Fuel assembly and structured grid division system and method using fuel assembly

Abstract

This invention provides a fuel assembly, comprising: a fuel rod bundle; the fuel rod bundle has a cross-shaped cross-section, a torsional structure around its axial direction, and anisotropic fluid flow within the fuel rod bundle; multiple fuel rod bundles are provided. This invention also provides a structured mesh generation system using the fuel assembly, comprising the above-mentioned fuel assembly, including a rod bundle region, a first flow channel region, and a second flow channel region; the rod bundle region is formed within the fuel rod bundle; the fuel rod bundle and the first flow channel region are rotated and combined along the fuel rod bundle axis to form a cylinder; the second flow channel region has multiple hollowed-out cylindrical regions. This invention also provides a structured mesh generation method using the fuel assembly, performing structured mesh generation using the above-mentioned fuel assembly. The fuel rod bundle in this invention has a periodic rotating structure, and the cross-section of the fuel rod bundle changes periodically. The geometric characteristics of the fuel rod bundle increase the difficulty of generating a structured mesh, avoiding subsequent calculation non-convergence.

Description

Technical Field

[0001] The present invention relates to the field of reactor thermal hydraulic technology, and in particular to a fuel assembly and a structured grid division system and method using the fuel assembly. Background Art

[0002] Nuclear fuel is the most important component of nuclear power plants. Thermal hydraulic analysis of nuclear fuel is of great significance to the safety and economy of reactors. Spiral cross fuel rods are a new type of advanced nuclear fuel that will be adopted by the new generation of reactors. Spiral cross fuel rods have the characteristics of self-support and self-mixing, and the fuel rod bundle does not need a spacer grid to fix it. At the same time, when the coolant passes through the spiral fins on the outside of the fuel rod, a swirl flow will be formed to promote the mixing between the sub-channels. Compared with the stirring wings of the spacer grid, the swirl formed by the spiral cross fuel rod fins will not attenuate. At present, there are relatively few research reports on the swirl flow characteristics, heat transfer enhancement mechanism, fuel swelling characteristics, etc. of spiral fuel. The above key issues are of great significance to the development and application of spiral cross fuel assemblies.

[0003] At present, CFD simulation analysis is an important method to obtain flow field information in rod bundle assemblies. Before performing CFD numerical simulation, it is necessary to first establish a geometric model and divide the grid according to the research objectives. The core fuel rod bundle is in a high temperature and high pressure environment, and the turbulent flow in the rod bundle channel has strong anisotropic characteristics. Since the longitudinal length of the rod bundle channel is generally 4-5 meters, the CFD analysis of the rod bundle channel is often computationally intensive. The number of structured grids is smaller and the grid quality is higher. According to the calculation examples related to the rod bundle, when using structured grids for CFD calculations, the calculation results are often easier to converge. In summary, establishing a reasonable structured body grid for the rod bundle channel is of great significance for subsequent CFD simulation analysis.

[0004] After searching, patent document CN104952492B discloses a gadolinium-loaded fuel rod, a fuel assembly with gadolinium-loaded fuel rods, and a pressurized water reactor core. The fuel core of the gadolinium-loaded fuel rod is composed of recycled uranium oxide and Gd2O3, the mass fraction of Gd2O3 is 3% to 5%, and the mass fraction of recycled uranium oxide is 95% to 97%. The fuel assembly with gadolinium-loaded fuel rods includes fuel rods, guide tubes, and instrument tubes. The fuel rods, guide tubes, and instrument tubes are arranged in a square structure, and the instrument tubes are arranged in the center of the fuel assembly; the fuel rods include gadolinium-free fuel rods and the aforementioned gadolinium-loaded fuel rods. The pressurized water reactor core uses the aforementioned fuel assembly with gadolinium-loaded fuel rods to assist in controlling the residual reactivity of the core. This prior art is a fuel rod with a guide tube, which will generate many grids during the meshing process, which will further cause the subsequent numerical calculation process to be huge in calculation amount or non-convergence.

