SK20094A3 - Distance lattice for nuclear fuel elements - Google Patents

Abstract

A spacer grid for nuclear fuel elements such as rods or pins which comprises mutually intersecting and interlocking sets of parallel strips, wherein the strips are in three sets 1a, 1b, 1c and the spaces between the strips comprise in transverse cross-section substantially regular hexagons separated by substantially equilateral triangles. <IMAGE>

Description

(57) A grid for nuclear fuel elements, such as fuel rods, consisting of intersecting and interlocking sets of parallel strips (1a, 1b, 1c). The strips (1a, 1b, 1c) are arranged in three sets, and the spaces between the strips (1a, 1b, 1c) have substantially transverse cross-sectional shape of substantially regular hexagons separated by substantially equilateral triangles.

Spacer for nuclear fuel elements

Technical field y

The invention relates to a spacer grid for nuclear fuel elements, such as fuel rods, consisting of intersecting and interlocking sets of parallel strips.

BACKGROUND OF THE INVENTION

Such spacer grids are used in nuclear reactors to hold and fasten the nuclear fuel rod bundles at a distance from each other to form a fuel system for the nuclear reactor. The fuel itself is usually contained in the outer casing of the fuel rod.

Many types of distance grids are known, but their production and control is very often complicated. For example, the individual cells forming the spacer grid are in some cases welded together along their lengths, and therefore the welding cannot be performed simply, which is time consuming and therefore expensive.

U.S. Patent No. 4,775,509 discloses a spacer grid arrangement in which fuel rods are held between parallel plates. running in different directions and at different distances or levels along the axis of the entire configuration, each portion of the desired contact points on each fuel rod being formed at each level along the axis by the array of plates at that level, contributing to the overall compactness of the embodiment. However, such an arrangement is complicated to manufacture and the location of the points of contact of the fuel rods with the plates is not ideal.

GB 2 081 961 A discloses a spacer grid arrangement in which a square grid is formed by two sets of interlocking plates. The rows of continuously assembled individual strips forming the springs run at an angle to the two sets of plates are fixed between the wells formed on the inner faces of the distance grid plates to facilitate holding the fuel rods. However, the individual strips forming the springs are difficult to manufacture and expensive to assemble. These belts contain a large number of loose parts prior to assembly. These springs contain the types of joints that need to be made with a relatively large push movement, and therefore the holding forces acting on the fuel rods and thus their fastening are not ideal.

SUMMARY OF THE INVENTION

The above-mentioned drawbacks are overcome by a spacer grid for nuclear fuel elements, such as fuel rods, consisting of intersecting and interlocking sets of parallel strips, according to the invention, which consists in that the strips are arranged in three sets and the spaces between the strips have transverse cross-sectional shape of substantially regular hexagons separated by substantially equilateral triangular km i.

In this distance grid, fuel elements such as fuel rods are located in hexagonal spaces between the bands. To hold the bundle of fuel elements in different positions, two or more spacers are provided along the fuel elements. Each fuel element may be a fuel regular cylinder.

rod that has

The hexagonal, if desired, the shape of the long space of the spacer grid may be such that each fuel rod is held in position by frictional contact with the six inner surfaces of the strips that form the hexagon.

The strips may advantageously be provided on their inner surfaces or selected areas of these areas forming each hexagon, for example on three of the six inner surfaces bounding each hexagon, with structures which assist in holding the fuel rod. Where necessary, these units are of essentially the same level. When the spacer is assembled and holds the fuel rods, certain surfaces, for example every other, i.e. 1. , 3.

and 5., in their sequential numbering, include a well, a well and a spring, the fuel rod in the given hexagonal metal space being pressed by a spring against both wells, and wherein the fuel rod axis lies preferably in the center of the hexagon.

Preferably, the intersecting strips form spaces.

having the shape of a regular hexagon, the geometric arrangement of the distance grid being such that each side of the hexagon is always part of a single equilateral triangle, so that the equilateral triangles always separate regular hexagons. Each triangle has an area that is (or is approximately) one sixth of the area of the hexagon.

The resulting envelope shape of about six equilateral triangles formed in this way around the hexagonal metal forms a six-pointed star.

In the triangular spaces, the coolant can preferably flow in a direction parallel to the fuel rods,

After assembling the spacer grid and fuel rods.

