JP2501338B2 - Reactor shutdown device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod of a nuclear reactor, and more particularly to a reactor shutdown device capable of coping with core deformation due to abnormal occurrence of a reactor in a fast breeder reactor.

[Prior Art] In a fast reactor, a control rod (hereinafter referred to as a main control rod) that has both a reactor power adjustment and a scrum function, and a control rod (hereinafter referred to as a backup control rod) that shares only the scrum function that backs up the control rod Some are equipped with. When the main control rod cannot be inserted into the core region, the backup control rod is inserted into the core region instead of the main control rod to stop the reactor.
Conventionally, when a detector detects a reactor abnormal event, the control rod converts it into an electric signal and transmits it to a control rod drive unit, and when the drive unit is activated, insertion into the core is started. In addition, since the control rod is generally structured to operate from about 10 m above the core, the reason why the main control rod cannot be inserted into the core is that the drive system does not operate due to a failure of the electrical system or mechanical system equipment. The control rod insertion is hindered by the thermal deformation of the core caused by the operation of the reactor and the deformation of the control rod guide tube due to an earthquake or the like. For this reason, the backup control rod has the function of spontaneously scramming (self-actuating) without being affected by electrical and mechanical equipment failures when the reactor is abnormal, and can be reliably inserted into the core deformed by an earthquake. A system is required.

Conventionally, in order to deal with abnormal events such as a coolant flow rate decrease accident and a reactivity insertion accident, many proposals for improving a control rod have been proposed in consideration of the above requirements. For example, in the event of a coolant flow rate drop accident, the coolant is a pressurized fluid, and the pressure difference due to this flow rate change is used to raise and lower a spherical neutron absorbing substance divided into multiple parts in the control rod guide pipe. The control rod of the method is one of them. An example of the structure is shown in FIG. In FIG. 10, the coolant (pressurized fluid) that has flowed in from the entrance nozzle 21 flows through the control rod guide pipe 9 from the lower side to the upper side and flows out from the upper outlet 22. Control rod guide tube 9
Inside, there are vertical partition plates 10 and 11, and a plurality of substantially spherical neutron absorbing materials 1 are housed between them. As the neutron absorbing material 1, one having a specific gravity larger than that of the coolant flowing in the control rod guide tube 9 is used, but during the operation of the reactor, due to the flow action of the coolant flowing from the lower part, the specific gravity of the neutron absorbing material 1 or more is exceeded. Since buoyancy acts, as shown in FIG. 10 (a), it floats above the core region 17. On the other hand, when an abnormality occurs or the reactor is shut down and the coolant flow rate drops below a certain level, the buoyancy decreases, and the neutron absorbing material 1 spontaneously falls into the core region 17 due to its own weight and is inserted, as shown in FIG. 10 (b). To build up. This causes the reactor to absorb neutrons and scram.

As the neutron absorbing material 1 used in this method, it is generally known to use boron carbide (B ₄ C) having a large neutron absorption cross section. However, since the boron carbide has a relatively small specific gravity, it does not fall into the core region when an abnormal event occurs such that the flow rate of the coolant decreases, or even if it falls, the falling speed is slow and it falls as a single mass. Instead, individual particles may fall randomly at intervals. Therefore, for example, as disclosed in Japanese Patent Laid-Open No. 56-135190, the upper and lower partition plates in the guide tube containing the neutron absorbing material can be forcibly moved up and down by an extension rod inserted from the upper part of the furnace,
There is a conventional example in which, when an abnormality occurs, the neutron absorbing material 1 is forcibly forced into the core region by a partition plate and pushed into the core region to scram the reactor.

[Problems to be solved by the invention] Originally, a backup control rod is not affected by electrical or mechanical equipment failure, can be reliably inserted into a deformed core, and has an emergency scrum function instead of the main control rod. Is required. However, the method of the conventional example requires electrical and mechanical equipment. Also, the use of long extension rods, etc. has a high possibility of losing the insertability when the core is deformed. Therefore, even if the scrum time becomes a little longer, it is necessary. It is desired to develop a control rod that uses a neutron absorbing material that has a large specific gravity and a fast falling speed that can obtain a scrum time. One of them is to make a neutron absorbing material from tantalum (Ta) or the like from the viewpoint of corrosion resistance against heavy metal materials such as sodium coolant. However, tantalum has a smaller neutron absorption cross section than boron carbide. Therefore, it is still preferable to use boron carbide as the neutron absorbing substance, but there was a problem of specific gravity.

