Air-cooled fast reactor core and air-cooled fast reactor system

Abstract

The invention discloses an air-cooled fast reactor core and an air-cooled fast reactor system. The air-cooled fast reactor core comprises a shell and a fuel assembly. The shell is provided with an inner cavity, the fuel assembly is installed in the inner cavity, the fuel assembly comprises a first fuel unit and a second fuel unit, and the first fuel unit and the second fuel unit extend in the axial direction of the shell. The number of the first fuel units is at least one, the number of the second fuel units is multiple, and the multiple second fuel units are arranged side by side and distributed on the periphery of the first fuel units. The first fuel unit comprises a first fuel pellet, the second fuel unit comprises a second fuel pellet, and the fuel enrichment degree of the second fuel pellet is higher than that of the first fuel pellet. The air-cooled fast reactor core has a relatively strong temperature negative feedback effect, so that the temperature in the reactor core is prevented from rapidly rising, a local power peak can be effectively reduced, and the air-cooled fast reactor core has relatively high safety performance.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear reactor engineering, and particularly relates to a gas-cooled fast reactor core and a gas-cooled fast reactor system. Background Art

[0002] The gas-cooled fast reactor (GFR) is one of the six advanced reactor types of the fourth-generation nuclear energy system. Its coolant is an inert gas such as helium or supercritical carbon dioxide, and the core mainly initiates fission reactions by fast neutrons. In other words, a gas coolant is used in the gas-cooled fast reactor. The advantage of the gas coolant is that there is no need to consider the phase change problem, and it can operate stably at high temperatures.

[0003] However, compared with liquid coolants, the heat transfer capacity and thermal conductivity of gases are relatively low. This makes it difficult for gas coolants to quickly and timely transfer the heat of the core in case of an emergency, and the temperature of the materials in the reactor is likely to rise rapidly. In severe cases, it may lead to fuel melting. Therefore, the existing gas-cooled fast reactors face significant safety problems. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a gas-cooled fast reactor core and a gas-cooled fast reactor system with high safety performance in view of the above deficiencies in the prior art.

[0005] According to an embodiment of the first aspect of the present invention, there is provided a gas-cooled fast reactor core, including: a housing and a fuel assembly; the housing is provided with an inner cavity, the fuel assembly is installed in the inner cavity, the fuel assembly includes a first fuel unit and a second fuel unit, and both the first fuel unit and the second fuel unit extend along the axial direction of the housing. The number of the first fuel units is at least one, the number of the second fuel units is multiple, and the multiple second fuel units are arranged side by side and disposed around the first fuel unit. The first fuel unit includes a first fuel pellet, the second fuel unit includes a second fuel pellet, and the fuel enrichment of the second fuel pellet is higher than that of the first fuel pellet.

[0006] Preferably, the first fuel unit further includes a first moderator, and a first coolant channel is provided inside the first moderator. The first coolant channel extends along the axial direction of the housing, and the first coolant channel is used for the gas coolant to flow through. The gas coolant flows along the axial direction of the first coolant channel; the first fuel pellets are all accommodated in the first moderator, and when the gas coolant flows through the first coolant channel, the first fuel pellets are cooled.

[0007] Preferably, the first fuel pellet is divided into a first inner ring pellet and a first outer ring pellet. Both the first inner ring pellet and the first outer ring pellet extend along the axial direction of the first coolant channel. The number of both the first inner ring pellets and the first outer ring pellets is multiple. The multiple first outer ring pellets are arranged side by side and are disposed around the multiple first inner ring pellets. The fuel enrichment of the first inner ring pellet is higher than that of the first outer ring pellet.

[0008] Preferably, the first fuel unit further includes a first cladding. The first cladding extends along the axial direction of the first coolant channel. The number of the first claddings is multiple, and the number of the first claddings is the same as the number of the first fuel pellets. Moreover, the first claddings and the first fuel pellets correspond to each other one by one, and each first cladding covers one first fuel pellet.

[0009] Preferably, first end units are respectively installed at both ends of the first cladding. The first fuel pellet is located between the two first end units. The first end unit includes a first axial reflector and a first axial shield. The first axial reflector is located inside the first axial shield. Gas cavities are respectively provided between both ends of the first fuel pellet and the two first end units.

[0010] Preferably, the first fuel unit further includes a first control rod guide tube and a first control rod. The first control rod guide tube is installed in the first coolant channel. The central axis of the first control rod guide tube and the central axis of the first coolant channel are on the same extension line. The first inner ring pellets and the first outer ring pellets are uniformly disposed around the first control rod guide tube. The first control rod is partially inserted into the first control rod guide tube, and the first control rod can move along the axial direction of the first control rod guide tube to adjust the insertion depth of the first control rod.

[0011] Preferably, the second fuel unit includes a second moderator. A second coolant channel is provided inside the second moderator. The second coolant channel extends along the axial direction of the housing. The second coolant channel is used for the gas coolant to flow through, and the gas coolant flows along the axial direction of the second coolant channel. The second fuel pellet is accommodated in the second moderator. When the gas coolant flows through the second coolant channel, the second fuel pellet is cooled.

[0012] Preferably, the second fuel pellet is divided into a second inner ring pellet and a second outer ring pellet, and both the second inner ring pellet and the second outer ring pellet extend along the axial direction of the second coolant channel. The number of both the second inner ring pellets and the second outer ring pellets is multiple. A plurality of the second outer ring pellets are arranged side by side and disposed around the plurality of second inner ring pellets. The fuel enrichment of the second inner ring pellet is higher than that of the second outer ring pellet, and the fuel enrichment of the second outer ring pellet is higher than that of the first inner ring pellet.

[0013] Preferably, the second fuel unit further includes a second cladding, which extends along the axial direction of the second coolant channel. The number of the second claddings is multiple, and the number of the second claddings is the same as that of the second fuel pellets, and the second claddings correspond to the second fuel pellets one by one, and each second cladding covers one second fuel pellet.

[0014] Preferably, second end units are respectively installed at both ends of the second cladding, and the second fuel pellet is located between the two second end units. The second end unit includes a second axial reflector and a second axial shield, and the second axial reflector is located inside the second axial shield. Gas cavities are respectively provided between both ends of the second fuel pellet and the two second end units.

[0015] Preferably, the first fuel pellet and the second fuel pellet have the same height. The ratio of the height of the fuel pellet to the diameter of the circumcircle of the fuel assembly is the height-diameter ratio of the active zone, and the height-diameter ratio of the active zone is 1.3 - 2.5.

[0016] Preferably, the housing includes a first reflector and a second reflector. The first reflector surrounds the fuel assembly, and the second reflector surrounds the first reflector. The first reflector is made of a first type of material, and the second reflector is made of a second type of material. The atomic mass of the nuclide of the first type of material is greater than that of the nuclide of the second type of material. The first reflector and the second reflector are used to reflect the escaping neutrons generated by the fuel assembly back into the inner cavity.

[0017] Preferably, the first type of material is zirconium carbide or zirconium silicide, and the second type of material is graphite or beryllium oxide.

[0018] Preferably, it further includes a second control rod guide tube and a second control rod. The second control rod guide tube extends along the axial direction of the inner cavity and is disposed through the first reflector. The number of the second control rod guide tubes is multiple, and the multiple second control rod guide tubes are arranged outside the fuel assembly. The second control rod is partially inserted into the second control rod guide tube, and the second control rod can move along the axial direction of the second control rod guide tube to adjust the insertion depth of the second control rod.

[0019] Preferably, the housing further includes a neutron shielding layer, which surrounds the outside of the reflector assembly and is used to absorb the leaked neutrons.

[0020] According to an embodiment of the second aspect of the present invention, there is provided a gas-cooled fast reactor system, including the above-mentioned gas-cooled fast reactor core.

[0021] By optimizing the fuel enrichment in the gas-cooled fast reactor core of the present invention, the fuel enrichment in the outer region is greater than that in the inner region, which can effectively improve the safety of the core. Specifically, from the cross-section of the gas-cooled fast reactor core, the second fuel unit is disposed around the first fuel unit, and the fuel enrichment of the fuel pellets in the second fuel unit is higher than that of the fuel pellets in the first fuel unit. It should be noted that usually the neutron flux in the inner region fuel assembly is relatively high, so the fission reaction rate in the inner region will be very high, which will lead to excessive power density and may cause local overheating or safety hazards. And in this gas-cooled fast reactor core, by placing the fuel assembly with lower fuel enrichment in the inner region, the fission cross-section and fission reaction rate in the inner region can be reduced, thereby slowing down the power generation in the inner region; at the same time, by setting the fuel assembly with higher fuel enrichment in the outer region, the fission cross-section and fission reaction rate in the outer region can be increased, improving the power generation in the outer region and making the internal and external power distributions more uniform. This design method can effectively flatten the power distribution in the core, reduce thermal stress and thermal flux imbalance, and improve the operation efficiency and stability of the nuclear reactor. Therefore, by making the fuel enrichment of the second fuel unit greater than that of the first fuel unit, the core power can be flattened, and thus the safety performance of the core can be improved.

