Beryllium-based (Be, BeO or Be2C) sleeve-enclosed fuel pellets / compacts for use in microreactors

Abstract

In one aspect, a nuclear reactor unit cell (202) is disclosed. The reactor unit cell includes a graphite moderator structure (204), a heat pipe (208) disposed within the graphite moderator structure, and a fuel assembly (210) disposed within the graphite moderator structure. The fuel assembly includes a beryllium oxide sleeve (212) and a nuclear fuel (214) disposed within the beryllium oxide sleeve.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Non-Provisional Patent Application No. 17 / 108,602, filed December 1, 2020, entitled "FUEL PELLETS / COMPACTS SURROUNDED BY BERYLIUM-BASED (Be or BeO or Be2C) SLEEVEFOR USE IN A MICRO-REACTOR," which is incorporated by reference in its entirety. government support

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

[0003] The present invention relates generally to nuclear reactors, such as small modular reactors (SMRs) and microreactors. [Background technology]

[0004] Electricity markets can be divided into centralized and decentralized. Centralized markets are based on large generators (hundreds of MWe) and high-capacity, high-density transmission and distribution grids. Distributed, or off-grid, markets, on the other hand, typically rely on small generators (less than 15 MWe) connected to small regional grids or microgrids. Examples of decentralized markets include remote mobile home communities, remote mines, military bases, and island communities. Currently, electricity in off-grid markets is primarily supplied by diesel generators. This leads to high electricity prices, dependence on fossil fuels, load shedding, complex fuel supply logistics, and aging infrastructure. Stringent requirements for off-grid markets include affordability, reliability, flexibility, resilience, sustainability (clean energy), energy security, rapid installation, and minimal maintenance. All of these requirements can be addressed by nuclear energy.

[0005] A microreactor is a nuclear reactor with a generating capacity of less than 10 MWe that can be applied to remote locations. Microreactors can be housed in a relatively small vessel, can be operated without active human involvement, and can operate without refueling for longer periods than conventional nuclear power plants.

[0006] One such microreactor is the eVinci microreactor system designed by Westinghouse Electric Company. Microreactors can be housed in a relatively small container, can operate without active human intervention, and can operate without refueling for longer periods of time than conventional nuclear power plants. Other examples of microreactors are described in commonly owned U.S. Provisional Patent Application No. 62 / 984,591, entitled "High Emperature Hydride Moderator Enabling Compact and Higher Power Density Cores Innuclear Microreactors," and U.S. Patent Application No. 14 / 773,405 (published as U.S. Patent Application Publication No. 2016 / 0027536), entitled "Mobile Heat Pipe Cooled Fastreactor System," both of which are incorporated herein by reference in their entireties.

[0007] 1 is a cross-sectional view of an exemplary micro-reactor 100. The micro-reactor 100 may include a core 102 that houses fuel, moderator channels, heat pipes, and reactivity control channels (shutdown rods). The fuel may be enclosed within the core 102, and the heat pipes may extend outside the core 102 to a secondary side of the micro-reactor 100 to extract heat generated by the fuel.

[0008] In one aspect, the core 102 is surrounded by an axial reflector and a radial reflector. As shown in FIG. 1 , six reactivity control drums 106 can be embedded in the radial reflector 118. In various other embodiments, the core 102 is surrounded by more than six control drums 106 (e.g., 12 control drums 106). Each control drum 106 can include a reflector portion 108 and an absorber portion 110. In use, the control drum 106 can be rotated between a reflector position, in which the reflector portion 108 of the control drum faces the monolith core 102 to increase reactivity within the core, and an absorber position, in which the absorber portion 110 faces the core 102 to decrease reactivity within the core. These reflectors can be contained within multiple housings, which can include a structural vessel 112, a neutron absorber vessel 122, and a boron carbide (gamma-ray and neutron shielding) 114. Additionally, an air gap 116 may be provided between the neutron absorber container 122 and the boron carbide shield 114 for shield cooling and gamma ray shielding. All of these components may be located within an outer structural canister 120.