On the other hand, in nuclear reactors that use sodium as a coolant, there is a radioactive corrosion product that occurs in the coolant sodium, and this must be effectively removed.

In order to solve these problems, an object of the present invention is to increase the apparent specific gravity of the neutron absorbing material using boron carbide, and further, to perform effective removal of radioactive corrosion products. To serve the stick.

[Means for Solving Problems] As a result of studies to achieve the above-mentioned object, nickel or nickel, which has been put into practical use for capturing and removing radioactive corrosion products existing in sodium coolant, has been developed. It has been found effective to coat a neutron absorbing material such as boron carbide with a nickel alloy.

The present invention is an accidental actuation type characterized in that a neutron absorbing material composed of boron carbide is coated with a coating material composed of nickel or a nickel-based alloy directly or through an intermediate substance formed on the surface of the neutron absorbing material. The idea is to use reactor control rods.

[Operation] According to the configuration of the present invention described above, the surface of the neutron absorbing material of boron carbide is coated with nickel or a nickel-based alloy having a specific gravity four times that of boron carbide, or is formed of the material. By encapsulating boron carbide in a spherical container, the apparent specific gravity of the neutron absorber can be increased. Therefore, the neutron absorber can surely drop even if the buoyancy of the pressurized fluid is slightly reduced when the reactor is abnormal. Since the surface is coated with a nickel-based metal with high mechanical strength,
The neutron-absorbing material prevents damage such as cracks on the surface and damage due to damage expansion due to the presence of the coating material, even against mutual collisions when repeating rising and falling due to the action of pressurized fluid in the control rod guide tube. It is also possible to prevent the tritium ( ³ H) generated in boron carbide from being damaged from being mixed into the coolant. In addition, since the nickel or nickel-based alloy used for the coating material serves as a trap, that is, a filter that adsorbs and removes the radioactive corrosion product (CP) in the coolant, that is, the control rod according to the present invention. It will also serve as a CP trap device. This is because the neutron absorbing substance has a spherical shape and thus has a large surface area, and the CP removal effect can be further enhanced by decreasing the particle size and increasing the number. In order to adjust this apparent specific gravity, it is possible to interpose tantalum (Ta), which has a specific gravity larger than that of nickel, as an intermediate layer between the neutron absorbing body and the coating material. Can be changed to Further, the thickness of the coating material can be reduced by interposing tantalum having a large specific gravity, and the amount of boron carbide to be coated need not be reduced more than necessary.

[Example] Next, an example of a self-actuated nuclear reactor control rod using the coated boron carbide neutron absorbing material according to the present invention will be described. In the following examples, the case where the neutron absorbing body is used for a fluid differential pressure type self-actuated reactor control rod will be described.

