JPH10170677A - Structure of grid spacer of fast reactor fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a structure avoiding wasteful flow of a coolant in gaps among fuel assemblies and allowing the fuel assemblies to be drawn out or inserted easily when a nuclear reactor is stopped, for use at the core of a fast breeder reactor. SOLUTION: In grid spacers 1, mounted on at least one part or more along the direction of the axis of each fuel assembly placed in a fast breeder reactor core, and restraining the movement of each fuel assembly in the direction of its axis and in the vertical direction, plates made of a bimetal 2 or shape memory alloy, in which a metal on the outside has a higher coefficient of thermal expansion than the fuel- assembly-side metal, are arranged circumferentially on the outer peripheries of at least those grid spacers 1 which are located at the top of a heating part. During power generation, the plates made of the bimetal 2 or shapememory alloy are each most deformed at its center in such a way as to project outwards, and the expanded plates made of the bimetal 2 or shape-memory alloy, mounted on the grid spacers 1 of adjacent fuel assemblies, make contact with one another to stop circulation of a coolant in gaps among the fuel assemblies, and are restored to their flat shapes during the stop of the fast breeder reactor, allowing the fuel assemblies to be freely drawn out or inserted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a restraining mechanism for a fuel assembly constituting a core portion of a fast reactor, and efficiently circulates a coolant through a heat generating portion of the fuel assembly during power generation to effectively cool the fuel assembly. The present invention relates to a structure of a grid spacer capable of easily removing and inserting a fuel assembly at rest.

[0002]

2. Description of the Related Art FIG. 6 is a partially cutaway perspective view showing the structure of a conventional fuel assembly.
3 is an entrance nozzle and 54 is a handling head. As shown in FIG. 6, conventionally, a duct called a wrapper tube 52 is disposed on the outer peripheral portion of the assembly of the fuel pins 51 to store and hold the fuel pins 51 and to distribute the coolant inside the duct. However, since the wrapper tube 52 is radioactive, it has a problem that it is not easy to safely process it after refueling.

In order to solve the above problem, a plurality of plate-like shapes called grid spacers are provided at intervals in the axial direction of the fuel assembly in place of the conventional wrapper tube and allow the fuel assembly to be inserted or tightened. Was thought to reduce the total amount of waste,
In the past, the coolant that had been used to accurately remove heat from the fuel assemblies by restricting the flow area by the wrapper pipe was allowed to freely flow through the gaps between the fuel assemblies with low flow resistance,
There is a possibility that a problem that the heat removal of the fuel is not sufficiently performed may occur.

[0004] As a first means for solving this problem,
For example, after loading the core components into the core, the core components are pulled toward the center of the core by driving in a wedge using a core restraint frame, and the gap between the fuel assemblies is reduced, so that the coolant is wasted. It is conceivable to prevent the flow.

[0005] As a second means, a method of increasing the number of fuel tubes and the like constituting a unit fuel assembly to increase the size of core components and thereby reducing the clearance around the core components can be considered.

[0006]

However, the first problem is to be solved.
In the means, it is possible to reduce the flow rate of the coolant flowing through the gaps between the fuel assemblies.However, when the fuel and the like are exchanged after use for a predetermined period, the entire core components are tied up. However, there is a problem that it is extremely difficult to extract or reinsert the fuel to be replaced.

In the above-mentioned second means, wasteful flow of the coolant can certainly be suppressed, but when the core components become too large, the heat generation rate per unit core component becomes large, and the fuel and the like become large. When performing the replacement, there is a disadvantage that it is difficult to accurately remove the heat.

The present invention has been made in view of the above situation, and has a simple structure that securely holds a core component such as a fuel pin and closes a gap between adjacent fuel assemblies during power generation. The coolant is prevented from flowing through the gap to allow the coolant to flow through the fuel pipe portion, and a sufficient gap is formed between the fuel assemblies when the fuel is stopped so that the fuel assembly can be easily removed and inserted. It is an object of the present invention to provide a grid spacer of a fast reactor fuel assembly that can be performed by a fuel cell.

[0009]

The above object is achieved by a structure of a grid spacer of a fast reactor fuel assembly according to the present invention. That is, (1) at least one or more grid spacers attached in the axial direction of the fuel assembly provided in the fast reactor core and restraining movement in the direction perpendicular to the axis of the fuel assembly; On the outer peripheral side of the located grid spacer, a plate material made of a bimetal, in which the coefficient of thermal expansion of the outer metal is larger than the coefficient of thermal expansion of the metal on the fuel assembly side, is disposed in the circumferential direction. At the time of power generation, the central part of the plate material is deformed most convexly outward,
The expanded bimetal plates attached to the grid spacers of the adjacent fuel assemblies come into contact with each other, preventing the coolant from flowing through the gaps between the fuel assemblies, and returning to a flat shape when the fast reactor is stopped. The grid spacer structure of the fast reactor fuel assembly allows the fuel assembly to be pulled out and inserted freely.

