Nuclear reactor with axially stratified fuel bed

Abstract

A nuclear reactor having an axially stratified fuel bed. The reactor is characterized by a reactor housing having a base, a top with an exhaust port, and an axis. The axially stratified fuel bed is within the reactor housing and includes: a first zone configured to operate at a first temperature T1, the first zone including a plurality of first fuel particles, each of which includes a first radioactive ceramic core and a first ceramic sealing coating; and a second zone configured to operate at a second temperature T2, wherein T2 exceeds T1, the second zone including a plurality of second fuel particles, each of which includes a second radioactive ceramic core and a second ceramic sealing coating. A coolant fluid flow channel transports coolant fluid from the reactor base to the exhaust port, sequentially passing through the first and second zones along the flow channel. The stability of the first ceramic sealing coating in T1 is greater than its stability in T2, while the stability of the second ceramic sealing coating in T2 is greater than the stability of the first ceramic sealing layer in T2.

Description

[0001] Government Support Statement

[0002] This invention was made with government support under Contract No. DE-NE0008745 awarded by the U.S. Department of Energy. The government may have certain rights in this invention. Technical Field

[0003] Various exemplary embodiments disclosed herein generally relate to a nuclear reactor with an axially stratified fuel bed suitable for high-temperature aerospace applications. The axially stratified fuel bed provides a grading method for pebbles or compact bodies with different TRISO layers that affect fuel properties such as temperature tolerance and irradiation reactivity. Background Art

[0004] A conventional high temperature gas-cooled nuclear reactor (HTGR) using spherical or cylindrical TRISO-based fuel elements is filled with a homogeneous bed of fuel elements during operation.

[0005] HTGR reactors operate at temperatures of approximately 900°C and use nuclear fuel elements consisting of pelletized nuclear fuel. The pelletized fuel is surrounded by a matrix material, which is molded into fuel elements of various shapes and geometries, such as spheres (pebbles) and cylinders (dense solids). The fuel elements are then stacked into a bed of pebbles or dense solids, through which coolant gas flows. The coolant gas can be either inert (such as nitrogen or argon) or non-inert (such as hydrogen or water). The fuel elements heat the coolant gas. The hot gas can be used for various purposes, such as power generation.

[0006] The granular nuclear fuel in an HTGR reactor may include a radioactive ceramic core (e.g., uranium oxide, thorium oxide, plutonium oxide, uranium carbide, thorium carbide, plutonium carbide, uranium nitride, thorium nitride, or plutonium nitride). In various embodiments, the radioactive ceramic core is coated with a ceramic sealing coating, such as a carbide or nitride of silicon, zirconium, or tungsten. The radioactive ceramic core is coated with multiple coatings. For example, the granular nuclear fuel may be a tristructural isotropic (TRISO) particle. TRISO nuclear fuel particles utilize continuous carbon laminar layers and a ceramic sealing coating. TRISO nuclear fuel particles include a uranium-based nuclear fuel particle core (e.g., UO3 or U3O8), which is initially coated with a porous low-density carbon layer (i.e., a buffer carbon layer), an inner dense pyrolytic carbon layer (IPyC), a ceramic sealing coating (e.g., silicon carbide), and an outer dense pyrolytic carbon layer (OPyC). These layers are deposited on the core using fluidized bed chemical vapor deposition technology.

[0007] Silicon-based ceramics are often used as sealing coatings for nuclear fuel particles, such as TRISO-type particles. However, in some applications, nuclear reactors are required to operate at extreme temperatures (i.e., temperatures exceeding 2000°C). For example, such reactors might be used in nuclear thermal propulsion (NTP) technology. Silicon-based ceramic sealing coatings are not suitable for high-temperature applications (i.e., temperatures exceeding 1500°C). Various carbide, boride, nitride, and / or oxide ceramics are stable at temperatures exceeding 2000°C. However, the production costs of many of these high-temperature ceramics are high, making their use as sealing coatings for all nuclear fuel particles within a fuel bed economically unfeasible. Furthermore, some ceramic materials, such as tungsten carbide, are very stable at extreme temperatures but do not possess the appropriate neutron cross-section to sustain criticality. Thus, for example, a bed composed of nuclear fuel particles with a tungsten carbide sealing coating might resist degradation at extreme temperatures but would not be able to sustain nuclear reactions.

[0008] There is a need in the art for an economically viable nuclear reactor whose fuel particles can both maintain criticality and withstand extreme temperatures. Summary of the Invention

[0009] Given the current demand for high-temperature nuclear reactors, various exemplary embodiments are briefly described. The following description may contain some simplifications and omissions, which are intended to highlight and introduce certain aspects of the various exemplary embodiments and are not intended to limit the scope of the present invention. In the following sections, preferred exemplary embodiments are described in sufficient detail to enable one of ordinary skill in the art to make and use the concepts of the present invention.

[0010] Various embodiments disclosed herein relate to a nuclear reactor having an axially stratified fuel bed, comprising a reactor housing having a base, a top having an exhaust port, and an axis. The axially stratified fuel bed comprises a plurality of zones:

[0011] a first zone configured to operate at a first temperature T1, the first zone comprising a plurality of first fuel particles, each first fuel particle having a first radioactive ceramic core and a first ceramic sealing coating;

[0012] a second zone configured to operate at a second temperature T2, wherein T2>T1, the second zone comprising a plurality of second fuel particles, each second fuel particle comprising a second radioactive ceramic core and a second ceramic sealing coating;

[0013] The stability of the first ceramic sealing coating in T1 is greater than that in T2, and the stability of the second ceramic sealing coating in T2 is greater than that of the first ceramic sealing coating in T2.

[0014] In various embodiments, the reactor includes a coolant fluid flow channel configured to transport a coolant fluid from a reactor base to an exhaust port, wherein the coolant fluid flow channel sequentially passes through the first region and the second region. The coolant fluid can be hydrogen, water, ammonia, oxygen, or carbon dioxide. The coolant fluid can also be an inert gas, such as nitrogen, argon, or helium. In a nuclear thermal propulsion (NTP) reactor for aerospace applications, the coolant fluid can flow through the reactor at high speeds, serving both as a propellant and removing heat from the reactor.

[0015] According to various embodiments, the first fuel particle includes a first ceramic sealing coating having a first neutron absorption cross section; the second fuel particle includes a second ceramic sealing coating having a second neutron absorption cross section, wherein the second neutron absorption cross section is higher than the first neutron absorption cross section.

