JPH05134077A - Control rod assembly - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] When the coolant temperature of the nuclear reactor rises, control rods are inserted into the core to keep up with the temperature rise and the position is maintained to suppress the reactor power. A control rod assembly for preventing re-pulling of the control rod body even when the temperature drops after the scram operation. [Structure] A control rod body, which is housed in the reactor core in a vertically movable manner, is suspended by an extension rod or the like at the upper end of the control rod body. A spring 21 that suspends the extension rod 17 that serves as a ratchet shaft and a bellows 22 that isolates the spring 21 and the temperature-sensitive member 25 from each other are provided to partition the spring and the temperature-sensitive member from each other. The temperature sensitive member 2 is connected to the outside through a resistor.
The temperature compensation holding mechanism is provided at the lower end of 5 with a ratchet mechanism 26 that locks the ratchet shaft 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod assembly for controlling the output of a fast breeder reactor.

[0002]

2. Description of the Related Art Generally, a core of a fast breeder reactor has a large number of fuel assemblies 1 and a plurality of control rod assemblies inserted between the fuel assemblies 1, as shown in the longitudinal sectional view of the core of FIG. 2, and a large number of shields (not shown) provided so as to surround the outermost portion of the fuel assembly 1.

The fuel assembly 1 is equipped with a large number of fuel pins 3 inside and is installed at a fixed position by inserting an entrance nozzle 4 provided at the lower end into an opening of a core support plate 5. Then, the coolant flows from the high-pressure plenum 6 in the core support plate 5 through the coolant inlet port 7 of the entrance nozzle 4, and then rises between the fuel pins 3 to remove heat from the fuel assembly 1.

The control rod assembly 2 includes a lower guide pipe 8, an upper guide pipe 9, a control rod body 10 and an extension pipe for suspending the control rod body 10.
The lower guide tube 8 is formed by 11 and is installed at a fixed position by inserting the entrance nozzle 12 formed at the lower end into the opening of the core support plate 5. The entrance nozzle 12 is provided with a coolant inlet 13 communicating with the high-pressure plenum 6 and a communication hole 14 communicating with the inside of the control rod body 10.

Further, the control rod body 10 has a neutron absorber built therein, and when the control rod body 10 is reinserted, the engaging portion 15 at the lower end is
Is inserted into the dash pot 16 of the lower guide tube 8 and placed. The control rod body 10 is suspended from the extension pipe 11 by grasping the lower end portion of the extension pipe 11 with a grip portion 18 formed on the upper end portion of the extension rod 17 extending upward from the upper end.

The control rod body 10 is inserted into or pulled out from the core by vertically moving the extension pipe 11 by a control rod drive mechanism (not shown). The heat of the control rod body 10 is removed by a coolant,
This coolant is from the high pressure plenum 6 to the entrance nozzle 12
The lower guide pipe 8 is elevated through the coolant inlet 13 and the communication hole 14.

[0007]

During normal operation of the fast breeder reactor, the degree of insertion of the control rod body 10 into the core is adjusted by vertically moving the extension pipe 11 to adjust the reactor power. Further, when an abnormality such as an abnormal increase in the reactor output or a decrease in the coolant occurs, a scram operation is carried out to urgently insert the control rod body 10 into the core and stop the furnace.

On the other hand, in the fast breeder reactor, the extension pipe 11 cannot be lowered for some reason in an emergency and the control rod body
It is necessary to keep the reactivity of the core below the critical level even in the unlikely event of a scrum failure, assuming a severe accident of scrum failure where 10 cannot be inserted into the core.

This is because if a scrum failure occurs, the reactor output will increase excessively and the coolant temperature will rise, which may lead to a core damage accident. Conventionally, there has been a demand for safety measures at the time of this severe accident. It had been.

An object of the present invention is to insert a control rod into a reactor core by a temperature compensating mechanism corresponding to the temperature rise when the coolant temperature rises due to an excessive increase in the reactor output, etc. This position is maintained by the control of the reactor power and the re-drawing of the control rod body in the core is prevented against the temperature drop around the control rod body after the scrum operation.
It is to provide a control rod assembly capable of reliably ensuring the safety of a nuclear reactor.