[0009] From a technical and operational standpoint, a reactor core must meet several criteria. One such criterion is that the core must be neutron flexible and capable of using a variety of moderator combinations. Examples of such moderators include a full graphite moderator, a graphite moderator with beryllium oxide (BeO) moderator pins, as described in commonly owned U.S. Patent Application No. 17 / 080,241, filed October 26, 2020, entitled "ENHANCED GRAPHITE NEUTRON REFLECTOR WITH BERYLLIUM OXIDE INCLUSIONS," which is incorporated herein by reference in its entirety, and a YH moderator. x or ZrH xExamples of suitable cores include graphite moderators with moderator pins. Another criterion is that the core must be thermomechanically self-sustaining, e.g., resilient to account for the loss of heat removal in the event of a heat pipe failure. Another criterion is that the core must be able to support the available manufacturing capacity. Another criterion is that the core must be compact enough to be integrated with other core components, such as radial reflectors like control drums, and transported as a unit while retaining its original shape. Another criterion is that the core must be usable in transportable, stationary, and mobile reactors.

[0010] In addition to meeting the above requirements, some applications require a more compact core, while at the same time simplifying fuel handling and reactor assembly.

[0011] In addition to the above, some reactors use triple-clad (TRISO) fuel particles. When TRISO fuel is used, there are concerns about protecting the TRISO fuel from the migration of several metal isotopes from the heat pipes via diffusion processes. Such isotopes, particularly nickel, can damage the silicon carbide layer in the TRISO particles, resulting in the release of fission products and gases from the TRISO core. In addition to the core design criteria mentioned above, minimizing the amount of fuel and its associated cost is very important when considering reactor design.

[0012] Therefore, there is a need to identify core modifications that will help address some, if not all, of the above technical issues and requirements, while preserving the developed unit cell and core designs. Summary of the Invention

[0013] In various embodiments, a nuclear reactor unit cell is disclosed that includes a graphite moderator structure, a heat pipe disposed within the graphite moderator structure, and a fuel assembly disposed within the graphite moderator structure, the fuel assembly including a beryllium oxide sleeve and a nuclear fuel disposed within the beryllium oxide sleeve.

[0014] In various embodiments, a nuclear reactor unit cell is disclosed that includes a graphite moderator substrate, a heat pipe disposed within the graphite moderator substrate, and a plurality of fuel assemblies disposed within the graphite moderator substrate, the plurality of fuel assemblies surrounding the heat pipe, and at least one of the plurality of fuel assemblies including a sleeve constructed of a beryllium-based material and nuclear fuel disposed within the sleeve.

[0015] In various embodiments, a nuclear reactor core is disclosed that includes a plurality of reactor unit cells, at least one of which includes a graphite moderator matrix, a heat pipe disposed within the graphite moderator matrix, and a plurality of fuel assemblies disposed within the graphite moderator matrix, at least one of the plurality of fuel assemblies including a beryllium oxide sleeve configured to contain nuclear fuel. [Brief explanation of the drawings]

[0016] Various features and advantages of the embodiments described herein are explained below with reference to the accompanying drawings, in which:

[0017] FIG. 1 illustrates an exemplary micro-reactor.

[0018] FIG. 2 illustrates multiple reactor unit cells of an enhanced core in accordance with at least one embodiment of the present disclosure.

[0019] FIG. 3 is a side view of a sleeve containing nuclear fuel, according to at least one embodiment of the present disclosure.

[0020] Like reference characters refer to corresponding parts throughout the several views. The examples described herein illustrate one form of various embodiments of the present invention, and such examples are not to be construed as limiting the scope of the present invention in any manner. DETAILED DESCRIPTION OF THE INVENTION

[0021] Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described herein and illustrated in the accompanying drawings. Well-known operations, components, and elements are not described in detail so as not to obscure the embodiments described herein. The reader should understand that the embodiments described and illustrated herein are non-limiting examples, and thus, the specific structural and functional details disclosed herein may be representative and illustrative. These embodiments may be modified or altered without departing from the scope of the claims herein.

