KR20080060801A - Improved fretting wear resistance spacer grid with w-type and m-type spring - Google Patents

Abstract

A spacer grid with W-type and M-type springs improved in fretting wear resistance is provided to reduce rigidity of the springs by including a parallel unit on a grid surface connection unit. A spacer grid with an M-type spring(64) is supported by a dimple formed on top and bottom portions of the spring to protrude in an opposite direction of the spring. The spring includes a fuel rod contact unit(641) and a grid surface connection unit(642). The fuel rod contact unit includes a depressed surface along a radius of curvature of a nuclear fuel rod. Two projection units(644) are formed on the depressed surface long in a vertical direction. The projection units and the nuclear fuel rod have a plurality of linear contacts. The grid surface connection unit connects the spacer grid and the fuel rod contact unit. The grid surface connection unit is formed in parallel with a spacer grid surface and further includes a parallel unit(643) to control rigidity of the spring according to a length.

Description

Fuel fretting wear resistance spacer grid with W-type and M-type spring}

1 is a schematic representation of a typical fuel assembly

2 is a plan view showing a general support grid

3 is a cutaway perspective view showing a general support grid

Figure 4a is a perspective view showing a conventional line contact type support grid

Figure 4b is a plan view showing a conventional line contact type support grid

Figure 5a is a perspective view showing a conventional surface contact type support grid

Figure 5b is a plan view showing a conventional surface contact type support grid

6A is a perspective view showing a lattice plane according to the first embodiment of the present invention;

6B is a side view showing a lattice plane according to the first embodiment of the present invention;

6C is a plan view illustrating a unit grid cell according to Embodiment 1 of the present invention.

7A is a perspective view showing a lattice plane according to a second embodiment of the present invention;

7B is a side view showing a lattice plane according to a second embodiment of the present invention.

7C is a plan view showing a unit grid cell according to a second embodiment of the present invention

<Description of the symbols for the main parts of the drawings>

631 fuel rod contact portion 632: grid connection portion

633: parallel portion 634: parallel portion

641: fuel rod contact 642: grid surface connection

643: parallel part 644: parallel part

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a support grid for supporting fuel rods of a nuclear fuel assembly, and more particularly, to a support grid having a spring characterized in preventing damage to fuel rods caused by fretting wear.

A nuclear reactor is a device made to be used for various purposes such as generating heat by producing artificial fission reaction of fissile material, producing radioisotopes and plutonium, or forming a radiation field.

In general, light water reactors use enriched uranium with an increased ratio of uranium-235 to 2-5%. In order to process the nuclear fuel used in nuclear reactors, uranium is molded into cylindrical pellets weighing about 5g. Hundreds of these pellets are bundled in bundles, charged in a vacuum in a zircaloy cladding tube, spring and helium gas are put therein, and the upper rod stopper is welded to make fuel rods. The fuel rods finally form a fuel assembly and are burned through a nuclear reaction in the reactor.

The fuel assembly and its components are shown in [FIG. 1] to [FIG. 3]. FIG. 1 is a schematic view showing a general fuel assembly, FIG. 2 is a plan view of a support grid from above, and FIG. 3 is a cutaway perspective view showing the support grid in detail.

Referring to FIG. 1, the nuclear fuel assembly includes a top body 4, a bottom body 5, a support grid 2, a guide tube 3, and a support grid 2. Supported by a spring 6 (see [FIG. 2] and [FIG. 3)) and dimples 7 (see [FIG. 2] and [FIG. 3]), which are inserted into the support grid 2 and formed in the support grid 2 It consists of the fuel rod (1). In order to prevent scratches on the surface of the fuel rod (1) and to prevent damage to the spring (6) in the support grid, the locker is applied to the surface of the fuel rod, charged into the skeletal body, and then fixed by attaching the upper and lower fixing bodies. After the assembly of the nuclear fuel assembly is completed, the assembly manufacturing process is completed by removing the lockers of the completed assembly and inspecting the spacing, warpage, electrical length, and dimensions between the fuel rods.

