WO1996010829A1

WO1996010829A1 - Warp-poor fuel element for pressurised water reactors

Info

Publication number: WO1996010829A1
Application number: PCT/DE1995/001285
Authority: WO
Inventors: Jürgen STABEL-WEINHEIMER
Original assignee: Siemens Aktiengesellschaft
Priority date: 1994-09-30
Filing date: 1995-09-18
Publication date: 1996-04-11
Also published as: DE29521378U1

Abstract

The fuel rods are laterally supported at the top and/or bottom in the meshes of a spacer by supporting elements (40, 41) that hold the fuel rods in a common horizontal plane (A-A). The end of a warped fuel rod can thus exert no overturning moment on the spacer capable of warping the guiding pipes secured to the spacer. In addition, fretting is avoided, since the warped fuel rod can rotate in said plane in the event of vibrations without rubbing against other supporting elements.

Description

description

Low bending fuel element of a pressurized water reactor

The invention is based on a fuel element, the structure of which is shown in principle in FIG. 1 in the case of pressurized water reactors is the customary prior art.

At its upper end it carries a head part 1 with a frame 2, which is attached to inner struts 3, for example, via spring struts

Superstructures of the reactor are supported and a plate 4 carries. The foot part 5 also has a corresponding frame, which stands with a feed connector on a support structure in the reactor core and carries a base plate 6. The base plate 6 is connected to the plate 4 via a plurality of guide tubes 7, which can be seen in the left part of FIG. 1, so that a support structure is formed which is open at the side, that is to say is not delimited by side walls (fuel element boxes with such side surfaces would be typical of fuel elements from Boiling water reactors).

A bundle of fuel rods 8 is arranged between the base plate 6 and the cover plate 4 and ends below the cover plate 4. These fuel rods 8 arranged around the guide tubes 7 can be seen in the right part of FIG. 1, while the fuel rods are not shown in the left part for better illustration.

The supporting structure also includes a plurality of lattice-shaped spacers 9 which are axially distributed over the length of the fuel element,

10, which are held on the guide tubes, for example by sleeves 11, which are fastened at some guide tube positions in the corresponding meshes of the spacers and which engage the guide tube running through these meshes. In addition, additional grids 12 can be provided at axial intermediate positions, which serve as supports for flow control surfaces (“mixing blades” 13) in order to mix and swirl the cooling water flowing between the fuel rods to increase the cooling effect.

Each fuel rod is laterally supported in a mesh of an uppermost spacer (spacer 9) and in one mesh of several spacers underneath (for example spacer 10) by support elements.

Figure 2 shows a cross section, Figure 3 shows a longitudinal section through a typical support mesh, that is, a grid mesh with support elements, which in this case are arranged equidistantly distributed over the circumference of the mesh. In this case, which is practically described in US Pat. No. 4,735,769, two springs 20, 21 made of Inconell are provided as support elements, which are arranged on abutting sides of the square meshes and approximately at a middle height of the spacer press the lying center plane AA with a spring force F onto the fuel rod 23 seated in this supporting mesh 22. A rigid support element, namely a pair of knobs 24, 25 and 26, 27, is arranged on each mesh side opposite this spring. The two knobs of a pair lie one above the other, so that the lower knobs 24, 26 in the plane BB (ie below the central plane AA), but the knobs 24, 27 in the plane CC (ie above the central plane AA) with one Press the contact force, which corresponds to half the spring force F, onto the fuel rod.

A similar construction is shown in FIG. 4 and FIG. 5, in which, however, the two springs 20, 21 of FIG. 2 are replaced by a diagonal spring 30 directed diagonally from a mesh corner onto the fuel rod, as described in US Pat. No. 5,311,564 is. The support elements (spring 30 and the corresponding pair of nubs 31, 32; 33, 34) are in this case mirror-symmetrical to the diagonal D above distributed and formed the circumference of the stitch 35 with its fuel rod 36.

Since the fuel rod is held in the appropriate mesh in several levels one above the other, the

Fuel rod in the respective spacer meshes is held practically perpendicular to the central plane A-A and thus practically parallel to the guide tubes.

The foot 5 (FIG. 1) carrying the entire weight of the fuel assembly rests immovably on a core grid plate. If the guide tubes are sufficiently straight and rigid, it can be assumed that the guide tubes 7 are fixed in the plane Aa-Aa and the lowest spacer 10 is aligned parallel to the plane Aa-Aa. In the non-irradiated state, all fuel rods are then also aligned vertically to a sufficient extent and all spacers are aligned horizontally, even if the fuel element heads 1 are held by the spring struts 3 only with a lower degree of rigidity, that is to say are easier to move in the plane Bb-Bb.

