Thermal Bridge

Abstract

A high-temperature gas-cooled reactor fuel block is provided, comprising a fuel channel and a coolant channel as a thermal bridge to improve heat transfer between a fuel element and a fuel block, wherein the fuel channel comprises a fuel element, and the fuel channel further comprises a thermal bridge that thermally connects the fuel element and the fuel channel, and the thermal bridge comprises a melting point higher than the operating temperature of the fuel block, thereby improving heat transfer from the fuel element to the fuel block, and thereby improving heat transfer to the coolant channel.

Description

Technology Field

[0001] The present invention relates to a thermal bridge for a high-temperature gas-cooled nuclear reactor. Background Technology

[0002] High-temperature gas-cooled reactors (HTGRs) are graphite-moderated fourth-generation reactors that typically use fuel elements such as uranium or plutonium in combination with an inert gas coolant to achieve very high outlet temperatures (typically exceeding 700°C). These HTGRs have been developed over the past 50 years, and two of them—the HTTR operated by the Japan Atomic Energy Agency and the HTR-10 operated by Tsinghua University in China—are still in operation.

[0003] HTGRs can be used in combination with a direct cycle design, in which the coolant flows through the core and is used to extract work energy through turbomachinery, without requiring a secondary coolant loop or associated heat exchanger typical in this field. Since this design offers high efficiency and allows for the minimization of the overall reactor size, it is particularly suitable for applications in confined spaces such as aircraft, ships, and submarines.

[0004] The most commonly used coolant fluid in HTGR direct cycle reactors is helium. Unlike alternative gases, helium is inert, which offers the significant advantage of enabling an inherently safe design in the event of a coolant leak. However, converting high-temperature gas into work energy is difficult due to the lack of modern turbomachinery designs suitable for helium. This lack of design expertise poses a significant commercial barrier to the commercialization of helium-based direct cycle gas reactors, as it results in high costs for the design and construction of these reactors and associated turbomachinery.

[0005] Furthermore, the efficiency of the reactor is reduced due to the fuel channel gap between the fuel element and the fuel channel. This fuel channel gap exists to accommodate manufacturing tolerance errors during the production of the fuel element, in addition to changes in the thermal volume of the fuel block caused by the difference in expansion rates between the fuel block and the fuel element resulting from heat generation, the accumulation of fission products, and time-dependent neutron irradiation.

[0006] According to a first embodiment, a high-temperature gas-cooled reactor fuel block comprising a fuel channel and a coolant channel is provided, wherein the fuel channel comprises a fuel element, and the fuel channel further comprises a thermal bridge that thermally connects the fuel element and the fuel channel, and the thermal bridge comprises a melting point higher than the operating temperature of the fuel block of the reactor, thereby improving heat transfer from the fuel element to the fuel block and thereby improving heat transfer to the coolant channel. A high-temperature gas reactor means an operating temperature of 600°C or higher.

[0007] Low-conductivity inert gases such as nitrogen are readily available in existing commercial off-the-shelf (COTS) turbomachine designs, which can reduce initial costs when designing and constructing such reactors. However, using nitrogen as a coolant gas introduces a trade-off in heat transfer between the fuel elements and the coolant because nitrogen (0.025 W / mK) has a lower thermal conductivity than helium (0.15 W / mK). To achieve the same thermal duty and work output as helium, it is necessary to significantly increase the mass flow rate of nitrogen.

[0008] In the prior art, the fuel channel typically includes a fuel element disposed therein, and there is a fuel channel gap between the fuel element and the wall forming the fuel channel. This gap allows for expansion as described above.

[0009] This deficiency in fuel channel gaps is not significant in helium-cooled reactors due to the high thermal conductivity of helium, but it becomes a significant thermal barrier when nitrogen is used as a reactor coolant. This is because nitrogen has lower thermal conductivity than helium, so using nitrogen lowers the coolant outlet temperature, which in turn reduces the overall efficiency of the reactor.

[0010] Reactor fuel blocks, fuel elements, fuel channels, and coolant channels can be designed in any way suitable for high-temperature gas-cooled reactors. One such example is the General Atomics® GT-MHR. The GT-MHR fuel block features a hexagonal cross-section and is provided with multiple fuel and coolant channels extending along normal axes from the hexagonal plane, and coolant gas flows through the coolant channels to absorb heat generated by the fuel elements during use.

