DE29809271U1 - Spacer for a fuel assembly - Google Patents

Description

GR 98 G 3359

Description

Spacer for a fuel element

The invention relates to a spacer for a fuel element of a nuclear reactor with an outer web in which inner webs are fastened.

Reactor core components made of zirconium alloys grow or shrink when irradiated with neutrons. These changes occur depending on the texture factor. An example of such nuclear reactor components is a spacer for a fuel element. Typical sheets, such as those used for spacers in nuclear reactors, especially boiling water reactors, grow in the direction of the longitudinal axis of the webs (i.e. across the fuel element), which can also be caused by other factors. This means that the external dimensions of such a spacer increase depending on the length of time it is used in the reactor core. When arranging fuel element bundles in square, closed boxes, the boxes must therefore have a corresponding excess of their clear width during production.

However, within the square formed by the inner walls of the box, the fuel rod bundle arranged therein can be arranged asymmetrically. The sub-channels and thus the distance to the boiling transition power of the edge fuel rods differ considerably in this case. Such a fuel element is thermohydraulically penalized in the first cycles, which means that full operation is not possible. In practice, compromises have therefore been made between the selected material properties and the intended dimensioning or centering.

GR 98 G 3359

Similar effects also occur in pressurized water reactors:

The fuel elements are arranged close to one another, with the spacers of adjacent fuel elements supporting one another via spacer elements (previously rigid studs formed from the webs), while the feet of the fuel elements are fixed to a core grid plate. The core grid plate changes its dimensions differently when heated or irradiated than the spacers, which can change the gap width between the fuel elements.

The invention is based on the object of specifying a spacer for a fuel element in which operating restrictions that are due to expansion-related geometric changes, in particular asymmetries, are prevented.

The object is achieved according to the invention with a spacer for a fuel element of a nuclear reactor with an outer web to which inner webs are attached, the outer web having a spring element at at least one attachment point to the respective inner web for generating a tension force between a box that can be arranged around the outer web and the outer web, and advantageously several spring elements are provided in such a way that the respective inner webs are arranged symmetrically in the box. In this way, the respective inner webs are held resiliently in the box, which provides automatic centering. Preferably, the spring elements are arranged symmetrically on the circumference of the outer web.

The distance between the spacer and the box is therefore ensured by a ■ spring element that is arranged at the point on the outer web where the end of an inner web is also attached. This end of the inner web can therefore be used to limit the movement of the spring element. When inserting the fuel element into the box,

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then the spring element cannot be bent to such an extent that it becomes inoperable; in addition, this way it can be ensured that the inner webs can expand in the longitudinal direction (i.e. in the plane of the spacer, ie perpendicular to the longitudinal axis of the fuel element), but a minimum distance from the box is guaranteed.

In a similar way, the distance between the outer web and an adjacent component is generally ensured (e.g. in the case of pressurized water fuel elements, which do not have a box but whose spacers must be kept at a specified minimum distance from one another). According to the invention, it is provided that the outer web carries a spring element at the point at which the end of an inner web is attached to it, which spring element is curved out on the side facing away from the inner web (the outside).

The spring element is advantageously curved in a W shape, which results in favorable spring forces. Preferably, the end of the inner web attached to the outer web carries a profile (e.g. a window), and only part of the profile is attached to the outer web, while the other part of the profile serves to limit an inward movement of the spring element (i.e. in the direction of the inner web).

These and other embodiments of the invention are set out in the claims.

Embodiments of the invention, further advantages and details are explained in more detail below with reference to the drawings. They show:

FIG 1 shows a schematic diagram of a fuel element in cross section with spring elements aligned parallel to the fuel rods;

GR 98 G 3359

FIGS 2 to 6 Detail A from Figure 1 in different representations;

FIG 7 shows a configuration corresponding to Figures 1 to 6.

. guide form with a in the outer web a

suspended spring element;

FIGS 8 and 9 embodiments with spring elements that are aligned perpendicular to the fuel rods; and

FIG 10 and 11 Different arrangements of the spring elements on the

Outer bridge. ·'

. Corresponding parts are provided with the same reference symbols in all figures.

