JPH11160475A - Reactor fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deflection of a lower part of a control rod guide tube and secure quick insertion of a control rod in a pressurized water reactor fuel assembly. SOLUTION: A reactor fuel assembly supports a plurality of fuel rods on grids 39, 40 connected through a plurality of control rod guide tubes mutually extending an upper nozzle and a lower nozzle in parallel, and fixed to receive the control rod guide tubes 35. In this case, the control rod guide tube 35 forms the dash pot of a small diameter on the lower end, the thickness of the dash pot pad is relatively largely formed and bending rigidity is enlarged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fuel assembly for a nuclear reactor, and more particularly to a structure of a fuel assembly for a pressurized water reactor.

[0002]

2. Description of the Prior Art Currently, fuel assemblies commonly used in pressurized water reactors have an upper nozzle and a lower nozzle each having a plurality of coolant through holes, which are hollow control rod guides. They are connected by a pipe (also called a guide thimble). There are a plurality of control rod guide tubes, each of which is inserted into and fixed to a lattice opening of a support lattice, which is a lattice structure. The plurality of fuel rods extending in parallel with the control rod guide tube are inserted one by one into the lattice openings of the support lattice and are elastically supported. A large number of such fuel assemblies are placed side by side in a nuclear reactor to form a core,
The control rod for controlling the nuclear reaction of the core is formed as an elongated rod-shaped body so as to be inserted into the control rod guide tube described above,
These are connected via a spider assembly to form a control rod cluster. As described above, the control rods that are inserted into or pulled out of the core are guided by the control rod guide pipes of the fuel assembly, but in an emergency, the control rods are quickly inserted by free fall or the like. A dash pot is formed at the lower part of the control rod guide tube to reduce the impact of the dash pot. 6 and 7 show the structure of the lower part of the control rod guide tube for that purpose.

In FIG. 6, a small-diameter dash pot portion 3 a of a control rod guide tube 3 is fixed to a lower nozzle 1 by a thimble screw 5. The end plug 7 screwed to the thimble screw 5 is fixed to the lower end of the dash pot portion 3a, and the bottomed cylindrical insert 9 sandwiched therebetween supports the lowermost support grid 11. The intermediate support grid 13 is
It is mechanically connected to the control rod guide tube 3 via a sleeve 15 welded thereto. When the control rod 17 is rapidly inserted as shown by the arrow, the dashpot 3
The reactor coolant in the area a is pushed out from the drain hole 3b, and a dashpot function is obtained. FIG. 7 is a structural example in which the intermediate support grid 13 is attached to the dash pot section 23a of the control rod guide tube 23. At the joint with the support grid 13, the diameter of the dash pot section 23a is enlarged as usual. The sleeve 15 fixed to the support lattice 13 is mechanically connected to the sleeve 15 by bulging or other suitable pipe expanding. The joining between the support lattice 13 and the control rod guide tubes 3 and 23 as described above is based on the fact that the constituent metal materials of the two are different.

[0004]

The fuel assembly constructed as described above is subjected to an upward lifting force by the coolant flowing upward in the reactor core. Such a lifting force is opposed by a pressing spring attached to the upper nozzle and in contact with the upper core plate in the reactor.
As described above, the elastic force of the pressing spring always acts on the fuel assembly, that is, the control rod guide tube. Thus, as is clear from the above description, the control rod guide tube is an elongated cylindrical body, and is easily bent in an arc shape under an axial load. In particular, the tendency is strong in a dash pot portion formed in a small diameter. When the amount of flexural deformation of the dash pot portion of the control rod guide tube increases, the gap between the control rod guide tube and the control rod is small, which may hinder the emergency insertion of the control rod. Accordingly, it is an object of the present invention to provide a reactor fuel assembly that can maintain a high control rod insertion property by keeping a control rod guide tube, particularly a dash pot portion, at a small amount of deflection.

[0005]

According to the present invention, an upper nozzle and a lower nozzle are connected via a plurality of control rod guide tubes extending in parallel with each other, and the control rod is connected to the upper nozzle. In a nuclear reactor fuel assembly having a plurality of fuel rods supported on a support grid fixed by receiving a guide tube, a control rod guide tube having a dashpot portion having a small inner diameter formed at a lower end thereof has a dashpot portion. Is relatively large, the section modulus is increased, and the flexural rigidity is increased. Further, according to the present invention, in a reactor fuel assembly having a similar configuration, the control rod guide tube in which the dashpot portion having a small inner diameter is formed at the lower end portion has a projection formed on the outer peripheral surface of the dashpot portion. The flexural rigidity is increased.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, the overall structure of the fuel assembly 30 will be described with reference to FIG. The upper nozzle 31 has a plurality of coolant through-holes (not shown), and has a hold-down leaf spring 31a at the upper part. Similarly, the lower nozzle 33 also has a plurality of coolant through holes (not shown).
, And has a leg mounted on the lower core plate A of the reactor internal structure. The upper nozzle 31 and the lower nozzle 33 are connected to both ends of a plurality of control rod guide tubes 35 to form a skeleton structure. Control rod guide tube 3
The positions of the arrangements 5 are determined in consideration of nuclear reactor physics and the like, and extend in parallel with each other. The uppermost support grid 37, the intermediate support grid 39, and the lowermost support grid 41, which are rectangular grid-like structures, are attached to the control rod guide tube 35 at predetermined intervals in the longitudinal direction. The uppermost support grid 37,
The intermediate support grid 39 and the lowermost support grid 41 are based on many prior patent applications and the application edition B of “Mechanical Engineering Handbook” edited by the Japan Society of Mechanical Engineers.
6 "Power Plant" as usual, as described on page 147. Each such support grid 37, 39,
A plurality of fuel rods 43 are inserted one by one into the lattice openings 41 and are elastically supported. Then, the leaf spring 31a is brought into contact with the upper core plate B and is given a predetermined deformation when the reactor is loaded.

