JPH0156715B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to fuel rod grid spacers that are assembled in a mesh pattern to form elongated hexagonal cross-sectional fuel assemblies.

Particularly in so-called improved nuclear reactors, such as fast breeder reactors and improved pressurized water reactors with a conversion ratio of less than 1, a spacer structure is required that provides relatively high accuracy in maintaining the spacing of the fuel rods. In particular, relatively high precision is required when the dimensions of the spacer must be increased to 20 cm or more depending on the cross section of the fuel assembly. The reason for the increased spacing accuracy of the fuel rods is that it is particularly important to have a relatively high packing density between the fuel rods and the intermediate chambers in which they are to be held in any case.

The grid spacers for such fuel rods are formed by a plurality of spacers distributed over the axial length of the fuel assembly, each of which is formed into a grid structure, each grid structure extending over a cross-section of the fuel assembly. the grid structure extends radially and has openings for the fuel rods, the grid structure further comprising a plurality of polygonal parts which are connected by central structural pins at inner corner positions. Such a spacer is known from DE 16 39 406, in particular from FIG. 3, which is connected by further structural pins at the outer corner positions. In this known spacer, reinforcing bridges arranged inside the grid structure run star-shaped from the center outwards. In the case of this lamellar grid spacer, the reinforcing bridges are connected to the respective parts of the grid structure. Therefore, when this spacer is observed from a plane, it is not formed into a homogeneous structure. A mass accumulation therefore occurs in the area of the reinforcing bridge inside the grid structure compared to the area of the grid structure. During a change in angle and therefore a change in temperature, this mass accumulation results in a different thermal expansion. As a result, bending and shear stresses can again occur in relatively thin fuel rods.

Due to their laminar construction, the known grid spacers described above pose various problems when it comes to accurately guiding closely packed fuel rods. Even if the tolerances are small, there is still the problem that thermal stresses must be avoided.

The grid spacer with a thin plate structure has a British patent no.
It is also known from Publication No. 962266. In this known grid construction, six sector-shaped parts are spot welded together, and the welded six
A circular grid spacer is formed by welding the parts and the support ring. The individual sectors are hereby formed by spot welding three cells.
Therefore, this thin plate-shaped spacer, which is formed by welding a plurality of components, lacks dimensional stability and also
The problem mentioned in the 1639406 publication is that it is not possible to obtain uniform thermal expansion in all directions of the cross section of the fuel assembly during operation.

By providing a spacer structure with a uniform coefficient of thermal expansion in all directions of the cross-section of the fuel assembly, the present invention achieves extremely small tolerances that are required despite the relatively large dimensions of the spacer. It is an object of the present invention to provide a grid spacer that ensures precise retention of fuel rods in the radial and axial directions of a fuel assembly.

According to the present invention, the grid structure of a single plate grid spacer is divided into six equilateral triangular parts, and these six parts are divided into two or three parts.
These two or three parts groups are arranged on two or three planes that are offset in the axial direction of the fuel assembly, and in this case, one parts group arranged on one plane. at least 2 of
parts are arranged at a distance from each other in the radial direction of the fuel assembly, and the at least two parts are arranged in two or three planes in a fuel assembly. It is arranged around a central structural pin so as to cover the entire cross-sectional area of the body, and two adjacent parts within the cross-sectional area are connected to each other by a common structural pin at an outer corner position. achieved by.

The effect of the spacer structure according to the invention is that two seemingly mutually exclusive demands are met, one of which is a greater precision in maintaining the spacing of the fuel rods, not only in the radial direction but also in the axial direction. is obtained, and the second has the effect that the volume or effective cross section of the spacer is neither increased nor decreased. By dividing the cross-section of the fuel assembly according to the invention, small unit parts are produced which can be easily machined with relatively high machining accuracy. This small component is then made smaller both in the radial and axial directions. Moreover, such units can be formed thinner than a single plate extending over the entire cross-section of the fuel assembly without compromising the stability of the fuel assembly, and therefore are better formed in terms of radiation and flow technology. can do.

Also, reinforcing the edges of each component always results in greater stiffness with good neutron economy. For this reinforcement, when the opening of the fuel rod has a polygonal cross-section, the edge has a rounded polygonal cross-section.

Next, the present invention will be explained with reference to the illustrated embodiments.

In FIG. 1, a spacer of a fuel assembly BE having a hexagonal cross section of a water-cooled high conversion reactor is designated as a whole by the reference numeral 1. In that case, due to the small fuel rod spacing,
Extreme precision is required to secure the fuel rods, which are approximately 2 meters long.