[0011] The fuel block may be manufactured from any suitable material that provides a suitable neutron moderator containing a small neutron absorption cross-section, for example, beryllium or graphite, more preferably graphite. The planar shape of the fuel block may be any suitable shape, such as circular, square, rectangular, pentagonal, hexagonal, octagonal, or any shape having more sides. In the example of the GT-MHR fuel block, it includes a hexagonal plan. Preferably, the planar shape of this fuel block enables multiple fuel blocks to be joined together in a mosaic pattern within the reactor. The fuel element is a material capable of undergoing nuclear fission and sustaining nuclear fission within the reactor. The fuel element may be a fissile material, for example, uranium or plutonium, and may include, for example, oxides, dioxides, or carbides of these elements, such as uranium oxide, plutonium oxide, uranium dioxide, plutonium dioxide, or uranium carbide, and salts thereof. The fuel may be a mixture of oxides to produce mixed oxide fuel (MOX). The fuel may be a tristructural-isotropic (TRISO) or quadstructural-isotropic (QUADRISO) fuel comprising a fuel kernel of uranium or plutonium oxide coated with a layer of isotropic material. This isotropic material may be selected from graphitic carbon or ceramics, for example, pyrolytic carbon or silicon carbide. Due to the isotropic layer present on the fuel kernel, this fuel is structurally resistant to neutron irradiation, corrosion, and oxidation, and thus can withstand higher operating temperatures and is also ideal for application in high-temperature gas-cooled reactors. Furthermore, these characteristics improve the stability characteristics of the reactor, as the fuel element does not melt even when the reactor's maximum operating temperature is exceeded—that is, meltdown is impossible.The fuel element may be provided with a granular consistency, such as particles that can be compressed into a fuel compact, such as a pebble compact, for use in a specific fuel rod assembly. Preferably, the fuel is TRISO fuel.

[0012] In this device, a thermal bridge thermally connects the fuel element and the fuel channel. By filling the gap between the fuel channel with the thermal bridge, the thermal bridge significantly increases heat transfer between the fuel element and the fuel channel, thereby improving heat transfer from the fuel element to the fuel block. This improves overall heat transfer to the coolant channel without the need to increase the mass flow rate of the coolant to achieve the same thermal efficiency, thereby having the effect of improving the overall efficiency of the reactor.

[0013] To efficiently transfer heat between fuel elements and fuel channels when the reactor typically operates at an operating temperature in the range of 600°C to 2000°C, the thermal bridge has a melting point higher than the operating temperature of the reactor fuel block. Preferably, the thermal bridge is solid at temperatures above 600°C. Preferably, the thermal bridge is solid at temperatures above 1000°C. More preferably, the thermal bridge is solid at temperatures up to 2000°C.

[0014] In use, fuel blocks and fuel elements may expand and contract due to temperature changes within the reactor, for example, during the operation of the reactor, i.e., during nuclear fission, and when the reactor is offline. Fuel blocks and channels may also expand and contract further due to the accumulation of fission products over time and neutron irradiation. Fuel elements and fuel channels also expand and contract at different rates. Therefore, the thermal bridge may be elastically compressible to accommodate volume changes in the fuel blocks, fuel channels, and fuel elements. Due to this elasticity, the thermal bridge can maintain simultaneous thermal contact, i.e., abutment state, with the fuel elements and fuel channels without creating air gaps that reduce heat transfer between the fuel elements and coolant channels during the expansion and contraction of the fuel blocks, fuel channels, and fuel elements.

[0015] The thermal bridge is a solid, but liquid or gaseous forms may also be considered. However, it will be understood that there may be significant manufacturing difficulties in containing compressible liquids or gases within a fuel block. Rapid expansion of the contained fluid inherently poses a risk of explosion. Furthermore, the fluid can penetrate microscopic cracks formed within the fuel block by neutron irradiation, thereby weakening the integrity of the fuel block. Preferably, the thermal bridge is a solid.

[0016] The thermal bridge may be a block of an elastic material, a foamed material, a powdered material, or a mixture thereof. Preferably, the thermal bridge is a powdered material.

[0017] The thermal bridge may preferably be a powdered material, and the powdered material may be particles, and the particles may be spherical, round, angular, flake-shaped, cylindrical, needle-shaped, hexahedral, or irregular. Preferably, the particles may be spherical.

[0018] The particle size of the powdered material may be within the range of 0.1 to 500 μm, preferably 1 to 200 μm, more preferably less than 100 μm, and more preferably 1 to 100 μm, which is a value determined by the average longest dimension of the particles. The thermal bridge may include a multimode or dual-mode size distribution of particles.