Figure 1 shows a fuel element 1 with a box 3 in which inner webs 5 are arranged in a checkerboard pattern. The inner webs 5 are surrounded by an outer web 7. Fuel rods can be inserted into the spaces formed by the inner webs 5. For example, a fuel rod 9 is arranged in one of the spaces. Studs and springs that center the fuel rod within the grid meshes 10 are provided, but not shown.

Such an arrangement of outer web 7 with inner webs 5 is also referred to as a spacer for fuel elements. The components of such a spacer are usually made of zirconium alloys or Inkonel. When irradiated with neutrons, the components change their dimensions in the direction of the arrows shown, depending on the texture factor of the alloy and/or other influences. In order to ensure a predetermined placement or centering of the fuel rods, it is provided here that the inner webs 5

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by spring elements. 11 are centered. The spring elements

11 are arranged on the outer web 7 and support each other
the box 3 of the fuel element 1.

The spring elements 11 can be part of the outer web 7 (designed as a structural unit) or as
elements. If necessary, in the case of a two-piece
In this version, only a loose mounting of the spring elements 11 on the outer web 7 is provided.

Below we will look at detail A in more detail, which
shows, by way of example, a contact point between an inner web 5 and the outer web 7 in more detail. In this case, reference is made to all figures 1 to 6 at the same time.

The embodiment according to Figures 2 to 6 shows a
Versions in which the spring element 11 forms a structural unit with the outer web 7. The spring element 11 is formed from the material of the outer web 7 (Zircaloy). The spring

The element 11 is designed as a W-shaped leaf spring, which is inserted at both its longitudinal ends into the outer web 7
Its central elevation 13 extends beyond the outer web 7 of the spacer and is used for contacting the box 3, which is not shown in Figures 2 to 6.

The two inner elevations 15 forming the W-shape extend into the interior of the outer web 7
and protrude into respective recesses 17 in
the respective inner webs 5. The recesses 17 can also be described as U- or V-shaped. In Figure 3 it can be seen that the inner elevations 15 are inserted into the respective recesses with a predetermined clearance or distance.
17. The elevations 15 cannot be bent further out of the outer web 15 and into the recesses than the recesses allow, which thus limit the movement of the spring element.

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Figure 4 shows a spring element 11 in a top view. Figures 5 and 6 each show a spring element 11 in the outer web 7 with an associated inner web 5 in a spatial representation. Figures 3 and 2 show the detail A of Figure 1 in a top view and the spring element 11 in a side view.

The play specified between spring element 11 and inner web 5 depends on the expected growth of the spacer. If the spacer, in particular the respective inner webs 5, grows, the spring elements 11 or their movement are limited by the externally surrounding box 3, whereby the play between spring element 11 and inner web 5 is reduced. In relative terms, the inner elevations 15 are pressed inwards into the interior of the outer web 7. At the end of the service life of such a spacer, the specified play is almost used up. During service measures (re-boxing) ■ the spring elements 11 cannot be inadvertently bent in such a way that they are permanently deformed. Even if the elasticity decreases and the centering effected by the spring elements 11 is deteriorated after a few cycles, there is initially a considerable advantage over a poorly centered, asymmetrical arrangement, and the lack of centering only occurs when the power of the fuel element 1 has already been reduced by burnout.

The embodiment shown is particularly characterized by a very favorable threading and sliding property of the spring elements 11, since these have no protruding ends and are connected to the outer web 7 on both sides. A particularly simple production is also provided, since the spring elements 11 are designed as a component within the outer web 7.

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It is also conceivable that the spring elements are installed as separate

Component 21 is held on the outer web 22 (Figure 7).

