JPH10142366A - Fixing structure of lower nozzle of nuclear fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce production cost by simplifying the mechanical structure of the connection part between a control rod guide tube and lower nozzle of fuel assembly for pressurized water reactor. SOLUTION: In a nuclear fuel assembly having an upper nozzle and a lower nozzle 25, a plurality of control rod guide tubes 23 connected at both ends to both nozzles, a support grid fixed to the control rod guide tube 23 and a plurality of nuclear fuel rods inserted in each opening of the support grid and extending in parallel to a control rod guide tube 23, the fixing structure of the lower nozzle 25 forms in a flat top cone shape the tip end of a connection bolt 39 with a penetration hole connecting the end plug with a screw hole fixed at the lower end of the control rod guide tube 23 and the lower nozzle 25. The tip end 39b of the bolt is constituted by projecting above the top end of the end plug 35.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art In a typical nuclear fuel assembly to be burned in a pressurized water reactor, a substantially rectangular upper nozzle and a lower nozzle having a plurality of coolant flow holes are arranged in parallel with each other. A plurality of control rod guide tubes are connected to form a so-called skeleton structure. A plurality of support grids having grid-like grid openings are mounted on the control rod guide tube at intervals in the longitudinal direction, and a plurality of nuclear fuel rods are inserted and supported one by one into each opening of the support grid. The nuclear fuel assembly has been constructed. Various structures are adopted for the connection between the upper nozzle and the control rod guide tube.For example, a short connection pipe having one end welded to the upper nozzle and the upper end of the control rod guide tube are mechanically expanded by swelling. It is also widely practiced to connect to. On the other hand, the connection between the control rod guide tube and the lower nozzle is performed by a structure as shown in FIG. That is, in the figure, the end plug 3 with a screw hole is provided at the lower end of the hollow control rod guide tube 1.
Is fixed, and a connection bolt 7 inserted into a mounting hole of the top plate of the lower nozzle 5 is screwed into the end plug 3. Code 9
Supports a lower support grid (not shown) at an upper portion by a tubular member called an insert. In order to secure an appropriate coolant distribution in the reactor core, a small through hole 11 for a coolant 13 having a predetermined fluid resistance characteristic is formed in the connecting bolt 7, and an assembly guide function is provided at an upper end. Small diameter part 15 to have
Are formed and cooperate with the end plug 3 to form a dust pocket around the periphery.

[0003]

Since the connecting bolt 7 having the conventional structure described above has a small diameter at the tip, it has a guiding function when inserted from below the lower nozzle 5 and screwed into the insert 9 and the end plug 3. In addition to the above, even when dust or the like settles during use, a problem of being guided to the outer dust pocket and closing the through-hole 11 is prevented. However, a considerable amount of machining is required to reduce the tip diameter as described above. In addition, an integrated nuclear fuel assembly uses several tens of connecting bolts, and the amount of use per reactor becomes enormous, so that considerable production costs were required. Therefore,
SUMMARY OF THE INVENTION It is an object of the present invention to provide a lower nozzle mounting structure for a nuclear fuel assembly using a connecting bolt which can achieve the same function and has a low manufacturing cost.

[0004]

According to the present invention, there is provided an upper and lower nozzle, a plurality of control rod guide tubes each having both ends connected to the nozzles, and a control rod guide tube. The lower nozzle mounting structure of a nuclear fuel assembly having a support grid fixed to the support grid and a plurality of nuclear fuel rods inserted through and supported by the respective openings of the support grid and extending in parallel with the control rod guide tube, comprises a lower end of the control rod guide tube. The tip end of a connection bolt with a through hole that connects the end plug with a screw hole attached to the lower nozzle and the lower nozzle is formed in a truncated cone shape, and the tip is formed to protrude upward from the upper end of the end plug. .