[0016] According to various embodiments, a first fuel particle includes a first radioactive ceramic core, and a second fuel particle includes a second radioactive ceramic core having a second neutron absorption cross section, wherein the first radioactive ceramic core and the second radioactive ceramic core are each an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium. The first radioactive ceramic core may be an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium; and the second radioactive ceramic core may be an oxide of uranium.

[0017] In various embodiments, the first fuel particles include a first ceramic sealing coating of silicon oxide, carbide, boride, carbon oxide or nitride; the second fuel particles include a second ceramic sealing coating of a high-temperature ceramic-based sealing coating, and the high-temperature ceramic-based sealing coating is selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, MoB, ZrC, HfC, TiC, TaC, MoC, Mo2C, WC, HfN, ZrN, TiN, SiBCN and Ta4HfC5.

[0018] In various embodiments, the first fuel particle includes a first ceramic sealing coating, which is an oxide, carbide, carbon oxide or nitride of silicon or zirconium; the second fuel particle includes a second ceramic sealing coating, which is an oxide, carbide, carbon oxide or nitride of zirconium or tungsten, wherein the first ceramic sealing coating and the second ceramic sealing coating do not contain zirconium at the same time.

[0019] The first ceramic sealing coating may be silicon carbide, configured for a first zone operating at a first temperature T1; the second ceramic sealing coating may be zirconium carbide, configured for a second zone operating at a second temperature T2, wherein T1 is between 750°C and 1200°C, and T2 is between 1200°C and 2000°C.

[0020] The first ceramic sealing coating may be silicon carbide, configured for a first zone operating at a first temperature T1; the second ceramic sealing coating may be tungsten carbide, configured for a second zone operating at a second temperature T2, wherein T1 is between 750°C and 1600°C, and T2 is between 1600°C and 2800°C.

[0021] The first ceramic sealing coating may be zirconium carbide, configured for a first zone operating at a first temperature T1; the second ceramic sealing coating may be tungsten carbide, configured for a second zone operating at a second temperature T2, wherein T1 is between 750°C and 2000°C, and T2 is between 2000°C and 2800°C.

[0022] Various embodiments disclosed herein relate to a nuclear reactor having an axially stratified fuel bed, comprising a plurality of zones. The axially stratified fuel bed comprises:

[0023] a first zone configured to operate at a first temperature T1, the first zone comprising a plurality of first fuel particles, each first fuel particle having a first radioactive ceramic core and a first ceramic sealing coating;

[0024] a second zone configured to operate at a second temperature T2, wherein T2>T1, the second zone comprising a plurality of second fuel particles, each second fuel particle comprising a second radioactive ceramic core and a second ceramic sealing coating;

[0025] a third zone configured to operate at a third temperature T3, wherein T3>T2, the third zone comprising a plurality of third fuel particles, each of the third fuel particles comprising a third radioactive ceramic core and a third ceramic sealing coating;

[0026] The stability of the first ceramic sealing coating in T1 is greater than that in T2, the stability of the second ceramic sealing coating in T2 is greater than that in T3, and the stability of the third ceramic sealing coating in T3 is greater than that of the second ceramic sealing coating in T3. The first ceramic sealing coating may have a first neutron absorption cross section, and the third ceramic sealing coating may have a third neutron absorption cross section, wherein the third neutron absorption cross section is higher than the first neutron absorption cross section.

[0027] In various embodiments, an axially stratified fuel bed comprises:

[0028] a plurality of first fuel particles, wherein the first fuel particles have:

[0029] a first ceramic sealing coating of silicon oxide, carbide, oxycarbide, or nitride;

[0030] a porous carbon layer, a pyrolytic carbon layer, or a combination thereof; or

[0031] an optional porous carbon layer, an optional inner pyrolytic carbon layer, a first ceramic sealing coating, and an outer pyrolytic carbon layer;

[0032] a plurality of second fuel particles, the second fuel particles having a second ceramic sealing coating of a first high temperature ceramic-based sealing coating of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, TaC, HfN, ZrN, TiN, SiBCN, or Ta4HfC5;

[0033] a plurality of third fuel particles, the third fuel particles having a third ceramic sealing coating selected from a second high temperature ceramic-based sealing coating of HfB2, ZrB2, TiB2, TaB2, HfC, TiC, TaC, WC, HfN, ZrN, TiN, or Ta4HfC5;

[0034] The second ceramic sealing coating and the third ceramic sealing coating are different. The second fuel particle may include an outer pyrolytic carbon layer on the second ceramic sealing coating. The third fuel particle may include an outer pyrolytic carbon layer on the third ceramic sealing coating.

[0035] In various embodiments, an axially stratified fuel bed comprises:

[0036] a plurality of first fuel particles having a first ceramic sealing coating, wherein the first ceramic sealing coating is silicon oxide, carbide, oxycarbide or nitride;

[0037] a plurality of second fuel particles having a second ceramic sealing coating, the second ceramic sealing coating being a first high temperature ceramic-based sealing coating of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, HfN, ZrN, TiN, or SiBCN;

[0038] A plurality of third fuel particles having a third ceramic sealing coating, wherein the third ceramic sealing coating is a second high temperature ceramic-based sealing coating selected from WC, TaC, HfC or Ta4HfC5.

[0039] In various embodiments, an axially stratified fuel bed comprises:

[0040] a first zone configured to operate at a first temperature T1, the first zone comprising a plurality of first fuel particles, each first fuel particle having a first radioactive ceramic core and a sealing coating of silicon oxide, carbide, oxycarbide, or nitride;

[0041] a second zone configured to operate at a second temperature T2, wherein T2>T1, the second zone comprising a plurality of second fuel particles, each second fuel particle comprising a second radioactive ceramic core and a sealing coating of zirconium oxide, carbide, oxycarbide, or nitride;

[0042] A third zone is configured to operate at a third temperature T3, where T3>T2. The third zone includes a plurality of third fuel particles, each of which includes a third radioactive ceramic core and a sealing coating of tungsten oxide, carbide, oxycarbide, or nitride. The first fuel particle may have a sealing coating of silicon carbide; the second fuel particle may have a sealing coating of zirconium carbide; and the third fuel particle may have a sealing coating of tungsten carbide. Different sealing coatings may be selected so that:

[0043] The first fuel particle operates at T1 of 750°C to 1200°C;

[0044] The second fuel particle operates at T2 of 1200°C to 2000°C;

[0045] The third fuel particle operates at T3 of 2000°C to 2800°C.

