KR101570473B1 - Burnable absorber-integrated guide thimble having self-shielding control ability - Google Patents

Abstract

According to an aspect of the present invention, in a control rod guide pipe which guides vertical movement of a control rod installed to control the reactivity of a reactor core, provided is a burnable absorber-integrated guide thimble (BigT) having self-shielding control ability, which comprises a combustible absorber layer which includes a combustible absorber which is spaced preset distance apart from the inside of the control guide pipe to be detachable therefrom, and a metal protection layer for protecting the combustible absorber layer.

Description

BigT {BURNABLE ABSORBER-INTEGRATED GUIDE THIMBLE HAVING SELF-SHIELDING CONTROL ABILITY}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod guide tube, and more particularly, to a burnable absorber-integrated guide thimble (BigT) including a combustible absorber for controlling reactivity and power distribution of a reactor core and having a self-shielding function.

A reactor is a furnace for obtaining energy by using nuclear fission of nuclear fuel. In the reactor, energy is obtained by a chain reaction in which neutrons released during fission cause another fission. At this time, in order to operate the reactor more safely and economically, it is necessary to appropriately control the reactivity of the reactor core and the distribution of the reactor power. In the case of a pressurized water reactor using water (H ₂ O) as a coolant, boron, which is a neutron absorbing material, is uniformly mixed with the coolant as a main means for controlling the reactivity of the core, and A burnable absorber is used as an auxiliary means of controlling the reactivity and power distribution. In addition, control rods formed of neutron absorbing materials are also used in pressurized light water reactors to control the density and reactivity of neutrons participating in reactor fission reactions.

Control of core reactivity using control rod is done by inserting control rod made of neutron absorbing material from top of reactor core to absorb nuclear neutrons and suppress nuclear fission in nuclear reactor core. The response control of the core by the control rod is advantageous in that the reaction speed of the core can be controlled more rapidly because the control rod is inserted or drawn at a higher speed than the method of controlling the concentration of the boron of the neutron absorber contained in the cooling water. However, in general, since the control rod is locally inserted into the core, there is a problem that it is difficult to simultaneously control the reactivity and the power distribution of the core simultaneously with the control rod alone. For this reason, in the case of pressurized light water reactors, the control of the core reactivity during normal operation is mostly performed by controlling the concentration of boron contained in the coolant. On the other hand, various types of combustible absorbers have been used for fuel assemblies in order to make the internal power distribution of the reactor more flat and to reduce the magnitude of the reaction to control using boric acid water.

Generally, a combustible absorber acts as a strong neutron absorbing material, but once absorbed by neutrons and converted into another nuclide, the neutron absorption cross-sectional area is greatly reduced. Gadolinium (Gd), Erbium (Er), Boron (B) and the like are used as typical combustible absorbers. In pressurized light water reactors, gadolinium (Gd) and erbium (Er) are generally used in the form of Gd ₂ O ₃ and Er ₂ O ₃ mixed properly with UO ₂ fuel. On the other hand, the void in the rare earth combustible absorber is not frequently used because it has a relatively large residual toxic effect, but can be used as a relatively efficient combustible absorber in a core having a very long cycle length. When gadolinium is mixed with nuclear fuel, the thermal conductivity of the fuel decreases, so that the fuel with mixed gadolinium is generally designed to have a very low power density There are disadvantages. Therefore, you can not use a large amount of Gadolinium when you use Gadolinium. Also, when the nuclear fuel and Gd ₂ O ₃ are mixed as in the present case, the doline is burned very quickly, so that it is difficult to apply when the cycle length of the core is long.