[0022] Furthermore, each fuel unit (the first fuel unit and the second fuel unit) is provided with a moderator (the first moderator and the second moderator), and the fuel pellets are arranged inside the moderator. By providing the moderator to soften the core energy spectrum, the temperature negative feedback effect of the core can be effectively improved, thereby further enhancing the safety of the core. It should be noted that in a nuclear reactor, when the temperature rises, due to the Doppler effect (i.e., the neutron resonance absorption cross-section increases with the increase in temperature), the probability of neutron absorption will increase, resulting in a decrease in the fission reaction rate. This decrease in the reaction rate will further lead to a decrease in the core temperature, that is, the temperature negative feedback of the core is realized. Through the temperature negative feedback effect, the rapid rise of the temperature inside the core can be avoided, thereby improving the safety of the core. The fuel units (the first fuel unit and the second fuel unit) in the core of this gas-cooled fast reactor both adopt fuel pellets with low enrichment levels to enhance the resonance absorption of the fuel and improve the Doppler effect, that is, the temperature negative feedback effect can be further improved.

[0023] In addition, at positions close to the moderator, the moderation effect of neutrons is strong and the fission cross-section is large. Therefore, in each fuel unit, the fuel enrichment level of the outer ring pellets is lower than that of the inner ring pellets. The advantage is that the power distribution of the core can be further flattened, thereby avoiding local power peaks.

[0024] Moreover, the gas coolant in the coolant channels (the first coolant channel and the second coolant channel) provided in the moderator can cool down the fuel pellets, avoiding the problem of fuel pellet melting.

[0025] In summary, the core of this gas-cooled fast reactor can effectively reduce local power peaks, and has a strong temperature negative feedback effect, thereby avoiding the rapid rise of the temperature inside the core and having high safety performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic radial structure diagram of the core of a gas-cooled fast reactor in some embodiments of the present invention;

[0027] Figure 2a is a schematic cross-sectional structure diagram of the first fuel unit in some embodiments of the present invention;

[0028] Figure 2b is a schematic cross-sectional structure diagram of the second fuel unit in some embodiments of the present invention;

[0029] Figure 3 is a schematic axial structure diagram of a fuel rod bundle in some embodiments of the present invention;

[0030] Figure 4 is a core burnup curve diagram in some embodiments of the present invention;

[0031] Figure 5It is the power distribution diagram of a 1 / 6 core assembly of the core of a gas-cooled fast reactor in some embodiments of the present invention;

[0032] Figure 6 It is the axial power distribution diagram of a gas-cooled fast reactor in some embodiments of the present invention.

[0033] In the figure: 1 - housing, 11 - first reflector, 12 - second reflector, 13 - neutron shielding layer, 2 - first fuel unit, 21 - first fuel rod bundle, 211 - first fuel pellet, 211a - first inner ring pellet, 211b - first outer ring pellet, 212 - gas cavity, 213 - first axial reflector, 214 - first axial shielding layer, 22 - first moderator, 23 - first control rod guide tube, 3 - second fuel unit, 31 - second fuel rod bundle, 311 - second fuel pellet, 311a - second inner ring pellet, 311b - second outer ring pellet, 32 - second moderator, 4 - second control rod guide tube. Detailed implementation manners

[0034] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the accompanying drawings in the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts belong to the scope of the present invention.

[0035] In the description of the present invention, it should be noted that the terms "upper", "lower", etc. indicating the orientation or position relationship are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of description and simplification, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0036] In the description of the present invention, the terms "first", "second", "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0037] In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "connected", "arranged", "installed", "fixed", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0038] It should be noted that the Gas-cooled Fast Reactor (GFR) is one of the six advanced reactor types of the fourth-generation nuclear energy system. Its coolant is an inert gas such as helium or supercritical carbon dioxide, and the fission reaction in the core is mainly initiated by fast neutrons. As a fast reactor, the GFR can achieve the breeding and transmutation of nuclear fuel and is a key link in the closed fuel cycle. At the same time, the inert gas coolant has stable chemical properties and no phase change problems, effectively avoiding problems such as sodium fires, sodium boiling, the need to set up an intermediate loop, and continuous heating during the shutdown process, which is conducive to improving the safety and economy during the operation of the fast reactor. Therefore, the GFR has good development prospects among the fourth-generation reactor types.

[0039] The main problem faced in the R & D process of the GFR reactor type is the poor heat transfer performance of the gaseous coolant. Once a loss-of-coolant accident occurs in the core, since it is difficult for the coolant to quickly and timely export the heat from the core, the temperature of the in-core materials is likely to rise rapidly, and in severe cases, it may lead to fuel melting. In the early GFR schemes, relatively conservative core parameters were mostly adopted, and the average in-core temperature was designed to be relatively low (the outlet temperature was 500°C - 600°C) to meet the heat export requirements. However, the low outlet temperature is not conducive to giving full play to the performance advantages of the gas coolant. In other words, the gas coolant can operate stably at a higher temperature, which helps to improve the thermal efficiency, enabling the GFR reactor type to convert thermal energy at a high temperature and thus achieve a higher thermoelectric efficiency. However, compared with liquid coolants, the heat transfer ability and thermal conductivity of gases are generally lower, which makes it difficult for the gas coolant to quickly and effectively remove the heat from the core in case of an emergency, increasing the melting risk. Since the beginning of the 21st century, with the development of high-temperature-resistant materials, new GFR schemes mostly adopt new high-temperature-resistant ceramic materials, thereby improving the thermal-hydraulic safety margin of the GFR core, and the core outlet temperature has also been increased to about 850°C. However, the design of heat export from the core under transient conditions is still the key issue restricting the development of the GFR.

[0040] In addition, the core energy spectrum of the gas-cooled fast reactor is hard and the neutron leakage is strong, which not only reduces the neutron economy of the core, but also easily causes strong irradiation damage to structural components such as the pressure vessel, affecting the service life of the structural components. In other words, the gas-cooled fast reactor has the advantage of a hard core energy spectrum (i.e., high neutron energy). However, a hard core energy spectrum means that neutrons are more likely to pass through the reflector and leak out of the reactor, resulting in a relatively high neutron leakage intensity. Strong neutron leakage not only reduces the neutron economy of the core, but also easily causes strong irradiation damage to structural components such as the pressure vessel, affecting the service life of the structural components, and further reducing the inherent safety of the core. Therefore, a relatively large reflector structure is often set in the fast reactor core to reduce the neutron leakage rate of the core. Existing fast reactor designs usually use "light reflectors" with strong moderation capabilities such as graphite and beryllium oxide, but these materials can easily lead to uneven core power distribution, a positive reflector temperature coefficient, and also have a greater impact on the core energy spectrum; to solve these problems, some European reactor types plan to use "heavy reflectors" represented by zirconium silicide (Zr2Si3), but the new problem that arises is that the reflection ability of the heavy reflector is weak, and the reflector thickness required for the core is large, increasing the overall size of the core and being unfavorable for achieving lightweight and miniaturization.

[0041] Enhancing the negative temperature feedback effect is an effective means to improve the inherent safety of the core. In the negative temperature feedback effect of the GFR, the largest proportion is the Doppler feedback of the fuel. However, in the traditional GFR design, the core energy spectrum is very hard and the Doppler effect of the fuel is weak, resulting in a very small reactivity temperature feedback for the overall GFR core. Therefore, appropriate optimization means need to be adopted to enhance the negative temperature feedback effect of the GFR core to improve the inherent safety of the core.

[0042] At the same time, in the design of the reflector, the present invention adopts a "heavy + light" double-layer reflector structure. The inner reflector close to the active region uses heavy materials to flatten the core power, and the outer reflector on the outside uses a layer of light material with strong moderation ability to reduce neutron leakage.

[0043] Embodiment 1

[0044] Please refer to Figure 1 , the present invention discloses a gas-cooled fast reactor core, including a housing 1 and a fuel assembly.

[0045] Among them, the housing 1 is provided with an inner cavity, and the fuel assembly is installed in the inner cavity. The fuel assembly includes a first fuel unit 2 and a second fuel unit 3. Both the first fuel unit 2 and the second fuel unit 3 extend along the axial direction of the housing 1, and the axial direction of the housing 1 is Figure 1The direction perpendicular to the paper surface. The number of the first fuel units 2 is at least one, and the number of the second fuel units 3 is multiple. The multiple second fuel units 3 are arranged side by side and disposed around the first fuel unit 2. The first fuel unit 2 includes a first fuel pellet 211, and the second fuel unit 3 includes a second fuel pellet 311. The fuel enrichment of the second fuel pellet 311 is higher than that of the first fuel pellet 211.

