Synergistically flow-controlled high-reliability fuel assembly

Abstract

A synergistically flow-controlled high-reliability fuel assembly, comprising a bottom nozzle (5), a framework and a top nozzle (6) which are sequentially arranged in the flow direction of a coolant, and fuel rods (1), guide thimbles (3), and an instrumentation tube (4) which are inserted into the framework. A plurality of axially curved spatially-penetrating elongated flow channels (505) having a width less than the spacing between adjacent fuel rods (1) are provided on a bottom nozzle adapter plate (501). Outlets of peripheral flow channels (5053) of the spatially-penetrating elongated flow channels (505) located at the periphery of the bottom nozzle adapter plate (501) are higher than outlets of intermediate flow channels (5054) located in the middle of the bottom nozzle adapter plate (501), so as to reduce an inter-assembly outlet crossflow. The fuel assembly can increase the degree of mixing of cold and hot coolants, thereby reducing the local hot spot temperature, and does not generate significant crossflow.

Description

A high-reliability fuel assembly with coordinated flow control Technical Field

[0001] This invention belongs to the field of nuclear fuel technology, specifically relating to a high-reliability fuel assembly with coordinated flow control. Background Technology

[0002] Fuel assemblies are the core components of nuclear power plants, operating in harsh aqueous chemical environments characterized by high temperature, high pressure, high flow rate, and strong radiation. Their performance directly affects the economy, safety, and reliability of the nuclear power plant. Among these, key safety indicators include the nuclear fuel assembly failure rate and thermal safety margin, which are closely related to the flow field within the nuclear fuel assembly. High safety is a continuous goal pursued in innovative nuclear fuel design.

[0003] Regarding nuclear fuel failure, according to the IAEA report "Review of Fuel Failures in Water Cooled Reactors," grid-to-rod fretting has long been the leading cause of fuel rod failure, primarily occurring in the first grid layer region. This is mainly due to significant lateral flow at the fuel rod inlet, leading to substantial vibration at the lower end of the fuel rods.

[0004] Regarding the thermal safety margin of nuclear fuel assemblies, the critical heat flux density is its core technical indicator. It means that when the local power density of a fuel assembly exceeds the critical heat flux density, the local temperature of the fuel assembly will rise sharply, leading to local burn-up failure and the breaching of the first safety barrier by radioactive materials. According to existing research, the critical heat flux density is closely related to the flow field of the fuel rod attachments and is strongly influenced by factors such as the design of the positioning grid structure and the deformation of the rod bundle due to irradiation. It is also closely related to the monitoring and feedback of the temperature and flow fields within the reactor core.

[0005] In addition, based on nuclear power plant operating experience, if the lateral flow at the nuclear fuel assembly outlet is large, it can easily cause wear on the control rod cladding and fuel assembly guide tubes in the control rod assembly.

[0006] The aforementioned phenomena are all related to the flow field design of the fuel assembly. The upstream flow field has a significant impact on the downstream flow field, and the extent of this impact is closely related to the hydraulic diameter. Therefore, it is urgent to propose a new design for the fuel assembly structure, starting from the overall flow field control, to improve the thermal safety performance of the fuel assembly. Summary of the Invention

[0007] This invention aims to provide a highly reliable fuel assembly with synergistic flow control. It reduces the lateral flow at the inlet and outlet of the fuel rod bundle, enhances coolant mixing in the near-wall region of the fuel rods, and increases the critical heat flux density. The entire fuel assembly, with the lower tube seat, upper tube seat, grid, and fuel rods as the core, has been synergistically and innovatively designed to achieve overall coordination of the flow field of the fuel assembly.

[0008] The technical solution of the present invention is as follows:

[0009] A high-reliability fuel assembly with coordinated flow control comprises a lower tube seat, a frame, an upper tube seat arranged sequentially along the coolant flow direction, and a fuel rod, a guide tube, and an instrument tube inserted into the frame.

[0010] The lower tube seat is a low-outlet lateral flow lower tube seat; on the lower tube seat connecting plate, there are several slender, through-flow channels with a width smaller than the fuel rod spacing and axial curvature; the outlet of the slender, through-flow channels in the periphery of the lower tube seat connecting plate is higher than the outlet of the middle channel area in the middle of the lower tube seat connecting plate, so as to reduce the lateral flow at the outlet between components.

[0011] An inlet chamfer is machined at the inlet end of the elongated, through-flow channel in the space to reduce flow resistance.

[0012] The aforementioned upper pipe seat is a low-outlet transverse flow upper pipe seat, which consists of an upper pipe seat connecting plate, an upper pipe seat surrounding plate fixed to the edge of the upper pipe seat connecting plate, and an upper pipe seat frame plate fixed to the upper pipe seat surrounding plate. The transverse flow between upper pipe seats is reduced by setting the upper pipe seat surrounding plate, and the flow resistance is reduced by opening frame plate through holes on the upper pipe seat frame plate.

[0013] The upper tube seat connecting plate is machined with a group of upper tube seat water flow holes. The group of upper tube seat water flow holes is distributed in a central rotational symmetry and axis symmetry on the upper tube seat connecting plate. The central part is densely arranged with curved through holes and the surrounding part is arranged with straight holes in parallel. The top and bottom of the holes are chamfered. Upper tube seat connecting holes are opened at the corresponding positions of the guide tube and the instrument tube on the upper tube seat connecting plate. The upper tube seat water flow holes near the upper tube seat connecting holes are distributed in a central rotational symmetry and axis symmetry with the upper tube seat connecting holes.

[0014] The upper tube seat is equipped with a variable stiffness, low-relaxation leaf spring clamping system to reduce the clamping load during fuel assembly operation.

[0015] The frame includes a structural mixing grid, with guide vanes on the inner and outer strips, and smoothly bent towards the inside of the fuel rod grid. The inner strip guide vanes project to cover the corner of the grid, and the projection edge is an arc concentric with the fuel rod, thereby guiding the coolant at the corner of the grid to flow around the fuel rod to achieve strong flow guidance.

[0016] The framework also includes a pure structural grid and a pure mixing grid; wherein, the pure structural grid is arranged at both ends of the turbulent flow field of the fuel rods, only positioning the fuel rods, without guiding the flow, and without setting guide vanes; the pure mixing grid is arranged between the structural mixing grids with insufficient guiding function, playing a role in enhancing the guiding flow, without positioning the fuel rods.

