JPH0376717B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a control device for a nuclear reactor, in particular, when an unstable phenomenon in which the output fluctuates over time occurs, detects the instability and corrects the instability. The present invention relates to a suitable nuclear reactor control device that solves the problem.

[Background of the invention]

Figure 1 shows a schematic diagram of a boiling water reactor. A boiling water reactor generates steam by the heat of nuclear fission generated in a core part 1, and sends the steam to a turbine part 2.
It is a system that guides and generates electricity. In general, when instability occurs in such a nuclear reactor, causing power fluctuations,
The following problem arises.

() Due to power fluctuations, the flow rate circulating through the reactor core 1 also fluctuates, making system control difficult.

() Due to flow vibration, the mechanical strength margin of the structure is reduced.

() As the flow rate fluctuates, the thermal margin in the core decreases. For this reason, nuclear reactors are designed to provide a sufficiently large margin for stability. However, there is some possibility that instability may occur due to human error and instrument malfunction. Therefore, in conventional boiling water reactors,
In order to detect instability and eliminate instability, the power monitoring system shown in Figure 2 is installed in the reactor core 1.
LPRM (Local Rower Range Monitor) 3 is used. When the core power is stable,
The LPRM signal is stable and has a small amplitude (see Figure 3), but the amplitude of the LPRM signal that becomes unstable becomes large (see Figure 4). Therefore, instability is detected by monitoring this LPRM signal, and when instability is detected, the instability is eliminated by inserting the control rod 4 or the like.

This conventional control method only takes into account the unstable region known up to now. However, it has become clear that when a nuclear reactor is in a natural circulation state or a state close to a natural circulation state (including some forced circulation states), an unstable mode other than the above-mentioned unstable mode exists. For this reason, the conventional reactor control methods have had the problem of being insufficient in terms of how to resolve instability.

[Purpose of the invention]

The purpose of the present invention is to avoid operating a nuclear reactor in both unstable modes: a first unstable mode that occurs in a region where the core exit quality is high, and a second unstable mode that occurs in a region where the exit quality is low. The purpose of the present invention is to provide a nuclear reactor control device that can perform the following steps.

[Summary of the invention]

The features of the present invention include a means for detecting thermal-hydraulic parameters in the core, and based on the detected thermal-hydraulic parameters, an unstable mode occurring in the core is detected in a region where the exit quality of the core is high. means for discriminating between a first unstable mode that occurs and a second unstable mode that occurs in a region where the exit quality is low; and when the first unstable mode is discriminated, the first unstable mode a first control means for controlling the reactor control parameters to eliminate the mode, and a second control for controlling the reactor control parameters to eliminate the second unstable mode when the second unstable mode is discriminated. It is in having the means.

[Embodiments of the invention]

The present invention was made in consideration of the conventionally known unstable mode (first unstable mode) of a nuclear reactor and the newly discovered second unstable mode. Hereinafter, the generation mechanism and characteristics of the second unstable mode, which is the motivation for the present invention, will be explained in relation to the first unstable mode.

() Phenomenon explanation Figure 5 shows the operating range in which the second unstable mode occurs using the reactor output and the subcooling degree of the coolant at the core inlet. The first unstable mode is also shown in the figure for comparison. While the first unstable mode occurs under conditions of low subcooling and high output, the second unstable mode, on the contrary, occurs under conditions of high subcooling and low output. That is, the first mode occurs under conditions where the core exit quality is high, and the second mode occurs under conditions where the quality is low. This mechanism is explained as follows.

The natural circulation flow rate is determined from the balance between the buoyant force F and the flow path loss ΔP in a flow path where natural circulation occurs due to boiling as shown in FIG. The buoyant force F is caused by the difference in the density of the coolant in the ascending passage 6 and the descending passage 5, and is a function F(α) of the void ratio α.
On the other hand, the flow path loss ΔP is mainly 2
It is phase flow friction loss, and can be considered as a function ΔP (X, Q) of quality X and flow rate Q. When the flow rate is constant, two-phase flow friction loss and quality generally have a monotonically increasing relationship as shown in FIG. Furthermore, the void ratio, which is a factor of buoyancy, has a relationship with quality as shown in FIG.

