Patent Number: 
Section: description

1. Field of the Invention The present invention relates to a fuel assembly for a nuclear reactor using a coolant such as a liquid metal, and particularly to a fuel assembly which is configured to store and hold a plurality of fuel pins in a wrapper tube by using grids and liner tubes and which suppresses an unnecessary flow of the coolant in an outer circumferential side in the wrapper tube and increases the flow volume of the coolant passing through interiorly disposed ones of the fuel pins to thereby increase the core power. 2. Related Art Generally, in a nuclear reactor, a fuel assembly is supported in a reactor core while being attached to a support member. In a nuclear reactor using a coolant such as a liquid metal, an electromagnetic pump is used as a drive source to circulate the coolant around a plurality of fuel pins included in the fuel assembly supported in the reactor core. In this case, if the nuclear reactor is small-sized, the fuel assembly is configured to store the fuel pins in a wrapper tube to enable the circulation of the coolant with no need for the drive source. The wrapper tube is configured to include an entrance nozzle at a lower end thereof for introducing the coolant, and an operation handling head at an upper end thereof. The wrapper tube includes therein grids for supporting the fuel pins in the radial direction of the wrapper tube, and liner tubes inserted in the wrapper tube for fixedly holding the respective grids in the axial direction of the wrapper tube. The intervals in the radial direction of the fuel pins are kept by the grids. Meanwhile, the intervals in the axial direction of the grids are kept by a tie rod, the liner tubes, or the like (see Japanese Unexamined Patent Application Publication No. 6-174882, for example). FIGS. 23 and 24 illustrate the configuration of this type of conventional fuel assembly. In the figures, a plurality of fuel pins 101 are stored in a wrapper tube 103, with the pin intervals of the fuel pins 101 being kept by grids 102. Each of the fuel pins 101 is fixed at a lower portion thereof by a lower pin support plate 105 and at an upper portion thereof by an upper pin support plate 106. The coolant such as a liquid metal flows in from a coolant inlet 108 of an entrance nozzle 104 and flows out from a coolant outlet 109 of a handling head 107. In the thus configured fuel assembly, as illustrated in FIG. 25, each of the grids 102, which has a low pressure drop, includes a grid frame 102a provided with a multitude of ring-shaped pin support members 110. As illustrated in FIG. 26, for example, each of the pin support members 110 is provided with three dimples 110a on the inside thereof such that the circumference of the corresponding fuel pin 101 is three-point supported, for example, by the dimples 110a.  FIG. 27 illustrates a deformation state in which the wrapper tube 103 is expanded by the thermal expansion. That is, the wrapper tube 103, the basic form of which is a regular hexagon as indicated by a virtual line in FIG. 27, is expanded when used due to the irradiation deformation and is deformed so as to expand toward the outer circumference thereof as indicated by a solid line. Conventionally, to cope with such deformation, liner tubes each formed by a thin hexagonal tube are provided outside a fuel bundle such that the liner tubes and the grids are alternately stacked. Thereby, the intervals in the axial direction of the grids are kept. In such a configuration, however, the flow passage area around the fuel bundle is large. Thus, the cladding temperature in a central area of the fuel bundle becomes relatively high in some cases. Therefore, there arises a need to keep the cladding temperature equal to or lower than a cladding temperature limit. As a result, the thermal efficiency is decreased. To address this issue, the inventors of the subject application have proposed a technique for reducing the cladding temperature, in which followers each having a triangular cross section are provided to reduce the flow passage area of a bundle edge sub-channel in a core heat generation unit for preventing a peripheral flow. That is, according to the technique, the liner tubes are provided with peripheral flow preventing projections to suppress the occurrence of the above-described phenomenon (see “Development of Densely. Packed and Low-Pressure-Drop Fuel Assembly for Non-Refueling Core (3),” 2004 Fall Meeting Preliminary Proceedings 307 of the Atomic Energy Society of Japan, for example). Meanwhile, in the above-described conventional configuration, the liner tubes and the grids are stacked and may be mutually misaligned in the radial direction. If the liner tubes and the grids are misaligned in the radial direction, an opening may be formed between the wrapper tube and the liner tubes to allow the coolant to flow from inside the liner tubes into the space on the wrapper tube side as a waste flow. Further, in the conventional fuel assembly, the bulging deformation occurs in the wrapper tube by the irradiation creep due to the inner pressure of the wrapper tube. It is therefore possible in the expanded portion that the flow passage area of a peripheral region around the fuel bundle is increased while the flow volume of the coolant in the central area of the fuel bundle is reduced, and thus that the cladding temperature is increased. It is also possible that the liner tubes are similarly expanded due to the inner pressure applied thereto. The present invention has been made in light of the above-described circumferences, and it is an object of the present invention to provide a fuel assembly which achieves a high thermal efficiency and a stable lifetime performance by preventing an unnecessary flow of a coolant in an outer circumferential area therein and by causing the coolant to effectively flow toward interiorly disposed fuel pins. To achieve the above object, the present invention provides a fuel assembly charged in a reactor core of a nuclear reactor using a liquid metal as a coolant. The fuel assembly includes a wrapper tube, grids, liner tubes, and a fixing device. The wrapper tube includes an entrance nozzle for introducing the coolant and an operation handling head, and stores a plurality of fuel pins. The grids are disposed in the wrapper tube to support the fuel pins in the radial direction of the wrapper tube. The liner tubes are inserted in the wrapper tube to fixedly hold the respective grids in the axial direction of the wrapper tube. The fixing device fixes the grids and the liner tubes in the radial direction of the wrapper