Patent Application
20150187445
This application claims priority to and the benefit of Korean Patent Application No. 10-2013-0163426 filed in the Korean Intellectual Property Office on Dec. 26, 2013 and Korean Patent Application No. 10-2014-0031399 filed in the Korean Intellectual Property Office on Mar. 18, 2014, the entire contents of which are incorporated herein by reference.
The present invention relates to a control rod guide thimble, and more particularly, to a burnable absorber-integrated control rod guide thimble that controls a power distribution and reactivity of a reactor core.
A nuclear reactor is a device that generates heat via fission chain reactions of nuclear fuels residing inside the reactor core. For a safe and economic operation of a nuclear reactor, an appropriate management of reactivity and power distribution of the reactor core is necessary. There are three common means of the reactivity and power distribution management in the state of the art commercial pressurized light water reactors. The first is by mixing the primary coolant uniformly with soluble boron, which is a neutron absorbent material. The second approach is by incorporating burnable absorbers in the fuel assembly lattices. The third mean in regulating the reactivity and density of neutrons involved in the fission chain reactions is by utilizing control rods that are made of neutron absorbent materials.
The management of reactor core reactivity with control rods is performed through either insertion or withdrawal of the neutron-absorbent rods from an upper end of the reactor core. Depth and speed of the said insertion or extraction dictate performance of the core reactivity management, whose impact is relatively quicker than the method of adjusting soluble boron concentration in the coolant. The control rods are, however, locally dispersed in the reactor core. When only control rods are used to regulate the core reactivity, local power peaking may arise around the core. As such, it may be difficult to simultaneously control power distribution and reactivity of the reactor core with control rods alone. In light of this control rod limitation, pressurized light water reactors mostly pursue the core reactivity management by controlling concentration of soluble boron in the coolant during normal operation. To smoothen control of power distribution and reduce dependency of the core reactivity management with borated water, various types of burnable absorbers are used extensively in the nuclear fuel assemblies.
Burnable absorber is, in general, a strong neutron absorber that is converted into a less neutron-absorbent nuclide after capturing a neutron. Selected examples of the burnable absorbers are gadolinium (Gd), erbium (Er), and Boron (B). In pressurized light water reactor, gadolinium (Gd) and erbium (Er) typically in Gd2O3 and Er2O3 compounds are appropriately mixed with UO2 nuclear fuels. In general, erbium is not used as frequently as gadolinium since erbium depletes relatively slower, which results in a larger poisonous residual at the same fuel burnup. Erbium may, therefore, be employed rather effectively as a burnable absorber in a reactor core having a very long cycle. On the other hand, gadolinium has a relatively smaller poisonous residual and is, thus, used more frequently as a burnable absorber. Mixing gadolinium with nuclear fuel, however, decreases the fuel heat conductivity which may result in a very low power density. As such, the amount of gadolinium mixed to the nuclear fuel may be quite limited. In addition, gadolinium is burnt very quickly in the Gd2O3 admixed nuclear fuels, making gadolinium difficult to be used in a reactor core having a long cycle.
Boron is, on the other hand, mostly used in the form of a so-called IFBA (Integrated Fuel Burnable Absorber) rod, which is essentially a UO2 fuel rod coated with a very thin layer of ZrB2. Boron may also be loaded into the control rod guide thimble in the form of a solid boron compound such as B4C, in a specific shape such as in the form of a WABA (Wet Annular Burnable Absorber) rod. When boron absorbs a neutron, helium gas is produced, making it difficult for boron to be directly mixed with the nuclear fuel. This is one of the reasons why boron is used in the form of an IFBA rod or is loaded in the guide thimble in the form of a WABA rod instead. Boron has a relatively small neutron absorption cross-sectional area. As such, when boron is used in the form of an IFBA rod, it needs to be integrated into a large number of fuel rods. On the other hand, when boron is loaded to the control rod guide thimble in the form of a WABA rod, the insertion of control rod in the said guide thimble may be denied. This unfavorable limitation restricts the use of boron as a burnable absorber in the form of the WABA rod. Accordingly, use of other variants of burnable absorbers are similarly and considerably restricted.
