CA2512708C - Control method for light water and helium gas flow rate in the liquid zone control system of candu reactor and candu reactor having improved tube support plate - Google Patents
Control method for light water and helium gas flow rate in the liquid zone control system of candu reactor and candu reactor having improved tube support plate Download PDFInfo
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- CA2512708C CA2512708C CA2512708A CA2512708A CA2512708C CA 2512708 C CA2512708 C CA 2512708C CA 2512708 A CA2512708 A CA 2512708A CA 2512708 A CA2512708 A CA 2512708A CA 2512708 C CA2512708 C CA 2512708C
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21C—NUCLEAR REACTORS
- G21C7/00—Control of nuclear reaction
- G21C7/005—Flux flattening
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- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21D—NUCLEAR POWER PLANT
- G21D3/00—Control of nuclear power plant
- G21D3/04—Safety arrangements
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
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Abstract
The present invention relates to a technique for improving instability in the output of a CANDU (Canadian Deuterium Uranium) reactor (pressurized heavy water reactor) by controlling flow rates of light water and helium gas fed into a compartment of a liquid zone control system (LZCS) of the reactor. The present invention also relates to a tube support plate in the compartment for improving the instability in the output of the reactor. A method of controlling flow rates of light water and helium gas passing through a plurality of compartments of a CANDU reactor according to the present invention comprises controlling the flow rates of the light water and the helium gas such that flow rates of the light water and the helium gas supplied to and discharged from a portion of each of the compartments below a tube support plate per unit time satisfy the following relationship (a) ~out - (~m)max < 0 or (b) ~out - (~in)max <=~He, where ~out is a flow rate of the light water discharged from the compartment per unit time, (~in)max is a maximum flow rate of the light water that can pass through the tube support plate per unit time, and ~He is a flow rate of the helium gas supplied.
Description
CONTROL METHOD FOR LIGHT WATER AND HELIUM GAS FLOW RATE IN
THE LIQUID ZONE CONTROL SYSTEM OF CANDU REACTOR AND CANDU
REACTOR HAVING IMPROVED TUBE SUPPORT PLATE
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a technique for improving instability in the output of a CANDU (Canadian Deuterium Uranium) reactor (pressurized heavy water reactor) by controlling flow rates of light water and helium gas fed into a compartment of a liquid zone control system (LZCS) of the reactor. The present invention also relates to a tube support plate in the compartment for improving the instability in the output of the reactor.
THE LIQUID ZONE CONTROL SYSTEM OF CANDU REACTOR AND CANDU
REACTOR HAVING IMPROVED TUBE SUPPORT PLATE
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a technique for improving instability in the output of a CANDU (Canadian Deuterium Uranium) reactor (pressurized heavy water reactor) by controlling flow rates of light water and helium gas fed into a compartment of a liquid zone control system (LZCS) of the reactor. The present invention also relates to a tube support plate in the compartment for improving the instability in the output of the reactor.
2. Description of the Background Art A liquid zone control system is one of reactivity control apparatuses for a CANDU
reactor and provides the function of smoothing an excess power tilt generated after replacement of nuclear fuel, which is implemented by controlling the amount of light water (H20) in a compartment thereof. Upon rapid rise of power tilt after the replacement of the nuclear fuel, the amount of the light water in the compartment is increased to reduce the power tilt. When output is stabilized, the amount of the light water is decreased accordingly. Thus, the output is smoothly distributed. In other words, when the LZCS
is normally operated, the power tilt is decreased according to a rise of the level of the water, while the power tilt is increased according to a drop in the level of the water.
Fig. 1 shows the structure of a compartment 100 of a conventional CANDU
reactor. The compartment 100 of Fig. 1 is shown as an upper outer compartment.
In Fig.
