A nuclear reactor having a fuel bed arranged in layers in the axial direction

Abstract

A nuclear reactor having a fuel bed arranged in layers in the axial direction, the reactor having a base, an upper part having an exhaust port, and a reactor shell having a shaft, the fuel bed arranged in layers in the axial direction within the reactor shell being set to operate at a first temperature T1 and including a first region containing a plurality of first fuel particles each having a first radioactive ceramic core and a first ceramic seal coating, and a second region set to operate at a second temperature T2 which is higher than T1 and containing a plurality of second fuel particles each having a second radioactive ceramic core and a second ceramic seal coating, the coolant fluid flow path being characterized in that it transports the coolant fluid from the base of the reactor to the exhaust port and is carried along a flow path that sequentially passes through the first region and the second region. Also, the first ceramic seal coating has better stability at T1 than at T2, and the second ceramic seal coating has better stability at T2 than the first ceramic seal coating at T2.

Description

Technical Field

[0001] The various embodiments disclosed herein generally relate to nuclear reactors having axially layered fuel beds suitable for high-temperature aerospace applications. The axially layered fuel beds provide a stepwise approach to pebbles and compacts with different TRISO layers and affect fuel properties such as temperature resistance and radiation reactivity.

Background Art

[0002] Conventional high-temperature gas-cooled nuclear reactors (HTGRs) use spherical or cylindrical TRISO-based fuel elements during operation and are filled with a homogeneous bed of fuel elements.

[0003] HTGR reactors operate at temperatures of about 900°C and use nuclear fuel elements containing granular nuclear fuel. The granular nuclear fuel is surrounded by a matrix material and formed and compressed into various shapes and geometries, such as spherical (e.g., pebbles) or cylindrical (e.g., compacts), to form nuclear fuel elements. These nuclear fuel elements are then stacked as a bed of pebbles or compacts, and a cooling gas flows through the bed. This cooling gas may be an inert gas such as nitrogen or argon, or a non-inert gas such as hydrogen or steam. The fuel elements heat the cooling gas, and the heated gas is used for multiple applications such as power generation.

[0004] The particulate nuclear fuel in an HTGR reactor may include a radioactive ceramic core such as, for example, 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 covered with a ceramic seal coat such as a carbide or nitride of silicon, zirconium, or tungsten. The radioactive ceramic core is covered with a plurality of coatings. For example, the particulate nuclear fuel may be a tristructural isotropic particle (TRISO particle). The TRISO nuclear fuel particle utilizes a continuous layered structure of carbon and a ceramic seal coat. The TRISO nuclear fuel particle includes, for example, a uranium-based nuclear fuel particle core such as UO3 or U3O8, and is first coated with a porous low-density carbon layer (buffer carbon layer), an inner dense pyrolytic carbon (IPyC), a ceramic seal coat (e.g., silicon carbide), and an outer dense pyrolytic carbon (OPyC). These layers are deposited on the core using fluidized bed chemical vapor deposition technology.

[0005] Silicon-based ceramics are frequently used as seal coatings for nuclear fuel particles such as TRISO-type particles. However, for certain applications, nuclear reactors may be required to operate at extremely high temperatures, i.e., temperatures exceeding 2000°C. Such reactors may be used, for example, in nuclear thermal propulsion (NTP) technology. Silicon-based ceramic seal coatings are not ideal for high-temperature applications exceeding 1500°C. There are carbide, boride, nitride, and / or oxide ceramics that are stable at temperatures exceeding 2000°C, but many of these high-temperature ceramics have high production costs and are not economically feasible to use as seal coatings for all nuclear fuel particles within a fuel bed. Furthermore, some ceramic materials, such as tungsten carbide, are very stable at extremely high temperatures but do not have an appropriate neutron cross-section to maintain criticality. Thus, a bed of nuclear fuel particles coated with, for example, tungsten carbide can withstand degradation at extreme temperatures but may not be able to sustain a nuclear reaction.

Summary of the Invention

Problems to be Solved by the Invention

[0006] As a technical requirement, there is a need for a nuclear reactor that can maintain criticality while withstanding extreme temperatures and is still economically feasible, having fuel particles that can withstand extreme temperatures while maintaining criticality.

Means for Solving the Problems

[0007] In view of the current demand for high-temperature nuclear reactors, a concise summary of various embodiments is presented. In the following summary, some simplifications and omissions may be made, which are intended to highlight and disclose some aspects of the various embodiments and do not limit the scope of the invention. A detailed description of the preferred embodiments will be provided in later paragraphs to enable those of ordinary skill in the art to make and use the inventive concept.

[0008] The various embodiments disclosed herein relate to a nuclear reactor that includes an axially layered fuel bed. The reactor vessel has a base, an upper portion with an exhaust port, and an axis. The axially layered fuel bed includes a plurality of regions. The first region includes a plurality of first fuel particles configured to operate at a first temperature T1 and having a first radioactive ceramic core and a first coating that is a carbon coating or a first ceramic seal coating or a combination thereof. The second region includes a plurality of second fuel particles configured to operate at a second temperature T2 that is higher than T1 and having a second radioactive ceramic core and a second ceramic seal coating. The first ceramic seal coating has better stability at T1 than at T2, and the second ceramic seal coating is configured to have better stability at T2 than the first ceramic seal coating at T2.

[0009] In various embodiments, the reactor has a coolant fluid flow path configured to carry a coolant fluid from the base of the reactor to the exhaust port and along a flow path that sequentially passes through the first region and the second region. The coolant may be hydrogen, water, ammonia, oxygen, or carbon dioxide. The coolant may also be an inert gas such as nitrogen, argon, or helium. In the case of a nuclear thermal propulsion (NTP) reactor for aerospace applications, the coolant passes through the reactor at high speed and serves as a propellant while also removing heat from the reactor.

[0010] According to various embodiments, the first fuel particles include a first ceramic seal coating having a first neutron absorption cross section, and the second fuel particles include a second ceramic seal coating having a second neutron absorption cross section. Here, the second neutron absorption cross section is larger than the first neutron absorption cross section.

