throughout the specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in an example or examples. Thus, the particular features, structures, or characteristics illustrated or described in connection with one example may be combined, in whole or in part, with the features, structures, or characteristics of another example or other examples without limitation. Such modifications and variations are intended to be included within the scope of the present examples.

[0018] In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as “forward,” “rearward,” “left,” “right,” “above,” “below,” “upwardly,” “downwardly,” and the like are words of convenience and are not to be construed as limiting terms. [0019] In a fuel assembly of a nuclear reactor core, fissile fuels such as, for example, uranium- 235 (sometimes referred to hereinafter as “²³⁵U”) interact with an incident neutron flux and upon absorbing an appropriately energetic neutron, such as a thermal neutron, can subsequently fission into a number of lighter nuclei fission products and/or fragments, thereby generating an emission of prompt neutrons and an amount of heat. These prompt neutrons can subsequently be absorbed by other nuclei to propagate another fission event, and so on and so forth. The lifetime of a prompt neutron occurs from the time it is emitted by a fission event to the time that it is absorbed by another nuclei. In order to maximize the likelihood of continued propagation of fission events, a neutron moderator can be positioned in the reactor core to effectively slow down neutrons born at high energies.

[0020] Generally, k_eff represents the ratio of neutrons in a generation to the number of neutrons in a previous generation be quantified as a neutron multiplication factor k_eff. Thus, a k_eff of a reactor core can be indicative of the criticality state thereof. During operation, a nuclear reactor is generally held in a critical state where k_eff = 1 and thus, neutrons are produced and consumed in a self-propagating chain reaction. In the context of nuclear fuel, excess reactivity can be defined as any available reactivity of the nuclear fuel above which is necessary to achieve critical conditions at a given point in time. In a conventional nuclear reactor design, such as a Pressurized Water Reactor (“PWR”), a failure to manage the excess reactivity in a nuclear fuel can result in a supercritical reactor where k_eff > 1 and the rate of neutrons produced exceeds the neutrons consumed. Thus, inadequate management of reactor conditions can result a potentially catastrophic runaway state. Accordingly, the management of excess reactivity and power distribution within a nuclear reactor is crucial in order to maintain safe operating conditions and/or economically adequate fuel cycle length.

[0021] Generally, in a conventional nuclear reactor, such as a PWR, the reactor core can include a large number of fuel assemblies, each of which includes a plurality of elongated fuel elements or fuel rods. For example, FIG. 1 illustrates a cross-sectional elevation view of a fuel assembly 10, according to at least one non-limiting aspect of this disclosure. The fuel assembly 10 includes an organized array of elongated fuel rods 22. The fuel rods 22 can house a plurality of fuel pellets 26 each comprising a fissile material capable of sustaining a nuclear fission chain reaction, such as an enriched uranium-based material. Each of the fuel rods 22 may include a plurality of nuclear fuel pellets 26. The fuel pellets 26 are housed within an elongated cladding 38 tube that is closed at opposite ends by an upper end plug 28 and a lower end plug 30. The pellets 26 may be maintained in a stack by a plenum spring 32 disposed between the upper end plug 28 and the top of the pellet stack. [0022] Still referring to FIG. 1 , the fuel rods 22 may be supported by one or more transverse grids 20 which attach to guide thimbles 18. The guide thimbles 18 extend longitudinally between top nozzle 16 and bottom nozzle 12 and are configured for discrete elongated control rods 34 to operably move therethrough. Opposite ends of the guide thimbles 18 can attach to the top nozzle 16 and bottom nozzle 12, respectively. The bottom nozzle 12 can be configured to support the fuel assembly 10 on a reactor vessel lower core plate 14 in the core region of a reactor. A liquid coolant, such as a solution of water and boric acid, may be pumped to the fuel assembly 10 upwardly through a plurality of flow openings in the lower core plate 14. The bottom nozzle 12 of the fuel assembly 10 may pass the coolant flow to and along the fuel rods 22 of the fuel assembly 10 in order to extract heat generated as a result of the fission reactions occurring therein. The water in the liquid coolant further provides neutron moderation while the soluble boron provides neutron absorption. Alternative reactor designs may operate at higher temperatures, thereby necessitating different coolant systems and/or fuel. For example, Tri-structural Isotropic (“TRISO”) particle based fuels can be employed in large Gas Cooled Reactors (“GCR”), which can operate at temperatures greater than about 700°C.