In the nuclear reactor, the fuel elements are exposed to intense neutron radiation during reactor operation. However, the neutron flow is not spatially distributed homogeneously over the fuel elements and is also not constant over time. On the other hand, the heat development (power) of the fuel rods is directly dependent on the neutron flux, so that the fuel rods heat up unevenly. This results in an uneven thermal expansion of the individual rod and the rods among one another, to which is added that the alloy (in general Zircaloy), from which the cladding tubes of the fuel rods consist, changes under irradiation, which changes leads to an inhomogeneous growth of the rods. Therefore, the fuel rods deform and often expand more on one side of the fuel rod bundle than on the opposite side. Forces are transmitted to the spacers via the non-positive mounting of the fuel rods, which also bends the guide tubes and thus the entire fuel elements.

Bent fuel assemblies are difficult to handle. For example, when changing fuel assemblies, it is then difficult to remove individual fuel assemblies from the reactor core, in which these fuel assemblies sit next to each other with a few millimeters spacing. In addition, these bends create uneven gaps between the individual fuel elements. These gaps are filled with cooling liquid, which influences the neutron flow both as a moderator and, as a result of the cooling of the fuel rods, the temperature-power distribution. Such effects are therefore not only troublesome because they make handling of the fuel elements more difficult, but must also be taken into account in the hydraulic and radiological design of the core.

This is particularly disruptive in the case of modern fuel elements which are designed with a view to high burn-up, higher operating temperatures and better use of the fuel.

There is therefore a growing need to avoid fuel element bending. It has already been proposed to reinforce the support structure of the fuel assemblies by using more stable control rod guide tubes or by replacing some fuel rods with additional structure tubes. Attempts are also being made to align the rods by tightening them more tightly in the spacers, for example by strengthening the connecting elements between the spacers and guide tubes and / or at least at the points where special bends are expected to be made of steel or alloys based on nickel. Special designs of spacers have also already been proposed, in which a special material combination (for example Zirkaloy and steel) enables a bimetal Effect should be exerted on the fuel rods in order to counteract the bends.

With increasing bending of the fuel element, the risk of so-called "fretting damage" - especially at the lowest spacer - seems to increase. This involves damage to the fuel rod cladding tube which results from friction corrosion between the fuel rods and the support elements when the fuel rods, which are set into considerable transverse vibrations by the high flow rate of the coolant, do not sit firmly between the support elements, but rub against them.

The invention is therefore based on the object of taking constructive measures to prevent the fuel elements from bending excessively during reactor operation and / or to avoid the risk of fretting damage at the lowest spacer of the fuel elements that are bent.

The invention provides a fuel assembly in which a base plate and a cover plate are connected to one another by guide tubes and form a laterally open support structure, with a plurality of grids approximately perpendicular to it being held at an axial distance from one another on the guide tubes. There is a between the base plate and the cover plate

Bundles of fuel rods are arranged, each of which passes through the mesh of the grids and the upper ends of which are not (or at least not rigidly) held on the cover plate and / or lower ends on the base plate. Several of the stitches lying one above the other, each penetrated by the same fuel rod, are designed as support meshes which have at least three support elements distributed approximately equidistantly (or at least approximately mirror-image to a diagonal of the support mesh) along the circumference of the fuel rod. These support elements support the

Remove the fuel rod from the side of the grid bars of the supporting mesh. To achieve the object, the top and / or bottom support mesh of this fuel rod only contain support elements which act practically in one plane on the fuel rod.

Advantageous developments of the invention are characterized in the subclaims and are explained in more detail with reference to ten figures and two exemplary embodiments. Show it:

FIG. 1 shows the side view of a fuel element for a pressurized water reactor already explained,

2 and FIG. 3 show an already described construction of a support mesh in longitudinal and cross section according to the prior art, which is also advantageous as one of the central support meshes of the fuel element according to the invention,

4 and FIG. 5 an already described, different structure of such a mesh according to the prior art and for use with the middle spacers of the fuel element according to the invention,

FIG. 6 shows the support mesh already shown in FIG. 3 when the fuel rod is bent,

FIG. 7 shows an upper or lower support mesh of a fuel rod designed according to the invention,

FIG. 8 a corresponding support mesh containing only resilient support elements,

9 and 10 show a cross-section and a longitudinal section through a support mesh containing only rigid support elements, and

