Use of combustibility absorbers in the graphite matrix of TRISO fuel compacts within the reactor core.

Abstract

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

Description

Technical Field

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

[0002] (Government Contracts) 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 this invention.

Background Art

[0003] Advanced nuclear reactors with a smaller installation area and lower output than conventional designs can be transported to remote locations and deployed locally for power generation. To operate a nuclear reactor safely, it is necessary to appropriately manage the state of the nuclear reactor to reduce the probability that the nuclear reactor will enter a catastrophic runaway state. Conventional reactivity management devices and methods may increase the dimensions and / or complexity of the overall installation area of the nuclear reactor, thus compromising portability and ease of deployment.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Therefore, there is a need to develop alternative reactivity management devices and systems to optimize the operating efficiency and safety of advanced nuclear reactors while maintaining the shape factor of the nuclear reactor.

[0005] The following summary is provided to facilitate understanding of some of the progressive features inherent in the aspects disclosed herein and is not intended to be a complete description. A comprehensive understanding of the various aspects disclosed herein requires a full reference to the specification, claims, and abstract.

[0006] In various embodiments, composite fuel compacts for nuclear reactors are disclosed. In some embodiments, the composite fuel compact comprises particles, a graphite matrix, and a neutron absorber. In some embodiments, the particles comprise fission fuel. In some embodiments, the graphite matrix forms a continuous phase surrounding the fission fuel. In some embodiments, the neutron absorber and the fission fuel are configured so that their relative positions do not move during reactor operation.

[0007] Fuel assemblies for reactor cores are disclosed in various embodiments. In some embodiments, the fuel assembly includes a first group of fixed fuel compacts and a second group of fixed fuel compacts. In some embodiments, each of the fixed fuel compacts includes TRISO fuel dispersed in a graphite matrix phase and a combustible absorbent. In some embodiments, each of the first and second groups of fixed fuel compacts is configured to control excess reactivity over the planned fuel cycle period of the fuel assembly.

[0008] Other purposes, features, and characteristics of this disclosure, as well as the operation and function of related structural elements, component combinations, and manufacturing economics, will become further apparent from the following description and accompanying drawings (which constitute part of this specification, with the same reference numerals indicating corresponding parts in each drawing). However, it should be clearly understood that the accompanying drawings are for illustrative and illustrative purposes only and are not intended to define any limitations of any aspect disclosed herein.

[0009] The various embodiments described herein, along with their purposes and advantages, will be best understood by referring to the following description and accompanying drawings. [Brief explanation of the drawing]

[0010] [Figure 1] A cross-sectional elevation view of a fuel assembly is shown according to at least one non-limiting aspect of this disclosure.

[0011] [Figure 2] A cross-sectional view of a micro reactor core is shown according to at least one non-limiting aspect of this disclosure.

[0012] [Figure 3] A perspective view of a fuel compact is shown according to at least one non-limiting aspect of this disclosure.

[0013] [Figure 4] Figure 3 shows a radial cross-sectional view of the fuel compact according to at least one non-limiting aspect of the present disclosure.

[0014] [Figure 5] Figure 3 shows an axial cross-sectional view of the fuel compact according to at least one non-limiting aspect of the present disclosure.

[0015] [Figure 6] A schematic cross-sectional view of a fuel assembly for a reactor core is shown according to at least one non-limiting aspect of this disclosure. [Modes for carrying out the invention]

[0016] In multiple figures, corresponding reference letters refer to the corresponding parts. The examples provided herein illustrate various aspects of the disclosure in one form and are not intended to limit the scope of the aspects disclosed herein.

[0017] To provide a full understanding of the structures, functions, manufactures, uses of structures, and methods described herein, specific exemplary embodiments of this disclosure are described below. Examples of such embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the structures, articles, and methods described herein and illustrated in the accompanying drawings are non-limiting embodiments, and the scope of the various examples of this disclosure is defined solely by the claims. Features illustrated or described in relation to one exemplary embodiment may be combined with features of other embodiments. Such variations and modifications are included within the scope of this disclosure.

[0018] References in this specification to “various examples,” “several examples,” “one example,” or “one example” mean that a particular feature, structure, or characteristic described in relation to that example is included in a particular example. Therefore, expressions such as “in various examples,” “several examples,” “one example,” or “one example” used throughout this specification do not necessarily all refer to the same example. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more examples. Accordingly, a particular feature, structure, or characteristic illustrated or described in relation to one example may be combined, in whole or in part, with features, structures, or characteristics of one or more other examples without limitation. Such modifications and variations are within the scope of this example.