[0023] A conventional reactivity control system can compensate for changes in excess reactivity over the lifetime of a fuel assembly. For example, each of the displaceable control rods 34 can comprise a neutron absorbing material, such as, for example, boron carbide, and the insertion depth thereof can be varied. A proportion of previously produced neutrons can be absorbed by the control rod 34 based on the insertion depth of a control rod 34, thereby decreasing the potential number of subsequent fission events and thus, introducing negative reactivity, in the vicinity of the inserted portion of the control rod 34. In addition to moving discrete control rods 34, the concentration of a dissolved neutron absorbing material, such as boric acid, in the liquid coolant and the flow rate thereof throughout the entire reactor core can be varied, thereby facilitating management of power distribution in the reactor vessel. In order to make timely and precise adjustments of neutron absorbing elements, reactivity control systems rely on dedicated drive mechanisms and/or pumping systems to manipulate the neutron absorbing components. However, the incorporation thereof into a reactor design generally increases the overall footprint of a reactor vessel and introduces complexities into the reactor core design.

[0024] Advanced nuclear reactor designs, such as, for example, nuclear microreactors, employing smaller scale architectures than traditional PWRs and GCRs, in both size and power output, are emerging as a solution for providing a reliable off-grid power source. For example, the eVinci™ microreactor currently being developed by Westinghouse is comprised of a microreactor vessel built into a dedicated container as an integral package. The space between the microreactor vessel and the container is minimized to provide a preassembled package having a footprint that is optimized for transportation via truck to a final destination. A perspective view of a microreactor vessel 100 cross-section is provided in FIG. 2, in accordance with at least one non-limiting aspect of the present disclosure. The reactor vessel 100 includes a core comprising a radial reflector 110, a fuel assembly 120, and control drums 130. The radial reflector 110 minimizes stray neutron leakage through the vessel 100. Generally, the fuel assembly 120 is operated at high temperatures of about 600°C or greater and can include a structure based on unit cells 122 comprised of a solid material, such as graphite, in which fueled compacts 124 can be inserted. Each of the fueled compacts 124 includes fissile fuel that is able to tolerate the operating temperatures, such as TRISO fuel which can maintain good thermal conductivity with surrounding structures thereby facilitating heat transfer during operation. Thus, the microreactor vessel 100 can maximize the potential power output therefrom while maintaining a space saving geometry. Other example microreactors and operation methods thereof are described in further detail in U.S. Patent Application No. 17/084,365 and in U.S. Patent Application No. 18/057,208, each of which is owned by the Applicant of the present disclosure, and each of which is herein incorporated by reference in its entirety.

[0025] An effective management of excess reactivity and spatial neutron flux distribution in a microreactor can be challenging due to the space constraints between the reactor vessel and the container. For example, drive systems for axially displacing a conventional control rod into a microreactor vessel can occupy a significant amount of space extending from the vessel surface, thereby complicating the logistics of transporting the microreactor to its final destination and/or increasing costs thereof. Thus, an implementation of a conventional control rod in a microreactor can compromise the economic and logistic benefits provided thereby.

[0026] Furthermore, it has been found by the inventors of the present disclosure that discrete movable neutron absorbers are not suitable for controlling reactor power in low neutron flux environments such as a microreactor core. Additionally, microreactor cores generally do not rely on liquid coolant based primary heat transfer systems and therefore, cannot rely on coolant to manage excess reactivity therein.

