JPH05297176A - Nuclear reactor stopping mechanism - Google Patents

Abstract

(57) [Abstract] [Purpose] Even if an accidental event such as a rapid coolant temperature rise occurs, by connecting the Curie point electromagnet connecting means and the axial thermal expansion device between the extension rod and the control rod, (EN) Provided is a reactor shutdown mechanism in which a control rod is inserted into a reactor core by a self-actuating response to suppress a temperature rise of a coolant to shut down the reactor. [Structure] In a reactor stop mechanism 20 for suspending and holding a control element 10 installed above a core 2 of a reactor vessel 1 and vertically movable in the core 2 in a coolant 3, an upper portion of the reactor vessel 1 It is characterized in that it comprises a connecting means by means of the Curie point electromagnet 7 attached to the extension rod 6 installed in the above, and an axial thermal expansion device 21 connected to this connecting means and extending by the temperature rise of the coolant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reactor stopping mechanism installed in a fast breeder reactor, which is capable of reliably operating a scram by operating by itself without relying on an external sensor or the like when the coolant temperature rises abnormally. Reactor shutdown mechanism that can

[0002]

2. Description of the Related Art Generally, in the core of a fast breeder reactor, an occurrence frequency of 10 ^-7 to 10 ^-10 / reactor year is extremely low, and a reactor shutdown failure event that is unlikely to occur from an engineering point of view occurs. Even if it does, measures are taken to prevent radioactive materials from being released into the environment. However, when such an event occurs, in the conventional nuclear reactor mechanism, although the accidental impact is stopped in the reactor vessel, damage to the entire core fuel may occur.

A path leading to core damage is assumed by taking an example of a flow stop type failure event of a reactor shutdown, which is one of the extremely low frequency events described above. (1) When the primary main circulation pump of a fast reactor is tripped due to the occurrence of some initiating event, but some signal or manual scrum is not possible, the temperature of the coolant due to the output-flow rate mismatch in the core fuel Rises.

(2) In the process of increasing the temperature of the coolant, when the scram cannot be further scrambled, the coolant boils and the sodium void reactivity is inserted, and when the void reactivity is a positive value, Furnace power may increase. (3) If the temperature of the fuel pellets rises due to the increase in reactor power, there is a possibility that fuel damage will occur throughout the core.

Therefore, conventionally, when such a temperature rise of the coolant occurs, a self-acting type furnace stopping mechanism has been devised which detects the excessive change by itself and scrams. As one of them, a reactor shutdown mechanism as disclosed in JP-A-60-66190 is disclosed. As shown in the schematic cross-sectional view of FIG. 4, this reactor shutdown mechanism has a reactor vessel 1 of a fast breeder reactor in which a core 2 composed of a large number of fuel assemblies is housed. It is immersed in the coolant 3 filled in the reactor vessel 1.

The upper opening of the reactor vessel 1 is a reactor plug 4
The reactor plug 4 is provided with a core upper part mechanism 5, and an extension rod is attached to the lower part of the core upper part mechanism 5. This extension rod is divided into upper and lower parts, and a Curie point electromagnet 7 made of a magnetic material having a Curie point, which is a connecting means, is installed at a lower end of the upper extension rod 6, and a lower extension rod 9 has an upper end. Is provided with an armature 8, and the Curie point electromagnet 7 and the armature 8 are provided.
It has a structure that adsorbs.

The Curie point electromagnets 7 and below are always immersed in the coolant 3, and a neutron absorber 10 which is a control element is hung below the lower extension rod 9, and inside the core 2. A lower guide tube 11 is installed so as to surround the neutron absorber 10. The operating principle of this self-actuating type reactor shutdown mechanism will be described by taking an example of the flow reduction type reactor shutdown failure event, which is one of the extremely low frequency events.

(A) When the primary main circulation pump of a fast reactor trips due to the occurrence of some cause event but some signal or manual scram is not possible, due to output-flow rate mismatch in the core fuel The temperature of the coolant 3 rises. (b) As the temperature of the coolant 3 rises, the temperature of the Curie point electromagnet 7 rises, and when the temperature of the Curie point electromagnet 7 exceeds the Curie point, the Curie point electromagnet 7 loses its coercive force,
The armature 8 is separated from the Curie point electromagnet 7, and the neutron absorber 10 is dropped and inserted into the core 2 through the lower guide tube 11.