[0017] The guide vanes of the structural mixing grid and the pure mixing grid bend inward toward the fuel rod grid with a radius of 2 to 10 mm; multiple openings are provided on the inner and outer strips of the structural mixing grid, namely inner strip flow holes and outer strip flow holes, to enhance the lateral flow in different grids.

[0018] At least one structural mixing grid is provided in the high-power area; if two or more structural mixing grids are provided, the distance between two adjacent structural mixing grids is 200-700mm, and one or two pure mixing grids can be added in between.

[0019] The fuel rod is a low-irradiation deformation fuel rod, which consists of a zirconium alloy cladding with creep resistance and low-irradiation growth characteristics, fuel pellets, and end plugs, and is filled with inert gas. The pressure of the inert gas is designed in a balanced iterative manner based on the external pressure of the fuel rod, the gas storage volume, the creep characteristics of the cladding, and the fuel rod burnup, and the maximum value is selected under the condition that the internal pressure of the fuel rod in hot state does not exceed the external pressure.

[0020] The active section of the fuel rod is composed of: uranium dioxide pellets, plutonium dioxide pellets, or uranium-plutonium mixed oxide pellets, or pellets containing gadolinium, boron, erbium, or dysprosium, or combinations thereof; or other compounds of uranium, plutonium, thorium, and other fissile nuclides, or compounds or mixtures containing gadolinium, boron, erbium, or dysprosium, and other neutron poison nuclides, or combinations thereof;

[0021] The isotopes of fissile nuclides account for 1–10% of the total content; the compounds or mixtures of neutron poison nuclides account for 0–12% of the total content.

[0022] The average grain size of the core blocks in the active section of the fuel rod is greater than 30 μm.

[0023] The fuel rod also includes a fuel positioning structure, which is a helical spring, a radial spring, or other elastic structure that compensates for the thermal expansion difference of the active section.

[0024] The cladding material is a zirconium-tin-niobium alloy, with the following weight percentages: Sn: 0.50%-1.20%, Nd: 0.60%-1.25%, Fe: 0.05%-0.55%, and the balance being Zr and impurities.

[0025] The zirconium-tin-niobium alloy is tightly covered with a corrosion-resistant and high-temperature-resistant chromium coating.

[0026] The guide tube is a zirconium alloy tube with a constant outer diameter and an inner neck; or a "tube-in-tube" assembly consisting of an outer tube with a constant wall thickness and an inner tube with a constant wall thickness fitted on its lower inner side, with a drop bar overflow hole opened on the outer tube near the upper end of the inner tube.

[0027] The guide tube is made of a creep-resistant, low-growth zirconium-tin-niobium alloy with an outer tube wall thickness ≥0.55mm. When a batch of guide tubes are assembled on the same fuel assembly, the diameter and wall thickness of the batch of guide tubes will vary due to machining tolerances. The guide tubes with larger diameters and wall thicknesses will be arranged around the fuel assembly.

[0028] The aforementioned spatially connected slender flow channel is symmetrical on both sides and has opposite bending directions, with an outlet DC section of ≥3mm in length at the outlet end.

[0029] The instrument tube is a temperature measuring instrument tube, which is arranged on a 12-foot fuel assembly operating at 5 m / s. There are 22 small holes with a diameter of 3 mm on the instrument tube at a height of 3 to 4 m from the lower surface of the active section of the fuel rod. Temperature measuring instruments or other core measuring instruments are inserted from the top of the instrument tube.

[0030] The significant advantages of this invention are:

[0031] (1) This invention achieves a high thermal safety margin for fuel assemblies. It employs a coordinated control of the entire flow field of the fuel assembly through inlet pipe seat flow equalization, intermediate grid mixing and flow control, and outlet pipe seat pressure reduction and flow guidance. This ensures uniform coolant distribution and guides the "low-temperature" coolant at the grid corners to flow around the "high-temperature" fuel rods, significantly improving the mixing degree of hot and cold coolants and thus reducing local hot spot temperatures. Compared to domestically used fuel assemblies, the coolant mixing effect is improved by 25%, and experimental data obtained from the fuel assembly critical heat flux density test proves that its average heat flux density is improved by approximately 5%. The grid outer strip guide vanes are arranged at varying heights, and their bent ends are embedded in the fuel rod gaps without protruding from the grid outer surface. Additionally, the inner strip ends are supported by welded guide vanes, improving the mixing uniformity between fuel assemblies and ensuring no snagging occurs during the lateral 500N extrusion test. This eliminates the snagging interference phenomenon of the unloading grid, resulting in structural stability.

[0032] (2) The lower tube seat structure of the present invention will not generate obvious transverse flow. Compared with the prior art, the average transverse flow velocity is reduced by 5%. At the same time, the comprehensive filtration efficiency exceeds 87% and the foreign object capture efficiency reaches 32%. Compared with the domestic fuel assembly, it can reduce the probability of foreign object abrasion, which accounts for the highest proportion of fuel rod breakage, by more than 40%, and has a good effect on preventing foreign object capture.

[0033] (3) This invention enables thermal safety monitoring and feedback of fuel assemblies. By setting up a temperature-measuring instrument tube, a temperature sensor can be placed inside to continuously measure the temperature of the coolant entering from the radially distributed water flow holes (the coolant has been uniformly mixed with the hot and cold coolant by the strong flow guide grid) in real time online. This allows the actual thermal safety margin of the fuel assembly to be known, and feedback can be provided to guide the optimization of the operation strategy.

[0034] (4) This invention achieves the maintenance of safety margin under heavy loads on fuel assemblies. High-strength zirconium-tin-niobium alloy is used as the cladding material and guide tube material. Combined with structural reinforcement measures such as "tube-in-tube" guide tubes, welded guide vanes supported at the ends of strips in the grid, and high-rigidity tube seats, the mechanical tests of the fuel assembly have proven that its pressure-bearing and bending stiffness (under pressure throughout the entire lifespan of the reactor) is 25% higher than that of domestically used fuel assemblies, meeting the seismic resistance requirements of 0.3g ultimate ground motion, and ensuring that the safety margin does not decrease after strong loads such as earthquakes.

[0035] (5) This invention achieves the maintenance of safety margin under high fuel consumption of fuel assemblies. By using creep-resistant and low-growth zirconium-tin-niobium alloy as cladding and guide tube materials, combined with load-reducing and strengthening measures in the structure such as large-grain fuel, "tube-in-tube" guide tube, variable stiffness leaf spring clamping system, low-pressure reduction frame and tube seat, the invention achieves small irradiation growth, small creep deformation, small bending deformation, strong containment capacity, safe and reliable geometric structure, and no reduction in safety margin under high fuel consumption of fuel assemblies.