[0011]

A control rod main body is mounted via an extension rod extending from an upper end of a control rod main body that is housed in a guide tube installed in a reactor core so as to be able to move up and down, and a grip portion provided on the upper portion of the extension rod. In a control rod assembly consisting of an extension pipe to be hung, a temperature sensing member having a double cylindrical periphery is hung on a part of the extension pipe or the extension rod, and an extension rod having a ratchet shaft at its tip is hung inside the temperature sensing member. A spring and a bellows for isolating the spring and the temperature-sensitive member are provided to partition it from the outside, and the spring portion and the temperature-sensitive member are sealed with a liquid metal so as to communicate with the outside via a flow resistor, and the temperature-sensitive A temperature compensation holding mechanism having a ratchet mechanism including a ratchet pawl for locking the ratchet shaft and a spring is provided at the lower end of the member.

[0012]

The operation of the temperature-sensitive member of the temperature compensation holding mechanism and the inside of the bellows are communicated with each other, and the liquid metal is enclosed in the inside.
Here, if the temperature sensitive member is formed of a thin member compared to the bellows, it responds quickly to the temperature of the liquid metal that is the surrounding coolant, and the liquid metal in the temperature sensitive member flows into the bellows,
When the excessive temperature of the coolant rises due to the abnormality of the nuclear reactor, the control rod body is inserted into the core by the expansion of the temperature compensation holding mechanism due to the expansion of the bellows.

At this time, since the ratchet mechanism is operable in the core insertion direction of the control rod body, and the ratchet shaft step and ratchet pawl are formed so as to prevent this in the core withdrawal direction, the ratchet mechanism is once inserted. The control rod body position is maintained as it is, and even if the coolant temperature drops due to the reactor shutdown, the reactor power output can be prevented from rising again and the safety of the reactor is ensured.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. It should be noted that the same components as those of the above-described conventional technique are designated by the same reference numerals and detailed description thereof will be omitted.

As shown in the longitudinal sectional view of the core of FIG. 2, the core of the fast breeder reactor has a large number of fuel assemblies 1, a plurality of control rod assemblies 2 inserted between the fuel assemblies 1, and a fuel assembly. It is configured by a large number of unillustrated shields provided so as to surround the outermost part of the body 1.

The fuel assembly 1 is internally provided with a large number of fuel pins 3, and is held in place by inserting an entrance nozzle 4 provided at the lower end into an opening of a core support plate 5. Then, the coolant flows into the fuel assembly 1 from the high-pressure plenum 6 in the core support plate 5 through the coolant inlet port 7 of the entrance nozzle 4, and rises between the fuel pins 3 to remove the fuel assembly 1. It is designed to remove heat.

The control rod assembly 2 comprises a lower guide pipe 8, an upper guide pipe 9, a control rod body 10 and an extension pipe 11 for suspending the control rod body 10. The entrance nozzle 12 formed at the lower end of the core is inserted into the opening of the core support plate 5 to hold it in place. The entrance nozzle 12 is provided with a coolant inlet 13 communicating with the high pressure plenum 6 and a communication hole 14 communicating with the inner side of the control rod body 10.

The control rod body 10 has a built-in neutron absorber inside, and at the time of re-insertion, the engagement portion 15 at the lower end is inserted into the dashpot 16 of the lower guide tube 8 to be inserted and arranged at a fixed position. It has become so.

Further, the control rod body 10 has an extension rod at its upper end.
The extension rod 17 is provided with a grip portion 18 at the upper end thereof, and the control rod main body 10 is suspended from the extension pipe 11 by gripping the lower end portion of the extension pipe 11 with the grip portion 18. It is supposed to be done.

Then, the extension rod 11 is moved up and down by a control rod drive mechanism (not shown) so that the control rod body 10 can be inserted into or pulled out from the core. Further, the heat removal of the control rod body 10 is performed by the high pressure plenum 6
Is performed by the coolant that rises in the lower guide tube 8 through the coolant inlet port 13 of the entrance nozzle 12 and the communication hole 14.