Figure 11 and Figure 12 is a cross and longitudinal section through circular support mesh. The invention assumes that can be represented as a superposition of three changes in geometry compared to its unflexed normal position in the plane of the top most spacer ^¬ the bending of a fuel rod. The increase in length leads to a displacement of the fuel rod end in the direction of the fuel element axis, that is to say perpendicular to the normal position of the spacer, * the lateral deflection of the end of a bent fuel rod corresponds to a displacement in the spacer plane, * due to the curvature of the fuel rod creates an oblique position, that is, an angle between the axis of the fuel rod on the one hand and the axis of the non-bent fuel element or the non-bent guide tube. Since the guide tubes are practically always connected orthogonally to the grids forming the supporting mesh, for example the spacers of FIG. 1, via the sleeves 11 (FIG. 1) and form the support structure of the fuel assembly, these different geometrical changes also have different effects on this support structure and the geometry of the entire fuel assembly.

When the length increases, the section of the fuel rod held by the support elements of a support mesh shifts in the vertical direction, that is, along the support elements, a friction force occurring which, once the static friction has been overcome, is proportional to the growth speed. The friction forces occurring in the individual support meshes add up in the entire grid (spacer), but they can be neglected (at least if the support meshes each contain only one diagonal spring 30 according to FIG. 4 and not two or more springs with correspondingly increased static friction) become. The fuel rods are not hindered in their length growth by a rigid clamping at both ends, so that there is also no fear of the fuel element bending.

The lateral (horizontal) displacement of the fuel rods leads to a force F (FIG. 3) of the support elements Force on the grid, which can be shifted parallel to the plane Bb-Bb (FIG. 1), for example in the case of the uppermost spacer 9. This shift is undesirable, but proves to be largely tolerable, but only assumes a largely tolerable extent, as long as no forces arise due to the inclination of the fuel rods, which further intensify the bending of the fuel element. The bottom spacer 10 is practically not moved anyway, since this can be regarded as rigidly fixed, as has already been explained.

However, the inclination of the fuel rod in the supporting mesh leads to the fuel rod exerting a "tilting moment" as explained below. In the vertical orientation of the fuel rod shown in FIG. 3, the force F of the spring 30 is absorbed in equal parts by the upper knobs 25, 27 and the lower knobs 24, 26; with a strong inclination, however, the upper knobs 25, 27, for example, no longer touch the fuel rod at all, and the lower knobs 24, 26 must absorb the entire force F. The support elements 20, 24 and 26 therefore exert a torque on the oblique fuel rod, which bends it back into the vertical normal position and has the amount F x R. This is desirable per se and an advantageous variant of the invention therefore provides support meshes for a plurality of grids lying between the uppermost and the lowest spacers, in which the support elements act on the fuel rod in different planes (ie R ≠ 0). The lowest support mesh can also be designed in this way, unless other reasons, which are explained below, lead to a different configuration. The "bending back" of the fuel rod and the corresponding torque are, however, associated with reaction forces on the supporting mesh, which can thereby be deflected out of its horizontal normal position, ie "tilted". In the grid, these moments occurring in the various support meshes add up to a total tilting moment and result in the guide tubes on which the grids (spacers) are fixed orthogonally, also tilted, i.e. bent from their vertical normal position.

This bending of the guide tubes acts particularly in the case of the spacer 9 carrying the uppermost support mesh, the position of which is not fixed with respect to the plane Bb-Bb by the relatively elastically mounted fuel element head. The guide tubes are therefore bent, starting from their upper end, and thus also deflect the grilles underneath from their horizontal normal position. This therefore leads to an inclined position of the support meshes in the grilles below, so that these support meshes now exert a bending moment on the fuel rod parts supported in them until these fuel rod parts also perpendicular to the support meshes in accordance with the force distribution shown in FIG the tilted grids stand and are bent parallel to the bent guide tubes.

The bending moments and tilting moments in the uppermost support mesh can be avoided if all the support elements of this

Attack stitches in the same plane A-A, i.e. the axial distance R of the contact surfaces on which the fuel rod 23

(Figure 3) or 36 (Figure 5) is supported, is made zero.

This results in the construction of the uppermost support mesh according to the invention.

For this construction of the uppermost support mesh, first calculations can be expected that the bending forces exerted on the entire fuel rod can be reduced by about half compared to the conventional clamping of the fuel rod. This makes it possible to design the fuel assemblies for higher erosion, higher operating temperatures and other properties to improve performance, without having to take particular account of changes caused by bending of the fuel assembly. Possible simple additions Additional measures, for example a slight reinforcement of the guide tube, are therefore often unnecessary.