In the case of the boron ZrB ₂ extremely thin are often used in a so-called IFBA concept called (Integrated Fuel Burnable Absorber) used to cover the UO ₂ fuel rods. In addition, in the case of boron, a boron compound such as B ₄ C is produced in a special shape and is loaded in the control rod guide tube. As a typical concept, WABA (Wet Annular Burnable Absorber) is exemplified. In the case of boron, it absorbs neutrons and helium gas is generated. Therefore, it is difficult to mix it with nuclear fuel, so it is used in the same way as IFBA, or in the same way as WABA. In the case of boron, the neutron absorption cross-sectional area is relatively small, so when using IFBA type, a relatively large number of fuel rods must be loaded with IFBA. In addition, the use of flammable absorbers is limited in WABA, as loading of flammable absorbers into the control rod guide tube, like WABA, limits the insertion of control rods. On the other hand, Korean Patent Registration No. 97-003787 discloses a technique of coating a combustible absorber on the inner surface of a control rod guide tube, but there is a problem in that it is difficult to control the reactivity and power distribution of the reactor core only by adding such a combustible absorber. Thus, as described above, various types of combustible absorbers are used in fuel assemblies, but they are used in a limited manner in their use.

In the case of controlling the reactivity of the core by controlling the concentration of boron contained in the cooling water, since the boron is uniformly mixed with the coolant of the reactor, it is advantageous to control the reactivity while minimizing distortion of the output distribution of the core . However, since it takes a lot of time to inject and dilute boron in the coolant, there is a problem in that it is not possible to use the reaction control of the core using boric acid when it is necessary to control the reactivity of the core rapidly, In the case of reactivity control, the process of lowering the boron concentration also generates a large amount of radioactive liquid waste. On the other hand, in order to control the concentration of water-soluble boron in the primary coolant system, an expensive and complicated device called " Chemical and Volumetric Control System (CVCS) " is required. In addition, the acidic boron-containing cooling water (boric acid water) is known to cause deterioration of reactor operation performance by inducing corrosion of structural materials and nuclear fuel cladding composing the reactor primary coolant system. In addition, if the concentration of boron in the coolant is very high, the coolant temperature coefficient can be very close to zero or positive. Since such a coolant temperature coefficient is not desirable from a safety point of view, It is one of safety related issues.

Because of the various problems described above, efforts to reduce the use of boron by actively utilizing flammable absorbers have been continuing, and ultimately, attempts to remove boric acid from the pressurized light water reactor Is also being carried out. If boron contained in the coolant can be greatly reduced or completely removed from the reactor, various problems related to boron can be largely mitigated or eliminated to enable economical and safe operation of the reactor. However, in order to successfully control the reactivity and power distribution of the core while reducing the amount of boron contained in the coolant, the dependence on the combustible absorber must be further increased. In this case, the performance of the reactor depends on the performance of the combustible absorber , A technique capable of compensating for the disadvantages of the above-described combustible absorber is required.

Accordingly, the applicant of the present invention has come up with a new concept of combustible absorber concept which can more effectively control the reactivity and power distribution of the core.

Korean Patent Registration No. 1997-003787 (Bulletin of Mar. 21, 1997)

Embodiments of the present invention provide a control rod guide including a combustible absorber that can be effectively used for reactivity and power distribution control of a reactor core in order to solve various problems associated with boric acid water and conventional combustible absorbers as described above .

Further, the combustible absorber provided on the control rod guide tube is designed to be able to be loaded and removed.

Further, the combustible absorber provided on the control-rod guide pipe is arranged in a non-uniform manner in the azimuthal direction inside the control-rod guide pipe.

According to an aspect of the present invention, there is provided a control rod guide tube for guiding the up and down movement of a control rod installed for controlling the reactivity of a core, the control rod guide tube comprising a combustible absorber including a combustible absorber detachably attached to an inner surface of the control- There is provided a combustible absorber having a self-shielding regulating function, wherein an absorber layer is provided, and a metal protective layer for protecting the combustible absorber layer is additionally provided. The flammable absorber concept newly developed according to the present invention is called BigT (Burnable absorber-Integrated Guide Thimble).

The combustible absorber layer can be formed into a ring-shaped cylindrical shape. In addition, the combustible absorber layer may be non-uniformly distributed in the azimuthal direction for self-shielding. In this case, the combustible absorber layer has a predetermined interval along the circumference of the combustible absorber layer, have.