[0036] (6) This invention achieves high safety under fuel assembly accident conditions. Using a creep-resistant zirconium-tin-niobium alloy as the cladding material improves the structural stability of the fuel rods under accident conditions, thereby maintaining the stability of the coolant flow channels, ensuring rapid heat removal, and enhancing the ability to cope with accident conditions. A tightly applied corrosion-resistant and high-temperature-resistant coating on the surface of the creep-resistant zirconium-tin-niobium alloy reduces the cladding corrosion rate, lowering the temperature of the cladding and fuel pellets compared to before the coating, increasing the safety margin under accident conditions, and delaying the zirconium-water reaction, thus exhibiting good accident resistance.

[0037] (7) This invention achieves high economic efficiency for fuel assemblies and nuclear power plants. The fuel rods designed according to this invention can achieve a burnup of 62 GWd / tU, a refueling cycle of 18–24 months, and a fuel rod breakage rate designed to be less than 10%. -6 It features high burnup, long refueling cycle, and low breakage rate, which can reduce the number of fuel assemblies and refueling frequency, improve the availability of nuclear power plants, and bring excellent economic benefits to nuclear power plants.

[0038] (8) This invention achieves high-quality mass production of fuel assemblies. By employing unique fuel assembly processes and mechanical mass production methods, such as the guide tube "expansion welding combination method", the spatial slender flow channel forming method, the three-layer skeleton welding method, the low-friction pulling rod method, and the guide tube reverse pulling method, the high fuel consumption, high safety, and high reliability performance of the fuel assemblies of this invention are ensured, and the product quality stability is improved. Attached Figure Description

[0039] Figure 1 is a schematic diagram of the fuel assembly structure of the present invention;

[0040] Figure 2 is a schematic diagram of the fuel rod structure of the present invention;

[0041] Figure 3 is a schematic diagram of the fuel assembly skeleton structure of the present invention;

[0042] Figure 4 is a schematic diagram of the overall structure of the mixing grid of the present invention;

[0043] Figure 5 is a schematic diagram of the spring structure of the present invention;

[0044] Figure 6 is a schematic diagram of the inner strip flow holes of the mixing grid structure of the present invention;

[0045] Figure 7 is a schematic projection of the inner strip guide vane of the mixing grid structure of the present invention;

[0046] Figure 8 is a schematic diagram of the outer strip structure of the mixing grid of the present invention;

[0047] Figure 9 is a schematic diagram of the inner strip structure of the mixing grid of the present invention;

[0048] Figure 10 is a partial schematic diagram of the inner and outer strip assembly of the mixing grid structure of the present invention;

[0049] Figure 11 is a schematic diagram of the instrument tube structure of the present invention;

[0050] Figure 12 is a schematic diagram of the lower tube seat structure of the present invention;

[0051] Figure 13 is a schematic diagram of the slender, through-flow channel in the lower tube seat of the present invention.

[0052] Figure 14 is a schematic diagram of the guide tube structure of the present invention;

[0053] Figure 15 is a schematic diagram of the overall structure of the upper tube seat of the present invention;

[0054] Figure 16 is a schematic diagram of the upper tube seat connecting plate and frame plate structure of the present invention.

[0055] In the diagram, 1. Fuel rod; 2. Structural mixing grid; 3. Guide tube; 4. Instrument tube; 5. Lower tube seat; 6. Upper tube seat; 7. Pure structural grid; 8. Pure mixing grid; 101. Sheath; 102. End plug; 103. Fuel pellet; 104. Fuel positioning structure; 201. Inner strip; 202. Outer strip; 2011. Inner strip guide vane; 2012. Inner strip spring; 2013. Inner strip rigid convexity; 2014. Support structure; 2015. Projection surface of inner strip guide vane; 2016. Inner strip flow hole; 2021. Outer strip low guide vane; 2022. Outer strip high guide vane; 2023. Outer strip rigid convexity; 2024. Outer strip flow hole; 301: Outer tube; 302: Inner tube; 401: Instrument tube coolant exchange hole; 402: Instrument cooling hole; 403: Instrument tube throttling hole; 501: Lower tube seat connecting plate; 502: Lower tube seat support leg; 503: Lower tube seat surrounding plate; 504: Lower tube seat connecting hole; 505: Spatial through-flow slender channel; 5011: Rib; 5012: Blade; 5051: Outlet direct flow section; 5052: Inlet chamfer; 5053: Surrounding flow channel; 5054: Middle flow channel; 601: Upper tube seat connecting plate; 602: Upper tube seat surrounding plate; 603: Upper tube seat frame plate; 604: Clamping system; 6011: Upper tube seat connecting hole; 6012: Upper tube seat water flow hole group; 6031: Frame plate through hole. Detailed Implementation

[0056] In the following, the terms “comprising” or “may include” as used in various embodiments of the invention indicate the presence of an inventive function, operation, or element, and do not limit the addition of one or more functions, operations, or elements. Furthermore, as used in various embodiments of the invention, the terms “comprising,” “having,” and their cognates are intended only to indicate a specific feature, number, step, operation, element, component, or combination of the foregoing, and should not be construed as primarily excluding the presence of one or more other features, numbers, steps, operations, elements, components, or combinations of the foregoing, or adding one or more combinations of the foregoing.

[0057] In various embodiments of the invention, the expression "or" or "at least one of A and / or B" includes any combination or all combinations of the words listed simultaneously. For example, the expression "A or B" or "at least one of A and / or B" may include A, may include B, or may include both A and B.

[0058] The expressions used in the various embodiments of the present invention (such as "first," "second," etc.) may modify various constituent elements in the various embodiments, but do not limit the corresponding constituent elements. For example, the above expressions do not limit the order and / or importance of the elements. The above expressions are only used for the purpose of distinguishing one element from other elements. For example, a first user device and a second user device refer to different user devices, although both are user devices. For example, a first element may be referred to as a second element without departing from the scope of the various embodiments of the present invention, and similarly, a second element may also be referred to as a first element.

[0059] It should be noted that if a description is made of "connecting" one component to another, then the first component can be directly connected to the second component, and a third component can be "connected" between the first and second components. Conversely, when a component is "directly connected" to another component, it can be understood that there is no third component between the first and second components.

[0060] The terminology used in the various embodiments of the invention is for the purpose of describing particular embodiments only and is not intended to limit the various embodiments of the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which the various embodiments of the invention pertain. The terms (such as those defined in commonly used dictionaries) are to be interpreted as having the same meaning as in the context of the relevant technical field and are not to be interpreted as having an idealized or overly formal meaning, unless clearly defined in the various embodiments of the invention.