1, black arrows indicate the flow of light water and white arrows indicate the flow of helium gas. The compartment 100 is a space between upper and lower bulkheads 10 and 11 that are fixedly installed within a pipe 80. A spreader 20 for causing the light water introduced through the bulkhead 10 to run down along the wall of the compartment is installed at an upper portion of the compartment 10, and a helium gas discharge hole 12 is formed in the lower bulkhead 11. Further, a double tube 40 for supplying the helium gas and discharging the light water penetrates through the upper bulkhead 10 and is then connected to the lower bulkhead. An outer tube of the double tube 40 connected to the lower bulkhead 11 is formed with a scavenger 41 for taking in and discharging the light water, and an inner tube of the double tube 40 is caused to communicate with the helium gas discharge hole 12 of the lower bulkhead 11. Moreover, a helium gas discharge tube 70 is fixed to the spreader 20 while penetrating through the upper bulkhead 10. Double tubes 50 and 60 for inflow and outflow of light water and helium gas for a lower compartment are installed within the compartment 100 while penetrating therethrough. In addition, a tube support plate 30 is installed within the compartment to prevent vibration caused by the fluids in the double tubes 40, 50 and 60 that have lengths considerably larger than diameters thereof. As shown in Fig. 2, the tube support plate 30 takes the shape of a disk that can be inserted into and fixed to the pipe 80 and is made of a porous metal material through which the light water and helium gas can pass vertically. The tube support plate is also formed with a through-hole 31 at the center thereof and a plurality of cut-away portions 32 at the periphery thereof so that the light water and the helium gas can easily pass through the tube support plate.
In the CANDU reactor constructed as above, the amount of the light water discharged from a lower portion of the compartment is fixed, while the amount of the light water introduced at the upper portion of the compartment is controlled to be changed in a predetermined range according to operating conditions. Therefore, if the amount of the light water introduced at the upper portion of the compartment is greater than the amount of the light water discharged from the lower portion of the compartment, the level of the water rises, whereas if the amount of the light water discharged from the lower portion of the compartment is greater than the amount of the light water introduced at the upper portion of the compartment, the level of the water is lowered. The output of the reactor is increased or decreased according to the changes in the level of the water. The LZCS
measures the changes in the level of the water within the compartment by measuring the difference in pressure between the upper and lower portions of the compartment, and maintains pressure according to the changes in the level of the water by causing a certain amount of helium gas to be introduced into the lower portion of the compartment and to be subsequently discharged from the upper portion of the compartment. If the level of the water in the compartment reaches 80%, the LZCS performs control such that a water level control logic has priority over an output control logic so that the level of the water cannot exceed 80%.
In the conventional CANDU reactor having the compartment structure and the LZCS control scheme, there is a phenomenon in which cycling or rapid drop periodically occurs when the level of the water W in an upper outer compartment rises over 80% after replacement of nuclear fuel, as shown in Fig. 3. Further, the power tilt P of the reactor also exhibits a variation similar to that in the level of the water. Such a variation is contrary to a variation in the output under normal control (the output is decreased upon rise of the level of the water under the normal control).
To alleviate an instability phenomenon in such a CANDU reactor, the following temporary methods are employed: a toxic material such as gadolinium (Gd) is input into the compartment or an operating scheme of a helium gas compressor is changed.
However, these methods have not fundamentally solved the instability phenomenon.
Such instability may cause sudden shutdown of the reactor. Further, the input toxic material becomes radioactive wastes after it absorbs neutrons, resulting in increase of the amount of wastes produced.
SUMMARY OF THE INVENTION
A primary object of the present invention is to examine a cause of an instability phenomenon such as rapid rise and drop or periodic cycling of a power tilt and the level of water in a CANDU reactor and to provide a method of controlling flow rates of light water and helium gas in a liquid zone control system to eliminate such an instability phenomenon.
According to the present invention to achieve the primary object, there is provided a method of controlling flow rates of light water and helium gas passing through a plurality of compartments of a CANDU reactor, each of the compartments having a tube support member installed therein, comprising the step of controlling the flow rates of the light water and the helium gas such that flow rates of the light water and the helium gas supplied to and discharged from a portion of each of the compartments below the tube support member per unit time satisfy the following relationship:
(a) QL - (01.). <0, or (b) QL - (Q:). SQxk where QL is a flow rate of the light water discharged from the compartment per unit time, (Qi.). is a maximum flow rate of the light water that can pass through the tube support member per unit time, and Q , is a flow rate of the helium gas supplied. The tube support member is normally used by a tube support plate having a plurality of holes, but the tube support member can be a tube support grid or an elastic spring.
