RU2249865C1 - Spacer grid for fuel assembly - Google Patents

Abstract

FIELD: nuclear engineering.

SUBSTANCE: proposed spacer grid designed for assembling fuel rods in triangular arrangement primarily used in type VVER reactors has encircling band and three groups of relatively intersecting strips arranged in parallel in each group and provided with slits at points of intersection, as well as with alignment stops disposed between adjacent alternate slits. Intersection of strips forms subchannels for receiving fuel elements and bonding elements. Elements are shaped as equilateral triangle in cross-sectional area and look like figure with convex sides whose centers of curvature radii are disposed on lines crossing intersection point of strips and axis of fuel element engageable with this side. Alignment stops on each strip are made in the form of bends. In addition mixing blades are provided on at least two groups of strips and unidirectionally positioned in one group of strips. Bends provided on each strip between adjacent alternate slits make it possible to obtain bonding elements of different type at strip intersection point thereby enabling formation of shaped subchannel with alignment stops of sufficient elasticity margin (elastic displacement margin or stiffness coefficient).

EFFECT: enhanced precision of fuel element alignment and stiffness of its locking in subchannel.

5 cl, 13 dwg

Description

The present invention relates to the field of nuclear engineering, namely to spacer devices of a fuel assembly (FA) with triangular packing of rods - fuel elements (fuel elements), and can be used in WWER reactors.

A known design of a spacer grid for assembly with a triangular packing of fuel rods, comprising three groups of strips located at an angle to each other and forming rows of cells of a triangular shape when they intersect to accommodate the fuel rods. The strips are aligned with each other with the help of slots and fastened at the intersection, and are also welded to the rim encircling them (see French application No. 25884109 A1, class G 21 C 3/32, publ. 10.04.87).

The fuel elements are centered in the cell by four rigid stops located at the upper and lower edges of each of two adjacent cell walls, and one elastic element in the form of a hydraulic spring located on the third cell wall. During the flow around the profile of such a spring, the pressure of the spring on the surface of the fuel element increases.

The disadvantages of this design of the lattice are the complexity of its manufacture and the insufficient operational reliability of the fuel assembly. This is due to the poor performance of hydraulic springs during vibration of fuel assemblies and the difficulty of manufacturing strips with such springs. The stock of elastic movements of these springs depends on the elasticity of the jumpers in the form of narrow strips connecting the springs to the side walls of the cells, and the flow rate of the coolant.

Known spacer grid for fuel assemblies with a triangular packing of fuel rods, containing a rim covering three groups of strips mutually intersecting at an angle of 120 ° to each other, parallel to each other in each group, equipped with slots at the intersection and centering elements located between adjacent slots with alternation through one, forming at the intersection of a hexagonal cell for the placement of fuel elements and their connecting elements with a cross section in the form of an equilateral triangle, mixing blades (see UK application No. 2277191 A, class G 21 C 3/352, publ. 10/19/94).

The technical solution for this application is chosen for the prototype, since it is the closest to the proposed technical essence and the achieved result.

The alignment of the fuel rod in the cell is carried out by three centering elements located at an angle of 120 ° to each other. Moreover, two of them are hard knuckles (half-ellipsoidal protrusions), and the third element is a loop spring located between two special parallel slots on the middle part of the strip surface.

The mixing blades are made on the upper edges of only one group of parallel strips and are bent towards the connecting triangular elements. In this case, the blades located on adjacent strips and bent towards the connecting triangular elements with a common vertex and two sides, have the opposite orientation.

The disadvantage of such a lattice is the complexity of its manufacture, due to the complexity of stamping strips with springs and the insufficient operational reliability of the fuel assemblies, due to the poor performance of these springs both during vibration of the fuel assemblies during the long-term operation of the reactor, and during technological operations with fuel assemblies. The stock of elastic displacements of the loop springs, and therefore the performance, depends on the elasticity of the jumpers in the form of narrow strips connecting the loop with the surface of the side walls of the cells. The low elasticity of such strips contributes to the appearance in the cell of backlash (gaps) between the surfaces of the fuel rod and the loop spring and subsequent mechanical damage to the surface of the fuel rod.

The insufficient operational reliability of the fuel assemblies is also due to the formation of mixing blades in the grill on only one group of strips, which limits the range of directions of action of the blades on the coolant flow for efficient mixing between the cells. Moreover, the mixing blades located on adjacent parallel strips and bent towards the connecting triangular elements with a common vertex and two lateral sides have the opposite orientation. Under the influence of these blades, on the cross section of the two connecting elements, oncoming short-range transverse flows of the coolant are formed, which mainly cause only its local mixing. This leads to insufficient alignment of the heat content and the temperature of the coolant over the entire passage section of the channel, and therefore to an insufficient margin in terms of critical power.