[0061] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0062] Example 1

[0063] As shown in Figure 1, this embodiment discloses a high-reliability fuel assembly with coordinated flow control, which is obtained by inserting fuel rods 1 into a frame.

[0064] As shown in Figure 2, the fuel rod 1 is formed by sealing the fuel active section containing fissile nuclides, the fuel positioning structure 104, and the thermally conductive gas at appropriate pressure within a zirconium alloy cladding 101 with creep resistance and low irradiation growth characteristics and an end plug 102. This ensures that the structure of the fuel rod 1 is stable during irradiation and that the safety margin does not decrease significantly.

[0065] As shown in Figure 3, the frame is composed of guide tube 3 and instrument tube 4 connected from bottom to top to lower tube seat 5, several structural mixing grids 2 and upper tube seat 6, and the distance between two adjacent structural mixing grids 2 is 200-700mm.

[0066] As shown in Figures 4 and 5, the structural mixing grid 2 is composed of inner strips 201 with springs 2012 and / or rigid protrusions 2013 interlocking with each other, and then an outer strip 202 with springs and / or outer strips rigid protrusions 2023 surrounding it; the grids formed by the inner strips 201 and the outer strips 202 are used to place fuel rods 1, guide tubes 3 or instrument tubes 4. A guide vane that bends smoothly into the grid is provided at the coolant outlet end of the strip fuel rod grid to guide the coolant to mix and flow, reduce the hot spot temperature and thus improve safety.

[0067] The guide tube 3 is a zirconium alloy tube with a constant outer diameter and an inner neck, or a "tube-in-tube" assembly consisting of a straight tube with a constant wall thickness and another straight tube with a constant wall thickness fitted inside the lower part of the inner side, so as to realize the rapid drop of the control rod and the hydraulic buffer at the end of the stroke to reduce the impact load and ensure the safety of the drop rod.

[0068] As shown in Figure 11, the instrument tube 4 is a temperature-measuring instrument tube, which is connected sequentially from bottom to top to the lower tube seat 5, several structural mixing grids 2, and the upper tube seat 6. In addition to the conventional instrument cooling holes 402 and instrument tube throttling holes 403, instrument tube coolant exchange holes 401 are opened at the hot section of the fuel assembly corresponding to the high heat generation power, so that coolant at the hot section can enter the instrument tube 4 to realize the temperature measurement of the fuel assembly and track the operating safety margin of the fuel assembly. The instrument tube 4 is arranged on a 12-foot fuel assembly operating at 5 m / s. There are 22 small holes with a diameter of 3 mm on the instrument tube 4 at a height of 3 to 4 m from the lower surface of the active section of the fuel rod 1. Temperature measuring instruments or other core measuring instruments are inserted from the upper part of the instrument tube 4.

[0069] As shown in Figure 12, the lower tube seat 5 includes a lower tube seat connecting plate 501, a lower tube seat support leg 502, and a lower tube seat surrounding plate 503. A flow guide surface is provided on the inner side of the lower tube seat support leg 502. The lower tube seat connecting plate 501 has lower tube seat connecting holes 504 corresponding to the positions of the guide tube 3 and / or the instrument tube 4. Several multi-layer transverse discontinuous structures with a width smaller than the fuel rod spacing are provided between the lower tube seat connecting holes 504. These structures are connected across layers to form an axially curved, through-flow, slender flow channel 505, which has the function of uniformly distributing the flow rate and can block and capture foreign objects in the coolant, avoiding erosion damage and reduced safety margin caused by foreign objects, and improving the safety and reliability of the fuel assembly.

[0070] Example 2

[0071] This embodiment further optimizes the spatially penetrating elongated flow channel 505 of the lower tube seat 5. The spatially penetrating elongated flow channel 505 is formed by multiple sets of parallel ribs 5011 and multiple sets of parallel blades 5012 orthogonally interlocked. On the blades 5012, at the midpoint between the two rows of fuel rods 1, a unidirectional protrusion structure is provided. This protrusion structure causes the flow channel to bend axially into a spatial curved surface structure, which serves to block and capture foreign objects in the flowing coolant, preventing erosion damage or reduced safety margin caused by foreign objects, and improving the safety and reliability of the fuel assembly.

[0072] Preferably, the blade 5012 has a thickness of 0.1-2 mm, a height of 5-20 mm, a convex structure radius of 2-10 mm, and a spacing of 1-5 mm.

[0073] As shown in Figure 13, the spatially penetrating elongated flow channel 505 is symmetrical on both sides with opposite bending directions. The outlet end is provided with an outlet direct flow section 5051, the length of which is ≥3mm, to reduce lateral flow at the lower pipe seat outlet. An inlet chamfer 5052 is machined at the inlet end of the spatially penetrating elongated flow channel 505 to reduce flow resistance.

[0074] The outlet of the slender, through-flow channel 505 located around the periphery of the lower tube connector plate 501 is higher than the outlet of the central channel 5054 located in the middle of the lower tube connector plate 501, with a height difference ≥1mm, to reduce the lateral flow at the outlet between the lower tube connectors of the assembly. According to CFD numerical analysis results, compared with the existing fuel assembly lower tube connector structure, the lateral flow inside the outlet of the lower tube connector 5 in this embodiment is reduced by 4.8%, the lateral flow between lower tube connectors is reduced by 2.3%, and the drag coefficient is reduced by 2.6%.

[0075] Example 3

[0076] This embodiment further optimizes the structural mixing grid 2 in the fuel assembly. The structural mixing grid 2 is composed of 2×(N-1) inner strips 201 with springs and / or rigid protrusions interlocking to form an (N-1)×(N-1) arrangement, and then surrounded by outer strips 202 with springs and / or rigid protrusions. The inner strips 201 and outer strips 202 form N×N grids at intervals, of which N×NM fuel rod grids hold fuel rods, and M non-fuel rod grids hold guide tubes 3 or instrument tubes 4; guide vanes can be set at the coolant outlet end at the fuel rod grid position of the strip. The springs can be directly stamped on the strip, or they can be made from a separate material and then clipped onto the strip, and the rigid protrusions are stamped on the strip. As shown in Figure 6, the rigid protrusions 2013 of the inner strips have flow holes 2016 on both sides. The shape of the holes is elongated or circular, which facilitates the lateral flow of coolant and enhances mixing. Each fuel assembly contains 2 to 20 grids arranged axially along the fuel rod 1. The guide vanes of the mixing grid 2 are smoothly bent inwards into the fuel rod grid cells with a bending radius of 5 mm. The inner strip guide vane 2011, after bending, has a projected surface 2015 covering the corner of the grid cell, with its projected edge forming an arc concentric with the fuel rod 1, as shown at point A in Figure 7. Through multiple bending processes, the "low-temperature" coolant at the grid cell corners is guided to flow around the "high-temperature" fuel rod 1, maximizing the reduction of hot spot temperatures and thus improving safety. The inner strip guide vane 2011 is formed by two bending processes, with a 3 mm distance between the horizontal baselines of the two bending processes.