A plate-shaped spacer 1 is composed of six parts 2 of the same shape, and the positions of these parts 2 are aligned with the plane E1.
Then, , is added sequentially in the direction of the clock hands,
In the plane E2, the numbers ',',' are similarly given in sequence (see FIG. 4). Each part 2 is equilaterally triangular and has 112 openings 3 for fuel rods (not shown) corresponding to the desired mesh pattern of the fuel rods. These openings 3 are likewise of hexagonal cross-section, as is clear from FIG. Each opening 3 has three protrusions 4 spaced equally apart by 120 degrees. This protrusion 4 has the function of guiding fuel rods 5 arranged as shown by the dashed line.

Each component 2 is arranged as two component groups as described above. That is, the parts 2 located at the positions , are arranged on the plane E1, and the parts 2 at the positions ',',' are arranged on the plane E2, which is offset from the plane E1 in the longitudinal direction of the fuel assembly. Therefore, part 2 of one part group at position , .
and the parts 2 of the other part group at positions ',',' will be arranged in two planes running at right angles to the longitudinal axis of the fuel assembly. Each Roman numeral (,',,',,') designates a component 2 at a position offset by 60° about the axis m of the central structural pin 15. Each part 2 is pierced at its central corner position 8 by a structural pin 15 (see FIG. 3). Therefore 2
The six parts 2 in two planes E1, E2 are connected to each other in a pairwise manner at the central corner position 8 by means of structural pins 15. In a further outer corner position 7, the connection between the part 2 in position 1 in the first plane E1 and the part 2 in position I' in the second plane E2 is effected by a further structural pin 7'. Similarly, the parts 2 located respectively in the plane E1 and the plane E2 in the angular position 6 are connected by structural pins 6'.

As is clear from FIGS. 1 and 4, the grid structure of the spacer 1 has three triangular shapes spaced apart from one another in the radial direction.
two planes E1, E2 running at right angles to the axial direction, in which the parts 2 are axially displaced from each other;
In this case, the central structural pin 1 is arranged so that the three parts 2 in the two parts groups cover the cross section of the fuel assembly BE.
They are arranged offset from each other with 5 at the center. Further, the first parts group is located at the position of one plane E1, , and the position of the other plane E2 is ',
The second group of parts located at ' and ' are connected by common structural pins 6' and 7' in the respective outer corner positions 6 and 7. Each six-part grid structure of the spacer 1 consists of a plate with a number of openings 3 for the fuel rods 5 (see also
figure).

The rings 10, 11 of the spacer part 2, which act as interlocking parts, extend over the entire thickness D of the part 2 in the longitudinal direction of the fuel assembly BE. On the other hand, the height of the ring 12 at the central corner position 8 is 1/3 of the thickness D, as can be seen from FIG. A structural pin 15, which is provided as the axis of the pair for centering purposes, supports the bush 16. This bush 16 positions the spacer 1 in the longitudinal direction of the fuel assembly BE.

According to the enlarged plan view in FIG. 2, the edge 18 of the spacer part 2 in position 1 is reinforced, so that its outer contour does not run parallel to the inner corner of the hexagonal opening 3. It is chamfered with a roundness of 20. This mechanical reinforcement has the advantage that it is easy to manufacture and provides the desired secure and precise retention of the fuel rods 5 in the illustrated hexagonal cross-section of 25 cm on a side.

[Brief explanation of drawings]

FIG. 1 is a plan view of a spacer according to the present invention, FIG.
The figure is an enlarged plan view of a part of Figure 1, Figure 3 is a vertical cross-sectional view taken along line A-B in Figure 1, and Figure 4 is a partially enlarged plan view of Figure 1.
FIG. 2 is a schematic perspective view of the figure; 1... Spacer, 2... Spacer parts, 3...
...Opening, 4...Protrusion, 5...Fuel rod, 6,7...
...Outer corner position, 8...Central corner position, 6', 7', 1
5... Structural pin, 10, 11, 12... Ring,
16...Button, 18...Edge of part 2, 20
...Roundness,,,,',','...Position of each part 2, BE...Fuel assembly, E1...Plane of part 2 at position ~, E2...Position '~
’ plane of part 2, D...thickness of part 2,
m... Axis of the central structural pin 15.

Claims (1)

[Claims] 1. In a grid spacer for maintaining the spacing between fuel rods that are combined in a mesh pattern and formed into a long hexagonal cross-sectional fuel assembly, a grid structure of a single plate-shaped grid spacer is divided into six equilateral triangle-shaped parts 2, these six parts 2 are divided into two or three parts groups, and these two or three parts groups are each shifted in the axial direction of the fuel assembly. at least two parts 2 of a group of parts arranged in one plane are arranged at a distance from each other in the radial direction of the fuel assembly;
Furthermore, the at least two parts are arranged around the central structural pin 15 such that the two or three parts arranged in two or three planes cover the entire cross-section of the fuel assembly. A grid spacer for a fuel rod, characterized in that the two parts adjacent in the cross section are connected to each other by a common structural pin at an outer corner position. 2. Grid spacer according to claim 1, characterized in that the edge 18 of the part 2 is reinforced.