(C) When the neutron absorber 10 is inserted into the core 2, a negative reactivity is introduced, the reactor power is reduced, and the reactor is safely shut down. Therefore, fuel damage due to overheating over the entire core is caused. Does not cause As described above, in the self-actuated reactor shutdown mechanism, the control element has a function of sensing an abnormal situation by itself only by a physical phenomenon and safely shutting down the reactor.

[0010]

In the conventional self-actuated reactor shutdown mechanism, a reactor shutdown failure event is started at time 12 as shown in the characteristic diagram of the temperature change at the core outlet of the coolant in FIG. Times 13 and 13a indicate that the Curie point of the Curie point electromagnet 7 has been reached, and temperature 14 indicates the Curie point temperature. Times 15 and 15a indicate loss of the coercive force of the Curie point electromagnet 7 and the core insertion time of the neutron absorber 10. Indicates.

Further, the times 16 and 16a indicate the times from the times 13 and 13a when the temperature of the coolant 3 exceeds the Curie point temperature 14 to the time when the Curie point electromagnet 7 reaches the Curie point temperature 14, and the solid line 17 is sharp. The change in the core outlet temperature at the reactor shutdown failure event accompanied by a significant coolant temperature rise, and the dashed-dotted line 18 represents the core outlet temperature in the case of the reactor shutdown failure event not accompanied by the abrupt coolant temperature rise.

When a reactor shutdown failure event accompanied by a sudden temperature of the coolant 3 occurs, the core outlet temperature of the coolant 3 is indicated by a solid line.
As shown by 17, the temperature rises sharply from the event start time 12, and the Curie point electromagnet 7 is heated with this temperature rise, but there is a time delay in the temperature rise. Also, in the event of a reactor shutdown failure that does not accompany a sudden temperature rise, the alternate long and short dash line
As shown by 18, the temperature rise of the coolant 3 starts from the event start time 12, and the Curie point electromagnet 7 is heated with the temperature rise of the coolant 3.

When the temperature of the coolant 3 exceeds the Curie point temperature 14 and time 16a elapses, the Curie point electromagnet 7 is heated to the Curie point temperature, the Curie point electromagnet 7 loses its coercive force, and the neutron absorber 10 becomes With the armature 8 and the lower extension rod 9, cut off from the adsorption surface of the Curie point electromagnet 7, inserted into the core 2 through the lower guide tube 11, and by inserting the neutron absorber 10 into the core 2, Negative reactivity is injected to reduce the reactor power and mitigate the core temperature rise.

However, even if the temperature of the coolant 3 reaches the Curie point temperature 14 as shown by the solid line 17 and the alternate long and short dash line 18, if the time 16, 16a does not elapse due to the delay of heat transfer, the Curie point electromagnet 7 is In the case of a reactor shutdown failure event indicated by the solid line 17 in which the Curie point temperature is not reached and therefore the temperature rise of the coolant 3 is sharp, the temperature of the coolant 3 before the Curie point electromagnet 7 reaches the Curie point temperature As indicated by point 17a, the temperature may be so high that it may lead to partial fuel damage within the reactor before the reactor shutdown is reached. Even if this sudden coolant temperature rise occurs, fuel damage may occur. There has been a demand for the development of a self-actuated reactor shutdown mechanism that does not reach the above.

An object of the present invention is to connect an extension rod and a control rod with a Curie point electromagnet connecting means and an axial thermal expansion device to prevent an accident event in which a rapid coolant temperature rise occurs. Even so, it is an object of the present invention to provide a reactor shutdown mechanism in which control rods are inserted into the core with a quick response due to self-operation to suppress the temperature rise of the coolant and shut down the reactor.

[0016]

[Means for Solving the Problems] In a reactor stop mechanism for suspending and holding a control element, which is installed above the core of a reactor vessel and is vertically movable in the core, in a coolant, the control element is installed above the reactor vessel. It is characterized by comprising a connecting means by means of a Curie point electromagnet attached to an installed extension rod, and an axial thermal expansion device which is connected to the connecting means and extends when the temperature of the coolant rises.