Now, consider instability under a natural circulation loop as shown in Figure 6. Whether or not instability occurs is determined by inputting a disturbance in a steady state, transmitting the disturbance in a chain, feeding it back again, and determining whether or not the fed-back output disturbance is larger than the input disturbance. Therefore, for instability to occur, it is necessary that the response to the disturbance in the feedback loop be sufficiently large.

First Unstable Mode The first unstable mode occurs under high quality conditions. As shown in FIG. 8, in the high quality region, the change in void ratio ∂α/∂X with respect to quality variation is small. Therefore, even for _{a fluctuation amount δ Q of} the same order as δ On the other hand, with respect to friction loss ΔP, in the high quality region, as shown in FIG. 7, the variation in friction loss with respect to quality fluctuation is ∂ΔP/∂X, which is large. Therefore, the change in friction loss ∂ΔP/∂Q with respect to the flow rate fluctuation δQ becomes large, and the fact that this change is large, that is, the friction loss fluctuates greatly, is thought to be the cause of the first unstable mode. .

Second Unstable Mode The second unstable mode occurs under low quality conditions. In the low quality region, the change in void ratio ∂α/∂X with respect to quality fluctuations is large (see Figure 8), while the change in friction loss ∂ΔP/∂X is small (see Figure 7). Recognize.

Therefore, contrary to the first unstable mode, fluctuations in the driving force (buoyancy) dominate. Hereinafter, the second unstable mode will be phenomenologically explained using FIG. 9.

In a state where the core power is low and the natural circulation flow rate is low, when a negative flow rate fluctuation δQ is applied, voids 7 suddenly occur in the reactor core 1 (state A). Therefore, buoyancy is generated and a driving force is generated to circulate the reactor water. On the other hand, as the flow velocity of the coolant increases in the reactor core 1, the heat removal ability of the coolant increases, so boiling stops (state B). However, when the void is removed, the buoyancy is lost and the flow of coolant becomes stagnant (state C), returning to its original state and continuing to repeat. As described above, the second mode is caused by the fluctuation of the flow rate and the generation and disappearance of voids which are periodically repeated, and the fluctuation of the driving force.

() How to detect instability The first unstable mode is caused by friction loss, and the second mode is caused by fluctuations in driving force. Therefore, in order to clarify the difference between the modes, the sixth
Experiments were conducted using a natural circulation device with a heat generating section and a cooling section as shown in the figure. Taking the flow rate measured at that time as an example, the raw waveforms for each code are shown in FIGS. 11 and 12. Furthermore, the waveform at stable time is shown in FIG. 10 for comparison. When stable, the flow rate is almost constant over time, and its amplitude is small (Figure 10). On the other hand, when it becomes unstable,
Its amplitude increases (Figures 11 and 12).
Therefore, instability can be detected by calculating and monitoring the standard deviation, which is an index of the amplitude.

() Discrimination of unstable modes The first unstable mode exhibits oscillations similar to a sin relationship around a certain average level (the 11th
figure). On the other hand, in the second unstable mode,
Because voids periodically repeat their disappearance and generation,
Shows a sawtooth waveform. Therefore, if we cut out the DC component of these waveforms and take the spectrum, we get the frequencies shown in FIGS. 13 and 14. That is, the first unstable mode exhibits a mountain-shaped distribution with a peak at a certain frequency. On the other hand, in the second mode, the peak value shows a double wavelength line spectrum. Therefore, to classify into the first and second unstable modes,
Spectral analysis of the recorded data can be performed and discrimination can be made based on whether the peak value of the spectrum is double wavelength.

