JPH11133177A - Nuclear reactor fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure smooth insertion of a control rod during the latter half of a life of a fuel assembly for a pressurized water reactor. SOLUTION: In a fuel assembly having an upper nozzle and lower nozzle 11 equipped with many coolant flow-through ports and confronting each other in spaced relation, a plurality of control-rod guide tubes 15 extending parallel to each other between the two nozzles and having opposite ends joined to the two nozzles respectively, a plurality of support grids disposed in spaced relation along the control-rod guide tubes 15 and including the lowest support grid 21, and a plurality of fuel rods inserted in and supported by grid openings in the support grids, an annular recess 27 is formed in an inner circumferential surface, excluding its upper end part, of an insert 25 for attaching the lowest support grid 21, fixed to the lower nozzle 11 and extending to surround a lower end part of the control-rod guide tube 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for combustion in a nuclear reactor, and more particularly, to a control rod guide tube mounting structure in a fuel assembly for a pressurized water reactor.

[0002]

2. Description of the Related Art In a pressurized water reactor which is currently widely used for power generation, a control rod assembly in which thin control rods are assembled in a cluster shape, a so-called control rod cluster, is used to control the nuclear reaction in the reactor. And Therefore, the fuel assembly constituting the reactor core also includes a tube for receiving and guiding the thin control rod, that is, a control rod guide tube. An outline of the general structure of the fuel assembly is as follows. That is, the reactor fuel assembly has an upper nozzle and a lower nozzle which are provided with a plurality of coolant flow-through ports and are opposed to each other at intervals, and which extend in parallel between these two nozzles and both ends are respectively connected to the two nozzles. It comprises a plurality of control rod guide tubes, a plurality of support lattices arranged at intervals along the control rod guide tubes, and a plurality of fuel rods inserted and supported in the lattice openings of the support lattice. Is done. The control rod and the control rod guide tube are used for the purpose of preventing the control rod from being subjected to coolant-induced vibration during operation as much as possible, and reducing the impact load when the control rod is urgently inserted (when it falls). Are generally set to be small. The plurality of support grids are fixed to the control rod guide tube by using bulging or welding, while the lowermost support grid is mounted in a state as shown in FIG.

FIG. 5 shows a conceptual structure, in which a lower end of a control rod guide tube 3 is fixed to a lower nozzle 1 via an unillustrated mounting bolt or the like. And the control rod guide tube 3
Cylindrical connecting member surrounding the lower end of the so-called insert 5
Connect and support the lowermost support grid 7.

[0004]

However, the fuel assembly is not necessarily subjected to uniform neutron irradiation over the entire area. For example, a portion facing the center of the core and a portion facing the outer periphery of the core are slightly irradiated. Therefore, as combustion proceeds, a deflection called bowing or the like occurs. FIG. 6 shows this situation in an exaggerated manner for explanation.
Although the fuel rods are not shown in this conceptual diagram, the lowermost support grid 7 and other support grids 9 are bent due to bending deformation of the fuel assembly.
Are displaced laterally from the normal position, and the control rod guide tube 3 is also bowed as a whole, but the lower end is restrained by the above-mentioned insert. On the other hand, since the control rod is also a very thin rod as described above, it has a property of easily following the entire bending deformation. However, if there is a singular point in the deformation, such as the lower end of the control rod guide tube 3, the interference occurs between the control rod guide tube 3 and the internal control rod 8, as shown in FIG. Inhibits sex. FIG. 7 is also greatly exaggerated for explanation. Accordingly, an object of the present invention is to provide a fuel assembly having a structure that can ensure smooth insertion of control rods even when the fuel assembly is deformed due to progress of combustion.

[0005]

