JPH02228591A - Fuel assembly and nuclear reactor - Google Patents

Abstract

PURPOSE:To avoid a larger size upper part of a nuclear reactor and to improve a neutron economy of whole reactor system by inserting a controlling-element assembly to an inside of a fuel element radially from the central part of a fuel bundle and upwardly from a lower part of a reactor core. CONSTITUTION:A fuel bundle consists of a plurality of fuel rods 1 which are arranged in a shape of a triangular grid, a plurality of spacers 12 provided axially, to keep horizontal intervals among fuel rods, and upper tie-plates 4 and lower tie-plates 13 to keep constant intervals among fuel rods. A controlling- element assembly 2 which consists of a Y-shaped thin plate controlling-element to be inserted into the fuel bundle, possesses a drive mechanism 16 at a rate of one such mechanism to three fuel assemblies. In such a controlling-element assembly consisting of thin plate controlling-elements, a structural stability can be well maintained. Moreover, by adding a follower part to an upper part of the controlling-elements, a high conversion ratio can be obtained, because no water comes into Y-shaped spatial regions in case of an operation with withdrawn controlling-elements.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a fuel assembly consisting of a plurality of fuel rods arranged in a triangular lattice. , control rod drive mechanism.

Reactor internal structures such as upper and lower grate plates, steam separators, and steam dryers can be easily constructed within the reactor vessel (and deformation due to fuel assembly irradiation is small, making it economical to use neutrons). The present invention relates to a fuel assembly and a core configuration thereof that improve the performance of the fuel assembly.

[Conventional technology]

Currently, the plutonium multiplier BWR (hereinafter abbreviated as Pu multiplier reactor) is a next-generation light water reactor that achieves a significant reduction in fuel cycle costs through high burnup and effective use of plutonium.
has been developed (Japanese Unexamined Patent Publication No. 129594/1983)
. In a Pu multiplier reactor, fissile Pu
The amount generated and the amount consumed are almost the same, and the ratio of the amount of fissile Pu in the newly loaded fuel to the reactor core and the amount of fissile Pu in the removed fuel after the end of the fuel life is approximately 1.0. In order to achieve such core characteristics, the fuel assemblies are arranged in a triangular arrangement and densified to increase the water-to-fuel volume ratio (H/U ratio) in the fuel assembly.

An example of a conventional design of a fuel assembly for a Pu multiplication reactor is shown in FIG. The fuel assembly mainly consists of an upper tie plate 4 and a spacer 5.
．． Multiple fuel bodies1. It is composed of a lower tie plate 6 and a channel box 3 that houses these. The fuel body 1 has a triangular close-packed structure in order to lower the H/U ratio.

Therefore, the horizontal cross section of the fuel assembly has a hexagonal shape. The upper and lower tie plates 6 fix both ends of the plurality of fuel bodies 1. A plurality of spacers 5 are provided at intervals in the axial direction within the channel box 3, and support a plurality of fuel bodies in alignment to form a fuel bundle. The channel box 3 surrounding the fuel bundle has a cylindrical shape with a hexagonal horizontal cross section.

The following three types of control rod shapes and insertion methods are currently being considered for use in the fuel assembly of such a Pu multiplication reactor.

(1) A type in which a control element assembly consisting of multiple cylindrical rod-shaped control elements (clusters) is inserted from the top of the reactor core (
Type 1).

(2) A type (type 2) in which a control element assembly consisting of a plurality of cylindrical rod-shaped control elements (clusters) is inserted from the bottom of the reactor core.

(3) A tie in which a control element assembly consisting of a plurality of thin plate-shaped control elements (blades) is inserted into the gap between adjacent channel boxes from the bottom of the reactor core.

(type 3).

A horizontal sectional view of a fuel assembly using type 1 and type 2 control rods is shown in FIG. In each fuel assembly, cluster control rods 7 are inserted into control rod guide tubes 8 in an array of fuel rods. Channel box 3 is installed between adjacent clusters.
There is a gap 9 delimited by a gap 9 through which a small amount of non-boiled cooling water flows.

