Improved nuclear fuel assembly with increased seismic performance and increased control rod efficiency

Abstract

A nuclear fuel assembly suitable for a small modular reactor (SMR), wherein the nuclear fuel assembly comprises a spacer grid provided with 17x17 lattice cells, a plurality of fuel rods, each arranged in an associated lattice cell of the spacer grid, guide tubes, each arranged and secured within an associated lattice cell of the spacer grid, and an upper nozzle and a lower nozzle secured to an upper end and a lower end of each guide tube, respectively, wherein, relative to a central cell of the spacer grid, each of eight guide tubes is arranged in an associated one of major axis directions Ixx and Iyy, each of four guide tubes is arranged in an associated one of on-diagonal directions Ixy, and each of 16 guide tubes is arranged in an associated one of off-diagonal directions Ixxy and Ixyy.

Description

Title of Invention: Improved nuclear fuel assembly with increased seismic performance and increased control rod efficiency Technical field

[0001] The present disclosure relates to a nuclear fuel assembly suitable for a small modular reactor (SMR). Description of associated art

[0002] A typical light water reactor core has a number of nuclear fuel assemblies, each of which has a plurality of long fuel rods and a plurality of guide tubes in the form of tubes used to guide control rods during their movement. The guide tubes are attached to an upper nozzle and a lower nozzle of the nuclear fuel assembly, providing a skeleton of the nuclear fuel assembly.

[0003] Control rods contain a material that absorbs neutrons generated during a fission process. Conventionally, materials with large neutron capture cross sections, such as boron carbide (B4C), hafnium (Hf), or silver-indium-cadmium (Ag-In-Cd), have been used.

[0004] Nuclear fuel assemblies of general light water reactors, including a domestic nuclear power plant model APR1400, are identical whether they are in control rod positions or non-control rod positions inside the core, and all the nuclear fuel assemblies have guide tubes even when used in the core position in which there is no guided control rod, and these guide tubes are used to provide a single skeleton of the nuclear fuel assemblies.

[0005] A typical nuclear fuel assembly consists of a 17x17 grid with a total of 289 cells. Of these, 264 cells contain fuel rods, 24 cells contain guide tubes, and the remaining cell, located in the center of the grid, contains an instrumentation tube.

[0006] A small modular reactor (SMR) that significantly reduces the size and output of traditional reactors is under development. Theoretically, the SMR has the advantage of being small, which facilitates output control and reactor cooling.

[0007] The foregoing description is intended only to facilitate understanding of the context of the present disclosure and is in no way intended to imply that the present disclosure falls within the scope of the related art which is already known to those skilled in the art. Related Art Documents

[0008] (Patent Document 1) Korean Patent Publication No. 10-1994-0003796 (published on May 3, 1994)

[0009] (Patent Document 2) Korean Patent Publication No. 10-1992-0007739 (published on September 16, 1992) SUMMARY

[0010] The present disclosure has been prepared in consideration of the above-mentioned problems of the related art, and the present disclosure is intended to provide a nuclear fuel assembly suitable for a small modular reactor (SMR).

[0011] To achieve the above-mentioned objective, a nuclear fuel assembly according to the present disclosure can be provided, the nuclear fuel assembly comprising: a spacer grid provided with 17x17 lattice cells; a plurality of fuel rods, each arranged in an associated lattice cell of the spacer grid; guide tubes, each arranged and fixed inside an associated lattice cell of the spacer grid;and an upper nozzle and a lower nozzle respectively attached to an upper end and a lower end of each guide tube, wherein, relative to a central cell of the spacer grid, each of eight guide tubes is arranged in a related one of main axis directions Ixx and lyy, each of four guide tubes is arranged in a related one of on-diagonal directions Ixy, and each of 16 guide tubes is arranged in a related one of off-diagonal directions Ixxy and Ixyy. ;

[0012] The guide tubes may each be arranged at (6,0), (3,0), (-3,0), (-6,0) and (0,6), (0,3), (0,-3), and (0,-6) in an associated one of the major axis directions.