In the core reactivity management by the control of soluble boron concentration in the cooling water, the boron is uniformly mixed with the coolant. It is, therefore, possible to control the core reactivity while simultaneously minimizing distortion on the core power distribution. It however takes some time to uniformly inject and dilute the soluble boron into and with the coolant, when it is necessary to quickly control reactivity of the reactor core. As such, the core reactivity may not properly be controlled using boric acid. Furthermore, a large amount of liquid waste may be generated during dilution of the soluble boron concentration. In addition, an expensive and complicated device called ‘chemical and volume control system (CVCS)’ is needed in order to regulate the concentration and water chemistry of soluble boron in the primary coolant system. Besides, the borated water is acidic, which accelerates corrosion of structural material constituting the primary coolant system and ultimately reduces operational performance of the nuclear reactor. Moreover, when concentration of soluble boron in the coolant is very high, the resulting coolant temperature coefficient may approach zero (0) or even become positive, which is not preferable for safety. This positive coolant temperature coefficient concern is an urgent issue related to the safety of a pressurized water reactor.
The abovementioned problems substantiate the needs to actively use more burnable absorbers in order to reduce dependency of reactor core reactivity management using soluble boron. A feasibility study on a soluble boron-free pressurized water reactor has been conducted. Should concentration of soluble boron in the coolant be largely decreased or even be completely removed, various problems related to the reactor operation with soluble boron is considerably alleviated or removed. This enables a safer and more economical operation of a nuclear reactor. However, in order to successively control power distribution and reactivity of the reactor core, while at the same time reducing the amount of soluble boron diluted in the coolant, use of burnable absorbers needs to be substantially increased. As such, a technology for remedying the above-described disadvantages of the burnable absorbers is needed.
Accordingly, the applicant of the present invention has considered a new burnable absorber that is capable of controlling power distribution and reactivity of the reactor core more effectively.
Patent Document 1: U.S. Patent Publication No. 2010-0040189 (published on Feb. 18, 2010)
In light of the various aforementioned problems related to the soluble boron dilution and the existing burnable absorbers, the present invention has been conceptualized in an effort to provide a burnable absorber-integrated control rod guide thimble, which when being used is capable to effectively control the power distribution and the reactivity of the core of a nuclear reactor.
The burnable absorber-integrated control rod guide thimble is designed as such it can be loaded or removed or replaced per operational specifications.
An exemplary embodiment of the present invention provides a burnable absorber-integrated control rod guide thimble that guides the up-and-down motions of a control rod which controls reactivity of a reactor core. The control rod guide thimble may contain burnable absorbers.
An inner surface of the control rod guide thimble may have a circular shape, which may contain integrated or separated burnable absorbers, so as to guide the control rod insertion and withdrawal, and an outer surface formed or extended so as to include or provide spaces for the burnable absorbers. Outer surface of the control rod guide thimble may be expanded to have four edges where burnable absorbers may be inserted or incorporated.
Spaces between the control rod guide thimble and the neighboring nuclear fuel rods allow cooling water to pass may be divided into four up, down, left and right regions as illustrated in
The burnable absorbers may be inserted or integrated at the extended edges of the control rod guide thimble. These burnable absorbers at the extended edges of the control rod guide thimble may be replaced during refueling interval of the reactor core when necessary.
In addition, burnable absorbers in the form of an integrated thin coating or a separately-attached ring may be loaded on the inner surface of the control rod guide thimble. The inside of the control rod guide thimble may still be used to drive insertion and withdrawal of the control rods. In this case, a metal protective layer may further be formed outside the integrated coating and the separately-attached ring in order to protect the burnable absorber layer. The burnable absorber itself can either be homogenous or azimuthally heterogeneous. As stated above, if necessary, the burnable absorber loaded onto the inside of the control rod guide thimble may be replaceable, or in the case of a separately-attached ring may be detached, removed and replaced from the guide thimble, during reactor refueling interval.
Furthermore, neutron-absorbent materials may also be directly admixed to the control rod guide thimble. As such, the control rod guide thimble may be made of an alloy of the burnable absorber and the material of the control rod guide thimble. The burnable absorber may be loaded in such a manner that the burnable absorber in the form of powders may be dispersed into the control rod guide thimble. In this case, if necessary, the control rod guide thimble itself may be replaced during refueling interval in order to reload fresh neutron-absorbent materials into the fuel assembly.
