Natural circulation nuclear reactor and method for controlling output of natural circulation nuclear reactor

Abstract

To provide a natural circulation nuclear reactor which can suppress reduction of stability of flow in the inside of a pressure container.SOLUTION: The present invention includes: a monitor device for acquiring measurement data of a reactor core flow rate, a water level, and a water supply flow rate; and a control device for controlling a control rod and a water supply flow rate. The control device calculates, by a formula (3), a time delay Δt of change of a reactor core flow rate to change of a reactor core output generated in a chimney on the basis of the reactor core flow rate acquired by the monitor device, and controls a water level by changing the water supply flow rate in the period of the time delay Δt.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a natural circulation type nuclear reactor and a method for controlling the output of a natural circulation type nuclear reactor.

Background Art

[0002] In a boiling water reactor, a coolant (liquid) flowing between fuel rods is boiled by a nuclear reaction using a fuel assembly in which nuclear fuel rods are arranged in a square pattern. Then, the generated steam is used for power generation in the boiling water reactor. In the boiling water reactor, the reaction heat of the fuel rods is directly transferred to the coolant. For this reason, in the boiling water reactor, efficient steam generation and cooling of the fuel rods are maintained.

[0003] Also, as a boiling water reactor, there is a natural circulation type boiling water reactor (hereinafter referred to as a natural circulation type nuclear reactor or a natural circulation reactor) that simplifies the in-reactor structure by eliminating the recirculation pump for cooling water and allows the coolant to circulate naturally (see, for example, Patent Document 1). The natural circulation reactor maintains the cooling of the reactor core by the natural circulation force of the cooling water. The natural circulation reactor mainly adjusts the thermal output of the reactor core by operating control rods to adjust the reactor core flow rate. The reactor core flow rate is determined based on the change rate of the reactor core output and the change in the average void fraction in the chimney.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in a natural circulation reactor, when the rate of change of the core power increases, there is a delay in the change of the average void fraction in the chimney with respect to the rate of change of the core power. For this reason, the change in the core flow rate lags behind the change in the core power. For example, when performing an output increase (control rod withdrawal) operation, there occurs a phenomenon in which the core power increases first, while the core flow rate increases with a delay. As a result, the core flow rate becomes low with respect to a state where the core power is high, and there is a possibility that the stability of the flow inside the pressure vessel decreases.

[0006] In order to solve the above-described problems, the present invention provides a natural circulation type nuclear reactor capable of suppressing a decrease in the stability of the flow inside the pressure vessel, and a method for controlling the output of the natural circulation type nuclear reactor.

[0007] Further, the above object and other objects of the present invention and the novel features of the present invention will be clarified by the description of this specification and the attached drawings.

Means for Solving the Problems

[0008] The natural circulation type nuclear reactor of the present invention is a natural circulation type nuclear reactor including a reactor pressure vessel, a core accommodated in the reactor pressure vessel, a shroud covering the core, a chimney provided above the core in the shroud, and control rods arranged in the core. The natural circulation type nuclear reactor has a core flow meter that measures the core flow rate of the coolant flowing into the core, a water level meter that measures the water level of the coolant in the reactor pressure vessel, and a feed water flow meter that measures the feed water flow rate of the coolant connected to the reactor pressure vessel. Further, the natural circulation type nuclear reactor has a monitoring device that acquires measurement data of the core flow rate, the water level, and the feed water flow rate, and a control device that controls the control rods and the feed water flow rate. The control device calculates a time delay Δt of the change in the core flow rate with respect to the change in the core power occurring in the chimney based on the core flow rate acquired by the monitoring device using the following formula (3), and controls the water level by changing the feed water flow rate during a period of the time delay Δt from the start of the operation of the control rod.

[0009]

Number

[0010] However, Hch is the chimney height, j is the average rising velocity of the coolant, ρL is the liquid phase density, Ac is the coolant flow area of the core, and Gc is the core flow rate.

[0011] Further, the output control method of the natural circulation type nuclear reactor of the present invention is a method for controlling the output of a natural circulation type nuclear reactor including a nuclear reactor pressure vessel, a core accommodated in the nuclear reactor pressure vessel, a shroud covering the core, a chimney provided above the core in the shroud, and a control rod disposed in the core. The output control method measures the core flow rate of the coolant flowing into the core, measures the water level of the coolant in the nuclear reactor pressure vessel, and measures the feed water flow rate of the coolant connected to the nuclear reactor pressure vessel. Then, based on the acquired core flow rate, the time delay Δt of the change in the core flow rate with respect to the change in the core output occurring in the chimney is calculated using the following formula (3), and the water level is controlled by changing the feed water flow rate during the period of the time delay Δt from the start of the control rod operation.

[0012]

Equation

[0013] (However, Hch is the chimney height, j is the average rising velocity of the coolant, ρL is the liquid phase density, Ac is the coolant flow area of the core, and Gc is the core flow rate.)

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a natural circulation type nuclear reactor capable of suppressing a decrease in the stability of the flow inside the pressure vessel, and an output control method for the natural circulation type nuclear reactor.

[0015] In addition, problems, configurations, and effects other than those described above will be clarified by the description of the following embodiments.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0017] Hereinafter, an example of a natural circulation type nuclear reactor and an output control method for a natural circulation type nuclear reactor according to an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following examples. In each of the drawings described below, common members are denoted by the same reference numerals. Further, in the drawings used in this specification, the same or corresponding components are denoted by the same reference numerals, and repeated descriptions of these components may be omitted.