() How to eliminate instability Figure 15 shows the dependence of flow amplitude on outlet quality. It can be seen that the point at which instability occurs is uniquely determined by the outlet quality of the boiling section. Therefore, it can be seen that in order to resolve the first or second instability, it is sufficient to decrease or increase the exit quality. Also, for pressure, an increase in pressure leads to an increase in saturation temperature, which is equivalent to a decrease in outlet quality. Therefore, in summary, the first method of resolving instability is: () Decrease the exit quality () Increase the pressure.

Also, the method to eliminate the second unstable mode is as follows. () Increase exit quality () Reduce pressure.

It is.

The present invention will be explained in more detail with reference to more specific examples.

FIG. 16 is a block diagram when this embodiment is applied to a boiling water reactor. The nuclear reactor includes a pressure vessel 10, a reactor core 1, a control rod 4, an LPRM 3, a recirculation pump 11, a turbine 2, a steam control valve 12, a feed water heater 13, a condensate pump 14, and the like.

The signal measured by the LPRM 3 is amplified by an amplifier 15, converted into a digital signal by an A/D converter 16, and stored in a buffer memory 17. Next, the signals selected by the selector 18 are sequentially guided to the computer. This computer has the following functions: () Detection of instability, () Discrimination of unstable modes, and () Instructions on how to resolve instability. perform tasks. When instability is detected, an instruction signal for how to resolve the instability according to the instability mode is sent to the control device, and the control system is activated so that the instability can be resolved.

The specific processing method within the computer is shown in PAD in Figure 17. The processing method will be explained below using the same figure.

() Method of detecting instability The data of the LPRM signal selected by the selector is sent to the CPU in the computer, and the average value x and standard deviation σ of the data are calculated.

Subsequently, the ratio σ/ between the standard deviation and the average value is determined and the ratio is compared with ε. If the ratio σ/ is larger than ε, the selected
The neighborhood of LPRM is treated as unstable, while when it is small, it is treated as stable. Furthermore, by switching the selector, for all LPRM signals,
Perform the above steps in sequence, and if all LPRM signals are unstable, treat the entire core as unstable. By the way, in the above method, ε is used as a judgment for detecting instability. This value is
Based on the result that the amplitude is in the unstable region when the amplitude is 0.2 or more as shown in FIG. 15, it is considered that ε=0.2 is appropriate.

() Method for discriminating unstable modes Spectrum analysis is performed on the LPRM signal determined to be unstable. In the second unstable mode, by using the fact that the peak value of the spectrum is a double-wavelength line spectrum (Figure 14), we compare it with the spectrum analysis results of the LPRM signal to determine whether the instability is in the first mode or not. 2 mode is discriminated.
Note that since the unstable frequency is 0.1 to 1 Hz, it is only necessary to analyze this frequency range.

() How to resolve instability Instruct how to resolve instability depending on the mode of instability. The first page shows the specific content as a PAD.
This will be explained below using FIG.

(a) When instability is the first instability mode Classify based on whether instability occurs locally or in the entire core. In the case of local instability,
A control rod 4 is inserted near the LPRM 3 where instability has been detected to reduce the output and eliminate the instability. On the other hand, if the entire core is unstable, stabilization is achieved by increasing the core pressure and reducing the exit quality. Specifically, the method of increasing the pressure is
The steam control valve 12 reduces the steam flow rate and increases the pressure. On the other hand, the method for reducing the outlet quality is as follows: (1) The rotation of the recirculation pump 14 is increased to increase the flow rate circulating through the reactor core 1, thereby reducing the outlet quality.

() The feed water heater 13 and the feed water pump 14 lower the feed water temperature and reduce the outlet quality.

() Insert control rod 4, reduce output,
Reduce exit quality.

Therefore, instability can be resolved by using the above methods in combination.