Each combustible absorber that is loaded along the combustible absorber layer can adjust its shielding effect according to the magnitude and / or thickness of the azimuth angle. Further, the sizes of the azimuth angles of the respective combustible absorbers loaded along the combustible absorber layer can be formed differently.

A reactor with a BigT comprising a combustible absorber according to embodiments of the present invention may enable a more efficient reactivity and power distribution control of the core in a unique manner.

In addition, since BigT including the combustible absorber according to the present invention is completely separated from the nuclear fuel and is independently manufactured, the manufacturing process is simple, and the cost is low, and the amount of the nuclear fuel in the fuel rod area is not adversely affected Can be maximized.

In addition, the BigT including the combustible absorber according to the present invention can be loaded so that the flammable absorber can be detached and attached to the inside of the guide tube without interfering with the insertion of the control rod. As a result, It is possible to control the reactivity and the power distribution.

Further, the BigT including the combustible absorber according to the present invention can be controlled in its own shielding by disposing the BigT in the azimuth direction inwardly of the control rod guide pipe, thereby effectively controlling the reactivity of the reactor according to the purpose Do.

1 and 2 show a nuclear fuel assembly according to the prior art and an enlarged view thereof.
Figure 3 shows BigT with its own shielding control function according to an embodiment of the present invention.
4 shows BigT with its own shielding control function according to another embodiment of the present invention.
FIG. 5 illustrates a BigT-AHR (Azimuthally Heterogeneous Ring) having a self-shielding function according to another embodiment of the present invention.
Figure 6 shows a fuel assembly and a BigT-AHR using the BigT-AHR with its own shielding control function of the present invention.
FIG. 7 shows an embodiment in which reactivity changes according to combustion of a nuclear fuel assembly using BigT having a self-shielding function according to an embodiment of the present invention.

Hereinafter, a BigT including a combustible absorber having a self-shielding function according to an embodiment of the present invention will be described with reference to the drawings. The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

A typical embodiment of a fuel assembly of a pressurized light water reactor is shown in Figs. 2 is an enlarged view of a portion A shown in Fig. Referring to FIGS. 1 and 2, a plurality of nuclear fuel rods 10 are installed in the nuclear fuel assembly, and a general control rod guide pipe 10 for guiding the up and down movement of the control rod is provided between the fuel rods 10, (Not shown). 1 and 2, a plurality of control rod guide tubes 20 slightly larger than the fuel rod 10 are used. In the figure, a space between the plurality of fuel rods 10 and the control rod guide tube 20 is a space formed so that the cooling water 30 flows. Conventionally, a conventional fuel assembly adopts a method of loading a conventional combustible absorber into the fuel region and the control rod guide pipe 20 as described above in the related art for controlling the reactivity of the reactor core and controlling the output distribution inside the nuclear fuel assembly. Also, as already described, the concentration of boron contained in the cooling water 30 is slowly controlled during operation of the reactor.

However, as described in the background art of the present invention, various combustible absorbers currently in use may adversely affect the performance of the nuclear fuel, or may interfere with the insertion of the control rod in the control rod guide tube 20 have. Particularly, in order to alleviate various problems caused by boron mixed with a coolant, it is necessary to use more combustible absorbers. However, it is difficult to effectively increase the combustible absorber in the fuel assembly due to the above-mentioned problems have. In addition, for the development of a future pressurized light water reactor core in which boric acid water is completely removed, there is a need for a very high-performance flammable absorber concept capable of detachably attaching a flammable absorber and not affecting the drive of the control rod.

The applicant of the present invention has solved the above-mentioned problem through the change of the simple configuration of the control rod guide tube 20, and more specifically, the control rod guide tube 20 installed to guide the control rod, It is possible to achieve the above technical objective by providing an additional combustible absorber within a range that does not affect the driving. In order to achieve such a technical objective, the combustible absorber added to the control rod should not be influenced by the addition of the combustible absorber, and the additional combustible absorber must be designed to be detachable and attachable. .