[0013] The guide tubes may each be arranged at (5,5), (-5,-5), (-5,5), and (5,-5) in one of the associated diagonal directions.

[0014] The guide tubes may each be arranged at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4) and (3,-6) in an associated one of the off-diagonal directions.

[0015] A spacer grid assembly may comprise: a spacer grid having 17x17 lattice cells; claddings, each of which is secured and assembled within an associated lattice cell of the spacer grid, wherein, relative to a central cell of the spacer grid, each of eight claddings is arranged in an associated one of major axis directions Ixx and lyy, each of four claddings is arranged in an associated one of diagonal directions Ixy, and each of 16 sheaths is arranged in one of the associated off-diagonal directions Ixxy and Ixyy.

[0016] The sheaths may each be arranged at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3), and (0,-6) in an associated one of the major axis directions.

[0017] The sheaths may each be arranged at (5,5), (-5,-5), (-5,5), and (5,-5) in one of the associated diagonal directions.

[0018] The sheaths may each be arranged at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4), and (3,-6) in one of the associated off-diagonal directions.

[0019] As mentioned above, a nuclear fuel assembly according to the present invention may comprise a spacer grid having 17x17 lattice cells, a plurality of fuel rods arranged within the lattice cells of the spacer grid, guide tubes arranged and fixed respectively within the associated lattice cells of the spacer grid, and an upper nozzle and a lower nozzle fixed respectively to an upper end and a lower end of each of the guide tubes,

[0020] wherein a total of 28 guide tubes are provided such that, with respect to a central cell of the spacer grid, eight guide tubes are arranged in main axis directions Ixx and lyy, four in on-diagonal directions Ixy, and 16 in off-diagonal directions Ixxy and Ixyy. Therefore, the present disclosure can achieve the following effects, which can increase control rod efficiency and seismic performance.

[0021] 1) Improving control rod efficiency for operation of heart without boric acid

[0022] A typical commercial pressurized water reactor operates with the control rods completely removed most of the time, and excess reactivity is controlled by soluble boric acid and fuel absorber rods. Small modular reactors are being developed primarily with a boric acid-free core. Insertion of control rods is required from the beginning of the cycle for small modular reactors. Remaining excess reactivity (not exceeding 1000 pcm) can be controlled into a critical state (keff = 1.0) by inserting regulating control rods. The present invention can therefore satisfy the requirement of subcriticality by improving control rod efficiency.

[0023] 2) Increased seismic performance

[0024] The present invention increases the number of guide tubes attached to the upper nozzle and the lower nozzle. This increases the bending rigidity of the nuclear fuel assembly skeleton, reduces the displacement of the nuclear fuel assembly at the same energy level with a natural frequency increased, and also reduces the generated load. The present invention can therefore increase seismic performance. Brief description of the drawings

[0025] The above-mentioned objectives, features and advantages, as well as other objectives, features and advantages of the present disclosure will be understood more clearly upon reading the following detailed description with reference to the accompanying drawings in which:

[0026] [Fig.l] is a front view of a nuclear fuel assembly according to one embodiment of the present disclosure;

[0027] [Fig.2] is a plan view showing an arrangement pattern of fuel rods in a spacer grid assembly according to the embodiment of the present disclosure;

[0028] [Fig. 3] is a plan view of a nuclear fuel assembly according to the embodiment of the present disclosure;

[0029] [Fig.4] is a graph showing results of subcriticality evaluation under ARI and Nl conditions based on the 24-rod control rod, wherein Nl conditions refer to insertion conditions of all (Nl) control rods excluding a control rod with the highest control rod efficiency among all N control rods; and

[0030] [Fig.5] is a graph showing results of a sub-evaluation criticality under ARI and Nl conditions based on the 28-rod control rod, wherein Nl conditions refer to insertion conditions of all (Nl) control rods excluding a control rod with the highest control rod efficiency among all N control rods. DETAILED DESCRIPTION

[0031] Specific structural or functional descriptions presented in the embodiments of the present disclosure are exemplified merely for the purpose of explaining embodiments according to the concept of the present disclosure, and embodiments according to the concept of the present disclosure may be implemented in various forms. Furthermore, the descriptions presented should not be construed as being limited to the embodiments described herein, but should be understood to include all modifications, equivalences, and substitutions included within the spirit and scope of the present disclosure.