[0022] Reference is now made to FIG. 2 , which illustrates a portion of an enhanced reactor core 200 in accordance with at least one embodiment of the present disclosure. The reactor core 200 may include multiple reactor unit cells 202 positioned adjacent to one another within the reactor core 200. In some embodiments, the reactor unit cells 202 may be similar to the reactor core blocks described in U.S. Provisional Patent Application No. 62 / 984,591, which is incorporated herein by reference. While five reactor unit cells 202 are shown for purposes of illustration, it should be understood that any number of reactor unit cells 202 may be included within the reactor core 200.

[0023] In various embodiments, each reactor unit cell 202 may include a graphite moderator structure (substrate) 204. In some embodiments, the graphite moderator structure 204 may be a unitary structure. In other embodiments, the graphite moderator structure 204 may be formed by assembling multiple smaller graphite support structure components. The graphite moderator structure 204 may define multiple channels 206 sized to accommodate multiple heat pipes 208 and multiple fuel assemblies 210, as described in more detail below. FIG. 2 shows only a portion of the channels 206, heat pipes 208, and fuel assemblies 210.

[0024] As shown in FIG. 2 , the channels 206 of the graphite support structure 204 can be arranged such that multiple fuel assemblies 210 surround one heat pipe 208. As an example, as shown in FIG. 2 , the channels 206 can be arranged such that six fuel assemblies 210 surround one heat pipe 208. In other embodiments, more than six fuel assemblies 210 (e.g., 8, 10, or 12 fuel assemblies 210) can surround one heat pipe 208. In other exemplary embodiments, fewer than six fuel assemblies 210 (e.g., 5, 4, or 3 fuel assemblies 210) can surround one heat pipe 208. In an exemplary embodiment, the ratio of the number of fuel assemblies 210 to the number of heat pipes 208 per reactor unit cell can be 24:7, as shown in FIG. 2 . Other example embodiments are contemplated in which the ratio of the number of fuel assemblies 210 to the number of heat pipes 208 is greater than 24:7, such as about 4:1, 5:1, 6:1, or 7:1. Other example embodiments are contemplated in which the ratio of the number of fuel assemblies 210 to the number of heat pipes 208 is less than 24:7, such as about 3:1, 2.5:1, or 2:1. Example embodiments are also contemplated in which the channel 206 is arranged such that multiple fuel assemblies 210 surround multiple heat pipes 208.

[0025] 2 and 3 , each of the fuel assemblies 210 may include a cylindrical sleeve 212 that may be positioned within a channel 206 defined by the graphite moderator structure 204. Additionally, each of the fuel assemblies 210 may include a nuclear fuel 214 that is positioned within the sleeve 212. In various embodiments, the sleeve 212 is positioned within the channel 206 such that at least a portion of the sleeve 212 is radially surrounded by the channel 206 and another portion of the sleeve 212 is not radially surrounded by the channel 206. In other words, the sleeve 212 may be positioned within the channel 206 such that at least a portion of the sleeve 212 is radially surrounded within the graphite support structure 204 and another portion of the sleeve 212 extends outside the graphite support structure 204. In various other embodiments, such as shown in FIG. 3 , the sleeve 212 may be positioned within the channel 206 such that the sleeve 212 is completely radially surrounded by the channel 206.

[0026] In various embodiments, the sleeve 212 can be constructed of a beryllium-based material, such as beryllium oxide (BeO) or beryllium carbide (BeC). Placing the beryllium-based sleeve 212 around the nuclear fuel 214 can be highly beneficial from a neutronics perspective. For example, the beryllium-based material not only moderates neutrons in the nuclear fuel 214, but also contributes in part to reactivity through the (n, 2n) reaction of neutrons with energies of about 1.8 MeV or greater.