[0046] The nuclear reactor of claim 1 further comprising:

[0047] a plurality of first fuel elements in the first zone, each first fuel element comprising a plurality of first fuel particles distributed in a first matrix material, wherein the first matrix material is stable in T1;

[0048] A plurality of second fuel elements in the second zone, each second fuel element comprising a plurality of second fuel particles distributed in a second matrix material, wherein the second matrix material is stable in T2.

[0049] Various embodiments disclosed herein relate to a nuclear reactor having an axially stratified fuel bed, comprising a plurality of zones:

[0050] a first zone configured to operate at a first temperature T1, the first zone comprising a plurality of first fuel elements, the first fuel elements comprising a plurality of first fuel particles distributed in a first matrix material, wherein the first matrix material is stable at T1;

[0051] A second zone configured to operate at a second temperature T2, where T2>T1, includes a plurality of second fuel elements, each of which includes a plurality of second fuel particles distributed in a second matrix material, wherein the second matrix material is stable at T2. The reactor may also include a third zone configured to operate at a third temperature T3, where T3>T2, includes a plurality of third fuel elements, each of which includes a plurality of third fuel particles distributed in a third matrix material, wherein the second matrix material is stable at T3.

[0052] The first matrix material may be ceramic, metal, ceramic-metal composite, composite material composed of at least two ceramics, graphite or phenolic resin. The second and / or third matrix material may be graphite, WC or tungsten. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] For a better understanding of various exemplary embodiments, reference is made to the accompanying drawings, in which:

[0054] Figure 1 An HTGR reactor with an axially stratified fuel bed is shown;

[0055] Figures 2A-2C Shows the application Figure 1 TRISO-type fuel particles for reactors;

[0056] Figures 3A-3C Shows the application Figure 1 seal-coated fuel particles for reactors;

[0057] Figure 4 The nuclear fuel elements are shown;

[0058] Figure 5 A HTGR reactor is shown having an axially stratified fuel bed containing nuclear fuel elements. DETAILED DESCRIPTION

[0059] Referring now to the drawings, wherein like numerals refer to like components or steps, broad aspects of various exemplary embodiments are disclosed.

[0060] As used herein, "TRISO particles" are defined as nuclear fuel particles that utilize continuous carbon laminar layers and a ceramic sealing coating. TRISO nuclear fuel particles include a nuclear fuel particle core that is initially coated with an optional porous low-density carbon layer (i.e., a buffer carbon layer), an inner dense pyrolytic carbon layer (IPyC), a ceramic sealing coating, and an optional outer dense pyrolytic carbon layer (OPyC). The nuclear fuel particle core can be a uranium, thorium, or plutonium ceramic. The ceramic sealing coating can be other high-temperature ceramics such as oxides, carbides, nitrides, or carbon oxides of silicon, zirconium, or tungsten.

[0061] While silicon-based ceramics can be used as sealing coatings for nuclear fuel particles (such as TRISO-type particles), they are not suitable for high-temperature applications. Silicon oxides are stable up to approximately 800°C, making them only suitable for nuclear reactors operating at low temperatures. Since high-gaseous thermal reactors (HGTR) reactors typically operate at temperatures up to 900°C, nuclear fuel particles with silicon oxide sealing coatings may not be suitable. Silicon carbide and silicon carbonitride are stable at temperatures of approximately 1400°C to 1500°C and are therefore more suitable for HGTR reactors. Silicon boron carbonitride (SiBCN) ceramics are stable up to 1800°C and can also be used in HGTR reactors. However, reactors used for nuclear thermal propulsion (NTP) technology operate at temperatures up to 2700°C. At these temperatures, silicon-based ceramic sealing coatings are unsuitable because they are not thermally stable. For example, silicon carbide dissociates at approximately 2450°C.

[0062] For better thermal stability, various carbides, borides, nitrides, and oxycarbides have acceptable thermal properties, including melting points approaching or exceeding 3000°C. Suitable borides include tantalum, titanium, hafnium, and zirconium borides, each of which has a melting point exceeding 3000°C. Carbides of tantalum, titanium, hafnium, zirconium, and tungsten, or nitrides of titanium, hafnium, and zirconium may also be used. In some embodiments, ceramic alloys, such as titanium hafnium carbide, may be used as high-temperature sealing coatings.

[0063] This disclosure describes a method for staging the fuel bed in a HTGR reactor, which allows operation at higher temperatures, achievable with a homogenous bed. This allows HTGR reactors with graded beds to operate at higher efficiencies and also provides a reactor for applications such as nuclear thermal propulsion, which requires temperatures exceeding 2000°C.

[0064] The graded fuel element bed uses a mixture of traditional TRISO particles and new TRISO particles with modified layer sequences and carbide materials, such as zirconium carbide (ZrC) and tungsten carbide (WC). These materials can be used as sealing coatings due to their improved mechanical properties (such as density). TRISO-like particles include a sealing coating with a thin layer of dense pyrolytic carbon and a subsequent thin layer of ceramic material (such as SiC, ZrC or WC), which is applied directly to the uranium-based fuel particles or through an intermediate buffer carbon layer. Before the TRISO particles are mixed with the matrix material and the resulting mixture is molded into nuclear fuel pebbles or compacts, these sealing coating-coated nuclear fuel particles can be used as is or an additional TRISO layer, such as an outer dense pyrolytic carbon layer, is applied to the sealing coating surface. Compared with SiC, ZrC or WC materials are generally not used in traditional HTGR applications due to their high production costs and poor irradiation performance.

[0065] Nuclear Thermal Propulsion (NTP) technology requires gas temperatures exceeding 2700°C. This presents a challenge for conventional homogeneous fuel bed designs, as the primary pressure layer and sealing layer (silicon carbide) are not suited to these extreme temperatures. However, for NTP designs, the highest temperatures occur in the exhaust region, while the lowest temperatures occur where the heated gas is introduced.

[0066] This disclosure describes a three-layer, graded fuel bed that utilizes conventional TRISO material with a SiC-based sealing coating at the bottom of the fuel bed. This is where gas is introduced and fuel temperatures are lowest. Because the conventional silicon carbide-based TRISO layer also exhibits ideal irradiation properties, this allows the fuel bed to react and generate heat under ideal irradiation conditions.

[0067] In the exhaust region of the fuel element bed, operating temperatures can exceed 2700°C. A TRISO material with a high-temperature ceramic-based sealing coating is used on top of the fuel bed. In various embodiments, the high-temperature ceramic-based sealing coating can be made of HfB2, ZrB2, TiB2, TaB2, TiC, TaC, WC, HfN, ZrN, TiN, or Ta4HfC5. In various embodiments, a TRISO material with a tungsten carbide-based sealing coating is used on top of the fuel bed. Compared to SiC, tungsten carbide has significantly improved mechanical properties and heat resistance. However, compared to SiC, tungsten carbide has poorer irradiation reactivity. However, since the TRISO material with the WC-based sealing coating is used in the exhaust port with lower reactivity, it does not affect the operation of the reactor.

