DE29919413U1 - Fuel assembly with different types of spacers - Google Patents

Description

GR 99 .G 3012

Description

Fuel element with different types of spacers

The invention relates to a fuel element for a nuclear reactor with a number of axially spaced spacers of different types.

- In a fuel element for a nuclear reactor, for example for a pressurized water or boiling water reactor, spacers are provided in the direction of the longitudinal axis of the fuel element, which are axially spaced apart and are designed as a grid. The fuel rods and supporting tubes are guided through these. The supporting tubes connect the fuel element base to the fuel element head and together form a stable framework for the fuel element. In a pressurized water reactor fuel element, guide tubes are provided for this purpose, which serve to guide control rods. In a boiling water reactor fuel element, for example, a central water tube is arranged. The spacers serve to hold the fuel rods and the guide tubes and to space them apart from one another. The spacers ensure that the fuel rods run largely parallel and that a uniform distance is maintained between them, through which the coolant flows during operation.

In the September 1998 issue of the magazine "Nuclear Engineering International", on pages 22 to 27, fuel elements currently in use by various manufacturers for different reactor types are shown. All fuel elements have a number of axially spaced spacers in common. Among other things, a fuel element is shown in which the bottom and top spacers are made of Inconel. The spacers arranged in between in the area active for power generation are made of Zircaloy. The area active for power generation is defined here as the area

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in the fuel element, in which the fuel is arranged in the fuel rods. The fuel rods are not completely filled with fuel, since an escape space for fission gases is provided within a fuel rod. In general, it can be assumed that at least the second lowest spacer is already in the active zone, which extends at least to the third highest or usually to the second highest spacer.

In a boiling water reactor, each fuel element is arranged individually in a fuel element casing. The cooling liquid flows into the fuel element casing from below and leaves it at the top after flowing through the fuel element.

In a pressurized water reactor, however, the fuel elements are arranged directly next to one another. In order to achieve sufficient stability of the entire fuel element arrangement in the nuclear reactor or in a fuel element storage rack, the spacers are mounted at the same height so that adjacent fuel elements support one another on the spacers. This gives the fuel element arrangement a high level of stability without the fuel elements being firmly connected to one another. This is a significant advantage for safety reasons in the event of vibrations that may occur, for example as a result of earthquakes.

The fuel rods and fuel elements must be at a precisely defined distance from each other so that the surrounding water (moderator) produces the exact moderation effect and the flow of thermal neutrons occurs that leads to the optimal use of the fuel. If a fuel element has too much play in the core, it can happen that it is too close to the neighboring fuel element on one side and too far away from the other. The fuel is then burned slowly on one side and is not

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fully utilized, whereas on the other side it is prematurely used up.

In particular, the optimal conditions (e.g. the ratio of moderator to fuel) in the individual areas of the fuel element and in the gaps between the fuel elements should not change during operation. Uneven burn-up in the fuel element leads to uneven thermal loading, which is small but is considered to be the cause of the well-known banana-shaped bending of the fuel elements and leads to uneven spacing between the fuel rods if the fuel elements are not stiffened by adjacent fuel elements or fuel element boxes, but are surrounded by gaps that are too large.

The external dimensions of the spacers must therefore be selected in such a way that the gaps between spacers of adjacent fuel elements are as small as possible. Even in a boiling water reactor fuel element, small gaps towards the fuel element casing are necessary.

On the other hand, the gap must be sufficiently large to allow the fuel elements to be moved relative to each other or the fuel element to be moved relative to its fuel element casing in the axial direction, as is necessary, for example, when changing the fuel elements.

When a nuclear reactor is in operation, the grids of the spacers grow larger due to radiation exposure over the course of the operating period. This particularly affects the spacers that are located in the area that is active for power generation, as this is where the highest radiation exposure occurs.
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The expansion of the spacer grids may lead to the fuel elements no longer being able to move against each other or against each other.

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can no longer be moved relative to the fuel element casing. This can cause significant disruptions, for example when changing the fuel elements.