(2) The plate material having a high coefficient of thermal expansion among the plate materials made of bimetal is composed of a plurality of plate materials which are long in the axial direction of the fuel assembly and short in the outer circumferential direction of the fuel assembly. Aggregate grid spacer structure.

(3) The plate made of bimetal is held at the longitudinal end of the plate so as to expand and contract.
The structure of the grid spacer of the fast reactor fuel assembly according to any one of (1) to (2).

(4) As for the plate material made of bimetal, a plate material having a small coefficient of thermal expansion is considerably martensitic stainless steel.
The grid spacer structure of a fast reactor fuel assembly according to any one of (1) to (3), wherein the plate material having a high coefficient of thermal expansion is made of austenitic stainless steel. It is.

(5) In a grid spacer which is attached at at least one position in the axial direction of the fuel assembly provided in the fast reactor core and restrains movement in the direction perpendicular to the axis of the fuel assembly, at least On the outer peripheral side of the grid spacer located at the top, a plate material made of a shape memory alloy is arranged in the circumferential direction, and the plate material made of the shape memory alloy is deformed so that the central portion of the plate material becomes the largest outwardly convex during power generation. The deformed shape memory alloy plates attached to the grid spacers of the adjacent fuel assemblies come into contact with each other, preventing the coolant from flowing through the gaps between the fuel assemblies, and having a flat shape when the fast reactor is stopped. The structure of the grid spacer of the fast reactor fuel assembly that allows the fuel assembly to be freely pulled out and inserted back into the fuel assembly.

(6) The plate member made of a shape memory alloy is composed of a plurality of plate members that are long in the axial direction of the fuel assembly and short in the outer peripheral direction of the fuel assembly. Construction.

(7) The plate made of a shape memory alloy is held at the longitudinal end of the plate so as to be deformable (5).
The structure of the grid spacer of the fast reactor fuel assembly according to any one of (1) to (6).

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 to 4 are views for explaining an embodiment of the present invention, in which a bimetal is used as a plate material which is deformed to be convex when the reactor is in operation and returns to a flat shape when the reactor is stopped. FIG. 1 is a partial perspective view of a fuel assembly in which a grid spacer having a bimetal is attached to an outer peripheral portion, and FIG. 2 is a perspective view of a grid spacer having a bimetal.
(a) is a diagram illustrating the shape of the bimetal during reactor shutdown, (b) is a diagram illustrating the shape of the bimetal during reactor operation, FIG. 3 is a diagram illustrating the shape and configuration of the bimetal itself, and FIG. A diagram schematically illustrating the shape and action of the bimetal at the time,
(a) shows when the reactor is shut down, and (b) shows when the reactor is operating.

FIG. 5 is a view for explaining the shape of a fuel assembly having no wrapper tube, which is the object of the present invention.

1 to 5, reference numeral 1 denotes a grid spacer, 2 denotes a bimetal, 3 denotes a fuel pin, 4 denotes a handling head, and 5 denotes a plate material of the bimetal, which is a plate material located inside from the center of the fuel assembly. , 6 is a B material which is a plate material of the bimetal which is located outside when viewed from the center of the fuel assembly, 7 is an evaporating portion, 8 is a fuel assembly, 9 is an entrance nozzle, 10 is a tie rod, and 11 is a bimetal holding portion. , 15 are bimetals in a flow path open state, and 16 is a bimetal in a flow path closed state.

As shown in FIG. 6, when a wrapper pipe (duct) 52 is provided on the outer peripheral portion of the fuel assembly, a coolant such as sodium for cooling a heat generating portion such as plutonium or uranium includes: A plate-like grid spacer that flows through the inside of the wrapper pipe and performs accurate cooling and heat removal, but instead of the wrapper pipe, the fuel pipe is inserted into a plurality of axially outer peripheral portions of the fuel assembly as shown in FIG. When 1 is disposed and held, the coolant such as sodium for cooling the fuel assembly flows through the gap between adjacent fuel assemblies having low flow resistance, so that the fuel pipe is not sufficiently cooled.