The "fretting" mentioned at the beginning is most likely to be feared in the bottom support mesh. The transverse vibrations of the fuel rod are strongest there, since the coolant emerging from the fuel element base still contains many eddies and other inhomogeneities in its flow, which excite such vibrations. In addition, as in the overlying support meshes, the vibrations are not damped from above and below by two adjacent support meshes.

Bent fuel rods in particular are particularly at risk from fretting, as is shown using the example of the mesh according to FIG. 6 for a bent fuel rod. In FIG. 6, the parts of the supporting mesh below the diagonal D-D of FIG. 4 have been removed in order to expose the bent fuel rod 36 '. For example, this can oscillate between a position indicated by solid lines and a position indicated by broken lines, which corresponds to the unbent normal position. In this case, it lies against the lower knob 31 or the lower knob 33 (indicated in FIG. 6 only by its contact surface 33 ') on the sides of the supporting mesh 36. It therefore moves in an angular range α around an axis of rotation lying in the plane B-B.

Consequently, this fuel rod 36 'is no longer firmly attached to the upper knobs 32 and 34, but instead rubs there. The scuff mark 34 'of the knob 34 which arises in the process can finally perforate the fuel rod. Similar scuff marks are also produced by knobs 32 and (albeit to a lesser extent) springs 30.

This perforation through the support members of the lowest support mesh can be largely avoided if all support elements attack elements of this support mesh in one plane on the fuel assembly (R = 0).

The invention therefore preferably provides for at least the uppermost and lowermost support mesh for each fuel rod, in FIG. 1 thus the meshes of the spacers 9 and 10, to be designed according to the principle “R = 0”, while the meshes of the intermediate spacers (in FIG 1 not shown) are designed as support meshes according to FIGS. 2, 3 and 4, 5. The intermediate grids 12, which only serve as supports for the flow control surfaces 13, on the other hand, often have meshes without support elements. If, for example, the fuel rods in the area of the lowest support mesh are not particularly excited by the construction of a flow-promoting foot, the construction according to FIGS. 2 to 5 can also be used for these lowermost support meshes. If, on the other hand, the design of the fuel assembly permits sufficient tolerances for bent fuel assemblies, the uppermost support mesh can also be designed in accordance with FIGS. 2 to 5, while only the lowest support mesh is designed according to the invention in order to avoid fretting damage .

A preferred example of modifying the support mesh according to FIGS. 2 to 5 in accordance with the principle “R = 0” is shown in FIG. 7. The cross section shown in FIG. 2 or 4 is not modified, so the support mesh in the case of the FIG 4 a resilient support element, in the case of FIG. 2 two resilient support elements (FIGS. 26, 27, * spring 40). Furthermore, two further rigid support elements are provided, which correspond to the pairs of nubs 24, 25 and 26, 27 (or 31, 32 and 33, 34) of FIGS. 2 and 4. However, while the support members (nubs) of FIGS. 2 and 4 act on the fuel rod in planes BB and CC (that is, above and below plane AA, in which the resilient support element 20 or 30 acts), the rigid supports provided in FIG. 7 engage - members 41 together with the elastic support member 40 in the same level AA. Instead of a pair of knobs, only a single knob 41 is used in each case. Apart from this deviation, these support members are still distributed practically equidistantly over the circumference of the fuel rod (FIG. 2) or at least approximately equidistantly distributed (FIG. 4). The mirror-symmetrical arrangement with respect to the diagonal DD of FIGS. 3 and 4 also remains unchanged.

It is not necessary to use elastic and rigid support elements in a support mesh at the same time. Thus, FIG. 8 shows that the rigid support element 41 of FIG. 7 can be replaced by a resilient support element 51. The support mesh then only contains resilient support members 50, 51.

From a fluidic point of view, it can be advantageous to use only one spring in each case in the supporting mesh (FIG. 4). In view of the differences in the spring and frictional forces that occur in FIGS. 2 and 4, it seems advantageous to use FIG. 4 only (or at least: at least) in the uppermost and / or lowermost support mesh. On the other hand, a support mesh projecting only diagonally into the mesh allows the fuel rod to perform weaker damped transverse vibrations in a preferred direction when vibrating than when using two support springs, so that with regard to fretting and

Vibration damping the training of Figure 2 can be more advantageous.

In some cases, however, resilient support members can be dispensed with and the support mesh can only be carried out with rigid support members. Such an example is shown in FIGS. 9 and 10, which is based on the support mesh of US-A-4,312,705. The support mesh 61 penetrated by the fuel rod 60 is formed by webs 62, 63 which cross each other, support rings 64, 65 being placed on the edges of these webs around the intersections of these webs are. These support rings are held in grooves in the webs 62, 63.

