JPH06123789A - Fast breeder reactor core - Google Patents

Abstract

(57) [Summary] [Objective] The object of the present invention is to achieve a high burnup and to increase the expansion amount of the core fuel portion at the time of a predictive transient event at the time of scrum failure, which is an inherent feature of a fast breeder reactor. It is intended to provide a core of a fast breeder reactor capable of improving the safety of the. [Configuration] The present invention is formed in a tubular shape having a hexagonal cross section,
A high-speed fuel cell having a trumpet tube 7 provided with a load pad 8 at least outside a portion located near the upper end of the core fuel part, and having a fuel assembly 6 made of ferritic steel. In the core 1 of the breeder reactor, the trumpet tube of the fuel assembly 5 located at the outermost periphery of the core 1 is formed of austenitic steel.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a core of a fast breeder reactor.

[0002]

2. Description of the Related Art Generally, the core of a fast breeder reactor is formed into a substantially cylindrical shape, with a core fuel part in the center thereof, an upper blanket part and a lower blanket part in the upper and lower parts of the core fuel part, and a core fuel part. A radial blanket portion is formed around the portion. Some core fuel parts are also provided with several layered blanket parts. Further, the core of such a fast breeder reactor is composed of a large number of fuel assemblies for the fast breeder reactor configured as follows.

That is, fuel rods in which a core fuel element and a blanket fuel element are enclosed in a fuel cladding tube are regularly arranged in a hexagonal lattice and bundled, and housed in a trumpet tube formed in a tubular shape having a hexagonal cross section. Has been done. An upper load pad (URP) is provided near the upper end on the outside of the trumpet pipe, and is located below the upper load pad and near the upper end of the core fuel part (near the lower end of the upper blanket part). Lower load pad (LR
P) is provided.

By the way, in the core of the fast breeder reactor constituted by the fuel assembly for the fast breeder reactor having the above-mentioned structure,
As shown in the characteristic diagram of FIG. 4 in which the vertical axis is temperature and the horizontal axis is radial distance from the core center, the temperature is high at the center of the core fuel part and the temperature starts to drop near the outer periphery of the core fuel part. 3 shows a temperature distribution in the furnace inner diameter direction. Due to such radial temperature distribution, the fuel assembly for a fast breeder reactor near the outer periphery of the core fuel portion, which has a large temperature change, causes a curve as shown in FIG.

That is, during the rated operation, as shown in FIG. 5 (A), the core fuel portion is bent toward the inside. Here, when an output increase accident (TOP) or a coolant flow rate decrease accident (LOF) occurs (at ATWS:
In the predictive transient event at the time of scrum failure), the ratio of the assembly output to the flow rate (P / F) increases and the temperature distribution in the in-core temperature direction further increases, and as shown in FIG. 5 (B), the core fuel part Causes a curve that goes to the outside. FIG. 5C is the same as FIG.
The difference between the case of (A) and the case of FIG. 5 (B) is shown.
As described above, during ATWS, the core fuel portion is substantially expanded as compared with during rated operation, and a negative reactivity is inserted. Such effects are important factors in enhancing the intrinsic safety of the furnace.

Generally, the greater the coefficient of thermal expansion of the trumpet tube material, the greater the core curvature. Therefore, the negative reactivity insertion amount at the ATWS is also the core composed of the trumpet tube made of the material having a large coefficient of thermal expansion. It is advantageous.

On the other hand, in a fast breeder reactor core aiming at high burnup, the trumpet tube must be made of a low swelling material. For this reason, ferritic steels having excellent swelling resistance are generally used in place of the austenitic steels used in the conventional trumpet tubes of fast breeder reactor cores.

However, the coefficient of thermal expansion of ferritic steel is about 40% smaller than that of austenitic steel, and regarding the magnitude of the negative reactivity insertion amount due to the bending of the core during ATWS, the core of an austenitic steel wrapper tube is used. Becomes smaller, which is not desirable from the viewpoint of increasing the intrinsic safety of the furnace.

[0009]

As described above, in the fast breeder reactor core which uses ferritic steel excellent in swelling resistance as the material of the trumpet tube in order to achieve high burnup, the core of the ATWS is used. The amount of insertion of the negative reactivity due to bending becomes smaller than that of the core made of austenitic steel trumpet tube, which is not desirable from the viewpoint of increasing the intrinsic safety of the furnace.