[0019] In the following explanation, similar reference letters in the drawings indicate the same or corresponding parts. Furthermore, please understand that terms such as "front," "rear," "left," "right," "up," "down," "top," and "bottom" are for convenience only and should not be interpreted as restrictive terms.

[0020] In the fuel assembly of a nuclear reactor core, when a fission fuel such as uranium-235 (hereinafter sometimes referred to as "235U") interacts with an incident neutron flux and absorbs neutrons having appropriate energy (e.g., thermal neutrons), it fissions into a large number of light fission products and / or fragments, thereby releasing prompt neutrons and generating heat. Thereafter, these prompt neutrons can be absorbed by other nuclei, triggering a chain of other fission events. The lifetime of a prompt neutron is the time from when it is released by a fission event until it is absorbed by another nucleus. To maximize the possibility of continuously sustaining fission events, a neutron moderator may be placed within the nuclear reactor core to effectively slow down neutrons born with high energy.

[0021] Generally, k eff represents the ratio of the number of neutrons in one generation to the number of neutrons in the previous generation and is quantified as the neutron multiplication factor. Therefore, the k eff of a nuclear reactor core can indicate the critical state of the core. During operation, a nuclear reactor is usually maintained in a critical state where k eff = 1, and neutrons are generated and consumed in a self-sustaining chain reaction. In the context of nuclear fuel, excess reactivity can be defined as the available reactivity of nuclear fuel that exceeds the amount required to achieve a critical state at a certain point. In conventional nuclear reactor designs such as pressurized water reactors (PWRs), if the excess reactivity of nuclear fuel cannot be managed, the reactor will enter a supercritical state where k eff > 1, and the neutron generation rate will exceed the neutron consumption rate. Therefore, if the reactor state is not properly managed, there is a risk of causing a catastrophic runaway state. Therefore, it is extremely important to manage the excess reactivity and power distribution within the nuclear reactor to maintain safe operating conditions and / or an economically viable fuel cycle period.

[0022] Generally, in a conventional nuclear reactor such as a PWR, a large number of fuel assemblies are included in the reactor core, and each fuel assembly includes a plurality of elongated fuel elements or fuel rods. For example, FIG. 1 shows a cross-sectional elevation view of a fuel assembly 10 according to at least one non-limiting aspect of the present disclosure. The fuel assembly 10 includes neatly arranged elongated fuel rods 22. The fuel rods 22 may contain a plurality of fuel pellets 26, and each fuel pellet 26 includes a fissionable material (e.g., an enriched uranium-based material) capable of maintaining a nuclear fission chain reaction. Each fuel rod 22 may include a plurality of nuclear fuel pellets 26. The fuel pellets 26 are housed in an elongated cladding tube 38 whose both ends are closed by an upper end plug 28 and a lower end plug 30. The pellets 26 may be maintained in a stacked state by a planar spring 32 disposed between the upper end plug 28 and the upper end of the pellet stack.

[0023] Referring further to Figure 1, the fuel rods 22 may be supported by one or more lateral grids 20 attached to a guide thimble 18. The guide thimble 18 extends longitudinally between the upper nozzle 16 and the lower nozzle 12 and is configured to allow discrete, elongated control rods 34 to move operably within the guide thimble 18. One end of the guide thimble 18 may be attached to the upper nozzle 16 and the other end to the lower nozzle 12. The lower nozzle 12 may be configured to support the fuel assembly 10 on the lower core plate 14 of the reactor vessel in the reactor core region. A liquid coolant (e.g., a solution of water and boric acid) may be delivered upward toward the fuel assembly 10 through a plurality of flow openings provided in the lower core plate 14. The lower nozzle 12 of the fuel assembly 10 flows the coolant along the fuel rods 22 of the fuel assembly 10 to remove the heat generated by the fission reaction occurring in the fuel rods 22. The water in the liquid coolant slows down neutrons, and the dissolved boron absorbs them. Alternative reactor designs may operate at higher temperatures and therefore may require different cooling systems and / or fuels. For example, large gas-cooled reactors (GCRs), which can operate at temperatures above approximately 700°C, may use three-structure isotropic (TRISO) particle-based fuels.