Referring to FIGS. 2 and 3, the support grid 2 includes slots 21 each having a strip (thin metal plate) formed at regular intervals so as to partition a space in which a plurality of fuel rods 1 can be charged. (Refer to [FIG. 6A]) to form a lattice shape. The support grating is arranged about 10 to 13 up and down and welded with a guide tube 3 of 4m length. Springs 6 and dimples 7 are regularly formed in each space portion partitioned by the support grid 2, and the springs 6 and dimples 7 are fuel rods 1 (see Fig. 1). By maintaining contact with the fuel rods 1 (see FIG. 1), the fuel rods 1 are secured by the elasticity of the springs 6 and arranged at a predetermined position.

Meanwhile, the development of nuclear fuel is being pursued with the aim of high combustion and integrity. In order to develop high-combustion fuels, methods for promoting heat transfer from fuel rods to cooling water have been proposed. The heat transfer promotion method is mainly to improve the flow of the reactor coolant flowing around the nuclear fuel rod, such as the attachment of the mixing wing and the design change thereof or the efficient configuration of the flow channel.

However, the method for promoting heat transfer mainly causes the coolant flowing around the nuclear fuel rod to become more turbulent, which is also the cause of the fluid-induced vibration that causes the nuclear fuel rod to vibrate.

Fluid-induced vibration of the fuel rod causes the fuel rod to slide relative to the support grid spring or dimple relative to each other, causing local wear on the contact surface of the fuel rod, resulting in fretting wear, in which the fuel rod is gradually damaged. . In other words, the method of improving thermal performance for the development of high-combustion fuels, on the one hand, promotes damage to the fuel rods.

On the other hand, as the combustion progresses, the support grid grows in the lateral direction. In addition, fuel rods have irregular outer diameters due to repeated creep in the radial direction during combustion in the reactor, that is, gradual phenomena due to high pressure of cooling water in the reactor and radial expansion due to expansion of the sintered body. The directionality may cause a gap between the spring / dimple of the support grid and the fuel rod, which intensifies the fretting wear.

In order to minimize the fretting wear phenomenon, methods for minimizing the wear depth under the same wear volume have been proposed even if fretting wear occurs by increasing the longitudinal contact length between the fuel rod and the spring / dimple or by making surface contact.

4 schematically shows a support grid with a conventional spring formed such that the length of the line contact in the longitudinal direction is increased.

In the case of the spring 61 formed on the support grid, fluid-induced vibration is generated, and the fuel rod 1 and the support are supported due to the irradiation growth of the support grid 21 and the roundness change of the fuel rod 1 (irregular directional shrinkage due to high pressure). When a gap is generated between the grids 21 and vibration of the fuel rod 1 occurs, even if the longitudinal contact length of the fuel rod 1 and the spring 61 increases, the fuel rod 1 can vibrate freely in the lateral direction. This is inefficient to minimize fretting wear. Rather, as the fuel rod 1 vibrates freely in the lateral direction, the vibration amplitude of the fuel rod from the support point at which the contact with the spring 21 occurs increases, thereby increasing the contact or impact load, thereby accelerating wear. have.

Meanwhile, a method of minimizing fretting wear by causing surface contact with a fuel rod is disclosed in US Patent No. US6606369, `` Nuclear reactor with improved grid '' (hereinafter referred to as prior art), filed March 6, 2002. .

5A and 5B, the prior art forms a curved surface on the spring 62 so as to be in surface contact in contact between the spring 62 and the fuel rod 1. It is characterized by preventing the axial and transverse shaking of the fuel rod by the fluid induced vibration than the contact method.

However, the prior art is intended to form a surface contacting the fuel rod 1 with the spring 62 and the dimple 72 to be the same as the radius of curvature of the fuel rod to make a surface contact, but in practice It is not possible to establish or maintain accurate surface contact or maintain a constant roundness and radius of curvature throughout the combustion process of the fuel rod due to manufacturing tolerances and changes in the roundness of the fuel rod 1 due to high-pressure cooling water.

In addition, when the contact portion of the spring 62 does not form a theoretically perfect surface, the fuel rod 1 may form irregular line contact or point contact, and thus may cause unexpected wear.

In addition, the width (A1) that can support the fuel rod by the spring 62 is only about 40 to 45 degrees due to structural problems, so when the gap between the fuel rod and the spring and / or dimple occurs, the lateral movement of the fuel rod It is difficult to support it stably.