FIG. 1 is a sectional view showing the structure of the embodiment.
The bonding state of the neutron absorbing material and the metal as the coating material is the first
As shown in the figure. Nickel 1 is a neutron absorbing material containing boron carbide as a main component, and 2 is a coating material, which is a metal material having a larger specific gravity than boron carbide and the like, a high temperature strength and a sodium corrosion resistance. Alternatively, it is a nickel alloy. As a method for providing the coating material 2 on the surface of the neutron absorbing material 1, a method of coating the surface of the spherical neutron absorbing material 1 with nickel by a method such as an ion plating method or a sputtering method is used. In this method, the coating material 2 is directly coated on the neutron absorbing material 1. FIG. 2 shows another embodiment in which a cylindrical neutron absorbing substance 1 is housed in a container 2 having a spherical outer shape. As shown in FIG. 3 (a), a spherical container 3 is formed in advance, and a cylindrical neutron absorbing substance 1 is inserted through an insertion hole 4 provided at one place of the coated container 3. Then, as shown in FIG. 3 (b), the coated container 3
Attach the sealing plug 5 made of the same material as the
Is fixed by welding. Alternatively, although not shown, it is possible to coat the entire surface of the spherical container 3 including the outer surface of the sealing stopper 5 with the same material by the method shown in FIG. Further, only the portion to which the sealing plug 5 is attached can be coated and sealed by the method shown in FIG.
As a result, the structure shown in FIG. 2 is obtained. In this way, the neutron absorbing material 1 is integrated with the coating material 2. As a result, the apparent specific gravity of the neutron-absorbing material 1 is larger than that of boron carbide alone, and as a result, a proper self-weight fall occurs even when the buoyancy is reduced due to a slight decrease in the flow rate of the pressurized fluid compared to the conventional case. It is obtained, and the falling speed is increased and the scrum time is shortened accordingly. Also,
According to the present invention, the surface of the neutron absorber is covered with the coating material 2 having high mechanical strength when the control rod guide tube 9 is moved up and down by the action of the pressurized fluid as described in the conventional example of FIG. Therefore, the neutron absorbing material 1 itself is prevented from being directly collided with each other, and is not damaged, that is, is not cracked on the surface due to the mutual collision, or destroyed by its expansion.

INDUSTRIAL APPLICABILITY The present invention has an effect that the radioactive corrosion products existing in the sodium coolant can be captured and removed by using nickel or a nickel alloy as the coating material. For example, as described in JP-A-58-20195, nickel has a property of adsorbing and capturing radioactive corrosion products such as manganese-54 (54Mn) and reducing the concentration of the products in the coolant. Because there is This lowers the radiation dose rate around the piping and the equipment through which the coolant flows in the nuclear power plant, lowers the exposure dose of the maintenance worker, and contributes to the reduction of the amount of material for shielding. That is, the neutron absorber according to the present invention also serves as a filter for removing the radioactive corrosion products.

In the above embodiment, when the neutron absorber has a certain size, the specific gravity of the neutron absorber and the amount of boron carbite that is the neutron absorbing substance can be adjusted by changing the coating thickness of the coating material 2. . For example, enlarging the insertion hole 4 shown in FIG. 3 (a) to reduce the volume of the coating container 3 and increase the volume of the neutron absorbing substance 1 contained therein is an example thereof, and FIG. Show. In this way, the specific gravity of the neutron absorber and the amount of boron carbide can be arbitrarily changed.

In the embodiment shown in FIG. 5, a material having a larger specific gravity than the covering material 2,
For example, the neutron absorbing material 1 is first coated with an intermediate material 6 such as tantalum metal, and the intermediate material is further coated with a coating material 2 made of nickel or a nickel alloy. Incidentally, the specific gravity of nickel is 8.9, but the specific gravity of tantalum is 16.6. In the neutron absorber in this case, the intermediate material 6 is coated on the surface of the neutron absorbing material 1 by the same method as the method of coating the neutron absorbing material 1 of the coating material 2 shown in FIG. 1, and the coating material 2 is coated thereon. It is made by further coating in the same way. In the technique using this intermediate material, the required specific gravity which is arbitrarily set cannot be secured or can be secured depending on the embodiment shown in FIGS.
This is suitable when the coating film of 2) becomes too thick and boron carbite that is the neutron absorbing material 1 cannot be incorporated in the scrum as much as necessary. That is, when a large amount of boron carbide is required as a neutron absorbing material in a limited control rod guide tube 9, one boron carbide is as close to the required spherical dimension as shown in FIG. Almost spherical shape is good. When the coating material 2 is coated on this as shown in FIG. 1, the thickness of the coating material 2 cannot be increased beyond a certain limit.
It may not be possible to increase the specific gravity of. In this case, as shown in FIG. 5, the surface of the neutron absorbing material 1 is coated with an intermediate material 6 such as tantalum having a larger specific gravity than that of the coating material 2 to increase the apparent specific gravity without increasing the thickness of the coating material 2. And the required apparent specific gravity can be secured. Further, the thickness of the intermediate material 6 can be reduced as the specific gravity of the material is increased. At this time, the nickel or nickel alloy material serving as the outermost film also serves as a protective coating for the intermediate material 6 in addition to the above-mentioned advantages.