[0017]

When the temperature of the coolant is suddenly increased, the thermal expansion material filled in the axial thermal expansion device is first heated and expanded by the heated coolant, and the bellows is expanded in the core direction. As a result, the control element is inserted into the core, the negative reactivity is input, the reactor power is reduced, and the reactor is settled.

During this time, the temperature of the Curie point electromagnet of the connecting means follows the temperature rise of the coolant with a sufficient margin, and when the core shutdown of the control element by the axial thermal expansion device does not sufficiently stop the reactor, the Curie When the point electromagnet exceeds the Curie point temperature at which it loses its coercive force, the control element is dropped into the core. As a result, a sufficient subcritical system is formed in the core, the temperature rise of the coolant is suppressed, and core damage is prevented in advance.

[0019]

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 denoted by the same reference numerals and detailed description thereof will be omitted. FIG. 1 is a partially cutaway enlarged front view of a reactor shutdown mechanism, and FIG. 2 is a schematic sectional view of a fast breeder reactor.

A reactor core 1 of a fast breeder reactor contains a core 2 composed of a number of fuel assemblies, and the core 2 is immersed in a coolant 3 filled in the reactor container 1. ing. Also, the upper opening of the reactor vessel 1 is the reactor plug 4
The reactor plug 4 is provided with an upper core mechanism 5 and an upper extension rod 6 is attached to the lower part of the upper core mechanism 5.

Further, a self-actuating reactor stop mechanism 20 is installed below the upper extension rod 6, and a neutron absorber 10 which is a control element is suspended via the reactor stop mechanism 20. There is. Further, a core 2 and a lower guide tube 11 provided so as to surround the neutron absorber 10 in the core 2 are installed below the neutron absorber 10.

As shown in FIG. 1, the self-actuating nuclear reactor shutdown mechanism 20 is provided with a Curie point electromagnet 7 made of a magnetic material having a Curie point as a connecting means at the lower end of the upper extension rod 6. , The attracting surface 7a of the Curie point electromagnet 7
It is composed of the armature 8 and the lower extension rod 9 and the axial thermal expansion device 21 which are adsorbed on the.

The Curie point electromagnet 7 is an electromagnet made of a magnetic material that loses its coercive force when the temperature reaches the Curie point temperature. Inside the Curie point electromagnet 7, a coil 24 for exciting the Curie point electromagnet 7 is built-in. The armature 8 is adsorbed by the coercive force of the Curie point electromagnet 7 and holds the neutron absorber 10. Axial thermal expansion device 21
Is formed of a bellows 23 which is filled with a thermal expansion material 22 and which expands due to expansion of the thermal expansion material 22. The reactor stop mechanism 20 including the Curie point electromagnet 7 is immersed in the coolant 3.

Next, the operation of the above configuration will be described. FIG. 3 is an operation characteristic diagram, FIG. 3 (a) shows the core outlet temperature of the coolant, FIG. 3 (b) shows the reactivity in the core, and FIG. 3 (c) shows the reactor power. Time 12 indicates the start of the reactor failure event, time 25 indicates the start of axial expansion by the axial thermal expansion device 21, and time 15 indicates the loss of coercive force of the Curie point electromagnet 7 and the insertion of the neutron absorber 10.

Here, assuming that a signal or manual scrum is not possible even though the primary main circulation pump (not shown) of the fast reactor has tripped due to the occurrence of some cause event in the reactor during rated operation, the core is Due to the output-flow rate mismatch in the fuel, the temperature of the coolant 3 from the event start time 12 is shown by the dotted arrow 26 as shown by the curve 26 in FIG.
It rises rapidly in the direction of a.

As the temperature of the coolant 3 rises, the coolant 3
The temperature of the Curie point electromagnet 7 and the axial thermal expansion device 21 immersed in is raised by the heat conduction from the coolant 3, and first, the axial thermal expansion device 21 rises in temperature of the thermal expansion material 22 filled inside. As a result, the thermal expansion material 22 expands. According to this, the bellows 23 extends in the axial direction, and the neutron absorber 10 held at the tip is displaced downward and inserted into the core 2.

