GB2277191A - Spacer grids 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

Spacer grids for nuclear fuel elements The present invention is related to spacer grids for nuclear fuel elements such as rods or pins.

Such grids are employed in the nuclear power industry to hold a collection of spaced nuclear fuel rods or pins to form a fuel assembly for use in a nuclear reactor. The fuel itself is usually contained in an outer casing of the rod or pin.

Many types of spacer grid are known in the prior art but these are often complicated to manufacture and inspect. For example, the individual cells making up the grid are in some cases welded together along their length and the welding operations are not easy to achieve and are very time consuming and therefore costly.

US 4775509 describes a spacer grid arrangement in which fuel pins are held between parallel running plates which run in different directions at different distances or levels along the axis of the arrangement whereby a fraction of the contact required on the pin is provided at each level along the axis by the sets of plates at that level the fractions adding to give the overall contact.

Such an arrangement is complicated to manufacture and assemble and the spacing of the points of contact on the pin by the plates is not ideal.

GB 2081961A describes a spacer grid arrangement in which a square grid is formed by two sets of mutually interlocking plates. Rows of continuous separately added individual strips forming springs running at an angle to the two sets of plates are fitted to facilitate holding the fuel pins between dimple abutments on the inner faces of the plates of the grid. The individual strips forming the springs are complicated to manufacture and expensive to assemble. Such strips comprise a large number of loose parts before assembly. The springs described comprise relatively large movement push away type bends in the strips and because of this the gripping forces on the pin by the springs and the abutments are not ideal.

According to the present invention there is provided 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 and the spaces between the strips comprise in transverse cross-section substantially regular hexagons separated by substantially equilateral triangles.

In the said grid, fuel elements such as rods or pins are located in use in the hexagonal spaces between the strips. Two or more grids may be used to hold a collection of fuel elements at different positions along the elements. Each fuel element may be a fuel rod or pin which is desirably a long right circular cylinder. The hexagonal spaces of the grid may be such that each rod or pin is held in position by frictional contact with the six inward facing strip surfaces which form the hexagon.

Conveniently, the strips may contain on or as their inward facing surfaces or selected regions forming each hexagon of the surfaces eg three of the six inward facing surfaces bounding each hexagon, formations which assist holding of the fuel rod or pin. Desirably the formations are all at substantially the same level. When the grid has been constructed and holds fuel pins. For example, in a given hexagon alternate surfaces (eg surfaces 1, 3 and 5 if numbered consecutively) may possess, respectively, a dimple, a dimple and a spring whereby a fuel rod or pin held in the hexagonal space is pressed by the spring against the dimples, the axis of the rod or pin thereby preferably coinciding with the centre of the hexagon.

The intersecting strips conveniently form spaces which are in the form of regular hexagons the geometrical arrangement of the grid being such that alternate sides of the hexagon meet to form equilateral triangles with the side of the hexagon between them, each triangle having an area which is (or is approximately) one sixth of the area of the hexagon. The enclosed shape around all six triangles formed in this way by the meeting sides of the hexagon encloses a six-sided star around the hexagon. The triangular spaces can conveniently permit coolant flow in a direction parallel to the axes of the rods or pins when the grid and rod/pin assembly is in use.

The shape of the outer enclosure, and thereby the enclosed formation of rods or pins held by the grid, is determined by the shape of the cavity in the reactor core into which the whole assembly (ie grid plus rods or pins held thereby) is to be located (in a known way). The grid may be enclosed by an outer enclosure strip which itself is in the form of a hexagon or alternatively in the form of a square.

The strips forming the hexagonal spaces of the grid may have at one or both of their intersecting edges (ie the edges running perpendicular to the axes of the hexagonal spaces) end tabs which facilitate welding of the strips. Further end tabs may be provided at the initial strip forming stage to allow mixing of coolant when in use in a nuclear reactor. For the latter use the tabs may provide mixing vanes on the downstream side of the grid (ie the side which faces away from the oncoming coolant flow). Such vanes can provide increased coolant flow turbulence when the coolant passes through the grid which improves the heat transfer capability of the coolant.

The said strips may interlock in the same manner as the cardboard grids used in the United Kingdom to pack eggs in an egg box. The strips in the invention have a series of regularly spaced slots to allow the strips to intersect and mutually interlock. The slots may for example be provided alternately at the top and bottom edges of the strips of one of the sets and at one only of the edges of the strips of the other two sets.

Alternatively, the strips of all three sets may be identical having the slots at both edges of the strips.

In each case the slots may extend inward to the centre of the strip desirably perpendicular to the long axis of the strips. The slots may have a length slightly greater than 0.5W, where W is the average depth of the strip, the depth not including any side tabs projecting from the edges of the strip. Such a slot length ensures adequate interlocking of the strips when fitted together. The slots may extend through the said tabs where provided.