It will be apparent to those skilled in the art from these statements that many spring and / or knob shapes are possible as support elements, which are proposed in the prior art with regard to flow resistance, holding force or other reasons.

Many deviations are also possible for the grids and their meshes through which the fuel rod is guided and which lie between the uppermost support mesh and the lowest support mesh. For example, meshes of the intermediate grids 12 in FIG. 1 can also have support members, but the intermediate grids can also be omitted entirely. In the context of the present invention, it is also not necessary for all the meshes of spacers which lie between the uppermost and lowermost support meshes to be support meshes with at least three support elements which act equidistantly on the fuel rod.

Likewise, a square cross section of the grid mesh is not necessary for the invention. For example, FIG. 11 shows a cross section through a grid which consists of welded sleeves 70, 71 through which the fuel rods (not shown) are guided. Correspondingly designed spacers are often used, springs 72 and knobs 74, 75 formed from the sleeves serving as support elements. FIG. 12 shows a longitudinal section along the plane XII-XII of FIG. 11, from which it can be seen that the knobs 74, 75 and the spring 72 also engage in a plane A-A in this case if this spacer is used as the uppermost spacer.

However, if this construction is used to form support meshes lying between the uppermost support mesh and the lowermost support mesh, the knobs 74, 75 are replaced by pairs of knobs which have support members acting on different planes (BB and CC), such as with through Broken lines show nubs 74 ', 74' * and 75 ', 75''.

According to the invention, therefore, when selecting the suitable support members, particular attention must be paid to the fact that the fuel element is particularly poor in bending or to have a special fretting safety at the lowest spacer, rather these properties can already be largely taken into account when the respective ones Supporting members of a mesh can be arranged in a single plane.

Claims

claims

1. Fuel element for a pressurized water reactor with the following features:

a) a base plate (6) and a cover plate (4) are connected to one another by guide tubes (7) to form a laterally open support structure, b) on the guide tubes there are several grids (8, 9, 10) running approximately perpendicularly thereto in the axial direction Held at a distance from one another, c) a bundle is arranged between the base plate (6) and the cover plate (4) to the guide tubes of approximately parallel fuel rods (8), the upper ends of which are not or at least not rigidly held on the cover plate, and each passing through meshes of the grids (8, 9, 10) one above the other, d) a plurality of the grating meshes, each of which is penetrated jointly by a fuel rod, are designed as support meshes which have at least three support elements approximately equidistantly distributed over the circumference of the fuel rod for lateral support of the fuel rod have, characterized in that the top and / or bottom support mesh of a fuel rod only support elements (40th , 41, Fig. 7), which act practically in one plane (A_A) on the fuel rod.

2. The fuel assembly according to claim 1, so that at least one of the support elements acting in one plane is a resilient support element (40).

3. Fuel element according to claim 1 or 2, characterized in that at least two of the supporting elements acting in one plane are rigid supporting elements (41).

4. Fuel element according to one of claims 1 to 3, so that the supporting elements (20, 24, 26) engaging in one plane are formed in a mirror-symmetrical manner to a diagonal (D-D) of the mesh and are distributed around the circumference of the fuel rod.

5. Fuel element according to claim 1 or one of claims 2 to 4, characterized in that between the top and the bottom support mesh is at least one central support mesh, the support elements (20, 24, 25) in several planes (AA, BB, CC) on Attack the fuel rod.

6. Fuel element according to claim 5, characterized in that the central support mesh contains at least one resilient support element (20) and two rigid support elements, that each rigid support element from a pair of practically one above the other arranged support members (24, 25; 26, 27th ) exists and the upper support members of the two pairs above and the two lower support members of the two pairs below the plane (AA), in which the resilient support element lies, engages the fuel rod.

7. Fuel element according to claim 6, d a d u r c h g e k e n n z e i c h n e t that the two rigid support members (31, 32) of each pair are arranged one above the other.

8. Fuel element according to claim 7, so that a resilient support element (21) is opposite a pair of rigid support members (26, 27).

9. Fuel element according to claim 7, characterized in that each a pair of rigid support members (33, 34; 31, 32) lie diagonally opposite a resilient support element (21).

10. Fuel element according to claim 6 or one of claims 7 to 9, d a d u r c h g e k e n n z e i c h n e t that the resilient support element is a spring, the rigid Stützglie¬ the rigid knobs.