The burnable absorber may be made of any one of the neutron-absorbent materials used in the existing burnable absorber technologies, such as boron, gadolinium, erbium, cadmium (Cd), samarium (Sm) and europium (Eu). The burnable absorber may have an appropriate chemical composition compound. For example, boron, metallic gadolinium and erbium, a carbide such as B4C or an oxide such as Gd2O3, and Er2O3 may be used when necessary. The burnable absorber may also be made of a combination of boron, gadolinium, erbium, cadmium, samarium and others as appropriately.
According to exemplary embodiments of the present invention, a more effective control of the power distribution and the reactivity of a nuclear reactor is uniquely possible with the burnable absorber-integrated control rod guide thimble.
Since the burnable absorber-integrated control rod guide thimble according to the present invention is independently manufactured by being completely separated from the nuclear fuel, its manufacturing process is simple and its manufacturing cost is low. Furthermore, the invention enables the possibility of maximizing the loading amount of the nuclear fuel in fuel rod region without adversely affecting performance of the nuclear fuel.
In the burnable absorber-integrated control rod guide thimble according to the present invention, the burnable absorber may be detachably attached to the inner surface of the control rod guide thimble so as not to disturb the driving of insertion and withdrawal of the control rods. The invention may offer the possibility of effectively controlling power distribution and reactivity of a soluble boron-free nuclear reactor.
Hereinafter, a burnable absorber-integrated control rod guide thimble according to an exemplary embodiment of the present invention will be described with reference to the drawings. A burnable absorber that is newly developed according to the present invention is referred to as a BigT (Burnable absorber-Intergrated Guide Thimble). Further, the accompanying drawings illustrate merely some of various exemplary embodiments of the present invention, and are illustrated larger than it actually is to easily describe exemplary embodiments of the present invention, but technical features of the present invention are not limited thereto.
A nuclear fuel assembly of a pressurized water reactor may be largely divided into two types depending on a design of a control rod guide thimble, and concepts of two types of typical nuclear fuel assemblies are illustrated in
However, as described in the “BACKGROUND OF THE INVENTION,” various burnable absorbers that are currently used may adversely affect performance of nuclear fuel, or may disturb insertion of the control rod. Particularly, in order to alleviate several problems caused by the mixing of soluble boron with the coolant, there is a growing need to use more burnable absorbers, but it may be difficult to effectively increase the amount of burnable absorbers due to the aforementioned problems. Furthermore, in order to develop a future reactor core of a pressurized water reactor from which boric acid solution is completely removed, there is a need for a high-performance burnable absorber capable of being attached or detached without affecting the driving of the control rods.
The applicant of the present invention has solved the aforementioned problems by simply changing the configurations of the control rod guide thimbles 20 and 20′, and more particularly, can achieve the aforementioned technical object by additionally providing a burnable absorber at the control rod guide thimbles 20 and 20′ provided so as to guide the control rods. In order to achieve the technical object, it is necessary to form a space where the burnable absorber is additionally provided at the control rod guide thimbles 20 and 20′, and in order to form the space, it is necessary to change shapes of the control rod guide thimbles 20 and 20′ to become larger or thicker. However, as illustrated in
The applicant of the present invention has invented a control rod guide thimble whose design has been changed as follows in order to load the burnable absorber to the control rod guide thimble while minimizing influence on a design of the typical nuclear fuel assembly and control rod.