[0077] The above design improves the coolant mixing effect by an average of about 25% (some numerical analysis data are shown in Table 1 below), and the test data obtained from the fuel assembly critical heat flux density test proves that the average heat flux density is improved by about 5%.

[0078] Table 1 Comparison of test data on coolant mixing effect

[0079] The outer strip guide vanes are configured with a high-low mix to guide the coolant to mix and flow, reducing hot spot temperatures and improving safety: when the crossflow direction generated by the guide vanes is the same as that of the inner strip 201, a high guide vane H1 with a size of 6-10 mm is used; when the crossflow direction generated by the guide vanes is opposite to that of the inner strip 201, a low guide vane H2 with a size of 2-6 mm is used. For example, when the crossflow direction generated by the outer strip guide vanes is the same as that of the inner strip 201, an 8 mm high outer strip guide vane 2022 is used; when the crossflow direction is opposite, a 3 mm low outer strip guide vane 2021 is used. Meanwhile, the outer strip protrusion 2013 includes outer strip flow holes 2024 on both sides. The holes are elongated or circular in shape, as shown in Figure 8. The outer strip guide vanes are arranged at varying heights, and their bent ends are embedded in the fuel rod gaps without protruding from the outer surface of the mixing grid 2. This improves the mixing uniformity between fuel assemblies and ensures that no snagging occurs during the lateral 500N extrusion test, eliminating grid snagging interference during unloading and ensuring structural stability. Fluid dynamics analysis was conducted on the high-low arrangement design of the outer strip guide vanes, and the mixing uniformity between grids was improved by approximately 10%.

[0080] Example 4

[0081] This embodiment further optimizes the structural mixing grid 2 in the fuel assembly. The spring is directly stamped on the strip and arranged along the axial direction of the fuel rod, contacting the fuel rod through a streamlined micro-concave arc surface. As shown in Figure 9, the inner strip spring 2012 is designed as a bridge-type spring, and the inner strip rigid protrusion 2013 is designed as a buckle-type rigid protrusion, etc., to guide the flow. A chamfer and other flow-guiding structures are added to the inflow end of the inner strip 201, and combined with matching measures such as increasing the coordination spacing, the coolant is better guided to gather at the corner of the grid element with the projection of the guide vane at the upper end, while reducing the degree of turbulence when the fluid reaches the guide vane.

[0082] As shown in Figure 10, the inner strip 201 has a support structure 2014 at both ends of the inner strip 201 that is in contact with the outer strip guide vane, and the outer strip guide vane rests on the support structure 2014. The top of the support structure 2014 can be fixed to the inner side of the outer strip guide vane it supports by welding or other means to form an integral structure, so as to avoid structural deformation during the operation of the fuel assembly and reduce the safety margin.

[0083] Example 5

[0084] This embodiment further optimizes the lower tube seat 5 in the fuel assembly. The through-flow elongated channel of the lower tube seat connecting plate is formed by two layers of parallel ribs with the same center distance as the fuel rod center distance of 12.6 mm, which hold a group of orthogonally arranged parallel blades with small spacing. A unidirectional protrusion structure is set on the blade between the two rows of fuel rods. This protrusion causes the elongated channel to bend axially into a spatial curved surface structure, which serves to block and capture foreign objects in the flowing coolant. The blade height is 18 mm, and the radius of the protruding sphere is 8 mm. They are arranged neatly and symmetrically at 2 mm intervals, and a spatial curved flow channel for filtering foreign objects is formed between the two parallel protrusions on two adjacent blades. Foreign object filtration test data shows that the overall filtration efficiency of this lower tube seat exceeds 87%, which is 9 percentage points higher than the 78% of the currently used fuel assemblies in China.

[0085] Example 6

[0086] This embodiment further optimizes the fuel rod 1 in the fuel assembly. The creep-resistant zirconium alloy cladding 101 is a zirconium-tin-niobium zirconium alloy with the following weight percentages: Sn: 0.50%-1.20%, Nd: 0.60%-1.25%, Fe: 0.05%-0.55%, and the balance being Zr and impurities. It has good strength, which makes the structure of the fuel rod 1 stable during irradiation and prevents a significant reduction in the safety margin.

[0087] The active section is composed of uranium dioxide pellets, or uranium dioxide pellets containing gadolinium / erbium, or combinations thereof; it can also be composed of uranium dioxide pellets, or uranium dioxide pellets coated with boron, or combinations thereof; it can also be composed of plutonium dioxide pellets, or plutonium dioxide pellets containing gadolinium / erbium, or combinations thereof; it can also be composed of uranium-plutonium mixed oxide pellets, or uranium-plutonium mixed oxide pellets containing gadolinium / erbium, or combinations thereof; it can also be composed of compounds of other uranium, plutonium, thorium and other fissile nuclides, or compounds / mixtures containing gadolinium / erbium / dysprosium and other neutron poison nuclides, or combinations thereof. The isotope content of the fissile nuclides is 1% to 10%. The pellets are prepared as single crystals or large-grain fuel pellets with an average grain size greater than 30 μm to improve the irradiation stability of the pellets and reduce the release of fission gases. The combination / mixture of neutron poison nuclides, ranging from 0% to 12% by mass, can provide diverse means to control the heat flux density of fuel rods to support safety margin control.

[0088] The fuel positioning structure 104 is a helical spring, a radial spring, or other elastic structure that can compensate for the thermal expansion difference of the active section, so as to achieve reliable positioning of the fuel active section during operation.

[0089] The heat-conducting gas is high-purity helium with good thermal conductivity that does not react with the fuel pellets 103 and the cladding 101. The gas pressure is designed through a balanced iterative process based on coolant pressure, gas storage volume, cladding creep performance, and fuel consumption. The maximum value is selected while ensuring that the internal pressure does not exceed the external pressure at the end of 95% of the fuel rod's lifespan. This reduces the creep effect caused by pressure difference, thereby ensuring the stability of the fuel rod structure during irradiation and preventing a significant reduction in safety margin.

