KR20080060795A - Anti-fretting wear spacer grid with integrated tubular spring and dimple - Google Patents

Abstract

An anti-fretting wear spacer grid with an integrated tubular spring and a dimple is provided to minimize wear of a fuel rod due to rotation deformation of the dimple by not independently including the dimple. An anti-fretting wear spacer grid(23) includes strips and a plurality of unit grid cells divided by inserting slots(231) formed on the strip at predetermined intervals. A nuclear fuel rod is charged and supported to the unit grid cell. The spacer grid further includes a spring receiving unit(232) and a cylindrical clip-shaped spring. The spring receiving unit is formed by cutting centers of metal plates corresponding to crossing points of the spacer grids in a square shape. The cylindrical clip-shaped spring is formed in a hollow cylindrical shape to be received in the spring receiving unit and a part of an outer circumferential surface thereof is cut along an axial direction.

Description

Anti-fretting wear spacer grid with integrated tubular spring and dimple}

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

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

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

6A is a front view showing a strip of a general support grid

6B is a schematic view showing rotational deformation in a general support grid

Figure 7a is a perspective view showing a support grid strip according to the present invention

Figure 7b is a perspective view showing a cylindrical clip spring of Example 1 according to the present invention

Figure 7c is a perspective view showing the coupling of the cylindrical clip spring and the support grid of Example 1 according to the present invention

Figure 7d is a plan view showing the coupling of the cylindrical clip spring and the support grid of Example 1 according to the present invention

Figure 8a is a perspective view showing a cylindrical clip spring of Example 2 according to the present invention

Figure 8b is a perspective view showing the coupling of the cylindrical clip spring and the support grid of Example 2 according to the present invention

Figure 9a is a perspective view showing a cylindrical clip spring of Example 3 according to the present invention

Figure 9b is a perspective view showing the coupling of the cylindrical clip spring and the support grid of Example 3 according to the present invention

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

63: cylindrical clip spring 232: spring receiving portion

631: fixing groove 633: spring cutout

642: rigidity control slot

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 springs and dimples which are characterized in preventing fuel rod damage due to 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 placed therein, and the upper rod stopper is welded to produce 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). When assembling the assembly, to prevent scratches on the surface of the fuel rod (1) and to prevent damage of the spring (6) in the support grid, apply a locker to the surface of the fuel rod, insert it into the skeleton, and then attach and fix 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 grids are arranged up to 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 fuel rods to become more turbulent, which is also the cause of the fluid-induced vibration that causes the fuel rods 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 occurs, and the fuel rod 1 and the fuel rod 1 are changed 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 support 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 may 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.

On the other hand, in general, the spring of the support grid is known to loosen the elasticity of the spring by about 40 to 50% within about 72 hours when the stress occurs in a high temperature environment, and the elasticity of the spring is further relaxed by neutron irradiation in the reactor.

It is known that the fretting wear is accelerated when the fluid rod vibration of the fuel rod occurs. In order to reduce the relaxation of the spring elasticity, it is necessary to increase the spring elasticity or reduce the plastic deformation of the spring.

On the other hand, if the stiffness of the support grid spring is too high, it may exceed the fuel rod stress standard by applying excessive scratches or local stresses to the cladding when the fuel rod is charged.If the spring stiffness is too low, the fuel rod will bend or excessive fuel rod fluid induced vibration may occur. May occur.

For this reason, the recently developed support grid springs tend to increase the spring's elastic displacement while maintaining relatively low spring stiffness.

By forming the spring long, it is possible to reduce the wear depth and reduce the rigidity of the spring by forming a long length of the support point, that is, the support point described above. However, in order to lengthen the spring, it is necessary to simultaneously form the spring and dimple at the height of the limited support grid, so that the overall height of the support grid is increased to increase the manufacturing cost due to the increase of the material. Unnecessarily absorbed amount is increased, which leads to an uneconomic problem.

Fuel rod fretting wear, on the other hand, occurs in dimples as well as springs. Dimples are designed to maintain three to five times the stiffness of springs because of their design characteristics that require minimum spacing between fuel rods. In addition, as described above, since the spring and the dimple must coexist on one side of the support grid plate, the contact length of the dimple is lengthened, resulting in a space limitation.