[0068] At the mid-bed temperature, where temperatures begin to approach the operating limits of silicon carbide-based TRISO layers, e.g., approximately 1400°C to 1500°C, nuclear fuel elements made of TRISO material with a high-temperature ceramic-based sealing coating are used. Suitable high-temperature ceramic-based sealing coatings at the mid-bed temperature can be made of HfB2, ZrB2, TiB2, TaB2, HfC, ZrC, TiC, TaC, HfN, ZrN, TiN, or Ta4HfC5. Depending on the mid-bed temperature, certain silicon-based ceramics can also be used as sealing coatings for nuclear fuel particles. For example, if the mid-bed temperature of the reactor operates at temperatures between 1400°C and 1800°C, silicon boron carbonitride (SiBCN) can be used as a sealing coating. In various embodiments, zirconium carbide is used as a sealing coating for TRISO particles at the mid-bed temperature. This zirconium carbide exhibits similar irradiation characteristics to silicon carbide but is more resistant to high temperatures. When the temperature approaches the operating limit of zirconium carbide, TRISO material with WC-based sealing coating is used instead of TRISO material with ZrC-based sealing coating.

[0069] In various embodiments, an axially stratified fuel bed comprises:

[0070] a plurality of first fuel particles having a first ceramic sealing coating, wherein the first ceramic sealing coating is silicon oxide, carbide, oxycarbide or nitride;

[0071] a plurality of second fuel particles having a second ceramic sealing coating, the second ceramic sealing coating being a first high temperature ceramic-based sealing coating of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, HfN, ZrN, TiN, or SiBCN;

[0072] A plurality of third fuel particles having a third ceramic sealing coating, wherein the third ceramic sealing coating is a second high-temperature ceramic-based sealing coating selected from WC, TaC, HfC, or Ta4HfC5. Due to the extreme temperatures at the exhaust port at the top of the fuel bed, tungsten carbide and high-melting point carbides of tantalum and / or hafnium (melting point > 4000°C) are ideal.

[0073] In various embodiments, the secondary fuel particles in the middle of the fuel bed may be made of a ceramic alloy coating. For example, a high-temperature boride ceramic may be co-deposited with 5% to 30% (by weight) SiC. The borides HfB2, ZrB2, and TiB2 may be co-deposited with SiC. While this lowers the service temperature of the resulting alloy, the alloy remains useful at the temperatures prevailing in the middle of the layered bed (perhaps between 1200°C and 2000°C). For example, ZrB2 is a high-temperature ceramic with a melting point exceeding 3000°C. A ZrB2 alloy containing 23% SiC (ZrB2-SiC alloy) has a melting point of 2270°C.

[0074] Accordingly, in various embodiments, an axially stratified fuel bed may include:

[0075] a plurality of first fuel particles having a first ceramic sealing coating, wherein the first ceramic sealing coating is SiC;

[0076] a plurality of second fuel particles having a second ceramic sealing coating, wherein the second ceramic sealing coating is a ceramic alloy ZrB2-SiC, HfB2-SiC, TaB2-SiC or TiB2-SiC;

[0077] A plurality of third fuel particles having a third ceramic sealing coating, wherein the third ceramic sealing coating is HfB2, ZrB2, TiB2 or TaB2.

[0078] This disclosure also describes a dual-layer, graded fuel bed that utilizes a conventional TRISO material with a SiC-based sealing coating at the bottom of the fuel bed, where gas is introduced and fuel temperatures are lowest. In the exhaust region of the fuel element bed, operating temperatures can reach 2000°C to 2700°C. In various embodiments, the high-temperature, ceramic-based sealing coating in the exhaust region of the bed can be made of HfB2, ZrB2, TiB2, TaB2, HfC, ZrC, TiC, TaC, WC, HfN, ZrN, TiN, or Ta4HfC5. For example, a TRISO material with a ZrC- or WC-based sealing coating can be used at the top of the fuel bed. If the exhaust region temperature is 2000°C or lower, a TRISO material with a ZrC- or WC-based sealing coating can be used in the exhaust region. If the exhaust region temperature exceeds 2000°C, a TRISO material with a WC-based sealing coating should be used in the exhaust region.

[0079] Generally, the ceramic sealing coating can be deposited onto the nuclear fuel particles by chemical vapor deposition from a suitable precursor using techniques known in the art. The ceramic sealing coating can be deposited directly onto the nuclear fuel particles or through an intermediate pyrolytic carbon layer. An outer pyrolytic carbon layer can optionally be deposited over the ceramic sealing coating.

[0080] As mentioned above, a graded fuel bed can operate at much higher temperatures than a conventional HTGR-type homogeneous fuel bed. This could potentially increase the efficiency of reactor designs for energy production and allow for specialized applications, such as NTP designs at extreme temperatures.

[0081] Figure 1 A HTGR reactor 1 having a graded fuel bed 2 is shown. The reactor includes a housing 1d having a bottom 1a, a top 1b, and an axis 1a. A coolant inlet 1f is located at or near the bottom 1a of the reactor housing 1a, and a coolant outlet 1c is located at or near the top 1c of the reactor housing 1d. Coolant gas flows into the coolant inlet 1f and out of the coolant outlet 1c. The coolant flows through the plurality of nuclear fuel particles in the graded fuel bed 2. The coolant fluid can be hydrogen, water, ammonia, oxygen, or carbon dioxide. The coolant fluid can also be an inert gas, such as nitrogen, argon, or helium.

[0082] like Figure 1 As shown, the graded fuel element bed 2 comprises a plurality of zones, including 2 to 4 zones. The graded fuel element bed 2 may comprise three zones, including Figure 1 The graded fuel element bed 2 may comprise two zones, including Figure 1 . The first zone 2a is located at the base of the reactor, near the gas inlet 1f. Zone 2a contains nuclear fuel particles 3. The nuclear fuel particles 3 are configured to operate at a first temperature T1, wherein each nuclear fuel particle 3 includes a first radioactive ceramic core and a first sealing coating. The first sealing coating can be a first ceramic sealing coating that is stable at T1. In various embodiments, the first ceramic sealing coating is an oxide, carbide, nitride or oxynitride of silicon or zirconium. The first ceramic sealing coating can be silicon carbide, silicon nitride, zirconium carbide or zirconium nitride.