For the same reason as above, it may be considered to make the spring contact portion wider in order to further increase the fuel rod support width. However, in this case, due to the characteristics of the sheet metal forming process, the thickness is mainly reduced in the bent portion, so that the production of the conformal bent spring of the prior art, such as cracking occurs during manufacture or combustion in the reactor, there is a lot of difficulties. In order to prevent such cracks, the radius of curvature of the bend should be 2.5 to 3 times or more of the thickness of the base material. The bend P1 formed at both ends of the fuel rod contact portion of the spring 62 has a bending radius of 2.5 times or more than the base material thickness. It is because the structure of the prior art is difficult to form.

In addition, when the bending portion P1 is formed, the smaller the radius of curvature, the more convex swelling of the outer side of the curved portion occurs, which is likely to cause local wear of the nuclear fuel rod.

As to solve the above problems,

An object of the present invention is to configure a spring so as to increase the number of support points in line contact between the fuel rod and the support grid.

It is also an object of the present invention to provide a means for lowering the stiffness of the spring as needed in the manufacture of the spring.

In order to achieve the above technical problem,

Strips are arranged at regular intervals vertically and horizontally, and a plurality of unit grid cells are partitioned by inserting slots formed at regular intervals into the strips. In the support grid supported by a spring formed to protrude into the upper and lower portions of the spring and the dimple formed to protrude in the opposite direction to the spring,

The spring has a concave surface is formed in accordance with the radius of curvature of the nuclear fuel rod and the fuel rod contact portion is formed in the longitudinal direction of the two to three convex portions on the concave surface and the convex portion and the nuclear fuel rods; And lattice surface connecting portion for connecting the support grid and the fuel rod contact portion; It consists of.

In addition, the grid surface connection portion may be formed in parallel with the support grid surface may have a parallel portion for adjusting the rigidity of the spring according to its length.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Unless otherwise defined or mentioned, terms indicating directions such as 'up, down, left and right' used in the present description are based on the states indicated in the drawings. In addition, the same reference numerals throughout the embodiments indicate the same member.

On the other hand, each space part divided by the lattice structure of the support grid is called a unit lattice cell, and one surface inside the lattice cell is defined as a lattice plane. If one lattice plane is defined as the longitudinal direction, all lattice planes parallel to the lattice plane become longitudinal lattice planes, and the lattice plane perpendicular to the longitudinal lattice plane is referred to as a transverse lattice plane. In addition, the axial direction means the direction in which the nuclear fuel rod is charged as the longitudinal direction of the unit cell.

Hereinafter, Example 1 of the present invention will be described in detail. In Embodiment 1 of the present invention, the spring 64 formed on the support grid is a main component thereof.

6A to 6C, the M-shaped spring 64 will be described. FIG. 6A is a perspective view showing a lattice plane constituting one side of a unit lattice cell of a support grid, FIG. 6B is a side view of the unit lattice cell, and FIG. 6C is a plan view of the unit lattice cell viewed from above.

Referring to FIG. 6A, the M-shaped spring 64 includes a fuel rod contact portion 641, a grid surface connecting portion 632, and a parallel portion 633 formed on the grid surface connecting portion 632.

The fuel rod contacting portion 641 is a portion for supporting the fuel rod 1 in contact with the fuel rod 1 afterwards, and is a convex portion that is continuously convexly formed at a constant height in the longitudinal direction for linear contact with the fuel rod 1. Two 644 are formed at the left and right, and as shown in FIG. 6C, when viewed from above, convex and concave portions are alternately formed to have an M shape.

The fuel rod contact portion 641 is connected to the support grid 21 through the grid surface connecting portion 642 at the top and bottom, respectively, the M-shaped spring 64 is generally arcuate when viewed from the side as shown in FIG. Has the shape of.

On the other hand, the center of the grid surface connecting portion 642 forms a parallel portion 643 formed in parallel with the support grid 21. The parallel portion 643 serves to lower the stiffness of the spring compared to the case where the entire grid surface connection portion 642 is formed to form a predetermined angle with the grid surface.