[0027] While alternative forms of particle based fuel incorporating a burnable absorber functionality into the particles themselves have been proposed, no extensive studies or testing have been performed to date. Additionally, an incorporation thereof into a microreactor design will require an extensive irradiation and new fuel qualification programs in order to be approved by regulatory agencies. Furthermore, as discussed hereinabove, implementing conventional methods and devices for controlling reactivity of a reactor core in a microreactor application can require added ex-vessel components and design complexities therein. Accordingly, various aspects of the present disclosure provide various methods and devices for managing excess reactivity and/or distribution of power in a microreactor core, for example, and maintaining the portability and/or simplicity of the microreactor.

[0028] Now referring to FIG. 3, a perspective view of a composite fueled compact 200 is provided, in accordance with at least one non-limiting aspect of the present disclosure. In various examples, the composite fueled compact 200 is configured with a cylindrical geometry. In some examples, the composite fueled compact 200 is configured to be inserted into a fuel assembly of a microreactor. For example, the composite fueled compact 200 can have an outer diameter substantially the same as, or slightly smaller than, a channel of a microreactor core graphite support structure or a tubular sleeve insertable therein. Other configurations of the composite fueled compact 200 are contemplated by the present disclosure. For example, in some implementations, the composite fueled compact 200 can have a spherical geometry, a tubular geometry, and/or a cross-section geometry circumscribable in a channel of a nuclear reactor designated for nuclear fuel.

[0029] Now referring to FIGs. 4-5, cross-section views of the composite fueled compact 200 of FIG. 3 are provided, in accordance with at least one non-limiting aspect of the present disclosure. FIG 4 depicts a radial cross-section view of the composite fueled compact 200 while FIG. 5 depicts an axial cross-section view of the composite fueled compact 200. The composite fueled compact 200 includes particles 210 and a matrix 220. The composition of the matrix 220 can be configured to moderate fast neutrons. For example, the matrix 220 can be primarily comprised of solid graphite. The matrix 220 defines a fixed continuous phase surrounding the particles 210. Thus, the positioning of each of the particles 210 relative to the matrix and to neighboring particles is fixed. In certain examples, the particles 210 occupy about 5% or more, or about 10% or more, or about 15% or more, or about 20% or more, or about 25% or more, or about 30% or more of the overall volume of the composite fueled compact 200. In one example, the particles 210 occupy about 37% of the overall volume of the composite fueled compact 200.

[0030] In various examples, at least a portion of the particles 210 can include fissile fuel 212. For example, the particles 210 can comprise a core of fissile fuel 212. In some examples, the particles 210 can comprise multiple layers. In certain examples, the fissile fuel 212 can be TRISO fuel. In some aspects, the fissile fuel 212 can be configured to provide optimized fuel cycle lengths in microreactors. For example, the fissile fuel 212 configured as TRISO fuel can include Uranium initially enriched to a level of 5% or more, or 10% or more, or up to about 20%. Fissile fuel 212 configured as TRISO fuel can be incorporated into existing high temperature gas-cooled reactor core designs and operating conditions thereof, each of which have previously undergone extensive irradiation and new fuel qualification programs. Thus, in some aspects, a composite fueled compact 200 incorporating this configuration can avoid a significant investment in time and/or cost associated with implementing untested forms of nuclear fuel.

[0031] Still referring to FIGs. 4-5, in various examples, the composite fueled compact 200 includes an amount of a neutron absorber 230. The neutron absorber 230 and the particles 210 are configured to be fixed in position relative to each other. For example, the neutron absorber 230 can be incorporated into the matrix 220. In certain examples, the neutron absorber 230 can be uniformly or homogeneously distributed throughout the matrix 220. In some aspects, a neutron absorber 230 incorporating this configuration can provide neutron absorption in the vicinity of the particles 210 throughout a planned lifetime of a fissile fuel 212 without requiring a dedicated ex-vessel motion control system. Thus, a fuel compact 200 can be configured to manage excess reactivity of a fissile fuel in a reactor environment, such as, a microreactor core, without increasing the overall footprint thereof. Other configurations of the neutron absorber 230 are contemplated by the present disclosure. For example, in some implementations, the neutron absorber can be incorporated into a separate layer surrounding the matrix 220 or into a graphite matrix of a stationary non-fueled compact.