[0083] In various embodiments, the graded fuel element bed 2 comprises two zones, including Figure 1Zones 2a and 2c are shown. The uppermost zone 2c is located at the top of the reactor, near the gas outlet 1c. Zone 2c contains nuclear fuel particles 5. The nuclear fuel particles 5 are configured to operate at a third temperature T3, wherein each nuclear fuel particle 5 includes a third radioactive ceramic core and a third sealing coating. The third sealing coating can be a third ceramic sealing coating that is stable at T3, wherein T3>T1. In various embodiments, the third ceramic sealing coating is an oxide, carbide, nitride or oxynitride of zirconium or tungsten. The third ceramic sealing coating can be zirconium carbide, zirconium nitride, tungsten carbide or tungsten oxynitride. The first radioactive ceramic core and the third radioactive ceramic core are each an oxide, carbide, oxycarbon or nitride of uranium, thorium or plutonium.

[0084] In various embodiments, the graded fuel element bed 2 includes two zones 2a and 2c. The nuclear fuel particles 3 in zone 2a include a first radioactive ceramic core and a ceramic sealing coating comprising silicon carbide. The nuclear fuel particles 5 in zone 2c include a third radioactive ceramic core and a ceramic sealing coating comprising zirconium carbide. The nuclear fuel particles 3 are configured to operate at a T1 temperature of 750°C to 1200°C, and the nuclear fuel particles 5 are configured to operate at a T2 temperature of 1200°C to 2000°C. The silicon carbide coating in the nuclear fuel particles 3 is unstable at temperatures exceeding 1600°C.

[0085] In various embodiments, nuclear fuel particles 3 in zone 2a include a first radioactive ceramic core and a ceramic sealing coating comprising silicon carbide. Nuclear fuel particles 5 in zone 2c include a third radioactive ceramic core and a ceramic sealing coating comprising tungsten carbide. Nuclear fuel particles 3 are configured to operate at a T1 temperature of 750°C to 1600°C, at which point the silicon carbide coating becomes thermally unstable. Nuclear fuel particles 5 are configured to operate at a T2 temperature of 1600°C to 2800°C.

[0086] In various embodiments, nuclear fuel particles 3 in zone 2a include a first radioactive ceramic core and a ceramic sealing coating comprising zirconium carbide. Nuclear fuel particles 5 in zone 2c include a third radioactive ceramic core and a ceramic sealing coating comprising tungsten carbide. Nuclear fuel particles 3 are configured to operate at a T1 temperature of 750°C to 2000°C, and nuclear fuel particles 5 are configured to operate at a T2 temperature of 2000°C to 2800°C.

[0087] The graded fuel element bed 2 may comprise three zones, including Figure 1Zones 2a, 2b, and 2c are shown. Zone 2a contains nuclear fuel particles 3 configured to operate at a first temperature T1. Zone 2b contains nuclear fuel particles 4 configured to operate at a second temperature T2, wherein T2>T1. Zone 2c contains nuclear fuel particles 5 configured to operate at a third temperature T3, wherein T3>T2. Nuclear fuel particles 3 have a first ceramic sealing coating, and the thermal stability of the first ceramic sealing coating in T1 is higher than the thermal stability in T2. ​​Nuclear fuel particles 4 have a second ceramic sealing coating, and the thermal stability of the second ceramic sealing coating in T2 is higher than the thermal stability of the first ceramic sealing coating in T2, and the thermal stability in T2 is higher than the thermal stability in T3. Nuclear fuel particles 5 have a third ceramic sealing coating, and the thermal stability of the third ceramic sealing coating in T3 is higher than the thermal stability of the second ceramic sealing coating in T3.

[0088] Figure 2A Shows suitability for use as Figure 1 Particles of nuclear fuel particles 3 in a first zone 2a of a reactor, wherein the first zone operates at a temperature of up to 1200°C or up to 1500°C. Figure 2A The particle 3 shown includes a radioactive core or core 11, which can be an oxide, carbide, oxycarbon, or nitride of uranium, thorium, or plutonium. Core 11 can be a uranium oxide-based nuclear fuel particle core, such as UO3 or U3O8. Core 11 is then coated with an optional buffer carbon layer 12, an inner dense pyrolytic carbon layer (IPyC) 13, a ceramic sealing coating 14a, and an optional outer dense pyrolytic carbon layer (OPyC) 15. In various embodiments, the nuclear fuel particle 3 is a TRISO particle comprising each of layers 12 to 15, wherein layer 14a is silicon carbide. In various embodiments, the nuclear fuel particle 3 is a TRISO-type particle comprising at least layers 13 and 14a, wherein layer 14a is silicon oxide, carbide, nitride, or oxynitride.

[0089] Figure 2B Shows suitability for use as Figure 1 Particles of nuclear fuel particles 4 in the middle zone 2b of the reactor, wherein the second zone operates at a temperature of up to 1600°C or up to 2000°C. Figure 2B The particle 4 shown includes a radioactive nucleus or core 11, similar to Figure 2AAs shown. The core 11 is then coated with an optional buffer carbon layer 12, an inner dense pyrolytic carbon layer (IPyC) 13, a ceramic sealing coating 14b, and an optional outer dense pyrolytic carbon layer (OPyC) 15. In various embodiments, the nuclear fuel particle 5 is a TRISO-type particle comprising at least layers 13 and 14b, wherein layer 14b is zirconium oxide, carbide, nitride, or oxynitride. In various embodiments, the first zone operates at temperatures up to 1200°C and the second zone operates at temperatures up to 1600°C. Layer 14b can be a layer of zirconium carbide, hafnium boride, titanium boride, or tantalum boride, or a boride layer selected from hafnium boride, titanium boride, tantalum boride, and zirconium boride. In various embodiments, the second zone operates at temperatures up to 2000°C. Layer 14b can be a layer of zirconium carbide, or a boride layer selected from hafnium boride, titanium boride, tantalum boride, and zirconium boride.

