JPH0423234B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for monitoring the output of a boiling water reactor, and in particular to an unstable phenomenon in which the output fluctuates over time due to changes in nuclear fission reactions. If an unstable phenomenon occurs, we need to detect it with high accuracy, and also develop a reactor output monitoring method to determine whether the unstable phenomenon is occurring throughout the reactor core or locally. This invention relates to a nuclear reactor output monitoring method for identifying the local position of a stability phenomenon that occurs locally.

[Background of the invention]

Conventionally, nuclear reactors have been designed and operated in such a way that unstable phenomena do not occur, but in the event that an unstable phenomenon occurs, it must be detected immediately and the type of instability and its It has become necessary to suppress unstable phenomena based on their location. This is because if the unstable phenomenon is left as it is, control becomes difficult. For this reason, reactor output has traditionally been monitored to detect unstable phenomena, but the conventional methods have good accuracy and cannot detect unstable phenomena. There is.

That is, the reactor power monitoring method according to the prior art is as follows:
LPRM (Local Power Range Monitor) detection signals and APRM as the average of these signals
(Averaged Power Range Monitor) signal to detect the output and temporal fluctuations in the output. Figure 1 shows the reactor core plane of a 1.1 million kW class BWR (size is shown in inches). In this case, the reactor core 1 has 185 control rods 3 between 764 fuel assemblies 2, , 43 LPRM4 are arranged as shown in the figure, and each LPRM4 also consists of four neutron detectors. As shown in Figure 2, the four neutron detectors A to D that make up the LPRM are installed at a distance (in inches) shown in parentheses from the bottom of the reactor, and are used to detect neutrons at that position. Therefore, core power is now being measured.

Therefore, if an unstable phenomenon occurs in which the amplitude of output fluctuation increases over time, it is necessary to detect this fact, the type and location of occurrence as soon as possible, and then control the output fluctuation. Measures such as inserting control rods are being taken to prevent this. However, in the conventional method, the amplitude of output fluctuation is LPRM
If the amplitude of the noise contained in the signal and APRM signal is the same or smaller than that, it is not possible to directly detect fluctuations from those signals, and it is difficult to determine in what region within the core the source of the power fluctuation is. This was due to a glitch in which observers had to compare the amplitudes of individual LPRM signals.

Therefore, apart from the above-mentioned method, recently it has been considered to apply spectrum analysis to the processing of LPRM signals (see Japanese Patent Laid-Open No. 57-50780). Obtain the spectrum in advance when no unstable phenomenon occurs, and compare this with the spectrum of the LPRM signal measured at any time. If the difference is larger than the set value, it is an unstable phenomenon. It is determined that this is occurring. However, when using this method, even if the spectrum changes due to noise etc., it may be incorrectly determined that an unstable phenomenon has occurred, which not only poses a problem in terms of detection accuracy, but also causes problems with local unstable phenomena. It has become impossible to detect the location of the occurrence.

Here, I would like to briefly explain the instability phenomena in general in nuclear reactors, although it may be late.The causes of instability phenomena can be broadly divided into thermo-hydraulic ones and reactivity-related ones. ing.
In thermo-hydraulic instability phenomena, even if the heat generation from the fuel rods is constant, the flow rate and void fraction oscillate periodically, and the nuclear fission reaction is not involved in this phenomenon. For example, thermal-hydraulic instability phenomena are beginning to occur in boilers and the like. This thermal-hydraulic instability phenomenon is generally considered to be a local instability phenomenon that occurs in some channels, but depending on the conditions, it can develop into an instability phenomenon that extends to the entire core. ing. on the other hand,
In the reactivity instability phenomenon, the amount of heat generated from the fuel rods oscillates with the flow rate and void ratio due to changes in the reaction during nuclear fission, and it may occur locally, or it may occur throughout the core. It is now possible for this to occur over a period of time.

[Purpose of the invention]

Therefore, the first object of the present invention is to be able to detect reactivity-related unstable phenomena with good accuracy, and when the unstable phenomenon is detected, to determine whether it is related to the entire core or local. The purpose of the present invention is to provide a method for monitoring nuclear reactor power that can be used to make judgments.

A second object of the present invention, in addition to the first object, is to provide a method for monitoring nuclear reactor power that can quickly determine the location of occurrence if the problem is local.

[Summary of the invention]

For this purpose, the frequency of the output oscillation when a reactivity instability phenomenon (hereinafter simply referred to as instability phenomenon) occurs is approximately 0.3, although there are some differences depending on the furnace type and operating conditions. ~0.5Hz,
Furthermore, we have focused on the fact that the waveforms of power oscillations differ depending on whether the instability phenomenon is related to the entire reactor core or when it is localized, and we aim to provide a method for monitoring reactor power as described below.