A secondary object of the present invention is to provide a CANDU reactor that has an improved tube support plate to alleviate the instability phenomenon such as rapid rise and drop or periodic cycling of the power tilt and the level of water.
According to the present invention for achieving the second object, there is provided a CANDU rector having a plurality of compartments, each of the compartments having a tube support plate installed therein, wherein the tube support plate is formed with a plurality of through-holes through which light water and helium gas pass.
The plurality of through holes of the support plate enables to satisfy flow conditions of light water and helium gas through the through-holes as follows:
(a) Q., - (O ). <0, or (b) Q.a - (Qr.). SQxu where Q. is a flow rate of the light water discharged from the compartment per unit time, (Qm ). is a maximum flow rate of the light water that can pass through the tube support member per unit time, and Qm is a flow rate of the helium gas supplied. . The support plate can be replaced by a support grid or a support elastic spring that satisfies the flow conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment in conjunction with the accompanying drawings, in which:
Fig. 1 is a view showing the structure of a compartment of a liquid zone control system in a CANDU reactor, and the flow of light water and helium gas therein;
Fig. 2 is a plan view of a tube support plate of a conventional CANDU reactor;
Fig. 3 is a graph showing variations in the level of the water and the output when 5 an instability phenomenon occurs in the conventional CANDU reactor;
Fig. 4 is an explanatory view illustrating a cause of the instability of the level of the water and the output when a conventional method of controlling flow rates of light water and helium gas is employed;
Fig. 5 is a graph showing stable and unstable operation regions according to the flow rates of the light water and the helium gas;
Fig. 6 is a plan view of an embodiment of a tube support plate of a CANDU
reactor according to the present invention; and Fig. 7 is a graph showing stable and unstable operation regions when the tube support plate shown in Fig. 6 is used.
DETAILED DESCRIPTION OF THE INVENTION
When nuclear fuel is replaced in a CANDU reactor, there is a case where an output in a specific region rises according to a reactivity property of the nuclear fuel. Since an excess power tilt adversely affects the operation of the reactor, a LZCS
control the operation so that a smooth output can be obtained. Although the control of the LZCS is normally performed in a normal operation, an instability phenomenon in which the control cannot be performed normally as shown in Fig. 3 occurs if the level of light water is very high to such an extent that the level of the water rises over a tube support plate (TSP).
If the level of the water is below the tube support plate as shown in Fig. 4 (a), there is no problem in view of the flow of the light water and the helium gas in a compartment.
However, if an operation is made in a state where the level of the water is higher than the level of the water as shown in Fig. 4 (b), there may be a phenomenon in which abnormality occurs in a stream of the light water flowing downward and a stream of the helium gas flowing upward. That is, the helium gas is accumulated below the tube support plate 30 as shown in Fig. 4 (c). This phenomenon depends on flow rates of the light water and the helium gas that pass through the tube support plate per unit time. That is, the phenomenon occurs if the amount of the light water that passes through the tube support plate per unit time is smaller than the amount of the water that is discharged below the tube support plate. The light water that has not passed through the tube support plate is held up on the tube support plate, while the helium gas introduced at a lower portion of the compartment cannot pass through the tube support plate and is also accumulated below the tube support plate, so that the amount of the light water below the tube support plate is gradually reduced. The level of the water within the compartment of the CANDU
reactor is controlled such that it cannot generally exceed over 80% to prevent the light water from overflowing toward the outside of the compartment. However, if a phenomenon in which the light water is held up on the tube support plate 30 occurs as shown in Fig. 4 (d), a water level meter for measuring the level of the water using the difference in pressure between upper and lower portions of the compartment outputs a measured value indicating that the level of the water is maintained at 80%. Actually, since the helium gas is accumulated below the tube support plate 30, the light water is held up on the tube support plate to such an extent that the level of the water reaches a spreader. Consequently, the level of the water measured by the water level meter is completely different from the actual level (position) of the water. The light water existing below the tube support plate that corresponds to the size of a space occupied by the helium gas accumulated below the tube support plate cannot be effectively used for controlling the reactivity.