[0032] The neutron absorber 230 can be based on boron, gadolinium, or a combination thereof. For example, the neutron absorber 230 can be comprised of boron carbide, gadolinium carbide, or a mixture thereof. Other configurations of the neutron absorber 230 are contemplated by the present disclosure. For example, in some implementations, the composition of the neutron absorber 230 can include erbium, hafnium, or any other material having a neutron absorption cross-section suitable for a desired range of neutron flux in a nuclear reactor.

[0033] Further to the above, the configuration of the neutron absorber 230 can be optimized based on a variety of factors including intended reactor application, neutron flux level, neutron spectrum, desired core life, and potential chemical and/or material interactions with existing core materials. For example, in some implementations of a fueled compact 200 intended for insertion into a microreactor core, a boron carbide and/or gadolinium carbide based neutron absorber 230 can be incorporated into a graphite-based matrix 220, at concentrations of about 2000 ppm or less based on the total mass of the matrix 220 and neutron absorber 230, to provide fuel cycles of about 5 years or longer. In some examples, a graphite-based matrix 220 can include about 1600 parts per million or less, or about 1000 parts per million or less, of a boron carbide and/or gadolinium carbide based neutron absorber 230 based on the weight of the matrix 220 and the neutron absorber 230. In certain examples, the neutron absorber 230 can be present in a matrix 220 based on solid graphite at a concentration in the range of about 300 parts per million to about 800 parts per million based on the weight of the matrix 220 and the neutron absorber 230. In some aspects, a composite fueled compact 200 incorporating this configuration can facilitate a management of excess reactivity and/or power distribution in a reactor core as discussed in further detail below.

[0034] For example, upon an initial fission event, a portion of neutron flux produced by fissile fuel 212 will interact with graphite in the surrounding matrix 220, thereby resulting in a moderated portion of neutrons more suitable for effecting subsequent fissions. The remaining portion of produced neutrons, representing an excess reactivity and/or amount of neutrons, can be absorbed by a neutron absorber 230, thereby diminishing the overall absorption capacity thereof. As discussed hereinabove, the initial excess reactivity must be managed to maintain reactor safety and efficiency. Since the rate of neutrons produced by a given amount of fissile fuel 212 diminishes as the fissile fuel is consumed over the lifetime of the fuel compact 200, so does the absorption rate required thereby to maintain a stable power output therefrom. Thus, even though the neutron absorber 230 is stationary with respect to the fissile fuel 212 and is consumable, the diminishing absorption requirements thereof allow the neutron absorber 230 to remain effective throughout the planned lifetime of the fuel compact 200, especially when attempting to maintain a relatively constant power output and/or a flattened reactivity curve. Accordingly, the neutron absorber 230, the matrix 220 and the fissile fuel 212 can cooperatively provide a stable reaction rate while maintaining safe operating conditions over the planned lifetime of the fuel compact 200 without requiring a replenishment, removal, and/or repositioning thereof.

[0035] Further to the above, based on a positioning of a fuel compact 200 within a vessel of a reactor, such as, for example, a microreactor, the fuel compact 200 can provide an ability to manage the power distribution within a reactor core. For example, since a microreactor generally exhibits denser neutron populations towards the center of the core at the beginning of life therein, a fuel compact 200 situated in a radially inner position of the core can be configured with a neutron absorber 230 greater in neutron capture cross-section area and/or amount included in the radially inner fuel compact in comparison to a fuel compact in a radially outer position. Additionally, since a temperature associated with a reactor region experiencing a greater fission rate is generally greater than that of a lower fission rate region, a number of fuel compacts 200 incorporating this configuration can also manage the temperature distribution with the reactor core. [0036] Now referring to FIG. 6, a cross-sectional schematic representation of a fuel assembly 1000 for a nuclear reactor core is provided, in accordance with at least one non-limiting embodiment of the present disclosure. The fuel assembly 1000 includes a first group of stationary fueled compacts 1100 and a second group of stationary fueled compacts 1200. In some examples, the fuel assembly 1000 can include a third group of stationary fueled compacts 1300. In certain examples, the fuel assembly 1000 can include a number of stationary non-fueled compacts 1400. The fuel assembly 1000 can be configured as a modular assembly. For example, the fuel assembly 1000 can include a number of discrete, modular unit cells 1010 as shown in FIG. 6.