[0005] Therefore, it is necessary to propose a fuel rod with geometric features that can increase the difficulty of generating structured body mesh division, so that when the structured mesh is used for subsequent CFD calculations, the calculation results are often easier to converge. Summary of the invention

[0006] In view of the defects in the prior art, the purpose of the present invention is to provide a fuel assembly and a structured grid division system and method for using the fuel assembly. During the grid division process, the present invention reduces the number of grids as much as possible while ensuring the grid quality, so as to avoid the situation where the subsequent numerical calculation process has a large amount of calculation or does not converge. Compared with unstructured grids, the number of grids is reduced by nearly 70%.

[0007] A fuel assembly provided according to the present invention comprises: a fuel rod bundle; the cross section of the fuel rod bundle is cross-shaped, the fuel rod bundle is a twisted structure around the axial direction, and the fluid flow in the fuel rod bundle is anisotropic; a plurality of fuel rod bundles are provided.

[0008] Preferably, the fuel rod bundle comprises arc-shaped fins, elbows and extension sections, the arc-shaped fins are symmetrically arranged to form protrusions of the fuel rods, and both ends of the arc-shaped fins are smoothly connected to the elbows through the extension sections.

[0009] Preferably, the fuel rod bundle further includes a rod center and a rod spacing, and the distance between the rod centers of the fuel rod bundles is the rod spacing.

[0010] Preferably, rod gaps are also included, and there are rod gaps between the fuel rod bundles.

[0011] A structured grid division system for applying a fuel assembly provided according to the present invention comprises the above-mentioned fuel assembly, including a rod bundle area, a first flow channel area and a second flow channel area; a rod bundle area is formed inside the fuel rod bundle; the fuel rod bundle and the first flow channel area are rotated and combined along the axis of the fuel rod bundle to form a cylinder; and there are multiple hollow cylindrical areas in the second flow channel area.

[0012] Preferably, the first flow channel region is a portion between the outside of the rod cluster region and a circumscribed circle of the rod cluster region.

[0013] Preferably, the second flow channel region is a portion outside the circumscribed circle of the rod bundle region.

[0014] According to a structured grid partitioning method using a fuel assembly provided by the present invention, structured grid partitioning is performed using the above-mentioned fuel assembly, comprising the following steps:

[0015] Step S1: establishing a two-dimensional geometric model according to the cross section of the fuel assembly;

[0016] Step S2: dividing the two-dimensional cross section into a rod bundle region, a first flow channel region and a second flow channel region;

[0017] Step S3: A two-dimensional topological structure is established for the rod bundle region (7), the first flow channel region (8) and the second flow channel region (9), and the distance from the first layer of grids in the near-wall region to the wall and the grid aspect ratio are adjusted according to the accuracy requirements of the numerical calculation.

[0018] The distance Δs from the first layer of mesh near the wall to the wall:

[0019]

[0020] Where μ is the fluid viscosity; ρ is the fluid density; τ is the wall shear force; y + is the dimensionless wall distance. When using the standard wall function, it is required that 30 < y + <100; When using enhanced wall function, y is required + ≈1.

[0021] Grid Aspect Ratio:

[0022] The ratio of two orthogonal sides of the hexahedral mesh controls the aspect ratio of the hexahedral mesh within 40.

[0023] Step S4: generating a two-dimensional structured network according to the two-dimensional topological structure;

[0024] Step S5: Rotate and stretch the two-dimensional structured grids in the rod bundle region and the first flow channel region along the axis of the fuel rod bundle, and vertically stretch the two-dimensional structured grid in the second flow channel region along the axis of the fuel rod bundle assembly;

[0025] Step S6: After stretching the rod bundle region, the first flow channel region and the second flow channel region, three groups of body grids are generated to obtain a complete fuel assembly body grid.

[0026] Preferably, in step S3, two-dimensional topological structures are respectively established for the rod bundle region, the first flow channel region and the second flow channel region, and meshing is performed using an O-type partitioning method. During the process of setting the mesh nodes, the boundary nodes of the rod bundle region and the first flow channel region are kept in one-to-one correspondence.

[0027] Preferably, the helical pitch L is the length of the cross-shaped fuel rod cross section twisted every 360 degrees. ° The corresponding length, the helical pitch L, is determined by the parameter setting of the grid rotation stretching in step S5. The calculation formula of the pitch L of the fuel rod bundle is as follows:

[0028]

[0029] Among them, n is the rotation angle of each layer of grid, and Δh is the height of each layer of grid.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention adopts a relatively narrow and long structure of the fuel rod bundle, so that in the process of mesh division, the number of meshes is reduced as much as possible under the premise of ensuring the quality of the meshes, otherwise the subsequent numerical calculation process will cause a huge amount of calculation or non-convergence.

