CZ37894A3 - Distance grate 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

Technical field

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 nuclear fuel rod bundles at a distance from each other to form a fuel system for a 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 welding is therefore not easy to do, which is time consuming and therefore expensive.

U.S. Pat. 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 assembly, each portion of the desired contact points on each fuel rod being formed in each level along the axis of the arrangement with a set of plates at this level, contributing to the overall compactness of the embodiment. However, such an arrangement is complicated in terms of production and assembly, 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 plates of the spacer grid to facilitate holding of the fuel rods. However, the individual strips forming the springs are complicated 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 because of this the holding forces acting on the fuel rods and hence their fastening are not ideal.

Summary of the Invention

The aforementioned 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, in which the strips are arranged in three sets and the spaces between the strips have cross-sectional shape of substantially regular hexagons, separated by substantially equilateral triangles.

In this spacer grid, fuel elements such as fuel rods are located in hexagonal spaces between the strips. Two or more spacer grids are arranged along the fuel elements to hold the bundle of fuel elements in different positions. Each fuel element may consist of a fuel rod having a long shape as desired

regular war. The hexagonal spaces 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 forming the hexagon.

Advantageously, the strips may be provided on their inner surfaces or selected areas of each hexagonal surface, for example on three of the six inner surfaces bounding each hexagon, with features that assist in holding the fuel rod. If necessary, these units are formed at essentially the same level. When the spacer is assembled and holds the fuel rods, certain surfaces, such as every second, i.e., 1, 3 and 5, as they are sequentially numbered, may include a dimple, a dimple and a spring, the fuel rod being pushed in the hexagonal space a spring against both pits, and wherein the axis of the fuel rod preferably lies in the center of the hexagon.

The intersecting strips preferably form a space having the shape of a regular hexagon, wherein the geometry of the spacer grid is such that each side of the hexagon is always part of an equilateral triangle, so that equilateral triangles always separate regular hexagons from each other. Each triangle has an area that is (or is approximately) one-sixth of the area

..... hexagon. The resulting envelope shape around six equilateral triangles, formed in this way around a hexagon, forms a six-pointed star. In triangular spaces, the coolant may preferably flow in a direction parallel to the fuel rods, after the spacer grid and the fuel rods have been assembled.

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 spacer rod with the fuel rods it holds) is to be inserted. The spacer grid may be surrounded by an outer jacket strip which itself has a hexagonal shape or alternatively a square shape.

The strips forming the hexagonal spaces of the spacer grid may have end protrusions on one or both of their intersecting edges (i.e. edges perpendicular to the axes of the hexagonal spaces) to facilitate welding of the strips. Additional end protrusions may be provided on the initial belt, forming a step to allow the coolant to be agitated during operation of the nuclear reactor. For this application, said protrusions may form mixing vanes on 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 a spacer grid, which improves the heat transfer of the refrigerant.

The strips fit 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 snap together and to cross them. The notches may be provided, for example, alternately on the upper and lower edges of the strips of one set and on. only one edge of the strips; the other two sets. Alternatively, the strips of all three sets can be made identically, with notches at their two edges. In any case, the indentations extend inwardly toward 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 width of the strip, this width comprising no lateral projections protruding from the edges of the strip. Such a length of the notches ensures that the notches interfere properly when assembled. The notches can be passed through the 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. If desired, the strips may be provided with further notches that do not extend to the edges of the strips, these notches being intended to facilitate coolant passage. These further notches can be made parallel to the previously mentioned notches. The above springs are formed by pressing the strips between said further notches.

The spacer grids according to the invention are preferably simpler to manufacture than the prior art spacer grids, and their construction is stronger, 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 interlocking strips are held firmly together by 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 workstations, so that the weld joints are easy to perform and use only a small amount of welding material as they do not need to be made along the entire length of the intersections between 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 of 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 refrigerant moves from the bottom of the fuel rod assembly with spacers to the top of the assembly and out into the heat exchangers. The fuel rod assembly with spacers causes only a small drop in coolant pressure, so that the coolant pressure passing through the reactor core can be minimized. Whenever the coolant comes into contact with a spacer grip retaining the fuel elements, its pressure is reduced by a certain amount. The spacer grid of the present invention constitutes an obstacle for the coolant with only a single strip thickness, and therefore causes a minimum pressure drop, which makes it possible 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 cost.