[0019] The thermal bridge may be made of any material having a low neutron cross-section and high thermal conductivity, for example, metals and their alloys, metalloids, carbon, or thermally conductive ceramics. Preferably, the thermal bridge may be made of a material selected from the group comprising molybdenum (isotopes 92 and 94), niobium, silicon carbide, or carbon (graphite). More preferably, the thermal bridge is made of graphitic carbon.

[0020] The thermal bridge may contain only graphite-like powder.

[0021] The inventors have discovered that graphitic materials are readily commercially available and possess low cost, a high melting point, high thermal conductivity, and a low neutron cross-section. Furthermore, since graphite does not burn upon contact with air or water, the risk is reduced compared to liquid sodium-cooled reactors.

[0022] The fuel channel gap may further contain combustible toxins. Such combustible toxins contain high neutron cross-sections to facilitate the absorption of neutrons caused by excessive reactivity early in the life of the nuclear fuel. The presence of combustible toxins decreases with the life of the reactor as the toxins 'burn,' i.e., absorb neutrons.

[0023] The flammable toxin may be selected from the group comprising compounds of boron or gadolinium. Preferably, the flammable toxin is a boron carbide.

[0024] The flammable toxin may be part of the thermal bridge, or separate from the thermal bridge, but may be co-deposited within the fuel channel gap as a separate layer adjacent to the thermal bridge surrounding the fuel element, for example. If the flammable toxin is part of the thermal bridge, the thermal bridge and the flammable toxin may be a homogeneous blend of powdered fine particles. Preferably, the thermal bridge contains the flammable toxin.

[0025] The thermal bridge may contain only boron carbide and flammable toxins as a thermal bridge.

[0026] Preferably, the thermal bridge comprises a graphite-like powder containing boron carbide, preferably as a flammable toxin, preferably as a homogeneous blend of fine powder particles.

[0027] According to a second embodiment, a high-temperature gas-cooled nuclear reactor system comprising a fuel block as defined in this specification is provided.

[0028] The coolant channels of the fuel in a high-temperature gas-cooled reactor system may contain a gas that can be readily used in a gas turbine, and this gas can be readily used in a gas turbine so that conventional gas turbine machinery can use the coolant gas without the need for mechanical modifications. One such desirable gas is nitrogen. This is a gas with lower conductivity compared to helium (thermal conductivity lower than 0.1 W / mK at 25°C).

[0029] This configuration allows for the use of helium, but using conventional gas turbine machinery requires a heat exchanger or a modified machine suitable for accepting helium. Conversely, the inventors have discovered that using nitrogen in a high-temperature gas-cooled reactor is suitable for existing gas turbines using air as the working fluid, and is therefore particularly useful for direct-cycle high-temperature gas-cooled reactors.

[0030] High-temperature gas-cooled reactor systems may include direct cycle systems. Such systems do not require a secondary coolant circuit and associated heat exchangers, and instead may utilize only a primary loop in which gas passes through the reactor, is heated, and then directly drives a turbomachine.

[0031] According to a third embodiment, a method for improving cooling in a high-temperature gas-cooled nuclear reactor as defined herein is provided, the method comprising the following steps:

[0032] I) a step of providing a thermal bridge between a fuel element and a fuel channel; and

[0033] II) A step of providing coolant flow within the coolant channel;

[0034] III) When in use, the thermal bridge improves heat transfer between the fuel element and the fuel block, and

[0035] IV) A step to improve heat transfer to the coolant flow within the coolant channel as a result.

[0036] The thermal bridge can be inserted into the fuel channel during the manufacturing stage of the fuel block or inserted into the original position inside the reactor after the fuel block is inserted.

[0037] According to a further embodiment, a direct cycle high-temperature nitrogen-cooled reactor system comprising a fuel block is provided, wherein the fuel block comprises fuel channels and coolant channels, the fuel channels comprise fissile material, the fuel channels further comprise a thermal bridge in the form of graphite powder that thermally connects the fissile material and the fuel channels, the thermal bridge comprises a melting point higher than the operating temperature of the fuel block of the reactor, and the thermal bridge further comprises a combustible toxin in the form of boron carbide, thereby improving heat transfer from the fuel element to the fuel block. Brief explanation of the drawing

[0038] A plurality of configurations of the present invention will be described below with reference to the attached drawings as examples. Figure 1 illustrates one configuration of a high-temperature gas-cooled nuclear reactor fuel block. FIG. 2 illustrates a plan view of a fuel channel including a fuel element filled with a thermal bridge according to the present invention. Figure 3 illustrates the thermal conductivity diagram of the fuel channel relative to the coolant channel. Figure 4 shows a graph comparing a nitrogen coolant with and without a thermal bridge and a helium gas coolant without a thermal bridge. Figure 5 illustrates a high-temperature gas-cooled nuclear reactor system. Specific details for implementing the invention