Further, a part of the light water held up on the tube support plate is pushed outside a fuel boundary and thus cannot contribute to control of the reactivity. Therefore, the output of the reactor cannot be normally controlled, thereby making the reactor unstable.
Referring to Fig. 4 (d), if a portion of the compartment below the tube support plate is taken as a control volume, flow rates of inflow and outflow of the light water and the helium gas (below the tube support plate satisfy the following equation:
QHe,` Out-Qin ......(1) where O is a flow rate of a fluid that passes through the compartment per unit time. The reason why such an abnormal phenomenon occurs while the level of the water is controlled to be maintained at 80% is that the amount Qin of the light water passing through the tube support plate is smaller than the amount Qout of the light water discharged at the lower portion of the compartment. That is, the abnormal phenomenon occurs if the following equation is satisfied:
QOnt - (Kin )max >0 ...... (2) Further, if the flow rate of the helium gas supplied is larger than an insufficient flow rate of the light water below the tube support plate, the helium gas passes through the tube support plate so that the abnormal phenomenon does not occur. If the maximum flow rate that can pass through the tube support plate is denoted by (Q;n )max , the abnormal phenomenon occurs when the following equation is satisfied:
Q0 - (Qin )max > QHe ...... (3) As an example, Fig. 5 shows calculation results according to the conditions of equations 1 and 2 and experiment results of a simulation of the abnormal phenomenon in connection with the flow rates of the light water and the helium gas in the liquid zone control system of the CANDU reactor. Fig. 5 presents the experiment results and the theoretical calculation results showing that stable and unstable operation regions exist according to the amount Qout of the light water discharged and the amount QHe of the helium gas introduced. These results show that the reactor can be always operated in the stable operation region by properly controlling the amount Quut of the light water discharged and the amount QHe of the helium gas introduced. When data of the experiment results are fitted between the stable and unstable operation regions, an experimental equation for the boundary (bold dotted line) of the stable operation region can be obtained as follows:
QHe =14.237 + 2.34 x ln(Qout - 0.458) (where, Qout > 0.458) ...... (4) Moreover, a general equation for the flow rate below the tube support plate is Q He = Q out - Qin , and the conditions of the occurrence of the abnormal phenomenon should satisfy both Q0 U, - (QQ, )max > 0 and Qout - (Qin )max >_ QHe Based on the aforementioned four equations, the stable operation region where the abnormal phenomenon occurs can be expressed as the following equation:
Q0 ut - (Qin )max <O or Qout - (Qin )max QHe ...... (5) Here, the flow rate of the helium gas is QHe =14.24 + ln(Qout - 0.46). If equation 5 is expressed in a different manner, the stable operation region can be expressed by the following equation:
QHe -14.237 + 2.34 x ln(Qo,u - 0.458) ====== (6) Furthermore, if the value of the amount (Q;,, ),,,ax of the light water passing through the tube support plate becomes sufficiently larger without controlling the amount Qout of the light water discharged and the amount He of the helium gas introduced, Qout _00m, becomes smaller. Therefore, the probability of occurrence of the abnormal phenomenon is lowered. Accordingly, the occurrence of the abnormal phenomenon can be prevented by increasing the areas of portions of the tube support plate corresponding to flow passages of the light water and the helium gas such that the light water and the helium gas cannot be accumulated. That is, as shown in Fig. 6, additional through-holes 33 are formed in the tube support plate 30 or the areas of cut-away portions of the through-hole 31 are increased to prevent the occurrence of the abnormal phenomenon. Fig. 7 shows results in which the stable operation region is increased when the areas of the flow passages of the light water and the helium gas are increased by about 10% by adding the through-holes 33 to the tube support plate of the liquid zone control system of the reactor.
According to the present invention, the flow rates of the light water and the helium gas are controlled so that a CANDU reactor can be operated in a stable region where an unstable phenomenon, which may occur in a liquid zone control system, dose not occur.