[0027] In certain aspects, the thickness of the sidewall 216 of the sleeve 212 can be determined based on the required moderation of the nuclear fuel 214, as well as the thermal gradients and stresses within the core 200. In some exemplary embodiments, the thickness of the sidewall 216 can range from about 0.15 cm to about 0.4 cm. Various other embodiments are contemplated in which the thickness of the sidewall 216 is greater than 0.4 cm (e.g., 0.5 cm, 0.6 cm, 0.7 cm, etc.). Various other embodiments are contemplated in which the thickness of the sidewall 216 is less than 0.15 cm (e.g., 0.125 cm, 0.1 cm, 0.075 cm, etc.).

[0028] As best shown in FIG. 3 , a first radial gap 218 can be defined between the sidewall 216 of the sleeve 212 and the inner edge 220 of the channel 206 of the graphite support structure 204 in which the sleeve 212 is housed. In certain aspects, the size of the first radial gap 218 can be determined based on the expansion of the beryllium-based material and the contraction of the graphite under irradiation, as well as the relative thermal expansion of the beryllium-based material and the graphite. In various embodiments, the first radial gap 218 can range from about 0.1 cm to about 0.2 cm. Other embodiments are contemplated in which the first radial gap 218 is greater than 0.2 cm (e.g., 0.25 cm, 0.3 cm, 0.4 cm, etc.). Other embodiments are contemplated in which the first radial gap 218 is less than 0.1 cm (e.g., 0.75 cm, 0.5 cm, 0.25 cm, etc.).

[0029] Continuing with reference to FIGS. 2 and 3 , as previously described, the fuel assembly 210 can include a nuclear fuel 214 disposed within a sleeve 212. In various embodiments, the nuclear fuel 214 can include TRISO fuel. In various embodiments, the nuclear fuel 214 can include any other suitable nuclear fuel compatible with nuclear reactors, such as a uranium-based fuel, e.g., UN. As best shown in FIG. 3 , a second radial gap 222 can be defined between a sidewall 216 of the sleeve 212 and a radial edge 224 of the nuclear fuel 214. In certain aspects, the size of the second radial gap 222 can account for fuel handling and assembly processes, as well as changes in the fuel compact under irradiation. In various embodiments, the second radial gap 222 can range from about 0.007 cm to about 0.01 cm. Various other embodiments are contemplated in which the second radial gap 222 is greater than 0.01 cm (e.g., 0.02 cm, 0.025 cm, 0.03 cm, etc.). Various other embodiments are also contemplated in which the second radial gap 222 is less than 0.007 cm (e.g., 0.006 cm, 0.005 cm, 0.004 cm, etc.).

[0030] As described above, a first radial gap 218 can be defined between the sidewall 216 of the sleeve 212 and an inner edge 220 of the channel 216 of the graphite support structure 204. Similarly, as described above, a second radial gap 222 can be defined between the sidewall 216 of the sleeve 212 and a radial edge 224 of the nuclear fuel 214. In certain aspects, the ratio of the size of the first radial gap 218 to the second radial gap 222 can range from about 10:1 to about 30:1. Various other embodiments are also contemplated in which the ratio of the size of the first radial gap 218 to the second radial gap 222 is less than 10:1 (e.g., 9:1, 7:1, 5:1, etc.). Various other embodiments are also contemplated in which the ratio of the size of the first radial gap 218 to the second radial gap 222 is greater than 30:1 (e.g., 32:1, 35:1, 40:1, etc.).

[0031] In certain aspects, the first radial gap 218 and the second radial gap 222 may be filled with a gas to maintain the size of the gap and to facilitate heat transfer out of the fuel assembly 210. In one exemplary embodiment, the first radial gap 218 and the second radial gap 222 may be filled with helium gas. Various other embodiments are contemplated in which the first radial gap 218 and the second radial gap 222 are filled with any suitable gas capable of facilitating heat transfer out of the fuel assembly 210.

