Patent Number: 058752234
Section: summary

TECHNICAL FIELD The present invention relates to a design of a spacer for retaining elongated elements in a fuel assembly for a light-water nuclear reactor. More particularly, the invention relates to the design of a spacer sleeve for such a spacer. BACKGROUND OF THE INVENTION A fuel assembly in a boiling water nuclear reactor comprises a long tubular container, often with rectangular or square cross section, which is open at both ends forming a continuous flow passageway, through which the coolant of the reactor may flow. The fuel assembly comprises a large number of equally long tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To allow coolant to flow past the fuel rods in the desired way, it is important that these be spaced from each other and prevented from bending or vibrating when the reactor is in operation. For this purpose, a plurality-of spacers are used, distributed along the fuel assembly in the longitudinal direction. A fuel assembly for a pressurized-water nuclear reactor has, in principle, the same construction as a fuel assembly for a boiling water nuclear reactor, except that the fuel rods are not enclosed by some tubular container and that the number of fuel rods is larger. In connection with repair and service of a nuclear reactor, foreign matter may enter the coolant. The foreign matter then moves with the coolant which is circulated in the reactor core. The foreign matter may consist of metal chips or pieces of metal wire. In those cases when the foreign matter adheres to the spacers or to other locations in the fuel assembly, it may give rise to wear damage on the elongated elements. The wear damage may have serious consequences, especially if it occurs on parts which are particularly fragile such as the fuel rods. This may be the case if the debris adheres to a spacer such that, because of the upwardly-flowing coolant, it is set into vibration thus wearing against one or more of the fuel rods which are positioned by the spacer. To avoid damage of the above kind, it is known to design various forms of debris-capturing means which are intended to be arranged below the fuel assembly in such a way that the coolant is forced to pass therethrough before it is passed into the fuel assembly. By arranging debris-separating means in this way, the main part of the foreign matter is prevented from entering the fuel assembly proper. A disadvantage with additional parts introduced in a fuel assembly is that they almost always lead to the increase of the pressure drop across the fuel assembly. The present invention relates to an alternative way of reducing the risk of wear on the fuel rods caused by foreign matter adhering to the upstream edge of the spacer. SUMMARY OF THE INVENTION According to one aspect of the present invention, a reduced risk of wear on the fuel rods by foreign matter is achieved by an alternative design of already known spacers. The invention is applicable to already-known spacers of the type comprising a grid structure of sleeves. By designing the upstream edge of such spacers with a wavy form, foreign matter captured towards the upstream edge of the spacer may be oriented such that it will not make contact with the elongated fuel rods positioned by the spacer. The wavy edge is suitably made such that that part of the upstream edge, which is arranged adjacent another sleeve to be joined together with this sleeve in the grid structure of the spacer, encounters the upwardly-flowing coolant after the coolant has encountered the edge between the joints. In this way, any foreign matter is oriented such that it is arranged between the elongated elements transversely of the flow direction when being captured. The captured foreign matter will thus make contact with that part of the wavy edge which is disposed at the joint with adjacently located sleeves in the grid structure and diagonally across the flow channel which is formed between the sleeves in an orthogonal grid structure, thus as far away from the rod surfaces as possible. In a particularly preferred embodiment of the invention, the sleeves are provided with elongated embossments adapted to coincide with that part of the edge which is arranged between the joints. The elongated embossments are adapted to substantially make contact with the fuel rods. By forming the spacer with a wavy edge, it can be given a low flow resistance while at the same time foreign matter which adheres to the upstream edge of the spacer is efficiently captured and oriented such that it does not make contact with and thus does not cause wear on the elongated elements. The low flow resistance is achieved by making the flow-preventing transverse area in a cross section through that part of the space which is provided with the wavy edge smaller than in a spacer with a substantially straight edge.