[0039] Referring to FIG. 1, an example of a conventional fuel block configuration (100) is provided. In this particular example, the fuel block (102) is a General Atomics GT-MHR high-temperature gas-cooled reactor fuel block comprising a plurality of fuel channels (106) and a coolant channel (104), wherein the fuel channels (106) comprise a plurality of fuel elements (not shown). The fuel block (102) comprises a hexagonal cross-section, thereby allowing adjacent fuel blocks (not shown) to be joined together in a mosaic fashion within the core. In use, the coolant gas flows along a longitudinal axis (Z) defined by the fuel channels (106) and coolant channels (104) that extend through the fuel block (102). The coolant gas also flows within the gap between the fuel channels (106) and the fuel elements, because there is a fuel channel gap that allows for thermal expansion and neutron irradiation expansion of the fuel elements in addition to accommodating manufacturing tolerance errors of the fuel elements.

[0040] Referring to FIG. 2, a planar configuration (200) of a fuel channel (206) containing a fuel element (208) is provided, and the fuel element (208) is surrounded by a thermal bridge (210). In this configuration, the fuel channel is a fissile material based on TRISO uranium oxide compressed into a cylindrical compact, and the fuel channel (206) is made of graphite. In this configuration, the thermal bridge is a solid graphite powder with a spherical particle size of 50 μm filled around the fuel element. In this configuration, the thermal bridge (210) is elastically compressible due to the gaps between the powder particles, thereby enabling thermal expansion and contraction of the fuel block, fuel channel, and fuel element and volume change of the fuel channel due to neutron irradiation. The thermal bridge (210) further contains a flammable toxin (not shown) in the form of boron carbide, which helps to offset additional reactivity of the fuel element (208) early in the life of the fuel element (208), and the toxin gradually burns over its life as neutrons are absorbed.

[0041] Referring to FIG. 3, a thermal conductivity diagram (300) between the fuel channel (306) and the coolant channel (304) is provided. In use, the fuel element (308) generates thermal energy due to nuclear fission when a fission product, for example, uranium (235), is fissioned. This thermal energy must be transferred from the center of the fuel element (308) to the edge of the fuel element (308), as indicated by resistance R1. In the absence of a thermal bridge, heat must traverse the fuel channel gas gap, indicated by resistance R2. This gas gap is filled with coolant gas flowing through the coolant channel (304). Since the thermal conductivity of the coolant gas in this gap is lower than that of the fuel block (302) and the fuel compact (308), heat transfer between the fuel element (308) and the wall (306) of the fuel channel is significantly reduced. The heat transferred subsequently travels across the fuel block (302) having resistance R3 and reaches the coolant channel (304), where the coolant gas is heated by convection. In the present invention, since a thermal bridge (310), which is substantially an extension of the fuel block, is provided to be in contact with the fuel element (308), the resistance R2 is greatly reduced. In this embodiment, the resistance R3 is greater than R2 due to the gaps between the graphite particles within the thermal bridge. In reality, the resistance R2 may be significantly higher than R3 because the gaps between the powder particles within the thermal bridge (310) are very small. It can be seen that when R2 is the thermal bridge (310), the resistance is always lower than when R2 is merely the coolant gas, so the heat transfer between the fuel element (308) and the coolant channel (304) is greatly improved by the provision of the thermal bridge (310).

[0042] Fig. 4 shows a graph (400) comparing a nitrogen coolant with or without a thermal bridge and a helium gas coolant without a thermal bridge. The Y-axis represents the temperature difference (in Kelvin units) between the fuel element and the fuel channel wall, plotted against the X-axis of the fuel block output (in kilowatt units). Line (420) represents a fuel block without a thermal bridge and using nitrogen as the coolant gas. Line (430) represents a fuel block without a thermal bridge and using helium as the coolant gas. Line (440) represents a fuel block with the thermal bridge of the present invention and using nitrogen as the coolant gas. As can be seen from this graph (400), when using a nitrogen coolant without a thermal bridge (310), the efficiency is significantly lower than that of a helium coolant without a thermal bridge (310) because the thermal conductivity of nitrogen is about 1 / 6 of the thermal conductivity of helium. Therefore, a significant temperature difference is seen through the fuel channel gap as a function of the fuel block output. When running a simulation using a thermal bridge with nitrogen coolant (440), the temperature difference is smaller than that of a high-conductivity helium coolant, so the thermal bridge can achieve higher thermal efficiency than using only helium coolant and is much more efficient than nitrogen coolant without a thermal bridge (420).