(b) When the instability is in the second instability mode As in the first mode, the cases are divided into whether the instability is in the whole core or locally. If instability occurs locally, the control rod 4 close to the LPRM 3 where instability has been detected is withdrawn, contrary to the first mode. On the other hand, if instability has occurred in the entire core, a method of reducing core pressure and a method of increasing outlet quality are used.
In the former case, the pressure is lowered by increasing the steam flow rate using the steam control valve 12. on the other hand,
The latter method is as follows: (1) The rotation of the recirculation system pump 11 is lowered, the core flow rate is reduced, and the outlet quality is increased.

(2) Increase the temperature of the feed water using the feed water heater 13 and the feed water pump 14 to increase the outlet quality.

(3) Withdraw the control rods, thereby increasing the output and exit quality.

By using these methods in combination, instability can be efficiently eliminated.

As another embodiment of the present invention, an example in which the present invention is applied to a natural circulation nuclear reactor shown in FIG. 19 can be considered. In a natural circulation reactor, pressure vessel 1
0, core 1, control rods 4, LPRM 3, subcooler 21, condenser 23, subcooler feed water pump 22, condenser feed water pump 24, etc. In such reactors, LPRM
As in the previous embodiment, the signal No. 3 is sent to the amplifier 15,
The signal is guided to a computer via an A/D converter 17 and a selector 18. The computer performs instability detection processing, and when instability is detected, it classifies the instability and sends an instruction signal to the control device on how to resolve the instability according to the type. The processing method within the computer is almost the same as in the previous embodiment, and is as shown in FIG. However, since the structure of the control system is different from the previous embodiment, the method for resolving instability is different. The solution method will be explained below using FIG. 20.

(a) When instability is in the first mode As in the previous embodiment, the instability is classified as to whether the entire core is local or not, and if the instability is local, the control rods 4 are inserted. On the other hand, in the case of the whole core, a method of increasing the core pressure and a method of reducing the outlet quality are used. A method for increasing the core pressure is to lower the rotation of the condenser feed water pump 23 and increase the pressure by decreasing the amount of heat exchange (condensation amount) in the condenser. On the other hand, the method for reducing the outlet quality is as follows: (1) The outlet quality is lowered by increasing the rotation of the subcooler water supply pump 22 and increasing the cooling effect in the subcooler.

() Lower the exit quality by inserting a control rod and lowering the output.

These methods can be used in combination to achieve efficient stabilization.

(b) When instability is in the second mode In this case, stabilization can be achieved by controlling the method for eliminating instability in the first mode in the opposite direction.

Another embodiment of the discrimination method based on the unstable mode is shown in FIG. The discrimination method according to this example is as follows:
Experimental results show that the representative frequency of the first unstable mode is higher than that of the second mode (11th
12). The calculation procedure is to find the representative frequency _x of the signal measured by LPRM. Then this frequency _x
Comparing the magnitude with a certain threshold frequency _th (for example, about 0.5Hz), if the frequency _x of the LPRM signal is large, the first unstable mode is selected, and if it is small, the second mode is selected. Distinguish. Therefore, in this example:
To distinguish the mode from the raw signal of LPRM,
The time required for analysis is reduced. Furthermore, since the time required to sample data is about two wavelengths (approximately 15 seconds), mode discrimination can be performed in a short time. As a result, instability can be detected at an early stage of occurrence, and instability can be resolved early. Hereinafter, the procedure for calculating the representative frequency _x will be explained using the 22nd frequency.

The LPRM signal has an oscillating waveform as shown in FIG. First, as a first step, two threshold values (high level V _h and low level V _l ) are provided. This threshold is
Using the maximum value V _nax and minimum value V _nio of the LPRM signal, the following values are set, for example.

V _h = εV _nax + (1-ε)V _nio V _l = (1-ε) V _nax +εV _nio ε=0.7 The crossing time of the threshold V _h , V _l and the LPRM signal is T _h and T _l In that case, T _h ,
Define T _l as follows. That is,
Let T _h be the time when the LPRM signal crosses the low level V _l and subsequently crosses the high level V _h . Therefore, for example, like point α
The LPRM signal falls below the high level V _h ,
High level V _h again without crossing the low level
Times that intersect with can be excluded. Similarly, regarding the crossing time with the low level,
LPRM signal crosses high level and continues
Let T _l be the time when V _l is crossed. As a result, a typical frequency _x can be expressed as follows using T _h and T _l .