[0032] The terms used herein are used only to describe specific embodiments and are not intended to limit the present disclosure. Singular expressions include plural expressions except explicit indication to the contrary in the context. It should be clearly understood that the terms “include” or “present”, as used herein, are intended to specify the presence of a feature, number, step, operation, component, part or combination thereof that is implemented, but does not exclude in advance the possibility of the presence or addition of one or more other numbers, features, steps, operations, components, parts or combinations thereof.

[0033] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0034] [Fig.l] is a front view of a nuclear fuel assembly according to one embodiment of the present disclosure, excluding the fuel rods.

[0035] Referring to [Fig.l], the nuclear fuel assembly 100 of the present embodiment comprises a spacer grid 110, guide tubes 130, each of which is arranged and fixed within an associated lattice cell of the spacer grid 110, and an upper nozzle 140 and a lower nozzle 150, fixed respectively to an upper end and a lower end of each guide tube 130. The fuel rods are supported on the spacer grid 110 and arranged axially in parallel with the guide tubes 130.

[0036] The upper nozzle 140 may be provided with a coil spring or a leaf spring to provide a pressure force against axial movement of the nuclear fuel assembly, and the lower nozzle 150 may be provided with a nozzle where coolant is introduced.

[0037] The spacer grid 110 is assembled such that a plurality of straps are assembled in the horizontal and vertical directions to present a lattice shape, and a fuel rod or a guide tube is placed inside each square lattice cell separated by the straps.

[0038] The spacer grid 110 elastically supports the fuel rods with cells and grid springs provided inside the lattice cells in each of which the fuel rod is inserted. The spacer grid 110 may also secure the guide tubes via sheaths, mounted inside cells and grid springs provided inside the lattice cells in each of which the guide tube 130 is inserted. Although [Fig.l] shows that the spacer grid 110 has a sheath 111 which is assembled with the guide tube 130, the spacer grid 110 and the guide tube 130 may also be secured by direct welding. 110 spacer grids are typically manufactured by processing and welding sheet metal, but they can also be produced with a metal 3D printing device and are not limited to a specific manufacturing process.

[0039] The spacer grid 110 may consist of 17x17 lattice cells, with a total of 28 guide tubes 130 which are arranged at specific positions therebetween. An embodiment thereof will be described in detail below.

[0040] Figure 2 is a plan view showing a fuel rod arrangement pattern in a spacer grid assembly according to the embodiment of the present disclosure, and for ease of understanding, the horizontal direction of the spacer grid 110 is the x-axis, the vertical direction is the y-axis, and the location of a specific array cell is indicated in a two-axis (x, y) coordinate system, and the coordinate of the center cell is indicated by (0,0). In Figure 2, i indicates -1.

[0041] Referring to [Fig. 2], the spacer grid 110 is composed of 17x17 array cells, the central cell (0,0) has an instrumentation tube positioned therein, a total of 28 guide tubes are arranged at specific locations, and fuel rods 120 are arranged in the remaining array cells. However, the fuel rods are omitted in [Fig. 2].

[0042] In the description of the present disclosure, based on the center cell (0,0), the major axis direction refers to the horizontal direction Ixx and the vertical direction lyy of the spacer grid 110, the on-diagonal direction refers to an on-diagonal direction Ixy of 45° (or 135°), and the off-diagonal direction refers to an on-diagonal direction Ixxy or Ixyy other than 45° (or 135°). The major axis direction, the on-diagonal direction, and the off-diagonal direction each include both the positive (+) direction and the negative (-) direction.