The applicant of the present invention has mounted the combustible absorber in such a manner that the combustible absorber can be detachably mounted inside the control rod guide tube while minimizing the influence on the existing fuel assembly and the control rod design. FIG. 3 shows BigT having a self-shielding adjustment function according to an embodiment of the present invention, and will be described in more detail with reference to the drawings.

3, the BigT having a self-shielding function according to an embodiment of the present invention can be loaded on a separate combustible absorber layer 300 on the inner surface of the control rod guide tube 200 . In contrast to the conventional combustible absorber, which occupies most of the interior of the control rod guide tube 200, in the above embodiment of the present invention, the combustible absorber 500 is disposed within the control rod guide tube 200 , A very thin combustible absorber layer 300 is installed along the inner side of the control rod guide tube 200 and a thin metal protective layer 400 is designed on the outer side thereof so that the combustible absorber layer 300 ). In Fig. 3, the combustible absorber layer 300 may be composed of a 100% combustible absorber 500, or only a portion thereof may be configured to include a combustible absorber 500.

In the case where 100% of the combustible absorber layer 300 is composed of the combustible absorber 500, as in the case of the above-described metal protective layer 400, the area between the combustible absorber layer 300 and the inner surface of the control- In addition, a thin metal protective layer may be designed in the spacing space 210 to protect the flammable absorber 500. By installing the combustible absorber 500 on the inner side of the control rod guide tube 200 in such a thin layer form, the control rod can be inserted and drawn out, and the combustible absorber 500 can be loaded on the inner side of the control rod guide tube 200 . The combustible absorber layer 300 including the combustible absorber 500 is designed to be ring-shaped cylindrical so as to be detachably attached to the inner side of the control-bar guide pipe 200 in order to detachably attach the combustible absorber 500 . The metal protective layer 400 can also be designed as a ring-shaped cylindrical shape.

As shown in FIG. 3, even if the combustible absorber 500 is loaded in the form of a layer on the inner side of the control rod guide tube 200, it may be necessary to slightly reduce the diameter of the control rod for smooth operation of the control rod. However, No major changes are required. In order to allow the combustible absorber 500 to be installed and removed on the inner surface of the control-rod guide tube 200 as described above, there is a gap between the combustible absorber layer 300 and the inner surface of the control- A slight spacing space 210 may be required. However, in the case of the fixed combustible absorber 500, which is not replaced, no additional space is required between the inner surface of the control-arm guide tube 200 and the combustible absorber layer 300, as shown in FIG. 4, .

FIG. 5 shows a BigT having its own shielding control function according to another embodiment of the present invention. Referring to the drawing, a combustible absorber layer 300 having a ring-shaped cylindrical shape with a spacing space 210 inside a control-rod guide tube 200 is provided with a combustible absorber 500 'in a non-homogeneous manner in the azimuth r direction . At this time, the combustible absorber 500 'has predetermined intervals along the circumference of the combustible absorber layer 300, and a plurality of the combustible absorbers 500' may be arranged at the same azimuth angle r. A technique capable of freely adjusting the length of the combustible absorber 500 'and the azimuth angle in this way is referred to as BigT-AHR (Azimuthally Heterogeneous Ring) in the present invention.

The BigT-AHR according to another embodiment of the present invention can freely adjust the length of the combustible absorber 500 'and the azimuth angle r, and the size of the azimuth r and the thickness of the combustible absorber 500' It is possible to control the self shielding effect by adjusting. Further, in order to obtain an effective self shielding effect suited to the technical purpose, the azimuth r may be formed to have various sizes, and it is possible to modify the combustible absorber material to be loaded, the length of charge, and the loading shape. In FIG. 5, reference numeral 310 denotes an outer protective layer, and 400 denotes an inner protective layer.