Next, the structure of the control rod guide tube 35 which is a characteristic configuration of the present invention will be described with reference to FIG. A mounting bolt 45 inserted through a mounting hole of the lower nozzle 33 is inserted into a threaded end plug 35 a fixed to the lower end of the control rod guide tube 35.
Are screwed together to securely connect and fix them. Lower end plug 3
A bottomed cylindrical insert 47 firmly held by mounting bolts 45 sandwiched between 5a and the lower nozzle 33 is welded to and held by the lowermost support grid 41. Further, the intermediate support grid 39 and the uppermost support grid (see FIG. 1) have a cylindrical sleeve 49 welded thereto.
Is mechanically connected to the control rod guide tube 35 by integral swelling. The illustrated intermediate support grid 39 is at the lowermost position, under which the control rod guide tube 35 is formed with a small inner diameter, in other words, forms a thick portion 35b,
This is the dashpot section.

In the fuel assembly 30 described above, the control rod guide tube 35 receives a control rod (not shown) and smoothly guides the control rod when the control rod is pulled out of the core or fully inserted. In the thick portion 35b, since the gap between the control rod and the outer surface of the inserted control rod is small due to the reduced inner diameter, when the control rod is rapidly inserted, water in the control rod guide pipe 35 is removed. Although discharged through the gap, the impact is reduced by receiving the throttle resistance.
Also, since the section modulus of the thick portion 35b is large,
The flexural rigidity is increased, and the amount of deflection is small even when the spring force of the leaf spring 31a acts.

In the above-described embodiment, the dash pot portion of the control rod guide tube 35 is formed as a thick portion 35b to increase the rigidity. However, as shown in FIG. An annular projection 135 is provided on the portion 135b.
Even if a plurality of c's are formed, the rigidity is increased and the same operation and effect can be obtained. Also, as shown in FIGS. 4 and 5, the same applies when a projection or a corrugation 235c extending in the axial direction is formed on the dashpot portion 235b of the control rod guide tube 235. In FIGS. 3 to 5, the structure of the fuel assembly other than that described above is the same as that of FIG.

[0010]

As described above, according to the present invention,
The dash pot formed at the lower end of the control rod guide tube of the fuel assembly is formed into a thick portion having a relatively large thickness, or a protrusion is formed on the outer peripheral surface so as to extend in the circumferential direction or the axial direction. Since the rigidity is increased, even if an axial compressive load is applied during operation of the reactor, the deflection is small, and therefore, the control rod insertion property can be favorably maintained.

[Brief description of the drawings]

FIG. 1 is an overall elevational view showing an embodiment of the present invention.

FIG. 2 is a partial cross-sectional view showing a main part of the embodiment.

FIG. 3 is a partial sectional view showing a modified embodiment in which a part of the embodiment is modified.

FIG. 4 is a partial sectional view showing another modified embodiment in which a part of the embodiment is modified.

FIG. 5 is a plan sectional view taken along the line VV of FIG. 4;

FIG. 6 is a partial sectional view showing a conventional structure.

FIG. 7 is a partial sectional view showing another conventional structure.

[Explanation of symbols]

 REFERENCE SIGNS LIST 30 fuel assembly 31 upper nozzle 33 lower nozzle 35 control rod guide pipe 35 a lower end plug 35 b thick part 37, 39, 41 support grid 43 fuel rod 45 mounting bolt 47 insert 49 sleeve 135 control rod guide pipe 135 b dash pot part 135 c Projection 235 Control rod guide tube 235b Dash pot 235c Corrugation

Claims (2)

[Claims] An upper nozzle and a lower nozzle are connected via a plurality of control rod guide tubes extending parallel to each other, and a plurality of fuel rods are supported on a support grid fixed by receiving the control rod guide tubes. In the nuclear reactor fuel assembly described above, the control rod guide tube has a dashpot portion having a small inner diameter formed at a lower end portion, and the dashpot portion has a relatively large thickness. Reactor fuel assembly. 2. An upper nozzle and a lower nozzle are connected via a plurality of control rod guide tubes extending in parallel with each other, and a plurality of fuel rods are supported on a support grid fixed by receiving the control rod guide tubes. A reactor fuel assembly comprising: a control rod guide tube having a dash pot portion having a small inner diameter formed at a lower end portion, and a projection formed on an outer peripheral surface of the dash pot portion. Aggregation.