When applying Type 1 control rods and insertion method to a Pu-multiplied BWR, as shown in Figure 4 (Japanese Patent Application Laid-Open No. 61-230078), the control rod guide tubes 8 are connected to the air-water separation system in the upper part of the core. It has a structure that penetrates through the container 11 and the steam dryer 1o.

Such a nuclear reactor has the disadvantage that it is complicated and the entire reactor vessel is also large.

When applying the type 2 control rod shape and insertion method to a Pu-multiplied BWR, the cluster control rods must be inserted from the bottom to the top against gravity. This complicates the autonomy of the cluster control rods and the handling of the assembly during refueling.

In the type 3 control rod and insertion method, control rods having plate-shaped wings are inserted between adjacent channel boxes 3, so it is necessary to provide a large gap 9 between the channel boxes. At the same time, in order to prevent gap closure due to deformation of the channel box 3, it is also necessary to thicken the channel box. Both the increase in the gap between the channel boxes and the thickening of the channel boxes deteriorate the nuclear properties of the Pu multiplier reactor.

[Problem to be solved by the invention]

The present invention relates to a control rod and a control rod drive mechanism in a core structure of a Pu multiplier 9 reactor, which is composed of a fuel assembly consisting of a plurality of fuel rods arranged in a triangular lattice.

Internal reactor structures such as the upper and lower core grate plates, steam separators, and steam dryers are constructed within the reactor vessel, and the deformation of the fuel assembly due to irradiation is small, allowing the entire reactor to be The purpose of the present invention is to provide a fuel assembly and a nuclear reactor that improve the economic efficiency of neutrons.

[Means to solve the problem]

The above purpose is to use a plurality of fuel rods arranged on a triangular lattice,
In order to maintain the horizontal spacing between these fuel rods, a plurality of spacers are provided in the axial direction, and in addition to keeping the axial spacing of the fuel rods constant, the cross section of the fuel bundle is made up of a lower tie plate. In the hexagonal fuel assembly, a control element assembly having a plurality of thin plate-like control elements radially extending from the center of the fuel bundle can be inserted from the bottom of the fuel assembly into the inner region of the cross section of the fuel assembly, A region capable of penetrating to the top of the fuel bundle is provided along the entire axial region inside the fuel rod array, and a plurality of thin plate-shaped parts are provided in the fuel assembly from the bottom of the core radially from the center of the fuel bundle. This is achieved by inserting a control element collection with control elements.

In a Pu multiplier reactor, similar to a conventional BWR, cooling water boils in the fuel assembly in a two-phase flow with steam. Therefore, a steam separator and a steam dryer are required above the core. Inserting cluster control rods from the top of the reactor core complicates the core configuration and increases the size of the top of the reactor vessel. Therefore, it is preferable to insert the control rods from the bottom of the core. Furthermore, even when the control rods are inserted from the lower part of the reactor core, it is necessary to have a control rod shape that is structurally stable and that improves core characteristics.

In the nuclear reactor and fuel assembly according to the present invention, the control element assembly having a plurality of thin plate-like control elements radially extending from the center of the fuel handle is inserted into the fuel assembly from the lower part of the reactor core. Since it is inserted from the bottom of the reactor core, it is possible to avoid complicating the upper core structure and increasing the size of the upper part of the reactor vessel, which occur in Type 1.

Furthermore, since the core upper structure of the conventional BWR can be used as is, it is possible to ensure the same structural stability as the current reactor.

Furthermore, since the control element is composed of a thin plate-like structure radiating from the center of the fuel bundle, it is different from the conventional type 2.
As shown in the figure, the independence of control elements and the handling when replacing fuel assemblies do not deteriorate, maintaining the same independence as the current BWR, and handling when replacing fuel assemblies is also improved compared to the current system. Available for use.