[0032] 3 , the fuel assembly 210 can further include a first end cap 226 and a second end cap 230. The first end cap 226 and the second end cap 230 can connect to a first open end 228 and a second open end 232 of the sleeve 212, respectively, to enclose the nuclear fuel 214 within the sleeve 212. In various embodiments, the first and second end caps 226, 230 can be press-fit into the first and second open ends 228, 232, respectively, to enclose the nuclear fuel 214 within the sleeve 212. In various embodiments, the first and second end caps 226, 230 can be welded to the first and second open ends 228, 232, respectively, to enclose the nuclear fuel 214 within the sleeve 212. In various embodiments, first and second end caps 226, 230 may be held against first and second open ends 228, 232, respectively, using a latching mechanism to enclose the nuclear fuel 214 within the sleeve 212. In various embodiments, the first and second end caps 226, 230 may be constructed from a beryllium-based material, such as BeO or BeC, to provide similar axial benefits to the nuclear fuel 214 as the sleeve 212.

[0033] The fuel assembly 210 can also include an axial reflector pellet 234 that can be positioned between the first and second end caps 226, 230 and an axial edge 236 of the nuclear fuel 214. In some embodiments, the axial reflector pellet 234 can be constructed of a beryllium-based material, such as BeO or BeC, and can provide similar benefits to the nuclear fuel 214 in the axial direction as the sleeve 212 and end caps 226, 230.

[0034] The above-described beryllium-based sleeve and other improvements described herein have been found to offer numerous advantages over the prior art and to meet many of the aforementioned requirements associated with nuclear reactor design and fuel considerations. Previously, beryllium-based materials have not been considered for nuclear reactor applications due to their limited availability and resulting high cost. Furthermore, beryllium-based materials have not been considered for nuclear reactor applications due to the tendency for them to crack and change shape under the high neutron fluxes encountered in large power reactors. However, smaller power reactors, such as microreactors, where the neutron flux is lower, are well suited to the use of beryllium-based materials.

[0035] As an example of advantages over prior art, the enhancements described above have been found to enable fuel reductions of approximately 5% to 12%, for example, for TRISCO fuel. This reduction translates into reduced fuel active length, core length, and the weight of the core and surrounding components. Furthermore, using 3D thermomechanical analysis that properly models the physical behavior of the heat pipes, it has been found that the enhancements described above enable the reactor system to meet the allowable fuel temperature range. The nuclear system performance also meets safety requirements under transient conditions, particularly for negative power reactivity coefficients.

[0036] Furthermore, the above-described enhancements enable protection of nuclear fuel, such as TRISO fuel, from migration of metal isotopes, such as nickel, that may occur from heat pipes and other metallic components within the reactor, such as reactor instrumentation. Additionally, the beryllium-based sleeve provides a "container" for the nuclear fuel that is placed within each fuel channel of the reactor, facilitating assembly of the reactor. Many other advantages will be readily apparent to those skilled in the art in light of the above disclosure.

[0037] Various aspects of the subject matter described in this application are described in the following examples.

[0038] [Example 1] A reactor unit cell comprising a graphite moderator structure, a heat pipe disposed within the graphite moderator structure, and a fuel assembly disposed within the graphite moderator structure, wherein the fuel assembly comprises a beryllium oxide sleeve and a nuclear fuel disposed within the beryllium oxide sleeve.

[0039] [Example 2] The reactor unit cell of Example 1, wherein the fuel assembly further includes a plurality of beryllium oxide pellets disposed within the beryllium oxide sleeve.

[0040] [Example 3] The reactor unit cell of Example 2, wherein the fuel assembly further comprises an end cap connectable to the beryllium oxide sleeve, and the beryllium oxide pellets are disposed between the end cap and the nuclear fuel.

[0041] [Example 4] The reactor unit cell of any one of Examples 1 to 3, wherein a radial gap is defined between the nuclear fuel and the beryllium oxide sleeve.

[0042] [Example 5] The reactor unit cell of Example 4, wherein the radial gap contains helium gas.

[0043] [Example 6] The reactor unit cell of Example 4 or 5, wherein the radial gap is in the range of about 0.007 cm to about 0.01 cm.