[0011] According to various embodiments, the first fuel particle has a first radioactive ceramic core, and the second fuel particle has a second radioactive ceramic core having a second neutron absorption cross section. 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 can be an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium, while the second radioactive ceramic core can be uranium oxide.

[0012] In various embodiments, the first fuel particle includes a first ceramic seal coating made of an oxide, carbide, boride, oxycarbide, or nitride of silicon. On the other hand, the second fuel particle includes a second ceramic seal coating of a high-temperature ceramic-based seal coat selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, MoB, ZrC, HfC, TiC, TaC, MoC, Mo2C, WC, HfN, ZrN, TiN, SiBCN, Ta4HfC5.

[0013] In various embodiments, the first ceramic seal coating is an oxide, carbide, boride, oxycarbide, or nitride of silicon or zirconium, the second ceramic seal coating is an oxide, boride, carbide, oxycarbide, or nitride of zirconium or tungsten, and neither the first ceramic seal coating nor the second ceramic seal coating contains zirconium.

[0014] The first ceramic seal coating can be silicon carbide for use in a first region set to operate at a first temperature T1, and the second ceramic seal coating can be zirconium carbide for use in a second region set to operate at a second temperature T2. Here, T1 ranges from 750°C to 1200°C, and T2 ranges from 1200°C to 2000°C.

[0015] The first ceramic seal coating may be silicon carbide for use in a first region configured to operate at a first temperature T1. On the other hand, the second ceramic seal coating may be tungsten carbide for use in a second region configured to operate at a second temperature T2. Here, T1 ranges from 750°C to 1600°C, and T2 ranges from 1600°C to 2800°C.

[0016] The first ceramic seal coating may be zirconium carbide for use in a first region configured to operate at a first temperature T1. On the other hand, the second ceramic seal coating may be tungsten carbide for use in a second region configured to operate at a second temperature T2. Here, T1 ranges from 750°C to 2000°C, and T2 ranges from 2000°C to 2800°C.

[0017] The various embodiments disclosed herein relate to a nuclear reactor having an axially layered fuel bed including a plurality of regions. The axially layered fuel bed a first region including a plurality of first fuel particles, each having a first radioactive ceramic core and a first coating that is a carbon coating or a first ceramic seal coating or a combination thereof, configured to operate at a first temperature T1; a second region including a plurality of second fuel particles, each having a second radioactive ceramic core and a second ceramic seal coating, configured to operate at a second temperature T2 that is higher than T1; a third region including a plurality of third fuel particles, each having a third radioactive ceramic core and a third ceramic seal coating, configured to operate at a third temperature T3 that is higher than T2. The first ceramic seal coating has higher stability at T1 than at T2, the second ceramic seal coating has higher stability at T2 than at T3, and the third ceramic seal coating has higher stability at T3 than the second ceramic seal coating. The first ceramic seal coating has a first neutron absorption cross-section, the third ceramic seal coating has a third neutron absorption cross-section, and the third neutron absorption cross-section may be higher than the first neutron absorption cross-section.

[0018] In various embodiments, the axially layered fuel bed includes: a number of first fuel particles having an oxide, carbide, oxycarbide, or nitride of silicon as the first ceramic seal coating; a porous carbon layer, a pyrolytic carbon layer, or a combination thereof; or an optional porous carbon layer, an optional inner pyrolytic carbon layer, the first ceramic seal coating, and an outer pyrolytic carbon layer; a number of second fuel particles having any of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, TaC, HfN, ZrN, TiN, SiBCN, or Ta4HfC5 as the second high-temperature ceramic-based seal coating; and a number of third fuel particles having any of HfB2, ZrB2, TiB2, TaB2, HfC, TiC, TaC, WC, HfN, ZrN, TiN, or Ta4HfC5 as the second high-temperature ceramic-based seal coating; Here, the second ceramic seal coating and the third ceramic seal coating are different. The second fuel particles may include an outer pyrolytic carbon layer on the second ceramic seal coating. The third fuel particles may include an outer pyrolytic carbon layer on the third ceramic seal coating.

[0019] In various embodiments, the axially layered fuel bed includes a number of first fuel particles having a first ceramic seal coating of an oxide, carbide, oxycarbide, or nitride of silicon, and A number of second fuel particles having a first high-temperature ceramic-based seal coating composed of any one of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, HfN, ZrN, TiN, or SiBCN, A number of third fuel particles having a second high-temperature ceramic-based seal coating selected from WC, TaC, HfC, or Ta4HfC5.

[0020] In various embodiments, the axially layered fuel bed includes the following: A first region including a plurality of first fuel particles configured to operate at a first temperature T1 and each having a first radioactive ceramic core and a seal coating of silicon oxide, carbide, oxycarbide, or nitride, A second region including a plurality of second fuel particles configured to operate at a second temperature T2 that is higher than T1 and each having a second radioactive ceramic core and a seal coating of zirconium oxide, carbide, oxycarbide, or nitride, A third region including a plurality of third fuel particles configured to operate at a third temperature T3 that is higher than T2 and each having a third radioactive ceramic core and a seal coating of tungsten oxide, carbide, oxycarbide, or nitride. The first fuel particles may have a seal coating of silicon carbide, the second fuel particles may have a seal coating of zirconium carbide, and the third fuel particles may have a seal coating of tungsten carbide. The various seal coatings may be selected as follows: The first fuel particles operate at T1 between 750°C and 1200°C, the second fuel particles operate at T2 between 1200°C and 2000°C, and the third fuel particles operate at T3 between 2000°C and 2800°C.

[0021] The nuclear reactor according to claim 1 further comprises A plurality of first fuel elements in which a plurality of first fuel particles are dispersed in a first matrix material that is stable at T1, It includes a plurality of second fuel elements in which a plurality of second fuel particles are dispersed in a second matrix material that is stable at T2.