For this purpose, the elongated spring part has lateral tabs 25, 26 at each of its two ends 23, 24, which are bent in a hook shape. For assembly, the spring part is first stretched, whereby the distance between the hook-shaped tabs 25, 26 becomes larger and these hooks can be placed around the upper edge and lower edge of the outer web 22, whereby the W-shaped middle part, which is aligned from top to bottom (i.e. parallel to the fuel rods), is placed in a recess 27 in the outer web. When the spring part is relaxed, the tabs 25, 26 enclose the edges of the outer web 22 in a claw-like manner and leave fastening points 28 free on the inside of the outer web for the end of the inner web. This end has a profile 29 which forms a window with the two recesses 29a into which the corresponding curvatures of the spring part 21 engage. At the top and bottom, the profile 29 forms edges 29b which can be welded or soldered at the points 28.

The key to the new idea is that the outer web 7 is elastically pressed against the box wall or a similar component adjacent to the web and that a certain minimum value is specified for the distance by an inner web, even if the spring force decreases or an excessive load occurs.

This minimum distance is designated d in Figure 8 and is provided by a protruding profile part 30 at the end of an inner web 31. The spring element is formed from the outer web 33 in the form of a band 32 perpendicular to the fuel rods, which protrudes with a curvature over the side facing away from the inner web, i.e. over the outside of the spacer and in the direction of the box wall 34

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(or the other component from which the distance is to be kept). While the inner web 31 is attached to the straight parts of the outer web at the top and bottom, e.g. via a weld seam 35, the curvature of the band 32 creates a recess in the outer web 33 through which the protruding profile part 30 protrudes.

While the band 32 (spring element) in Figure 8 shows only a single (convex) curvature, the corresponding band 42 (spring element) in Figure 9 is curved in a W shape, similar to Figures 2 to 6. However, this band 42 runs perpendicular to the fuel rods, i.e. rotated by 90° compared to Figures 2 to 6.

The spring elements ensure that the spacer is mounted symmetrically in the box. For this purpose, the spring elements 50 (Figure 10) are preferably arranged symmetrically on the outer web in such a way that the left half of the spacer is symmetrical to the right half and the front half is symmetrical to the rear half.

Figure 11 shows that such lines of thought are also useful for fuel elements 60, 61 arranged next to one another in pressurized water reactors. Here, the spring elements of adjacent fuel elements can lie directly against one another; in the case shown, two spring elements 62, 63 are arranged between two fuel element corners at slightly different distances from the corners. However, over the entire circumference of the fuel element 60, the spring elements are distributed rotationally symmetrically in such a way that their positions merge into one another after a rotation of 90°. Between two adjacent fuel elements 60, 61 and their outer webs 66, 67, the spring elements 62, 63 of one fuel element and the corresponding spring elements 64, 65 of the other fuel element therefore maintain a distance, with the ends of the inner webs

ensure that this distance does not fall below a minimum value.

Claims (8)

GR 98 G 3359 .. .··..·' • · ·■ w - - • · &iacgr;&ogr; Protection claims 1. Spacer for a fuel element (1) of a nuclear reactor with an outer web (7) to which inner webs (5) are fastened, wherein the outer web (7) has a spring element (11) at at least one fastening point of an inner web (5) for generating a tension force between a box (3) that can be arranged around the outer web and the outer web (7). 2. Spacer according to claim 1, wherein several of the spring elements (11) are arranged on the outer web such that the respective inner webs (5) are arranged symmetrically in the box (3). 3. Spacer for a fuel element of a nuclear reactor, with an outer web to which inner webs are attached, wherein the outer web carries a spring element at at least one attachment point of an inner web, which is curved out on the outside of the outer web facing away from the inner web. 4. Spacer according to one of claims 1 to 3, characterized in that the spring element is curved in a w-shape. 5. Spacer according to one of claims 1 to 4, characterized in that the end of the inner web attached to the outer web carries a profile and is attached to the outer web with only a part of the profile, while another part of the profile limits a movement of the spring element directed towards the inner web. 6. Spacer according to claim 5, characterized in that the profile is on the inside of the outer GR 98 G 3359 .. . _# ··, 11
bridge forms a window into which a on the inside of the The curvature of the spring element protrudes from the outer web 7. Spacer according to one of claims 1 to 6, characterized in that the spring element is a part of the outer web that is formed from the outer web. 8. Spacer according to one of claims 1 to 7, wherein the spring elements (11) are arranged symmetrically on the circumference of the outer web (7).