[0024] Conventional reactivity control systems can address changes in excess reactivity over the lifespan of the fuel assembly. For example, each of the movable control rods 34 may be equipped with a neutron absorber such as boron carbide, and the insertion depth of the control rods 34 can be varied. Some of the neutrons already generated are absorbed by the control rods 34 depending on their insertion depth, so the number of subsequent fission events decreases (i.e., negative reactivity) near the insertion point of the control rods 34. In addition to moving the discrete control rods 34, the concentration of the dissolved neutron absorber (e.g., boric acid) in the liquid coolant and its flow rate in the core can be changed, making it easier to manage the power distribution within the reactor vessel. Reactivity control systems rely on dedicated drive mechanisms and / or pump systems to operate the neutron absorption components in order to adjust the neutron absorption elements in a timely and precise manner. However, incorporating these into the reactor design usually increases the overall installation area of ​​the reactor vessel and complicates the core design.

[0025] As a solution to provide reliable off-grid power, advanced reactor designs (e.g., microreactors) that employ smaller structures in both size and power than conventional PWRs and GCRs are emerging. For example, the eVinci® microreactor currently under development by Westinghouse features a microreactor vessel integrated into a dedicated container as an integrated package. The space between the microreactor vessel and the container is minimized to create an assembled package with an optimal footprint for truck transport to the final destination. Figure 2 shows a cross-sectional perspective view of a microreactor vessel 100 according to at least one non-limiting aspect of this disclosure. The reactor vessel 100 includes a core comprising a radial reflector 110, a fuel assembly 120, and a control drum 130. The radial reflector 110 minimizes neutron leakage from the vessel 100. Generally, the fuel assembly 120 may include a structure based on a unit cell 122 made of a solid material (e.g., graphite) that operates at high temperatures of approximately 600°C or higher, into which a fuel compact 124 can be inserted. Each fuel compact 124 contains a high-temperature-resistant fission fuel, such as TRISO fuel, which can maintain good thermal conductivity with the surrounding structure, thereby facilitating heat transfer during operation. Thus, the micro reactor vessel 100 can maximize output while maintaining a space-saving shape. Other examples of micro reactors and methods of operating them are described in more detail in U.S. Patent Applications 17 / 084,365 and 18 / 057,208, owned by the applicant of this disclosure, which are incorporated herein by reference in full.

[0026] Due to the limited space between the reactor vessel and the container, effectively managing excess reactivity and spatial neutron flux distribution in a micro-reactor is difficult. For example, a conventional drive system that moves control rods axially into the micro-reactor vessel may occupy a large space extending from the surface of the reactor vessel, which complicates the logistics of transporting the micro-reactor to its final destination and / or increases logistics costs. Therefore, implementing conventional control rods in a micro-reactor may undermine its economic and logistical advantages.

[0027] Furthermore, the inventors of this disclosure have shown that discretely moving neutron absorbers are not suitable for controlling output in low neutron flux environments (e.g., micro reactor cores). Moreover, since micro reactor cores generally do not rely on liquid coolant-based primary heat transfer systems, excess reactivity cannot be controlled by the coolant.

[0028] Although alternative forms of particle-based fuel incorporating the function of a combustible absorber into the particles themselves have been proposed, extensive research and testing have not yet been conducted. Furthermore, incorporating such fuels into micro-reactor designs requires extensive irradiation and new fuel qualification programs to obtain regulatory approval. Moreover, as mentioned above, implementing conventional methods and devices for controlling core reactivity in a micro-reactor would require components outside the reactor vessel, potentially complicating the design. Accordingly, various aspects of this disclosure provide various methods and devices for managing excess reactivity and / or power distribution in a micro-reactor core, etc., while maintaining the portability and simplicity of the micro-reactor.

[0029] Referring to Figure 3, a perspective view of a composite fuel compact 200 according to at least one non-limiting aspect of the present disclosure is shown. In various examples, the composite fuel compact 200 is configured in a cylindrical shape. In some examples, the composite fuel compact 200 is configured to be inserted into a fuel assembly of a micro reactor. For example, the composite fuel compact 200 may have an outer diameter that is substantially the same as, or slightly smaller than, the channel of the micro reactor core graphite support structure or a tubular sleeve that can be inserted into said channel. Other configurations of the composite fuel compact 200 are also envisioned in the present disclosure. For example, in some embodiments, the composite fuel compact 200 may be spherical, tubular, and / or have a cross-sectional shape that can be inscribed in a channel allocated for reactor fuel.