Further, as another example of the above, as shown in FIG. 6, tantalum or the like of the same material as the intermediate material 6 shown in FIG. After being incorporated, the entire surface of the sphere is coated by the method shown in FIG. 1 with the coating material 2 of nickel or the like made of the same material as shown in FIG. This is to cope with the case where the amount of boron carbide is not required as a neutron absorbing substance but the required specific gravity cannot be secured by the method shown in FIGS. 1 to 3, and the intermediate container 7 It is possible to increase the apparent specific gravity by increasing the volume (weight).

As yet another embodiment, as shown in FIG. 7, the neutron absorbing material 1 is formed by using an alloy of nickel and titanium having a function of a shape memory alloy as a coating material for the neutron absorbing material 1 and molding the same into a container. Shape memory alloy container 8 that can store
It is what In FIG. 7 (a), the sealing plug 5 is made of nickel or a nickel-based alloy (the same material as the shape memory alloy container 8 may be used). The coating method of the neutron absorbing material 1 according to the present embodiment is shown in FIG.
7B, the shape memory alloy container 8 has an insertion hole 7 for accommodating the neutron absorbing substance 1 and the sealing plug 5 which are contained therein, as shown in FIG. 7B. The temperature that was processed to increase the internal volume of the, and stored in the insertion hole 7 and then heated to memorize the shape (the temperature for memorizing the shape can be arbitrarily selected by the mixing ratio of Ni and Ti, etc.)
When the temperature is raised above the shape, the shape is recovered and the state shown in FIG. The coated neutron absorber material 1 obtained by this method has the same advantages as those described with reference to FIGS. 1 to 6, and the shape memory alloy container 8 has an insertion hole by adjusting the shape to be memorized. The inner dimension of 7, that is, the inner volume can be arbitrarily selected. According to this, since the gap between the outer surface of the neutron absorbing material 1 and the surface of the insertion hole 7 can be eliminated and both surfaces can be brought into close contact with each other, the neutron absorbing material 1 remains in the insertion hole 7 even if vibration or the like is applied. It cannot move, and there is no risk of colliding with the inner surface of the insertion hole 7 and damaging it.

The above-mentioned coated neutron absorbing material according to the present embodiment is, of course, applied to the conventional free fall method shown in FIG. 10, but in the free fall method, the neutrons deposited in the core region are generally deposited. When the absorber is again moved to the upper part outside the core region for the operation of the reactor, the flow rate of the coolant, which is the pressurized fluid, is increased to increase the buoyancy with respect to the neutron absorbing material. However, it may be necessary to intentionally operate the reactor at a low flow rate for testing a plant including a nuclear reactor, and in that case, it is difficult to move the neutron absorber to the upper part by buoyancy. On the other hand, if the coolant temperature suddenly rises due to an abnormality in the core, it may not be possible to deal with it by the free fall method. Therefore, in order to enable these countermeasures, an example of a reactor shutdown device comprising a self-actuated reactor control rod of an improved operation system using the coated neutron absorber according to the present invention will be described below. Disclose.

FIG. 8 is a sectional view showing the structure of the embodiment.
In FIG. 8, the coated neutron absorber 1a according to the present invention is held in the control rod guide tube 9 inside the upper and lower holding members 12 and 13 having the flow holes through which the coolant as the pressurized fluid flows. The upper and lower holding members 12, 13 are connected by a universal joint 14. During operation of the reactor, as shown in FIG. 8 (a), the neutron absorber 1a held in the upper and lower holding members 12, 13 is suspended by a universal joint 14 extending from the upper part of the control rod guide tube 9 and has an upper side. The hole partition plate (10) is floated and held at a position where the upper surface of the upper holding member (12) is in contact with the lower surface of the pulling member (15) held at the lower surface position. Although the upper and lower holding members 12 and 13 in this embodiment are usually made of nickel or a nickel-based alloy material, they may or may not be the same as the coating material of the neutron absorber 1a. Three forces can be used to levitate and hold the total neutron absorber held by the upper and lower holding members 12, 13 at the position shown in FIG. 8 (a). The first is to increase the weight of all members including the neutron absorber by the buoyancy of the pressurized fluid. In this case, the weight of the neutron absorber according to the present invention described in FIGS. 1 to 7, that is, the apparent specific gravity. Also, the weight is adjusted by changing the materials used for the upper and lower holding members. Secondly, in addition to the first buoyancy, all the members including the neutron absorber are held in the target position by the magnetic attraction of the magnet 16 having the Curie point at a certain temperature provided in the pulling member 15. Done by.
In this case, the weight of all members is made larger than the buoyancy. In this case, it is also possible to use an electromagnet as the magnet and make the magnetic attraction force variable so that the whole can be held only by the magnetic attraction force.

