JPH0651082A - Nuclear reactor of reflector control type - Google Patents

Abstract

PURPOSE:To enhance the reaction degree controlling ability of a neutron reflecting body without prolonging the length of the neutron reflecting body. CONSTITUTION:A neutron reflecting body 9 is installed outside of the core 2 of a nuclear reactor immersed in a cooling material 4 and moved up and down so as to adjust the leak of neutrons from the core 2, and thereby the reaction degree at the core 2 is controlled. In this nuclear reactor of reflecting body control type, the core in its portion positioned above the neutron reflecting body 9 is surrounded by a substance 24 whose neutron reflecting ability is inferior to the cooling material 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reactor of a reflector control system in which the reactivity of a core is controlled by moving a neutron reflector up and down, and in particular, without changing the length of the neutron reflector itself. The present invention relates to a reflector-controlled nuclear reactor in which the reactivity control capability is increased and the reactivity life of the core is extended to prolong the operating period without refueling.

[0002]

2. Description of the Related Art Among reactors of the reflector control system described above, FIGS. 8 to 10 show a conventional general structure of a reactor employing a so-called external reflector system.

That is, inside the reactor vessel 1, a core 2 is arranged at a central portion, a neutron shield 3 is arranged at a position surrounding the core 2, and liquid sodium and the like are arranged. Is filled with the coolant 4.

As shown in FIG. 9, the core 2 is composed of, for example, 18 hexagonal fuel assemblies 5, and in the center of this core, neutrons are drawn upward during operation for controlling the reactivity of the core 2. A channel 6 for an absorber rod is arranged and surrounded by a core barrel 7.

A partition wall 8 is arranged outside the core barrel 7 so as to divide the flow path of the coolant 4 at a predetermined distance. The space between the core barrel 7 and the partition wall 8 provides a space for the core 2. Moving region 1 of the neutron reflector 9 used for the operation of the
0 is formed.

Here, the coolant 4 flows from the bottom to the top in the inside of the partition wall 8 and enters the core 2 on the way to take away the heat generated by the nuclear fission to raise the temperature. Then, the coolant 4 whose temperature has risen enters the inside of an intermediate heat exchanger (not shown), where it exchanges heat with the secondary sodium, and then flows downward from the intermediate heat exchanger. The cooled coolant 4 after heat exchange passes through the outside of the partition wall 8 and
It goes around to the lower part of and is introduced into the core 2 again.

The neutron shield 3 is for limiting the neutron irradiation dose of the reactor vessel 1 to a predetermined value or less over the entire life of the plant, and a plurality of neutron shields 3 are arranged between the reactor vessel 1 and the partition walls 5. Neutron shielding rod 11 of

As the structure of the neutron shield 3, in addition to a structure made of stainless steel or the like, a pin accommodating a B ₄ C ceramic containing boron having a large neutron absorption capacity is arranged, or hafnium, tantalum, or the like. The metal or a compound thereof can be included. Further, by disposing the neutron absorber, the reactivity control capability of the neutron reflector 9 can be increased.

Reference numeral 12 is a guard vessel surrounding the reactor vessel 1.

The fuel assembly 5 is, for example, as shown in FIG. 10, a hexagonal trumpet tube 1 made of stainless steel.
A large number of fuel pins 14 are regularly arranged in the inside of the fuel cell 3, and neutron shields 15a and 15b are arranged on the upper and lower portions of the trumpet tube 13.

In the figure, one of a large number of fuel pins 14 is taken out and shown, and the fuel pin 14 has a fuel portion 14a and a plenum portion 14b for confining gas components generated by nuclear fission. It is equipped. Further, the fuel pin 14 is configured to promote the mixing of the coolant 4 by a wire wrap or a grid (not shown), and be joined and fixed to the trumpet tube 13 at the lower end plug portion.

The fuel assembly 5 is constructed so as to be inserted into and fixed to a core support 17 through an entrance nozzle 16 having a small diameter, and is provided with a coolant population 18 and a coolant outlet 19. . In addition, the perforated trumpet tube in which a part of the trumpet tube of the fuel assembly is removed, the partial ductless or the ductless assembly has essentially the same configuration.

A neutron reflector 9 is arranged in the moving region 10 between the core barrel 7 and the partition wall 8 as shown in FIG.
Is suspended from and supported by the lower end of the drive rod 20, and the drive rod 20 extends upward through the shield plug 21 that closes the upper end opening of the reactor vessel 1 and is installed on the upper surface of the shield plug 21. The device 22 is configured to move up and down.

That is, as the drive unit 22 is driven, the drive rod 20, and thus the neutron reflector 9, moves vertically in the moving region 10 between the core barrel 7 and the partition wall 8. ing.

The liquid surface 4a of the coolant 4 and the shield plug 2
Between 1 and 2, a cover gas space 2 filled with cover gas
It is 3.