[0030] Referring to Figures 4 and 5, cross-sectional views of the composite fuel compact 200 of Figure 3 are shown according to at least one non-limiting aspect of the present disclosure. Figure 4 shows a radial cross-sectional view of the composite fuel compact 200, and Figure 5 shows an axial cross-sectional view of the composite fuel compact 200. The composite fuel compact 200 comprises particles 210 and a matrix 220. The composition of the matrix 220 may be configured to slow down fast neutrons. For example, the matrix 220 may consist mainly of solid graphite. The matrix 220 forms a stationary continuous phase surrounding the particles 210. Thus, the position of each particle 210 is fixed with respect to the matrix and adjacent particles. In some examples, the particles 210 occupy about 5% or more, about 10% or more, about 15% or more, about 20% or more, about 25% or more, or about 30% or more of the total volume of the composite fuel compact 200. In some examples, the particles 210 occupy about 37% of the total volume of the composite fuel compact 200.

[0031] In various examples, at least a portion of the particle 210 may contain fission fuel 212. For example, the particle 210 may have a core of fission fuel 212. In some examples, the particle 210 may have multiple layers. In some examples, the fission fuel 212 may be TRISO fuel. In some embodiments, the fission fuel 212 may be configured to optimize the fuel cycle length in a microreactor. For example, fission fuel 212 configured as TRISO fuel may initially contain uranium enriched to a level of 5% or more, 10% or more, or up to approximately 20%. Fission fuel 212 configured as TRISO fuel can be incorporated into existing high-temperature gas-cooled core designs and their operating conditions, which have already undergone extensive irradiation and new fuel qualification programs. Therefore, in some embodiments, using a composite fuel compact 200 having this configuration can avoid significant investment in time and / or cost associated with implementing an untested fuel configuration.

[0032] Continuing to refer to Figures 4 and 5, in various examples, the composite fuel compact 200 includes a certain amount of 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 may be incorporated into a matrix 220. In one example, the neutron absorber 230 may be uniformly or homogeneously spread throughout the matrix 220. In one embodiment, the neutron absorber 230 having this configuration can absorb neutrons in the vicinity of the particles 210 over the intended lifetime of the fission fuel 212 without requiring a dedicated movement control system outside the reactor vessel. Thus, the fuel compact 200 may be configured to manage the excess reactivity of fission fuel in a reactor environment such as a micro-reactor core without increasing its overall installation area. Other configurations of the neutron absorber 230 are also envisioned in this disclosure. For example, in some embodiments, the neutron absorber may be incorporated into a separate layer surrounding the matrix 220, or it may be incorporated into a fuel-free, fixed-compact graphite matrix.

[0033] The neutron absorber 230 may be based on boron, gadolinium, or a combination thereof. For example, the neutron absorber 230 may consist of boron carbide, gadolinium carbide, or a mixture thereof. Other configurations of the neutron absorber 230 are also envisioned in this disclosure. For example, in some embodiments, the composition of the neutron absorber 230 may include erbium, hafnium, or any other material having a neutron absorption cross-section suitable for a desired neutron flux region of a reactor.

[0034] In addition to the above, the composition of the neutron absorber 230 may be optimized based on various factors including the reactor application, neutron flux level, neutron spectrum, desired core lifetime, and possible chemical and / or material interactions with existing core materials. For example, in some embodiments in which the fuel compact 200 is inserted into a micro-reactor core, a fuel cycle of about 5 years or more may be obtained by incorporating a boron carbide-based and / or gadolinium carbide-based neutron absorber 230 into a graphite-based matrix 220 at a concentration of about 2000 ppm or less based on the total weight of the matrix 220 and the neutron absorber 230. In some examples, the graphite-based matrix 220 may contain boron carbide-based and / or gadolinium carbide-based neutron absorber 230 at a concentration of about 1600 parts per million or less, or about 1000 parts per million or less, based on the weight of the matrix 220 and the neutron absorber 230. In one example, the neutron absorber 230 may be present in a solid graphite-based matrix 220 at a concentration of approximately 300 parts per million to approximately 800 parts per million, based on the weights of the matrix 220 and the neutron absorber 230. In some embodiments, a composite fuel compact 200 having this configuration can facilitate the control of excess reactivity and / or power distribution in the core, as detailed below.