[0090] Figure 2C Shows suitability for use as Figure 1 Particles of nuclear fuel particles 5 in the upper zone 2c of the reactor, wherein the third zone operates at a temperature of up to 2000°C or up to 2800°C. Figure 2C The illustrated particle 5 includes a radioactive nucleus or core 11. Core 11 is then coated with an optional buffer carbon layer 12, an inner dense pyrolytic carbon layer (IPyC) 13, a ceramic sealing coating 14c, and an optional outer dense pyrolytic carbon layer (OPyC) 15. In various embodiments, the nuclear fuel particle 5 is a TRISO-type particle comprising at least layers 13 and 14c, wherein layer 14c is a tungsten oxide, carbide, nitride, or oxynitride. In various embodiments, the second zone operates at temperatures up to 1600°C and the second zone operates at temperatures up to 2000°C, and layer 14c can be a layer of zirconium carbide or tungsten carbide, or a boride selected from hafnium boride, titanium boride, tantalum boride, and zirconium boride. In various embodiments, the second zone operates at temperatures up to 2800°C, and layer 14b is a tungsten carbide layer.

[0091] As mentioned above, the graded fuel element bed 2 may comprise two zones, including Figure 1Zones 2a and 2c are shown. The materials for the ceramic sealing coatings used for the nuclear fuel particles 3 and 5 are selected based on the operating temperatures of each zone. If zone 2a reaches temperatures up to 1200°C and zone 2c reaches temperatures up to 2000°C, zone 2a may contain nuclear fuel particles 3 using a silicon-based ceramic layer as layer 14a, while zone 2c may contain nuclear fuel particles 5 using a zirconium-based or tungsten-based ceramic layer as layer 14c. If zone 2a reaches temperatures up to 1600°C and zone 2c reaches temperatures up to 2800°C, zone 2a may contain nuclear fuel particles 3 having a zirconium-based or silicon-based ceramic layer 14a, while zone 2c may contain nuclear fuel particles 5 having a tungsten-based ceramic layer 14c. If zone 2a reaches temperatures up to 2000°C and zone 2c reaches temperatures up to 2800°C, zone 2a may contain nuclear fuel particles 3 having a zirconium-based ceramic layer 14a, while zone 2c may contain nuclear fuel particles 5 having a tungsten-based ceramic layer 14c. Tungsten-based ceramics have higher thermal stability than zirconium-based ceramics, which in turn have higher thermal stability than silicon-based ceramics. However, the precursors for zirconium- and tungsten-based ceramics are more expensive than those for silicon-based ceramics. Therefore, a more economical silicon-based sealing coating 14a can be used on the particles 3 in the low-temperature zone 2a, while a zirconium- or tungsten-based ceramic sealing coating can be used in the high-temperature zone 2c. In certain embodiments, the low-temperature zone 2a operates at temperatures exceeding 1200°C or exceeding 1600°C. In this case, a zirconium-based sealing coating 14a can be used on the particles 3 in the low-temperature zone 2a, while a tungsten-based ceramic sealing coating can be used in the high-temperature zone 2c.

[0092] Figures 3A to 3C Shows suitability for use as Figure 1 An embodiment of particle replacement of nuclear fuel particles in a reactor. Figure 3A The pellet 3 shown can be used in the lower region 2a of the reactor. The pellet 3 includes a radioactive nucleus or core 21, which can be an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium. A ceramic sealing coating 22a is then applied to the core 21. In various embodiments, the ceramic sealing coating 22a is an oxide, carbide, nitride, or oxynitride of silicon, configured to operate at temperatures up to 1200°C. Figure 3B Shows suitability for use as Figure 1 Particle replacement embodiment of nuclear fuel particles 4 in the middle region 2b of the reactor. Figure 3B The particle 4 shown includes a radioactive nucleus or core 21 and a ceramic sealing coating 22b, wherein the layer 22b is an oxide, carbide, nitride or oxynitride of zirconium, which is configured to operate at temperatures up to 2000°C. Figure 3C Shows suitability for use as Figure 1 Particle replacement embodiment of nuclear fuel particles 5 in the upper zone 2c of the reactor. Figure 3CThe particle 5 shown includes a radioactive nucleus or core 21 and a ceramic sealing layer 22c, wherein the layer 22c is a tungsten oxide, carbide, nitride or oxynitride, configured to operate at temperatures up to 2800°C.

[0093] In various embodiments, the first ceramic sealing coating (e.g. Figure 2A The layer 14a in the embodiment has a first neutron absorption cross section; the second ceramic sealing coating (e.g. Figure 2B The layer 14b in the embodiment has a second neutron absorption cross section; the third ceramic sealing coating (eg Figure 2C The layer 14c in the third zone 2c has a third neutron absorption cross section. The third neutron absorption cross section may be lower than the first and / or second neutron absorption cross sections. At the temperatures prevailing in the reactor zone 2a, silicon carbide has good thermal stability and favorable irradiation properties, such as a high neutron absorption cross section, enabling the core particles 3 with the SiC layer 14a in the fuel bed zone 2a to react and generate heat. At the temperatures prevailing in the reactor zone 2b (which exceed the prevailing temperature in zone 2a), zirconium carbide has good thermal stability and acceptable irradiation properties, so core particles 4 with the ZrC layer 14b can be used in zone 2a. In various embodiments, the temperature of the third zone 2c can reach 2500°C to 2500°C. Most ceramic coatings are thermally unstable under such extreme conditions. Therefore, core particles 5 with a tungsten carbide layer 14c are used in zone 2c. The tungsten carbide layer 14c has good thermal stability at the temperatures prevailing in the reactor zone 2c. Although WC has worse radiation characteristics (e.g., lower neutron absorption cross section) than ZrC or SiC, in the high temperature region 2c, radiation characteristics are less important than thermal stability, so the use of the WC layer 14c is acceptable. The same considerations apply to Figures 3A to 3C Ceramic sealing coatings 22a-22c in.

[0094] Back to Figure 1 , Figures 3A to 3C Single-layer coated nuclear fuel particles and Figures 2A to 2C Combinations of multiple layers of coated nuclear fuel particles can be used in different zones of the reactor. In the lower zone 2a, particles 3 having a radioactive core or core 21 and a single silicon carbide outer layer 22a can be used, wherein zone 2a operates at temperatures up to 1200°C. In the middle zone 2b, particles 3 having a radioactive core or core 21 and a single silicon carbide outer layer 22a can be used. Figure 2AA TRISO particle 3 comprising a radioactive core 11, a silicon carbide layer 14a, and an outer pyrolytic carbon layer 15 is shown, wherein the intermediate zone 2b operates at temperatures up to 1600°C. In the lower zone 2a, the exposed silicon carbide layer 22a on the particle is thermally stable up to 1200°C; above this temperature, the silicon carbide surface may oxidize if the coolant gas contains an oxygen source. The silicon carbide layer 14a on the particle in the intermediate zone 2b is protected from exposure to oxidants by the dense pyrolytic carbon layer 15 and is therefore less susceptible to oxidation. Figure 2C or Figure 3C Nuclear fuel particles 5 having a zirconium-based or tungsten-based ceramic sealing coating 14c or 22c therein can be used in the upper zone 2c of the reactor.