Here, N is the total number of crossing times.

〔Effect of the invention〕

According to the present invention, the reactor can be operated while avoiding both the first unstable mode, which occurs in a region where the core exit quality is high, and the second unstable mode, which occurs in a region where the exit quality is low. This makes it easier to operate the reactor core.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a conventional boiling water reactor;
The figure is a schematic diagram showing the installation position of the LPRM, Figure 3 is a waveform diagram of the LPRM signal when it is stable, Figure 4 is a waveform diagram of the LPRM signal when it is unstable, and Figure 5 is the area where the second unstable mode occurs. Figure 6 is a schematic diagram of a natural circulation device, Figure 7 is a diagram showing the relationship between quality and two-phase flow friction loss, Figure 8 is a diagram showing the relationship between quality and void ratio, and Figure 9 is a diagram showing the relationship between quality and void ratio. A diagram showing the generation mechanism of the second unstable mode, Figure 10 is a flow rate waveform diagram in stable mode, Figure 11 is a flow rate waveform diagram in the first unstable mode, and Figure 12 is the second unstable mode. Fig. 13 shows the spectrum of the first unstable mode, Fig. 14 shows the spectrum of the second unstable mode, and Fig. 15 shows the outlet quality and the conditions for the occurrence of instability. Figure 16 is a diagram showing an example of this embodiment applied to a boiling water reactor, Figure 17,
Figure 18 shows the computer processing algorithm, Figure 19.
20 is a diagram showing an algorithm, FIG. 21 is a flowchart showing an algorithm of a discrimination method in another embodiment of the present invention, and FIG. 22 is a diagram showing an embodiment applied to a natural circulation nuclear reactor. FIG. 7 is a graph diagram showing a threshold level in an LPRM signal waveform diagram in another embodiment of the present invention. 1... Core, 2... Turbine, 3... LPRM
(Local Power Range Monitor), 4...Control rod, 5...Downflow path, 6...Upflow path, 7...Void, 8...Heating part, 9...Cooling part, 10...Pressure vessel, 11... Recirculation pump, 12... Steam control valve, 13... Feed water heater, 14... Feed water pump, 15... Amplifier, 16... A/D converter, 17... Buffer memory, 18... Selector, 19...computer, 20...control device, 21...
...Subcooler, 22...Subcooler water supply pump,
23...Condenser, 24...Condenser water supply pump.

Claims (1)

[Scope of Claims] 1. Means for detecting thermal-hydraulic parameters in the reactor core, and based on the detected thermal-hydraulic parameters, an unstable mode occurring in the reactor core is detected when the outlet quality of the core is The first occurrence occurs in high areas.
mode discrimination means for discriminating between an unstable mode and a second unstable mode occurring in a region where the exit quality is low; and when the first unstable mode is discriminated, the first unstable mode; a first control means for controlling reactor control parameters to eliminate the second unstable mode; and when the second unstable mode is discriminated, to eliminate the second unstable mode;
1. A nuclear reactor control device comprising: second control means for controlling reactor control parameters. 2 The thermal-hydraulic parameter is the pressure within the reactor core,
The reactor control parameter is a coolant flow rate, a void ratio, or a pressure loss, and the reactor control parameter is a pressure in the reactor core, a coolant flow rate, a main steam flow rate, a control rod operation amount, or a coolant temperature. control equipment for nuclear reactors. 3. The control of a nuclear reactor according to claim 1, wherein the mode discrimination means performs a spectral analysis of the detected thermal-hydraulic parameter and discriminates between two unstable modes based on the difference in the spectra. Device.