[0043] Referring to FIG. 5, a high-temperature gas-cooled reactor system (500) comprising a reactor (516) is provided, which comprises at least one graphite fuel block (502). In the present invention, the graphite fuel block (502) is a General Atomics® GT-MHR fuel block comprising a fuel channel (506) and a coolant channel (504), wherein the fuel channel comprises a plurality of TRISO fuel elements (508), and the fuel elements (508) are formed within a cylindrical capsule mounted in series within the fuel channel (506). The fuel channel (306) further comprises a thermal bridge (510) surrounding each fuel element (508), wherein the thermal bridge is graphite powder. In the present configuration, the high-temperature gas-cooled reactor system (500) is a direct cycle system and is provided with a primary circuit loop (512) that directly drives a turbomachine (514) without requiring a secondary coolant loop or associated heat exchanger. In this embodiment, the coolant gas is nitrogen.

[0044] Although several desirable configurations have been illustrated and described, those skilled in the art will understand that various changes and modifications can be made without departing from the scope of the invention, as defined in the appended claims.

[0045] Attention should be paid to all documents and papers submitted simultaneously with or prior to this specification and made available for public viewing together with this specification in connection with this application, the contents of all such documents and papers are incorporated herein by reference.

[0046] All features described in this specification, including the appended claims, summary, and drawings, and / or all steps or processes of any disclosed method may be combined in any combination, but at least some of these features and / or steps are mutually exclusive.

[0047] Each feature disclosed in this specification (including the appended claims, summary, and drawings) may be replaced by an alternative feature that serves the same, equivalent, or similar purpose unless otherwise specified. Accordingly, unless otherwise explicitly stated, each disclosed feature is an example of a general series of equivalent or similar features.

[0048] The present invention is not limited to the details of the configurations described above. The present invention extends to any new feature disclosed in this specification (including any appended claims, summary, and drawings), any new combination of any new feature, or any new combination of any disclosed method step or process step.

Claims

Claim 1 A high-temperature gas-cooled reactor fuel block (100) comprising: a fuel channel (206); and a coolant channel (304), wherein the fuel channel comprises a fuel element (208), and the fuel channel further comprises a thermal bridge (210) that thermally connects the fuel element and the fuel channel, wherein the thermal bridge comprises a melting point higher than the operating temperature of the reactor fuel block, thereby improving heat transfer from the fuel element to the fuel block and thereby improving heat transfer to the coolant channel, wherein the thermal bridge is in the form of a powdered material that thermally connects the fissile material and the fuel channel, thereby allowing thermal expansion and contraction of the fuel block, the fuel channel and the fuel element, and wherein the volume change of the fuel channel is caused by neutron irradiation. Claim 2 A high-temperature gas-cooled nuclear reactor fuel block, wherein the powder-type material is a particle and the particle size is less than 100 μm. Claim 3 A high-temperature gas-cooled nuclear reactor fuel block according to either claim 1 or 2, wherein the thermal bridge is selected from the group comprising metals and their alloys, metalloids, carbon, and thermally conductive ceramics. Claim 4 In claim 3, the thermal bridge is graphite, a high-temperature gas-cooled reactor fuel block. Claim 5 A high-temperature gas-cooled reactor fuel block, wherein, in either claim 1 or 2, the thermal bridge further comprises a flammable poison. Claim 6 In claim 5, the flammable toxin is a high-temperature gas-cooled reactor fuel block, which is boron carbide. Claim 7 A high-temperature gas-cooled nuclear reactor system comprising a fuel block (100) according to claim 1 or 2. Claim 8 In claim 7, the reactor is a high-temperature gas-cooled reactor system that is a direct cycle system. Claim 9 A direct cycle high-temperature nitrogen-cooled reactor system (500) comprising a fuel block (100) according to claim 1 or 2, wherein the fuel block comprises: a fuel channel (206); and a coolant channel (304), wherein the fuel channel comprises a fissile material, and the fuel channel further comprises a thermal bridge (210) in the form of graphite powder that thermally connects the fissile material and the fuel channel, wherein the thermal bridge comprises a melting point higher than the operating temperature of the reactor fuel block, and the thermal bridge further comprises a combustible toxin in the form of boron carbide, thereby improving heat transfer from the fuel element to the fuel block, and thereby improving heat transfer to the coolant channel. Claim 10 delete Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete

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