One M-shaped spring 64 is formed in each of the four lattice planes 21 in the unit lattice cell, and the M-shaped spring 64 is formed to protrude in a predetermined direction through the entire support grid. The two springs protrude inwards and the two springs protrude outwards, i.e., inside the neighboring unit grid cells.

The dimple 71 has the same configuration as that of the conventionally known bar, and thus detailed description thereof will be omitted.

The operation of Example 1 will be described with reference to FIG. 6C.

The M-shaped spring 64 formed on one lattice surface 21 and two dimples 71 formed on the lattice surface 21 facing the same contact the nuclear fuel rod 1 from both sides, but the dimple 71 When the fuel rods 1 are inserted into a plurality of strong stiffness to maintain a constant distance from each other and the M-shaped spring 64 is to support the fuel rods 1 by pressing the fuel rods 1 with a constant elasticity. The M-shaped spring 64 and the dimple 71 formed at right angles also support the fuel rod 1 through the same action.

At this time, each support wing creates two line contact points per spring. In this case, considering only the support point of the spring, the support point is doubled than the support method by the conventional line contact, and the wear depth is further reduced under the same fretting wear condition.

A second embodiment will be described with reference to Figs. 7A to 7C. FIG. 7A is a perspective view showing one lattice plane with a spring according to the second embodiment, FIG. 7B is a side view of the lattice plane, and FIG. 7C is a spring with a second embodiment A plan view of a unit grid cell viewed from above.

The second embodiment is almost the same in configuration as the first embodiment, but there is a difference in the configuration of the fuel rod contact portion 631. That is, as shown in FIG. 7A, three convex portions 634 are formed in the fuel rod contact portion 631, unlike the first embodiment.

Referring to FIG. 7C, the operation of the second embodiment will be described basically the same as that of the first embodiment. However, when considering the spring-only contact and support scores compared to the conventional line contact method, three times the support points are formed, and the wear depth is further reduced under the same fretting wear conditions.

Although the preferred embodiment of the present invention has been described above, the technical idea of the present invention is not limited to the above-described preferred embodiment, and implemented with various support grids within the scope not departing from the technical idea of the present invention specified in the claims. Can be.

As above,

The support grid according to the present invention is to make a plurality of line contact between the fuel rod and the support grid to reduce the wear depth under the same wear volume.

In addition, the support grid according to the present invention can include a parallel portion in the grid surface connection portion can lower the rigidity of the spring.

Claims (4)

Strips are arranged at regular intervals vertically and horizontally, and a plurality of unit grid cells are partitioned by inserting slots formed at regular intervals into the strips. In the support grid supported by a spring formed to protrude into the upper and lower portions of the spring and the dimple formed to protrude in the opposite direction to the spring, The spring is A fuel rod contact portion having a concave surface formed according to a radius of curvature of the nuclear fuel rod and having two convex portions extending longitudinally on the concave surface to make a plurality of line contacts between the convex portion and the nuclear fuel rod; And A lattice plane connection portion connecting the support grid and the fuel rod contact portion; A support grid with an M-type spring with improved fuel rod fretting wear resistance, characterized in that consisting of. The method of claim 1, The grid surface connection portion is provided in parallel with the support grid surface is provided with a W-type and M-type spring with improved fuel rod fretting wear resistance, characterized in that it further comprises a parallel portion that can adjust the stiffness of the spring according to its length Support grid. Strips are arranged at regular intervals vertically and horizontally, and a plurality of unit grid cells are partitioned by inserting slots formed at regular intervals into the strips. In the support grid supported by a spring formed to protrude into the upper and lower portions of the spring and the dimple formed to protrude in the opposite direction to the spring, The spring is A fuel rod contact portion having a concave surface formed according to a radius of curvature of the nuclear fuel rod, wherein three convex portions are elongated in the longitudinal direction on the concave surface to make a plurality of line contacts between the convex portion and the fuel rod; And A lattice plane connection portion connecting the support grid and the fuel rod contact portion; A support grid with a W-shaped spring, characterized in that the fuel rod fretting wear resistance is improved. The method of claim 3, The grid surface connection portion is provided in parallel with the support grid surface is provided with a W-type and M-type spring with improved fuel rod fretting wear resistance, characterized in that it further comprises a parallel portion that can adjust the stiffness of the spring according to its length Support grid.

Images