[0046] It should be noted that for the poor heat transfer performance of the gaseous coolant, in the transient condition, when the heat in the core of the gas-cooled fast reactor suddenly rises, the gaseous coolant cannot export the heat in time. Further, once a loss-of-coolant accident occurs in the core, since it is difficult for the gaseous coolant to export the heat in the core to the outside of the core in time and quickly, the temperature of the materials in the reactor is likely to rise rapidly, and in severe cases, it may lead to the melting of (cladding) fuel.

[0047] The core of this gas-cooled fast reactor optimizes the fuel enrichment in the core of the gas-cooled fast reactor, making the fuel enrichment in the outer region greater than that in the inner region, which can effectively improve the safety of the core. Specifically, as Figure 1 shown, from the cross-section of the core of the gas-cooled fast reactor, the second fuel unit 3 is disposed around the first fuel unit 2, and the fuel enrichment of the fuel pellets in the second fuel unit 3 is higher than that of the fuel pellets in the first fuel unit 2.

[0048] In other words, the first fuel unit 2 is the inner-region fuel unit, and the second fuel unit 3 is the outer-region fuel unit. By optimizing the fuel enrichment of (the first fuel unit 2 and the second fuel unit 3), low-enriched uranium is used to enhance the resonance absorption of the fuel and improve the Doppler effect.

[0049] Preferably, the enrichment of the inner-region fuel assembly is 14% to 17%, and the enrichment of the outer-region assembly is 18% to 20%. As an example, the enrichment of the inner-region fuel assembly can be 14%, 16% or 17%, and the enrichment of the outer-region assembly is 18%, 19.75% or 20%.

[0050] The fuel enrichment refers to the concentration level of fissile nuclides in the fuel elements in a nuclear reactor. Usually, the fuel used in a nuclear reactor is fissile materials such as uranium or plutonium. In this embodiment, the fuel used in the fuel assembly is uranium dioxide (UO2), and the fuel enrichment represents the relative content of fissile nuclides (such as uranium-235) in the fuel.

[0051] Generally, the neutron flux in the inner region fuel assembly is relatively high. Therefore, the fission reaction rate in the inner region will be very high, which will lead to an excessive power density and may cause local overheating or safety hazards. In this gas-cooled fast reactor core, by placing fuel assemblies with a lower fuel enrichment in the inner region, the fission cross-section and fission reaction rate in the inner region can be reduced, thereby slowing down the power generation in the inner region. At the same time, by setting fuel assemblies with a higher fuel enrichment in the outer region, the fission cross-section and fission reaction rate in the outer region can be increased, improving the power generation in the outer region and making the internal and external power distribution more uniform. This design method can effectively flatten the core power distribution, reduce the thermal stress and the imbalance of heat flux, and improve the operation efficiency and stability of the nuclear reactor. Therefore, by making the fuel enrichment of the second fuel unit greater than that of the first fuel unit, the core power can be flattened, and thus the safety performance of the core can be improved.

[0052] Furthermore, each fuel unit (the first fuel unit and the second fuel unit) is provided with a moderator (the first moderator and the second moderator respectively), and the fuel pellets are arranged inside the moderator. By setting the moderator to soften the core energy spectrum, the temperature negative feedback effect of the core can be effectively improved, thereby further enhancing the safety of the core. It should be noted that in a nuclear reactor, when the temperature rises, due to the Doppler effect (i.e., the neutron resonance absorption cross-section increases with the increase in temperature), the probability of neutrons being absorbed will increase, resulting in a decrease in the fission reaction rate. This decrease in the reaction rate will further lead to a decrease in the core temperature, that is, the temperature negative feedback of the core is realized. Through the temperature negative feedback effect, the rapid rise of the temperature inside the core can be avoided, thereby improving the safety of the core. The fuel units (the first fuel unit and the second fuel unit) in this gas-cooled fast reactor core all use low-enrichment fuel pellets to enhance the resonance absorption of the fuel and improve the Doppler effect, that is, the temperature negative feedback effect can be further improved.

[0053] In addition, at the position close to the moderator, the moderation effect of neutrons is strong and the fission cross-section is large. Therefore, in each fuel unit, the fuel enrichment of the outer ring pellets is lower than that of the inner ring pellets. The advantage is that it can further flatten the core power distribution and thus reduce the local power peak. Moreover, the gas coolant in the coolant channels (the first coolant channel and the second coolant channel) arranged in the moderator can cool the fuel pellets, avoiding the problem of fuel pellet melting.

[0054] It should also be noted that the fact that the fuel enrichment of the above-mentioned second fuel pellet 311 is higher than that of the first fuel pellet 211 means that the fuel enrichment of any pellet among the multiple second fuel pellets 311 is higher than that of the first fuel pellet 211.

[0055] In summary, the core of this gas-cooled fast reactor can effectively reduce the local power peak, and has a strong negative temperature feedback effect, thus avoiding the rapid rise of the temperature in the core and having high safety performance.

[0056] In some embodiments, in order to further improve the safety of the core, a fully ceramic core design is adopted to increase the temperature limit value and the safety margin of the core. Specifically, in this embodiment, uranium dioxide (UO2) fuel is used, that is, both the first fuel pellet 211 and the second fuel pellet 311 are made of uranium dioxide (UO2) material; the cladding material is silicon carbide (SiC) ceramic material with good high-temperature resistance, so that the core has a high temperature limit value.

[0057] Please refer to Figure 2a , in some embodiments, the first fuel unit 2 includes a first moderator 22, and a first coolant channel is provided inside the first moderator 22. The first coolant channel extends along the axial direction of the housing 1, and the first coolant channel is used for the gas coolant to flow through. The gas coolant flows along the axial direction of the first coolant channel. The first fuel pellets 211 are all accommodated in the first moderator 22. When the gas coolant flows through the first coolant channel, it cools the first fuel pellets 211.

[0058] It should be noted that enhancing the negative temperature feedback effect is an effective means to improve the inherent safety of the core. In the negative temperature feedback effect of GFR, the Doppler feedback of the fuel accounts for the largest proportion. However, in the traditional GFR scheme, the core energy spectrum is very hard and the Doppler effect of the fuel is weak, resulting in a very small reactivity temperature feedback of the overall GFR core. Therefore, appropriate optimization means need to be taken to enhance the negative temperature feedback effect of the GFR core to improve the inherent safety of the core.

[0059] In this embodiment, the first moderator 22 is used to soften the energy spectrum of the first fuel unit 2, thereby enhancing the negative temperature feedback effect of the core. The negative temperature feedback effect refers to the phenomenon in a nuclear reactor that when the temperature of the reactor rises, the rate of the nuclear reaction decreases, thereby reducing the generated heat. This phenomenon helps to control the temperature of the nuclear reactor, maintain its stable operation, and prevent the core from overheating. When the temperature of the nuclear reactor rises, the neutron resonance absorption cross section of the nuclear fuel will increase, resulting in an increase in neutron absorption, a decrease in the nuclear reaction, and a decrease in the heat generation of the fuel, thereby reducing the temperature of the reactor.

[0060] Furthermore, as Figure 2a and Figure 3 shown, the first fuel pellet 211 is divided into a first inner ring pellet 211a and a first outer ring pellet 211b. Both the first inner ring pellet 211a and the first outer ring pellet 211b extend along the axial direction of the first coolant channel, that is, Figure 2a the direction perpendicular to the paper surface in Figure 3in the vertical direction. The number of the first inner core blocks 211a and the first outer core blocks 211b is multiple. The multiple first outer core blocks 211b are arranged in parallel and disposed around the multiple first inner core blocks 211a. The fuel enrichment of the first inner core blocks 211a is higher than that of the first outer core blocks 211b.

[0061] In this embodiment, from the cross-section of the first moderator 22, by arranging the core blocks with lower fuel enrichment in the outer circle, that is, the first outer core blocks 211b, and arranging the core blocks with higher fuel enrichment in the inner circle, that is, the first inner core blocks 211a, the sizes (height, diameter) of the first outer core blocks 211b and the first inner core blocks 211a are the same, and the main differences lie in the position and fuel enrichment. Since at the position close to the moderator, the moderation effect of neutrons is strong and the fission cross-section is large, therefore, this is beneficial to flatten the power distribution in the first fuel unit and reduce the local power peak.

[0062] In some embodiments, the first fuel unit 2 further includes a first cladding. The first cladding extends along the axial direction of the first coolant channel. The number of the first claddings is multiple, and the number of the first claddings is the same as the number of the first fuel core blocks 211, and the first claddings correspond to the first fuel core blocks 211 one by one. Each first cladding covers one first fuel core block 211.