[0035] For example, in the initial fission event, a portion of the neutron flux produced by the fission fuel 212 interacts with the graphite in the surrounding matrix 220, resulting in slowed neutrons more suitable for subsequent fission. The remaining portion of the neutrons, representing excess reactivity and / or excess neutron quantity, is absorbed by the neutron absorber 230, resulting in a reduction in the overall absorption capacity of the neutron absorber 230. As mentioned above, the initial excess reactivity must be managed to maintain the safety and efficiency of the reactor. The neutron generation rate produced by a certain amount of fission fuel 212 decreases as the fuel is consumed during the lifetime of the fuel compact 200, but the absorption rate required to maintain stable power also decreases in the same way. Therefore, even though the neutron absorber 230 is fixed to the fission fuel 212 and is consumed, its absorption requirements decrease, allowing the neutron absorber 230 to remain effective throughout the planned lifetime of the fuel compact 200, especially when trying to maintain a relatively constant power and / or flat reactivity curve. Therefore, the neutron absorber 230, the matrix 220, and the fission fuel 212 together can provide a stable reaction rate while maintaining safe operation over the planned lifespan of the fuel compact 200, without requiring replenishment, removal, and / or rearrangement of the fuel compact 200.

[0036] In addition to the above, the position of the fuel compact 200 within the reactor vessel, such as a micro-reactor, can allow the fuel compact 200 to control the power distribution within the reactor core. For example, in a micro-reactor, the neutron density is generally higher towards the center of the core at the start of its life cycle. Therefore, a fuel compact 200 located radially inward from the core may have a larger neutron absorption cross-section and / or quantity of neutron absorber 230 compared to a fuel compact located radially outward. Furthermore, since the temperature in the reactor region with a high fission rate is usually higher than the temperature in the region with a low fission rate, having multiple fuel compacts 200 with this configuration can also control the temperature distribution within the core.

[0037] Referring next to Figure 6, a schematic cross-sectional view of a fuel assembly 1000 for a reactor core according to at least one non-limiting embodiment of the present disclosure is shown. The fuel assembly 1000 includes a first group of fixed fuel compacts 1100 and a second group of fixed fuel compacts 1200. In some examples, the fuel assembly 1000 may include a third group of fixed fuel compacts 1300. In some examples, the fuel assembly 1000 may include a number of fixed non-fuel compacts 1400. The fuel assembly 1000 may be configured as a modular assembly. For example, as shown in Figure 6, the fuel assembly 1000 may include a number of discrete modular unit cells 1010.

[0038] Each of the fixed fuel compacts 1100 and 1200 is similar in many respects to other composite fuel compacts described elsewhere in this disclosure, but for brevity, the descriptions of these composite fuel compacts are not repeated here. In various examples, each of the fixed fuel compacts in groups 1100 and 1200 includes a combustible absorber and TRISO fuel dispersed in a graphite matrix phase configured to slow neutrons. Each of the fixed fuel compacts 1100 and 1200 may be configured independently, as with the composite fuel compact 200 described above. Thus, each of the fixed fuel compacts 1100 and 1200 may be configured independently to manage excess reactivity in the vicinity of their location in the reactor core. For example, the first group of fixed fuel compacts 1100 may include a combustible absorber containing boron carbide and gadolinium carbide, and the second group of fixed fuel compacts 1200 may include a combustible absorber containing boron carbide alone. When groups of fixed fuel compacts 1100 and 1200 having this configuration are inserted into a microreactor core such that the first group 1100 surrounds the second group 1200, the difference in neutron absorption capacity between the groups of fixed fuel compacts follows the radial difference in reactivity of the microreactor core. Therefore, the fuel assembly 1000 may be configured to manage excess reactivity in the core while maintaining a desirable power distribution.

[0039] Continuing to refer to Figure 6, each of the third group's fixed fuel compacts 1300 is similar in several respects to the fixed fuel compacts 1100 and / or 1200 described above. For example, each of the third group's fixed fuel compacts 1300 contains TRISO fuel dispersed in a graphite matrix phase. However, each of the third group's fixed fuel compacts 1300 is configured to have a lower level of reactivity control than either the first group's 1100 or the second group's 1200. For example, compared to either the first group's 1100 or the second group's 1200, each of the third group's fixed fuel compacts 1300 may have a smaller content of combustible absorbent and / or may contain combustible absorbent with a smaller neutron absorption cross-section. In some cases, some of the third group's fixed fuel compacts 1300 may contain no combustible absorbent at all. The third group of fixed fuel compacts 1300 having this configuration may be used together with fixed fuel compacts 1100 and 1200 that are located in the radially inward portion of the fuel assembly 1000, which exhibits a high neutron flux, in a region of the reactor vessel that generally receives a low neutron flux (for example, the radially outer region of the micro-reactor core at the start of its life). In one example, the fuel assembly 1000 having this configuration can provide a fuel cycle length of at least 5 years without refueling in a micro-reactor. Thus, the fuel assembly 1000 is configured to manage excess reactivity while maintaining a desirable power distribution without relying on neutron absorbers in the form of movable rods or liquid coolants.