As a result of various studies to solve this problem, the present inventors focused on the temperature change of the fuel assembly between the operation of the reactor and the shutdown of the reactor, and utilized the temperature difference to make use of the temperature difference. During reactor operation, the gap between the fuel assemblies is closed to prevent waste flow of the coolant, and when the reactor is stopped, the gap between the fuel assemblies is sufficiently secured.
A structure has been invented that allows easy and reliable removal and reinsertion of an arbitrary fuel assembly during a fuel replacement operation.

That is, of the grid spacers 1, at least the grid spacer 1 located at the uppermost part of the heat generating portion
This is a structure in which plate members made of a bimetal are arranged side by side in the circumferential direction on the outer peripheral portion.

First, as shown in FIG. 3, the bimetal is manufactured by joining an A material 5 made of a material having a small coefficient of thermal expansion and a B material 6 made of a material having a large coefficient of thermal expansion by diffusion bonding or the like. Although various materials can be considered as the A material 5 and the B material 6, according to the study of the present inventors, as an example of the combination, the A material 5 is a martensitic stainless steel (for example, C: 0.13%, Cr : 13%, Ni: 0.14
%), And austenitic stainless steel (for example, Cr: 18%, Ni: 8%) or the like is suitable as the B material.

A holding portion for the bimetal 2 is formed on the outer periphery of the grid spacer 1, and the bimetal 2 is mounted. At this time, the material A 5 having a small coefficient of thermal expansion is positioned on the fuel assembly 8 side such that the material B 6 having a large coefficient of thermal expansion is located outside.

When the A material 5 and the B material 6 are made of the above-mentioned materials, the bimetal 2 is set to have an ambient temperature of about 200 ° C. when the nuclear reactor is stopped.
As shown in FIG. 4A, since it is stored in the holding portion 11 of the grid spacer 1, the space between the fuel assemblies 8 is as shown in FIG.
A sufficient gap is formed as shown in the bimetal portion 15 in the open channel state of (a), and a predetermined fuel assembly 8 can be freely removed or a new fuel assembly 8 can be removed as required for fuel exchange or the like. It can be easily inserted.

On the other hand, during the operation of the reactor, the ambient temperature of the bimetal 2 rises to about 500 ° C., so that the bimetal 2 is deformed in a convex shape toward the B material 6 having a large coefficient of thermal expansion.
As shown in FIG. 4 (b), the top of the protrusion protrudes outward from the supporting portion 11 of the bimetal 2, and as shown in the bimetal portion 16 in the closed state of the flow path in FIG. 8 makes contact with the tops of the bimetals 2 attached to the grid spacers 1 to close the gaps between the fuel assemblies 8.

Thus, during the operation of the nuclear reactor, accurate cooling can be performed without causing insufficient cooling of the heat generating portion due to the coolant flowing through the gaps between the fuel assemblies 8. FIG. 1 shows a state in which the bimetal 2 is deformed during the operation of the reactor.

During the operation of the reactor, the temperature distribution in the core portion is not always uniform, and the interval between the fuel assemblies 8 is slightly different. The forces acting on the bimetals 8 in contact with each other are not uniform.

To cope with this, for example, as shown in FIG.
The dimensions of the holding portion 11 of the bimetal 2 of the grid spacer 1 shown in FIGS. 1 and 2 (b) can be adjusted even if the bimetal 2 expands during the operation of the reactor and comes into contact with the bimetal 2 of the adjacent fuel assembly 8 and is pushed back. Bimetal 2 is sufficient for holding part 1
The structure is such that a margin is provided in the direction in which the bimetal 2 extends so that the bimetal 2 can be stored in the bimetal 2.

In the embodiment of the present invention, a case has been described where at least the plate-shaped member disposed on the outer peripheral portion of the grid spacer located at the uppermost portion of the heat generating portion is made of a bimetal. A plate material made of a memory alloy is arranged, and the central portion of the plate material made of a shape memory alloy is deformed most protrudingly outward during power generation, and is attached to a grid spacer of each adjacent fuel assembly and deformed. The contact between the fuel assemblies prevents the coolant from flowing through the gap between the fuel assemblies, and when the fast reactor is stopped, it returns to a flat shape so that the fuel assemblies can be freely removed and inserted. However, it is possible to obtain the same effect as a plate material made of a bimetal.

[0030]

As described above, according to the present invention, the following effects can be obtained. A plate made of bimetal or shape memory alloy is arranged on the outer periphery of the fuel assembly, and it operates only linearly using the temperature change of the fuel assembly during reactor operation and shutdown. Despite the configuration, the gap between each fuel assembly is closed during operation of the reactor to prevent the coolant from flowing unnecessarily, and the heating section is accurately cooled. There is an excellent effect that the fuel assemblies can be formed sufficiently to easily remove and insert each fuel assembly.