Now, in the case where the reactor is abnormal in the state of FIG. 8 (a) in the present embodiment which also uses the magnetic attraction force, all members including the neutron absorber 1a fall into the core region 17 and scrub the reactor. Then, the state shown in FIG. 8 (b) is obtained. The abnormality of the reactor that causes the neutron absorber to fall in this example is an abnormal increase in the coolant temperature due to abnormal heating of the core due to a coolant flow rate reduction accident or a reactivity insertion accident, etc. Can handle. That is, in the case of a coolant flow rate reduction accident, in the present invention, as described above, in the state of FIG. 8 (a), the total weight of the upper and lower holding members 10 and 11 and the built-in neutron absorber 1a is a pressurized fluid. It is supported by the buoyancy caused by the flow of a certain coolant and the magnetic attractive force of the magnet provided in the member 16 for pulling up, and if the buoyancy due to the decrease in the flow rate of the coolant decreases, the holding force will decrease correspondingly, so the neutron absorption All members, including the body, fall due to their gravity. On the other hand, when the temperature of the coolant becomes abnormally high, when the temperature becomes higher than the Curie point of the electromagnet 16, the magnetic attraction force disappears, so the holding force for all members suddenly decreases, and again the weight has overcome the buoyancy of the coolant. All members fall by their own weight.

Next, when core deformation occurs and the control rod guide tube 9 is deformed, all members including the neutron absorber according to the present embodiment are dropped, and the same members located in the core region 17 are pulled up. This will be described with reference to FIGS. 8 (a) and 8 (b). When the control rod guide tube 9 is deformed, all the members including the neutron absorber fall free from the upper and lower holding members 12 and 13 connected by the universal joint 14 as shown in FIG. 8 (a). However, since the outer peripheral portions of the holding members 12 and 13 themselves serve as a guard that slides down along the inner wall surface of the control rod guide tube 9, the insertion of all the members into the core region 17 is reliably performed. On the other hand, when pulling up all members, the universal joint 14 is extended to lower the pulling member 15 to the upper surface of the holding member 12, where the magnetic attraction force of the electromagnet 16 is set to a value capable of holding all members, and the pulling member All members are pulled up to the uppermost position by pulling up the universal joint 14 of 15. When the pressurized fluid is flowing at a steady flow rate, the magnetic force of the electromagnet 16 may be a value that cancels the buoyancy of the fluid. In this case, even if the control rod guide tube 9 is deformed as shown in FIG. 8 (d), it can be inserted without any problem by the same function as when all the members are dropped. The same is true for times.

Yet another embodiment of the present invention is shown in FIG. In this embodiment, in addition to the embodiment shown in FIG. 8, an electromagnet 16 having a Kuriy point in the lower holding member 13 is incorporated, and the flow passage shutoff valve 19 is normally attracted and held by the magnetic force of the electromagnet 16 to cool. Material flow path
20 is secured, and when the temperature of the coolant rises above the Curie point due to the temperature rise of the coolant in the event of a furnace failure, the flow passage cutoff valve 19 falls apart from the electromagnet 16 and drops the coolant flow passage 20 as described above. It reduces the buoyancy of the pressurized liquid on all members. This causes all members to fall into the core area 17. According to the present embodiment, the position of the electromagnet 16 is closer to the core region 17 as compared with the embodiment shown in FIG. 8, the time to catch an abnormal rise in the coolant temperature is earlier, and the time to start scram is shortened. It is a thing. In addition, the fall and insertability of all members and the pulling up in the case of using this embodiment are the same as those of the embodiment shown in FIG.