[0005]

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, the entire structure of the nuclear fuel assembly 20 will be described with reference to FIG. In FIG. 2, a rectangular box-shaped upper nozzle 21 having an upper opening has a coolant flow hole formed in a portion (not shown) corresponding to a bottom plate, and a plurality of control rod guide tubes 23 and a plurality of control rod guide tubes 23 as described later. Are linked. The control rod guide tube 23 is connected to a lower nozzle 25 as described later.
It has a shape like a rectangular step with four corners 25a at the four corners, and a coolant flow hole is formed in the step plate or the top plate. An upper grid 27 as a support grid and a plurality of intermediate grids 29 are attached and fixed to the control rod guide tube 23 at intervals, and a nuclear fuel rod 31 is inserted and supported in the grid opening to form a fuel assembly as a whole. The body 20 is constituted. The lower grid 33, which is one of the support grids, is supported by the control rod guide tube 23 and the lower nozzle 25 in another manner.

Next, a connection structure between the lower nozzle 25 and the control rod guide tube 23 will be described with reference to FIG. Control rod guide tube 2
The end plug with screw hole 35 connected and fixed to the lower end of 3 is shorter than the conventional one. Mounting hole 37 for lower nozzle 25
The surrounding dimensions and shape are the same as those of the related art. A connecting bolt 39 is inserted into the mounting hole 37 and screwed into a screw hole of the end plug 35. The shape and size of the small through hole 41 penetrating the connecting bolt 39 are the same as those of the conventional one, but a truncated conical surface 39a is formed at the tip. The insert 43 supporting the lower grid 33 (not shown in FIG. 1) has the same shape and dimensions as the conventional insert, and the tip surface 39b of the connecting bolt 39 protrudes above the end plug 35 from such a dimensional relationship. A refuse pool, that is, a refuse pocket 45 is defined around the rim. In this way, if fine dust contained in the coolant in the control rod guide tube 23 sinks, it is guided by the frustoconical surface 39a and falls into the dust pocket 45. Then, during operation of the nuclear reactor, a part of the rising coolant 47 flows through the small through hole 41 and rises in the control rod guide tube 23.

On the other hand, as shown in FIG. 3, a connecting short pipe 49 whose upper end is fixed in the hole of the upper nozzle 21 is inserted into the control rod guide pipe 23 above the upper grid 27, and both are bulged. Are mechanically linked by Because of such a connection structure, the control rod guide tube 23 accepts a control rod (not shown) without any trouble. In addition, even if the use of the connecting short pipe 49 is stopped, the tip of the control rod guide pipe 23 is extended to the inside of the hole of the upper nozzle 21, and the control rod guide pipe is fixed in the groove on the inner peripheral surface of the hole by expanding the pipe. Good. In short, various connection structures between the control rod guide tube and the upper nozzle can be adopted.

[0008]

As described above, according to the present invention,
Since the tip of the connecting bolt for attaching the lower nozzle to the control rod guide tube is formed in a truncated conical surface and the small diameter portion is eliminated, the amount of machining can be reduced and the cost can be reduced. In addition, the tip of the connecting bolt protrudes above the end plug, and a garbage pocket is formed around the frusto-conical surface. Thus, an appropriate core state can be maintained.

[Brief description of the drawings]

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

FIG. 2 is a partially omitted elevation view showing the entire structure of the nuclear fuel assembly according to the embodiment of the present invention.

FIG. 3 is a partial sectional view of another portion of the nuclear fuel assembly of FIG. 2;

FIG. 4 is a partial sectional view showing a conventional lower nozzle mounting structure.

[Explanation of symbols]

 Reference Signs List 20 nuclear fuel assembly 21 upper nozzle 23 control rod guide tube 25 lower nozzle 27 upper grid 29 intermediate grid 31 fuel rod 33 lower grid 35 end plug 37 mounting hole 39 connecting bolt 39a truncated conical surface 39b tip 41 through small hole 43 insert 45 Garbage pocket 47 Coolant

Claims (1)

[Claims] 1. An upper and lower nozzle, a plurality of control rod guide tubes each having both ends connected to the nozzles, a support grid fixed to the control rod guide pipe, and a support inserted through each opening of the support grid. A nuclear fuel assembly having a plurality of nuclear fuel rods extending in parallel with the control rod guide tube, a connection with a through hole connecting the end plug with a screw hole attached to a lower end of the control rod guide tube and the lower nozzle. A lower nozzle mounting structure for a nuclear fuel assembly, wherein a tip of a bolt is formed in a truncated conical shape, and a tip protrudes above an upper end of the end plug.