[0032] 2. The present invention combines the point contact between fuel rod bundles, the control nodes cannot be directly embedded in the contact points, and the grid cannot be generated. By setting rod gaps between fuel rod bundles, point contacts are eliminated, and grids can be embedded in point contact areas, so that the fluid flow in the fuel rod bundle is anisotropic, and the calculation structure is easier to converge and more accurate.

[0033] 3. The present invention generates a cross-sectional mesh first and then generates a volume mesh by twisting and stretching, without using mesh node correspondence at the axial position, so that the numerical calculation can be better converged.

[0034] 4. The present invention divides the fuel assembly into hexahedral grids, so that the grid quality is higher and the number of grids is smaller, thereby reducing the amount of calculation for the numerical simulation of the rod bundle assembly.

[0035] 5. The hexahedral grid division of the fuel assembly of the present invention can be evenly embedded in the rod gaps, restoring the geometric features of the spiral cross-shaped fuel assembly.

[0036] 6. The present invention can flexibly adjust the number and distribution of nodes in the grid according to actual computing conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0038] Figure 1 A flowchart of the structured grid division method for applying a fuel assembly in the present invention;

[0039] Figure 2 is a cross-sectional view of a spiral cross-shaped fuel bundle in the present invention;

[0040] Figure 3 It is a schematic diagram of the area division of the spiral cross-shaped fuel bundle in the present invention;

[0041] Figure 4 It is a schematic diagram of the mesh division of the spiral cross fuel bundle area in the present invention;

[0042] Figure 5 A schematic diagram of the mesh division of the spiral cross-shaped fuel flow channel area in the present invention;

[0043] Figure 6 It is a schematic diagram of the mesh division of the spiral cross fuel bundle region in the present invention;

[0044] Figure 7 A schematic diagram of the mesh division of the spiral cross-shaped fuel flow channel region in the present invention; DETAILED DESCRIPTION

[0045] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0046] like Figure 2 As shown, the present invention provides a fuel assembly, comprising: a fuel rod bundle; the cross section of the fuel rod bundle is cross-shaped, the fuel rod bundle is a twisted structure around the axial direction, and the fluid flow in the fuel rod bundle is anisotropic; a plurality of fuel rod bundles are provided.

[0047] The fuel rod bundle includes an arcuate fin 1, an elbow 2, an extension section 3, a rod center 4, a rod gap 5 and a rod spacing 6. The arcuate fin 1 is symmetrically arranged to form a protrusion of the fuel rod, and both ends of the arcuate fin 1 are smoothly connected to the elbow 2 through the extension section 3. The distance between the rod centers 4 between the fuel rod bundles is the rod spacing 6. There is a rod gap 5 between the fuel rod bundles.

[0048] like Figures 3 to 7 As shown, the present invention also provides a structured grid division system using a fuel assembly, comprising the above-mentioned fuel assembly, including a rod bundle area 7, a first flow channel area 8 and a second flow channel area 9; the rod bundle area 7 is formed in the fuel rod bundle; the fuel rod bundle and the first flow channel area 8 are rotated and combined along the axis of the fuel rod bundle to form a cylindrical shape; and there are multiple hollow cylindrical areas in the second flow channel area 9. The first flow channel area 8 is the part between the outside of the rod bundle area 7 and the circumscribed circle of the rod bundle area 7. The second flow channel area 9 is the part outside the circumscribed circle of the rod bundle area 7.

[0049] like Figures 1 to 7 As shown, the present invention further provides a structured grid division method using a fuel assembly, and the structured grid division is performed using the above-mentioned fuel assembly, comprising the following steps:

[0050] Step S1: Establish a two-dimensional geometric model according to the cross section of the fuel assembly.

[0051] Step S2: Divide the two-dimensional cross section into a rod bundle region 7, a first flow channel region 8 and a second flow channel region 9.