As a result, the neutron reflector 9 is driven by the driving device 2
The neutron leakage from the core 2 is adjusted by moving the core 2 in the up and down direction, and thereby the reactivity of the core 2 is controlled.

In a so-called external reflector type nuclear reactor in which the neutron reflector is installed outside the reactor vessel, the neutron reflector is arranged in a gas.

[0018]

However, in the case of the conventional reflector moving type reactor, in order to increase the controllability of the neutron reflector, it is necessary to increase the length of the neutron reflector itself. As the length of the neutron reflector is increased in order to meet this request, not only the self-weight of the neutron reflector increases, but also the length of the neutron reflector is restricted due to many restrictions on the reactor structure. There was a problem that it was difficult to make it longer.

In particular, in the case of a so-called in-reactor type reactor in which the neutron reflector is installed inside the reactor vessel, the length of the neutron reflector can be made sufficiently long due to restrictions on the internal structure of the reactor. However, it is the current situation that the above-mentioned problems have been conspicuously manifested.

In view of the above, it is an object of the present invention to provide a neutron reflector whose reactivity control capability can be enhanced without increasing the length of the neutron reflector itself.

[0021]

In order to achieve the above object, a reflector-controlled nuclear reactor according to the present invention has a neutron reflector arranged outside the core of a reactor immersed in a coolant in a vertical direction. In the reactor of the reflector control system to control the reactivity of the core by adjusting the leakage of neutrons from the core by moving the core around the core above the neutron reflector than the coolant. It is characterized in that it is surrounded by a substance having a poor neutron reflection ability.

[0022]

According to the present invention configured as described above, since the core located above the neutron reflector is surrounded by a substance having a neutron reflecting ability lower than that of the coolant, the neutron reflector In the early stage of combustion (BOL), which is located below the core, the periphery of the core is covered with the substance, and the reactivity is suppressed to a low level as compared with the conventional case where the entire periphery is covered with a coolant, The fuel enrichment can be increased accordingly. Further, when the neutron reflector moves upward, in addition to the increase in reactivity due to the change in the relative position of the neutron reflector and the core, it is surrounded by the coolant while reducing the range surrounded by the substance. The range can be gradually widened, whereby the reactivity due to the difference in the neutron reflecting ability of both parts due to the replacement of the substance with the coolant can be added.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. The same members as those in the conventional example shown in FIGS. 8 to 10 are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 shows a first embodiment of the present invention, which is applied to a sodium-cooled small fast reactor of an internal reflector type having a core equivalent diameter of about 83 cm and an effective length of about 400 cm.
The neutron reflector is made of stainless steel and has a length of approximately 17
The one having a thickness of 0 cm and a thickness of 15 cm is used, and the points different from the above conventional example are as follows.

That is, on the upper portion of the neutron reflector 9, a neutron absorber 24, such as natural boron, which has substantially the same cross-sectional shape as that of the neutron reflector 9 and whose neutron reflecting ability is lower than that of the coolant 4, is integrally connected. At the same time, the lower end of the drive rod 20 is connected to the upper end of the neutron absorber 24. As a result, the driving rod 20, and hence the neutron reflector 9 and the neutron absorber 24 are integrated together with the driving of the driving device 22, and along the inside of the moving region 10 between the core barrel 7 and the partition wall 8. It is like moving vertically.

Next, the operation of the above embodiment will be described.

First, based on FIG. 2, the reaction when the neutron reflector 9 and the neutron absorber 24 are moved upward from the positional relationship shown in FIG. 2A to the positional relationship shown in FIG. The change in degree will be described. At this time, the neutron reflector 9
The reactivity increases, of course, due to the change in the relative position between the core 2 and the core 2. However, the reactivity of the core 2 surrounded by the coolant 4 and the area surrounded by the neutron absorber 24 are different from each other. Changes.

[0028] That is, in FIG. (A), the periphery of the core 2, the coolant 4 in the length field L _1, but is surrounded respectively neutron absorber 24 in the length field L _2, Fig. ( in b), a coolant 4 in the length field L _3, the length region L ₄ will be respectively surrounded by the neutron absorber 24, part of the difference _{(L 3 -L 1 = L 2} -L ₄ = a) is replaced by the coolant 4 from the neutron absorber 24. As a result, on the side surface of the core 2, the difference in neutron reflecting ability between the neutron absorber 24 and the coolant 4 in the replaced portion a is added to the reactivity.

FIG. 3 shows the state of the initial combustion (BOL) in which the neutron reflector 9 is located below the core 2. In this case, the periphery of the core 9 is covered with the neutron absorber 24 having a lower neutron reflecting ability than the coolant 4, so that the reactivity is suppressed to be lower than that in the conventional case where the neutron absorber 24 is covered with the coolant 4. Therefore, the fuel enrichment can be increased by that much, which can contribute to the extension of the reactivity life of the core 2.

In this case, it is preferable that the entire periphery of the core 2 is surrounded by the neutron absorber 24 as shown in FIG.