[0063] In other words, the outer sides of each first fuel core block 211 are covered with the first cladding, thereby forming a first fuel rod bundle 21. As Figure 2a shown, among them, the fuel rod bundle corresponding to the first inner core block 211a is the first inner fuel rod; the fuel rod bundle corresponding to the first outer core block 211b is the first outer fuel rod. The first cladding provides additional mechanical strength for the first fuel core block 211, can withstand the pressure and temperature changes in the reactor core, and ensures the integrity of the fuel rod bundle structure under extreme conditions. Specifically, in this embodiment, the first fuel core block 211 (the first inner core block 211a and the first outer core block 211b) uses uranium dioxide (UO2) fuel; the first cladding material is a silicon carbide (SiC) ceramic material with good high-temperature resistance performance, so that the reactor core has a higher temperature limit.

[0064] Furthermore, first end units are respectively installed at both ends of the first cladding, and the first fuel core block 211 is located between the two first end units. The first end unit includes a first axial reflector 213 and a first axial shield 214, and the first axial reflector 213 is located inside the first axial shield 214. Gas cavities are respectively provided between both ends of the first fuel core block 211 and the two first end units.

[0065] Among them, the axial reflection layer is used to reflect neutrons and increase the utilization rate of neutrons. By redirecting neutrons back into the fuel rods, the reflection layer can promote more efficient nuclear fission reactions, thereby enhancing the energy output of the reactor. The axial shielding layer is used to absorb and attenuate high-energy radiation (such as γ-rays and neutrons) from the reactor, reduce the radiation dose in the surrounding environment, and ensure safety. The function of the gas cavity is to accommodate fission products and fission gases. After the nuclear fission reaction, the number of atoms increases, resulting in an increase in the volume of the fuel pellets, that is, fuel swelling occurs. At the same time, gaseous fission products are generated, so a gas cavity structure is required to accommodate the fission gases and the volume swelling of the fuel, avoiding excessive pressure on the cladding and causing the cladding to rupture.

[0066] As Figure 2a shown, the first fuel unit 2 further includes a first control rod guide tube 23 and a first control rod. The first control rod guide tube 23 is installed in the first coolant channel. The central axis of the first control rod guide tube 23 and the central axis of the first coolant channel are on the same extension line. The first inner ring pellets 211a and the first outer ring pellets 211b are evenly distributed around the first control rod guide tube 23. The first control rod is partially inserted into the first control rod guide tube 23, and the first control rod can move axially along the first control rod guide tube 23 to adjust the insertion depth of the first control rod.

[0067] Among them, the first control rod is a safety rod and can be made of materials that are easy to absorb neutrons, such as boron, boron carbide, cadmium, etc. The safety rod is used to be fully inserted into the first control rod guide tube 23 to quickly stop the chain fission reaction in the reactor core.

[0068] In this embodiment, by placing the safety rod in the middle of multiple first fuel rod bundles 21, uniform distribution of neutron flux and power can be achieved, preventing local overheating or abnormal reactions. In case of an emergency, by regulating the position and movement of the safety rod, the overall neutron flux distribution of the reactor can be better controlled to ensure the stability of reactor shutdown.

[0069] Please refer to Figure 2b , in some embodiments, the second fuel unit 3 includes a second moderator 32. The inside of the second moderator 32 is provided with a second coolant channel. The second coolant channel extends along the axial direction of the housing 1. The second coolant channel is used for the gas coolant to flow through, and the gas coolant flows along the axial direction of the second coolant channel. The second fuel pellets 311 are accommodated in the second moderator 32. When the gas coolant flows through the second coolant channel, it cools the second fuel pellets 311.

[0070] In this embodiment, the second moderator 32 is used to soften the energy spectrum of the second fuel unit 3, thereby enhancing the temperature negative feedback effect of the reactor core. The temperature negative feedback effect refers to the phenomenon in a nuclear reactor that when the temperature of the reactor rises, the rate of the nuclear reaction decreases, thereby reducing the heat generated. This phenomenon helps to control the temperature of the nuclear reactor, maintain its stable operation, and prevent the reactor core from overheating. When the temperature of the nuclear reactor rises, the neutron resonance absorption cross-section of the nuclear fuel increases, resulting in an increase in neutron absorption, a decrease in the nuclear reaction, and a decrease in the heat generation of the fuel, thereby reducing the temperature of the reactor.

[0071] The second fuel pellets 311 are divided into second inner ring pellets 311a and second outer ring pellets 311b, and both the second inner ring pellets 311a and the second outer ring pellets 311b extend along the axial direction of the housing 1. The number of both the second inner ring pellets 311a and the second outer ring pellets 311b is multiple. The multiple second outer ring pellets 311b are arranged in parallel and are disposed on the periphery of the multiple second inner ring pellets 311a. The fuel enrichment of the second inner ring pellets 311a is higher than that of the second outer ring pellets 311b, and the fuel enrichment of the second outer ring pellets 311b is higher than that of the second inner ring pellets 311a.

[0072] In this embodiment, from the cross-section of the second moderator 32, by arranging the pellets with lower fuel enrichment on the outer ring, that is, the second outer ring pellets 311b; and arranging the pellets with higher fuel enrichment on the inner periphery, that is, the second inner ring pellets 311a. Similar to the first fuel pellets 211, the second outer ring pellets 311b and the second inner ring pellets 311a have the same size, and their main differences lie in the position and fuel enrichment. Since at the position close to the moderator, the moderation effect of neutrons is strong and the fission cross-section is large, thus, this is beneficial to flatten the power distribution in the second fuel unit and reduce the local power peak.

[0073] The second fuel unit 3 further includes a second cladding. The second cladding extends along the axial direction of the second coolant channel. The number of the second claddings is multiple, and the number of the second claddings is the same as the number of the second fuel pellets 311, and the second claddings correspond to the second fuel pellets 311 one by one. Each second cladding covers one second fuel pellet 311.

[0074] In other words, the outer side of each second fuel pellet 311 is covered with a second cladding, thereby forming a second fuel rod bundle 31. As Figure 2bAs shown in the figure, among them, the fuel rod bundle corresponding to the second inner ring fuel pellet 311a is the second inner ring fuel rod; the fuel rod bundle corresponding to the second outer ring fuel pellet 311b is the second outer ring fuel rod. The second cladding provides additional mechanical strength for the second fuel pellet 311, can withstand the pressure and temperature changes in the reactor core, and ensures the integrity of the fuel rod bundle structure under extreme conditions. Specifically, in this embodiment, the second fuel pellet 311 (the second inner ring fuel pellet 311a and the second outer ring fuel pellet 311b) uses uranium dioxide (UO2) fuel; the second cladding material is silicon carbide (SiC) ceramic material with good high-temperature resistance, so that the reactor core has a higher temperature limit.

[0075] Second end units are respectively installed at both ends of the second cladding, and the second fuel pellet 311 is located between the two second end units. The second end unit includes a second axial reflector and a second axial shield, and the second axial reflector is located inside the second axial shield. Gas cavities are respectively provided between both ends of the second fuel pellet 311 and the two second end units.

[0076] Among them, the axial reflector is used to reflect neutrons and increase the utilization rate of neutrons. By redirecting neutrons back into the fuel rod, the reflector can promote a more efficient nuclear fission reaction, thereby increasing the energy output of the reactor. The axial shield is used to absorb and attenuate high-energy radiation (such as γ rays and neutrons) from the reactor, reduce the radiation dose in the surrounding environment, and ensure safety. The function of the gas cavity is to accommodate fission products and fission gases. After the nuclear fission reaction, the number of atoms increases, resulting in an increase in the volume of the fuel pellet, that is, fuel swelling occurs. At the same time, gaseous fission products are generated, so a gas cavity structure is required to accommodate the fission gases and the volume swelling of the fuel, avoiding excessive pressure on the cladding and causing the cladding to rupture.

[0077] In some embodiments, fitting surfaces are provided on the outer side walls of the first fuel unit 2 and the second fuel unit 3. The first fuel unit 2 and the second fuel unit 3 are arranged side by side, and the outer side walls are fitted together through the fitting surfaces. The cross-sections of the first fuel unit 2 and the second fuel unit 3 can be circular, semi-circular ring-shaped, regular quadrilateral, regular hexagon, or regular octagon, etc. For example: both the first fuel unit 2 and the second fuel unit 3 adopt regular quadrangular prisms, which can achieve a compact layout; it is also possible that the first fuel unit 2 adopts a regular quadrangular prism, while the second fuel unit 3 adopts a regular octagonal prism, and four second fuel units 3 surround one first fuel unit 2, which can also achieve a compact layout. Another example: the first fuel unit 2 can also be set as circular, and the second fuel unit 3 is set as semi-circular ring-shaped. Two semi-circular ring-shaped second fuel units 3 are combined into a ring, and the second fuel unit 3 is arranged around the first fuel unit 2, which can also achieve a compact layout.