[0052] Step S3: Establish two-dimensional topological structures for the rod bundle region 7, the first flow channel region 8 and the second flow channel region 9 respectively, and adjust the distance from the first layer of grids in the near-wall region to the wall and the grid aspect ratio according to the accuracy requirements of the numerical calculation.

[0053] The distance Δs from the first layer of mesh near the wall to the wall:

[0054]

[0055] Where μ is the fluid viscosity; ρ is the fluid density; τ is the wall shear force; y + is the dimensionless wall distance. When using the standard wall function, it is required that 30 < y + <100; When using enhanced wall function, y is required + ≈1.

[0056] Mesh aspect ratio: the ratio of two orthogonal sides of the hexahedral mesh. The aspect ratio is required to be within 40.

[0057] Two-dimensional topological structures are established for the rod bundle region 7, the first flow channel region 8 and the second flow channel region 9 respectively, and the mesh is divided by O-type partitioning. In the process of setting the mesh nodes, the boundary nodes of the rod bundle region 7 and the first flow channel region 8 are kept in one-to-one correspondence.

[0058] Step S4: generating a two-dimensional structured network according to the two-dimensional topological structure;

[0059] Step S5: Rotate and stretch the two-dimensional structured grids of the rod bundle area 7 and the first flow channel area 8 along the axis of the fuel rod bundle, and vertically stretch the two-dimensional structured grid of the second flow channel area 9 along the axis of the fuel rod bundle assembly. The helical pitch L is the length corresponding to each 360° twist of the cross section of the spiral cross-shaped fuel rod. The helical pitch L is determined by the parameter setting of the grid rotation and stretching in step S5. The calculation formula of the pitch L of the fuel rod bundle is as follows:

[0060]

[0061] Among them, n is the rotation angle of each layer of grid, and Δh is the height of each layer of grid.

[0062] Step S6: After stretching the rod bundle region 7, the first flow channel region 8 and the second flow channel region 9, three groups of body grids are generated to obtain a complete fuel assembly body grid.

[0063] The present invention Preferred Example , for further explanation.

[0064] Based on the above basic embodiments, the fuel rod bundle in the present invention adopts a spiral cross-shaped fuel rod bundle. The special geometric structure of the spiral cross-shaped fuel rod bundle is that the fuel rods rotate around the rod axis, causing the cross-section of the rod bundle assembly to change periodically. For such a complex geometric structure, unstructured grid division is often used. The space filling rate of the unstructured grid is low, resulting in the number of grids being much larger than the structured grid, which will increase the amount of calculation for numerical calculations. At the same time, due to the small rod gap of the spiral cross-shaped fuel assembly, the use of a structured grid cannot accurately restore the geometric structure, causing the numerical calculation results to deviate from the actual value. The structured grid division method suitable for the spiral cross-shaped fuel rod bundle provided by the present invention can better restore the geometric features of the spiral cross-shaped fuel rod bundle, and compared with the unstructured grid, the number of grids is reduced by nearly 70%.

[0065] The present invention takes a 2×2 spiral cross fuel rod bundle assembly as an example. The spiral cross fuel rod includes a spiral cross fuel rod arc fin 1, an elbow 2 and an extension 3. Before dividing the grid, a two-dimensional geometric model needs to be established. The parameters that need to be marked include: the radius of the arc fin, the radius of the elbow, the length of the extension, the rod gap, the rod spacing, the number of fuel rod bundles and their arrangement. The two-dimensional geometric model is divided into three areas: the rod bundle area 7, the first flow channel area 8 and the second flow channel area 9. A two-dimensional topological structure is established for these three parts respectively and an O-type center division is adopted. After setting the nodes of the topological structure, a two-dimensional grid is generated. The two-dimensional grid is stretched along the axis of the rod bundle channel to generate a corresponding body grid. Rotational stretching is performed for the rod bundle area 7 and the first flow channel area 8; vertical stretching is performed for the second flow channel area 9. The rotation angle of each layer of the grid is adjusted to change the pitch L of the spiral cross fuel rod. The body grids of the three parts are combined and the interface is set to obtain the final body grid.