FIG. 4 shows a second embodiment, which is the neutron absorber 24 in the first embodiment.
In place of the above, a vacuum or a gas having a neutron reflecting ability lower than that of the coolant 4 is used. That is, a space 26 sealed by the box body 25 is formed above the neutron reflector 9, and the space 26 is evacuated or filled with gas.

In order to compare the effects of the first and second embodiments with the conventional example, the reactivity life of each of the results of the combustion calculation of these cores is shown in the table below.

[0033]

[Table 1] From this table, it can be seen that the reactivity life of the core can be extended to about 1.5 times longer than that of the conventional example by each of the examples.

In each of the above embodiments, the neutron reflector 9 and the substance having a lower neutron reflecting ability than the coolant 4 provided above the neutron reflector 9 move together in the vertical direction, and are accommodated. It is necessary to preliminarily provide a space above the core. However, it is often difficult to provide such a space in the narrow reactor vessel 1 due to the reactor structure.

FIG. 5 shows a third embodiment in which such a drawback is prevented. In this embodiment, a downward moving region extending laterally of the core 2 over substantially the entire length of the core 2 is shown. A guide box 27 having an open U-shaped cross section is arranged and fixed, and the upper end of the guide box 27 and the cover gas space 2 are also fixed.
3 is made to communicate with each other through a communication pipe 28, and a bellows 29 for sealing is arranged between the upper end of the neutron reflector 9 and the upper inner peripheral surface of the guide box 27, whereby the guide box 27 While the neutron reflector 9 moves in the vertical direction inside, the cover gas in the cover gas space 24 is introduced to the upper part of the guide box 27, and the boundary between the introduced cover gas and the coolant 4 is sealed by the bellows 29. It was done like this.

The drive rod 20 connected to the upper end of the neutron reflector 9 is inserted through the communication tube 28. Instead of the bellows 29, another sealing means such as a metal ring may be used to seal the area.

In this embodiment, the guide box 27 is the core 2
When the volume inside the upper guide box 27 decreases as the neutron reflector 9 rises, the coolant 4 is supplemented below the neutron reflector 9 by the reduced amount. become.

FIG. 6 shows a fourth embodiment. The difference from the third embodiment is that the communicating pipe 28 extends through the shield plug 21 to the upper side, and the guide from the upper end. Gas and the like can be introduced into the box 27, and the bellows 29 is omitted.

In the case of this embodiment, the pressure inside the guide box 27 can be controlled from the outside of the reactor vessel 1, whereby the coolant 4 located below the neutron reflector 9 can be controlled.
The height of can be adjusted arbitrarily.

FIG. 7 shows a fifth embodiment, which has the features of the third and fourth embodiments. That is, the communication pipe 28 is connected to the shield plug 21.
And extends to the upper side so that gas or the like can be introduced into the inside of the guide box 27 from this upper end, and for sealing between the upper end of the neutron reflector 9 and the upper inner peripheral surface of the guide box 27. The bellows 29 is arranged so that the boundary between the gas introduced from the outside into the guide box 27 and the coolant 4 is sealed by the bellows 29.

[0041]

Since the present invention has the above-mentioned structure,
The reactivity control capability can be increased without changing the length of the neutron reflector itself, which can prolong the reactivity life of the core and thus the operating time without refueling.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation when the neutron reflector moves upward.

FIG. 3 is a diagram similarly attached to the description of the action at the early stage of combustion (BOL).

FIG. 4 is a schematic diagram showing a second embodiment.

FIG. 5 is a schematic diagram showing a third embodiment.

FIG. 6 is a schematic diagram showing a fourth embodiment.

FIG. 7 is a schematic diagram showing a fifth embodiment.

FIG. 8 is a schematic diagram showing a conventional example.

FIG. 9 is a horizontal sectional view of the same.

FIG. 10 is also an enlarged view of the fuel assembly.

[Explanation of Codes] 1 reactor vessel 2 core 3 neutron shield 4 coolant 5 fuel assembly 9 neutron reflector 20 drive rod 22 drive device 24 neutron absorber 25 box 26 space 27 guide box 28 communicating tube 29 bellows

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tsuguo Yokoyama 8 Shinsita-cho, Isogo-ku, Yokohama, Kanagawa Pref., Toshiba Corporation Yokohama Works (72) Inventor Shigeo Kasai Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa No. 8 Incorporated company Toshiba Yokohama Works (72) Inventor Sadao Hattori 1-6-1, Otemachi, Chiyoda-ku, Tokyo Inside Central Research Institute of Electric Power Industry

Claims (1)

[Claims] 1. A reflector for controlling the reactivity of a core by adjusting the leakage of neutrons from the core by vertically moving a neutron reflector arranged outside the core of a nuclear reactor immersed in a coolant. In a control system reactor, a reactor control system reactor characterized in that the periphery of the core located above the neutron reflector is surrounded by a substance having a neutron reflecting ability inferior to that of the coolant. .