[0018] 〈1. First Embodiment〉 [Configuration of Natural Circulation Reactor] A first embodiment of a natural circulation type nuclear reactor and a method for controlling the output of the natural circulation type nuclear reactor will be described. FIG. 1 shows the configuration of a reactor pressure vessel of a natural circulation reactor according to the first embodiment. In the natural circulation reactor, a cylindrical shroud 2 installed concentrically with the reactor pressure vessel 1 is provided on the lower side of the reactor pressure vessel 1 having a cylindrical shape, and a reactor core 3 composed of a large number of nuclear fuel assemblies covered by the shroud 2 is installed. Below the reactor core 3 including the fuel assemblies, the coolant that has passed through the lower type plate flows into the reactor core 3. Then, the coolant boils in the reactor core 3.

[0019] The natural circulation reactor is provided with a space called a chimney 4 above the reactor core 3 in order to promote the natural circulation force. In the chimney 4, steam 8 generated by the boiling of the coolant in the fuel assemblies flows in. For this reason, the density of the coolant in the chimney 4 is lower than that outside the chimney 4 in terms of the steam volume fraction.

[0020] Above the chimney 4, a separator 6 for performing gas-liquid separation of the coolant that has passed through the chimney 4 and a dryer 5 for drying the steam separated by the separator 6 are installed. Also, at the position of the dryer 5 above the reactor pressure vessel 1, a main steam line 10 is connected to the reactor pressure vessel 1. Above the reactor core 3, a gas-liquid two-phase flow in which the gas phase and the liquid phase of the boiled coolant are mixed is formed. The gas-liquid two-phase flow is moisture-separated through the separator 6 and the dryer 5 installed in the upper part of the reactor pressure vessel 1. The separated steam is sent from the main steam line 10 to the turbine building. The hot water stays in a space outside the separator 6 or outside the shroud 2 (hereinafter referred to as the downcomer 7). Then, the hot water flows downward through the downcomer 7 and flows into the reactor core again.

[0021] In the downcomer 7 through the cylindrical shroud 2, there is no steam and only hot water. Therefore, a density difference occurs between the chimney 4 inside the shroud 2 and the downcomer 7 outside. Due to this density difference, the natural circulation reactor obtains the natural circulation force of the cooling water by gravity. Therefore, in the natural circulation reactor, the recirculation flow rate and the core flow rate of the cooling water are determined by the average steam volume ratio (hereinafter referred to as the average void fraction) inside the chimney 4 (or the shroud 2). For this reason, the natural circulation reactor does not require direct control of the core flow rate by a recirculation pump or the like.

[0022] The feed water line 13 is connected to the reactor pressure vessel 1 via the feed water pump 11 and the feed water flow rate control valve 19. The feed water line 13 is connected to the position of the downcomer 7 between the shroud 2 and the inner wall of the reactor pressure vessel 1. In the feed water line 13, hot water is sent to the downcomer 7 in the reactor pressure vessel 1 by the feed water pump 11 through a turbine condenser or the like.

[0023] Control rods 15 are installed in the lower core 3 inside the reactor pressure vessel 1. The control rods 15 are connected to a driving device (not shown) that drives the control rods 15. In a natural circulation reactor, an output control operation (hereinafter referred to as a load following operation) for changing the output of the reactor is performed in response to a startup operation or a demand command. The natural circulation reactor cannot directly adjust the core flow rate when performing the load following operation. For this reason, in the natural circulation reactor, the heat output of the core is mainly adjusted by operating the control rods 15 to adjust the core flow rate. In practice, the output and the core flow rate are determined by subsequent nuclear feedback.

[0024] The natural circulation reactor includes a core flow meter 14 that measures the flow rate of the coolant flowing into the core 3, and a water level meter 18 that measures the water level 9 inside the pressure vessel. The natural circulation reactor also includes a feed water flow meter 12 that measures the feed water flow rate, and a monitoring device 16 that acquires and monitors the measurement data of the feed water flow meter 12. Furthermore, the natural circulation reactor includes a control device 17 that processes the data of the monitoring device 16 and transmits instructions to the driving devices of the feed water pump 11, the feed water flow rate control valve 19, and the control rods 15 based on the calculation results for control.

[0025] The monitoring device 16 and the control device 17 are constituted by a known arithmetic unit. For example, the monitoring device 16 and the control device 17 are constituted by a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), a storage unit, etc. The CPU reads out the program codes of the software related to various processes of core power control stored in the ROM (an example of a recording medium) and expands them in the RAM. Then, the CPU implements core power control according to the expanded program. Note that the monitoring device 16 and the control device 17 may be provided with another arithmetic unit such as an MPU (Micro Processing Unit) as the arithmetic unit instead of the CPU. The storage unit is an example of a recording medium composed of a non-volatile storage or the like. The storage unit stores various processing programs executed by the CPU, programs such as an OS, information related to the functions of core power control necessary for the execution of the programs, various data tables used by each arithmetic unit for calculations, etc. In addition, the storage unit stores various measurement data acquired by the monitoring device 16, various calculation results calculated by the control device 17, etc. Furthermore, the storage unit stores data such as the target output value of the natural circulation reactor input by the operator from an external instruction device or the like together with various data to be stored.

[0026] [Core flow rate] Next, the core flow rate of the natural circulation reactor will be described. Fig. 2 shows a relationship diagram between the core flow rate and the core power of the natural circulation reactor. The horizontal axis is the core flow rate normalized by the core flow rate at 100% rated operation. The vertical axis is the core power normalized by the core power at rated operation. The graph indicated by the broken line A is a steady state line representing the steady state. Usually, the relationship between the core power output and the core flow rate is one-to-one, and the output control is performed within a range planned based on the steady state line by prior prediction.