[0040] In an example where the fuel assembly 1000 includes a number of fixed non-fuel compacts 1400, each fixed non-fuel compact 1400 contains a certain amount of combustible absorbent material that is similar in some respects to the neutron absorbent materials described elsewhere in this specification. In some examples, the amount of combustible absorbent material in the fixed non-fuel compact 1400 may be incorporated into a graphite matrix. Since the non-fuel compacts do not contain any fission fuel, they do not actively generate neutrons. However, when placed in the initial high-over-reactivity region of the reactor core, the combustible absorbent material of the fixed non-fuel compact 1400 can assist the combustible absorbent material of other fuel compacts to further reduce over-reactivity without requiring a dedicated control assembly outside the reactor vessel. Furthermore, in an example where the fixed non-fuel compact 1400 includes a graphite matrix, the fixed non-fuel compact 1400 can avoid the loss of unreacted fast neutrons by maximizing the amount of thermal neutrons in the high-neutron flux region of the reactor core (e.g., the radially inner portion of a micro-reactor core). Accordingly, in some embodiments, a fuel assembly 1000 including a fixed non-fueled compact 1400 can optimize the fuel cycle length and / or optimize the neutron economy within the reactor vessel without increasing the overall installation area of ​​the reactor package.

[0041] Various aspects of this disclosure include, but are not limited to, the following numbered sections.

[0042] Item 1 - A composite fuel compact for a nuclear reactor, comprising a particle containing fission fuel, a graphite matrix forming a continuous phase surrounding the fission fuel, and a neutron absorber, wherein the neutron absorber and the fission fuel are configured to remain fixed in position relative to each other during reactor operation.

[0043] Item 2 - The composite fuel compact according to Item 1, wherein each particle is configured as a multilayer particle having a fissile core.

[0044] Item 3 - A composite fuel compact as described in Item 2, comprising TRISO fuel.

[0045] Item 4 - A composite fuel compact according to any one of items 1-3, wherein the fission fuel has an initial enrichment level greater than 5%.

[0046] Item 5 - A composite fuel compact as described in Item 4, wherein the fission fuel has an initial enrichment level of approximately 10% or more.

[0047] Item 6 - A composite fuel compact according to any one of items 1 to 5, comprising a graphite matrix and a neutron absorber.

[0048] Item 7 - The composite fuel compact according to Item 6, wherein the neutron absorber is uniformly distributed throughout the graphite matrix.

[0049] Item 8 - The neutron absorber is composed of boron, gadolinium, or a combination thereof, as described in any one of Items 1 to 7.

[0050] Item 9 - The composite fuel compact according to Item 8, wherein the neutron absorber is composed of boron carbide, gadolinium carbide, or a combination thereof.

[0051] Item 10 - The neutron absorber is present at a concentration of approximately 2000 parts per million or less based on the weight of the matrix and the neutron absorber, as described in any one of items 1-9 of the composite fuel compact.

[0052] Item 11 - The neutron absorber is present at a concentration of approximately 1600 parts per million or less based on the weight of the matrix and the neutron absorber, as described in Item 10 of the composite fuel compact.

[0053] Item 12 - The neutron absorber is present in a composite fuel compact as described in Item 10, at a concentration of approximately 300 parts per million to approximately 800 parts per million based on the weight of the matrix and the neutron absorber.

[0054] Item 13 - The composite fuel compact according to any one of items 1 to 12, comprising a layer surrounding a graphite matrix, the layer comprising a portion of a neutron absorber.

[0055] Item 14 - A composite fuel compact having a cylindrical shape, as described in any one of items 1 to 13.

[0056] Item 15 - A composite fuel compact is spherical, as described in any one of items 1 to 14.