[0037] Each of the stationary fueled compacts 1100 and 1200 is similar in many respects to other composite fueled compacts described elsewhere in the present disclosure which are not repeated for the sake of brevity. In various examples, each of the stationary fueled compacts in groups 1100 and 1200 includes a burnable absorber and TRISO fuel dispersed in a graphite matrix phase configured to moderate neutrons. Each of the stationary fueled compacts 1100 and 1200 can be independently configured similarly to a composite fueled compact 200 as described hereinabove. Thus, each of the stationary fueled compacts 1100 and 1200 can be independently configured to manage an excess reactivity in the vicinity of their respective locations within the reactor core. For example, the first group of stationary fueled compacts 1100 can include a burnable absorber based on boron carbide and gadolinium carbide while the second group of stationary fueled compacts 1200 can include a burnable absorber based on boron carbide alone. When the groups of stationary fueled compacts 1100 and 1200 incorporating this configuration are inserted into a microreactor core so that the first group 1100 surrounds the second group 1200, the variance in neutron absorption capacity of the groups of stationary fueled compacts will follow the radial variance in reactivity of the microreactor core. Accordingly, a fuel assembly 1000 can be configured to manage excess reactivity in a reactor core while maintaining a desirable power distribution therein.

[0038] Still referring to FIG. 6, each of the stationary fueled compacts 1300 of the third group is similar in some aspects to other stationary fueled compacts 1100 and/or 1200 as described hereinabove. For example, each of the stationary fueled compacts 1300 of the third group includes TRISO fuel dispersed in a graphite matrix phase. However, each of the stationary fueled compacts 1300 of the third group is configured to provide less reactivity management than either of the first group 1100 or the second group 1200. For example, in comparison with either of the first group 1100 or the second group 1200, each of the stationary fueled compacts 1300 of the third group can include a lower burnable absorber content and/or a burnable absorber having a lower neutron absorption cross-section. In certain examples, a portion of the stationary fueled compacts 1300 of the third group can exclude a burnable absorber altogether. A stationary fueled compact 1300 of the third group incorporating this configuration can be employed in regions of a reactor vessel which generally experience lower neutron flux, such as a radially outer region of a microreactor core at the beginning of life, and in conjunction with stationary fueled compacts 1100 and 1200 positioned at radially inward portions of the fuel assembly 1000 exhibiting higher neutron flux. In certain examples, a fuel assembly 1000 incorporating this configuration can provide a fuel cycle length for a microreactor without refueling for at least 5 years. Thus, a fuel assembly 1000 can be configured to manage excess reactivity while maintaining a desired power distribution without relying on a neutron absorber in the form of a displaceable rod or liquid coolant.

[0039] In examples where the fuel assembly 1000 includes a number of stationary non-fueled compacts 1400, each of the stationary non-fueled compacts 1400 includes an amount of burnable absorber material similar in some respects to other neutron absorbing materials described elsewhere in the present disclosure. In some examples, an amount of burnable absorber material of a stationary non-fueled compact 1400 can be incorporated into a graphite matrix. A non-fueled compact does not include any fissile fuel and thus, does not actively produce neutrons. However, when incorporated in areas of a nuclear reactor core having high initial excess reactivity, the burnable absorber of a stationary non-fueled compact 1400 can supplement a burnable absorber of other fueled compacts to further diminish any excess reactivity without requiring a dedicated ex-vessel control assembly. Additionally, in examples where the stationary non-fueled compacts 1400 include a graphite matrix, the stationary nonfueled compacts 1400 can maximize the amount of thermal neutrons in high neutron flux regions of a nuclear reactor core, such as a radially inner portion of a microreactor core, thereby avoiding loss of unreacted fast neutrons. Thus, in some aspects, a fuel assembly 1000 including stationary non-fueled compacts 1400 can provide an optimized fuel cycle length and/or an optimized neutron economy within a reactor vessel of a nuclear reactor, without increasing the overall footprint of the reactor package.