[0090] In a single fuel assembly, the fuel rods with high H in the active section are arranged in a square of N×NM. The diameter of the fuel rods is D, and the center distance is L, where 100mm≤H≤6000mm, 3≤N≤25, 1≤M≤50 and M≤N×N / 4, 3mm≤D≤15mm, and 4mm≤L≤20mm.

[0091] Example 7

[0092] This embodiment further optimizes fuel rod 1 in the fuel assembly. The creep-resistant zirconium-tin-niobium zirconium alloy cladding of the fuel rod, by weight percentage, has the following composition: Sn: 0.68-0.75, Nb: 0.75-1.00, Fe+Cr: 0.10-0.50, Fe / (Nb+Fe): 0.20-0.35, Cu or Bi: 0.01-0.10, Si or S: 0.002-0.01, O: 0.06-0.15, C: less than 0.008, N: less than 0.006; the balance is Zr and impurities. It has good mechanical properties, creep resistance, and low-irradiation growth characteristics, ensuring that the structure, shape, and size of the fuel rod remain stable during irradiation, and the safety margin does not decrease significantly.

[0093] The excellent properties of the fuel rods have been confirmed by commercial in-reactor irradiation of over 60,000 fuel rod cladding tubes, with a burnup depth of 57 GWd / tU achieved. Specifically, this includes: structural integrity after four 18-month long-cycle irradiation sessions within a megawatt-class nuclear power unit; and a thermal creep rate of less than 2.4 × 10⁻⁶ at 400°C and 130 MPa without irradiation. -5 / h, the creep rate after irradiation at 400℃ and 130MPa is less than 2.2×10 -6 / h; creep rate at 850℃ and 40MPa is less than 5.6×10 -3 / s; after oxidation at 1204℃ for more than 17% and rapid cooling, it still maintains plasticity; the irradiation growth is less than 0.55%, which makes the axial dimension of the fuel rod stable during irradiation and the safety margin does not decrease significantly.

[0094] The fuel rods 1 are arranged in a 17×17-25 square, with a diameter of 9.5 mm. The height of the active section is 12 feet, 14 feet, 4.2 m or 2.15 m. The center distance of the fuel rods 1 is 12.6 mm. It has good power and coolant distribution uniformity and mutual matching, and supports good safety.

[0095] Example 8

[0096] This embodiment further optimizes the fuel rod 1 in the fuel assembly. The surface of the creep-resistant zirconium-tin-niobium zirconium alloy cladding of the fuel rod is tightly covered with a corrosion-resistant and high-temperature-resistant chromium coating to improve the corrosion resistance and high-temperature oxidation resistance of the zirconium alloy cladding and enhance safety.

[0097] Example 9

[0098] This embodiment categorizes and optimizes the arrangement of grids in the fuel assembly according to the different requirements of different axial positions within the fuel assembly. Based on the axial position and temperature distribution within the fuel assembly, the grids can be divided into pure structural grids (7), structural mixing grids (2), and pure mixing grids (8). Pure structural grids (7) are located at the two ends of the fuel assembly where power is lower and the flow field is turbulent; they only position the fuel rods and have no guiding effect, nor are they equipped with guide vanes. Structural mixing grids (2) are located in the middle of the fuel assembly, serving both to position the fuel rods and guide the flow. Pure mixing grids (8) are arranged between structural mixing grids (2) with insufficient guiding function, typically in the high-temperature zone of the active section, to enhance guiding flow, but do not position the fuel rods. Pure mixing grids (8) can be arranged or omitted between two structural mixing grids (2) as needed. The spacing of the pure mixing grids (8) matches the coolant flow rate, representing the distance at which their guiding effect disappears or becomes insufficient. Through the reasonable arrangement and spacing of the grids, the coolant is guided to flow mixed along the entire length of the fuel assembly, reducing hot spot temperatures and thus improving safety.

[0099] Preferably, the grid is specifically arranged in the axial direction of the 12-foot fuel assembly as 2 pure structural grids 7, 6 structural mixing grids 2 and 3 pure mixing grids 8. The distance between the pure structural grids 7 and the adjacent structural mixing grids 2 is 400-650 mm, preferably 522 mm. One pure mixing grid 8 is arranged between the 3rd to 6th structural mixing grids 2 from bottom to top.

[0100] Preferably, the 14-foot fuel assembly is specifically arranged axially as 2 pure structural grids 7, 8 structural mixing grids 2 and 4 pure mixing grids 8, with the spacing between the pure structural grids 7 and the structural mixing grids 2 being 400mm to 650mm, preferably 516mm, and one pure mixing grid 8 is arranged between each of the 4th to 8th structural mixing grids 2 from bottom to top.

[0101] Example 10

[0102] As shown in Figure 14, this embodiment further optimizes the guide tube 3 in the fuel assembly. The "pipe-in-pipe" guide tube is prepared by an "expansion-welding combination method" in which the inner tube 302 is first expanded and connected to the lower inner side of the outer tube 301, and then the two layers of tubes are integrally welded to the end plug, so as to reduce the difficulty of the manufacturing process. The outer tube 301 has a drop rod overflow hole at the upper end of the inner tube 302. The expansion joint spacing and the spacing between expansion joint and welding are such that the inner tube 302 and the outer tube 301 theoretically do not contact each other when the guide tube is subjected to the maximum axial load, so as to achieve appropriate structural strength and reduce the number of expansion joints. The outer diameter of the outer tube 301 is as large as possible and is optimally coordinated with the cooling of the surrounding fuel rods, while the inner diameter of the inner tube 302 is as small as possible and is optimally coordinated with the drop rod of the control rod. A cold-fitting process can be selected to minimize the initial gap requirement and thus increase the inner tube wall thickness and improve the structural strength.

[0103] Preferably, the guide tube material is the aforementioned creep-resistant, low-growth zirconium-tin-niobium alloy, and the outer tube 301 has a wall thickness ≥0.55mm. When a batch of guide tubes are assembled on the same fuel assembly, the diameter and wall thickness of the guide tubes may vary due to machining tolerances. The guide tubes with relatively larger diameters and wall thicknesses are arranged on the periphery of the fuel assembly to improve rigidity and prevent deformation of the fuel assembly during irradiation operation, thus reducing the safety margin.