[0066] In the description of the present application, it should be understood that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0067] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A structured grid generation system for a fuel assembly, characterized in that: The fuel assembly comprises: a fuel rod bundle; The cross section of the fuel rod bundle is cross-shaped, the fuel rod bundle is in a twisted structure around the axial direction, and the fluid flow in the fuel rod bundle is anisotropic; a plurality of fuel rod bundles are provided; The system comprises a rod bundle region (7), a first flow channel region (8) and a second flow channel region (9); A fuel bundle region (7) is formed in the fuel bundle; The fuel rod bundle and the first flow channel region (8) are rotated and combined along the axis of the fuel rod bundle to form a cylindrical shape; The second flow channel region (9) has a plurality of hollow cylindrical regions; The first flow channel region (8) is a portion between the outside of the rod bundle region (7) and the circumscribed circle of the rod bundle region (7); The second flow channel region (9) is a portion outside the circumscribed circle of the rod bundle region (7).

2. The structured grid division system for fuel assembly application according to claim 1, characterized in that: The fuel rod bundle comprises an arc-shaped fin (1), an elbow (2) and an extension section (3); the arc-shaped fin (1) is symmetrically arranged to form a protrusion of the fuel rod; and two ends of the arc-shaped fin (1) are smoothly connected to the elbow (2) through the extension section (3).

3. The structured grid division system for applying fuel assembly according to claim 1, characterized in that: The fuel rod bundle further comprises a rod center (4) and a rod spacing (6), and the distance between the rod centers (4) of the fuel rod bundle is the rod spacing (6).

4. The structured grid division system for applying fuel assembly according to claim 1, characterized in that: It also comprises rod gaps (5), where there are rod gaps (5) between the fuel rod bundles.

5. A structured grid generation method for a fuel assembly, characterized in that: The fuel assembly comprises: a fuel rod bundle; The cross section of the fuel rod bundle is cross-shaped, the fuel rod bundle is in a twisted structure around the axial direction, and the fluid flow in the fuel rod bundle is anisotropic; a plurality of fuel rod bundles are provided; The structured grid division of the applied fuel assembly includes a rod bundle region (7), a first flow channel region (8) and a second flow channel region (9); A fuel bundle region (7) is formed in the fuel bundle; The first flow channel region (8) is a portion between the outside of the rod bundle region (7) and the circumscribed circle of the rod bundle region (7); The second flow channel region (9) is a portion outside the circumscribed circle of the rod bundle region (7); Using the fuel assembly to perform structured meshing includes the following steps: Step S1: establishing a two-dimensional geometric model according to the cross section of the fuel assembly; Step S2: dividing the two-dimensional cross section into a rod bundle region (7), a first flow channel region (8) and a second flow channel region (9); Step S3: establishing two-dimensional topological structures for the rod bundle region (7), the first flow channel region (8) and the second flow channel region (9), respectively, and adjusting the distance from the first layer of grids in the near-wall region to the wall and the grid aspect ratio according to the accuracy requirements of the numerical calculation; Step S4: generating a two-dimensional structured network according to the two-dimensional topological structure; Step S5: rotating and stretching the two-dimensional structured grids of the rod bundle region (7) and the first flow channel region (8) along the axis of the fuel rod bundle, and vertically stretching the two-dimensional structured grid of the second flow channel region (9) along the axis of the fuel rod bundle assembly; Step S6: After stretching the rod bundle region (7), the first flow channel region (8) and the second flow channel region (9), three groups of body grids are generated to obtain a complete fuel assembly body grid.

6. The structured grid generation method for fuel assembly according to claim 5, characterized in that: In the step S3, two-dimensional topological structures are respectively established for the rod bundle region (7), the first flow channel region (8) and the second flow channel region (9), and meshing is performed using an O-type partitioning method. During the process of setting the mesh nodes, the boundary nodes of the rod bundle region (7) and the first flow channel region (8) are kept in one-to-one correspondence.

7. The structured grid generation method for fuel assembly according to claim 5, characterized in that: The helical pitch L is the length corresponding to each 360° twist of the cross section of the helical cross-shaped fuel rod. The helical pitch L is determined by the parameter setting of the grid rotation stretching in step S5. The calculation formula of the pitch L of the fuel rod bundle is as follows: Among them, n is the rotation angle of each layer of grid, and Δh is the height of each layer of grid.

Images