[0040] Various aspects of the present disclosure include, but are not limited to, the aspects listed in the following numbered clauses.

[0041] Clause 1 - A composite fueled compact for a nuclear reactor, the composite fueled compact comprising particles comprising a fissile fuel; a graphite matrix, wherein the graphite matrix defines a continuous phase surrounding the fissile fuel; and a neutron absorber, wherein the neutron absorber and the fissile fuel are configured to remain fixed in position relative to each other during reactor operation. [0042] Clause 2 - The composite fueled compact of clause 1 , wherein each of the particles is configured as a layered particle comprising a fissile core.

[0043] Clause 3 - The composite fueled compact of any one of clauses 1-2, wherein the fissile fuel comprises TRISO fuel.

[0044] Clause 4 - The composite fueled compact of any one of clause 1-3, wherein the fissile fuel has an initial enrichment level of greater than 5%.

[0045] Clause 5 - The composite fueled compact of clause 4, wherein the fissile fuel has an initial enrichment level of about 10% or greater.

[0046] Clause 6 - The composite fueled compact of any one of clauses 1-5, wherein the graphite matrix comprises the neutron absorber.

[0047] Clause 7 - The composite fueled compact of clause 6, wherein the neutron absorber is uniformly distributed throughout the graphite matrix.

[0048] Clause 8 - The composite fueled compact of any one of clauses 1-7, wherein the neutron absorber is comprised of boron, gadolinium, or a combination thereof.

[0049] Clause 9 - The composite fueled compact of clause 8, wherein the neutron absorber is comprised of boron carbide, gadolinium carbide, or a combination thereof.

[0050] Clause 10 - The composite fueled compact of any one of clauses 1-9, wherein the neutron absorber is present at a concentration of about 2000 parts per million or less based on the weight of the matrix and the neutron absorber.

[0051] Clause 11 - The composite fueled compact of clause 10, wherein the neutron absorber is present at a concentration of about 1600 parts per million or less based on the weight of the matrix and the neutron absorber.

[0052] Clause 12 - The composite fueled compact of clause 10, wherein the neutron absorber is present at a concentration in the range of about 300 parts per million to about 800 parts per million based on the weight of the matrix and the neutron absorber.

[0053] Clause 13 - The composite fueled compact of any one of clauses 1-12, wherein the composite fueled compact comprises a layer surrounding the graphite matrix, wherein the layer comprises a portion of the neutron absorber. [0054] Clause 14 - The composite fueled compact of any one of clauses 1-13, wherein the composite fueled compact has a cylindrical geometry.

[0055] Clause 15 - The composite fueled compact of any one of clauses 1-14, wherein the composite fueled compact has a spherical geometry.

[0056] Clause 16 - A fuel assembly for a nuclear reactor core, the fuel assembly comprising a first group of stationary fueled compacts and a second group of stationary fueled compacts. Each of the stationary fueled compacts comprises TRISO fuel dispersed in a graphite matrix phase; and a burnable absorber. Each of the first group and second group of stationary fueled compacts are configured to manage an excess reactivity for the duration of a planned fuel cycle length of the fuel assembly.

[0057] Clause 17 - The fuel assembly of clause 16, wherein the burnable absorber of each of the first group and the second group of stationary fueled compacts are independently configured to absorb neutrons based on a positioning thereof in the nuclear reactor core.

[0058] Clause 18 - The fuel assembly of any one of clauses 16-17, wherein the number of compacts comprises a third group of stationary fueled compacts configured to provide less reactivity control than either of the first group or the second group of stationary fueled compacts.