[0104] Example 11

[0105] As shown in Figures 15 and 16, this embodiment further optimizes the upper pipe seat 6 in the fuel assembly. The upper pipe seat 6 consists of an upper pipe seat connecting plate 601, an upper pipe seat surrounding plate 602 fixed to the edge of the upper pipe seat connecting plate 601, and an upper pipe seat frame plate 603 fixed to the upper pipe seat surrounding plate 602. The upper pipe seat connecting plate 601 has an upper pipe seat connecting hole 6011 for installing the guide pipe 3 and the instrument pipe 4, as well as an upper pipe seat water flow hole group 6012. The upper pipe seat water flow hole group 6012 is centrally symmetrical and axisymmetrically distributed, with densely arranged curved through holes in the center and parallel straight holes around it. The water flow holes are chamfered at the top and bottom, exhibiting comprehensive characteristics of high flow rate, low pressure drop, high rigidity, low stress, and high symmetrical flow rate, which can avoid the reduction of safety margin due to uneven or deformed outlet flow field. The lateral flow between the upper tube seats is reduced by setting the upper tube seat enclosure plate 602, and the flow resistance is reduced by opening the frame plate through hole 6031 on the upper tube seat frame plate 603.

[0106] Example 12

[0107] This embodiment further optimizes the upper tube seat 6 in the fuel assembly. The upper tube seat 6 is equipped with a variable stiffness, low relaxation, high stability leaf spring clamping system 604 for in-core clamping and positioning of the fuel assembly, so as to reduce the clamping load during the operation of the fuel assembly, improve the structural stability of the fuel assembly under high burnup, and avoid deformation leading to a reduction in safety margin.

[0108] Example 13

[0109] This embodiment further optimizes the fuel assembly. The fuel assembly consists of 264 fuel rods 1, 24 guide tubes 3, 1 temperature measuring instrument tube 4, 11 layers of grid, 1 upper tube seat 6, and 1 lower tube seat 5 and their connecting structures, arranged in a 17×17 square. The fuel rods 1 have a diameter of 9.5 mm, an active section height of 12 feet, and a center-to-center distance of 12.6 mm. Through the arrangement of the fuel rods 1 and the grid structure, the coolant is guided to flow mixed throughout the entire length of the fuel assembly, reducing the hot spot temperature and thus improving safety.

[0110] The aforementioned fuel rod 1 has a burnup of up to 62 GWd / tU. Tests and in-core applications show that this fuel assembly, used in existing reactor cores such as Hualong One, has a thermal safety margin greater than 15%, meets the seismic resistance requirements for a 0.3g ultimate ground motion, a refueling cycle of 18–24 months, irradiation growth of less than 0.3%, a bending deformation to assembly length ratio of less than 0.1%, and a fuel rod design-controlled failure rate of less than 10%. -6 .

[0111] Example 14

[0112] This embodiment discloses a fuel assembly method: one end of the guide tube 3 and the instrument tube 4 pass through several structural mixing grids 2 in sequence, and then the other end is pulled back through the lowest structural mixing grid 2 and then installed on the lower tube seat 5, so that the guide tube 3, the instrument tube 4 and the structural mixing grid 2 are fixedly connected; the lower tube seat 5 is removed, and the fuel rods 1, whose surfaces have been coated with water-soluble lubricant or pyrolytic grease to reduce the coefficient of friction, are pulled into the grid cells of the structural mixing grid 2 in batches; the upper tube seat 6 is installed and the lower tube seat 5 is reinstalled to form a fuel assembly.

[0113] The guide tube 3 is a "tube-in-tube" guide tube, which is prepared by the "expansion welding combination method" of first expanding the inner tube to the lower part of the inner side of the outer tube, and then welding the two layers of tubes and the end plug as a whole.

[0114] The guide vanes of the structural mixing grid 2 are smoothly bent by die stamping to improve manufacturing efficiency. The inner strip guide vanes of the structural mixing grid 2 are formed by Q bending, wherein the first bending is along the first horizontal baseline of the upper edge of the inner strip, or along the first oblique baseline at an angle of 10-20° to the first horizontal baseline; the distance between the horizontal baseline of the Qth bending and the Q-1th bending is 2-5mm; the distance between the oblique baseline of the Qth bending and the Q-1th bending is 2-5mm; the angle of the Qth bending is smaller than the angle of the Q-1th bending, and the continuous Q bending guides the coolant to mix and flow, maximizing the reduction of hot spot temperature and thus improving safety.

[0115] The lower tube seat 5 is manufactured using additive manufacturing methods such as brazing or printing to ensure manufacturing efficiency and economy.

[0116] The zirconium-tin-niobium alloy cladding 101 of the fuel rod 1 is processed using a low-temperature process and delivered in a fully recrystallized state to ensure good mechanical and corrosion resistance.

[0117] Example 15

[0118] This embodiment further optimizes the fuel assembly. The fuel assembly can be used in conjunction with a control rod assembly with M-1 control rods to better flatten the power level within the fuel assembly, achieving a higher safety margin while also achieving higher fuel consumption.

[0119] Example 16

[0120] The reactor core composed of fuel assemblies described in any of the above embodiments can be arranged in a 15×15 pattern to form 177 cores and 157 cores, in a 16×16 pattern to form 193 cores, in a 17×17 pattern to form 241 cores, in a 14×14 pattern to form 121 cores, or in a 9×9 pattern to form 57 cores.

[0121] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-reliability fuel assembly with coordinated flow control, characterized in that: A lower tube seat (5), a frame, an upper tube seat (6), and fuel rods (1), guide tubes (3), and instrument tubes (4) inserted into the frame are arranged sequentially along the coolant flow direction. The lower tube seat (5) is a low-outlet lateral flow lower tube seat. On the lower tube seat connecting plate (501), there are several spatially penetrating slender flow channels (505) with a width smaller than the fuel rod spacing and axial curvature. The outlet of the spatially penetrating slender flow channel (505) located in the periphery of the lower tube seat connecting plate (501) is higher than the outlet of the middle flow channel (5054) located in the middle of the lower tube seat connecting plate (501) to reduce the lateral flow at the outlet between components. The frame includes a structural mixing grid (2), with guide vanes of the structural mixing grid (2) disposed on the inner and outer strips and smoothly bent toward the inner side of the fuel rod grid cell; the inner strip guide vane projection (2015) covers the corner of the grid cell, and the projection edge is an arc concentric with the fuel rod (1), thereby guiding the coolant at the corner of the grid cell to flow around the fuel rod (1) to achieve strong flow guidance; The skeleton also includes a pure structural lattice (7) and a pure mixed lattice (8); Among them, the pure structural grid (7) is arranged at both ends of the turbulent flow field of the fuel rod (1), which only positions the fuel rod (1) and has no guiding effect, and no guide vanes are set; the pure mixing grid (8) is arranged between the structural mixing grids (2) with insufficient guiding function, which plays a role in enhancing the guiding effect, and does not position the fuel rod (1). The guide vanes of the structural mixing grid (2) and the pure mixing grid (8) bend towards the inside of the fuel rod grid with a radius of 2 to 10 mm; multiple openings are provided on the inner strip (201) and outer strip (202) of the structural mixing grid (2), namely the inner strip flow hole (2016) and the outer strip flow hole (2024), to enhance the lateral flow in different grids; At least one structural mixing grid (2) is provided in the high power area; if two or more structural mixing grids (2) are provided, the distance between two adjacent structural mixing grids (2) is 200 to 700 mm, and one or two pure mixing grids (8) can be added between them.