[0022] Various embodiments disclosed herein relate to a nuclear reactor having an axially layered fuel bed including a plurality of regions. The axially layered fuel bed is set to operate at a first temperature T1 and has a first region having a plurality of first fuel elements in which a plurality of first fuel particles are dispersed in a first matrix material that is stable at T1, is set to operate at a second temperature T2 which is higher than T1 and has a second region having a plurality of first fuel elements in which a plurality of second fuel particles are dispersed in a second matrix material that is stable at T2, is set to operate at a third temperature T3 which is higher than T2 and has a third region having a plurality of third fuel elements in which a plurality of third fuel particles are dispersed in a third matrix material that is stable at T3.

[0023] The first matrix material may be a ceramic, a metal, a ceramic-metal composite material, a composite material composed of at least two ceramics, graphite, or a phenolic resin. The second and / or third matrix materials may be graphite, WC, or tungsten.

Brief Description of the Drawings

[0024] To better understand the various embodiments, reference is made to the accompanying drawings.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0025] Referring to the drawings, like numbers refer to like parts or steps, and broad aspects of various embodiments are disclosed.

[0026] As used herein, the term "TRISO particle" refers to a nuclear fuel particle that utilizes a continuous layer of carbon and ceramic seal coating. A TRISO nuclear fuel particle includes a core of the nuclear fuel particle and is first coated with an optional porous low-density carbon layer, i.e., a buffer carbon layer, an inner high-density pyrolytic carbon (IPyC), a ceramic seal coating, and an optional outer high-density pyrolytic carbon (OPyC). The core of the nuclear fuel particle may be a ceramic of uranium, thorium, or plutonium. The ceramic seal coating may be a high-temperature ceramic such as an oxide, carbide, nitride, or oxycarbide of silicon, zirconium, or tungsten.

[0027] Silicon-based ceramics can be used as seal coatings for nuclear fuel particles such as TRISO-type particles, but silicon-based ceramic seal coatings are not suitable for high-temperature applications. Silicon oxide is stable at temperatures up to about 800°C, and the use of silicon oxide is only suitable for reactors operating at low temperatures. High-temperature gas-cooled reactor (HTGR) reactors typically operate at temperatures up to 900°C, so nuclear fuel particles with silicon oxide seal coatings may not be suitable. Silicon carbide and silicon carbonitride are stable at temperatures from about 1400°C to 1500°C and are more suitable for HTGR reactors. Silicon boron carbonitride (SiBCN) ceramics are stable at temperatures up to 1800°C and can also be used in HTGR reactors. However, reactors used in nuclear thermal propulsion (NTP) technology operate at temperatures up to 2700°C. At these temperatures, silicon-based ceramic seal coatings are not suitable because they are not thermally stable. For example, silicon carbide decomposes at about 2450°C.

[0028] To achieve excellent thermal stability, various carbides, borides, nitrides, and oxycarbides with acceptable thermal properties and melting points close to or exceeding 3000°C are suitable. Suitable borides include borides of tantalum, titanium, hafnium, and zirconium, each with 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 cases. In some embodiments, ceramic alloys such as titanium hafnium carbide may be used as high-temperature seal coatings.

[0029] The present disclosure describes a staged approach to fuel element beds in an HTGR-style reactor, enabling operation at higher temperatures than achievable with a homogeneous bed. This allows an HTGR reactor with a staged bed to operate with higher efficiency and provides a reactor suitable for applications such as nuclear thermal propulsion, which requires temperatures above 2000°C.

[0030] The staged fuel element bed utilizes a mixture of conventional TRISO particles and new TRISO particles with altered layer sequences and carbide materials (e.g., zirconium carbide (ZrC) and tungsten carbide (WC)). Due to improved mechanical properties such as the density of these materials, they can be applied as a seal coat. Particles similar to TRISO include a seal coat applied directly to the uranium-based fuel particle or via a buffer carbon layer, with a thin high-density pyrolytic carbon layer followed by a thin ceramic material layer (e.g., SiC, ZrC, or WC). These sealed nuclear fuel particles can be used as is or additional TRISO layers such as an outer high-density pyrolytic carbon layer can be applied over the seal coat. Subsequently, the TRISO particles are mixed with a matrix material and the resulting mixture is formed into nuclear fuel pebbles or compacts. ZrC and WC materials are not typically used as they have high manufacturing costs for conventional HTGR applications and degraded irradiation characteristics compared to SiC.

[0031] Nuclear thermal propulsion (NTP) technology requires gas temperatures above 2700°C. This is a challenge for conventional homogeneous fuel bed designs, and the main pressure and containment layer, silicon carbide, cannot withstand these extreme temperatures. However, in an NTP design, the highest temperatures are seen in the exhaust area and the lowest temperatures are seen where the heated gas is introduced.

[0032] The present disclosure describes a three - layer staged fuel bed. At the bottom of the fuel bed, where gas is introduced and the fuel temperature is the lowest, a conventional TRISO material with an SiC - based seal coat is utilized. In this way, since the conventional SiC - based TRISO layer is also ideal for irradiation characteristics, the fuel bed can react under ideal irradiation conditions and generate heat.

[0033] In the exhaust area of the fuel element bed, the operating temperature may exceed 2700 °C. Therefore, a TRISO material with a high - temperature ceramic - based seal coat is used at the upper part of the fuel bed. In various embodiments, the high - temperature ceramic - based seal coat may be made of HfB2, ZrB2, TiB2, TaB2, TiC, TaC, W, Hf, ZrN, TiN, or Ta4HfC5. In some embodiments, a TRISO material with a tungsten carbide - based seal coat is used at the upper part of the fuel bed. Tungsten carbide has significantly improved mechanical properties and heat resistance compared to SiC. However, tungsten carbide has inferior irradiation reactivity compared to SiC. However, since the TRISO material is used in the exhaust area, and the reactivity is not as important, the use of a TRISO material with a tungsten carbide - based seal coat does not affect the operation of the nuclear reactor.