[0057] Item 16 - A fuel assembly for a reactor core comprising a first group of fixed fuel compacts and a second group of fixed fuel compacts. Each fixed fuel compact comprises TRISO fuel dispersed in a graphite matrix phase and a combustible absorbent. The fuel assembly is configured such that each of the first and second groups of fixed fuel compacts controls excess reactivity over the duration of the planned fuel cycle length of the fuel assembly.

[0058] Item 17 - The fuel assembly according to Item 16, wherein the first and second groups of combustible absorbers of the fixed fuel compact are individually configured to absorb neutrons based on the position of the combustible absorbers within the reactor core.

[0059] Item 18 - The fuel assembly according to any one of items 16-17, comprising a third group of fixed fuel compacts configured to provide a lower reactivity control level than either of the first or second group of fixed fuel compacts.

[0060] Section 19 - Each group of fixed fuel compacts is a fuel assembly as described in any one of Sections 16-18, located in the corresponding unit cell.

[0061] Item 20 - A fuel assembly according to any one of items 16-19, comprising a number of fixed non-fuel compacts having a flammable absorbent material.

[0062] This specification describes various features and characteristics to provide an understanding of the configuration, structure, manufacture, function, and / or operation of the disclosed methods and systems. It will be understood that these various features and characteristics of the disclosed as described herein may be appropriately combined, whether or not such combinations of features and characteristics are expressly described herein. The inventors and applicants expressly intend that such combinations of features and characteristics fall within the scope of the disclosure described herein. Accordingly, the claims can be amended to describe any combination of features and characteristics expressly or essentially described herein, or features and characteristics expressly or essentially supported herein. Furthermore, the applicant has the right to amend the claims to affirmatively disallow features and characteristics that may exist in the prior art, even if such features and characteristics are not expressly described herein. Accordingly, any such amendment shall not add new matter to the specification or claims, and shall conform to the requirements of specification, sufficiency of specification, and additional matter.

[0063] Those skilled in the art will understand that, with respect to the attached claims, the operations described herein may generally be performed in any order. Furthermore, while various operation flows are shown sequentially, it should be understood that these various operations may be performed in an order other than that illustrated, or may be performed simultaneously. Examples of such alternative orders include, unless otherwise specified in the context, repetition, interleaving, interruption, reordering, incremental, preparation, supplementation, simultaneous, reverse, or other variant orders. Additionally, words such as "depending on," "related to," and other past tense adjectives are not usually intended to exclude such variants unless otherwise specified in the context.

[0064] The inventions described herein may have, consist of, or essentially consist of the various features and characteristics described herein. The words “to have” (and any form of “to have,” such as “has” or “is equipped”), “to possess” (and any form of “has” such as “has” or “is possess”), “to include” (and any form of “include,” such as “has” or “is containing”), and “to contain” (and any form of “contains,” such as “contains” or “is containing”) are open-ended linking verbs. Thus, a method or system that “has,” “has,” “includes,” or “contains” one or more features and / or characteristics has, but is not limited to having only, such one or more features and / or characteristics. Similarly, any element of a composition, coating, or process that “has,” “includes,” or “contains” one or more features and / or properties has, but is not limited to having only, one or more features and / or properties, and may have other features and / or properties.

[0065] As used herein, including in the claims, the grammatical articles "a," "an," and "the" are intended to include "at least one" or "one or more" unless otherwise specified. Thus, in this specification, articles are used to refer to one or more (i.e., "at least one") grammatical objects of the article. For example, "a component" means one or more components and therefore can be one or more components that may be employed or used in the implementation of the compositions, coatings, and processes described. However, it should be understood that the absence of the phrase "at least one" or "one or more" in some cases does not imply that the objects of the grammatical articles "a," "an," and "the" are limited to one. Furthermore, the use of singular nouns includes plural forms, and the use of plural nouns includes singular forms.

[0066] In this specification, unless otherwise specified, all numerical parameters should be understood to be preceded and modified in all cases by the word “approximately.” In this case, the numerical parameters have inherent variability characteristic of the underlying measurement technique used to determine the numerical value of the parameter. Not to the effect of limiting the application of the doctrine of equivalents to the claims, each of the numerical parameters described herein should be interpreted using ordinary rounding techniques, taking into account at least the number of significant figures reported.