According to the present invention, there is provided, in accordance with the present invention, an upper nozzle and a lower nozzle which are provided with a plurality of coolant through-holes and which are opposed to each other at intervals. A plurality of control rod guide tubes extending parallel to each other and having both ends connected to the nozzles; a plurality of support lattices including a lowermost support lattice disposed at intervals along the control rod guide tubes; In a reactor fuel assembly having a plurality of fuel rods inserted through and supported by a grid opening of a support grid, a lowermost support grid mounting insert fixed to a lower nozzle and extending around a lower end of a control rod guide tube. An annular recess is formed on the peripheral surface except for the upper end.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, the overall structure of the fuel assembly 10 for a pressurized water reactor will be described with reference to FIG. The lower nozzle 11 has legs at four corner lower surfaces at a portion in contact with the lower core plate at the time of fuel loading, and is provided with a number of holes through which light water as a reactor coolant passes. An upper nozzle 13 is provided at an upper end position opposite to the lower nozzle 11, and a plurality of light water through holes are similarly formed. However, a nozzle spring 13a is further provided at an upper surface edge, and the upper core plate is used for loading. Pressed. Then, the upper nozzle 13 and the lower nozzle 1
A plurality of control rod guide tubes 15 each having both ends connected to 1
Extend parallel to each other. An instrumentation guide tube 17, which is a similar hollow tube, extends axially through the center of the fuel assembly 10,
Used for guiding nuclear instrumentation detectors. Multiple support grids 19
Are fixed to the control rod guide tube 15 at intervals in the longitudinal direction, and the lowermost support grid 21 is supported as described later. The control rod guide tube 15 and the instrumentation guide tube 17 are inserted through the corresponding grid openings of the support grid 19, and the fuel rods 23 are inserted and supported one by one through the other grid openings. I have.

FIG. 1 shows a main part of the present invention. A control rod guide tube 15 is fixed to the lower nozzle 11 by a mounting bolt (not shown), and a tubular insert 25 fixed between the lower nozzle 11 and the control rod guide tube 15 fixes the lower end of the control rod guide tube 15. Surrounding. The upper end of the insert 25 is connected to and supports the lowermost support grid 21 and forms an inward flange 25a inside. In other words, an annular recess 27 is formed between the insert 25 and the control rod guide tube 15 except for the inward flange 25a at the upper end. The width of such an annular recess 27 may be about 2 mm in a practical furnace.

FIG. 3 is an exaggerated view of the state in which the fuel assembly 10 having the above-described structure is subjected to combustion and is bent and deformed. As before, the fuel rods are not shown, but the control rod guide tube 15 is gently bowed over its entire length. In this case, since the insert 25 surrounding the lower end has a large gap and does not restrict the deformation, no singular point of the deformation curve is formed at that portion. Therefore,
When the control rod 8 is inserted into the control rod guide tube 15, a state as shown in FIG. 4 is obtained. Although FIG. 4 is also exaggerated in order to make the explanation easy to understand, the control rod 8 is gently deformed following the inner surface of the control rod guide tube 15. As described above, since the control rod 8 is guided and deformed by the control rod guide tube 15, the sliding resistance is reduced and the control rod 8 is inserted smoothly.

[0009]

As described above, according to the present invention,
Since the insert surrounding the lower part of the control rod guide tube at the lower part of the fuel assembly does not restrain the bending deformation, the deformation becomes gentle, so the control rod also deforms according to this, the frictional resistance between them is small, The control rod is inserted smoothly.

[Brief description of the drawings]

FIG. 1 is a conceptual sectional view showing a main part of an embodiment of the present invention.

FIG. 2 is an overall perspective view in which a part of the fuel assembly according to the embodiment is cut away.

FIG. 3 is an operation explanatory view of the embodiment.

FIG. 4 is a partial sectional view for explaining the operation of the embodiment.

FIG. 5 is a schematic partial cross section showing the structure of a conventional device.

FIG. 6 is a conceptual diagram for explaining a problem of a conventional device.

FIG. 7 is a partial cross-sectional view for explaining a problem of the conventional device.

[Explanation of symbols]

 Reference Signs List 10 fuel assembly 11 lower nozzle 13 upper nozzle 15 control rod guide pipe 17 instrumentation guide pipe 19 support grid 21 bottom support grid 23 fuel rod 25 insert 25a inward flange 27 annular recess

Claims (1)

[Claims] 1. An upper nozzle and a lower nozzle which are provided with a plurality of coolant through-holes and are opposed to each other at intervals, and a plurality of control rods which extend in parallel between the two nozzles and both ends of which are respectively connected to the two nozzles. A guide pipe, a plurality of support grids including a lowermost support grid arranged at intervals along the control rod guide pipe, and a plurality of fuel rods inserted and supported through the grid openings of the support grid. A reactor fuel assembly having
A nuclear recess formed in the inner peripheral surface of the lowermost support grid mounting insert fixed to the lower nozzle and extending around the lower end of the control rod guide tube, leaving an upper end at the lower end thereof; Fuel assembly.