The technical task is to create the design of a spacer grid with an increased margin of elastic displacements of the centering stops and a more uniform distribution of coolant temperatures over the entire passage section of the channel.

The solution of this problem allows to increase the operational reliability of fuel assemblies and reduce the complexity of its manufacture.

The problem is solved due to the fact that in a spacing grid for assembly with a triangular packing of fuel rods containing a girdle rim, three groups of mutually intersecting and parallel to each other in each group of strips equipped with slots at the intersection, centering stops located between adjacent slots with alternation through one, and forming at the intersection of the cell for the placement of fuel rods and their connecting elements with a cross section in the form of an equilateral triangle, mixing blades, centering The support stops on each strip are made in the form of bends, forming connecting elements at the intersection of the strips, having in cross section a figure with convex sides, the centers of radius of curvature of which are located on the lines passing through the intersection of the strips and the axis of the fuel element interacting with this side. In addition, the mixing blades are made simultaneously on at least two groups of strips and are oriented in one group of strips in one direction, the center of radius of curvature of the convex sides of the connecting elements is located at the intersection of the strips, and additional protrusions are made on the convex sides of the connecting elements, for example, form of corrugation.

Performing bends on each strip between adjacent slots with alternating through one makes it possible to obtain connecting elements of another type when crossing the stripes, which makes it possible to form a figured cell with centering stops having a sufficient margin of elasticity (margin of elastic displacements - stiffness coefficient). This allows for more accurate centering and rigid fixation of the fuel rod in the cell compared to the prototype.

The stiffness of any geometric figure (contour) is determined by the stiffness coefficient C = P / V, where: P is the force, and V is the radial displacement.

A comparative calculation of stiffness coefficients for elements with the same thickness of the material used and their height, made in the form of continuous rings C _k , half rings C _pc and with a cross section in the form of a figure, with convex identical sides whose centers of radius of curvature lie at the vertices of this figure ( at the points of intersection of the bands), C _f shows:

_To C / C _f = 7.24 and C _nk = (1-0,7) × C _to

Therefore, the effect obtained when using such a bonding element, compared with the ring and half ring will be greater.

Based on the above calculation results, we can conclude that the connecting elements have a sufficient margin of elasticity and will be more efficient. The stock of elastic displacements of such elements depends only on the radius of curvature of their lateral sides. The location of the center of radius of curvature at the point of intersection of the strips (at the top of the figure with convex sides) allows you to get the most rigidity.

Thus, the proposed technical solution in comparison with the prototype increases the operational reliability of the fuel assemblies due to the reduction of mechanical damage to the surface of the fuel rods both during assembly of the fuel assemblies and in reactor conditions (with vibration of the fuel assemblies).

In addition, this embodiment reduces the degree of overlap of the passage section of the cell, which leads to a decrease in the hydraulic resistance of the lattice.

The placement of the mixing blades simultaneously on two or three groups of strips with one-side orientation on one group of strips allows to obtain extended transverse flows, which leads to more efficient mixing of the coolant between the cells in the passage section of the channel compared to the prototype.

The implementation of additional centering protrusions, for example, in the form of corrugations reduces the contact area of the convex sides (stops) and a fuel rod, which leads to a decrease in temperature and voltage at their contact points, and, consequently, to increase the reliability of fuel assemblies.

The complexity of manufacturing the lattice is reduced due to the exclusion of complex in the manufacture of the springs of the prototype.

The essence of the invention is illustrated by drawings, where:

figure 1 shows a top view (without mixing blades);

figure 2 shows a fragment of the lattice of figure 1;

figure 3 shows the connecting elements of the cells;

figure 4 shows a top view of the strip without mixing blades;

figure 5 shows a General view of the strip of figure 4;

figure 6 shows a General view of the strip with mixing blades;

Fig.7 shows a section bB strip 6;

on Fig shows a top view of a strip with protrusions;

figure 9 shows a General view of a strip with protrusions;

figure 10 shows a fragment of the lattice with mixing blades located on two groups of strips;

figure 11 shows the flow lines of the coolant in figure 10;

on Fig depicts a fragment of the lattice with mixing blades located on three groups of strips;

in Fig.13 shows the flow lines of the coolant in Fig.12.