[0034] At the midpoint of the bed, in the range where the temperature approaches the operating limit of the silicon carbide-based TRISO layer, for example, in the range of about 1400°C to 1500°C, nuclear fuel elements utilizing TRISO materials with a high-temperature ceramic-based seal coat are used. Suitable high-temperature ceramic-based seal coats for the middle part of the bed may be made of HfB2, ZrB2, TiB2, TaB2, HfC, ZrC, TiC, TaC, HfN, ZrN, TiN, or Ta4HfC5. Depending on the temperature in the middle part of the bed, certain silicon-based ceramics may also be used as seal coats for the nuclear fuel particles. For example, when the middle part of the reactor operates at a temperature of 1400°C to 1800°C, silicon boron carbonitride (SiBCN) may be used as the seal coating. In various embodiments, zirconium carbide is used for the seal coating of the TRISO particles in the midpoint bed. This zirconium carbide exhibits irradiation characteristics similar to those of silicon carbide but is more resistant to higher temperatures. When the temperature approaches the operating limit of zirconium carbide, the TRISO material with a ZrC-based seal coat is replaced by a TRISO material with a WC-based seal coat.

[0035] In various embodiments, an axially layered fuel bed includes the following: A plurality of first fuel particles having a first ceramic seal coat of silicon oxide, carbide, oxycarbide, or nitride, A plurality of second fuel particles having a first high-temperature ceramic-based seal coat of HfB2, ZrB, TiB2, TaB2, Zr, HfC, TiC, Hf, ZrN, Ti, or SiBCN, A plurality of third fuel particles having a second high-temperature ceramic-based seal coat selected from WC, Ta, HfC, or Ta4HfC5. In the exhaust portion at the top of the fuel bed, extreme temperatures occur, so tungsten carbide and high melting point carbides of tantalum and hafnium (melting point > 4000°C) are desirable.

[0036] In various embodiments, the second fuel particles in the middle portion 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% SiC by weight. HfB2, ZrB2, and TiB2 may be co-deposited with SiC. By doing so, the use temperature of the resulting alloy is reduced, but it is still useful in the middle portion of the layered bed, i.e., in the range of 1200°C to 2000°C. For example, ZrB2 is a high-temperature ceramic with a melting point exceeding 3000°C, but a ZrB2 alloy (ZrB2-SiC alloy) containing 23% by weight of SiC has a melting point of 2270°C.

[0037] Thus, in various embodiments, an axially layered fuel bed may include the following. A plurality of first fuel particles having a first ceramic seal coating of SiC. A plurality of second fuel particles having a second ceramic seal coating of a ceramic alloy of ZrB2-SiC, HfB2-SiC, TaB2-SiC, or TiB2-SiC. A plurality of third fuel particles having a third ceramic seal coating of HfB2, ZrB2, TiB2, or TaB2.

[0038] The present disclosure also describes a two - layer, staged fuel bed that uses a conventional TRISO material and a SiC - based seal coat at the bottom of the fuel bed. Here, gas is introduced and the fuel temperature is the lowest. In the exhaust area of the fuel element bed, the operating temperature can reach from 2000°C to 2700°C. In various embodiments, the high - temperature ceramic - based seal coat in the exhaust area of the bed may 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 - based or WC - based seal coat may be used at the upper part of the fuel bed. When the temperature in the exhaust area is 2000°C or less, either a TRISO material with a ZrC - based seal coat or a TRISO material with a WC - based seal coat may be used in the exhaust area. When the temperature in the exhaust area exceeds 2000°C, a TRISO material with a WC - based seal coat should be used in the exhaust area.

[0039] Generally, the ceramic seal coating can be deposited on the nuclear fuel particles using a chemical vapor deposition (CVD) method from a suitable precursor. This technique is carried out using known techniques. The ceramic seal coating can be deposited directly on the nuclear fuel particles or through an intervening pyrolytic carbon layer. Also, an outer pyrolytic carbon layer can optionally be deposited on the ceramic seal coating.

[0040] As described above, the staged fuel element bed can operate at much higher temperatures than a conventional high - temperature gas - cooled reactor (HTGR) - style homogeneous fuel bed. This improves the efficiency of the reactor design for energy generation and enables it to accommodate special applications involving extreme temperatures, such as nuclear thermal propulsion (NTP) designs.

[0041] Figure 1 shows a high-temperature gas-cooled reactor (HTGR) nuclear reactor 1 with a staged fuel element bed 2. The nuclear reactor comprises a shell 1d including a bottom 1a, an upper part 1b, and an axis 1e. A coolant inlet 1f is arranged at or near the bottom 1a of the nuclear reactor shell, and a coolant outlet 1c is arranged at or near the upper part of the nuclear reactor shell 1d. The gas coolant flows in from the coolant inlet 1f and flows out from the coolant outlet 1c. The coolant passes through a plurality of nuclear fuel particles in the staged fuel element bed 2. The coolant fluid may be, for example, hydrogen, water, ammonia, oxygen, or carbon dioxide. Also, the coolant fluid may be an inert gas such as nitrogen, argon, or helium.

[0042] As shown in Figure 1, the staged fuel element bed 2 includes a plurality of regions and can have two to four regions. The staged fuel element bed 2 may have three regions including regions 2a, 2b, and 2c, as shown in Figure 1. Also, as shown in Figure 1, it may have two regions including regions 2a and 2c. The first region 2a is arranged near the bottom of the nuclear reactor and near the gas inlet 1f. The region 2a contains nuclear fuel particles 3. The nuclear fuel particles 3 are configured to operate at a first temperature T1, and each nuclear fuel particle 3 includes a first radioactive ceramic core and a first seal coating. The first seal coating may be a first ceramic seal coating that is stable at T1. In various embodiments, the first ceramic seal coating may be an oxide, carbide, nitride, or oxynitride of silicon or zirconium. The first ceramic seal coating may be silicon carbide, silicon nitride, zirconium carbide, or zirconium nitride.