As illustrated in
Meanwhile, the burnable absorbers 500 each having a pad shape may be provided at the four edges 230 of the control rod guide thimble 200 illustrated in
As illustrated in
As mentioned above, the reactor core provided with the control rod guide thimble 200, 200′ or 200″ integrated with the burnable absorber 500, 500′ or 500″ according to the present invention adopts a method in which the additional burnable absorbers are loaded without affecting the performance of the nuclear fuel, so that it is possible to more effectively control the power distribution and the reactivity of the reactor core. Particularly, the burnable absorbers 500, 500′ or 500″ according to the present invention can be loaded or removed from the control rod guide thimble 200, 200′ or 200″. A previously burned burnable absorber may be naturally replaced by a fresh burnable absorber 500 even in the case
As one example of the present invention, the burnable absorbers 500, 500′ or 500″ may be replaced by fresh burnable absorber whenever the nuclear fuel of the reactor core is reloaded. Accordingly, the reactor core provided with the control rod guide thimble 200, 200′ or 200″ integrated with the burnable absorbers 500, 500′ or 500″ according to the present invention can control the reactivity of the reactor core through the burnable absorbers 500, 500′ or 500″ provided within the control rod guide thimble 200 in addition to the controlling of the reactivity of the reactor core by the existing burnable absorbers and the cooling water 300 with which boric acid is mixed, it is possible to more easily control the power distribution and the reactivity of the nuclear reactor in comparison to the related art.
As illustrated in
Since the burnable absorber as the BigT suggested in the present invention is directly attached to the control rod guide thimble, a reactivity worth of the control rod may be largely decreased. A reactivity worth of the control rod using a typical B4C neutron absorber in the nuclear fuel assembly model used in the result of
As can be seen from Table 1, since control rod worth in both of the burnable absorber having the rod shape and the burnable absorber having the pad shape as the BigT according to the present invention are far more than 40,000 pcm, it is possible to achieve a very high control rod worth even when the burnable absorber as the BigT is used. The reason why the very high control rod worth can be achieved even though the strong burnable absorber 500 or 500′ is directly loaded to the control rod guide thimble 200 or 200′ is because the burnable absorber 500 or 500′ is loaded to only a position near a vertex of the modified control rod guide thimble 200 or 200′.
Meanwhile, as represented in Table 1, a power peaking factor within the nuclear fuel assembly when the strong burnable absorber as the BigT according to the present invention is fueled is about 1.08 under the maximum condition at an initial cycle. The power peaking factor is gradually decreased as the nuclear fuel is burned, and is decreased to about 1.05 after 510 days. Such a magnitude of power peaking factor approximates to a power peaking factor of the actual reactor core design. The nuclear fuel assembly used for the power peaking factor of Table 1 has a constant uranium enrichment of 4.5% in all of the fuel rods 100. In the actual nuclear fuel assembly, in order to reduce the power peaking factor, a slightly low uranium enrichment may be used in a partial region. Accordingly, even when the burnable absorber as the BigT according to the present invention is used, if a slightly low enrichment is applied in a generation region of the peak power, the power peaking factor can further be reduced.
On the other hand, the burnable absorbers 500, 500′ and 500″ used in the exemplary embodiments of the present invention are materials that have very large neutron absorption cross-sectional areas and that are converted into materials having small neutron absorption cross-sectional areas after capturing a neutron. The burnable absorbers 500, 500′ or 500″ inserted in the nuclear reactor absorb neutrons during the operation of the nuclear reactor, so that it is possible to adjust the reactivity of the nuclear reactor. The burnable absorbers 500, 500′ and 500″ used in the exemplary embodiments of the present invention may be made of any one of materials which can be used as existing burnable absorbers, such as boron, gadolinium, erbium, cadmium, samarium, and europium. Further, the burnable absorber may be appropriately used by mixing boron, gadolinium, erbium, cadmium, samarium, and europium in an appropriate chemical composition.
As still another exemplary embodiment of the present invention, the burnable absorber may be dispersed and loaded within the extended control rod guide thimble 200, 200′ or 200″, or may be made of an alloy of a metallic material such as gadolinium or erbium and a material of the control rod guide thimble 200, 200′ or 200″. In addition, the burnable absorber as the BigT suggested in the present invention has a general configuration in an axial direction. That is, the burnable absorber in the axial direction may cover the entire activation region where the nuclear fuel is presented, or a region where the burnable absorbers are not loaded may be considered as partial upper and lower portions thereof.
Although the exemplary embodiments of the present invention have been described, it is to be appreciated that those skilled in the art can variously modify and change the present invention through supplement, change, deletion, and addition of the constituent element without departing from the spirit of the present invention defined in the claims, and the modifications and the changes are included in the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2013-0163426 | Dec 2013 | KR | national |
| 10-2014-0031399 | Mar 2014 | KR | national |