[0043] A total of 28 guide tubes are provided in the spacer grid 110. With their arrangement based on the center cell (0, 0), eight guide tubes are positioned in the main axis directions Ixx and lyy, four in the on-diagonal directions Ixy, and 16 in the off-diagonal directions Ixxy and Ixyy. As mentioned above, at each guide tube arrangement position of the spacer grid, the guide tube can be attached directly to the spacer grid or attached to the spacer grid via the sheath.

[0044] The off-diagonal directions Ixxy and Ixyy may be arranged at an angle equal to 0 based on the on-diagonal direction Ixy. These may include a first off-diagonal direction Ixxy biased toward the x-axis and a second off-diagonal direction Ixyy biased toward the y-axis. In the present embodiment, the first off-diagonal direction Ixxy and the second off-diagonal direction Ixyy may have an angle 0 of 18° between each of them and the on-diagonal direction Ixy.

[0045] The guide tubes may be arranged at (6,0), (3,0), (-3,0), and (-6,0) in the horizontal direction Ixx and at (0,6), (0,3), (0,-3), and (0,-6) in the vertical direction lyy.

[0046] In addition, the guide tubes may be arranged at (5,5), (-5,-5), (-5,5), (5,-5) in the diagonal direction Ixy.

[0047] The guide tubes may be arranged at (6,3), (4,2), (-4,-2), and (-6,-3) and at (-6,3), (-4,2), (4,-2) in the first off-diagonal direction Ixxy, and (6,-3) and at (3,6), (2,4), (-2,-4), and (-3,-6) and at (-3,6), (-2,4), (2,-4), and (3,-6) in the second off-diagonal direction Ixyy.

[0048] [Fig. 3] is a plan view of a nuclear fuel assembly according to the embodiment of the present disclosure.

[0049] As shown in [Fig.3], a control rod assembly 200, according to An embodiment of the present disclosure, comprises: a cylindrical spider body 210; a plurality of spider blades 220 extending radially from the spider body 210; and a plurality of spider fingers 230, each provided on an associated one of the plurality of spider blades 220 and configured to respectively fix an associated one of the guide tubes. In addition, each of the spider fingers 230 corresponds to an associated one of the aforementioned 28 guide tubes and is assembled with an associated control rod 210.

[0050] In this manner, the present disclosure may improve boric acid-free operation and seismic performance by allowing the insertion of 28 control rods at specific locations in the spacer grid having 17x17 lattice cells.

[0051] Specifically, from the perspective of nuclear fuel assembly design, it is confirmed that, by applying 28 control rods to the nuclear fuel assembly of the 17x17 lattice cell spacing grid, the control rod efficiency has increased by 17%, the nuclear fuel loading amount has decreased by 1.5%, and the seismic performance is improved. It is also confirmed that the reactivity is relatively high, and the core cycle length is increased compared with the conventional nuclear fuel assembly employing 24-rod control rods.

[0052] Furthermore, in the subcriticality evaluation results based on the 24-rod control rod, it is evaluated that the effective multiplication factor keff is not more than 0.95 in the ARI condition and is not more than 0.99 in the Nl condition, which means that the subcriticality condition is satisfied at a level with practically no margin (see [Fig. 4]). It has been evaluated that the subcriticality evaluation results based on the 28-rod control rod under conservative conditions have a margin of 0.98 for the effective multiplication factor keff in the Nl condition (see [Fig. 5]). The safety of a stop margin is further strengthened, and 4 additional empty spaces for the nozzle top-mounted in-core instrumentation (TM-ICI) or control rod assembly can be secured with respect to the 24-rod control rod.

[0053] From a mechanical design perspective, the 28-rod control rod is intended to increase control rod efficiency in connection with boric acid-free operation, and it has the effect of facilitating TM-ICI acceptance and control rod loading / removal. In conventional nuclear power plants, core reactivity is controlled by dilution of boron, a toxic substance, in the coolant, which is accompanied by undesirable effects such as deposition of dirt not only on the nuclear fuel but also on major components of the reactor core. The present disclosure requires core reactivity control only with one control rod for boron-free operation of a small modular reactor.The control rod efficiency of the present disclosure is therefore increased over the existing one, making boron-free operation efficient.