That is, by obtaining the power spectral density function (PSD) by performing spectrum analysis on the LPRM signal, and then determining the coherence and transfer relationship for all combinations of any two signals, it is possible to determine the occurrence of an unstable phenomenon. In this system, it is determined based on the absolute value of the transfer function and the phase difference whether an unstable phenomenon is occurring or not, and whether it is local or related to the entire core.

Furthermore, when it is determined that the unstable phenomenon is local, the position where it occurs is determined based on the absolute value and phase difference of the transfer function.

[Embodiments of the invention]

The present invention will be explained below with reference to FIGS. 3 to 20.

First, before specifically explaining the present invention, the theoretical background thereof will be explained as follows.

In other words, in general, a reactor core tends to exhibit unstable phenomena as its power increases and as the power distribution in the axial direction of the core reaches a lower peak (a state in which the peak position of the power is closer to the lower end of the core). ing. Here, as the analysis conditions, the reactor core is set to 4 as shown in Figure 3.
If we assume that the region is divided into two regions and the output distribution is given to each region as shown in Figure 4, then the region has a higher output than the other regions and has a lower peak. Therefore, it is found that it is unstable, and the region ~ is found to be stable as a result of analysis. In such a state, a local instability phenomenon occurs in the core.

Here, the changes in core power in each region ~ 3
The analysis results using the dimensional nuclear thermal-hydraulic stability analysis code are shown in Figure 5a as relative values to the initial values. In addition, Figure 5b is an enlarged view of a part of the area, and as can be seen, the amplitude of the vibration in this area is larger than that in other areas, and the phase is also larger than that in other areas. It is later than that and the waveform is different. The output also oscillates in the region ~, but this is due to neutron diffusion from the region. In addition, the reason why the phase is delayed is that the output is larger in the region than in other regions, so there is a large amount of voids (steam bubbles) generated, and the time that these voids exist in the flow path is greater than in other regions.
This is because it is longer than . If there are many voids, the effect of moderating neutrons will be reduced and the output will not increase.

FIG. 6 shows the output response, which was analyzed using the same three-dimensional nuclear thermal-hydraulic stability analysis code, as a relative value to the initial value when an instability phenomenon occurs in the entire reactor core. In this case, the instability phenomenon is considered to be distributed throughout the core, so both the amplitude and phase are approximately equal.

The present invention has been made against the above background, and will be specifically explained below.

FIG. 7 shows the reactor power monitoring device according to the present invention together with the reactor core and LPRM. In this case, the measurement signal (LPRM signal) from the neutron detector configuring each of the multiple LPRM4s is
The signal is converted into a digital signal via amplifiers 5, ₅₁ to _5N corresponding to the neutron detector, a multiplexer 6, and an A/D converter 7, and is temporarily stored in the buffer memory 8. The measurement signal stored in the buffer memory 8 is then selectively given to the spectrum analyzer 10 by the selector 9, and the spectrum is analyzed by the spectrum analyzer 10 to determine the PSD, transfer function, and coherence. The PSD, transfer function, etc. from the spectrum analyzer 10 are then subjected to predetermined processing and calculations in the calculation circuit 11. Note that in this example, the number of neutron detectors is N, but all N measurement signals from the neutron detectors are not necessarily required, and only the necessary ones can be selected as necessary. It is. If a measurement signal is appropriately selected and subjected to processing, the determination process can be performed more quickly. In addition, the spectrum analyzer 10
In this case, PSD etc. are required, but their physical meanings are as follows. That is, PSD expresses the contribution rate of each frequency component to the average power of random fluctuations, and therefore shows a large value in the frequency band of unstable vibration. Next is coherence, which is an index that expresses the degree of association between two measurement signals, and is defined as ``1'' if the relationship is perfect, and ``0'' if there is no relationship at all. It is something. The final transfer function also represents the absolute value ratio and phase difference between the two measurement signals.