The "grid" of a spacer is understood here only to mean the structure formed by the webs on which the fuel rods are supported, which does not include so-called flow guide vanes, which are provided in the upper area of the active zone on the upper edges of the webs in order to impose certain flows on the flowing coolant (water), while in the lower area such flow guide vanes only represent an unnecessary flow resistance and are omitted. The webs of the spacers themselves (i.e. the "grids") are, however, the same in such cases throughout the entire active zone.

Likewise, the term "spacers" does not refer to the so-called "intermediate grids" which are arranged in some pressurized water fuel elements between the positions of the usual spacers and only serve as supports for such flow guide vanes, but do not have a supporting function for the fuel rods and do not serve to support the fuel elements against each other. Such intermediate grids are generally weaker than the spacers themselves.

The present invention is based on the object of specifying a fuel element in which the spacers have a suitable distance from adjacent spacers or from the fuel element casing during the operating period.

The object is achieved according to the invention by a fuel element in which a spacer with a grid type A and a maximum external dimension and a spacer with a grid type B are provided in the area active for power generation, which is designed in such a way that it does not exceed the maximum external dimension when expanded due to operation of the nuclear reactor.

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The invention is based on the basic idea of using different spacer grids in order to be able to adapt them to the requirements associated with their arrangement in the fuel element. Special spacers are provided where the loads and requirements are particularly high, and where particularly strong growth is to be expected, spacers are used whose growth can be tolerated. This in particular prevents the expansion of the spacers caused by the operation of the reactor as a result of radiation-induced growth, creep or corrosion from leading to jamming of the fuel elements among each other or of the fuel element in the fuel element casing. At the same time, losses in fuel utilization due to dimensions are reduced.

In order not to exceed the maximum external dimensions, the following three particularly advantageous variants are proposed according to the subclaims, which can also be combined with each other:

a) The grid type B has a different and in particular smaller dimension than the grid type A.

b) Grille type B has at least one removable spacer element.

c) Grid type B consists of a material that has only a small expansion when exposed to radiation and other expansion-promoting factors arising during reactor operation.

The two grid types A and B can have the same geometric shape, provided they differ at least in their material.

In variant a), the spacer with grid type B, hereinafter referred to as type B spacer, is designed from the outset due to its small external dimensions in such a way that

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exceeding the maximum external dimensions is not possible even over the course of several years of operation. The type B spacer or spacers is/are preferably only arranged in the area of the highest load, where the highest growth rates of the spacers are to be expected. In the remaining areas with a lower growth rate, spacers with grid type A, hereinafter referred to as type A spacers, with larger external dimensions are provided. Type A spacers are provided in particular in the lower area of the fuel element in order to achieve sufficient stability of the fuel element arrangement.

Preferably, the external dimensions of several Type B spacers become increasingly smaller or larger in the direction of the fuel element axis, depending on the areas in which the highest growth rates are expected. For example, the external dimensions continuously increase or decrease.

In its new condition, i.e. before it is first exposed to thermal stress or radiation during operation, the Type B spacer has an external dimension that is approximately 0.5 mm to 3 mm smaller than the maximum external dimension of the Type A spacer.

The advantage of variant b) is that, even when new, the spacer elements mean that there is only a small gap between the spacers of adjacent fuel elements, which results in high mechanical stability. After a certain period of operation, when the spacers have expanded in the active area, the spacer elements can be easily removed so that the fuel elements do not jam together. In a preferred embodiment, the spacer element is designed as a circumferential band or as a latchable strip or as a tag that can be removed from the spacer to make it easy to remove.

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The advantage of variant c) is that the choice of material means that the maximum external dimensions cannot be exceeded. Inconel is preferably used as the material for the type B spacer, as it has the required low expansion. However, Inconel has the disadvantage that it has a high neutron absorption and thus partially inhibits power generation. Using Inconel for all spacers is therefore disadvantageous with regard to power development in the fuel element, so that preferably only some of the spacers are made of Inconel. Steel can also be used for the type B spacer. In an advantageous embodiment, the type A spacer is made of Zircaloy, which has a lower neutron absorption capacity than Inconel. Outside the active area, Inconel is suitable due to its high strength. Its high neutron absorption is insignificant in this non-active area.