Further, a temperature compensation holding mechanism 20 is interposed below the grip portion 18 in the middle of the extension pipe 11. As shown in the enlarged vertical sectional view of FIG. 1, this temperature compensation holding mechanism 20 is a bellows suspended by a spring 21 attached to the lower portion of the grip portion 18.
22 and ratchet shaft 23 are arranged, and bellows 22
An upper end is joined to a ratchet shaft 23 and a lower end is joined to a temperature-sensitive member 25 through a support plate 24 provided at upper and lower end portions of the above to form a sealing structure.

A double cylindrical temperature sensitive member 25 is arranged on the outer peripheral portion of the bellows 22. The temperature sensitive member 25 is connected to the grip portion 18 on the upper side and to the ratchet mechanism 26 on the lower side. ing. Further, the inside of the double cylinder communicates with the outer peripheral portion of the bellows 22 through a communication hole 27 formed in the grip portion 18,
A liquid metal 28 is enclosed inside these, and the enclosed liquid metal 28 is provided at the upper end of the double cylinder. 29
Are formed.

The ratchet mechanism 26 has a ratchet claw 26a.
Is configured to be pressed against the ratchet shaft 23 by the leaf spring 26b, the lower end of the ratchet shaft 23 is connected to the extension rod 17, and the extension rod 17 is connected to the control rod body 10 as shown in FIG. It is configured. The spring 21 is stretched by the weight of the control rod body 10 during normal operation. Next, the operation of the above configuration will be described.

In the fast breeder reactor, when abnormalities such as an abnormal increase in reactor output and a decrease in coolant occur, the extension pipe 11 is lowered to insert the control rod body 10 into the core urgently, A scrum operation is performed to stop the.

However, in this emergency, when the extension pipe 11 does not descend for some reason and the control rod body 10 cannot be inserted into the core, the coolant temperature rises excessively and the temperature compensation holding mechanism 20 feels unsatisfactory. The temperature around the temperature member 25 also rises. Since the temperature-sensitive member 25 has a thin cylindrical shape, the temperature of the liquid metal 28 inside the temperature-sensitive member 25 also rises with almost no time delay. As a result, the liquid metal 28 in the temperature-sensitive member 25 rapidly expands, penetrates the communication hole 27, and enters the outer peripheral portions of the spring 21 and the bellows 22.

For this reason, the bellows 22 is extended and the control rod body is extended.
10 is inserted in the core. At this time, the ratchet shaft 23 has a taper portion 23a and a stepped portion 23b. When the ratchet shaft 23a is inserted into the core, the ratchet shaft 23 is pushed downward by the taper portion 23a while pushing and opening the ratchet pawl 26a. The spring 26b presses the ratchet pawl 26a against the ratchet shaft 23 to hold the position of the ratchet shaft 23. While repeating the lowering operation and the holding operation in the stepped portion 23a, the control rod body 10 is inserted into the core by the amount of thermal expansion stroke corresponding to the temperature rise.

Here, when the control rod body 10 is inserted into the core, the output of the nuclear reactor is reduced and the temperature of the coolant around the control rod body 10 is lowered as compared with that before the control rod was inserted. Due to this temperature drop, the liquid metal 28 in the temperature sensitive member 25 of the temperature compensation holding mechanism 20 is
And the expanded liquid metal 28 contracts.

As a result, the force for inserting the spring 21 and the bellows 22 into the core is reduced, and the pulling force of the spring 21 tends to cause the control rod body 10 to be pulled out from the core. The ratchet mechanism 26 prevents the pull-out operation, and the control rod body 10 holds the position inserted in the core.

When the scrum fails in the fast breeder reactor, as shown in FIG.
As shown in the control rod reactivity characteristic graph of No. 3, a positive reactivity indicated by a line A in which the control rod reactivity is higher than the critical value of 0 is generated by the Doppler effect and the coolant density effect in the core. However, in the present invention, the bellows 22 of the temperature compensation holding mechanism 20 expands and the control rod body 10 is inserted into the core, so that the negative reactivity of the line B is generated. The total reactivity obtained by superposing both the positive (line A) and the negative (line B) reactivity is a negative reactivity region lower than the critical level as shown by line C, and the reactor secures safety.