[0078] Specifically, the first moderators 22 of the first fuel unit 2 and the second moderators 32 of the second fuel unit 3 both adopt a hexagonal columnar assembly structure design to achieve a compact arrangement of the fuel in the reactor, thereby reducing the core volume and facilitating the miniaturization of the core. The first fuel unit 2 is attached to the fitting portions on the outer sidewalls of a plurality of second fuel units 3 to achieve a compact arrangement. As Figure 1 shown, the number of the first fuel units 2 is 7, and the number of the second fuel units 3 is 30.

[0079] The advantages of arranging the fuel assemblies with a higher fuel enrichment (the second fuel unit 3) in the outer region and the fuel assemblies with a lower fuel enrichment (the first fuel unit 2) in the inner region are described as follows:

[0080] Generally, the neutron flux of the fuel assemblies in the inner region is relatively high. By placing the fuel assemblies with a lower fuel enrichment in the inner region, the fission cross-section and fission reaction rate in the inner region can be reduced, thereby slowing down the power generation in the inner region; at the same time, by arranging the fuel assemblies with a higher fuel enrichment in the outer region, the fission cross-section and fission reaction rate in the outer region can be increased, improving the power generation in the outer region and making the internal and external power distributions more uniform. This design method can effectively flatten the core power distribution, reduce the thermal stress and the imbalance of the heat flux, and improve the operation efficiency and stability of the nuclear reactor. Therefore, by making the fuel enrichment of the second fuel unit 3 greater than that of the first fuel unit 2, the core power can be flattened, and thus the safety performance of the core can be improved.

[0081] At the same time, to flatten the core power distribution, the fuel assemblies in the inner region adopt a lower fuel enrichment, and the outermost fuel rods of each assembly close to the moderator layer have a lower enrichment than the inner fuel rods because the moderation is strong and the fission cross-section is large at this position. In this embodiment, the enrichments of the fuel assemblies in the inner region are 16% and 17%, and the enrichments of the assemblies in the outer region are 18% and 19.75%.

[0082] Please refer to Figure 2a and Figure 2b , Figure 1 which is a schematic structural diagram of the gas-cooled fast reactor assembly of this embodiment. The entire core contains two different types of assemblies, which respectively constitute the outer region and the inner region of the core. Considering adding moderator materials in the assemblies, through screening and optimization of various different moderator materials and different arrangement methods of the moderator materials in the assemblies, the core energy spectrum can be adjusted and the Doppler feedback can be improved. In other words, both the outer region assemblies (i.e., the second fuel unit 3) and the inner region assemblies (i.e., the first fuel unit 2) are provided with moderator layers.

[0083] Figure 2b Shown is the outer region assembly (i.e., the second fuel unit 3), which is composed of a fuel rod bundle and a moderator layer. In this embodiment, the outer region assembly contains a total of 91 fuel rods; Figure 2aShown is the inner region component (i.e., the first fuel unit 2), and a control rod guide tube is also provided at the center, occupying the space of 7 fuel rods. Therefore, the inner region component contains a total of 84 fuel rods.

[0084] As Figure 2a shown, the first fuel unit 2 includes a first moderator 22 and a first fuel rod bundle 21. The first moderator 22 (i.e., the moderator layer) extends axially along the inner cavity of the housing 1, and a first coolant channel is provided inside it. The central axis of the first coolant channel is on the same extension line as the central axis of the first moderator 22. The first fuel rod bundle 21 extends axially along the first coolant channel and is installed inside the first coolant channel. Among them, the first moderator 22 is in the shape of a regular hexagonal prism. In this embodiment, the first moderator is made of beryllium oxide (BeO) with a thickness of 1 cm. The BeO moderator layer can not only serve as the support structure of the component to ensure the structural stability of the fuel assembly, but also can achieve the effect of adjusting the energy spectrum and improving the Doppler feedback while not having too much influence on the power distribution inside the component.

[0085] As Figure 2b shown, the second fuel unit 3 includes a second moderator 32 and a second fuel rod bundle 31. The second moderator 32 extends axially along the inner cavity, and a second coolant channel is provided inside it. The central axis of the second coolant channel is on the same extension line as the central axis of the second moderator 32. The second fuel rod bundle 31 extends axially along the second coolant channel and is installed inside the second coolant channel. Among them, similar to the first moderator 22, the second moderator 32 is also in the shape of a regular hexagonal prism. And the second moderator 32 has the same size as the first moderator 22, and the second moderator 32 also uses beryllium oxide (BeO) with a thickness of 1 cm. The BeO moderator layer can not only serve as the support structure of the component to ensure the structural stability of the fuel assembly, but also can achieve the effect of adjusting the energy spectrum and improving the Doppler feedback while not having too much influence on the power distribution inside the component.

[0086] Please refer to Figure 3 , in this embodiment, the fuel rod adopts the "pellet - cladding" structure with a high level of technology maturity. The pellet material can be selected from oxides, carbides, nitrides, etc. containing uranium or plutonium. In this embodiment, uranium dioxide (UO2) fuel is used. Preferably, the cladding material is a silicon carbide (SiC) ceramic material with good high - temperature resistance, so that the reactor core has a relatively high temperature limit.

[0087] In other words, the first fuel rod bundle 21 includes a first cladding and first fuel pellets 211; the first fuel pellets 211 are installed inside the first cladding, first end units are respectively installed at both ends of the first cladding, the first fuel pellets 211 are located between the two first end units, and gas cavities 212 are provided between both ends of the first fuel pellets 211 and the two first end units. The first end unit includes a first axial reflector 213 and a first axial shield 214, and the first axial reflector 213 is located inside the first axial shield 214.

[0088] Similarly to the first fuel rod bundle 21, the second fuel rod bundle 31 includes a second cladding and second fuel pellets 311. The second fuel pellets 311 are installed inside the second cladding, second end units are respectively installed at both ends of the second cladding, the second fuel pellets 311 are located between the two second end units, and gas cavities are provided between both ends of the second fuel pellets 311 and the two second end units; the second end unit includes a second axial reflector and a second axial shield, and the second axial reflector is located inside the second axial shield.

[0089] The axial structures inside the claddings of the first fuel rod bundle 21 and the second fuel rod bundle 31 are as Figure 3 shown, from the center to both ends in sequence are the fuel zones (i.e., the first fuel pellets 211 and the second fuel pellets 311), gas cavities, axial reflectors 213, and axial shields 214. In this embodiment, the (first and second) axial reflector material is zirconium carbide (ZrC). Compared with common materials such as graphite and BeO, the moderation ability of ZrC is slightly weaker, which is beneficial for flattening the core power and at the same time avoiding generating a large positive reactivity temperature coefficient. The shield material is boron carbide (B4C).

[0090] It can be seen that the differences between the second fuel assembly and the first fuel assembly are as follows: 1. The fuel enrichment of the second fuel unit 3 is greater than that of the first fuel unit 2; 2. A first control rod guide tube 23 is provided inside the first fuel unit 2.

[0091] This is because the inner region of the core usually has a higher neutron flux density, and thus has a relatively higher fission reaction rate. Therefore, arranging the fuel assembly with a higher fuel enrichment in the outer region can effectively flatten the core power. At the same time, the inner region fuel assembly requires more control rods to maintain the stability of the reaction. By arranging the control rods inside the assembly at the positions around the power peak of the core fuel assembly, the power distribution of the assembly can be flattened and the value of the control rods can be increased. The neutron flux density in the outer region of the core is relatively small, and relatively no control rods need to be provided for adjustment.

[0092] The height of the first fuel pellet 211 is equal to that of the second fuel pellet 311. The ratio of the height of the fuel pellet to the diameter of the circumscribed circle of the fuel assembly is the height-to-diameter ratio of the active region, and the height-to-diameter ratio of the active region is 1.3 - 2.5.

[0093] Specifically, by optimizing the height-to-diameter ratio of the active region, while meeting the requirements of the core's excess reactivity, a larger height-to-diameter ratio is adopted to increase the neutron leakage rate in the active region, thereby enhancing the temperature negative feedback effect. Based on this, to increase the neutron leakage rate in the active region and enhance the temperature negative feedback effect, a larger height-to-diameter ratio of the active region is used, and the height-to-diameter ratio is 1.3 - 1.5. It should be noted that the active region refers to the area containing all the fuel assemblies in the core, excluding structures such as the reflector, gas cavity, and shielding layer. As an example, the height-to-diameter ratio can be 1.3, 1.4, or 1.5. Preferably, the height-to-diameter ratio of the core in this embodiment is 1.3.