Thus, it is possible to prevent a power tilt, thereby improving the stability and operatability of the reactor. That is, sudden rise or drop and periodic cycling of the output and the level of the water in a compartment of the CANDU reactor are solved, thereby eliminating a feeling of uneasiness of an operator and enhancing the economical efficiency, stability and operatability of a nuclear power station.
Further, the reactor can be stably operated without inputting a toxic material into the light water so that the amount of radioactive wastes generated can be reduced.
The embodiment of the present invention described above herein and illustrated in the drawings should not be construed as defining the technical spirit of the present invention. The scope of the present invention is limited only by the appended claims.
Those skilled in the art can make various changes and modifications within the technical spirit and scope of the present invention. Therefore, such embodiments and changes fall within the scope of the present invention so far as they are obvious to those skilled in the art.
reactor and provides the function of smoothing an excess power tilt generated after replacement of nuclear fuel, which is implemented by controlling the amount of light water (H20) in a compartment thereof. Upon rapid rise of power tilt after the replacement of the nuclear fuel, the amount of the light water in the compartment is increased to reduce the power tilt. When output is stabilized, the amount of the light water is decreased accordingly. Thus, the output is smoothly distributed. In other words, when the LZCS
is normally operated, the power tilt is decreased according to a rise of the level of the water, while the power tilt is increased according to a drop in the level of the water.
Fig. 1 shows the structure of a compartment 100 of a conventional CANDU
reactor. The compartment 100 of Fig. 1 is shown as an upper outer compartment.
In Fig.
1, black arrows indicate the flow of light water and white arrows indicate the flow of helium gas. The compartment 100 is a space between upper and lower bulkheads 10 and 11 that are fixedly installed within a pipe 80. A spreader 20 for causing the light water introduced through the bulkhead 10 to run down along the wall of the compartment is installed at an upper portion of the compartment 10, and a helium gas discharge hole 12 is formed in the lower bulkhead 11. Further, a double tube 40 for supplying the helium gas and discharging the light water penetrates through the upper bulkhead 10 and is then connected to the lower bulkhead. An outer tube of the double tube 40 connected to the lower bulkhead 11 is formed with a scavenger 41 for taking in and discharging the light water, and an inner tube of the double tube 40 is caused to communicate with the helium gas discharge hole 12 of the lower bulkhead 11. Moreover, a helium gas discharge tube 70 is fixed to the spreader 20 while penetrating through the upper bulkhead 10. Double tubes 50 and 60 for inflow and outflow of light water and helium gas for a lower compartment are installed within the compartment 100 while penetrating therethrough. In addition, a tube support plate 30 is installed within the compartment to prevent vibration caused by the fluids in the double tubes 40, 50 and 60 that have lengths considerably larger than diameters thereof. As shown in Fig. 2, the tube support plate 30 takes the shape of a disk that can be inserted into and fixed to the pipe 80 and is made of a porous metal material through which the light water and helium gas can pass vertically. The tube support plate is also formed with a through-hole 31 at the center thereof and a plurality of cut-away portions 32 at the periphery thereof so that the light water and the helium gas can easily pass through the tube support plate.
In the CANDU reactor constructed as above, the amount of the light water discharged from a lower portion of the compartment is fixed, while the amount of the light water introduced at the upper portion of the compartment is controlled to be changed in a predetermined range according to operating conditions. Therefore, if the amount of the light water introduced at the upper portion of the compartment is greater than the amount of the light water discharged from the lower portion of the compartment, the level of the water rises, whereas if the amount of the light water discharged from the lower portion of the compartment is greater than the amount of the light water introduced at the upper portion of the compartment, the level of the water is lowered. The output of the reactor is increased or decreased according to the changes in the level of the water. The LZCS
measures the changes in the level of the water within the compartment by measuring the difference in pressure between the upper and lower portions of the compartment, and maintains pressure according to the changes in the level of the water by causing a certain amount of helium gas to be introduced into the lower portion of the compartment and to be subsequently discharged from the upper portion of the compartment. If the level of the water in the compartment reaches 80%, the LZCS performs control such that a water level control logic has priority over an output control logic so that the level of the water cannot exceed 80%.
