Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-01-27
2001-11-20
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S434000, C376S442000, C376S448000, C376S462000
Reexamination Certificate
active
06320925
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a spacer for a fuel assembly of a nuclear power station, in particular for a fuel assembly of a light water reactor.
Spacers are used for fixing fuel rods in a fuel assembly. A spacer forms a matrix of intersecting webs and a base area covered by the spacer corresponds essentially to the cross sectional area of the fuel assembly. Each web has an assembly gap which is disposed at an intersection location with an intersecting web and receives the intersecting web. The webs which are guided in the assembly gaps and subsequently fixed, for example by welding, form cells having an essentially rectangular or square base area. The fuel rods, as well as guide tubes in the case of pressurized water fuel assemblies and possibly water rods in the case of boiling water fuel assemblies, project through the cells formed by the spacer and are held there. In other words, they are fixed in their position relative to the center axis of the fuel assembly.
In certain circumstances, under the operating conditions in the core of the nuclear reactor, the spacers of a fuel assembly may undergo longitudinal expansion which may lead to an increase in the external dimensions of the spacers and consequently of the fuel assembly. In an extreme case, the result of the increase in the external dimensions may be that a fuel rod bundle formed by the spacer can no longer be removed from a fuel assembly box, for example in the case of a fuel assembly for a boiling water reactor.
In the case of fuel assemblies of pressurized water reactors, the longitudinal expansion of the spacers is unusually high. The increase in the external dimensions may cause complications with the adjacent fuel assemblies during servicing work and during loading and unloading of the reactor core.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a spacer for a fuel assembly of a nuclear power station, which overcomes the heretofore-mentioned disadvantages of the heretofore-known devices of this general type and which undergoes insignificant longitudinal expansion during an operating period.
With the foregoing and other objects in view there is provided, in accordance with the invention, a spacer for a fuel assembly of a nuclear power station, comprising webs disposed in a grid defining intersection locations, each of the webs having a wall thickness and having an assembly gap receiving an intersecting web at one of the intersection locations; the assembly gap in each of the webs having regions through which parts of the other of the webs pass in the intersection location of two webs, at least two of the regions having different widths; and the assembly gap in each of the webs having a total length formed by the regions through which the parts of the other of the webs pass in the intersection location of two webs, at most a fraction of the total length having a width substantially corresponding to the wall thickness of the other of the webs.
Therefore, the assembly gap has a width, at most over a fraction of its total length, which corresponds essentially to the wall thickness of the intersecting web, but in a remaining region is wider than the wall thickness of the intersecting web.
The invention proceeds from the knowledge that, under normal operating conditions, the longitudinal expansion of a spacer may be caused by corrosion of the spacer. A corrosion layer may form on the web wall in the assembly gap, particularly at assembly gaps of a spacer web formed of a zirconium alloy, for example, a Zircalloy plate. In order to achieve a high accuracy of fit of the spacer composed of the webs, the assembly gaps have heretofore been dimensioned in such a way that their width corresponds essentially to the wall thickness of the intersecting web. With that structure of the spacer, there is the risk that the corrosion layers, which grow toward one another from the edges of the assembly gaps and from the surface of the inserted web, will meet one another before the end of the period of use. The corrosion layers then exert a solid pressure on the respective web which may lead to a lengthening of the web.
Since each web has a multiplicity of assembly gaps with corrosion-endangered regions, the resulting total longitudinal expansion of the webs may become so great that the change in the external dimension of the spacer exceeds a critical value. Moreover, a varying longitudinal expansion of different webs may lead to the warping of individual webs and/or of parts of the spacer, thus adversely influencing the flow properties of the spacer.
The invention proceeds, then, from the notion of reducing the solid pressure exerted on a web by the corrosion layers.
According to the invention this is achieved, on one hand, in such a way that the assembly gaps of two webs which are provided at an intersection location with another web, have a width only over a fraction of their total length, through which parts of a web pass in each case, that corresponds to the wall thickness of the intersecting web. By virtue of the reduced bearing surface between the edges of the assembly gaps and the intersecting webs, the solid pressure on the web decreases, thus resulting in a considerable reduction in the undesirable longitudinal expansion of the web. The length of the regions in which the width of the assembly gap corresponds essentially to the wall thickness of the intersecting web is selected in such a way that the spacer matrix has a strength which is sufficient for subsequent machining steps, such as, for example, the welding of the webs at the intersection locations, and for the loads which occur under operating conditions. This affords the advantage of ensuring that, after the webs have been welded together, the assembly gaps do not have to be widened in an additional operation, for example by pickling or corroding, in order to reduce the solid pressure on the web which occurs as a result of corrosion.
In accordance with another feature of the invention, each web has a recess on at least one side of each assembly gap, in the region of the narrowest cross section of the latter, wherein the recess is adjacent the assembly gap. This affords the advantage of permitting web deformation caused by corrosion in the region of the narrowest cross section of the assembly gap to be absorbed by the recess. The solid pressure on the remaining part of the web can thereby be further reduced.
If the web is to have only minimal longitudinal expansion due to corrosion, without an additional recess, the width of the assembly gap is dimensioned, virtually over its entire length, to be greater than the wall thickness of the intersecting web. In order to acquire the necessary stability of the spacer matrix formed from the webs, the assembly gap has only a few (but at least three) support locations along its axis or principal extent, for example bearing points or bearing regions, at which the assembly gap touches the intersecting web. The support locations are disposed on both sides of each assembly gap, in such a way that the intersecting web is supported in each case on only one side over a sufficiently small area. The advantage of this is that a solid pressure may build up at the few support locations during the operating period and could partially deform the intersecting web transversely to its direction of principal extent, and consequently only some of the solid pressure contributes to the longitudinal expansion of the web.
In accordance with a further feature of the invention, a further reduction in the solid pressure leading to longitudinal expansion is achieved through the use of apertures in the middle of the web. Preferably, each web has at least one aperture disposed on an assembly axis of each assembly gap. Advantageously, each assembly gap opens in each case into at least one of the apertures. Thus, corrosion-endangered locations in the web are cut out and, moreover, it becomes possible to have a sufficient cooling water stream for the regions of the spacer which are lo
Bokers Franz-Josef
Dewes Peter
Garzarolli Friedrich
Jahreiss Werner
Manzel Reiner
Carone Michael J.
Greenberg Laurence A.
Lerner Herbert L.
Richardson John
Siemens Aktiengesellschaft
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