The slots desirably have a width slightly greater than the thickness of the strips to provide a clearance fit of the strips through the slots. The said strips may optionally include further slots formed therein not extending to the edges of the strips which further slits further facilitate coolant flow in use. The further slots may extend parallel to the first mentioned slots. The aforementioned springs where provided may be formed by pressing of the strips between the said further slots.

The spacer grids according to the present invention advantageously are simpler to manufacture than those of the prior art and have an inherently stronger construction giving better contact with the fuel pins being retained.

As the strips always remain unbent the maximum strip strength is maintained. This overcomes any inherent problems associated with bending of the strip material.

The interlocking strips are strongly held together by mutual frictional forces. Welded joints between strips may be provided at the side edges of strips where the edges intersect but, because of the accessibility of the welded sites, such joints are easy to apply, and consume only small quantities of the welding materials because they are not required to extend along the length of the intersections between strips. Additional welds at some or all of the strip intersections on the upper or lower edges may be added for further strength. The strips according to the present invention do not require large numbers of components to be assembled as in the prior art.

In a nuclear reactor the coolant moves from the bottom of the fuel assembly to the top of the assembly and out to the heat exchangers. The fuel assembly desirably causes a low drop in coolant pressure so that the pressure of the coolant passed through the reactor core can be minimised.

Each time the coolant comes into contact with a fuel element spacer grid it loses a certain amount of pressure.

The grid according to the present invention presents to the coolant only single strip thickness obstructions and therefore causes minimal pressure drop which allows the efficiency of the reactor to be minimised.

The amount of parasitic neutron absorption is also minimised and this and the minimised pressure drop give improved reactor fuel cycle costs.

The following is a summary of the features and benefits that the grids according to the present invention can offer. They can be of very strong and rigid construction formed from a simple interlocking assembly.

The pattern of slots ensures easy manufacture. The number and location of welding sites is minimised thereby reducing manufacturing costs. The grids can be manufactured (eg in Zircaloy 4) with low neutron absorption. The quantity of material used in the grid is minimised thereby reducing parasitic neutron absorption.

The spring loading of the fuel elements is readily adjustable in the grid. The fuel elements are free to increase in size in the grid during irradiation in the reactor. The grids form a suitable support for the fuel elements during transportation and handling. The grids offer minimal coolant pressure drop although coolant mixing vanes where required can be provided in a simple manner.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of a nuclear fuel pin spacer and holder grid; Figure 2 is side view of a strip as employed in the grid shown in Figure 1; Figure 3 is a sectional view on the line III-III of the strip shown in Figure 2; Figure 4 is a sectional view on the line IV-IV of the strip shown in Figure 2.

Figure 5 is a side view of another form of strip as employed in the grid shown in Figure 1; Figure 6 is a Sectional view on the line VI-VI in Figure 5 of the strip shown in Figure 5; Figure 7 is a sectional view on the line VII-VII in Figure 5 of the strip shown in Figure 5.

As shown in Figure 1 a grid is formed from a row of strips la running in a first direction intersecting and interlocking with a row of strips lb running in a second direction at an angle of 1200 to the first direction and a row of strips ic running in a third direction at an angle of 1200 to the first and second directions. The strips la, ib and ic may be formed of a strong, machinable metal having a suitable neutron absorbance, eg stainless steel or Zircaloy.

Hexagonal spaces 3 for holding nuclear fuel rods or pins (not shown) are formed between the strips la, lb, lc and triangular spaces 5 are formed between the hexagonal spaces 3. The spaces 5 assist coolant flow between the fuel pins during use.

Dimples 7a are formed on the side walls of the strips la which face inward to and bound the hexagonal spaces 3 and dimples 7b are formed on the side walls of the strips lb which also face inward to and bound the spaces 3.

Springs 9 comprising inwardly bent strips are formed on the side walls of the strips lc which face inward to and bound the hexagonal spaces 3.

A hexagonal outer strip 10 bounds the entire configuration of hexagons and triangles formed by the strips la, lb and lc. The outer strip 10 has on its inward facing walls dimples 12 (similar to the dimples 7a) where the strip 10 is parallel to the strips la and lb respectively and springs 14 where the strip 10 is parallel to the strips ic.

Figures 2 to 4 show one kind of the strips, viz the strips lc, from which the grid shown in Figure 1 is formed. The length of each strip ic is chosen according to the position the strip lc will occupy in the grid (the requirement for various lengths of strip lc being evident from the arrangement of the strips shown in Figure 1 within the overall hexagonal envelope of the strip 10).

This is the same for the strips la and lb.