For all three variants, a preferred embodiment is that the type A spacer is designed for high mechanical load-bearing capacity in order to give the fuel element arrangement a high level of mechanical stability. For this purpose, the type A spacer has, for example, a particularly thick sheet. Zircaloy is primarily suitable for this due to the problem of neutron absorption capacity.

In a fuel element intended for a pressurized water reactor, type A spacers are preferably provided in the lower active region oriented towards the fuel element base and type B spacers are provided in the upper active region oriented towards the fuel element head. In particular, the type B spacers are located exclusively in the upper active region, where the highest growth rates occur.

In contrast, in an advantageous embodiment of a fuel element intended for a boiling water reactor, the type B spacers are only present in a central area.

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because, in contrast to the pressurized water reactor, the highest growth rates occur in the middle range.

It is particularly advantageous if the type B provided in the middle area according to variant C is not made of Zircaloy, for example, like type A, but of Inconel, which has practically no radiation growth; its external dimensions can therefore be selected as desired for the final state after a longer service life, i.e. after the spacers of type A in the other areas have grown accordingly. The external dimensions of type B therefore do not need to be smaller than those of type A; in fact, they can even be larger in the initial state. This also reduces the risk of the banana-shaped bends mentioned.

Embodiments, further details and advantages of the invention are explained in more detail below with reference to the drawing. They show in schematic representation: 20

FIGS 1 and 2 show a fuel element in a longitudinal view, FIG 3 shows a partial view of an arrangement of several fuel elements for a pressurized water reactor in a plan view, FIG 4 shows a partial view of an arrangement of four fuel elements each arranged in a fuel element box for a boiling water reactor in a plan view and

FIGS 5 to 7 each show a spacer with differently designed spacer elements in a

view,

FIG 8 shows a particularly advantageous variant of the fuel element.

According to Figures 1 and 2, a fuel element 2 extends along a longitudinal axis 4 from a fuel element base 6 to a fuel element head 8. Between fuel element base 6 and

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A number of fuel rods 10 are arranged on the fuel element head 8, which run essentially parallel and equidistant from one another. When a nuclear reactor is operating, a cooling and moderator liquid F flows through the fuel element 2 along its longitudinal axis 4. If the fuel element 2 is intended for a pressurized water reactor, in addition to the fuel rods 10, guide tubes (not shown in detail) are provided for receiving control rods, which together with the fuel element base 6 and the fuel element head 8 form a load-bearing frame for the fuel element 2. The embodiments according to Figures 1 and 2 show typical embodiments of a fuel element 2 for a pressurized water reactor.

The fuel rods 10 and, if applicable, the guide tubes are guided by a number of spacers A which are axially spaced apart from one another in the direction of the longitudinal axis 4. These are numbered 1 to 9 (Figure 1) or 1 to 8 (Figure 2). The spacers A are generally designed as grid plates (see Figures 5 and 6). They serve to space the individual fuel rods 10 apart from one another so that the distances between the individual fuel rods 10 remain largely constant in order to ensure the most homogeneous power generation possible and thus the most homogeneous loading of the fuel rods 10 possible. Some (or all) of the spacers can carry the flow guide vanes already mentioned.

The fuel rods 10 are generally designed as cladding tubes into which the fuel (not shown in detail) is introduced. However, the fuel rod 10 is not completely filled with fuel, so that the ends of the fuel rods in particular do not contribute to power generation. The arrangement of the fuel in the individual fuel rods 10 defines an active area 12 for power generation.