[0027] In a natural circulation reactor, when in the range to the left of the dashed line A, that is, in a state of having a higher core output and a lower core flow rate than the steady state line, fluctuations in the core output and fluctuations in the natural circulation force due to the void fraction generation are likely to occur. For this reason, in the natural circulation reactor, the flow stability of the coolant may decrease inside the reactor pressure vessel 1. On the other hand, in a natural circulation reactor, when in the range to the right of the dashed line A, that is, in a state of having a lower core output and a higher core flow rate than the steady state line, the fluidity of the coolant due to the natural circulation force is stabilized inside the reactor pressure vessel 1.

[0028] In a natural circulation reactor, when the output change rate increases, a delay occurs in the change of the average void fraction in the chimney 4 with respect to the change rate of the core output. For this reason, the core flow rate changes with a delay with respect to the change in the core output. In the case of an output increase operation such as the temperature and pressure increase process during startup, first the core output increases, and then the core flow rate increases with a delay. For this reason, a state of high core output and low core flow rate is reached. Thus, the time delay in the change of the core flow rate with respect to the change in the core output in the chimney until the change from the average void fraction of one state to the average void fraction of another state may lead to a decrease in the flow stability of the coolant.

[0029] Therefore, the natural circulation reactor of this embodiment predicts the time delay occurring in the chimney 4 from the coolant flow rate in the reactor acquired by the monitoring device 16, and adjusts the water level 9 in the reactor by the control device 17 to change the core flow rate. The core flow rate of the natural circulation reactor is determined such that the static pressure difference inside and outside the shroud 2 mainly balances the frictional loss. The static pressure outside the shroud 2 is determined by the water level in the reactor pressure vessel. For this reason, the core flow rate changes when the water level in the reactor pressure vessel changes.

[0030] In a natural circulation reactor, it is operated so as to keep the water level 9 constant by adjusting the feed water flow rate. For this reason, generally, the water level 9 is not used for controlling the core flow rate. There is a relationship between the water level H and the core flow rate Gc as shown in the following formula (1).

[0031]

Equation

[0032] In Equation (1), Δρ is the density difference between the gas and the liquid, and g is the acceleration due to gravity. In addition, the change in the feed water flow rate ΔQ and the change in the water level ΔH are related by the following Equation (2).

[0033]

Number

[0034] In Equation (2), A is the cross-sectional area outside the shroud 2, and Δt is the time difference. The time delay Δt of the change in the core flow rate with respect to the change in the core output occurring inside the chimney 4 is caused by the gradual change in the void fraction from the lower part to the upper part of the chimney 4. Therefore, the time delay Δt is determined by the upward velocity of the coolant inside the chimney 4. Using the average upward velocity j inside the chimney 4, the time delay Δt can be expressed by the following Equation (3).

[0035]

Number

[0036] In Equation (3), Hch is the chimney height, and j is the average upward velocity of the coolant. ρL is the liquid phase density, and Ac is the cooling water flow path area of the core. The above Equation (3) assumes j as the average velocity of the liquid phase. In addition, the above Equation (3) can also be expressed as the following Equation (4) taking into account other elements of the natural circulation path.

[0037]

Number

[0038] In Equation (4), a is an arbitrary coefficient, and b is an arbitrary intercept. In an actual natural circulation path, a part of the coolant becomes vapor and expands in volume, so the velocity of the liquid phase increases and the time delay becomes shorter. Therefore, the above equation is a conservative evaluation equation for the time delay.

[0039] [Method for Controlling Output of Natural Circulation Path] Fig. 3 shows a flowchart of the method for controlling the output of the natural circulation path. First, the monitoring device 16 acquires a request for output change (target output) and measurement data (step S1). In this process, the monitoring device 16 acquires the target output value of the natural circulation reactor from an instruction device or an input device (not shown) installed outside. The instruction device and the input device are communicably connected to the monitoring device 16 or the control device 17. The target output value is input to the instruction device or the like by, for example, an operator or the like. The measurement data acquired by the monitoring device 16 is, for example, the core flow rate, the position of the control rod, and the like. The monitoring device 16 acquires each measurement data from the core flow meter 14, the feed water flow meter 12, and the control device 17.

[0040] Next, the control device 17 calculates a target core flow rate from the target output value based on the relationship between the core flow rate and the core output in a known steady state (step S2). The relationship between the core flow rate and the core output is, for example, the steady state line shown in Fig. 2 above. In this process, based on the steady state line shown in Fig. 2, the core flow rate when the core output becomes 100% is calculated as the target core flow rate.

[0041] Next, the control device 17 calculates a change amount ΔH of the water level required for the output change from the difference between the measured current core flow rate and the target core flow rate based on the relationship between the core flow rate and the water level calculated in advance (step S3). In this process, the relationship between the core flow rate and the water level calculated in advance is the relational expression between the water level H and the core flow rate Gc shown in the above formula (1). The control device 17 calculates the current water level H from the current core flow rate using the formula (1). Further, the control device 17 calculates the water level H for achieving the target core flow rate from the target core flow rate using the formula (1). Then, the control device 17 calculates the change amount ΔH of the water level required for the output change from the difference between the current water level H and the water level H for achieving the target core flow rate.

[0042] Next, the control device 17 calculates the time delay Δt of the core flow rate with respect to the core output generated in the chimney 4 based on the measurement data, and calculates the necessary feed water change flow rate ΔQ (step S4). In this process, the control device 17 calculates the time delay Δt from the change amount ΔH of the water level calculated in step S3 and the current core flow rate using the above formula (3). Further, the control device 17 calculates the feed water change flow rate ΔQ, which is the difference between the current feed water flow rate and the feed water flow rate for achieving the change amount ΔH of the water level calculated in step S3, from the calculated time delay Δt using the above formula (2).