[0043] In various embodiments, the staged fuel element bed 2 may have two regions including regions 2a and 2c shown in FIG. 1. The uppermost region 2c is disposed near the upper part of the reactor and near the gas outlet 1c. Region 2c contains nuclear fuel particles 5. The nuclear fuel particles 5 are configured to operate at a third temperature T3, and each nuclear fuel particle 5 includes a third radioactive ceramic core and a third seal coating. The third seal coating is a third ceramic seal coating that is stable at T3, and T3 is a temperature higher than T1. In various embodiments, the third ceramic seal coating may be an oxide, carbide, nitride, or oxynitride of zirconium or tungsten. The third ceramic seal coating may 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, oxycarbide, or nitride of uranium, thorium, or plutonium.

[0044] In various embodiments, the staged fuel element bed 2 includes regions 2a and 2c. The nuclear fuel particles 3 in region 2a include a first radioactive ceramic core and a ceramic seal coating including silicon carbide. The nuclear fuel particles 5 in region 2c include a third radioactive ceramic core and a ceramic seal coating including zirconium carbide. The nuclear fuel particles 3 are configured to operate at a temperature T1 of 750°C to 1200°C. The nuclear fuel particles 5 are configured to operate at a temperature T2 of 1200°C to 2000°C. The silicon carbide coating in the nuclear fuel particles 3 is not stable at temperatures exceeding 1600°C.

[0045] In various embodiments, the nuclear fuel particles 3 in region 2a include a first radioactive ceramic core and a ceramic seal coating including silicon carbide. The nuclear fuel particles 5 in region 2c include a third radioactive ceramic core and a ceramic seal coating including tungsten carbide. The nuclear fuel particles 3 are configured to operate at a temperature T1 from 750°C to 1600°C, and above this temperature, the silicon carbide coating becomes thermally unstable. The nuclear fuel particles 5 are configured to operate at a temperature T2 from 1600°C to 2800°C.

[0046] In various embodiments, the nuclear fuel particles 3 in region 2a include a first radioactive ceramic core and a ceramic seal coating including zirconium carbide. The nuclear fuel particles 5 in region 2c include a third radioactive ceramic core and a ceramic seal coating including tungsten carbide. The nuclear fuel particles 3 are configured to operate at a temperature T1 from 750°C to 2000°C. The nuclear fuel particles 5 are configured to operate at a temperature T2 from 2000°C to 2800°C.

[0047] The staged fuel element bed 2 may have three regions including regions 2a, 2b, and 2c, as shown in FIG. 1. Region 2a includes nuclear fuel particles 3 configured to operate at a first temperature T1. Region 2b includes nuclear fuel particles 4 configured to operate at a second temperature T2, where T2 is higher than T1. Region 2c includes nuclear fuel particles 5 configured to operate at a third temperature T3, where T3 is higher than T2. The nuclear fuel particles 3 have a first ceramic seal coating with higher thermal stability at T1 than at T2. The nuclear fuel particles 4 have a second ceramic seal coating with higher thermal stability at T2 than the first ceramic seal coating and higher thermal stability at T3 than the first ceramic seal coating. The nuclear fuel particles 5 have a third ceramic seal coating with higher thermal stability at T3 than the second ceramic seal coating.

[0048] Figure 2A shows a nuclear fuel particle 3 suitable for use in the first region 2a of the nuclear reactor of FIG. 1, the first region operating at a temperature of up to 1200 °C, or up to 1500 °C. The particle 3 shown in FIG. 2A contains a radioactive kernel or core 11 that is an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium. The core 11 may be, for example, a uranium oxide-based nuclear fuel particle core such as UO3 or U3O8. The core 11 is optionally coated with a buffer carbon layer 12, an inner high density pyrolytic carbon (IPyC) layer 13, a ceramic seal coat 14a, and optionally an outer high density pyrolytic carbon (OPyC) 15. In various embodiments, the nuclear fuel particle 3 is a TRISO particle including each of the layers 12 to 15, and the layer 14a is silicon carbide. In various embodiments, the nuclear fuel particle 3 is a TRISO type particle including at least the layers 13 and 14a, and the layer 14a is an oxide, carbide, nitride, or oxynitride of silicon.

[0049] Figure 2B shows a nuclear fuel particle 4 suitable for use in the intermediate region 2b of the reactor of FIG. 1, and the second region operates at a temperature of up to 1600 °C, or up to 2000 °C. The particle 4 shown in FIG. 2B includes a radioactive kernel or core 11 similar to that shown in FIG. 2A. The core 11 is coated with an optional buffer carbon layer 12, an inner high-density pyrolytic carbon (IPyC) layer 13, a ceramic seal coat 14b, and an optional outer high-density pyrolytic carbon (OPyC) 15. In various embodiments, the nuclear fuel particle 5 is a TRISO-type particle including at least layers 13 and 14b, and the layer 14b is an oxide, carbide, nitride, or oxynitride of zirconium. In embodiments where the first region operates at up to 1200 °C, the second region operates at up to 1600 °C, and the layer 14b may be a layer of a boride selected from zirconium carbide, hafnium boride, titanium boride, tantalum boride, or hafnium boride, titanium boride, tantalum boride, zirconium boride. In embodiments where the second region operates at up to 2000 °C, the layer 14b may be a layer of zirconium carbide, or a layer of a boride selected from hafnium boride, titanium boride, tantalum boride, zirconium boride.

[0050] Figure 2C shows a nuclear fuel particle 5 suitable for use in the upper region 2c of the reactor of FIG. 1, and the third region operates at a temperature of up to 2000°C or up to 2800°C. The particle 5 shown in FIG. 2C includes a radioactive kernel or core 11. The core 11 is coated with an optional buffer carbon layer 12, an inner high-density pyrolytic carbon (IPyC) layer 13, a ceramic seal coat 14c, and an optional outer high-density pyrolytic carbon (OPyC) 15. In various embodiments, the nuclear fuel particle 5 is a TRISO-type particle including at least layers 13 and 14c, and the layer 14c is an oxide, carbide, nitride, or oxynitride of tungsten. In embodiments where the second region operates at up to 1600°C, the second region operates at up to 2000°C and the layer 14c may be a layer of zirconium carbide or tungsten carbide, or a layer of a boride selected from hafnium boride, titanium boride, tantalum boride, zirconium boride. In embodiments where the second region operates at up to 2800°C, the layer 14c is a layer of tungsten carbide.