As shown in Figure 2 the strip lc has a pattern of repeating units of pitch P. Each unit within the pattern has at the upper edge of the strip lc two large tabs 11 which form mixing vanes and two small tabs 13 to be used for welding sites the tabs 11 and 13 being provided alternately along the upper edge. At every other tab 13, labelled 13a in Figure 2, a slot 15 is provided through the tab 13a and extends perpendicular to the elongate axis of the strip lc (ie, the axis coinciding with the line IV IV shown in Figure 2). If required, the mixing vane tabs 11 may be bent into the regions which will form the triangular spaces 5 as shown in Figure 1 in an alternating pattern along the strip lc. Each slot 15 extends substantially to the centre of the strip ic, ie to the line IV-IV, ie approximately half the average width of the strip lc (not taking into account the extra width due to the tabs provided at the edges of the strip lc). Small tabs 17 also to allow welding are provided on the lower edge of the strip lc opposite the small tabs 13 at the upper edge. At every other tab 17, labelled 17a, a slot 19 is provided through the tab 17a and extends parallel to the slots 15. Each slot 19 extends substantially to the centre of the strip lc. The slots 15 and 19 are provided alternately along the length of the strip lc.

As seen in Figures 3 and 4 springs 9 are formed in the strip lc in the regions between alternate pairs of the slots 15 and 19 (ie between the first, third, fifth pairs etc but not the second, fourth, sixth pairs etc). This ensures that springs 9 are formed in regions which will comprise the hexagonal spaces 3. The positions of the springs 9 are level with alternate large tabs 11. The springs 9 are formed between pairs of slots 21 (not extending to the edges of the strip lc) provided in the strip lc, the springs 9 comprising bent portions formed by pressing of the strip lc in a direction perpendicular to the plane of the strip lc as seen in Figure 2.

Figures 5 to 7 show strips 1b as used in the grid shown in Figure 1. The strips la are identical to the strips lb. The lengths of the strips la and lb are chosen according to their position in the grid. In the strips ib there is again a pattern of repeating units of pitch P.

Each unit within the pattern has at the upper edge of the strip lb two small tabs 23 for welding similar to the tabs 13a, slots 25 similar to the slots 15 extend through the tabs 23 substantially to the centre of the strip lb. In this case there are two slots 25 per unit of pitch P and the slots 25 extend through each of the tabs 23 (not alternate tabs as in Figure 2). At the lower edge of the strip lb there are provided tabs 27 (similar to the tabs 13 in Figure 2) which are opposite the tabs 23. In this case, dimples 29 are formed at the centre of the strip lb (at a frequency of one per repeating unit to ensure that the dimples 29 are present in regions which will form the hexagonal spaces 3) by pressing of the strip lb.

The frequency of the slots 25 in the strips la is different from that of the slots 19 in the strip lc (Figure 2) because this is a function of the grid assembly design. It allows easy assembly/interlocking of the strips at different positions across the grid.

In use, the strips la, lb and lc are fitted together using a locating jig to form the grid as shown in Figure 1. In assembling the grid the strips la are initially placed in the jig (not shown) and strips lc are slotted into these strips. Next, strips la are inverted and rotated through 120O to fit into the remaining slots in the strips lc and strips lb. The slots 25 in the strips lb and the corresponding slots in the strips la receive the strips lc and the slots 15 and 19 in the strips lc receive respectively the strips la and lb. The tabs 13 and 17 on the strips lc, the tabs 23 and 27 on the strips lb and the corresponding tabs on the strips la serve to facilitate spot welding of the strips together at their edges.

In use nuclear fuel rods or pins and grids as shown in Figure 1 are assembled in a known way. The rods or pins of a given collection are held at suitable locations along their length by a suitable arrangement of grids. The whole assembly formed thereby is fitted into a receptor in a nuclear reactor core by virtue of the hexagonal shape of the grids.

In use the large tabs 11 (Figure 2) serve as coolant mixer vanes in the reactor core in the manner described above.

If no mixing vanes are used then the three sets of strips used in the grid can all be the same thereby reducing manufacturing costs.

Claims (8)

Claims

1. 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 and the spaces between the strips comprise in transverse cross-section substantially regular hexagons separated by substantially equilateral triangles.

2. A grid as in Claim 1 and wherein nuclear fuel elements are located in the hexagonal spaces between the strips.

3. A grid as in Claim 1 or Claim 2 and wherein the strips contain on or as their surfaces or selected regions forming each hexagonal space, integral formations which assist holding of the fuel elements.

4. A grid as in Claim 3 and wherein the formations comprise springs and dimples.

5. A grid as in any one of the preceding Claims and which is enclosed by an outer enclosure strip which is in the form of a hexagon or a square.

6. A grid as in any one of the preceding Claims and wherein the strips forming the hexagonal spaces have at one or both of their intersecting edges end tabs which facilitate welding of the strips and mixing of coolant when in use in a nuclear reactor.

7. A grid as in any one of the preceding Claims and wherein the strips have regularly spaced slots to allow the strips to interlock mutually, the slots having a width slightly greater than the thickness of the strips to provide a clearance fit of the strips through the slots.

8. A grid as in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.