The advantages of the spacer grid according to the invention are as follows. The spacer grille has a very strong and rigid construction made of a simple interlocking assembly. The design of the notches facilitates their production. The number and positioning of the welding sites is minimized, thereby reducing production costs. The spacer grid may be made (e.g., zircaloy 4) with low neutron absorption. The quantity of material used for the spacer grid ¹ is minimal, thereby reducing undesired neutron absorption. The springs loading the fuel elements are easily adjustable in the distance grid. The fuel elements are held in the spacer grid so that they can freely increase in size when radiated in the reactor. The spacer grids provide a suitable support for the fuel elements during transportation and other handling. Spacer Grid - Causes a minimum drop in refrigerant pressure, even if it is provided with refrigerant mixing knobs, if needed, since they can be made simply.

BRIEF DESCRIPTION OF THE DRAWINGS

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a spacer bar for fuel rods; FIG. 2 is a side view of the strip used in the spacer grid shown in FIG. 1;

Fig. 3 is a sectional view along the line III-III of Fig. 2; Fig. 4 is a sectional view along the line IV-IV in Fig. 2;

FIG. 5 is a side view of another embodiment of the belt used in the spacer grid shown in FIG. 1; FIG. 6 is a cross-sectional view of the belt taken along line VI-VI of FIG. 5, and FIG. 7 is a cross-sectional view of the strip taken along the line VII-VII in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the spacer grid is formed from a plurality of first strips 1a extending in a first direction and intersecting and interlocking with a plurality of second strips 1b. extending in a second direction inclined at an angle of 120 'to the first direction and with a plurality of third strips 1c extending in a third direction inclined at an angle of 120 ° to the first and second directions. The strips 1a, 1b, 1c are formed of a solid machinable metal having a suitable neutron absorption capacity, for example of stainless steel or zircaloy alloy.

Hexagonal spaces 3 for fuel rods (not shown) are formed between the strips 1a, 1b, 1c and triangular spaces 5 are formed between the hexagonal spaces 3. Triangular spaces 5 facilitate the passage of coolant between the fuel rods.

On the side walls of the first strips 1a that face inwards and delimit hexagonal spaces 3, first pits Ta are formed, and on the side walls of the second strips 1b that also face inward and delimit hexagonal spaces 3, second pits 7b are formed.

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 hexagons and triangles formed by the strips 1a, 1b, 1c is bounded by a hexagonal outer strip 10. This outer strip 10 is also provided with dimples 12 (similar to the first dimples 7a,) at its inwardly facing walls at the points where the outer strip 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.

FIGS. 2 to 4 show one type of belt represented here by the third belt 1c forming part of the spacer grid shown in FIG. 1. The length of each third belt 1c is selected according to its position it occupies in the spacer grid (different length requirements) The third belt 1c is evident from the arrangement of the belts 1a, 1b, 1c in Figure 1 in the hexagonal outer belt 10). The same applies to the first and second belts 1a and 1b.

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As shown in FIG. 2, the third strip 1c has an embodiment Z 'of repeating units at intervals P. Each inside the upper edge of the third strip 1c is provided with two large projections 11 constituting the mixing vanes and two small projections 13 which are intended. as welding sites, the protrusions 11 and 13 being provided alternately along the upper edge. Every second small protrusion 13a. see FIG. 2, 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 the line IV-IV in FIG. 2). If desired, the large projections 11 forming the mixing vanes may be bent into the region that will form the triangular spaces 5, see FIG. 1, alternately along the third strip 1c. Each notch 15 extends substantially to the center of the third strip 1c, i.e. to the line IV-IV, i.e. to about half the average width of the third strip 1c (disregarding the special widths formed by the protrusions 11 and 13 provided at the edges of the third strip 1c). At the lower edge of the third strip 1c opposite the small protrusions 13 in its upper edge, further small protrusions 17 are provided, also to facilitate welding. Each second small projection 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 notches 15 and 19 are provided alternately along the length of the third strip 1c.