[0067] Numerical ranges described herein include all subranges encompassed within the described range. For example, the range "1 to 10" includes all subranges (and inclusive) between the stated minimum value "1" and the stated maximum value "10," i.e., all subranges where the minimum value is 1 or greater and the maximum value is 10 or less. Furthermore, all ranges described herein include their endpoints. For example, the range "1 to 10" includes endpoints 1 and 10. The maximum numerical limit described herein is intended to include all subranges encompassed therewith, and the minimum numerical limit described herein is intended to include all upper numerical limits encompassed therewith. Accordingly, the applicant has the right to amend this specification, including the claims, to explicitly describe any subranges encompassed within an explicitly described range. All such ranges are essentially described herein.

[0068] In this specification, when used particularly in relation to layers, “on top of,” “above,” “across,” and variations thereof (e.g., “applied on top,” “formed on top,” “deposited on top,” “placed on top,” “located on top,” etc.) mean that they are applied, formed, deposited, placed on, or positioned on the surface of a substrate, but do not necessarily have to be in contact with the surface of the substrate. For example, a layer “applied” on a substrate does not preclude the presence of another layer or other layer of the same or different composition between the applied layer and the substrate. Similarly, a second layer “applied” on a first layer does not preclude the presence of another layer or other layer of the same or different composition between the applied second layer and the applied first layer.

[0069] While specific examples of this disclosure have been described above for illustrative purposes, it will be apparent to those skilled in the art that the details of this disclosure may be modified in various ways without departing from the scope of the disclosure as defined in the appended claims.

Claims

1. A compact composite fuel for nuclear reactors, Particles containing nuclear fission fuel, A graphite matrix forming a continuous phase surrounding the fission fuel, Equipped with a neutron absorber, The neutron absorber and the fission fuel are configured to remain fixed in position relative to each other during reactor operation. Compact multi-fuel design.

2. The composite fuel compact according to claim 1, wherein each of the aforementioned particles is configured as a multilayer particle having a fissile core.

3. The composite fuel compact according to claim 2, wherein the fission fuel comprises TRISO fuel.

4. The composite fuel compact according to claim 1, wherein the fission fuel has an initial enrichment level of more than approximately 5%.

5. The composite fuel compact according to claim 4, wherein the fission fuel has an initial enrichment level of about 10% or more.

6. The composite fuel compact according to claim 1, wherein the graphite matrix comprises the neutron absorber.

7. The composite fuel compact according to claim 6, wherein the neutron absorber is uniformly distributed throughout the entire graphite matrix.

8. The composite fuel compact according to claim 1, wherein the neutron absorber is composed of boron, gadolinium, or a combination thereof.

9. The composite fuel compact according to claim 8, wherein the neutron absorber is composed of boron carbide, gadolinium carbide, or a combination thereof.

10. The composite fuel compact according to claim 8, wherein the neutron absorber is present at a concentration of approximately 2,000 parts per million or less based on the weight of the matrix and the neutron absorber.

11. The composite fuel compact according to claim 10, wherein the neutron absorber is present at a concentration of approximately 1600 parts per million or less based on the weight of the matrix and the neutron absorber.

12. The composite fuel compact according to claim 10, wherein the neutron absorber is present at a concentration 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 fuel compact comprises a layer surrounding the graphite matrix, The composite fuel compact according to claim 1, wherein the layer comprises a portion of the neutron absorber.

14. The composite fuel compact according to claim 1, wherein the composite fuel compact has a cylindrical shape.

15. The composite fuel compact according to claim 1, wherein the composite fuel compact is spherical.

16. A fuel assembly for a nuclear reactor core, The fuel assembly comprises a first group of fixed fuel compacts and a second group of fixed fuel compacts. Each of the aforementioned fixed fuel compacts is TRISO fuel dispersed in the graphite matrix phase, Equipped with a flammable absorbent, Each of the first and second fixed fuel compacts is configured to manage the degree of excess reactivity over the duration of the planned fuel cycle length of the fuel assembly. Fuel assembly.

17. The fuel assembly according to claim 16, wherein the combustible absorbent in each of the first and second group fixed fuel compacts is individually configured to absorb neutrons based on the position of the combustible absorbent in the reactor core.

18. The fuel assembly according to claim 16, further comprising a third group of fixed fuel compacts configured to provide a lower reactivity control level than either of the first or second group of fixed fuel compacts.

19. The fuel assembly according to claim 16, wherein each of the groups of fixed fuel compacts is arranged in a corresponding unit cell.

20. The fuel assembly according to claim 16, comprising a plurality of fixed non-fuel compacts having a flammable absorbent material.

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