In the conventional CANDU reactor having the compartment structure and the LZCS control scheme, there is a phenomenon in which cycling or rapid drop periodically occurs when the level of the water W in an upper outer compartment rises over 80% after replacement of nuclear fuel, as shown in Fig. 3. Further, the power tilt P of the reactor also exhibits a variation similar to that in the level of the water. Such a variation is contrary to a variation in the output under normal control (the output is decreased upon rise of the level of the water under the normal control).
To alleviate an instability phenomenon in such a CANDU reactor, the following temporary methods are employed: a toxic material such as gadolinium (Gd) is input into the compartment or an operating scheme of a helium gas compressor is changed.
However, these methods have not fundamentally solved the instability phenomenon.
Such instability may cause sudden shutdown of the reactor. Further, the input toxic material becomes radioactive wastes after it absorbs neutrons, resulting in increase of the amount of wastes produced.
SUMMARY OF THE INVENTION
A primary object of the present invention is to examine a cause of an instability phenomenon such as rapid rise and drop or periodic cycling of a power tilt and the level of water in a CANDU reactor and to provide a method of controlling flow rates of light water and helium gas in a liquid zone control system to eliminate such an instability phenomenon.
According to the present invention to achieve the primary object, there is provided a method of controlling flow rates of light water and helium gas passing through a plurality of compartments of a CANDU reactor, each of the compartments having a tube support member installed therein, comprising the step of controlling the flow rates of the light water and the helium gas such that flow rates of the light water and the helium gas supplied to and discharged from a portion of each of the compartments below the tube support member per unit time satisfy the following relationship:
(a) QL - (01.). <0, or (b) QL - (Q:). SQxk where QL is a flow rate of the light water discharged from the compartment per unit time, (Qi.). is a maximum flow rate of the light water that can pass through the tube support member per unit time, and Q , is a flow rate of the helium gas supplied. The tube support member is normally used by a tube support plate having a plurality of holes, but the tube support member can be a tube support grid or an elastic spring.
A secondary object of the present invention is to provide a CANDU reactor that has an improved tube support plate to alleviate the instability phenomenon such as rapid rise and drop or periodic cycling of the power tilt and the level of water.
According to the present invention for achieving the second object, there is provided a CANDU rector having a plurality of compartments, each of the compartments having a tube support plate installed therein, wherein the tube support plate is formed with a plurality of through-holes through which light water and helium gas pass.
The plurality of through holes of the support plate enables to satisfy flow conditions of light water and helium gas through the through-holes as follows:
(a) Q., - (O ). <0, or (b) Q.a - (Qr.). SQxu where Q. is a flow rate of the light water discharged from the compartment per unit time, (Qm ). is a maximum flow rate of the light water that can pass through the tube support member per unit time, and Qm is a flow rate of the helium gas supplied. . The support plate can be replaced by a support grid or a support elastic spring that satisfies the flow conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment in conjunction with the accompanying drawings, in which:
Fig. 1 is a view showing the structure of a compartment of a liquid zone control system in a CANDU reactor, and the flow of light water and helium gas therein;
Fig. 2 is a plan view of a tube support plate of a conventional CANDU reactor;
Fig. 3 is a graph showing variations in the level of the water and the output when 5 an instability phenomenon occurs in the conventional CANDU reactor;
Fig. 4 is an explanatory view illustrating a cause of the instability of the level of the water and the output when a conventional method of controlling flow rates of light water and helium gas is employed;
Fig. 5 is a graph showing stable and unstable operation regions according to the flow rates of the light water and the helium gas;
Fig. 6 is a plan view of an embodiment of a tube support plate of a CANDU
reactor according to the present invention; and Fig. 7 is a graph showing stable and unstable operation regions when the tube support plate shown in Fig. 6 is used.
DETAILED DESCRIPTION OF THE INVENTION
When nuclear fuel is replaced in a CANDU reactor, there is a case where an output in a specific region rises according to a reactivity property of the nuclear fuel. Since an excess power tilt adversely affects the operation of the reactor, a LZCS
control the operation so that a smooth output can be obtained. Although the control of the LZCS is normally performed in a normal operation, an instability phenomenon in which the control cannot be performed normally as shown in Fig. 3 occurs if the level of light water is very high to such an extent that the level of the water rises over a tube support plate (TSP).