[0043] Next, simultaneously with starting the control rod operation, the control device 17 changes the feed water flow rate by ΔQ in the time of Δt (step S5). In this process, the control device 17 starts a control rod operation that changes (increases or decreases) the core output by the operation of the control rod 15. Further, simultaneously with the start of the control rod operation, the control device 17 controls the feed water pump 11 and the feed water flow rate regulating valve 19, and increases the feed water flow rate by the feed water change flow rate ΔQ calculated in step S4 from the current value during the period from the start of the control rod operation until Δt elapses.

[0044] Thereafter, after the core flow rate reaches the target core flow rate, the control device 17 changes the feed water flow rate to control the water level and maintains the core flow rate constant (step S6). In this process, the control device 17 acquires a signal from the monitoring device 16 and determines whether the core flow rate has reached the target core flow rate. The monitoring device 16 acquires measurement data from the core flow meter 14 and outputs the acquired core flow rate data to the control device 17. Usually, the core flow rate reaches the target core flow rate during the period from the start of the control rod operation until Δt calculated in step S5 elapses. Also, after the core flow rate reaches the target core flow rate, that is, after Δt from the start of the control rod operation, the control device 17 acquires the water level data from the monitoring device 16 and controls the feed water flow rate so that the water level becomes constant. Specifically, the core flow rate reaches the target core flow rate after Δt from the start of the control rod operation due to the increase in the natural circulation amount caused by the increase in the core output and the increase in the water level caused by the increase in the feed water flow rate. After the start of the control rod operation, the natural circulation amount further increases due to the increase in the core output, resulting in an increase in the core flow rate. Therefore, after Δt has elapsed since the start of the control rod operation, in order to maintain the target core flow rate, the control device 17 needs to offset the increase in the core flow rate due to the increase in the water level by the increase in the natural circulation amount of the core flow rate caused by the increase in the core output and keep the core flow rate constant. Accordingly, the control device 17 reduces the water level by reducing the supply water flow rate and decreases the increase in the core flow rate due to the increase in the water level. In this process, for example, the control device 17 adjusts the feed water flow rate while monitoring the water level data acquired from the monitoring device 16, and controls the feed water pump 11 and the feed water flow rate regulating valve 19 to a feed water flow rate that can keep the core water volume and the water level constant. Control the core flow rate to be constant, and after performing the control rod operation, end the processing according to this flowchart.

[0045] [Core Flow Rate Control: Water Level, Feed Water Flow Rate] Next, the changes in the core output (core output), core flow rate, water level, and feed water flow rate due to the processing of the above flowchart will be described. FIG. 4 shows graphs of the changes in the core output (core output) and the core flow rate. Also, FIG. 5 shows graphs of the changes in the core output (core output), core flow rate, water level, and feed water flow rate. In FIGS. 4 and 5, the horizontal axis represents the elapsed time, and the vertical axis represents the core output, core flow rate, water level, or feed water flow rate, respectively.

[0046] FIG. 4 is a graph showing the time course of core power and core flow rate in a natural circulation reactor with a conventional configuration. In the conventional configuration, after the start ts of control rod operation, the core power increases from state 1 to state 2. At this time, the core flow rate begins to increase with a delay of Δt from the start ts of control rod operation. This Δt is the time delay Δt calculated in step S4 described above.

[0047] On the other hand, FIG. 5 is a graph showing the time course of core power, core flow rate, water level, and feedwater flow rate in the natural circulation reactor of the present embodiment. Also in the configuration of the present embodiment, after the start ts of control rod operation, the core power increases from state 1 to state 2. At the same time as the start ts of control rod operation, the feedwater flow rate increases by ΔQ calculated by the above formula (2). Therefore, at t1 after Δt has elapsed from the start ts of control rod operation, the water level increases by ΔH calculated by the above formula (1). In this way, by increasing the water level by ΔH after Δt from the start ts, the core flow rate increases simultaneously with the start ts of control rod operation, and the core flow rate reaches the target core flow rate (state 2) after Δt from the start ts. As a result, the time delay Δt in the increase timing of the core flow rate with respect to the increase in core power can be eliminated. And in the natural circulation path, a state of high core power and low core flow rate can be avoided, and destabilization of the operating state can be suppressed.

[0048] Also, after Δt, the core flow rate increases as the core power increases. Therefore, it is necessary to reduce the increase in the core flow rate due to the increase in the water level by the amount of increase in the natural circulation amount of the core flow rate due to the increase in the core power. That is, after Δt, it is necessary to lower the water level to lower the water level to the state 1 at the start ts. For this reason, after t1 when Δt has elapsed from the start ts, the feedwater flow rate is reduced to lower the water level. For example, as described above, the control device 17 acquires water level data from the monitoring device 16 and controls the feedwater pump 11 and the feedwater flow rate control valve 19 to make the water level constant. Also, the operator of the nuclear reactor or the like may adjust the feedwater flow rate so that the water level becomes constant while checking the water level data. Note that the feedwater amount increases in response to the increase in the generated steam amount due to the increase in the core power. Therefore, the feedwater amount after t1 becomes a value higher than the flow rate in the initial state 1. In addition, in the example shown in FIG. 5, the operation start time ts of the control rod is set as the adjustment start time of the feedwater flow rate control. However, the same effect can be obtained as long as the adjustment start time of the feedwater flow rate control is within the period from the start to the end of the control rod operation. Furthermore, when continuously changing the core output from state 2 to state 3, it is desirable that the control rod operation start time and the start of the control rod operation are simultaneous.