[0054] In addition, the present disclosure can strengthen the mechanical characteristics of the skeleton of a nuclear fuel assembly by increasing the number of guide tubes attached to the upper nozzle and the lower nozzle.

[0055] The present disclosure described above is not limited to the aforementioned embodiments and the accompanying drawings, and those skilled in the art will appreciate that various substitutions, modifications and changes may be made to the present disclosure within a scope consistent with the technical spirit of the present disclosure.]

Claims

Claims

1. A nuclear fuel assembly comprising: a spacer grid having 17x17 lattice cells; a plurality of fuel rods, each arranged in an associated lattice cell of the spacer grid; guide tubes, each arranged and fixed inside an associated lattice cell of the spacer grid; and an upper nozzle and a lower nozzle fixed respectively to an upper end and a lower end of each guide tube, wherein, with respect to a center cell of the spacer grid, each of eight guide tubes is arranged in an associated one of major axis directions Ixx and lyy, each of four guide tubes is arranged in an associated one of on-diagonal directions Ixy, and each of 16 guide tubes is arranged in an associated one of off-diagonal directions Ixxy and Ixyy.

2. The nuclear fuel assembly of claim 1, wherein the guide tubes are each arranged at (6,0), (3,0), (-3,0), (-6,0) and (0,6), (0,3), (0,-3) and (0,-6) in an associated one of the major axis directions. wherein, in an (x,y) coordinate system, x represents a horizontal lattice cell location relative to a center cell (0,0), and y represents a vertical lattice cell location relative to the center cell (0,0).

3. The nuclear fuel assembly of claim 1, wherein the guide tubes are each arranged at (5,5), (-5,-5), (-5,5), and (5,-5) in an associated one of the diagonal directions, wherein, in an (x,y) coordinate system, x represents a horizontal lattice cell location relative to a center cell (0,0), and y represents a vertical lattice cell location relative to the center cell (0,0).

4. A nuclear fuel assembly according to claim 1, wherein the guide tubes are each arranged at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4) and (3,-6) in an associated one of the off-diagonal directions, wherein, in an (x,y) coordinate system, x represents a horizontal lattice cell location relative to a center cell (0,0), and y represents a vertical lattice cell location relative to the center cell (0,0).

5. A spacer grid assembly comprising: a spacer grid provided with 17x17 lattice cells; claddings, each of which is secured and assembled within an associated lattice cell of the spacer grid, wherein, with respect to a central cell of the spacer grid, each of eight claddings is arranged in an associated one of major axis directions Ixx and lyy, each of four claddings is arranged in an associated one of on-diagonal directions Ixy, and each of 16 claddings is arranged in an associated one of off-diagonal directions Ixxy and Ixyy.

6. The spacer grid assembly of claim 5, wherein the claddings are each arranged at (6,0), (3,0), (-3,0), (-6,0), (0,6), (0,3), (0,-3) and (0,-6) in an associated one of the major axis directions, wherein in an (x,y) coordinate system, x represents a horizontal lattice cell location relative to a center cell (0,0), and y represents a vertical lattice cell location relative to the center cell (0,0).

7. The spacer grid assembly of claim 5, wherein the claddings are each arranged at (5,5), (-5,-5), (-5,5), and (5,-5) in an associated one of the diagonal directions, wherein, in an (x,y) coordinate system, x represents a horizontal lattice cell location relative to a center cell (0,0), and y represents a vertical lattice cell location relative to the center cell (0,0).

8. The spacer grid assembly of claim 5, wherein the claddings are each arranged at (6,3), (4,2), (-4,-2), (-6,-3), (-6,3), (-4,2), (4,-2), (6,-3), (3,6), (2,4), (-2,-4), (-3,-6), (-3,6), (-2,4), (2,-4), and (3,-6) in an associated one of the off-diagonal directions, wherein in an (x,y) coordinate system, x represents a horizontal lattice cell location relative to a center cell (0,0), and y represents a vertical lattice cell location relative to the center cell (0,0).

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