[0094] It should be noted that increasing the neutron leakage rate can help enhance the temperature negative feedback, that is, the higher the temperature, the longer the neutron mean free path, the increased leakage, and the decreased reaction rate, thereby achieving a negative reactivity temperature coefficient. Here, it can be found that for enhancing the temperature negative feedback, the neutron leakage of the core design is increased. However, if the neutron leakage rate is too high, it will lead to a lower reactor efficiency, which is not conducive to the efficient operation of the reactor core; to reduce the neutron leakage rate (maintaining the neutron leakage rate at a reasonable level and avoiding too high a neutron leakage rate), the neutron leakage is reduced through the design of the reflector, and the reflector is used as an auxiliary means.

[0095] In some embodiments, the core of this gas-cooled fast reactor adopts a "heavy + light" double-layer reflector structure. Specifically, the housing 1 is provided with an inner cavity, and the fuel assemblies are installed in the inner cavity. The housing 1 includes a first reflector 11 and a second reflector 12. The first reflector 11 surrounds the fuel assemblies, and the second reflector 12 surrounds the first reflector 11. The first reflector 11 is made of a first type of material, and the second reflector 12 is made of a second type of material. The atomic mass of the nuclide of the first type of material is greater than that of the nuclide of the second type of material. The first reflector 11 and the second reflector 12 are used to reflect the escaping neutrons generated by the fuel assemblies back into the inner cavity.

[0096] Among them, the first type of material is zirconium carbide or zirconium silicide, and the second type of material is graphite or beryllium oxide.

[0097] Furthermore, to flatten the core power, reduce neutron leakage, and reduce the usage amount of reflector materials, the present invention uses a "heavy + light" double-layer reflector structure. In this embodiment, the inner reflector material is ZrC, and the outer reflector material is graphite.

[0098] In this embodiment, the double-layer reflection layer can reflect the escaping neutrons back to the core, increasing the possibility of neutron-induced fission reactions. Through the action of the reflection layer, neutrons are well retained in the core, improving the thermal power of the nuclear reactor.

[0099] Furthermore, by setting up a heavy reflection layer to flatten the core power, the inherent safety of the core can also be improved. Specifically: By flattening the core power, the stability of the reactor can be improved. Keeping the core power at a relatively stable level can reduce the fluctuations of the reactor, improve the stability of the reactor, and reduce the operation risks. Moreover, by flattening the core power, it is possible to avoid local overheating caused by local heat release in the core, so as to protect the equipment life. Avoiding frequent power fluctuations can also reduce the damage to the reactor equipment, extend the service life of the equipment, and reduce the maintenance cost. In addition, by flattening the core power, the fuel consumption can also be reduced, that is, a stable core power can reduce the rapid consumption of fuel in a short time, extend the service life of the fuel, and reduce the operation cost.

[0100] Even further, aiming at the problem of the weak reflection ability of the heavy reflection layer, if only increasing the thickness of the reflection layer, the overall size of the core will be increased, which is not conducive to achieving light weight and miniaturization. By setting a light reflection layer outside the heavy reflection layer, while being beneficial to flatten the core power distribution, the material usage of the reflection layer can be reduced, which is conducive to the miniaturization of the core.

[0101] In other words, the core of this gas-cooled fast reactor adopts a double-layer reflection layer structure of "heavy + light" in the radial direction. The inner heavy reflection layer uses zirconium carbide or zirconium silicide, and the outer light reflection layer uses graphite or beryllium oxide. Outside the reflection layer is the neutron shielding layer.

[0102] In the gas-cooled fast reactor core of this embodiment, by providing a first reflector 11 and a second reflector 12, the first reflector 11 and the second reflector 12 are annular. The first reflector 11 surrounds the fuel assembly, and the second reflector 12 surrounds the first reflector 11. Among them, the first reflector 11 is a heavy reflector (for example: a reflector made of zirconium carbide or zirconium silicide), and the second reflector 12 is a light reflector (for example: a reflector made of graphite or beryllium oxide). It should be noted that the light reflector has good neutron reflection and moderation effects, and can effectively reduce the neutron leakage rate of the core. However, since the light reflector moderates or reflects neutrons, the fission rate increases at the position close to the light reflector, and the power also increases accordingly. In other words, simply using the light reflector to reduce the neutron leakage rate is likely to cause the core energy spectrum to become softer, which will further lead to uneven power distribution in the core and affect the safety of the core. Further, the heavy reflector has a weak neutron reflection ability and has little impact on the core energy spectrum, which can avoid energy spectrum softening and is thus beneficial to flattening the core power. Therefore, the gas-cooled fast reactor core of this embodiment is provided with a double-layer reflector. The inner reflector is made of a material with a heavy atomic mass, which can avoid the reduction of neutron energy and flatten the core power; the outer reflector is made of a material with a light atomic mass, which can reduce the neutron leakage of the core.

[0103] In summary, the gas-cooled fast reactor core can maintain the neutron leakage rate at an ideal level, that is, improve the neutron reflectivity; at the same time, it can weaken the influence of the reflector on the core energy spectrum, which is beneficial to flattening the core power and further improving the safety of the core.

[0104] The gas-cooled fast reactor has the hardest energy spectrum among various fast reactor types, so it has better breeding and transmutation performance and can better meet the sustainable development goals of the fourth-generation reactor. However, the harder core energy spectrum leads to stronger neutron leakage, which not only reduces the neutron economy of the core, but also easily causes strong irradiation damage to structural components such as pressure vessels, affecting the service life of the structural components. Therefore, fast reactor cores often use a relatively large-volume reflector structure to reduce the neutron leakage rate of the core. Existing fast reactor schemes usually use "light reflectors" with strong moderation capabilities such as graphite and beryllium oxide. However, these materials have a greater impact on the core energy spectrum, and the proportion of thermal neutrons near the reflector is larger, so the power at this position is higher, resulting in uneven core power distribution and a positive reflector temperature coefficient. Further, the uneven core distribution is likely to cause local heating of the core, affecting the safety of the core. To solve these problems (that is, the problem of uneven core power distribution caused by using a light reflector), some reactor types plan to use a "heavy reflector" represented by zirconium silicide (Zr2Si3) to weaken the influence of the reflector on the core energy spectrum; but the new problem that follows is that the reflection ability of the heavy reflector is weak, and the reflector thickness required for the core is large, increasing the overall size of the core and being not conducive to achieving lightweight and miniaturization.

[0105] In the design of the core of this gas-cooled fast reactor, a double-layer reflector structure of "heavy + light" is adopted. The inner reflector close to the active zone uses heavy materials to flatten the core power, and the outer reflector on the outside uses a layer of light material with strong moderation ability to reduce neutron leakage.

[0106] Furthermore, the housing 1 further includes a neutron shielding layer 13. The neutron shielding layer 13 surrounds the outside of the reflector assembly, and the neutron shielding layer 13 is used to absorb the leaked neutrons.

[0107] In some embodiments, the core of this gas-cooled fast reactor further includes a second control rod guide tube 4 and a second control rod. The second control rod guide tube 4 extends along the axial direction of the inner cavity and is disposed through the first reflector 11. The number of the second control rod guide tubes 4 is multiple, and the multiple second control rod guide tubes 4 are arranged outside the fuel assembly. The second control rod is partially inserted into the second control rod guide tube 4, and the second control rod can move along the axial direction of the second control rod guide tube 4 to adjust the insertion depth of the second control rod.

[0108] Please refer to Figure 1 , which shows the schematic diagram of the core structure of the gas-cooled fast reactor. The core is composed of an inner zone fuel assembly, an outer zone fuel assembly, a radial inner reflector, a radial outer reflector, and a (neutron) shielding layer. Two groups of a total of 19 control rods are arranged in the core, among which 12 regulating rods are located in 12 reflector assemblies close to the active zone, and 7 safety rods are located at the center of the inner zone fuel assembly.

[0109] In other words, the core of this gas-cooled fast reactor further includes a second control rod guide tube 4 and a second control rod. The second control rod guide tube 4 extends along the axial direction of the inner cavity and is disposed through the first reflector 11. The number of the second control rod guide tubes 4 is multiple, and the multiple second control rod guide tubes 4 are arranged outside the fuel assembly. The second control rod is partially inserted into the second control rod guide tube 4, and the second control rod can move along the axial direction of the second control rod guide tube 4 to adjust the insertion depth of the second control rod. The second control rod is a regulating rod, and the regulating rod is mainly used to adjust the power of the nuclear reactor and control the nuclear reaction rate, and compensate for the reactivity loss caused by burnup. By adjusting the insertion depth of the regulating rod to control the neutron flux, thereby adjusting the stability of the nuclear reaction process and the power output.