[Brief description of the drawings]

FIG. 1 is a partial perspective view of a fuel assembly according to the present invention with a grid spacer having a bimetal on its outer periphery.

FIGS. 2A and 2B are perspective views of a grid spacer having a bimetal according to the present invention, in which FIG. 2A illustrates the shape of the bimetal during reactor shutdown and FIG. 2B illustrates the shape of the bimetal during reactor operation.

FIG. 3 is a diagram illustrating a shape and a configuration of a bimetal itself for describing an embodiment of the present invention.

FIGS. 4A and 4B are diagrams schematically illustrating the shape and operation of a bimetal during a reactor shutdown and a reactor operation according to an embodiment of the present invention, wherein FIG. 4A illustrates a reactor shutdown, and FIG. The reactor is in operation.

FIG. 5 is a view for explaining the shape of a fuel assembly having no trumpet tube to which the present invention is directed.

FIG. 6 is a diagram showing an example of a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Grid spacer 2 Bimetal 3 Fuel pin 4 Handling head 5 A material of plate material which constitutes bimetal 6 B material of plate material which constitutes bimetal 7 Heat generation part 8 Fuel assembly 9 Entrance nozzle 10 Tie rod 11 Bimetal holding part 15 Flow path opening Bimetal part in the state 16 Bimetal part in the flow path closed state

Claims (7)

[Claims] 1. A grid spacer, which is attached to at least one position in the axial direction of a fuel assembly provided in a fast reactor core and restrains movement of the fuel assembly in a direction perpendicular to the axis of the fuel assembly, comprises: On the outer peripheral side of the grid spacer located at a position, a plate material made of a bimetal in which the coefficient of thermal expansion of the outer metal is larger than the coefficient of thermal expansion of the metal on the fuel assembly side is disposed in the circumferential direction. At the time of power generation, the central portion of the plate material is deformed to the largest convex shape outward, and the expanded bimetal plates attached to the grid spacers of the adjacent fuel assemblies come into contact with each other, and the gap between the fuel assemblies is cooled by the coolant. The fuel grid of the fast reactor fuel assembly is characterized in that the fuel assembly is prevented from flowing, and when the fast reactor is stopped, it returns to a flat shape so that the fuel assembly can be freely removed and inserted. Structure of Dosupesa. 2. A plate material having a high coefficient of thermal expansion among plate materials made of bimetals is composed of a plurality of plate materials which are long in the axial direction of the fuel assembly and short in the outer circumferential direction of the fuel assembly. Of the grid spacer of the fast reactor fuel assembly in Japan. 3. The fast reactor fuel assembly according to claim 1, wherein the plate made of bimetal is held so as to be able to expand and contract at a longitudinal end of the plate. The structure of the body grid spacer. 4. A plate material made of a bimetal, wherein a plate material having a small coefficient of thermal expansion is considerably martensitic stainless steel, and a sheet material having a large coefficient of thermal expansion is made of austenitic stainless steel. 2. The structure of the grid spacer of the fast reactor fuel assembly according to claim 1. 5. A grid spacer, which is attached to at least one location in the axial direction of a fuel assembly disposed in a fast reactor core and restrains movement in a direction perpendicular to the axis of the fuel assembly, wherein at least the uppermost portion of the heat generating portion A plate material made of a shape memory alloy is arranged in the circumferential direction on the outer peripheral side of the grid spacer located at the position of the grid spacer. In the plate material made of the shape memory alloy, the central portion of the plate material is most greatly deformed outwardly during power generation, The deformed shape memory alloy plates attached to the grid spacers of adjacent fuel assemblies come into contact with each other, preventing the coolant from flowing through the gaps between the fuel assemblies, and forming a flat shape when the fast reactor is stopped. A grid structure of a fuel assembly for a fast reactor, wherein the grid spacer is capable of returning and allowing the fuel assembly to be freely removed and inserted. 6. The fast reactor fuel assembly according to claim 5, wherein the plate material made of the shape memory alloy is composed of a plurality of plate materials that are long in the axial direction of the fuel assembly and short in the outer circumferential direction of the fuel assembly. Grid spacer structure. 7. The fuel assembly for a fast reactor according to claim 5, wherein the plate made of a shape memory alloy is held so as to be deformable at a longitudinal end of the plate. The structure of the body grid spacer.