[0049] [Relationship between core flow rate and core output] The effect of the output control of the natural circulation reactor described above will be described with reference to FIG. 6. FIG. 6 is a relationship diagram (graph) between the core flow rate and the core output of the natural circulation reactor shown in FIG. 2 above, and the broken line A is the steady state line. The arrow B represents the relationship between the core flow rate and the core output in the case of the configuration of the natural circulation reactor of the present embodiment shown in FIG. 5 above. The arrow C represents the relationship between the core flow rate and the core output of the natural circulation reactor of the conventional configuration shown in FIG. 4 above. Also, the arrows B and C show an example of the relationship between the core flow rate and the core output when increasing the core output from 50% to 100%.

[0050] In the conventional configuration of FIG. 4, with respect to the start ts of the control rod operation, the core flow rate starts to increase with a time delay Δt. Therefore, during the period from ts to t1, the core flow rate does not increase with respect to the increase in the core output. Accordingly, the arrow C represents a state of high core output and low core flow rate, and moves to a range on the left side of the steady state line of the broken line A. As a result, in the conventional configuration of FIG. 4, the flow stability of the coolant inside the reactor pressure vessel 1 may decrease.

[0051] On the other hand, in the case of the configuration of the present embodiment shown in FIG. 5, with respect to the start ts of the control rod operation, the core flow rate starts to increase simultaneously. And during the period from ts to t1, the core flow rate increases to the target core flow rate when the core output is 100%. For this reason, the arrow B in the case of the configuration of the present embodiment represents a state of low core output and high core flow rate rather than the steady state line, and moves to a range on the right side of the steady state line of the broken line A. Accordingly, in the case of the configuration of the present embodiment, the fluidity of the coolant is stabilized inside the reactor pressure vessel 1.

[0052] In the above-described natural circulation path and the output control method of the natural circulation path, the case of increasing the core output at startup is taken as an example to explain the case of increasing the core output. The above-described natural circulation path and the output control method of the natural circulation path can be applied not only to the case of increasing the above-described core output but also to the case of decreasing the output. For example, the control device 17 calculates the target core flow rate from the target output value to be decreased. Then, the control device 17 calculates the decrease amount ΔH of the water level based on the relationship between the core flow rate and the water level calculated in advance from the difference between the current core flow rate and the target core flow rate. Further, the control device 17 calculates the time delay Δt and the feed water change flow rate ΔQ from the decrease amount ΔH of the water level. Then, in accordance with the start of the control rod operation, the feed water flow rate is decreased by ΔQ in the time of Δt to make the core flow rate reach the target core flow rate. Thereby, the decrease of the core output can be realized in a state where the fluidity of the coolant inside the pressure vessel 1 is stabilized without being affected by the time delay Δt.

[0053] 〈2. Second Embodiment〉 Next, a second embodiment of the natural circulation type nuclear reactor and the output control method of the natural circulation type nuclear reactor will be described. FIG. 7 shows the configuration of the reactor pressure vessel of the natural circulation type nuclear reactor according to the second embodiment. Note that the configuration of the reactor pressure vessel of the second embodiment is different from the configuration of the reactor pressure vessel of the first embodiment described above only in that a configuration for measuring the core output is added. Therefore, in the following description, detailed description of the same configuration as that of the first embodiment described above will be omitted.

[0054] As shown in FIG. 7, the natural circulation type nuclear reactor is provided with an output measurement unit 31 in the core 3 inside the reactor pressure vessel 1. The output measurement unit 31 measures the core output of the core 3. Further, the output measurement unit 31 outputs the measurement data to the monitoring device 16. The control device 17 takes in the core output measured by the output measurement unit 31 as measurement data from the monitoring device 16 and calculates the void fraction in the predicted chimney 4.

[0055] The time delay Δt is caused by the gradual change in the void fraction from the lower part to the upper part of the chimney. Therefore, using the calculated void fraction in the chimney 4 and the average upward velocity j in the chimney 4, the time delay Δt can be expressed by the following equation (5). The following equation (5) is an equation that takes into account the influence of the void fraction in the chimney on the above equation (3).

[0056] [Number]

[0057] In equation (5), α is the void fraction in the chimney. The above equation (5) evaluates the time delay Δt from the velocity when the liquid phase rises together with the vapor, using the actual velocity of the liquid phase. Since the void fraction is between 0 and 1, the time delay Δt calculated by equation (5) is smaller than the time delay Δt calculated by the above equation (3).

[0058] In a natural circulation type nuclear reactor, the control device 17 can calculate the time delay Δt using the above equation (5). For example, the control device 17 can accurately calculate the time delay Δt by using the above equation (5) instead of the above equation (3) in step S4 of the flowchart shown in FIG. 3 above.

[0059] 〈3. Third Embodiment〉 Next, a third embodiment of a natural circulation type nuclear reactor and an output control method of the natural circulation type nuclear reactor will be described. Note that the configuration of the natural circulation type nuclear reactor of the third embodiment can apply the same configuration as that of the first embodiment and the second embodiment described above. Also, the output control method of the natural circulation type nuclear reactor of the third embodiment can apply the same control process as that of the first embodiment described above, except that a process considering the upper limit H high and the lower limit H low of the water level is added. Therefore, in the following description, detailed descriptions of the same configurations as those of the first embodiment and the second embodiment described above will be omitted.