The shape of the outer shell, and thus the shape of the fuel rod assembly retained by the spacer grid, is determined by the shape of the cavity in the core of the reactor in which the entire assembly (i.e., the fuel rod spacer it retains) is inserted. The spacer grid may be surrounded by an outer sheath strip, which itself has a hexagonal shape or alternatively a square shape.

The strips forming the hexagonal metal spaces of the spacer grid may be provided at one or both of their intersecting edges (i.e. edges perpendicular to the axes of the hexagonal spaces) with end projections which facilitate the welding of the strips. Other end protrusions may be provided on the initial belt, forming a step to allow mixing and coolant in the operation of the nuclear reactor. For this use, said projections may form mixing e vanes at the outlet side of the spacer grid (i.e., the side opposite to the coolant flow inlet). These vanes can create increased turbulence of the refrigerant flow as the refrigerant passes through the spacer grid, which improves the heat transfer of the refrigerant.

Said strips will snap together in the same way as cardboard grids used in the United Kingdom to pack eggs into boxes. The belts according to the invention are provided with a series of regularly spaced notches which allow the belts to be interlocked and crossed. For example, the notches may be provided alternately at the top and bottom edges of the strips of one set and at only one edge of the strips of the other two sets. Alternatively, the strips of all three sets may be made identically, having notches at their two edges. In any case, the slits extend inwards to the center of the strip perpendicular to its longitudinal axis. The notches may have a length slightly greater than 0.5 W. where W is the average web width.

the width does not include any lateral projections projecting from the edges of the strips. Such a length of the notches ensures that the notches interfere properly when assembled. Notches with projections.

The notches may have a width slightly greater than the thickness of the strips to ensure that the strips are seated in the notches with clearance.

The belts may, if desired, be provided with further notches not extending to the edges of the belts, these additional notches being intended to facilitate the passage of the refrigerant.

These further notches may be made parallel to the aforementioned notches.

The aforementioned springs are formed by pressing the strips with the other notches.

The spacers according to the invention are preferably simpler to manufacture than the spacers known hitherto, and are more rigid in construction, making better contact with the retained fuel rods. Since the strips always remain bent, their maximum strength is maintained. This eliminates all the problems associated with bending the web material. The strips, when assembled together, are held together by means of frictional forces. Weld joints can be made at the lateral edges of the strips where these edges intersect due to the good accessibility of the sites to be made, so that 2-weld joints are easy to perform and only use material because they do not have to intersect between small amounts Welding performed along the entire length of the strips. To further increase the strength, additional welds may be made at some or all of the intersections of the strips at the top or bottom edge. The belts according to the invention do not require the use of a large number of components to be assembled, as in the known embodiments.

In the nuclear reactor, the coolant moves from the bottom of the spacer rod fuel rod assembly to the top of the assembly and out into the heat exchangers. The spacer rod fuel rod assembly causes only a small amount of accumulator coolant so that the pressure of the coolant passing through the reactor core can be minimized. Whenever the coolant comes into contact with a distance grille holding the fuel elements, its pressure is reduced by a certain amount. The spacer grid of the present invention constitutes an obstacle for the coolant at only one thickness, allowing pressure to minimize reactor performance.

The amount of unwanted neutron absorption is also minimized, and this, together with minimal pressure drop, reduces the reactor fuel cycle costs.

The advantages of the spacer grid according to the invention are as follows. The distance grid has a very rigid and rigid construction made of a simple interlocking assembly. Making the notches facilitates their production. The number and placement of the welding sites is minimized, thereby reducing production costs. The spacer grid can be made (e.g., zircaloy 4) with low neutron absorption.

The amount of spacer material used is minimal, thus reducing unwanted neutron absorption

The springs loading the fuel elements are easily adjustable in the distance grid.

The fuel elements are held in the distance grid so that they can freely increase in size as the reactor radiates.

Spacer grids provide suitable support for fuel elements during transport and other maneuvering.

u. Spacer grids cause a minimum pressure drop of cold, even if cold is provided, if necessary.

mixing blades »because they can be made simply.