As shown by the curve 27 in FIG. 3 (b), the negative reactivity is introduced into the core 2 from the axial expansion start time 25. As a result, as shown by the curve 28 in FIG. 5 (c), the reactor power starts to decrease from the axial expansion start time 25. As a result of this decrease in output, as indicated by the curve 26 in FIG. 3 (a), the rising gradient of the core outlet coolant temperature from the axial expansion start time 25 is indicated by the dotted arrow.
It becomes the direction shown by 26b, and the temperature rise rate decreases.

However, if the temperature of the coolant 3 continues to be higher than the Curie point temperature of the Curie point electromagnet 7, the temperature of the Curie point electromagnet 7 becomes higher than the Curie point temperature, the coercive force is lost, and the Curie point electromagnet 7 and the armature are lost. 8 separates, and the structure below the armature 8 held by the Curie point electromagnet 7 by its own weight falls freely.

By this dropping operation, the neutron absorber 10 is inserted into the core 2 along the lower guide tube 11. As a result, as shown by the curve 27 in FIG. 3 (b), a large negative reactivity is injected into the core 2 at the time of insertion 15 of the neutron absorber 10, which results in the curve in FIG. 3 (c). Like 28, the furnace power is greatly reduced.

Due to this output reduction, the temperature of the coolant 3 becomes
As shown by the curve 26 in FIG. 3A, after the time 15, the temperature rise of the coolant 3 is moderated and the temperature does not become excessively high.
Sufficient subcriticality is secured and the temperature of the coolant 3 drops,
Safely transition to the reactor shutdown state. As a result, core 2
Since overall overheating does not occur, fuel damage due to high temperature inside the furnace can be prevented.

[0031]

As described above, according to the present invention, the axial thermal expansion device having a good responsiveness and the mechanism of the Curie point electromagnet capable of sufficiently obtaining the negative reactivity are combined, and the conventional Curie point electromagnet has a problem. It is possible to prevent fuel damage over the entire core even for a reactor shutdown failure event which is an extremely low frequency event in which a rapid coolant temperature rise occurs. Furthermore, since there is a margin regarding the temperature setting of the Curie point, the degree of freedom in designing the Curie point electromagnet can be expanded,
It has the effects of improving reliability and reducing costs.

[Brief description of drawings]

FIG. 1 is a partially cutaway enlarged front view of a reactor shutdown mechanism showing an embodiment according to the present invention.

FIG. 2 is a schematic sectional view of a fast breeder reactor according to the present invention.

FIG. 3 is an operational characteristic diagram of one embodiment according to the present invention (see FIG.
(A) is the core outlet temperature of the coolant, FIG. 3 (b) is the reactivity in the core, and FIG. 3 (c) is the reactor power).

FIG. 4 is a schematic sectional view of a conventional fast breeder reactor equipped with a reactor shutdown mechanism.

FIG. 5 is a core outlet temperature characteristic diagram of a coolant in a conventional fast breeder reactor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor vessel, 2 ... Reactor core, 3 ... Coolant, 5 ... Reactor upper mechanism, 6 ... Upper extension rod, 7 ... Curie point electromagnet, 7a ...
8 ... Armature, 9 ... Lower extension rod, 10 ... Neutron absorber, 11 ... Lower guide tube, 12 ... Reactor failure event start time, 1
3, 13a ... Curie point temperature reaching time, 14 ... Curie point temperature, 15, 15a ... Curie point electromagnet loss time and neutron absorber core insertion time, 16, 16a ... Curie point electromagnet heating time, 17 … Failure event of reactor shutdown accompanied by sudden temperature rise of coolant, 18… Failure event of reactor shutdown not accompanied by sudden rise of coolant temperature, 20… Reactor shutdown mechanism, 21… Axial thermal expansion device, 22… Thermal expansion material, 23 … Bellows, 24… Coil, 25… Axial expansion start time, 26… Coolant core outlet temperature curve, 27
… Reactor curve in the core, 28… Reactor power curve.

Claims (1)

[Claims] 1. A reactor stop mechanism for suspending and holding, in a coolant, a control element which is installed above a core of a reactor vessel and is vertically movable within the core. An extension rod installed above the reactor vessel. A nuclear reactor shutdown mechanism comprising: a connecting means by means of a Curie point electromagnet attached to the shaft; and an axial thermal expansion device which is connected to the connecting means and extends when the temperature of the coolant rises.