[0060] [Output Control Method of Natural Circulation Loop] Fig. 8 shows a flowchart of the output control method for the natural circulation path of the third embodiment. First, perform the same processing as steps S1 to S4 of the flowchart shown in Fig. 3 described above to calculate the change amount ΔH of the water level required for the output change n , the time delay Δt n , and the feed water change flow rate ΔQ n (steps S10 to S13). Note that these processes are performed with n = 1. That is, the processes here calculate the change amount ΔH 1 , the time delay Δt 1 , and the feed water change flow rate ΔQ 1 .

[0061] Next, when the control device 17 changes the current water level H by the change amount ΔH n , it determines whether it exceeds the upper limit H high of the water level in the reactor pressure vessel 1 or falls below the lower limit H low (step S14). Upper limit H high is exceeded, or when it falls below the lower limit H low (Yes in step S14), the control device 17 starts the control rod operation and at the same time changes the feed water flow rate by ΔQ n in the time of Δt (step S15). This process is the same as the process of step S5 of the flowchart shown in Fig. 3 described above.

[0062] Next, before the water level H reaches the upper limit H high or the lower limit H low , the control device 17 stops the control rod operation and the feed water flow rate operation for the period of Δt n (step S15). In this process, the control device 17 keeps the core output constant by stopping the control rod operation that changes (increases or decreases) the core output. Also, the control device 17 controls the feed water flow rate by controlling the feed water pump 11 and the feed water flow rate regulating valve 19 to be ΔQ nStop the process of increasing. That is, the control device 17 controls the feed water pump 11 and the feed water flow control valve 19 so as to return the feed water flow rate to the state of state 1. However, in this state, the feed water flow rate increases more than that in state 1 in response to the increase in the amount of generated steam due to the increase in the core output. After stopping the control rod operation and the feed water flow rate operation, return to the process of step S12 with n = n + 1. Then, the control device 17 determines that the water level H does not exceed the upper limit H high and does not fall below the lower limit H low of the water level, and repeats the processes from step S12 to step S16 until the water level falls below the lower limit H

[0063] Upper limit H high and does not fall below the lower limit H low of the water level (No in step S14), the control device 17 starts the control rod operation and changes the feed water flow rate by ΔQ n in a time of Δt (step S17). This process is the same as the process of step S5 in the flowchart shown in FIG. 3 above. After the core flow rate reaches the target core flow rate, the control device 17 controls the water level by changing the feed water flow rate and maintains the core flow rate constant (step S18). The process of step S18 is the same as the process of step S6 in the flowchart shown in FIG. 3 above. After controlling the core flow rate to be constant and performing the control rod operation, end the process according to this flowchart.

[0064] [Core Flow Rate Control: Water Level, Feed Water Flow Rate] Next, the changes in the core output (core output), core flow rate, water level, and feed water flow rate due to the processes of the above flowchart will be described. FIG. 9 and FIG. 10 show graphs of the changes in the core output (core output), core flow rate, water level, and feed water flow rate. In FIG. 9 and FIG. 10, the horizontal axis represents the elapsed time, and the vertical axes represent the core output, core flow rate, water level, and feed water flow rate, respectively.

[0065] FIG. 9 is a graph showing the passage of time of the core output, core flow rate, water level, and feed water flow rate according to this embodiment when the core output is increased from state 1 to state 2. FIG. 10 is a graph showing the passage of time of the core output, core flow rate, water level, and feed water flow rate according to this embodiment when the core output is decreased from state 1 to state 2.

[0066] As shown in FIG. 9, from the start time ts of the control rod operation, the core output increases from state 1. At the same time as the start time ts of the control rod operation, the feed water flow rate is increased by ΔQ calculated by the above formula (2). 1 Accordingly, the water level H also increases. However, the water level H reaches the upper limit H of the water level before increasing by the change amount ΔH. That is, the water level H reaches the upper limit H of the water level at time t1 before the time delay Δt has elapsed from the start time ts of the control rod operation. 1 high 1 high

[0067] When it is predicted that the water level H reaches the upper limit H, the control device 17 stops the control rod operation for increasing the core output at t1, and temporarily stops the output increase. Then, the state 2' is maintained while the time delay Δt elapses from t1 when the control rod operation is stopped. Also, the control device 17 temporarily stops the feed water flow rate operation for increasing the feed water flow rate by ΔQ from t1 to Δt. high 1 n n

[0068] Even when the above control rod operation and feed water flow rate operation are stopped, the core flow rate increases by the increase in the natural circulation amount due to the increase in the core output while the time delay Δt elapses from t1. Therefore, the control device 17 performs a feed water flow rate operation for decreasing the feed water flow rate and the water level so as to keep the core flow rate constant. The control device 17 acquires the measurement data of the core flow rate from the monitoring device 16, and controls the feed water pump 11 and the feed water flow rate regulating valve 19 so that the core flow rate becomes constant. That is, in the above process, the control device 17 has a time delay Δt from the start time ts of the control rod operation. 1 1 ​​​​​​​​​During the passage of time, not only change it in the direction of increasing the feed water flow rate, but also change it in the direction of decreasing it. Note that this process may be such that an operator of the nuclear reactor or the like adjusts the feed water flow rate so that the core flow rate becomes constant while checking the measurement data of the core flow rate.

[0069] Also, in the above state 2', the core output is increased compared to state 1. Therefore, even in a state where the core output and the core flow rate are kept constant, the water level H decreases due to the evaporation of the coolant. Therefore, the water level H decreases during the passage of the time delay Δt 1 from t1.