BRIEF DESCRIPTION OF THE DRAWINGS

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which: FIG. 1 shows a plan view of a fuel rod spacer, FIG. 2 is a side view of the strip used in the spacer grid shown in FIG. 1, FIG. 3 is a cross-sectional view along the line III-III of FIG. 2, FIG. 4 is a cross-sectional view along the line IV-IV of FIG. 2, FIG. 5 is a side view of another embodiment of the strip used in the spacer grid shown in FIG. 1, FIG. 6 shows a sectional view along the line VI-VI of FIG. 5, and FIG. 7 is a sectional view along the line VII-VII of FIG. 5th

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the spacer grid is formed from a series of first strips 1a extending in a first direction and overloaded.

hired a interlocking with a number of second belts lb, take over the accounts in a second direction inclined at 120 ° to in the first direction, and with a series of third belts 1c extending

in a third direction inclined to the first and second directions at an angle of 120 °. The strips 1a, 1b, ΐc are formed of a solid machinable metal having a suitable neutron absorption capacity, for example of stainless steel or zircaloy alloy.

Between the strips 1a, 1b, 1c, hexagonal spaces are formed in the fuel rod esters (not shown), and triangular spaces 5 are formed between the hexagonal spaces 5. Triangular spaces 5 assist in the passage of both cold and cold between the fuel rods.

First wells 7a are formed on the side walls of the first strips 1a, which inwardly and delimit hexagonal spaces 3, and second wells 7b are formed on the side walls of the second strips 1b, which also face inwardly and delimit the hexagonal spaces 3b.

Springs 9 are formed on the side walls of the third strips 1c, which face inwards and delimit hexagonal spaces 3.

The entire arrangement of hexagonal metal and triangles formed by strips 1a, 1b. 1c is bounded by a hexagonal outer belt 10. This outer belt 10 is also provided with wells 12 (similar to the first wells 7a) on its inwardly facing walls, in places where the outer belt 10 is parallel to the first and second belts 1a and 1b. . and springs 14 at locations where the outer belt 10 is parallel to the third belts 1c.

In FIG. 2 to 4, there is shown one type of belt, represented here by the third belt 1c forming part of the distance grid shown in FIG. The length of each third strip 1c is selected according to its position it occupies in the distance grid (the requirement for different lengths of the third strip 1c is apparent from the arrangement of the strips 1a, 1b, 1c in Fig. 1 in the hexagonal metal outer strip 10). ). The same applies to the first and second belts 1a and 1b.

As shown in FIG. 2. has a third embodiment belt 1c consisting of repeating units at intervals P. Each unit is provided within the upper edge of the third belt 1c with two large projections 11 forming the mixing e-blades and two small projections 13 which are intended as welding points, the protrusions 11 and 13 being arranged alternately along the upper edge. Every second small protrusion 13a, see FIG. 2 is provided with a notch 15 formed therein which is perpendicular to the longitudinal axis of the third strip 1c (i.e., the axis coinciding with line IV-IV in FIG. 2). If necessary, the large protrusions 11 forming the mixing e-blades may be bent into the area that will form the triangular spaces 5, see FIG. 1, alternately along the third belt 1 c. Each notch 15 extends substantially to the center of the third strip 1c, i.e. to line IV-IV, that is to say about half the average width of the third strip 1c (no special widths formed by the protrusions 11 and 13 provided at the edges of the third strip 1 c). At the lower edge of the third strip 1c opposite the small protrusions 13 at its upper edge there are further small protrusions 17 also provided to facilitate the welding. Each second small protrusion 17a is provided with a notch 19 formed therein which is parallel to the notches 15. Each notch 19 extends substantially to the center of the third strip 1c. The two slits 15 and 19 are arranged alternately along the length of the third strip 1c.

As shown in FIG. 3 and 4, springs 9 are arranged in the third strip 1c in the regions between each second pair of slits 15 and 19, i.e. between the first, third, fifth, etc., but not between the second, fourth, sixth, etc.). it is ensured that the springs 9 are formed in regions which will form hexagonal spaces 3. The positions of the springs 9 are aligned with every second large projection 11. The springs 9 are formed between pairs of further notches 21 (not extending to the edges of the third belt 1c) formed in the third belt 1c, the springs 9 being formed by bent portions formed by pressing the third belt 1c in a direction perpendicular to the plane of the third belt 1c in FIG. Second

In FIG.

1 to 1, used in the distance grid 1 of FIG. 1. Here, the first strips 1a are identical to the second strips 1b.