[0110] The advantages of such a setting include: arranging the control rods outside the fuel assembly in the gaps between adjacent second fuel units 3 to weaken the power peaks that may be brought about by moderation between the core fuel assemblies, thereby further flattening the core power; at the same time, the space between the core fuel assemblies can be fully utilized to improve the utilization rate of the in-core space.

[0111] Such as Figure 1As shown, in this embodiment, the shell 1 further includes a neutron shielding layer 13. The neutron shielding layer 13 surrounds the outer side of the reflective layer assembly, and the neutron shielding layer 13 is used to absorb leaked neutrons.

[0112] See also Figure 4 , Figure 5 and Figure 6 , the optimization effect of the gas-cooled fast reactor core performance in this embodiment will be demonstrated through experimental data.

[0113] As an example, the core thermal power is 10MW, the design life is 900 days, and the core active area volume is 0.5m 3 The core volume including the reflector and shield layers is 3.3m 3 , making it easy to miniaturize the core. The core has a relatively uniform power distribution. When all control rods are removed from the core, the radial rod power peak factor is 1.25, and the axial power is cosine-distributed, with an axial power peak factor of 1.16. The core has two independent control rod groups, both of which can achieve safe cold shutdown of the core under rod jam conditions; the core has strong negative temperature feedback. When the coolant flow rate decreases or even completely loses the coolant under transient conditions, the core can automatically reach subcriticality using negative temperature feedback without the control mechanism acting, and then achieve safe shutdown.

[0114] The gas-cooled fast reactor assembly and core design scheme proposed in the present invention can realize the design of cores with different powers and different operating lives through reasonable fuel rod design, moderation layer arrangement, adjustment of parameters such as active zone size and fuel enrichment; the core size can be further reduced by optimizing the assembly grid diameter ratio; the proliferation performance of the core can be enhanced by reasonable arrangement of the proliferation zone; the neutron economy of the core can be improved and the core power can be further flattened by optimizing the selection of reflector layer materials and sizes; effective control of reactivity can be achieved through reasonable design of control rod structure and materials; this scheme has excellent design flexibility.

[0115] The Monte Carlo program was used to model and analyze the core design of the present embodiment, and the calculation results of the neutronics characteristic parameters were given.

[0116] Figure 4 The effective proliferation factor k of the core is given as effThe curve of the change with burnup days. As can be seen from the figure, the burnup curve of the core is approximately a monotonically decreasing straight line, which conforms to the burnup characteristics of the fast reactor core and verifies the rationality of the moderation layer structure setting. At the beginning of life (zero burnup), the core has the maximum residual reactivity, about 1480 pcm, and the initial residual reactivity is relatively low, which is convenient for using the negative temperature feedback to achieve subcriticality of the core; at a thermal power of 10 MW, the core life is about 925 EFPD, and the residual reactivity at the end of life (900 EFPD) is 140 pcm. The reactivity change range during the entire operating life is 1340 pcm, and the small change range is convenient for operation control.

[0117] Figure 5 The power factor distribution of each component of 1 / 6 core at zero burnup and with all control rods withdrawn is given. As can be seen from the figure, the component power factors are all within 1.1; the maximum value of the power factor of the whole core is 1.25, and the minimum value is 0.79. The radial power distribution is relatively uniform; the axial power is approximately cosine-distributed, but the power slightly increases at both ends near the reflector, and the axial power peak factor is 1.16.

[0118] The reactivity temperature coefficient of the core at the design temperature is about -2.2 pcm / K, which has been significantly improved compared with the existing fast reactor schemes (about -1 pcm / K). The present invention adopts a gaseous coolant and a fully ceramic core, and the temperature limit of the core is the failure temperature of the fuel and cladding materials, and the temperature limit is relatively high. When the core reaches the design temperature limit, it already has a subcriticality of 500 pcm, indicating that the core can use the negative temperature feedback effect to achieve automatic safe shutdown within the temperature limit.

[0119] Please refer to Figure 2a 、 Figure 2b and Figure 1 , and the working principle of the core of this gas-cooled fast reactor is described as follows:

[0120] First of all, the gas-cooled fast reactor does not need to consider the phase change problem of the coolant, and the nuclear design limit is mainly the temperature limit of the materials in the reactor. Ceramic materials have good high-temperature resistance performance. Adopting a fully ceramic core design can increase the temperature limit, thereby enhancing the core safety. Secondly, through the reasonable arrangement of the moderation materials and the optimized screening of the fuel enrichment, the Doppler feedback of the fuel is improved. Finally, through the optimized research on the height-diameter ratio of the active area, the neutron leakage of the core is increased, thereby further enhancing the negative temperature feedback effect. In transient conditions such as loss of cooling, the reactivity loss caused by the core temperature rise can directly trigger subcriticality of the core and then achieve safe shutdown, greatly improving the inherent safety of the core. The structural design of the double-layer reflector is beneficial to flattening the core power distribution while reducing the material consumption of the reflector, which is beneficial to the miniaturization of the core.

[0121] In summary, after the structural optimization of the core of this gas-cooled fast reactor, it has the following advantages:

[0122] 1. By reasonably arranging the moderating materials within the fuel assembly, optimizing the fuel enrichment, optimizing the height-diameter ratio of the core active zone, etc., the negative temperature feedback effect of the core is enhanced, and the initial excess reactivity of the core is appropriately controlled, achieving the safety design goal of using the negative temperature feedback to make the core subcritical.

[0123] 2. Meanwhile, the "light + heavy" double-layer reflector structure is used, effectively reducing the total size of the core and being able to effectively address the challenges of safety and lightweight design faced during the R & D process of the core of the gas-cooled fast reactor.

[0124] Example 2

[0125] The present invention also discloses a gas-cooled fast reactor, including the core of the gas-cooled fast reactor in Example 1.

[0126] The working process of the gas-cooled fast reactor is as follows: Inside the core, uranium-235 or plutonium-239 nuclei undergo fission with neutrons, releasing a large amount of energy (heat), more neutrons, and fission products. The released neutrons will further trigger other nuclear fissions, forming a chain reaction. Helium gas or other gases act as coolants and flow through the core, carrying away the heat generated by nuclear fission. The heated coolant flows through pipes to a heat exchanger, transferring the heat to a secondary cooling system or directly for power generation. The heat exchanger converts the heat of the gas coolant into steam, and the steam drives a turbine generator to generate electricity. If a secondary cooling system is adopted, the heat of the cooling gas can be transferred through water or other fluids to generate steam. By adjusting the position of the control rods, a certain number of neutrons can be absorbed, thereby controlling the power output of the reactor and ensuring that the reactor operates within the safe efficiency range. The gas-cooled fast reactor usually has a negative temperature feedback mechanism. When the core temperature rises, it may cause changes in the coolant fluidity (such as a decrease in density or flow rate), reducing the ability to transfer heat, thereby suppressing the power output of the reactor to a certain extent and ensuring safety.

[0127] It should be noted that the current problem with gas-cooled fast reactors is that the heat transfer performance of the gaseous coolant is poor. Once a loss-of-coolant accident occurs in the core, since it is difficult for the coolant to quickly and timely transfer the core heat out of the core, the temperature of the materials inside the reactor is likely to rise rapidly, and in severe cases, it may lead to fuel melting.

[0128] In addition, the hard energy spectrum of the core of a gas-cooled fast reactor results in strong neutron leakage. Strong neutron leakage not only reduces the neutron economy of the core, but also easily causes strong irradiation damage to structural components such as the pressure vessel, affecting the service life of the structural components. Therefore, a relatively large reflector structure is often set in the fast reactor core to reduce the neutron leakage rate of the core. Existing fast reactor schemes usually use "light reflectors" with strong moderation capabilities such as graphite and beryllium oxide. However, these materials easily lead to uneven power distribution in the core, produce a positive reflector temperature coefficient, and also have a greater impact on the core energy spectrum. To solve these problems, some reactor types plan to use "heavy reflectors" represented by zirconium silicide (Zr2Si3). However, the new problem that follows is that the reflection ability of the heavy reflector is weak, and the reflector thickness required for the core is large, increasing the overall size of the core and being unfavorable for achieving lightweight and miniaturization.

[0129] By adopting the gas-cooled fast reactor core in Embodiment 1, the negative temperature feedback effect of the GFR core can be enhanced to improve the inherent safety of the core. Further, it can also flatten the core power while reducing the neutron leakage rate.

[0130] It should be noted that the gas-cooled fast reactor in this embodiment further includes a first driving device and a second driving device. Both the first driving device and the second driving device can adopt existing hydraulic driving devices. The first driving device and the second driving device are respectively connected to the first control rod and the second control rod in Embodiment 1, and are respectively used to drive the first control rod and the second control rod to move axially, thereby controlling the insertion depth of the first control rod and the second control rod.