[0059] Clause 19 - The fuel assembly of any one of clauses 16-18, wherein each group of stationary fueled compacts is positioned in a respective unit cell.

[0060] Clause 20 - The fuel assembly of any one of clauses 16-19, wherein the number of compacts comprises a number of stationary non-fueled compacts comprising a burnable absorber material.

[0061] Various features and characteristics are described in this specification to provide an understanding of the composition, structure, production, function, and/or operation of the disclosure, which includes the disclosed methods and systems. It is understood that the various features and characteristics of the disclosure described in this specification can be combined in any suitable manner, regardless of whether such features and characteristics are expressly described in combination in this specification. The Inventors and the Applicant expressly intend such combinations of features and characteristics to be included within the scope of the disclosure described in this specification. As such, the claims can be amended to recite, in any combination, any features and characteristics expressly or inherently described in, or otherwise expressly or inherently supported by, this specification. Furthermore, the Applicant reserves the right to amend the claims to affirmatively disclaim features and characteristics that may be present in the prior art, even if those features and characteristics are not expressly described in this specification. Therefore, any such amendments will not add new matter to the specification or claims and will comply with the written description, sufficiency of description, and added matter requirements.

[0062] With respect to the appended claims, those skilled in the art will appreciate that recited operations therein may generally be performed in any order. Also, although various operational flows are presented in a sequence(s), it should be understood that the various operations may be performed in other orders than those that are illustrated or may be performed concurrently. Examples of such alternate orderings may include overlapping, interleaved, interrupted, reordered, incremental, preparatory, supplemental, simultaneous, reverse, or other variant orderings, unless context dictates otherwise. Furthermore, terms like “responsive to,” “related to,” or other past-tense adjectives are generally not intended to exclude such variants, unless context dictates otherwise.

[0063] The invention(s) described in this specification can comprise, consist of, or consist essentially of the various features and characteristics described in this specification. The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. Thus, a method or system that “comprises,” “has,” “includes,” or “contains” a feature or features and/or characteristics possesses the feature or those features and/or characteristics but is not limited to possessing only the feature or those features and/or characteristics. Likewise, an element of a composition, coating, or process that “comprises,” “has,” “includes,” or “contains” the feature or features and/or characteristics possesses the feature or those features and/or characteristics but is not limited to possessing only the feature or those features and/or characteristics and may possess additional features and/or characteristics.

[0064] The grammatical articles “a,” “an,” and “the,” as used in this specification, including the claims, are intended to include “at least one” or “one or more” unless otherwise indicated. Thus, the articles are used in this specification to refer to one or more than one (i.e. , to “at least one”) of the grammatical objects of the article. By way of example, “a component” means one or more components and, thus, possibly more than one component is contemplated and can be employed or used in an implementation of the described compositions, coatings, and processes. Nevertheless, it is understood that use of the terms “at least one” or “one or more” in some instances, but not others, will not result in any interpretation where failure to use the terms limits objects of the grammatical articles “a,” “an,” and “the” to just one. Further, the use of a singular noun includes the plural, and the use of a plural noun includes the singular, unless the context of the usage requires otherwise.

[0065] In this specification, unless otherwise indicated, all numerical parameters are to be understood as being prefaced and modified in all instances by the term “about,” in which the numerical parameters possess the inherent variability characteristic of the underlying measurement techniques used to determine the numerical value of the parameter. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter described herein should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0066] Any numerical range recited herein includes all sub-ranges subsumed within the recited range. For example, a range of “1 to 10” includes all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value equal to or less than 10. Also, all ranges recited herein are inclusive of the end points of the recited ranges. For example, a range of “1 to 10” includes the end points 1 and 10. Any maximum numerical limitation recited in this specification is intended to include all lower numerical limitations subsumed therein, and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. Accordingly, Applicant reserves the right to amend this specification, including the claims, to expressly recite any subrange subsumed within the ranges expressly recited. All such ranges are inherently described in this specification.