As described above, the BigT having the self-shielding function including the combustible absorbers 500 and 500 'according to the present invention provides an additional combustible absorber 500, 500' loading method without affecting the performance of the nuclear fuel It is possible to more effectively control the reactivity of the core and the output distribution control. Particularly, the combustible absorbers 500 and 500 'according to the present invention can be detachably attached to the inside of the control-rod guide pipe 200. In this case, the combustible absorbers 500 and 500 'of the BigT concept already used are removed and new combustible absorbers 500 and 500' are already loaded on the burned fuel assemblies to maximize the use of the combustible absorbers 500 and 500 ' It is possible.

The BigT having the self shielding function according to the embodiment of the present invention can replace the combustible absorbers 500 and 500 'with the control rod guide tube 200 every nuclear fuel reloading period of the core. Therefore, in addition to the control of core reactivity by the cooling water mixed with the existing combustible absorber and boric acid, the reactor core having the control rod guide tube 200 including the combustible absorbers 500 and 500 ' The reactivity of the reactors and the power distribution control can be made much easier than before in that the reactivity of the reactor core can be controlled through the combustible absorbers 500 and 500 'installed in the reactor 200.

FIG. 6 shows a nuclear fuel assembly including BigT having a self-shielding function including a combustible absorber according to an embodiment of the present invention. The nuclear fuel assembly was a standard 17 × 17 nuclear fuel assembly of Westinghouse Company, the enrichment degree of uranium (U) was 4.5%, and the metallic gadolinium (Gd, Gadolinium) was used as the combustible absorber 500 '. Five types of BigT-AHR were used to analyze the changes in the reactivity of nuclear fuel assemblies with self-shielding control according to azimuth angle (r) and thickness variation of BigT flammable absorber in the nuclear fuel assembly. The same amount of combustible absorber 500 'is loaded in all BigT designs used in the numerical analysis, and the specific thickness and azimuth of the BigT-AHR used in this numerical experiment are shown in Table 1 below. The density of gadolinium used in this numerical calculation is 7.9 g / cm ³ . On the other hand, the concrete numerical values of the nuclear fuel assembly and the BigT-AHR shown in Table 1 are illustrative examples for explaining the BigT-AHR according to one embodiment of the present invention, and the technical scope of the present invention is limited within the above- It is not.

BigT Design Thickness (mm) azimuth AHR10 0.9515 10 ° AHR20 0.4522 20 ° AHR30 0.2969 30 ° AHR40 0.2211 40 ° AHR90 ^* 0.0971 90 °

* Corresponds to homogeneous ring in azimuth direction

FIG. 7 shows the result of calculation of the combustion of the nuclear fuel assembly equipped with the BigT-AHR according to the present invention. FIG. 7 shows the change of the reactivity of the nuclear fuel assembly according to the combustion time. Referring to the figure, it can be seen that although the same amount of gauldroid absorber is loaded in each BigT-AHR, the decrease in the reactivity of the initial nuclear fuel assembly is very different from the maximum of about 20,000 pcm to the minimum of about 10,000 pcm. According to FIG. 7, as the azimuth angle r increases, the decrease in initial reactivity decreases. As the azimuth angle r increases, the thickness of the combustible absorber 500 'becomes thinner, As the surface area of the combustible absorber increases, the self shielding phenomenon of the gadolinium is weakened, so that a larger amount of gadolinium absorbs the neutrons.