[0070] Next, when the time delay Δt 1 elapses from t1, the control device 17 calculates again the change amount ΔH 2 of the water level required for the output change, the time delay Δt 2 , and the feed water change flow rate ΔQ 2 . Then, the control device 17 starts the control rod operation at the time t2 after the elapse of the time delay Δt 1 , and causes the core output to reach the target core output (state 2) after Δt 2 . Also, the control device 17 increases the feed water flow rate by ΔQ 2 calculated by the above formula (2) simultaneously with the start t2 of the control rod operation. Thereby, the water level H is increased by the calculated ΔH 2 . As a result, the core flow rate increases simultaneously with the start t2 of the control rod operation, and the core flow rate reaches the target core flow rate (state 2) after Δt 2 . Therefore, the time delay Δt of the increase timing of the core flow rate with respect to the increase of the core output can be eliminated, and the destabilization of the operation state can be suppressed. The processing after the core flow rate reaches the target core flow rate is performed in the same manner as in the above-described first embodiment.

[0071] Also, as shown in FIG. 10, when reducing the core output from state 1 to state 2, the feed water flow rate is decreased by ΔQ 1 calculated by the above formula (2) simultaneously with the start ts of the control rod operation. Accordingly, the water level H also decreases. However, the water level H has a change amount ΔH 1Before only decreasing, the lower limit H of the water level low is reached. That is, the water level H reaches the lower limit H of the water level at the time t1 before the time delay Δt 1 has elapsed since the start ts of the control rod operation. low is reached.

[0072] When it is predicted that the water level H reaches the lower limit H low , the control device 17 stops the control rod operation for reducing the core output at t1 and temporarily stops the output reduction. Then, while the time delay Δt 1 elapses from t1 when the control rod operation is stopped, the state 2' is maintained. Also, the control device 17 primarily stops the feedwater flow rate operation for reducing the feedwater flow rate by the feedwater change flow rate ΔQ n from t1 to Δt n .

[0073] Even when the above control rod operation and feedwater flow rate operation are stopped, the core flow rate decreases only by the decrease in the natural circulation amount due to the decrease in the core output during the time delay Δt 1 elapsing from t1. Therefore, the control device 17 performs a feedwater flow rate operation for increasing the feedwater flow rate and the water level so as to keep the core flow rate constant. The control device 17 acquires the measurement data of the core flow rate from the monitoring device 16 and controls the feedwater pump 11 and the feedwater flow rate control valve 19 so that the core flow rate becomes constant. That is, in the above process, the control device 17 not only changes the feedwater flow rate in the decreasing direction but also changes it in the increasing direction during the time delay Δt 1 elapsing from the start ts of the control rod operation. Note that this process may be such that the reactor operator adjusts the feedwater flow rate so that the core flow rate becomes constant while checking the measurement data of the core flow rate.

[0074] Also, in the above state 2', the core output is lower than that in state 1. Therefore, even in the state where the core output and the core flow rate are kept constant, the water level H increases due to the decrease in the evaporation amount of the coolant. Therefore, the water level H increases during the time delay Δt 1 elapsing from t1.

[0075] Next, when a time delay Δt elapses from t1 1 the control device 17 calculates again the amount of change in water level ΔH necessary for the output change 2 , the time delay Δt 2 , and the feedwater change flow rate ΔQ 2 . Then, the control device 17 starts control rod operation at time t2 after the elapse of the time delay Δt, and causes the core output to reach the target core output (state 2) after Δt 1 . Also, the control device 17 reduces the feedwater flow rate by ΔQ calculated by the above formula (2) simultaneously with the start ts of the control rod operation 2 . As a result, the water level H is reduced by ΔH calculated for the water level calculation 2 . As a result, the core flow rate decreases simultaneously with the start t2 of the control rod operation, and the core flow rate reaches the target core flow rate (state 2) after Δt 2 . Therefore, the time delay Δt in the reduction timing of the core flow rate with respect to the reduction of the core output can be eliminated, and the destabilization of the operating state can be suppressed 2 . The processing after the core flow rate reaches the target core flow rate is performed in the same manner as in the first embodiment described above

[0076] In the above processing, after receiving a change command for the core output, it is possible to predict in advance that the water level H that changes by adjusting the feedwater flow rate reaches the upper limit / lower limit. When it is predicted that the water level H reaches the upper limit / lower limit, the feedwater flow rate and the water level are changed for a time of Δt1 simultaneously with the start of the control rod operation, but the control rod operation is temporarily stopped for a time of Δt1 before reaching the upper limit / lower limit. As a result, the water level can be restored and a margin for the amount of change in the water level can be ensured

[0077] 〈4. Fourth Embodiment〉 Next, a fourth embodiment of a natural circulation type nuclear reactor and an output control method for a natural circulation type nuclear reactor will be described. FIG. 11 shows the configuration of a reactor pressure vessel of a natural circulation type nuclear reactor according to the fourth embodiment. The configuration of the reactor pressure vessel of the fourth embodiment is the same as that of the first embodiment described above. Therefore, in the following description, detailed description of the same configuration as that of the first embodiment described above will be omitted

[0078] As shown in FIG. 11, the natural circulation type reactor defines the water level H in the reactor pressure vessel 1 and the distance L from the feed water pump to the reactor pressure vessel in the feed water line. This water level H and distance L are stored in advance in the monitoring device 16 and the control device 17.

[0079] Since there is a distance in the feed water line in the natural circulation type reactor, it takes time from when the control device 17 instructs a change in the feed water flow rate until it is actually reflected in the change in the water level. Also, in the downcomer 7, it takes time from when the water level H fluctuates until the core flow rate changes. Therefore, in calculating the time delay Δt using the above formula (3), by taking into account the time delay of the feed water line and the time delay of the downcomer, a more accurate Δt can be calculated. Using the distance L from the feed water pump to the reactor pressure vessel in the feed water line and the water level H, the time delay Δt can be expressed by the following formula (6). The following formula (6) is a formula obtained by taking into account the influence of the time delay of the feed water line and the time delay of the downcomer on the above formula (3).