Furthermore, compared to Type 3, in which a thin plate-shaped control element is inserted into the gap between adjacent channel boxes, in the present invention, the control element is inserted inside the fuel assembly, so the control element is inserted into the channel. The gap between the boxes can be reduced, and at the same time, the thickness of the channel box can be reduced, and the nuclear properties of the reactor core can be improved.

[Effect]

According to the nuclear reactor and fuel assembly of the present invention, by inserting the control element assembly having a plurality of thin plate-shaped control elements radially from the center of the fuel bundle from the lower part of the fuel assembly, the Pu multiplier reactor The reactor configuration is as easy to configure as a conventional BWR, and.

It is possible to provide structural stability, simultaneously achieve the nuclear properties necessary for Pu multiplication, and ensure the structural stability of the fuel assembly.

〔Example〕

Hereinafter, the present invention will be explained in detail based on examples. 1st
The cross section of the fuel assembly according to the present invention is shown in FIG.
A longitudinal cross-sectional view is shown. The fuel bundle includes a plurality of fuel rods 1 arranged in a triangular lattice shape, a plurality of spacers 12 provided in the axial direction to maintain horizontal spacing between these fuel rods, and spacers 12 arranged in the axial direction of the fuel rods 1. Upper tie plate with constant spacing4. It is composed of a lower tie plate 13.

Since the fuel rods are arranged in a triangular lattice pattern, the cross section of the fuel bundle is hexagonal. Therefore, the channel box 3 containing the fuel bundle also has a hexagonal shape. In the internal region of the cross section of the fuel bundle, a control element assembly 2 having a plurality of thin plate-like control elements radially extending from the center of the fuel bundle can be inserted from the bottom of the fuel bundle,
A region is provided along the entire axial extent within the array of fuel rods that is capable of penetrating to the top of the fuel bundle. In the embodiment shown in FIG. 1, the control element aggregate 2 is
It consists of a thin plate-shaped control element. The control element assembly 2 includes a control rod made of boron carbide and hafnium in a thin plate-like control element. This control element assembly 2 is located at the lower part of the reactor core, and is inserted into the fuel bundle from the lower part of the fuel assembly.

Inside the fuel bundle, there is a region in the same Y shape where the fuel rods 2 have been removed so that a Y-shaped thin plate control element can be inserted therein. In order to form such a region and maintain the horizontal spacing between the fuel rods 1, the shape of the spacers provided in the axial direction is also Y-shaped, as shown in FIG. A region without grid 14 is formed. A band 15 similar to the band 15 provided at the outermost periphery of the spacer is provided inside the Y-shape to prevent the Y-shaped space within the fuel bundle from being blocked due to bending of the fuel rods 1 or the like. Also,
The lower tie plate supporting the fuel rods at its lower end is also provided with a Y-shaped hole in the plate surface so that the Y-shaped control element can pass therethrough.

A control element assembly 2 consisting of a Y-shaped thin plate-like control element inserted into a fuel bundle has an integrated drive mechanism 16 at the lower part of the fuel assembly. FIG. 7 shows the control element assembly 2 and the control rod drive mechanism 16.

The control element assembly 2 may have not only a control element part made of a normal control rod, but also a control element having a structure in which a follower part 2' made of carbon is provided on the upper part of this part. FIG. 8 shows this control element assembly with follower section. When cluster-type control rods are inserted from the bottom of the core, adding a follower section to the top of the control element increases the length of the control rods, making them more likely to buckle. In the control element assembly 2 made of thin plate-like control elements of the present invention, such a problem does not occur and structural stability can be maintained. In addition, by adding the follower part 2' to the top of the control element, when the control element part is pulled out and operated, water does not enter the Y-shaped space area, so the H/U ratio in the fuel assembly is reduced. is reduced, resulting in a high conversion ratio.