[0131] The gas-cooled fast reactor further includes a pressure vessel, and the gas-cooled fast reactor core in Embodiment 1 is arranged in the pressure vessel. As Figure 2a shown, the first fuel rod bundle 21 in the first coolant channel is cylindrical. Therefore, there are gaps between multiple first fuel rod bundles 21, and the gaps extend along the axial direction of the first coolant channel to form a first gas passage. Similarly, as Figure 2b shown, the second fuel rod bundle 31 in the second coolant channel is cylindrical. Therefore, there are gaps between multiple second fuel rod bundles 31, and the gaps extend along the axial direction of the second coolant channel to form a second gas passage.

[0132] The pressure vessel is provided with an air inlet and an air outlet. The air inlet is used to transport inert gas into the accommodating cavity. One end of the first gas passage is communicated with the air inlet, and the other end is communicated with the air outlet. Similarly, one end of the second gas passage is communicated with the air inlet, and the other end is communicated with the air outlet. The inert gas flows through the first gas passage and the second gas passage, and the flow direction of the inert gas in the first gas passage and the second gas passage is the same. Through heat exchange between the inert gas and the fuel assembly, the heat-exchanged inert gas flows out from the air outlet.

[0133] It is understood that the above embodiments are merely exemplary embodiments adopted to illustrate the principles of the present invention, but the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also regarded as the protection scope of the present invention.

Claims

1. A core of a gas-cooled fast reactor, characterized in that, Comprising: A housing (1) and a fuel assembly; The housing (1) is provided with an inner cavity, and the fuel assembly is installed in the inner cavity. The fuel assembly includes a first fuel unit (2) and a second fuel unit (3), and both the first fuel unit (2) and the second fuel unit (3) extend along the axial direction of the housing (1); The number of the first fuel units (2) is at least one, the number of the second fuel units (3) is multiple, and the multiple second fuel units (3) are arranged side by side and disposed around the first fuel unit (2); The first fuel unit (2) includes a first fuel pellet (211), the second fuel unit (3) includes a second fuel pellet (311), and the fuel enrichment of the second fuel pellet (311) is higher than that of the first fuel pellet (211).

2. The gas-cooled fast reactor core according to claim 1, wherein The first fuel unit (2) further includes a first moderator (22). A first coolant channel is provided inside the first moderator (22). The first coolant channel extends along the axial direction of the housing (1). The first coolant channel is used for the circulation of a gas coolant, and the gas coolant flows along the axial direction of the first coolant channel; All the first fuel pellets (211) are accommodated in the first moderator (22). When the gas coolant flows through the first coolant channel, the first fuel pellets (211) are cooled.

3. The gas-cooled fast reactor core according to claim 2, characterized in that, The first fuel pellets (211) are divided into first inner ring pellets (211a) and first outer ring pellets (211b), and both the first inner ring pellets (211a) and the first outer ring pellets (211b) extend along the axial direction of the first coolant channel, The number of both the first inner ring pellets (211a) and the first outer ring pellets (211b) is multiple. The multiple first outer ring pellets (211b) are arranged side by side and disposed around the multiple first inner ring pellets (211a), and the fuel enrichment of the first inner ring pellets (211a) is higher than that of the first outer ring pellets (211b).

4. The gas-cooled fast reactor core according to claim 3, characterized in that, The first fuel unit (2) further includes a first cladding. The first cladding extends along the axial direction of the first coolant channel. The number of the first claddings is multiple, and the number of the first claddings is the same as the number of the first fuel pellets (211), and the first claddings correspond to the first fuel pellets (211) one by one, and each first cladding covers one first fuel pellet (211).

5. The gas-cooled fast reactor core according to claim 4, characterized in that, First end units are respectively installed at both ends of the first cladding, and the first fuel pellet (211) is located between the two first end units; The first end unit includes a first axial reflector and a first axial shield, and the first axial reflector is located inside the first axial shield; Gas cavities are respectively provided between both ends of the first fuel pellet (211) and the two first end units.

6. The gas-cooled fast reactor core according to claim 3, characterized in that, The first fuel unit (2) further includes a first control rod guide tube (23) and a first control rod. The first control rod guide tube (23) is installed in the first coolant channel, and the central axis of the first control rod guide tube (23) is on the same extension line as the central axis of the first coolant channel. The first inner ring fuel pellets (211a) and the first outer ring fuel pellets (211b) are evenly distributed around the first control rod guide tube (23). The first control rod is partially inserted into the first control rod guide tube (23), and the first control rod can move axially along the first control rod guide tube (23) to adjust the insertion depth of the first control rod.

7. The gas-cooled fast reactor core according to claim 3, characterized in that The second fuel unit (3) includes a second moderator (32). A second coolant channel is provided inside the second moderator (32). The second coolant channel extends along the axial direction of the housing (1) and is used for the gas coolant to flow through. The gas coolant flows along the axial direction of the second coolant channel. The second fuel pellets (311) are accommodated in the second moderator (32). When the gas coolant flows through the second coolant channel, the second fuel pellets (311) are cooled.

8. The gas-cooled fast reactor core according to claim 7, characterized in that, The second fuel pellets (311) are divided into a second inner ring fuel pellets (311a) and a second outer ring fuel pellets (311b), and both the second inner ring fuel pellets (311a) and the second outer ring fuel pellets (311b) extend along the axial direction of the second coolant channel. The number of the second inner ring fuel pellets (311a) and the second outer ring fuel pellets (311b) is multiple. The multiple second outer ring fuel pellets (311b) are arranged in parallel and are disposed on the periphery of the multiple second inner ring fuel pellets (311a). The fuel enrichment of the second inner ring fuel pellets (311a) is higher than that of the second outer ring fuel pellets (311b), and the fuel enrichment of the second outer ring fuel pellets (311b) is higher than that of the first inner ring fuel pellets (211a).

9. The gas-cooled fast reactor core according to claim 7, characterized in that, The second fuel unit (3) further includes a second cladding. The second cladding extends along the axial direction of the second coolant channel. The number of the second claddings is multiple, and the number of the second claddings is the same as the number of the second fuel pellets (311). The second claddings correspond to the second fuel pellets (311) one by one, and each second cladding covers one second fuel pellet (311).

10. The gas-cooled fast reactor core according to claim 9, characterized in that, Second end units are respectively installed at both ends of the second cladding, and the second fuel pellets (311) are located between the two second end units. The second end unit includes a second axial reflector and a second axial shield, and the second axial reflector is located inside the second axial shield. Gas cavities are respectively provided between both ends of the second fuel pellets (311) and the two second end units.

11. The gas-cooled fast reactor core according to any one of claims 1 to 10, characterized in that, The height of the first fuel pellets (211) is equal to that of the second fuel pellets (311). The ratio of the height of the fuel pellets to the diameter of the circumscribed circle of the fuel assembly is the height-diameter ratio of the active zone, and the height-diameter ratio of the active zone is 1.3 - 2.

5.

12. The gas-cooled fast reactor core according to claim 1, characterized in that, The said housing (1) comprises a first reflector layer (11) and a second reflector layer (12). The first reflector layer (11) surrounds the fuel assembly, and the second reflector layer (12) surrounds the first reflector layer (11). The first reflector layer (11) is made of a first type of material, and the second reflector layer (12) is made of a second type of material. The atomic mass of the nuclide of the first type of material is greater than that of the nuclide of the second type of material. The first reflector layer (11) and the second reflector layer (12) are used to reflect the escaping neutrons generated by the fuel assembly back into the inner cavity.

13. The gas-cooled fast reactor core according to claim 12, characterized in that, The first type of material is zirconium carbide or zirconium silicide, and the second type of material is graphite or beryllium oxide.

14. The gas-cooled fast reactor core according to claim 12, characterized in that, It further comprises a second control rod guide tube (4) and a second control rod. The second control rod guide tube (4) extends along the axial direction of the inner cavity and penetrates through the first reflector layer (11). The number of the second control rod guide tubes (4) is multiple, and the multiple second control rod guide tubes (4) are arranged on the outer side of the fuel assembly. The second control rod is partially inserted into the second control rod guide tube (4), and the second control rod can move along the axial direction of the second control rod guide tube (4) to adjust the insertion depth of the second control rod.

15. The gas-cooled fast reactor core according to claim 14, characterized in that, The housing (1) further comprises a neutron shielding layer (13). The neutron shielding layer (13) surrounds the outer side of the reflector assembly, and the neutron shielding layer (13) is used to absorb the leaked neutrons.

16. A gas-cooled fast reactor system, characterized in that, It includes the gas-cooled fast reactor core according to any one of claims 1 to 15.

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