Strip lengths

1a, 1b are selected according to their position in the distance grid. The second belt 1b is also made up of repeating units with a P pitch.

Each unit is provided at the upper edge of the second strip 1b with two small projections 23, also intended to be welded, similar to each second projection 13a.

notches 25 through said center of the second belt wherein like the notches 15 extend through the small protrusions 23 substantially up to 1b. In this case, two notches 25 are provided per unit with a pitch P, the notches 25 being provided in each small projection 23 (not in each other as in Fig. 2). At the lower edges of the second strip 1b, there are further small protrusions 27 (similar to the small protrusions 1.3 of Fig. 2), located opposite the small protrusions 23. In this case, holes 29 (in each second securing unit) are formed by pressing it in the middle that the wells 29 are formed in areas that will form hexagonal spaces 3).

The positioning of the notches 25 in the first belt 1a is different from the positioning of the notches 19 in the third belt 1c (see FIG. 2) since it is a function of the embodiment of the spacer grid assembly. It makes it possible to easily assemble / insert the belts 1 a, 1b, 1c at different positions across the distance grid.

The strips 1a, 1b, 1c are assembled together using a positioning device so as to form the spacer grid shown on the

1, FIG. 1. In order to assemble the spacer grid firstly, the first strips 1a and the notches of these strips 1 are inserted into the positioning device (not shown) and the third strips 1c are inserted. Then, the first strips 1a are rotated and rotated 120 ° to be inserted into the remaining slits in the third strips 1c and the second strips 1b. A third strip 1c is inserted into the slits 25 in the second strips 1b and the corresponding slits in the first strips 1a and the respective first strips 1a and the second strips 1b are inserted into the slits 15 and 19 in the third strips 1c. The projections 13 and 17 on the third strips 1 c. the projections 23 and 27 on the second strips 1b and the corresponding projections on the first strips 1a serve to facilitate spot welding of the strips 1a, 1b, 1c to each other at their edges.

Fuel rods and spacers, see fig. 1, are assembled together in a known manner. The fuel rods of said bundle are held at respective locations along their length by a suitable arrangement of spacers. The entire assembly thus formed is placed in a reservoir in the core of the nuclear reactor, which also has a hexagonal shape as spacers.

In the operation of the nuclear reactor, the large protrusions 11 (FIG. 2) serve as mixing coolant blades in the core of the nuclear reactor in the manner described above.

If the mixing blades are not used, then the three sets of strips 1a, 1b, 1c used to form the spacer grid themselves are a measure of reducing production costs.

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PATENT CLAIMS

Claims (7)

Spacer for nuclear fuel elements, such as fuel rods, consisting of 20 intersecting and interlocking sets of parallel strips, characterized in that the strips are arranged in three sets and the spaces between the strips have a cross-sectional shape substantially regular hexagonal metal separated by substantially equilateral and triangular km i. Spacer according to claim 1, characterized in that the nuclear fuel elements are located in hexagonal spaces between the strips. Spacer according to claim 1 or 2, characterized in that the strips have integral shapes on their faces or selected portions constituting each hexagonal space to assist in retaining the fuel elements. • J Spacer according to claim 3, characterized in that the formations are formed by springs and wells. j Spacer according to any one of the preceding claims, characterized in that it is surrounded by an outer strip having a hexagonal or square shape. Spacer according to any one of the preceding claims, characterized in that the strips forming the hexagonal spaces are provided at one or both of their intersecting edges with end protrusions which facilitate welding of the strips and mixing of the coolant during operation of the nuclear reactor. 1 3 v Spacer according to any one of the preceding claims, characterized in that the strips are provided with regularly spaced notches to facilitate insertion and the strips are mutually spaced, the notches having a width slightly less than the thickness of the strips to ensure that the strips are seated in the notches. will. List of reference marks first strip 1a second strip lb rubs strip 1c hexagonal space 3 triangular space 5 first hole 7a second hole 7b spring 1 on outer belt 10 large projection 11 hole 12 small projection 13 second small projection 13a spring i 14 notch 15 small projection 17 second small projection 17a notch 19 notch 21 small projection 23 notch 25 small projection 27 well 29 Pl 0BR.1 ί'4 ~ ' FIG. Third FIG. 4th