[0095] In an alternative embodiment, Figure 1 In the reactor, different zones can use a combination of nuclear fuel particles lacking a sealing coating and nuclear fuel particles with a ceramic sealing coating. In the lower zone 2a, particles 3 having a radioactive core or core 12, a porous carbon layer 12 and a pyrolytic carbon layer 13 can be used, wherein the particles 3 in zone 2a are similar to the nuclear fuel particles except that the outer layers 14a and 15 are missing. Figure 2A The particles in the zone 2a are similar. Zone 2a operates at temperatures up to 1200°C. In the intermediate zone 2b, Figure 2A A TRISO particle 3 in the middle zone 2b comprises a radioactive core 11, inner carbon layers 12 and 13, a silicon carbide or zirconium carbide layer 14a, and an outer pyrolytic carbon layer 15, wherein zone 2b operates at temperatures up to 1600° C. The silicon carbide layer 14a on the particle in the middle zone 2b is protected from exposure to oxidants by the dense pyrolytic carbon layer 15. Figure 2C or Figure 3C The nuclear fuel particles 5 having the zirconium-based or tungsten-based ceramic sealing coating 14c or 22c can be used in the upper zone 2c of the reactor. Generally speaking, the zirconium-based ceramic sealing coating is not used in both the zone 2b and the zone 2c.

[0096] Although Figure 1 While shown is a graded fuel element bed 2 having a plurality of regions 2a-2c of nuclear fuel particles, various embodiments disclosed herein include a graded fuel element bed in which the fuel elements comprise nuclear fuel particles dispersed in a matrix material. Figure 4 A nuclear fuel compact 31 is shown with nuclear fuel particles 33 dispersed in a matrix material 32. The nuclear fuel particles 33 may be Figures 2A to 2C Any of the particles 3, 4 and 5 shown, wherein the ceramic layer 14a, 14b or 14c is selected according to the required operating temperature of the compact. Alternatively, the nuclear fuel particle 33 may be Figures 3A to 3CIn any of the illustrated particles 3, 4, and 5, ceramic layer 22a, 22b, or 22c is selected based on the desired operating temperature. If the desired operating temperature of nuclear fuel density body 31 is below 1200°C, matrix material 32 can be a ceramic material; a metal; a ceramic-metal composite (e.g., titanium carbide / nickel-cobalt cermet, tungsten carbide / cobalt cermet); a composite material consisting of at least two ceramics; graphite; or a phenolic resin. If the desired operating temperature of nuclear fuel density body 31 exceeds 1200°C, matrix material 32 can be a thermally stable material such as graphite, WC, or tungsten.

[0097] The nuclear fuel compact 31 may be a cube or a cylinder. In various embodiments, the nuclear fuel compact 31 may be a generally spherical pebble with the nuclear fuel particles 33 evenly distributed therein.

[0098] Figure 5 Shows the Figure 1 A similar HTGR reactor 1 with a graded fuel element bed 2 is shown. The reactor includes a housing 41d. A coolant inlet 41f is located at or near the bottom 41a of the reactor housing 41a, and a coolant outlet 41c is located at or near the top 41b of the reactor housing 1d. Figure 5 As shown, the graded fuel element bed 42 comprises a plurality of zones, including zones 42a, 42b and 42c. The first zone 42a is located at the bottom of the reactor, near the gas inlet 41f. The third zone 42c is located at the top of the reactor, near the gas outlet 41c.

[0099] Region 42a contains a core secret entity 31a, wherein the secret entity 31a is similar to Figure 4 The dense body 31a includes nuclear fuel particles 33 dispersed in a matrix 32, wherein the nuclear fuel particles 33 may be particles 3 of FIG. 2a or FIG. 3a, wherein the particles 3 include a ceramic sealing coating 14a or 22a of silicon carbide. The zone 42a containing the nuclear dense body 31a is configured to operate at a temperature of up to 1200° C. or up to 1600° C.

[0100] The region 42b includes a nuclear dense body 31b. The dense body 31b includes nuclear fuel particles 33 dispersed in a matrix 32, wherein the nuclear fuel particles 33 may be Figure 2B or Figure 3B The particles 4 in the embodiment of the present invention are characterized in that the particles 3 include a ceramic sealing coating 14b or 22b of zirconium carbide. The zone 42b including the core dense body 31b is configured to operate at a temperature of up to 2000°C.

[0101] The region 42c includes a nuclear dense body 31c. The dense body 31c includes nuclear fuel particles 33 dispersed in a matrix 32, wherein the nuclear fuel particles 33 may be Figure 2C or Figure 3CThe particles 5 in the embodiment of the present invention are characterized in that the particles 3 include a ceramic sealing coating 14c or 22c of tungsten carbide. The zone 42c including the core dense body 31c is configured to operate at a temperature of up to 2800°C.

[0102] The compact bodies 31a, 31b and 31c in the regions 42a, 42b and 42b contain sufficient mass to avoid fluidization of the coolant gas.

[0103] Although various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects, it will be understood that the invention is capable of other embodiments and that its details are capable of modification in various obvious respects. It will be apparent to those skilled in the art that variations and modifications can be made to the invention without departing from the spirit and scope of the invention. Therefore, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined solely by the claims.

Claims

1. A nuclear reactor having an axially stratified fuel bed, comprising: a reactor housing having a base, a top having an exhaust port, and a shaft; The axially stratified fuel bed comprises: a first zone configured to operate at a first temperature T1, the first zone comprising a plurality of first fuel particles, each first fuel particle comprising a first radioactive ceramic core and a first coating; wherein the first coating is a first ceramic sealing coating; and a second zone configured to operate at a second temperature T2, wherein T2>T1, the second zone comprising a plurality of second fuel particles, each second fuel particle comprising a second radioactive ceramic core and a second ceramic sealing coating; a coolant fluid flow channel configured to transport coolant fluid from a reactor base to an exhaust port, wherein the coolant fluid flow channel sequentially passes through the first zone and the second zone; in: The stability of the first ceramic sealing coating in T1 is greater than the stability in T2; The stability of the second ceramic sealing coating in T2 is greater than the stability of the first ceramic sealing coating in T2.

2. The nuclear reactor of claim 1, wherein the coolant fluid is nitrogen, argon, helium, hydrogen, water, ammonia, oxygen, or carbon dioxide.