2. The high-reliability fuel assembly with coordinated flow control as described in claim 1, characterized in that: An inlet chamfer (5052) is machined at the inlet end of the through-flow channel (505) to reduce flow resistance.

3. The high-reliability fuel assembly with coordinated flow control as described in claim 1, characterized in that: The upper tube seat (6) is a low-outlet transverse flow upper tube seat, which consists of an upper tube seat connecting plate (601), an upper tube seat surrounding plate (602) fixed to the edge of the upper tube seat connecting plate (601), and an upper tube seat frame plate (603) fixed to the upper tube seat surrounding plate (602). The transverse flow between upper tube seats is reduced by setting the upper tube seat surrounding plate (602), and the flow resistance is reduced by opening a frame plate through hole (6031) on the upper tube seat frame plate (603).

4. The high-reliability fuel assembly with coordinated flow control as described in claim 3, characterized in that: A group of upper tube seat water flow holes (6012) is machined on the upper tube seat connecting plate (601). The group of upper tube seat water flow holes (6012) is distributed in a central rotational symmetry and axis symmetry on the upper tube seat connecting plate (601). The central part is densely arranged with curved through holes and the surrounding part is arranged with straight holes in parallel. The top and bottom of the holes are chamfered. Upper tube seat connecting holes (6011) are opened on the upper tube seat connecting plate (601) at the corresponding positions of the guide tube (3) and the instrument tube (4). The upper tube seat water flow holes near the upper tube seat connecting holes (6011) are distributed in a central rotational symmetry and axis symmetry with respect to the upper tube seat connecting holes (6011).

5. A high-reliability fuel assembly with coordinated flow control as described in claim 4, characterized in that: The upper tube seat (6) is equipped with a variable stiffness low relaxation leaf spring clamping system (604) to reduce the clamping load during fuel assembly operation.

6. The high-reliability fuel assembly with coordinated flow control as described in claim 1, characterized in that: The fuel rod (1) is a low-irradiation deformation fuel rod, which consists of a zirconium alloy cladding (101) with creep resistance and low irradiation growth characteristics, a fuel pellet (103), and an end plug (102), and is filled with inert gas. The pressure of the inert gas is designed in a balanced iterative manner based on the external pressure of the fuel rod, the gas storage volume, the creep characteristics of the cladding, and the fuel rod burnup, and the maximum value is selected under the condition that the internal pressure of the fuel rod in hot state does not exceed the external pressure.

7. A high-reliability fuel assembly with coordinated flow control as described in claim 6, characterized in that: The active section of the fuel rod (1) is composed of: uranium dioxide pellets, plutonium dioxide pellets, or uranium-plutonium mixed oxide pellets, or pellets containing gadolinium, boron, erbium, or dysprosium, or combinations thereof; or other compounds of uranium, plutonium, thorium, and other fissile nuclides, or compounds or mixtures containing gadolinium, boron, erbium, or dysprosium, and other neutron poison nuclides, or combinations thereof; The isotopes of fissile nuclides account for 1–10% of the total content; the compounds or mixtures of neutron poison nuclides account for 0–12% of the total content.

8. The high-reliability fuel assembly with coordinated flow control as described in claim 7, characterized in that: The average grain size of the core block in the active section of the fuel rod (1) is greater than 30 μm.

9. A high-reliability fuel assembly with coordinated flow control as described in claim 6, characterized in that: The fuel rod (1) further includes a fuel positioning structure (104), which is a helical spring, a radial spring, or other elastic structure that compensates for the thermal expansion difference of the active section.

10. A high-reliability fuel assembly with coordinated flow control as described in claim 9, characterized in that: The cladding (101) is made of zirconium-tin-niobium zirconium alloy, with the following weight percentages: Sn: 0.50%-1.20%, Nd: 0.60%-1.25%, Fe: 0.05%-0.55%, and the balance being Zr and impurities.

11. A high-reliability fuel assembly with coordinated flow control as described in claim 10, characterized in that: The zirconium-tin-niobium alloy is tightly covered with a corrosion-resistant and high-temperature-resistant chromium coating.

12. The high-reliability fuel assembly with coordinated flow control as described in claim 1, characterized in that: The guide tube (3) is a zirconium alloy tube with an equal outer diameter and an inner neck; or a "tube-in-tube" assembly consisting of an outer tube with equal wall thickness and an inner tube with equal wall thickness fitted on its lower inner side, with a drop bar overflow hole opened on the outer tube near the upper end of the inner tube.

13. A high-reliability fuel assembly with coordinated flow control as described in claim 12, characterized in that: The guide tube (3) is made of a creep-resistant, low-growth zirconium-tin-niobium alloy with an outer tube wall thickness ≥0.55mm. When a batch of guide tubes (3) are assembled on the same fuel assembly, the diameter and wall thickness of the batch of guide tubes will differ due to the processing tolerance. The guide tubes (3) with larger diameter and wall thickness will be arranged around the fuel assembly.

14. The high-reliability fuel assembly with coordinated flow control as described in claim 1, characterized in that: The aforementioned spatially penetrating slender flow channel (505) is symmetrical on both sides and has opposite bending directions, and the outlet end is provided with an outlet DC section (5051) with a length ≥3mm.

15. A high-reliability fuel assembly with coordinated flow control according to claim 1, characterized in that: The instrument tube (4) is a temperature measuring instrument tube, which is arranged on a 12-foot fuel assembly running at 5 m / s. There are 22 small holes with a diameter of 3 mm on the instrument tube (4) at a height of 3 to 4 m from the lower surface of the active section of the fuel rod (1). Temperature measuring instruments or other core measuring instruments are inserted from the top of the instrument tube (4).

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