tube. Further, in a preferable embodiment of the fuel assembly, the fixing device may include pins for fixing joining ends of the grids and the liner tubes along the radial direction of the wrapper tube. Furthermore, the fixing device may further include pin support portions, which are through holes formed on an outer circumferential side of a grid frame of each of the grids at positions corresponding to positions of engaging portions of the liner tubes, and through which the pins can be inserted in the vertical direction. The fuel assembly may further include a coolant blocking member for preventing the coolant from flowing in a gap between the inner circumference of the wrapper tube and the outer circumference of each of the liner tubes. The coolant blocking member may include contact pieces, which project from an outer circumferential side of the liner tube to come in contact with the inner surface of the wrapper tube, and which are formed of an elastic material capable of increasing the range of closure in accordance with the expansion of the wrapper tube. The coolant blocking member may be a skirt-shaped member hanging from an upper end portion of the liner tube along the outer circumferential surface of the liner tube, and may include a plurality of divided pieces divided by vertically extending grooves to individually come in contact with the inner circumferential surface of the wrapper tube. It is preferable to form the coolant blocking member from a high nickel steel. Further, the inner circumferential surface of a grid frame of each of the grids may be formed with a plurality of projections for closing gaps between outer peripherally disposed ones of the fuel pins. The projections may be formed in accordance with the pin pitch of the fuel pins. Furthermore, the fuel assembly may have a structure in which at least either one of a grid frame of each of the grids and a peripheral wall of each of the liner tubes is formed as a concave and convex wall bent toward the inner circumference thereof, and in which parts of the concave and convex wall projecting toward the inner circumference thereof close gaps between outer peripherally disposed ones of the fuel pins. An end portion of either one of the grid frame and the liner tube may be provided with closure portions for closing a space on the outer circumferential side of the parts of the concave and convex wall closing the gaps between the outer peripherally disposed ones of the fuel pins. The inner circumferential surface of each of the liner tubes may be provided with a plurality of rod members extending along the axial direction. Each of the rod members may have a substantially angular cross section and be disposed in accordance with the pin pitch of the fuel pins to close gaps between outer peripherally disposed ones of the fuel pins. Further, an upper end portion in the wrapper tube may be provided with an upper pin support plate for supporting the fuel pins, and the upper pin support plate may be pierced through by a tie rod, the upper end of which presses and holds downward the grids and the liner tubes via an elastic member. It is preferable to form the elastic member by a compression coil spring. Furthermore, a peripheral wall of each of the liner tubes may be drilled with a plurality of holes piercing through the peripheral wall to allow the coolant to flow between a space on the side of the wrapper tube and a space on the side of the fuel pins. According to the present invention, with the provision of the fixing device for fixing the end portions of the grids and the liner tubes in the radial direction, a gap can be prevented from being formed between the grids and the liner tubes by a positional misalignment in the radial direction. Therefore, the unnecessary flow of the coolant can be prevented, and the improvement of the thermal efficiency of the fuel assembly and the stabilization of the lifetime performance of the fuel assembly can be achieved. Further characteristics of the present invention will be made clearer from the following detailed description with reference to the accompanying drawings. Embodiments of a fuel assembly according to the present invention will be described below with reference to FIGS. 1 to 22.  FIG. 20 is a schematic view illustrating a fourth embodiment of the present invention, and FIG. 21 is a partially enlarged cross-sectional view of FIG. 20. Further, FIG. 22 is an enlarged view of main parts of FIG. 21. The present embodiment is configured such that an upper end portion in the wrapper tube 2 is provided with the upper pin support plate 6 for supporting the fuel pins 5, and that a tie rod 21 penetrating the upper pin support plate 6 has an upper end which can be pressed down by an upper end plug 25 via an elastic member 24 such as a compression coil spring. The grids 9 and the liner tubes 8 are pressed and held downward by the elastic member 24. That is, an upper pin support ring 23 is provided at an upper end position in the wrapper tube 2, and the upper end of the tie rod 21 penetrating the upper pin support ring 23 is pressed down by the upper end plug 25 via the elastic member 24 such as a compression coil spring. Thus, the grids 9 and the liner tubes 8 are pressed and held downward by the elastic member 24 with the elastic force. According to the above-described configuration, the entirety of the components can be held by causing the upper end plug 25 of the tie rod 21 (the fuel pin 5) to press the uppermost grid 9 via the elastic member 24 in a manner such that the liner tubes 8 and the grids 9 will not be misaligned. Accordingly, even if the expansion occurs due to the heat of the fuel and the irradiation, the entirety of the components can be reliably held by causing the fuel pin 5 itself to pull the entirety of the components. At the same time, the other fuel pins 5 are allowed to freely expand. It is preferable to provide a ring having the same shape as the shape of the outer diameter of the ring element to properly apply the elastic force of the elastic member 24 to the grid 9 to thereby reliably apply the pressing force to the grid 9. With the liner tubes 8 and the grids 9 thus held with the elastic member 24 by the upper end plug 25 of one of the fuel pins 5, the fuel pins 5, the grids 9, and the liner tubes 8 can be integrally handled, and the free expansion of the other fuel pins 5 is not interrupted. The present invention is not limited to the embodiments described above, and other alterations and modifications may be made in the present invention as long as not departing from the scope of the appended claims.