[0051] As described above, the staged fuel element bed 2 may have two regions including regions 2a and 2c shown in FIG. 1. The materials used for the ceramic seal coats of the nuclear fuel particles 3 and 5 are selected based on the operating temperatures of the respective regions. When region 2a reaches a temperature of up to 1200°C and region 2c reaches a temperature of up to 2000°C, region 2a may contain nuclear fuel particles 3 with a silicon-based ceramic layer 14a, and region 2c may contain nuclear fuel particles 5 with a zirconium or tungsten-based ceramic layer 14c. When region 2a reaches a temperature of up to 1600°C and region 2c reaches a temperature of up to 2800°C, region 2a may contain nuclear fuel particles 3 with a zirconium or silicon-based ceramic layer 14a, and region 2c may contain nuclear fuel particles 5 with a tungsten-based ceramic layer 14c. When region 2a reaches a temperature of up to 2000°C and region 2c reaches a temperature of up to 2800°C, region 2a may contain nuclear fuel particles 3 with a zirconium-based ceramic layer 14a, and region 2c may contain nuclear fuel particles 5 with a tungsten-based ceramic layer 14c. Tungsten-based ceramics have higher thermal stability than zirconium-based ceramics, and zirconium-based ceramics have higher thermal stability than silicon-based ceramics. However, the precursors of zirconium and tungsten-based ceramics are more expensive than the precursors of silicon-based ceramics. Therefore, a more economical silicon-based seal coating 14a may be used for the particles 3 in the low-temperature region 2a, and a zirconium or tungsten-based ceramic seal coating may be used in the high-temperature region 2c. When the low-temperature region 2a operates at a temperature exceeding 1200°C or exceeding 1600°C, the particles 3 in the low-temperature region 2a may use a zirconium-based seal coating 14a, and a tungsten-based ceramic seal coating may be used in the high-temperature region 2c.

[0052] Figures 3A through 3C show another embodiment of a particle suitable for use as a nuclear fuel particle in the nuclear reactor of FIG. 1. The particle 3 shown in FIG. 3A may be used in the lower region 2a of the nuclear reactor and includes a radioactive kernel or core 21 that is an oxide, carbide, oxycarbide, or nitride of uranium, thorium, or plutonium. The core 21 is coated with a ceramic seal coat 22a. In various embodiments, the ceramic seal coat 22a is an oxide, carbide, nitride, or oxynitride of silicon and is configured to operate at temperatures up to 1200°C. FIG. 3B shows another embodiment of a particle suitable for use as a nuclear fuel particle 4 in the intermediate region 2b of the nuclear reactor of FIG. 1. The particle 4 shown in FIG. 3B includes a radioactive kernel or core 21 and a ceramic seal coat 22b, and the layer 22b is an oxide, carbide, nitride, or oxynitride of zirconium and is configured to operate at temperatures up to 2000°C. FIG. 3C shows another embodiment of a particle suitable for use as a nuclear fuel particle 5 in the upper region 2c of the nuclear reactor of FIG. 1. The particle 5 shown in FIG. 3C includes a radioactive kernel or core 21 and a ceramic seal coat 22c, and the layer 22c is an oxide, carbide, nitride, or oxynitride of tungsten and is configured to operate at temperatures up to 2800°C.

[0053] In various embodiments, a first ceramic seal coating, such as layer 14a of FIG. 2A, has a first neutron absorption cross section, a second ceramic seal coating, such as layer 14b of FIG. 2B, has a second neutron absorption cross section, and a third ceramic seal coating, such as layer 14c of FIG. 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. Silicon carbide has good thermal stability at the temperatures found in region 2a of the reactor and also has good irradiation characteristics with a high neutron absorption cross section, and nuclear fuel particles 3 having an SiC layer 14a in region 2a of fuel element bed 2 can react to generate heat. Zirconium carbide has good thermal stability at the temperatures found in region 2b that exceed the temperatures found in region 2a and also has acceptable irradiation characteristics, and nuclear fuel particles 4 having a ZrC layer 14b can be used in region 2b. In various embodiments, the temperature in the third region 2c may reach from 2500°C to 2800°C. Under these extreme conditions, most ceramic coatings are not thermally stable. Thus, nuclear fuel particles 5 having a tungsten carbide layer 14c are used in region 2c. The tungsten carbide layer 14c has good thermal stability at the temperatures found in region 2c of the reactor. WC has inferior irradiation characteristics compared to ZrC and SiC, for example, a low neutron absorption cross section, but the use of the WC layer 14c is acceptable because thermal stability is more important than irradiation characteristics in the high temperature region 2c. Similar considerations apply to the ceramic seal coats 22a - 22c of FIGS. 3A through 3C.

[0054] Returning to FIG. 1, a combination of single-layer coated nuclear fuel particles (FIGS. 3A through 3C) and multi-layer coated nuclear fuel particles (FIGS. 2A through 2C) may be used in different zones of a nuclear reactor. In the lower region 2a, particles 3 having a radioactive kernel or core 21 and a single outer silicon carbide layer 22a may be used, and this region 2a operates at a temperature up to 1200° C. In the intermediate region 2b, the TRISO particles 3 of FIG. 2A may be used, which include a radioactive kernel 11, a silicon carbide layer 14a, and an outer pyrolytic carbon layer 15, and this region 2b operates at a temperature up to 1600° C. The exposed silicon carbide layer 22a of the particles in the lower region 2a is thermally stable up to 1200° C, but above this temperature, the silicon carbide surface may oxidize if an oxygen source is mixed into the coolant gas. The silicon carbide layer 14a of the particles in the intermediate region 2b is protected from exposure to oxides by a dense pyrolytic carbon layer 15 and is less susceptible to the effects of oxidation. In the upper region 2c, the nuclear fuel particles 5 of FIG. 2C or FIG. 3C may be used, which include a zirconium or tungsten-based ceramic seal coating 14c or 22c.