If the level of the water is below the tube support plate as shown in Fig. 4 (a), there is no problem in view of the flow of the light water and the helium gas in a compartment.
However, if an operation is made in a state where the level of the water is higher than the level of the water as shown in Fig. 4 (b), there may be a phenomenon in which abnormality occurs in a stream of the light water flowing downward and a stream of the helium gas flowing upward. That is, the helium gas is accumulated below the tube support plate 30 as shown in Fig. 4 (c). This phenomenon depends on flow rates of the light water and the helium gas that pass through the tube support plate per unit time. That is, the phenomenon occurs if the amount of the light water that passes through the tube support plate per unit time is smaller than the amount of the water that is discharged below the tube support plate. The light water that has not passed through the tube support plate is held up on the tube support plate, while the helium gas introduced at a lower portion of the compartment cannot pass through the tube support plate and is also accumulated below the tube support plate, so that the amount of the light water below the tube support plate is gradually reduced. The level of the water within the compartment of the CANDU
reactor is controlled such that it cannot generally exceed over 80% to prevent the light water from overflowing toward the outside of the compartment. However, if a phenomenon in which the light water is held up on the tube support plate 30 occurs as shown in Fig. 4 (d), a water level meter for measuring the level of the water using the difference in pressure between upper and lower portions of the compartment outputs a measured value indicating that the level of the water is maintained at 80%. Actually, since the helium gas is accumulated below the tube support plate 30, the light water is held up on the tube support plate to such an extent that the level of the water reaches a spreader. Consequently, the level of the water measured by the water level meter is completely different from the actual level (position) of the water. The light water existing below the tube support plate that corresponds to the size of a space occupied by the helium gas accumulated below the tube support plate cannot be effectively used for controlling the reactivity.
Further, a part of the light water held up on the tube support plate is pushed outside a fuel boundary and thus cannot contribute to control of the reactivity. Therefore, the output of the reactor cannot be normally controlled, thereby making the reactor unstable.
Referring to Fig. 4 (d), if a portion of the compartment below the tube support plate is taken as a control volume, flow rates of inflow and outflow of the light water and the helium gas (below the tube support plate satisfy the following equation:
QHe,` Out-Qin ......(1) where O is a flow rate of a fluid that passes through the compartment per unit time. The reason why such an abnormal phenomenon occurs while the level of the water is controlled to be maintained at 80% is that the amount Qin of the light water passing through the tube support plate is smaller than the amount Qout of the light water discharged at the lower portion of the compartment. That is, the abnormal phenomenon occurs if the following equation is satisfied:
QOnt - (Kin )max >0 ...... (2) Further, if the flow rate of the helium gas supplied is larger than an insufficient flow rate of the light water below the tube support plate, the helium gas passes through the tube support plate so that the abnormal phenomenon does not occur. If the maximum flow rate that can pass through the tube support plate is denoted by (Q;n )max , the abnormal phenomenon occurs when the following equation is satisfied:
Q0 - (Qin )max > QHe ...... (3) As an example, Fig. 5 shows calculation results according to the conditions of equations 1 and 2 and experiment results of a simulation of the abnormal phenomenon in connection with the flow rates of the light water and the helium gas in the liquid zone control system of the CANDU reactor. Fig. 5 presents the experiment results and the theoretical calculation results showing that stable and unstable operation regions exist according to the amount Qout of the light water discharged and the amount QHe of the helium gas introduced. These results show that the reactor can be always operated in the stable operation region by properly controlling the amount Quut of the light water discharged and the amount QHe of the helium gas introduced. When data of the experiment results are fitted between the stable and unstable operation regions, an experimental equation for the boundary (bold dotted line) of the stable operation region can be obtained as follows:
QHe =14.237 + 2.34 x ln(Qout - 0.458) (where, Qout > 0.458) ...... (4) Moreover, a general equation for the flow rate below the tube support plate is Q He = Q out - Qin , and the conditions of the occurrence of the abnormal phenomenon should satisfy both Q0 U, - (QQ, )max > 0 and Qout - (Qin )max >_ QHe Based on the aforementioned four equations, the stable operation region where the abnormal phenomenon occurs can be expressed as the following equation:
Q0 ut - (Qin )max <O or Qout - (Qin )max QHe ...... (5) Here, the flow rate of the helium gas is QHe =14.24 + ln(Qout - 0.46). If equation 5 is expressed in a different manner, the stable operation region can be expressed by the following equation:
QHe -14.237 + 2.34 x ln(Qo,u - 0.458) ====== (6) Furthermore, if the value of the amount (Q;,, ),,,ax of the light water passing through the tube support plate becomes sufficiently larger without controlling the amount Qout of the light water discharged and the amount He of the helium gas introduced, Qout _00m, becomes smaller. Therefore, the probability of occurrence of the abnormal phenomenon is lowered. Accordingly, the occurrence of the abnormal phenomenon can be prevented by increasing the areas of portions of the tube support plate corresponding to flow passages of the light water and the helium gas such that the light water and the helium gas cannot be accumulated. That is, as shown in Fig. 6, additional through-holes 33 are formed in the tube support plate 30 or the areas of cut-away portions of the through-hole 31 are increased to prevent the occurrence of the abnormal phenomenon. Fig. 7 shows results in which the stable operation region is increased when the areas of the flow passages of the light water and the helium gas are increased by about 10% by adding the through-holes 33 to the tube support plate of the liquid zone control system of the reactor.
According to the present invention, the flow rates of the light water and the helium gas are controlled so that a CANDU reactor can be operated in a stable region where an unstable phenomenon, which may occur in a liquid zone control system, dose not occur.
Thus, it is possible to prevent a power tilt, thereby improving the stability and operatability of the reactor. That is, sudden rise or drop and periodic cycling of the output and the level of the water in a compartment of the CANDU reactor are solved, thereby eliminating a feeling of uneasiness of an operator and enhancing the economical efficiency, stability and operatability of a nuclear power station.
Further, the reactor can be stably operated without inputting a toxic material into the light water so that the amount of radioactive wastes generated can be reduced.
The embodiment of the present invention described above herein and illustrated in the drawings should not be construed as defining the technical spirit of the present invention. The scope of the present invention is limited only by the appended claims.
Those skilled in the art can make various changes and modifications within the technical spirit and scope of the present invention. Therefore, such embodiments and changes fall within the scope of the present invention so far as they are obvious to those skilled in the art.
Claims
1. A method of controlling flow rates of light water and helium gas passing through a plurality of compartments of a CANDU reactor, each of the compartments having a tube support member installed therein, comprising the step of:
controlling the flow rates of the light water and the helium gas such that flow rates of the light water and the helium gas supplied to and discharged from a portion of each of the compartments below the tube support member per unit time satisfy the following relationship:
where ~ out is a flow rate of the light water discharged from the compartment per unit time, (~ in)max is a maximum flow rate of the light water that can pass through the tube support member per unit time, and ~ He is a flow rate of the helium gas supplied.
controlling the flow rates of the light water and the helium gas such that flow rates of the light water and the helium gas supplied to and discharged from a portion of each of the compartments below the tube support member per unit time satisfy the following relationship:
where ~ out is a flow rate of the light water discharged from the compartment per unit time, (~ in)max is a maximum flow rate of the light water that can pass through the tube support member per unit time, and ~ He is a flow rate of the helium gas supplied.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| KR10/2004/0117768 | 2004-12-31 | ||
| KR1020040117768A KR100646701B1 (en) | 2004-12-31 | 2004-12-31 | Control method for light water and helium gas flow rate in the liquid zone control system of candu reactor and candu reactor having improved tube support plate |
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| CA2512708A1 CA2512708A1 (en) | 2006-06-30 |
| CA2512708C true CA2512708C (en) | 2011-10-04 |
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| KR101146948B1 (en) * | 2009-10-30 | 2012-05-22 | 한국수력원자력 주식회사 | Reactivity measurement method of liquid zone control system in CANDU plants using neutron detector |
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| CA2512708A1 (en) | 2006-06-30 |
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