Also, referring to FIG. 7, it can be seen that the reactivity of the nuclear fuel assemblies after 150 days to 200 days increases again, except for a very small azimuth angle (10 °), because the gadolinium is a combustible absorber Because. In particular, in the case of the BigT-AHR having a large azimuth angle (r) with a small self-shielding effect, a large amount of the combustible absorber 500 'is burned in the early stage and after about 200 days, the burnt combustible absorber 500' It can be seen that the reactivity of the aggregate is similar to the case where the combustible absorber is not loaded. On the other hand, when the azimuth angle (r) is very small, such as 10 °, the reduction of the initial reactivity is relatively small due to the enhancement of the self shielding property of the gadolinium, and as a result, the gadolinium is burned very slowly. In the case of all the combustion calculations in Fig. 7, the reactivity of the grid loaded with BigT does not completely restore the reactivity of the lattice that is not loaded with the combustible absorber, which is the loss caused by the remaining combustible absorber 500 ' The ultimate reactivity loss due to the gadolinium flammable absorber can be increased in proportion to the total amount of gadolinium flammable absorber. Meanwhile, as described above, BigT can be designed to be removed after use, and in FIG. 7, the loss of reactivity of the nuclear fuel assembly can be minimized by removing BigT after the gadolinium combustible absorber is sufficiently burnt.

As described above, in the case of BigT-AHR according to the embodiment of the present invention, as the azimuth angle (r) increases, weakening of the self shielding effect can absorb more neutrons and significantly reduce the reactivity of the initial lattice. However, when the azimuth angle r of the combustible absorber is too large, a large amount of the combustible absorber 500 'is burned at an early stage, so that it is confirmed that the reactivity may be recovered too early in the middle of the cycle. As can be seen from the above numerical experiment results, the large azimuth angle (r) shows a considerable decrease in the initial reactivity, but thereafter, a considerable increase in the reactivity is observed. . However, as can be seen from the above results, when the shielding is appropriately adjusted through adjustment of the azimuth (r), it is possible to appropriately adjust the response of the grating at a desired timing, or appropriately use the self- It is also possible to make the change more flat. Therefore, the BigT according to the present invention can improve the reactivity control characteristic through optimization of the shapes of the combustible absorbers 500 and 500 '.

Meanwhile, the combustible absorbers 500 and 500 'used in the embodiments of the present invention may be formed of any one of materials that can be used as existing combustible absorbers such as boron, gadolinium, air bubble, cadmium, samarium, It is possible. In addition, the combustible absorber can be optimally used by mixing boron, gadolinium, uvium, cadmium, samarium, europium, etc. in an appropriate chemical composition.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit of the invention as set forth in the appended claims. The present invention can be variously modified and changed by those skilled in the art, and it is also within the scope of the present invention.

10: Nuclear fuel rod 200: Control rod guide tube
30: cooling water 300: combustible absorber layer
400: metal protection layer 500, 500 ': combustible absorber

Claims (11)

A control rod guide tube for guiding up and down movement of a control rod provided for controlling the reactivity of a core,
A combustible absorber layer having a combustible absorber spaced a predetermined distance from the inner surface of the control-
Wherein the combustible absorber layer is heterogeneously distributed in the azimuthal direction for combustible absorbers for self shielding optimization. The method according to claim 1,
Wherein the combustible absorber layer is formed in a ring-shaped cylindrical shape. The method according to claim 1,
Wherein the combustible absorber layer is formed entirely of a combustible absorber. The method of claim 3,
And a metal protective layer for protecting the combustible absorber layer is additionally provided in a spaced-apart space between the combustible absorber layer and the inner surface of the control rod guide tube. delete The method according to claim 1,
Wherein the combustible absorber has a predetermined spacing along the circumference of the combustible absorber layer, and a plurality of the combustible absorbers are disposed. The method according to claim 1,
Wherein each combustible absorber loaded along the combustible absorber layer is capable of controlling its own shielding effect according to the magnitude or thickness of the azimuth angle. 8. The method of claim 7,
Wherein a size of an azimuth angle of each combustible absorber to be loaded along the combustible absorber layer is differently formed. A nuclear fuel assembly comprising a plurality of BigTs having a self-shielding function according to claim 1. 10. The method of claim 9,
Wherein the plurality of BigTs are formed to have various azimuth angles, respectively. 9. A nuclear fuel assembly comprising a plurality of BigTs having a self-shielding function according to any one of claims 6 to 8.

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