Furthermore, in this embodiment, since there is no need to insert a control element into the gap 9 between adjacent channel boxes, this gap can be made smaller. At the same time, since there is no need to increase the thickness of the channel box 3 and suppress the amount of deformation in order to maintain the gap interval, the channel box 3 can be made thinner. By reducing the gap 9 between the channel boxes and making them thinner, the neutron economy in configuring the reactor core is improved.

Furthermore, in this embodiment, since the control element is inserted into the fuel bundle, it can also be applied to a fuel assembly that does not use a channel box.

As a second embodiment, FIG. 9 shows a case where the control element assembly 2 is composed of a star-shaped thin plate-like control element 17. The configuration of the fuel bundle and the structure of the control element assembly are the same as in the first embodiment. In this embodiment, since the thin plate-shaped control element 17 is point symmetrical with respect to the center of the fuel bundle, the orientation of the fuel assembly with respect to the center of the reactor core can be freely selected when loading the reactor core.

As a third embodiment, FIG. 10 shows a case where the control element assembly 2 is composed of an X-shaped thin plate-like control element 18. In this embodiment as well, the configuration of the fuel bundle and the structure of the control element assembly are as follows.

This is the same as the first embodiment.

Furthermore, by adjusting the orientation of the control element assembly 2 within the fuel assembly during core configuration, the value of each control rod relative to the reactivity of the entire core can be reduced. FIG. 11 shows an example applied to the first embodiment.

(Effects of the Invention) According to the present invention, a fuel assembly in which reactor internals are easy to configure in a reactor vessel, deformation of the fuel assembly due to irradiation is small, and the neutron economy of the entire reactor is improved. body, and
A nuclear reactor can be provided.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of one embodiment of the fuel assembly of the present invention, FIG. 2 is a vertical cross-sectional view of a fuel assembly for a conventional Pu multiplier reactor, and FIG.
The figure is a cross-sectional view of the fuel assembly of a conventional Pu multiplier reactor, and Figure 4 is a cross-sectional view of the fuel assembly of a conventional Pu multiplication reactor.
FIG. 5 is a vertical cross-sectional view of an embodiment of the fuel assembly according to the present invention, FIG. 6 is a cross-sectional view of a spacer used in the fuel assembly according to the present invention, and FIG. 7 is a cross-sectional view of the core of the multiplier reactor according to the present invention. II
A perspective view of a Jfil element assembly, FIG. 8 a perspective view of an embodiment of a control element assembly according to the present invention, and FIGS. 9 and 10 a horizontal and vertical sectional view of an embodiment of a fuel assembly according to the present invention. , FIG. 11 is a partial cross-sectional view of a reactor core using the fuel assembly of the present invention.

Claims (1)

[Claims] 1. A plurality of fuel rods arranged in a triangular lattice, a plurality of spacers provided in the axial direction to maintain the horizontal spacing between the fuel rods, and the fuel rods. In a fuel assembly in which the cross section of the fuel bundle is hexagonal, the cross section of the fuel bundle is kept constant, and the cross section of the fuel bundle is hexagonal. A control element assembly having a plurality of thin plate-like control elements radially from the center is inserted from the bottom of the fuel assembly and penetrates to the top of the fuel bundle, inside the array of fuel rods. A fuel assembly characterized in that it is provided along the entire axial region of the fuel assembly. 2. In claim 1, the control element assembly inserted into the fuel assembly has a thin plate-like control element radially extending from the center of the fuel bundle, and the control element assembly is located in the reactor core. A nuclear reactor located at a lower part and insertable from the lower part of the fuel assembly. 3. The fuel assembly according to claim 1, wherein the spacer constituting the fuel bundle is provided with a partial region in which no grid exists so that the control element can pass through. 4. The fuel assembly according to claim 1, wherein the lower tie plate is provided with a through hole therein so as to secure an insertion space for a plate-shaped control rod. 5. In claim 2, when a cross section of the inserted portion of the control element assembly of the fuel assembly is taken in the reactor core, the direction of the fuel assembly with respect to the center of the reactor core has a specific directionality. A nuclear reactor characterized by being loaded with a fuel assembly.