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The fuel element 2 in Figure 1 differs from that in Figure 2 in addition to the number of spacers A (8 instead of 9) and in addition to the design of the fuel element base 6 and the fuel element head 8, in particular with regard to the extent of the active region 12. While in Figure 2 the active region 12 extends close to the fuel element base 6 and the first spacer Al is already arranged in the active region 12, the active region 12 according to Figure 1 is further away from the fuel element base 6 and the spacer Al is arranged in the non-active region. The number of spacers A depends, among other things, on the length of the fuel element 2, which is generally between 4 and 6 m.

The fuel element 2 and thus also the spacers A are square in cross-section, as can be seen in Figure 3. When arranging several fuel elements 2 in a nuclear reactor, a nominal distance N is provided for the fuel elements, which is slightly larger than the maximum external dimension M of a fuel element 2. The maximum external dimension M is also the maximum external dimension M of the spacers A. The nominal distance N is, for example, 230 or 215 mm, and the associated maximum external dimension M is approximately 229 or 214 mm. A gap of approximately 1 mm is therefore provided between two adjacent fuel elements, which allows the fuel elements 2 to be moved against each other, for example when changing the fuel elements. The small gap also ensures that the fuel elements 2 support each other on their spacers and cannot tip over. The spacers A of the individual fuel elements 2 are preferably arranged at the same height for this purpose.

In a boiling water reactor, the fuel elements 2 are each arranged in a fuel element box 14, as can be seen in Figure 4. The nominal distance N in this case is formed by the inner dimensions of the fuel element box 14. Here too, the maximum outer dimension M is smaller than the

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nominal distance N in order to be able to move the fuel elements 2 relative to the fuel element boxes 14. The four fuel element boxes 14 shown in Figure 4 are arranged in such a way that they form a cross-shaped gap into which a control rod (not shown in detail) can be inserted for power regulation.

The thermal stress that occurs during operation, and in particular the radiation exposure, leads to growth or expansion of the components used to construct the fuel element over the course of the operating period. This also applies to the spacers A1 to A9, whose external dimensions A increase. This can lead to fuel elements 2 jamming against one another or to a fuel element 2 jamming in a fuel element casing 14.

In order to prevent this, according to a first variant (variant a), at least some of the spacers in the active region 12 are designed with a reduced external dimension R, as is indicated in Figures 1 and 2 for some spacers A. The reduced external dimension R is approximately 1 to 3 mm smaller than the maximum external dimension M. Since the growth rate in the active region varies depending on the power density, the spacers A with the reduced external dimension R are preferably arranged only in the region of the highest power density in order to influence the mechanical stability of the fuel element arrangement as little as possible. In a fuel element 2 for a pressurized water reactor, for example, only the last three spacers A6 to A8 (Figure 1) or the last two spacers A6, A7 in the active region 12 (Figure 2) preferably have a reduced external dimension R. The spacers A can also have graduated reduced external dimensions R, which extend, for example, over the entire active region 12 and have the smallest external dimensions in the region in which the greatest expansions are expected. The external dimensions decrease in particular continuously.

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According to a second variant (variant c), at least some spacers A in the active region 12 are made of a material that has only very low expansion rates. Inconel is particularly suitable as such a material. Inconel is a registered trademark and refers to a nickel-chromium alloy. As with variant a), the Inconel spacers A are preferably arranged in the area of the highest power density. The Inconel spacers A can also have a reduced external dimension R. The remaining spacers A in the active region 12 and in the non-active region are preferably made of Zircaloy, also a registered trademark, for a zirconium alloy. The advantage of the Zircaloy spacers A is that they have only a low neutron absorption, so that the power generation is only slightly influenced by the spacers. In the non-active region, the neutron absorption of the spacers plays no role, so that Inconel, for example, can also be used as a material there.

According to a third variant (variant b), as shown in Figures 5 to 7, a spacer element 16A to 16D is attached to the outside of the spacer A. The spacer A consists essentially of interlocking sheets 18 which are arranged perpendicular to one another and form a grid with grid openings 20 through which the fuel rods 10 (only a few are shown as examples) or the guide tubes are guided.