The present invention has been made in consideration of the above-mentioned conventional circumstances, and it is possible to achieve a high burnup and increase the expansion amount of the core fuel portion during ATWS. It aims to provide a fast breeder reactor core capable of improving the safety of

[0011]

That is, the present invention has a trumpet tube which is formed in a tubular shape having a hexagonal cross section and which is provided with a load pad at least outside a portion located near the upper end of the core fuel section. In the core of a fast breeder reactor in which a fuel assembly made of a ferritic steel is provided, the fuel assembly wrapping pipe located at the outermost periphery of the core is made of austenitic steel. And

[0012]

In the core of the fast breeder reactor of the present invention having the above-mentioned structure, the core of the core having a large neutron flux that promotes the swelling expansion of the trumpet tube is provided with a ferrite steel trumpet tube excellent in swelling resistance. A high burnup is achieved by loading the core fuel assembly. On the other hand, ATW
A wrapper pipe made of austenitic steel having a large coefficient of thermal expansion is provided at the outermost periphery of the core where the temperature gradient in the radial direction of the wrapper pipe, which causes the substantial expansion of the core fuel portion due to the bending of the core during S, is large. The inherent safety can be improved by loading the core fuel assembly.
Since the neutron flux at the outermost periphery of the core is smaller than that at the center of the core, even in a core fuel assembly consisting of an austenitic steel trumpet tube with poor swelling resistance, it is difficult to achieve high burnup of the reactor. There is no. Therefore, in the fast breeder reactor core of the present invention, both achievement of high burnup and improvement of inherent safety can be satisfied.

[0013]

EXAMPLES Examples of the fast breeder reactor core of the present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of a core of a fast breeder reactor according to an embodiment of the present invention. A core fuel region 1 is provided at the center of the core, and a radial blanket region 2 and a neutron shield are provided around the core fuel region 1. A region 3 is arranged, and a control rod assembly 4 is loaded in the core fuel region 1. The trumpet tubes of the outermost peripheral core fuel assemblies 5 arranged in the outermost peripheral portion of the core fuel region 1 are made of austenitic steel, and the other wrapper pipes of the fuel assemblies 6 of the core fuel region 1 are made of ferritic steel. ing.

The graph of FIG. 2 shows the axial change in the temperature difference between the trumpet tube 7 on the furnace center side and the outer trumpet tube 7, which causes the bending of the fuel assembly for a fast breeder reactor. In this graph, the vertical axis is the axial position, the horizontal axis is the temperature difference, the solid line is P / F = 1.0 (during rated operation), and the dotted line is P / F.
It shows F> 1.0 (at ATWS). As shown in this graph, the temperature difference gradually increases from the lower end of the core fuel part, which is the heat generating part, reaches its maximum near the upper end of the core fuel part (near lower load pad 9), and reaches the upper part of the upper part of the trumpet tube. This temperature difference continues up to the load pad 8.

In the fast breeder reactor core of this embodiment having the above-described structure, as described above, the trumpet tube of the outermost peripheral fuel assembly 5 in which the temperature difference between the trumpet tube 7 on the core side and the trumpet tube 7 on the outer side is large is made of ferritic steel. Is also made of a material having a large thermal expansion. Therefore, the bending moment based on the temperature difference can be increased. Therefore, as shown by the solid line in FIG. 3 (B), the amount of S-shaped curve of the trumpet pipe 7 during ATWS can be made larger than in the conventional case (ferrite steel) shown by the dotted line, and the core fuel The amount of expansion of the part can be increased to increase the amount of negative reactivity insertion. Like FIG. 5 described above, FIG. 3A shows the difference between the rated operation and FIG. 3C shows the difference between the rated operation and the ATWS.

That is, in the core of the fast breeder reactor of this embodiment, the S-shaped curve amount of the trumpet tube 7 of the outermost peripheral fuel assembly 5 during ATWS can be made larger than that of the conventional case, and the core Since the expansion amount of the fuel portion can be increased to increase the negative reactivity insertion amount, it is possible to improve the inherent safety of the fast breeder reactor as compared with the conventional case.

[0018]

As described above, according to the fast breeder reactor core of the present invention, it is possible to further increase the expansion amount of the core fuel portion at the time of ATWS while achieving high burnup. Therefore, the inherent safety of the fast breeder reactor can be improved.

[Brief description of drawings]

FIG. 1 is a schematic plan view showing the structure of a fast breeder reactor core according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing an axial change in temperature difference between a trumpet tube on the center side and an outer trumpet tube.

3A and 3B show a state of deformation of a fuel assembly of a core of the fast breeder reactor shown in FIG. 1, (A) during rated operation, (B) during ATWS,
(C) is a characteristic diagram for explaining the difference.

FIG. 4 is a characteristic diagram showing a temperature distribution in a furnace inner diameter direction.

FIG. 5 shows the deformation of the fuel assembly of the core of the conventional fast breeder reactor (A) during rated operation, (B) during ATWS,
(C) is a characteristic diagram for explaining the difference.

[Explanation of symbols]

 1 ... Core fuel region 2 ... Radial blanket region 3 ... Neutron shield region 4 ... Control rod assembly 5 ... Core fuel assembly (made of austenitic steel wrapper pipe) 6 ... Core fuel assembly (made of ferrite steel wrapper pipe) 7 … Trumpet pipe 8… Upper load pad (URP) 9… Lower load pad (LRP)

Claims (1)

[Claims] 1. A trumpet tube formed in a tubular shape having a hexagonal cross section and provided with a load pad outside at least a portion located near the upper end of the core fuel section, the trumpet tube being formed of ferritic steel. A core of a fast breeder reactor in which a fuel assembly located at the outermost periphery of the core is formed of austenitic steel in the core of the fast breeder reactor.