[0044] [Example 7] The reactor unit cell of any one of Examples 1 to 6, wherein a radial gap is defined between the fuel assembly and the graphite moderator structure.

[0045] [Example 8] The reactor unit cell of Example 7, wherein the radial gap contains helium gas.

[0046] [Example 9] The reactor unit cell of Example 7 or 8, wherein the radial gap is in the range of about 0.1 cm to about 0.2 cm.

[0047] Example 10: The reactor unit cell of any one of Examples 1 to 9, wherein the beryllium oxide sleeve has a wall thickness ranging from about 0.15 cm to about 0.4 cm.

[0048] [Example 11] The reactor unit cell of any one of Examples 1 to 10, wherein the nuclear fuel comprises TRISO fuel.

[0049] [Example 12] A reactor unit cell comprising a graphite moderator substrate, a heat pipe disposed within the graphite moderator substrate, and a plurality of fuel assemblies disposed within the graphite moderator substrate, wherein the plurality of fuel assemblies surround the heat pipe, and at least one of the plurality of fuel assemblies comprises a sleeve made of a beryllium-based material and a nuclear fuel disposed within the sleeve.

[0050] [Example 13] The reactor unit cell of Example 12, wherein the at least one fuel assembly further includes a plurality of pellets made of a beryllium-based material, the pellets being disposed within the sleeve.

[0051] [Example 14] The reactor unit cell of Example 13, wherein the at least one fuel assembly further comprises an end cap connectable to the sleeve, and the pellet is disposed between the end cap and the nuclear fuel.

[0052] [Example 15] A reactor unit cell of any one of Examples 12 to 14, wherein a first radial gap is defined between the sleeve and the graphite moderator substrate, and a second radial gap is defined between the nuclear fuel and the sleeve, and the first radial gap is different from the second radial gap.

[0053] [Example 16] The reactor unit cell of Example 15, wherein the first radial gap is in the range of about 0.1 cm to about 0.2 cm, and the second radial gap is in the range of about 0.007 cm to about 0.1 cm.

[0054] [Example 17] The reactor unit cell of Example 15 or 16, wherein the first radial gap and the second radial gap contain helium gas.

[0055] [Example 18] A nuclear reactor core including a plurality of reactor unit cells, at least one of the reactor unit cells comprising a graphite moderator substrate, a plurality of heat pipes disposed within the graphite moderator substrate, and a plurality of fuel assemblies disposed within the graphite moderator substrate, wherein at least one of the plurality of fuel assemblies comprises a beryllium oxide sleeve configured to contain nuclear fuel.

[0056] [Example 19] The nuclear reactor core of Example 18, wherein a radial gap is defined between the at least one fuel assembly and the graphite moderator substrate.

[0057] Example 20: The reactor core of Example 18 or 19, wherein the beryllium oxide sleeve has a wall thickness ranging from about 0.15 cm to about 0.4 cm.

[0058] As is apparent from the foregoing disclosure, unless otherwise specified, discussions throughout the foregoing disclosure using terms such as "processing," "computing," "calculating," "determining," "displaying," and the like, are understood to refer to operations and processes of a computer system or similar electronic computing device in which data represented as physical (electronic) quantities in the registers and memory of that computer system are manipulated and transformed into other data similarly represented as physical quantities in the memory or registers or other information storage, transmission, or display devices of that computer system.

[0059] In this application, one or more components may be referred to as being "configured to," "configurable to," "operable / operative to," "adaptable," "able to," "conformable / conformed to," etc. Those skilled in the art will appreciate that the phrase "configured to" may generally encompass active components and / or inactive components and / or standby components, unless the context requires otherwise.

[0060] Generally, those skilled in the art will recognize that terms used in this application, particularly in the appended claims (e.g., the body of the appended claims), are generally intended to be "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "including but not limited to," etc.). Furthermore, those skilled in the art will recognize that if a specific number of recitations introduced in a claim is intended, such intention will be expressly set forth in the claim; otherwise, no such intention exists. For example, as an aid to understanding, the appended claims may use the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed as suggesting that when a claim recitation is introduced by the indefinite article "a" or "an," any particular claim containing such introduced claim recitation is limited to claims containing only one such recitation, even if the introductory phrase "one or more" or "at least one" and the indefinite article "a" or "an" are both included in the same claim (e.g., "a" and / or "an" should generally be construed to mean "at least one" or "one or more"). The same applies when a definite article is used to introduce a claim recitation.