The spacer grid for the fuel assembly with the triangular packing of the fuel rods is made in the form of cells 2 located inside the girdle rim 1, designed to accommodate the fuel rods 3, and connecting elements of two types 4 and 5. Cells 2 and connecting elements 4 and 5 are formed when they intersect at an angle of 120 ° to each other of three groups of parallel bands 6, 7 and 8.

Full or partial alignment of the strips 6, 7 and 8 with each other is provided by the slots 9. On each strip between the slots 9 with alternating through one slot (one flat section) made centering stops 10 in the form of bends. Performing bends allows you to get the connecting elements of two types 4 and 5 at the intersection of the bands (see figure 2 and figure 3). In cross section, element 4 has the form of a normal equilateral triangle, and element 5 is a figure with convex identical sides, the centers of radii of curvature of which lie on lines passing through the intersection points of the strips and the axis of the fuel elements interacting with these sides. Due to the connecting elements 5, made in the form of a figure with convex sides, cells 2 are formed with three elastic centering stops 10 for installing fuel rods 3. Mixing blades 11 are made on the strips, which are bent towards the connecting elements 4 by a given angle α relative to the flat surface of the strips . The mixing blades 11 are simultaneously made on at least two groups of strips. When using two groups of bands 5 and 6, the blades 11 are made on each of them through two bends. When using three groups of strips 5, 6 and 7, the blades 11 on strips 5 and 6 are made through four bends, and for strip 7 - through two bends. Within the same group of strips, the blades have the same orientation.

To form the specified cells 2, the bends are located at a distance L equal to the spacing of the fuel rods 3 (see FIG. 4) and have the same radius of curvature r, the value of which is determined from the condition for reliable spacing of the fuel rods 3. If necessary, the formation of larger cells designed for installation in the fuel assemblies of special guide pipes, bends in the places of the specified placement of these pipes do not.

Slots 9 are made with a given height (depth) h. They can be made only on one edge of the strip (upper or lower), depending on its position when aligned, for example 6 and 7. To align with them the strip 8 of the third set of slots 9 are made respectively on both the upper and lower edges.

If necessary, the contact area of the convex sides of the connecting elements 5 with the fuel rods can be reduced by forming additional centering protrusions 12 on them, for example, in the form of corrugations.

On the basis of the proposed base lattice (Fig. 1), by means of local reduction of the length and number of used strips, its modifications can be obtained.

The proposed spacer grid operates as follows.

Using spacer grids, fuel rods 3 are installed in the fuel assemblies at a given distance L from each other. The fuel rods 3 are installed in cells 2, in which they are centered and fixed due to bends (the convex sides of the connecting elements 5) to prevent them from bending and pushing out of their place during the entire operation of the fuel assembly.

In the active zone of a nuclear reactor, a fuel assembly is washed with a coolant, with the help of which the surface of the fuel rods 3 is cooled. Under the action of the heat carrier flux, the fuel rods 3 vibrate, which is damped due to a sufficient elastic reserve of the bends 10.

Under the action of the mixing blades 11 on the flow of coolant between the rows of fuel rods, extended transverse flows are formed, which cause effective mixing of the coolant between the cells. This leads to a more complete alignment of the coolant temperatures in the passage section of the channel, which leads to an increase in the margin in terms of the critical power and, consequently, the operational reliability of the fuel assemblies.

Claims (5)

1. A spacer grid for a fuel assembly with a triangular packing of fuel rods, containing a girdle rim, three groups of mutually intersecting and parallel to each other in each group of bands equipped with slots at the intersection, centering stops located between adjacent slots alternating through one, and forming at the intersection of the cell for the placement of fuel rods and their connecting elements with a cross-section in the form of an equilateral triangle, mixing blades, characterized in that the centering Frames on each strip are made in the form of bends, forming connecting elements at the intersection of the strips, having in cross section a figure with convex sides, the center of radius of curvature of which is located on a line passing through the intersection of the strips and the axis of the fuel element interacting with this side. 2. The spacer grid for the fuel assembly according to claim 1, characterized in that the mixing blades are made simultaneously on at least two groups of strips. 3. The spacer grid for the fuel assembly according to claim 1 or 2, characterized in that the mixing blades of one group of strips are oriented in one direction. 4. A spacer grid for a fuel assembly according to any one of claims 1 to 3, characterized in that the center of radius of curvature of the convex sides of the connecting elements is located at the intersection of the strips. 5. A spacer grid for the fuel assembly according to any one of claims 1 to 4, characterized in that additional protrusions are made on the convex sides of the connecting elements, for example, in the form of corrugations.

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