Now, Figures 8 and 9 show the normalized PSD of the measurement signals expected to be measured by any two neutron detectors 5 _I and 5 _J when an unstable phenomenon occurs. be. From these figures, the peak of the spectrum appears around 0.3 to 0.5 Hz, but from this fact alone it cannot be determined that an unstable phenomenon is occurring. According to the noise analysis of the LPRM signal measured in the actual reactor even when stable,
This is because results have been obtained in which, depending on the furnace type, a spectrum peak appears at 0.35Hz as shown in Figure 11 (Application of Noise
Analyzes is a method to monitor stability of
Boiling Water Reactor; BRUpadhyaya and 3 others, Progress in Nuclear Energy1982.Vo19,
pp657-664). Therefore, even though the presence of a peak in a spectrum can serve as a rough guide, it is a reliability problem to immediately determine that an unstable phenomenon has occurred based on the presence of a peak. Therefore, in order to improve reliability, the coherence COH _IJ is calculated for all combinations of arbitrary two signals. If an unstable phenomenon occurs, the coherence COH _IJ will show a value close to "1" in the frequency band of 0.3 to 0.5 Hz, as shown in FIG. At this time, it is necessary to set a threshold value to determine whether or not an unstable phenomenon is occurring, and a value of about "0.5" is considered appropriate as the threshold value. Figure 12 shows two typical cases inside the furnace.
It shows the coherence of noise measured between points (Noise Investigations in Boiling).
Water and Pressurized−Water Reactors; G.
Kosaly, Progress in Nuclear Energy Vol.5,
pp145−199). Of course, this is in a stable state, but since the coherence shows a maximum value of "0.4", it is considered appropriate to set it as described above.

FIG. 13 shows the processing flow of spectrum analysis in the spectrum analyzer, and FIG. 14 shows the flow of the stability/unstable determination processing performed in the arithmetic circuit. According to this, in spectrum analysis, the PSD is first determined for the measurement signals from N neutron detectors, and then the transfer function is calculated for all combinations of two of these measurement signals.
T _IJ and coherence COH _IJ are becoming sought after. When the processing in the spectrum analyzer is completed, the arithmetic circuit is activated next and the processing shown in FIG. 14 is executed. In this stable/unstable processing, the presence or absence of a peak is determined within the range of 0.3 to 0.5 Hz for the PSD corresponding to all measurement signals, and if it is determined that there is a peak, it is determined that there is a peak. The coherence for all combinations is the standard value (0.5
In addition to determining whether or not the value is greater than or equal to the reference value, the number of combinations that are greater than or equal to the reference value is counted. In this case, the criterion for determining the presence or absence of a peak is 0.3 to 0.5Hz.
The spectrum in one band is 1.1 of that in other bands.
Although it is said to be more than twice that, this value should be appropriately set depending on the furnace type and operating conditions, and the same applies to the reference value of coherence.

In this way, the number of combinations with coherence exceeding the reference value is counted,
Once the process of determining whether the coherence is greater than or equal to the reference value is completed, an unstable phenomenon occurs based on the ratio of the counted number to the total number of combinations ( _N C ₂ (N-1)/2). It is now decided whether or not it is. In this example, the number of items counted is _N
It is determined whether an unstable phenomenon is occurring depending on whether or not it is larger than C ₂ /2.
This reference value should also be optimally set depending on the furnace type.

As described above, it is possible to know whether or not an unstable phenomenon is occurring.Next, when an unstable phenomenon occurs, the process of determining whether it is related to the entire core or local. explain.

FIG. 15 shows the flow of the process. As already explained in Fig. 6, when the instability phenomenon is related to the entire core, the relative values of the amplitude of the output in each region within the reactor are almost equal, and there is almost no phase difference. I understand. That is, assuming the transfer function T _IJ of any two measurement signals, its absolute value |T _IJ | and phase difference arg (T _IJ ) are each the 16th
As shown in FIG. 17, the absolute value is 1 (0 dB) and the phase difference is approximately equal to 0 in the frequency band of 0.3 to 0.5 Hz.

Therefore, if it is determined that an unstable phenomenon has occurred in the flow shown in FIG.
The flow shown in Figure 5 is executed, and how many of the combinations related to _{2 N} C meet the above condition (1-ε<|T _IJ |<1+
It is counted whether ε and −φ<arg( _TIJ )<φ) are satisfied. If the finally obtained counting result is greater than _N C ₂ /2, it is determined that the instability phenomenon is related to the entire core. In this case, the values of ε and φ are generally set as 0.05 and π/20, but should be set appropriately depending on the furnace type and operating conditions. Also, _N
The same applies to the value of C ₂ /2.

If an instability phenomenon occurs and it is determined that it does not affect the entire core,
After all, the unstable phenomenon is occurring locally, and in such a case, it is necessary to specify the local position. FIG. 18 shows a flow for determining the local position. As is clear from Figures 5a, b and 6, the judgment principle in this flow is that when an unstable phenomenon occurs locally, the difference between the measurement signal related to the unstable region and that related to the stable region is The absolute value and phase difference of the transfer function are as shown in FIGS. 19 and 20. In other words, if the measurement signals from the neutron detectors 5 _I and 5 _J are both related to the stable region, the transfer function
The absolute value of T _IJ is close to 1 (0 dB), and the phase difference is 0.
However, if the measurement signal from the neutron detector 5 _I is related to the unstable region, the absolute value of the transfer function T _IJ from I to J will be considerably smaller than 1, and moreover, The phase difference arg ( _TIJ ) increases. Therefore, by detecting the transfer function that has the smallest absolute value and the largest phase difference among the transfer functions related to all combinations of two measurement signals, the local position where the unstable phenomenon is occurring can be determined. It is.