[0061] Additionally, even when a specific number of recitations introduced in a claim is explicitly recited, one of ordinary skill in the art would understand that such recitation should be interpreted to mean at least the recited number (e.g., "two recitations" without any other modifier means at least two recitations or more than two recitations). Furthermore, in instances where a conventional expression similar to "at least one of A, B, and C, etc." is used, such syntax is typically intended to mean the meaning that one of ordinary skill in the art would understand the conventional expression (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or all of A, B, and C, etc.). Furthermore, in instances where conventional language similar to "at least one of A, B, or C, etc." is used, such syntax is generally intended in the sense that one of ordinary skill in the art would understand the conventional language (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or all of A, B, and C, etc.). Those of ordinary skill in the art will also recognize that disjunctive words and / or phrases expressing two or more alternative terms should generally be understood to contemplate the possibility of including one of the terms, either of the terms, or both of the terms, unless the context requires otherwise. For example, the phrase "A or B" would generally be understood to include the possibilities of "A," "B," or "A and B."

[0062] With respect to the appended claims, those skilled in the art will understand that the actions described therein may generally be performed in any order. Also, while flow diagrams of various actions are shown in a certain order, it should be understood that the various actions may be performed in an order other than that shown, or may be performed simultaneously. Examples of such alternative orderings may include overlapping, interleaving, interrupting, reordering, augmenting, preliminary, additional, concurrent, reverse, or various other orderings, unless the context requires otherwise. Furthermore, terms such as "responsive to," "related to," and other past-tense adjectives are not generally intended to exclude such variations, unless the context requires otherwise.

[0063] It is worth noting that references to "one aspect," "an aspect," "an exemplification," "one exemplification," etc. mean that a particular feature, structure, or characteristic described in connection with that aspect is included in at least one aspect. Thus, the appearances of the phrases "in one aspect," "in an aspect," "in an exemplification," and "in one exemplification" in various places throughout this application do not necessarily all refer to the same aspect. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects.

[0064] Any patent application, patent, non-patent publication, or other disclosure material referenced in this application and / or listed in any Application Data Sheet is incorporated herein by reference to the extent the incorporated material is not inconsistent with this application. Therefore, to the extent necessary, the disclosure as expressly set forth in this application shall supersede any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated herein by reference but that conflicts with existing definitions, opinions, or other disclosure material set forth in this application shall be incorporated only to the extent that there is no conflict between the incorporated material and the existing disclosure material.

[0065] The words "comprise" and its derivatives (e.g., "comprises," "comprising"), "have" and its derivatives (e.g., "has," "having"), "include" and its derivatives (e.g., "includes," "including"), and "contain" and its derivatives (e.g., "contains," "containing") are open-ended linking verbs. That is, a system that "comprises," "has," "contains," or "contains" one or more elements has, but is not limited to, having only, one or more of those elements. Similarly, a system, device, or equipment element that "comprises," "has," "contains," or "contains" one or more features has, but is not limited to, having only one or more of those features.

[0066] As used in this disclosure, unless otherwise specified, the terms "substantially," "about," or "approximately" refer to a tolerance for a particular value as determined by one of ordinary skill in the art, which depends in part on how the value is measured or determined. In certain embodiments, the terms "substantially," "about," or "about" refer to within 1, 2, 3, or 4 standard deviations. In certain embodiments, the terms "substantially," "about," or "about" refer to a range within 50%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value.