[0055] In another embodiment, a combination of nuclear fuel particles lacking a seal coat and nuclear fuel particles having a ceramic seal coat may be used in different regions of the reactor of FIG. 1. In the lower region 2a, particles 3 having a radioactive kernel or core 12, a porous carbon layer 12, and a pyrolytic carbon layer 13 may be used, and these particles 3 are similar to the particles of FIG. 2A but lack the outer layers 14a and 15. Region 2a operates at a temperature of up to 1200°C. In the intermediate region 2b, the TRISO particles 3 of FIG. 2A may be used, which include a radioactive kernel 11, inner carbon layers 12 and 13, a silicon carbide or zirconium carbide layer 14a, and an outer pyrolytic carbon layer 15, and this region 2b operates at a temperature of up to 1600°C. The silicon carbide layer 14a of the particles in the intermediate region 2b is protected from exposure to oxides by the dense pyrolytic carbon layer 15. In the upper region 2c, the nuclear fuel particles 5 of FIG. 2C or FIG. 3C may be used, which include a zirconium or tungsten-based ceramic seal coating 14c or 22c. Generally, a zirconium-based ceramic seal coating is not used in both region 2b and region 2c.

[0056] FIG. 1 shows a staged fuel element bed 2 containing nuclear fuel particles in a plurality of regions 2a - 2c, but various embodiments disclosed herein include a staged fuel element bed including fuel elements containing nuclear fuel particles dispersed within a matrix material. FIG. 4 shows a nuclear fuel compact 31 in which nuclear fuel particles 33 are dispersed within a matrix material 32. The nuclear fuel particles 33 are any of the particles 3, 4, and 5 shown in FIGS. 2A - 2C, and the ceramic layer 14a, 14b, or 14c is selected based on the desired operating temperature of the compact. Alternatively, the nuclear fuel particles 33 are any of the particles 3, 4, and 5 shown in FIGS. 3A - 3C, and the ceramic layer 22a, 22b, or 22c is selected based on the desired operating temperature. If the desired operating temperature of the nuclear fuel compact 31 is less than 1200°C, the matrix material 32 may be a ceramic material, a metal, a ceramic - metal composite material (e.g., titanium carbide / nickel - cobalt cermet, tungsten carbide / cobalt cermet, etc.), a composite material composed of at least two ceramics, graphite, or a phenolic resin. If the desired operating temperature of the nuclear fuel compact 31 exceeds 1200°C, the matrix material 32 may be a thermally stable material such as graphite, WC (tungsten carbide), or tungsten. The nuclear fuel compact 31 may be cubic or cylindrical. In various embodiments, the nuclear fuel compact 31 may be a generally spherical pebble containing nuclear fuel particles 33 evenly dispersed therein.

[0057] FIG. 5 shows an HTGR reactor 1 with a staged fuel element bed 2 similar to that of FIG. 1. The reactor includes a shell 41d. The coolant inlet 41f is disposed at the bottom 41a of the reactor shell 41d, and the coolant outlet 41c is disposed at the top 41b of the reactor shell 41d. As shown in FIG. 5, the staged fuel element bed 42 includes a plurality of regions, including regions 42a, 42b, and 42c. The first region 42a is disposed at the bottom of the reactor and is located near the gas inlet 41f. The third region 42c is disposed at the top of the reactor and is located near the gas outlet 41c.

[0058] Region 42a contains nuclear fuel compact 31a, which is similar to compact 31 in FIG. 4. Compact 31a contains nuclear fuel particles 33 dispersed in matrix 32. The nuclear fuel particles 33 may be the particles 3 in FIG. 2a or FIG. 3a, and these particles 3 contain a silicon carbide ceramic seal coating 14a or 22a. The nuclear fuel compact 31a contained in region 42a is configured to operate at a temperature of up to 1200°C or up to 1600°C.

[0059] Region 42b contains nuclear fuel compact 31b. Compact 31b contains nuclear fuel particles 33 dispersed in matrix 32. The nuclear fuel particles 33 may be the particles 4 in FIG. 2B or FIG. 3B, and these particles 4 contain a zirconium carbide ceramic seal coating 14b or 22b. The nuclear fuel compact 31b contained in region 42b is configured to operate at a temperature of up to 2000°C.

[0060] Region 42c contains nuclear fuel compact 31c. Compact 31c contains nuclear fuel particles 33 dispersed in matrix 32. The nuclear fuel particles 33 may be the particles 5 in FIG. 2C or FIG. 3C, and these particles 5 contain a tungsten carbide ceramic seal coating 14c or 22c. The nuclear fuel compact 31c contained in region 42c is configured to operate at a temperature of up to 2800°C.

[0061] The compacts 31a, 31b, and 31c in regions 42a, 42b, and 42c have a mass sufficient to avoid fluidization by the cooling gas.

[0062] Although various embodiments have been described in detail with specific reference to particular examples, it should be understood that the present invention is capable of other embodiments and that details thereof can be modified in various obvious respects. As will be apparent to those skilled in the art, variations and modifications can be made within the spirit and scope of the present invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not limit the invention in any way, and the invention is defined only by the claims.

