Induced nuclear reactions: processes – systems – and elements – Grids
Reexamination Certificate
2002-05-30
2004-04-27
Keith, Jack (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Grids
C376S438000, C376S439000, C376S442000, C376S443000, C376S435000
Reexamination Certificate
active
06728329
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly, and more particularly to a fuel assembly, including fuel spacers, used for a boiling water reactor.
A fuel assembly used for a boiling water reactor has been disclosed, for example, in Japanese Patent Laid-open No. Hei 2-163695. This fuel assembly includes a plurality of fuel rods and two water rods. The fuel rods are arranged in a square lattice array of 9 rows×9 columns. These fuel rods and water rods, which form a fuel bundle, are held at mutual specific intervals and are kept immovable by a plurality of fuel spacers arranged in the axial direction.
The fuel spacer includes a large number of cylindrical members; one band member; a plurality of loop-shaped springs; and water rod holding members, each being formed into an approximately &OHgr;-shape in transverse cross-section, for holding the water rods. The large number of cylindrical members are joined to each other and are bundled. Each cylindrical member is provided at a lattice position at which a fuel rod is to be inserted, and an associated one of the fuel rods is inserted in each cylindrical member. The band member, formed into a square shape, surrounds the outer periphery of the bundle of the large number of cylindrical members.
The band member has projecting members (bath-tubs) which project on the inner peripheral side of a fuel channel box and is brought in contact with the inner surface of the fuel channel box. The loop-shaped spring is provided at a joined portion between a pair of the adjacent cylindrical members for pressingly supporting the fuel rods inserted in the adjacent cylindrical members. In addition, the loop-shaped spring functions to generate pressing forces when the fuel rods are inserted in the adjacent cylindrical members.
The water rod holding member is joined to two of the cylindrical members adjacent to the water rod. In the case where the loop-shaped springs are intended to be arranged for holding the fuel rods in the two cylindrical members adjacent to the water rod, since the mating one paired with each of the two cylindrical members is not present from the viewpoint of arrangement of springs over the fuel spacer, the spring function cannot be achieved. To cope with such an inconvenience, a spring pressing projecting piece and a spring holding projecting piece are provided at a joined portion between the water rod holding member and each of the above two cylindrical members, so that a pressing force necessary for holding the fuel rod is generated by holding the loop-shaped spring using the spring holding projecting piece and pressing it using the spring pressing projecting piece.
On the other hand, to improve nuclear characteristics of a fuel assembly for a boiling water reactor, as described in Japanese Patent Laid-open No. Hei 5-232273, there is a known configuration provided with a plurality of fuel rods including rods (hereinafter, referred to as short-length fuel rods) each having a length shorter than that of each of the remaining ones of the fuel rods. In the fuel assembly disclosed in the above-mentioned document, to improve the controllability of the reactivity due to reduction in the void coefficient, the short-length fuel rods are arranged in the outer peripheral region of the square lattice array and adjacent to the water rods.
The fuel spaces, as described above, are provided at a plurality of positions in the axial direction. As a result, of the fuel spacers used for the above fuel assembly, including the short-length fuel rods, those positioned above the upper ends of the short-length fuel rods have no fuel rod portions at lattice positions associated with the short-length fuel rods. In this regard, there has been already proposed a structure in which, in the fuel spacer positioned above the upper ends of the short-length fuel rods, the cylindrical members located at the lattice positions associated with the short-length fuel rods are removed to reduce the pressure loss.
However, in the fuel assembly disclosed in Japanese Patent Laid-open No. Hei 5-232273 in which, the short-length fuel rods are arranged in the outermost peripheral region of the square lattice array and adjacent to the water rods, if it is intended to simply remove the cylindrical members located at the lattice positions associated with the short-length fuel rods in the fuel spacer positioned above the upper ends of the short-length fuel rods, the following two problems are likely to occur:
(1) Problem in Terms of Strength
In a usual fuel spacer, short-length fuel rods are located in an inner region (not outermost peripheral region) of a square lattice array of fuel rods. As a result, even by removing cylindrical members located at the lattice positions associated with the short-length fuel rods to reduce the pressure loss, the cylindrical members in the outermost region are continuously in contact with a band member which surrounds the outer periphery of the fuel spacer, so that the structural strength of the entire fuel assembly is little reduced.
For example, if an external force is applied to the fuel spacer via a fuel channel box in case of an earthquake or upon handling of the fuel assembly, the load is first transmitted to projecting members provided on the band member. After that, the load is transmitted, via the band member, to the cylindrical members in the outermost peripheral region of the square lattice array joined to the inner side of the band member, and then the force is sequentially transmitted to the cylindrical members arranged on the inner peripheral side of the square lattice array. In the case where the band member and the cylindrical members are substantially continuously arranged in the transmission path of the load, as described above, the joined body of the band member and the cylindrical members exhibits, as one body, an effect of ensuring the strength, to thereby sufficiently ensure the structural strength of the entire fuel spacer.
On the contrary, in the case where the cylindrical members located at the lattice positions associated with the short-length fuel rods in the outer peripheral region are removed, the arrangement of the cylindrical members in the outermost peripheral region of the square lattice array becomes discontinuous at the positions where the cylindrical members are removed. This makes the strength ensuring effect of the joined body, which is composed of a large number of the cylindrical members, insufficient, and thereby reduces the structural strength of the entire fuel spacer.
To minimize such a reduction in strength, for example, Japanese Patent Laid-open No. Hei 6-3473 discloses a fuel spacer having a structure in which eight bath-tubs for mainly receiving forces applied from a fuel channel box are provided on a square-shaped band member. To be more specific, two of the bath-tubs are provided on each side of the band member in such a manner as to face to two of the bath-tubs provided on the opposed side of the band member; and cylindrical members are necessarily provided at all of the lattice positions between the two of the facing bath-tubs to ensure the strength, and the cylindrical members located at the other lattice positions are removed to reduce the pressure loss.
The fuel spacer disclosed in Japanese Patent Laid-open No. Hei 6-3473, however, causes another problem.
That is to say, in the design of a fuel assembly including short-length fuel rods, the arrangement of the short-length fuel rods varies depending on the required nuclear characteristics. However, in the above fuel spacer, the cylindrical members located at all of the lattice positions between the two of the facing bath-tubs cannot be omitted, and accordingly, if the short-length fuel rod is arranged at one of the lattice positions between the two of the facing bath-tubs, it fails to sufficiently reduce the pressure loss. Conversely, in the case of giving precedence to a sufficient reduction in pressure loss, the short-length fuel rods cannot be arranged at all of the lattice positions between two of the fa
Aizawa Yasuhiro
Fukumoto Takashi
Hirano Atsuya
Irube Makoto
Kanno Satoshi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Keith Jack
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