[0067] As used in this specification, particularly in connection with layers, the terms “on,” “onto,” “over,” and variants thereof (e.g., “applied over,” “formed over,” “deposited over,” “provided over,” “located over,” and the like) mean applied, formed, deposited, provided, or otherwise located over a surface of a substrate but not necessarily in contact with the surface of the substrate. For example, a layer “applied over” a substrate does not preclude the presence of another layer or other layers of the same or different composition located between the applied layer and the substrate. Likewise, a second layer “applied over” a first layer does not preclude the presence of another layer or other layers of the same or different composition located between the applied second layer and the applied first layer.

[0068] Whereas particular examples of this disclosure have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present disclosure may be made without departing from the disclosure as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A composite fueled compact for a nuclear reactor, the composite fueled compact comprising: particles comprising a fissile fuel; a graphite matrix, wherein the graphite matrix defines a continuous phase surrounding the fissile fuel; and a neutron absorber, wherein the neutron absorber and the fissile fuel are configured to remain fixed in position relative to each other during reactor operation.

2. The composite fueled compact as claimed in claim 1 , wherein each of the particles is configured as a layered particle comprising a fissile core.

3. The composite fueled compact as claimed in claim 2, wherein the fissile fuel comprises TRISO fuel.

4. The composite fueled compact as claimed in claim 1 , wherein the fissile fuel has an initial enrichment level of greater than about 5%.

5. The composite fueled compact as claimed in claim 4, wherein the fissile fuel has an initial enrichment level of about 10% or greater.

6. The composite fueled compact as claimed in claim 1 , wherein the graphite matrix comprises the neutron absorber.

7. The composite fueled compact as claimed in claim 6, wherein the neutron absorber is uniformly distributed throughout the graphite matrix.

8. The composite fueled compact as claimed in claim 1 , wherein the neutron absorber is comprised of boron, gadolinium, or a combination thereof.

9. The composite fueled compact as claimed in claim 8, wherein the neutron absorber is comprised of boron carbide, gadolinium carbide, or a combination thereof.

10. The composite fueled compact as claimed in claim 8, wherein the neutron absorber is present at a concentration of about 2000 parts per million or less based on the weight of the matrix and the neutron absorber.

11. The composite fueled compact as claimed in claim 10, wherein the neutron absorber is present at a concentration of about 1600 parts per million or less based on the weight of the matrix and the neutron absorber.

12. The composite fueled compact as claimed in claim 10, wherein the neutron absorber is present at a concentration in the range of about 300 parts per million to about 800 parts per million based on the weight of the matrix and the neutron absorber.

13. The composite fueled compact as claimed in claim 1 , wherein the composite fueled compact comprises a layer surrounding the graphite matrix, wherein the layer comprises a portion of the neutron absorber.

14. The composite fueled compact as claimed in claim 1 , wherein the composite fueled compact has a cylindrical geometry.

15. The composite fueled compact as claimed in claim 1 , wherein the composite fueled compact has a spherical geometry.

16. A fuel assembly for a nuclear reactor core, the fuel assembly comprising a first group of stationary fueled compacts and a second group of stationary fueled compacts, wherein each of the stationary fueled compacts comprises:

TRISO fuel dispersed in a graphite matrix phase; and a burnable absorber; wherein each of the first group and second group of stationary fueled compacts are configured to manage an excess reactivity for the duration of a planned fuel cycle length of the fuel assembly.

17. The fuel assembly as claimed in claim 16, wherein the burnable absorber of each of the first group and the second group of stationary fueled compacts are independently configured to absorb neutrons based on a positioning thereof in the nuclear reactor core.

18. The fuel assembly as claimed in claim 16, wherein the number of compacts comprises a third group of stationary fueled compacts configured to provide less reactivity control than either of the first group or the second group of stationary fueled compacts.

19. The fuel assembly as claimed in claim 16, wherein each group of stationary fueled compacts is positioned in a respective unit cell.

20. The fuel assembly as claimed in claim 16, wherein the number of compacts comprises a number of stationary non-fueled compacts comprising a burnable absorber material.