6A and 6B, since the support grid plate 2 should be fitted at right angles to each other, one slot 201 must be formed at the top or the bottom thereof. Therefore, the dimples 7 which are positioned symmetrically with respect to the spring 6 have different support conditions. That is, the dimple 7 positioned on the side where the slot 201 is formed has a flexible support condition due to the elasticity of the support grid 2 due to the slot 201 compared to the dimple 7 formed on the side without the slot 201. For this reason, when a fuel rod (not shown) is charged in the support grid, different reaction forces are generated in the upper and lower dimples 7, and in particular, rotational deformation A2 occurs in the dimples 7 in which the slots 201 are formed. When such a rotational deformation (A2) occurs, the wear of the fuel rod (not shown) is caused by the edge (P2) of the hard dimple to increase the likelihood of penetration damage.

As to solve the above problems,

An object of the present invention is to prevent the loss of the bearing capacity of the spring caused by the elastic relaxation of the spring by heat or irradiation.

In addition, an object of the present invention is to support the nuclear fuel rod by a stable line contact, to form the support length by the line contact as long as possible within the longitudinal length of the limited support grid.

In addition, an object of the present invention is to prevent as much as possible through the damage of the fuel rod through the edge of the dimple caused by the rotational deformation in the dimple on the slot side under the influence of the slot for forming the grid formed on the support grid.

In order to achieve the above technical problem, the present invention,

A support grid having nuclear fuel rods loaded and supported in a plurality of unit grid cells partitioned by mutually inserting slots formed at regular intervals in the strip, the strips being arranged at regular intervals in the longitudinal and horizontal directions,

A spring accommodating part formed by cutting each of the centers of the metal plates corresponding to the intersection points of the support lattice crossing longitudinally and horizontally in a square; Cylindrical clip-shaped spring is formed in a hollow cylindrical shape so as to be accommodated in the spring accommodating portion and cut a portion of the outer circumferential surface along the axial direction; It characterized by having a.

In addition, the cylindrical clip spring may be provided with a fixing groove in the upper and lower parts to be fixed to the support grid.

In addition, the cylindrical clip spring may form a stiffness control slot in which a portion of the outer peripheral surface is cut.

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. The axial direction refers to the direction in which the nuclear fuel rod is charged as the longitudinal direction of the unit cell.

Hereinafter, Embodiment 1 of the present invention will be described in detail with reference to FIGS. 7A to 7D. According to the first embodiment of the present invention, the support grid 23 and the cylindrical clip spring 63 in which the spring accommodation portion 232 is formed are the main components thereof.

7A shows the support grid according to the first embodiment, FIG. 7B shows the cylindrical clip spring according to the first embodiment, and FIG. 7C shows the cylindrical clip spring received in the support grid. Figure 7d shows the combined state, and the support grid and the cylindrical clip-shaped spring is shown from above seen.

The support grid 23 is demonstrated with reference to FIG. 7A.

As described above, the support grid 23 has a lattice structure in such a manner that the strips having the slots 231 for insertion and engagement are arranged at regular intervals in the longitudinal and horizontal directions, and then the slots 231 are respectively inserted. The support grid 23 is formed.

The support grid 23 of Embodiment 1 according to the present invention forms a spring accommodation part 232 by cutting a central portion corresponding to the intersection of the support grid 23 in a square.

A cylindrical clip spring 63 will be described with reference to FIG. 7B.

Cylindrical clip-type spring (63) according to the first embodiment has a hollow hollow cylindrical shape of a thin iron plate as a whole, that is, a cylindrical shape in which the center thereof is axially penetrated, and a spring cutout portion in which any part of the outer circumferential surface is cut along the axial direction. 633 is formed and has a cylindrical clip shape. In addition, the end of the cylindrical clip-type spring (63) forms a fixing groove (631) for fixing the cylindrical clip-type spring (63) to the support grid (23).

With reference to FIG. 7C and FIG. 7D, the connection relationship and operation | movement of the support grid 23 and the cylindrical clip spring 63 are demonstrated.

The cylindrical clip spring 63 is accommodated in the spring receiving portion 232 of the support grid 23 formed in a lattice structure and the fixing groove 631 of the cylindrical clip spring 63 is fitted into the support grid 23. It is fixed. The cylindrical clip springs span four unit grid cells and protrude from four corner portions of the unit grid cells. After the fuel rod 1 is charged, the outer portion of the fuel rod 1 contacts the outer circumferential surface of the cylindrical clip spring 63 and is supported by the elasticity of the cylindrical clip spring 63.