[0067] In summary, many advantages have been described by employing the concepts described herein. The foregoing description of one or more embodiments has been presented for purposes of illustration and description and is not intended to be exhaustive or limiting of the precise forms disclosed. Modifications or variations are possible in light of the above teachings. The one or more embodiments have been selected and described to illustrate principles and practical applications, thereby enabling those skilled in the art to utilize various embodiments, with various modifications, as suitable for the particular use contemplated. It is intended that the claims presented herewith define the overall scope.

Claims

1. a graphite moderator structure; a heat pipe disposed within the graphite moderator structure; a fuel assembly disposed within the graphite moderator structure; A reactor unit cell comprising: a beryllium oxide sleeve; a nuclear fuel disposed within the beryllium oxide sleeve; A reactor unit cell comprising:

2. 2. The reactor unit cell of claim 1, wherein said fuel assembly further comprises a plurality of beryllium oxide pellets disposed within said beryllium oxide sleeve.

3. 3. The nuclear reactor unit cell of claim 2, wherein said fuel assembly further comprises an end cap mateable with said beryllium oxide sleeve, said beryllium oxide pellets being disposed between said end cap and said nuclear fuel.

4. 2. The nuclear reactor unit cell of claim 1, wherein a radial gap is defined between said nuclear fuel and said beryllium oxide sleeve.

5. 5. The reactor unit cell of claim 4, wherein said radial gap contains helium gas.

6. 5. The reactor unit cell of claim 4, wherein said radial gap is in the range of about 0.007 cm to about 0.01 cm.

7. 2. The reactor unit cell of claim 1, wherein a radial gap is defined between said fuel assemblies and said graphite moderator structure.

8. 8. The reactor unit cell of claim 7, wherein said radial gap contains helium gas.

9. 8. The reactor unit cell of claim 7, wherein said radial gap is in the range of about 0.1 cm to about 0.2 cm.

10. 2. The reactor unit cell of claim 1, wherein said beryllium oxide sleeve has a wall thickness ranging from about 0.15 cm to about 0.4 cm.

11. The nuclear reactor unit cell of claim 1 , wherein the nuclear fuel comprises TRISO fuel.

12. a graphite moderator substrate; a heat pipe disposed within the graphite moderator substrate; a plurality of fuel assemblies disposed within the graphite moderator substrate; a plurality of fuel assemblies surrounding the heat pipe, and at least one fuel assembly of the plurality of fuel assemblies a sleeve made of a beryllium-based material; a nuclear fuel disposed within the sleeve; A reactor unit cell comprising:

13. 13. The reactor unit cell of claim 12, wherein said at least one fuel assembly further includes a plurality of pellets comprised of a beryllium-based material, said pellets being disposed within said sleeve.

14. 14. The nuclear reactor unit cell of claim 13, wherein said at least one fuel assembly further comprises an end cap connectable to said sleeve, said pellets being disposed between said end cap and said nuclear fuel.

15. 13. The nuclear reactor unit cell of claim 12, wherein a first radial gap is defined between the sleeve and the graphite moderator substrate, and a second radial gap is defined between the nuclear fuel and the sleeve, the first radial gap being different from the second radial gap.

16. 16. The nuclear reactor unit cell of claim 15, wherein said first radial gap is in the range of about 0.1 cm to about 0.2 cm, and said second radial gap is in the range of about 0.007 cm to about 0.1 cm.

17. 16. The nuclear reactor unit cell of claim 15, wherein the first radial gap and the second radial gap contain helium gas.

18. A nuclear reactor core comprising a plurality of reactor unit cells, At least one of the reactor unit cells comprises: a graphite moderator substrate; a plurality of heat pipes disposed within the graphite moderator matrix; a plurality of fuel assemblies disposed within the graphite moderator substrate; wherein at least one of the plurality of fuel assemblies includes a beryllium oxide sleeve configured to contain nuclear fuel.

19. 20. The nuclear reactor core of claim 18, wherein a radial gap is defined between the at least one fuel assembly and the graphite moderator substrate.

20. 20. The nuclear reactor core of claim 18, wherein said beryllium oxide sleeve has a wall thickness in the range of about 0.15 cm to about 0.4 cm.

Images