As shown in Figures 3 and 4, there are in the third strip 1c in the areas between each second pair of notches 15 and 19 (i.e. between the first, third, fifth pair, etc., but not between the second, fourth, sixth pair, etc.) This ensures that the springs 9 are formed in areas that will form hexagonal spaces. The positions of the springs 9 are aligned with each second large projection 11. The springs 9 are formed between pairs of further notches 21 (not extending to the edges of the third strip 1c) made in the third strip 1c, the springs 9 being formed by bent portions formed by pressing the third strip 1c perpendicular to the plane of the third belt 1c in FIG. 2.

5 to 7 show a second strip 1b used in the spacer grid of FIG. 1. The first strips 1a are identical to the second strips 1b. The lengths of the strips 1a, 1b are selected according to their position in the spacer grid. The second strip 1b is also made of repeating units with a P pitch. Each unit is provided at the upper edge of the second strip 1b with two small protrusions 23, intended for welding, similar to each second protrusion 13a. wherein, similar to the notches 15, the notches 25 extend through the small protrusions 23 substantially to the center of the second strip 1b. In this case, two notches 25 are provided per unit with a pitch P, the notches 25 being provided in each small protrusion 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 13 of FIG. 2) located opposite the small protrusions 23. In this case, pits 29 are formed in the center of the second belt 1b by pressing it. ensuring that the pits 29 are formed in areas that will form hexagonal spaces 2) ·

The positioning of the notches 25 in the first strip 1a is different from the positioning of the notches 19 in the third strip 1c (see FIG. 2) because it is a function of the embodiment of the spacer grid assembly. It makes it possible to easily assemble / insert the belts la, lb, le in different positions across the spacer grids.

The strips 1a, 1b, 1c are assembled together using a positioning device so as to form the spacer grid shown in Fig. 1. When assembling the spacer grid, the first strips 1a (not shown) are first inserted into the notches of these strips 1a and inserted third strips lc. 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. The notches 25 in the second belts 1b. and the corresponding notches in the first strips 1a insert the third strip 1c and the notches 15 and 19 in the third strips 1c insert the respective first strips 1a and the second strips 1b. The projections 13 and 17 on the third belts 1c, the projections 23 and 27 on the second belts 1b and the corresponding projections on the first belts 1a serve to facilitate spot welding of the belts 1a. lb. lc to each other at their edges.

The fuel rods and spacers, as shown in FIG. 1, are assembled together in a known manner. The fuel rods of the bundle are held at appropriate 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 spacer grids.

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

If the mixing vanes are not used, then the three sets of strips 1a, 1b, 1c used to form the spacer grid themselves are measures that reduce production costs.

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Claims (7)

PATENTOVÉ NÁ A nuclear spacer is fuel rods consisting of 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 substantially regular hexagons in cross-section, separated by essentially with equilateral triangles. A spacer grid according to claim 1, characterized in that the nuclear fuel elements are located in hexagonal spaces between the strips. Spacer grate according to claim 1 or 2, characterized in that the strips comprise integral shapes on their faces or selected portions forming each hexagonal space, which help to hold the fuel elements. 4. The grid according to claim 3, wherein the formations are formed by springs and dimples. A spacer grid 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 the welding of the strips and the mixing of the coolant during operation of the nuclear reactor. Spacer according to any one of the preceding claims, characterized in that the strips are provided with regularly spaced notches to facilitate the insertion of the strips into each other, the notches having a width slightly greater than the thickness of the strips to ensure that the strips are seated in the notches with clearance. .