Embodiment 2 will be described with reference to Figs. 8A and 8B.

8A shows the cylindrical clip spring 64 according to the second embodiment, and FIG. 8B shows the cylindrical clip spring 64 coupled with the support grid 23.

Example 2 is substantially the same as the configuration of Example 1, but there is a difference in that the rigid control slot 642 is formed by cutting a portion of the outer circumferential surface of the cylindrical clip spring 64 except for the portion where the fuel rod is in contact.

Increasing the width of the stiffness control slot 64 reduces the rigidity of the cylindrical clip-shaped spring 64 to have a more flexible support condition.

Embodiment 3 will be described with reference to FIG. 9A and FIG. 9B.

9A illustrates a cylindrical clip spring 65 according to the third embodiment of the present invention, and FIG. 9B illustrates a state in which the cylindrical clip spring 65 is coupled to the support grid 23.

Example 3 according to the present invention has a configuration for adjusting the stiffness of the spring as in the case of Example 2 shows another cutting method. Same as Example 2 in terms of cutting off a portion of the outer circumferential surface of the cylindrical clip spring 64 except for the portion where the fuel rod contacts, but maintaining the contact portion with the fuel rod 1 while maintaining the rigidity of the cylindrical clip spring 65. Since the length of the non-incision 656 can be further reduced, more flexible support conditions can be realized as compared to the spring 64 according to the second embodiment (see FIG. 8A).

Hereinafter, the operation of the support grid according to the present invention will be described with reference to FIGS. 7C and 7D.

When the fuel rod 1 is charged into the support grid 23, the outer circumferential surface of the fuel rod 1 touches the outer circumferential surface of the cylindrical clip spring 63 protruding from the four edges of the unit lattice cell, and the elasticity of the cylindrical spring 63 is increased. Is supported. In this case, as shown in [FIG. 7D], among the cylindrical clip springs 63 divided into four parts by the support grid 23, P5 and P6 have spring cutouts 633 (see FIG. 7A). Stiffness is small compared with P3 and P4 in which the spring cutout 633 is not formed. Therefore, P5 and P6 act as springs and P3 and P4 act as dimples.

In addition, the cylindrical clip-type spring 63 is formed in a cylindrical shape, the radial rigidity is larger than a general spring, the effective elastic range of the spring, that is, the depth of the spring is small. Cylindrical clip-type spring (63) according to the present invention has a very small plastic deformation amount to minimize the phenomenon that the elasticity is relaxed, but since the rigidity as described above has a small effective range having a large elasticity as described above, the cylindrical clip-type spring (64) or practice The cylindrical clip spring 65 of Example 3 provided a means to reduce the stiffness of the spring itself.

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.

Spring according to the present invention is provided with a cylindrical clip spring consisting of a spring and a dimple integrally and the rigidity adjusting means of the cylindrical clip spring to support the fuel rod even at a small spring displacement, thereby loosening the elasticity of the spring by heat or irradiation and The loss of bearing capacity of the spring as a cause was minimized.

In addition, the spring according to the present invention was configured to integrally constitute the spring and the dimple to ensure the maximum support length within the longitudinal length of the limited support grid.

In addition, the present invention does not have an independent dimple to minimize fuel rod wear due to the rotational deformation of the dimple.

Claims (3)

A support grid having nuclear fuel rods loaded and supported in a plurality of unit grid cells partitioned by inserting slots formed at regular intervals into the strips, the strips being arranged at regular intervals vertically and horizontally, A spring accommodating part formed by cutting each of the centers of the metal plates corresponding to the intersection points of the support lattice crossing longitudinally and horizontally in a square; Cylindrical clip-shaped spring is formed in a hollow cylindrical shape so as to be accommodated in the spring accommodating portion and cut a portion of the outer circumferential surface along the axial direction; A support grid with a cylindrical clip-like spring and dimples for preventing fuel rod fretting wear. The method of claim 1, The cylindrical clip spring is a support grid having a cylindrical integral spring and dimples for preventing fuel rod fretting wear, characterized in that it has a fixing groove in the upper and lower portions so as to be fixed to the support grid. The method according to claim 1 or 2, The cylindrical clip spring is a support grid with a cylindrical clip-shaped integral spring and dimples for preventing fuel rod fretting wear, characterized in that the rigid adjustment slot formed by cutting a portion of the outer peripheral surface.

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