The spacer element 16A to 16D is preferably attached to the spacer A in such a way that it can be easily removed from the spacer A if necessary. According to Figure 5, it is designed as a band 16A that runs around the spacer A and is connected to the spacer A via a number of easily detachable fastening means 22. The fastening means 22 are, for example, knobs or spring elements that fit into recesses provided for them in the spacer

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holder A engages and locks into place, or also adhesives or welding points.

The same fastening means 22 can also be used to fasten the strip 16B shown in Figure 6 or the buttons 16C shown. The spacer element 16A to 16D does not necessarily have to run around the entire outer surface of the spacer A. It is sufficient if spacer elements 16A to 16D are only fastened at a few points.

According to Figure 7, a pendant 16D is used as a spacer element, which is U-shaped, for example, and is hung over the outermost sheet 18 of the spacer A. The inner leg of the U-shaped pendant engages in the grid opening 20 formed by the sheets 18. Its outer leg serves to enlarge the external dimensions of the spacer A.

The spacer elements 16A to 16D are provided for spacers A with a reduced external dimension R. The spacers A are made of Zircaloy or Inconel, for example. After a certain period of operation, when the gap dimensions between individual fuel elements 2 reach a value below which displacement of the fuel elements against each other can no longer be easily guaranteed, the spacer elements 16A to 16D are either completely or partially removed from the relevant spacers A. For partial removal, the spacer elements 16 are constructed in layers, for example. It can be particularly advantageous to provide spacers A for a fuel element 2 that have spacer elements 16A to 16D with different thicknesses in order to achieve the greatest possible stability of the fuel element arrangement while at the same time ensuring a sufficient gap dimension.

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A particularly advantageous embodiment is shown in Figure 1 if, according to Figure 8, 12 spacers made of Zircaloy A2, A3, A7, A8 and practically identical spacers made of Inconel A4, A5, A6 are used within the active zone. 5

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Claims (11)

1. Fuel element ( 2 ) for a nuclear reactor with a number of axially spaced-apart grid-shaped spacers (A) of different types, and with an active region ( 12 ) for power generation, wherein in the active region ( 12 ) a spacer with a grid type A and a maximum external dimension (M) and a spacer with a grid type B are provided, the latter being designed in such a way that it does not exceed the maximum external dimension (M) when expanded due to operation of the nuclear reactor. 2. Fuel element ( 2 ) according to claim 1, wherein the grid type B has a different, and in particular smaller, external dimension than the grid type A. 3. Fuel element ( 2 ) according to claim 2, in which several grid types B are provided, the external dimensions of which become increasingly smaller or larger in the direction of the fuel element axis. 4. Fuel element ( 2 ) according to one of claims 1 to 3, wherein the grid type B in the new state has an external dimension which is approximately 1 mm to 3 mm smaller than the maximum external dimension (M) of the grid type A. 5. Fuel element ( 2 ) according to one of the preceding claims, in which the grid type B has a removable spacer element ( 16A to LED) for increasing the external dimensions. 6. Fuel element ( 2 ) according to claim 5, wherein the spacer element ( 16 A to 16 D) is designed as a circumferential band ( 16 A), or as a latchable strip ( 16 B), or as a tag ( 16 D) removable from the spacer (A). 7. Fuel element ( 2 ) according to one of the preceding claims, in which the grid type B consists of a corrosion-resistant material which has only a small expansion under radiation exposure. 8. Fuel element ( 2 ) according to one of the preceding claims, in which the grid type A consists of Zircalloy and the grid type B consists of Inconel. 9. Fuel element ( 2 ) according to one of the preceding claims, in which the grid type A is designed for high mechanical load-bearing capacity. 10. Fuel element ( 2 ) according to one of the preceding claims for a pressurized water reactor, wherein grid type A spacers are provided in the lower region oriented towards the fuel element base ( 6 ) and grid type B spacers are provided in the upper region. 11. Fuel element ( 2 ) according to one of the preceding claims for a boiling water reactor, wherein the grid type B is provided only in a central region.