[0080]

Equation

[0081] In formula (6), v is the flow velocity of the feed water pipe, and v = volume flow rate / flow cross-sectional area. In the natural circulation type reactor, the control device 17 can calculate the time delay Δt using the above formula (6). For example, the control device 17 can accurately calculate the time delay Δt by using the above formula (6) instead of the above formula (3) in step S4 of the flowchart shown in FIG. 3 above.

[0082] Note that the present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the above embodiments have been described in detail for easy understanding of the present invention, and the present invention is not necessarily limited to the embodiments having all the configurations described. Also, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment. Further, it is possible to add the configuration of another embodiment to the configuration of one embodiment. Also, for a part of the configuration of each embodiment, it is possible to delete it, or add or replace other configurations.

Explanation of Reference Numerals

[0083] 1 Reactor pressure vessel, 2 Shroud, 3 Core, 4 Chimney, 5 Dryer, 6 Separator, 7 Downcomer, 8 Steam, 9 Water level, 10 Main steam system line, 11 Feed water pump, 12 Feed water flow meter, 13 Feed water system line, 14 Core flow meter, 15 Control rod, 16 Monitoring device, 17 Control device, 18 Water level gauge, 19 Feed water flow control valve, 31 Output measurement unit

Claims

1. A natural circulation type nuclear reactor comprising a reactor pressure vessel, a reactor core accommodated in the reactor pressure vessel, a shroud covering the reactor core, a chimney provided above the reactor core within the shroud, and control rods arranged in the reactor core, wherein: a core flow meter for measuring the core flow rate of the coolant flowing into the reactor core; a water level gauge for measuring the water level of the coolant in the reactor pressure vessel; a feed water flow meter for measuring the feed water flow rate of the coolant connected to the reactor pressure vessel; a monitoring device for acquiring measurement data of the core flow rate, the water level, and the feed water flow rate; a control device for controlling the control rods and the feed water flow rate; and the control device calculates a time delay Δt of the change in the core flow rate with respect to the change in the core output occurring in the chimney based on the core flow rate acquired by the monitoring device using the following formula (3), and controls the water level by changing the feed water flow rate during the period of the time delay Δt from the start of the operation of the control rods. A natural circulation type nuclear reactor. 【Number 1】 〔However, Hch is the chimney height, j is the average upward velocity of the coolant, ρL is the liquid phase density, Ac is the cooling water flow area of the reactor core, and Gc is the core flow rate.〕

2. The control device calculates the change amount of the feed water flow rate and the change amount of the water level based on the difference between the measured current core flow rate and the target core flow rate. The natural circulation type nuclear reactor according to Claim 1.

3. After changing the feed water flow rate, the control device controls the feed water flow rate to keep the core flow rate constant. The natural circulation type nuclear reactor according to Claim 1.

4. The control device changes the feed water flow rate from the start to the end of the control rod operation. The natural circulation type nuclear reactor according to Claim 1.

5. having an output measurement unit for measuring the core output, and the control device calculates the time delay Δt using the following formula (5) that takes into account the influence of the void fraction in the chimney in the formula (3). The natural circulation type nuclear reactor according to Claim 1. 【Number 2】 〔However, Hch is the chimney height, j is the average upward velocity of the coolant, ρL is the liquid phase density, Ac is the cooling water flow area of the reactor core, Gc is the core flow rate, and α is the void fraction in the chimney.〕

6. When the control device determines that the water level reaches the upper limit due to the calculated change amount of the feed water flow rate, after the start of the control rod operation, the control device temporarily stops the control rod operation and reduces the feed water flow rate. The natural circulation type nuclear reactor according to Claim 2.

7. When the control device determines that the water level reaches the lower limit based on the calculated change amount of the feed water flow rate, after the start of the control rod operation, the control rod operation is temporarily stopped, and the feed water flow rate is increased. The natural circulation type nuclear reactor according to claim 2.

8. The control device calculates the time delay Δt using the following formula (5) considering the influence of the time delay of the feed water system line and the time delay of the downcomer in the formula (3). The time difference of the core flow rate is calculated by the following formula (6) using the water level and the distance from the feed water pump to the pressure vessel. The natural circulation type nuclear reactor according to claim 1. 【Number 3】 〔However, Hch is the chimney height, j is the average rising speed of the coolant, L is the distance from the feed water pump to the nuclear reactor pressure vessel, H is the water level, and v is the flow velocity of the feed water system piping.〕

9. An output control method for a natural circulation type nuclear reactor including a nuclear reactor pressure vessel, a core accommodated in the nuclear reactor pressure vessel, a shroud covering the core, a chimney provided above the core in the shroud, and control rods arranged in the core, comprising: measuring the core flow rate of the coolant flowing into the core; measuring the water level of the coolant in the nuclear reactor pressure vessel; measuring the feed water flow rate of the coolant connected to the nuclear reactor pressure vessel; calculating a time delay Δt of the change in the core flow rate with respect to the change in the core output occurring in the chimney using the following formula (3) based on the obtained core flow rate, and controlling the water level by changing the feed water flow rate during the period of the time delay Δt from the start of the control rod operation; An output control method for a natural circulation type nuclear reactor. 【Number 4】 〔However, Hch is the chimney height, j is the average rising speed of the coolant, ρL is the liquid phase density, Ac is the cooling water flow area of the core, and Gc is the core flow rate.〕

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