3. The nuclear reactor according to claim 1, wherein: The first ceramic sealing coating has a first neutron absorption cross section; The second ceramic sealing coating has a second neutron absorption cross section; The second neutron absorption cross section is higher than the first neutron absorption cross section.

4. The nuclear reactor according to claim 1, wherein: The first radioactive ceramic core and the second radioactive ceramic core are each an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium.

5. The nuclear reactor according to claim 1, wherein: The first radioactive ceramic core is selected from the group consisting of oxides, carbides, oxycarbides, or nitrides of uranium, thorium, or plutonium; The second radioactive ceramic core is uranium oxide.

6. The nuclear reactor according to claim 1, wherein: The first ceramic sealing coating is selected from the group consisting of silicon oxide, carbide, boride, oxycarbide or nitride; The second ceramic sealing coating is a high temperature ceramic-based sealing coating selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, TaC, WC, HfN, ZrN, TiN, SiBCN and Ta4HfC5.

7. The nuclear reactor according to claim 1, wherein: The first ceramic sealing coating is an oxide, carbide, boride, carbon oxide or nitride of silicon or zirconium; The second ceramic sealing coating is an oxide, boride, carbide, oxycarbide or nitride of zirconium or tungsten; The first ceramic sealing coating and the second ceramic sealing coating do not contain zirconium at the same time.

8. The nuclear reactor according to claim 7, wherein: The first ceramic sealing coating is silicon carbide; The second ceramic sealing coating is zirconium carbide; T1 is between 750°C and 1200°C, and T2 is between 1200°C and 2000°C.

9. The nuclear reactor according to claim 7, wherein: The first ceramic sealing coating is silicon carbide; The second ceramic sealing coating is tungsten carbide; T1 is between 750°C and 1600°C, and T2 is between 1600°C and 2800°C.

10. The nuclear reactor according to claim 7, wherein: The first ceramic sealing coating is zirconium carbide; The second ceramic sealing coating is tungsten carbide; Among them, T1 is between 750°C and 2000°C, and T2 is between 2000°C and 2800°C.

11. The nuclear reactor of claim 1 , further comprising: a third zone configured to operate at a third temperature T3, wherein T3>T2, the third zone comprising a plurality of third fuel particles, each of the third fuel particles comprising a third radioactive ceramic core and a third ceramic sealing coating; in: The stability of the second ceramic sealing coating in T2 is greater than the stability in T3; The stability of the third ceramic sealing coating in T3 is greater than the stability of the second ceramic sealing coating in T3.

12. The nuclear reactor according to claim 11, wherein: The first ceramic sealing coating has a first neutron absorption cross section; The third ceramic sealing coating has a third neutron absorption cross section; The third neutron absorption cross section is higher than the first neutron absorption cross section.

13. The nuclear reactor according to claim 12, wherein: The first ceramic sealing coating is silicon carbide; The second ceramic sealing coating is zirconium carbide; The third ceramic sealing coating is tungsten carbide.

14. The nuclear reactor according to claim 13, wherein: T1 is between 750°C and 1200°C; T2 is between 1200°C and 2000°C; T3 is between 2000°C and 2800°C.

15. The nuclear reactor according to claim 11, wherein: The first ceramic sealing coating is silicon oxide, carbide, carbon oxide or nitride; The second ceramic sealing coating is a first high temperature ceramic-based sealing coating, and the first high temperature ceramic-based sealing coating is selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, TaC, HfN, ZrN, TiN, SiBCN and Ta4HfC5; The third ceramic sealing coating is a second high temperature ceramic-based sealing coating, and the second high temperature ceramic-based sealing coating is selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, TiC, TaC, WC, HfN, ZrN, TiN or Ta4HfC5; The second ceramic sealing coating and the third ceramic sealing coating are different.

16. The nuclear reactor of claim 11, wherein: The first ceramic sealing coating is silicon oxide, carbide, carbon oxide or nitride; The second ceramic sealing coating is a first high temperature ceramic-based sealing coating, and the first high temperature ceramic-based sealing coating is selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, ZrC, TiC, HfN, ZrN, TiN, SiBCN and Ta4HfC5; The third ceramic sealing coating is a second high-temperature ceramic-based sealing coating, and the second high-temperature ceramic-based sealing coating is selected from the group consisting of WC, TaC, HfC and Ta4HfC5.

17. The nuclear reactor of claim 11, wherein: The first ceramic sealing coating is selected from the group consisting of silicon oxide, carbide, oxycarbide or nitride; The second ceramic sealing coating is zirconium oxide, carbide, carbon oxide or nitride; The third ceramic sealing coating is tungsten oxide, carbide, carbon oxide or nitride.

18. The nuclear reactor of claim 11, further comprising: a plurality of first fuel elements in the first zone, each first fuel element comprising a plurality of first fuel particles distributed in a first matrix material, wherein the first matrix material is stable at T1; a plurality of second fuel elements in the second zone, each second fuel element comprising a plurality of second fuel particles distributed in a second matrix material, wherein the second matrix material is stable at T2; and A plurality of third fuel elements in the third zone, each third fuel element comprising a plurality of third fuel particles distributed in a third matrix material, wherein the third matrix material is stable at T3.

19. The nuclear reactor of claim 11, wherein: Each first fuel particle includes a first radioactive ceramic core, a first ceramic seal coating, and at least one carbon coating; Each second fuel particle includes a second radioactive ceramic core, a second ceramic seal coating, and at least one carbon coating; Each of the third fuel particles includes a third radioactive ceramic core, a third ceramic seal coating, and at least one carbon coating.

20. The nuclear reactor of claim 1 , further comprising: a plurality of first fuel elements in the first zone, each first fuel element comprising a plurality of first fuel particles distributed in a first matrix material, wherein the first matrix material is stable at T1; and A plurality of second fuel elements in the second zone, each second fuel element comprising a plurality of second fuel particles distributed in a second matrix material, wherein the second matrix material is stable at T2.

21. The nuclear reactor of claim 20, wherein the first matrix material is selected from the group consisting of ceramics; metals; ceramic-metal composites; composites composed of at least two ceramics; graphite; and phenolic resins.

22. The nuclear reactor of claim 20, wherein the second matrix material is selected from the group consisting of graphite, WC, and tungsten.

23. The nuclear reactor of claim 1, wherein: Each first fuel particle includes a first radioactive ceramic core, a first ceramic seal coating, and at least one carbon coating; Each second fuel particle includes a second radioactive ceramic core, a second ceramic seal coating, and at least one carbon coating.

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