The above has described the case where the present invention is applied to a fast reactor using sodium as a coolant, but it is needless to say that the present invention can be applied to a nuclear reactor using water or gas as a coolant. It is obvious that many modifications and changes can be made within the scope of the idea of the present invention without being limited to the above-described embodiments.

[Effects of the Invention] As described above, according to the present invention, nickel or nickel-based alloy is directly used on the surface of boron carbide having a large neutron absorption cross-section used as a neutron absorbing material, or an intermediate material such as tantalum having a larger specific gravity than these. The neutron absorbing material has a large apparent specific gravity, and even if the coolant flow rate is slightly reduced, it is possible to safely and surely drop the neutron absorbing material into the core region to scrub the reactor and to increase the thickness of the coating material. By changing the combination, any operating characteristics can be obtained, the damage due to the mutual collision of the neutron absorbing materials can be prevented by the nickel or nickel alloy material coated on the surface, and the radioactive corrosion products in the coolant by this coating material. It is also possible to have a function of a filter that adsorbs and captures also and removes it from the coolant.

[Brief description of drawings]

FIG. 1 is a sectional view of a neutron absorber according to an embodiment of the present invention, FIG. 2 is a sectional view of another embodiment, and FIGS. 3 to 4 are fabrication of the embodiment shown in FIG. A bird's-eye view and a sectional state diagram showing the method, FIGS. 5 and 6 and 7 are sectional views of still another embodiment, and FIGS. 8 and 9 are self-actuated using a neutron absorber according to the present invention. FIG. 10 is a cross-sectional view of an embodiment of a reactor control rod for a nuclear reactor, and FIG. 10 is a cross-sectional configuration diagram of a conventional self-actuating reactor control rod. 1 ... neutron absorbing material, 1a ... neutron absorber, 2 ... coating layer, 3 ... coating container, 6 ... intermediate layer or container, 8 ...
… Guide tubes, 11 …… partition plates, 12 …… core area.

Claims (9)

(57) [Claims] 1. A large number of spherical neutron absorbing spheres are housed inside a guide tube extending from the inside of the core to above the core, and the neutron absorbing body flows from the bottom to the top during normal operation of the reactor. The buoyancy generated by the above is held at the outer upper part of the core by a holding force that is at least part of it,
In order to stop the core by dropping due to loss or reduction of the holding force at abnormal times, in which the neutron absorbing substance of the neutron absorber is coated with nickel-based metal, the inside of the nickel-based metal coating A reactor shutdown device characterized in that a layer of a material having a specific gravity larger than that of nickel-based metal is interposed between the two. 2. The nuclear reactor shutdown device according to claim 1, wherein tantalum is used as the substance having a specific gravity larger than that of the nickel-based metal. 3. The nuclear reactor shutdown device according to claim 1, wherein an alloy of nickel and titanium is used as the nickel-based metal. 4. The nuclear reactor shutdown device according to claim 1, wherein a spherical container made of nickel-based metal is used as the coating of the neutron absorbing material. 5. The upper and lower holding members for sandwiching and holding a large number of spherical neutron absorbers from the upper and lower portions in the guide tube are provided, and these upper and lower holding members are joined by a bendable joining member. The nuclear reactor shutdown device according to claim 1. 6. A pull-up member for pulling up a neutron absorber is provided inside the guide tube, and the pull-up member and the holding member are connected by a holding means that loses or reduces the holding force in the event of a reactor abnormality. The reactor shutdown device according to claim 5, characterized in that 7. The nuclear reactor according to claim 6, wherein a magnetic force of a magnetic material that loses magnetism at a predetermined temperature is used as a holding force of the pulling member and the holding means of the holding member. Stop device. 8. A temperature sensitive cutoff valve for shutting off the flow passage at a predetermined temperature is provided in a coolant flow passage provided in a lower holding member of the holding members. Reactor shutdown device according to claim 5. 9. The nuclear reactor shutdown device according to claim 8, wherein a magnetic substance that loses magnetism at a predetermined temperature is used as the temperature-sensitive shutoff operation means of the temperature-sensitive shutoff valve.