Explanation of Signs

[0063] 1…High-temperature gas-cooled reactor, 1a…Bottom, 1b…Top, 1c…Coolant outlet, 1d…Reactor shell, 1e…Axis, 1f…Coolant inlet, 2…Staged fuel element bed, 3, 4, 5…Nuclear fuel particles, 11…Core, 12…Buffer carbon layer, 13…High-density pyrolytic carbon layer, 15…High-density pyrolytic carbon

Claims

1. A nuclear reactor having a fuel bed arranged in layers in the axial direction, wherein the reactor shell has a base, an upper part having an exhaust port, and a shaft, the fuel bed arranged in layers in the axial direction is set to operate at a first temperature T1, and includes a first region containing a plurality of first fuel particles each having a first radioactive ceramic core and a first coating which is a carbon coating or a first ceramic seal coating or a combination thereof, a second region containing a plurality of second fuel particles each having a second radioactive ceramic core and a second ceramic seal coating, the second region being set to operate at a second temperature T2 which is higher than T1, the coolant fluid flow path is configured to carry the coolant fluid from the base of the reactor to the exhaust port and to be carried along a flow path passing sequentially through the first region and the second region, the first ceramic seal coating has better stability at T1 than at T2, and the second ceramic seal coating has better stability at T2 than the first ceramic seal coating at T2. A nuclear reactor characterized by this.

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

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

4. The first radioactive ceramic core and the second radioactive ceramic core are oxides, carbides, oxycarbides, or nitrides of uranium, thorium, or plutonium. The nuclear reactor according to claim 1, characterized by this.

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

6. The first ceramic seal coating is selected from the group consisting of oxides, carbides, borides, oxycarbides, or nitrides of silicon, The second ceramic seal coating is a high-temperature ceramic-based seal coat selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, TaC, WC, HfN, ZrN, TiN, SiBCN, and Ta4HfC5. The nuclear reactor according to claim 1, characterized in that.

7. The first ceramic seal coating is an oxide, carbide, boride, oxycarbide, or nitride of silicon or zirconium, The second ceramic seal coating is an oxide, boride, carbide, oxycarbide, or nitride of zirconium or tungsten, The nuclear reactor according to claim 1, characterized in that neither the first ceramic seal coating nor the second ceramic seal coating contains zirconium.

8. The first ceramic seal coating is silicon carbide, The second ceramic seal coating is zirconium carbide, The nuclear reactor according to claim 7, characterized in that T1 ranges from 750°C to 1200°C and T2 ranges from 1200°C to 2000°C.

9. The first ceramic seal coating is silicon carbide, The second ceramic seal coating is tungsten carbide, The nuclear reactor according to claim 7, characterized in that T1 ranges from 750°C to 1600°C and T2 ranges from 1600°C to 2800°C.

10. The first ceramic seal coating is zirconium carbide, The second ceramic seal coating is tungsten carbide, The nuclear reactor according to claim 7, characterized in that T1 ranges from 750°C to 2000°C and T2 ranges from 2000°C to 2800°C.

11. In the nuclear reactor according to claim 1, further comprising a third region containing a plurality of third fuel particles each having a third radioactive ceramic core and a third ceramic seal coating, and being set to operate at a third temperature T3 which is higher than T2. The second ceramic seal coating has better stability at T2 than at T3, and the third ceramic seal coating has better stability at T3 than the second ceramic seal coating, a nuclear reactor characterized by this.

12. The first ceramic seal coating has the first neutron absorption cross section, The third ceramic seal coating has a third neutron absorption cross section, The nuclear reactor according to claim 11, characterized in that the third neutron absorption cross section is higher than the first neutron absorption cross section.

13. The first ceramic seal coating is an oxide, carbide, oxycarbide, or nitride of silicon, The second ceramic seal coating is a first high-temperature ceramic-based seal coat selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, ZrC, HfC, TiC, TaC, HfN, ZrN, TiN, SiBCN, and Ta4HfC5, The third ceramic seal coating is a second high-temperature ceramic-based seal coat selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, TiC, TaC, WC, HfN, ZrN, TiN, or Ta4HfC5, The nuclear reactor according to claim 11, characterized in that the second and third ceramic seal coatings are different.

14. The first ceramic seal coating is an oxide, carbide, oxycarbide, or nitride of silicon, The second ceramic seal coating is a first high-temperature ceramic-based seal coat selected from the group consisting of HfB2, ZrB2, TiB2, TaB2, ZrC, TiC, HfN, ZrN, TiN, SiBCN, and Ta4HfC5, The nuclear reactor according to claim 11, characterized in that the third ceramic seal coating is a second high-temperature ceramic-based seal coat selected from the group consisting of WC, TaC, HfC, and Ta4HfC5.

15. The first ceramic seal coating is selected from the group consisting of an oxide, carbide, oxycarbide, or nitride of silicon, The second ceramic seal coating is an oxide, carbide, oxycarbide, or nitride of zirconium, The nuclear reactor according to claim 11, wherein the third ceramic seal coating is tungsten oxide, carbide, oxycarbide, or nitride.

16. The first ceramic seal coating is silicon carbide, The second ceramic seal coating is zirconium carbide, The nuclear reactor according to claim 12, wherein the third ceramic seal coating is tungsten carbide.

17. T1 ranges from 750°C to 1200°C, T2 ranges from 1200°C to 2000°C, The nuclear reactor according to claim 16, wherein T3 ranges from 2000°C to 2800°C.

18. In the nuclear reactor of claim 1, further, a plurality of first fuel elements in which the plurality of first fuel particles are dispersed in a first matrix material that is stable at T1 in the first region, A plurality of second fuel elements in which a plurality of the second fuel particles are dispersed in a second matrix material that is stable at T2 in the second region, characterized by including.

19. In the nuclear reactor of claim 11, further, a plurality of first fuel elements in which the plurality of first fuel particles are dispersed in a first matrix material that is stable at T1 in the first region, A plurality of second fuel elements in which a plurality of the second fuel particles are dispersed in a second matrix material that is stable at T2 in the second region, A plurality of third fuel elements in which a plurality of the third fuel particles are dispersed in a third matrix material that is stable at T3 in the third region, characterized by including.

20. The nuclear reactor according to claim 18, wherein the first matrix material is selected from the group consisting of ceramic, metal, ceramic-metal composite material, composite material composed of at least two kinds of ceramics, graphite, and phenolic resin.

21. The nuclear reactor according to claim 18, wherein the second matrix material is selected from the group consisting of graphite, WC, and tungsten.

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