However, as described above, the bellows 22 expands in the axial direction once the coolant temperature rises, and the control rod body 10
Is inserted into the core to generate a negative reactivity,
When the reactor output is suppressed by the generation of this negative reactivity, the temperature of the coolant around the control rod body 10 drops. As a result, the liquid metal 28 which has been expanded in the temperature sensitive member 25 is contracted,
As a result, it is conceivable that the control rod body 10 inserted into the core operates in the pulling direction and the reactor power again increases.

However, since the control rod body 10 is prevented from being pulled out by the ratchet mechanism 26 and the inserted position in the reactor core is maintained, the control rod body 10 is re-pulled and the reactor is re-pulled. Output does not occur,
The safety of the nuclear reactor can be reliably maintained. It should be noted that the slit portion 29 provided with the flow resistance element allows the liquid metal 28 to escape to the outside when the temperature rises gently during normal operation and the ratchet shaft 23.
It is provided to prevent lowering.

[0032]

As described above, according to the present invention, when the reactor coolant temperature rises excessively, the temperature rise is used to insert the control rod body into the core to suppress the rise of the reactor output and to increase the ambient temperature. Since the control rod body does not have to be re-pulled out even if the temperature drops, the reactor output can be prevented from rising again.

For this reason, a larger negative control rod reactivity can be given to the core, and even if a scrum failure accident occurs, the reactor power is automatically attenuated, and the subcritical state is maintained for a long time. Can be ensured, which has the effect of improving the safety and reliability of reactor operation.

[Brief description of drawings]

FIG. 1 is an enlarged vertical sectional view of a temperature compensation holding mechanism according to an embodiment of the present invention.

FIG. 2 is a longitudinal sectional view of the core of the present invention.

FIG. 3 is a characteristic diagram of control rod reactivity and accident time.

FIG. 4 is a vertical cross-sectional view of a conventional core.

[Explanation of symbols]

1 ... Fuel assembly, 2 ... Control rod assembly, 10 ... Control rod body,
11 ... extension pipe, 17 ... extension rod, 18 ... grip, 20 ... temperature compensation holding mechanism, 21 ... spring, 22 ... bellows, 23 ... ratchet shaft,
23a ... tapered portion, 23b ... stepped portion, 24 ... support plate, 25 ... temperature sensitive member, 26 ... ratchet mechanism, 26a ... ratchet pawl, 26b
… Leaf springs, 27… communication holes, 28… liquid metal, 29… slits.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Inokakura 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Stock company Toshiba Research Institute

Claims (1)

[Claims] 1. A control rod body which is housed in a guide tube installed in a core so as to be able to move up and down, an extension rod extending from an upper end of the control rod body, and a grip portion provided on an upper portion of the extension rod. In a control rod assembly including an extension member including an extension pipe for suspending a main body, the extension member includes an upper extension member, a lower extension member, and a temperature compensation holding mechanism provided between these extension members. However, the temperature compensation holding mechanism includes a spring connecting the lower portion of the upper extension member and a ratchet shaft formed on the upper portion of the lower extension member, and the spring and the ratchet shaft coaxially covering the lower portion of the upper extension member. The attached double-cylindrical temperature-sensing member is liquid-tightly mounted between the lower part of the temperature-sensing member and the upper part of the ratchet shaft, and the inside of the temperature-sensing member and the outside of the temperature-sensing member. Bellows to isolate the temperature A ratchet mechanism that is provided at the lower part of the member and engages with the ratchet shaft to allow only the downward movement of the ratchet shaft, and connects the annular space of the double cylindrical temperature-sensitive member and the space where the spring exists. A first communication hole,
A control rod assembly characterized in that the space in which the spring exists and the outer space of the temperature sensitive member are constituted by a second communication hole that communicates with each other via a flow resistor.