Now, to explain the flow shown in FIG. 18, first, from the transfer function T _IJ obtained in the flow shown in FIG. 13, the transfer function T _JI is |T _JI |=1/|T _IJ
|, arg (T _JI ) = −arg (T _IJ ). For example, the transfer function T ₂₁ is determined from the transfer function T ₁₂ . In the flow of this example, the transfer functions T ₁₁ , T _{22 ,} .
Once transfer functions related to all required combinations have been found, the next transfer function T _IJ with the smallest absolute value and largest phase difference is detected as updatable. If a transfer function T _IJ that satisfies the above conditions is detected from all the transfer functions, it can be determined that an unstable phenomenon is occurring in the region centered on the neutron detector 5 _I. Therefore, by taking measures such as inserting control rods corresponding to the area, unstable vibrations in the output can be suppressed.

〔Effect of the invention〕

As explained above, in the case of the present invention, the PSD is obtained by spectral analysis of the LPRM signal as a measurement signal, and the coherence and transfer function related to all combinations of any two LPRM signals are obtained, and the PSD spectrum is If it is determined that an unstable phenomenon is occurring, the type of unstable phenomenon is determined based on the transfer function, and if the unstable phenomenon is local. In this case, the local position is determined. Therefore, the present invention has the advantage of being able to detect the presence or absence of an unstable phenomenon and its type, as well as the local position related to the occurrence of the unstable phenomenon, with good accuracy.

[Brief explanation of drawings]

Figure 1 shows an example of the BWR reactor core plane, and Figure 2 shows the LPRM installed in the reactor core.
Figures 3 and 4 are for explaining the configuration of
A diagram showing the case where the core is divided into four regions as an analysis condition and a power distribution is given to each region.
Figures 5a and b are diagrams showing an enlarged view of the output response when local instability occurs in the nuclear reactor obtained through analysis, and Figure 6 shows the output response when local instability occurs in the reactor core. FIG. 7 is a diagram showing the configuration of the reactor power monitoring device according to the present invention together with the reactor core, etc., and FIGS. 8 and 9 are diagrams showing the output response when an unstable phenomenon occurs. Figure 10 shows the PSD of the measurement signal. Figure 10 shows the coherence between any two measurement signals when an unstable phenomenon occurs. Figure 11 shows the LPRM signal in a stable state measured in an actual reactor. Figure 12 shows the noise coherence measured between two typical points in the furnace during stable conditions. Figure 13 shows the process flow of spectrum analysis in the spectrum analyzer. 14, 15, and 18 show the flowcharts of the process of determining the presence or absence of an unstable phenomenon, the process of determining the type of unstable phenomenon, and the process of determining an unstable local position, which are executed by the arithmetic circuit. Figures shown, Figures 16 and 17, respectively.
The figure is a diagram for explaining the principle for determining the type of unstable phenomenon, and FIGS. 19 and 20 are diagrams for explaining the determination principle for determining the unstable local position. 1...Core, 4...LPRM, 5,5 _I ~ 5 _N ...
Amplifier, 6... Multiplexer, 7... A/D converter, 8... Buffer memory, 9... Selector, 10... Spectrum analyzer, 11... Arithmetic circuit.

Claims (1)

[Claims] 1. After determining the presence or absence of an unstable reactivity phenomenon accompanying changes in nuclear fission reaction in a boiling water reactor, if it is determined that an unstable phenomenon exists, the entire reactor core is classified as the type of the phenomenon. A reactor output monitoring method that determines whether the output is local or related to the reactor, by performing spectrum analysis of measurement signals from multiple neutron detectors installed scattered within the reactor. After finding the power spectral density function, we find the coherence and transfer function for all combinations of any two measurement signals, and on the premise that the spectrum peak exists in a specific frequency band of the power spectral density function, we calculate the coherence from the coherence. A reactor output monitoring method in which, when it is determined that an unstable phenomenon exists, the type of unstable phenomenon is determined based on the absolute value and phase difference of the transfer function. 2. After determining whether there is a reactivity instability phenomenon associated with changes in the nuclear fission reaction in a boiling water reactor, if it is determined that there is an unstable phenomenon, the type of the phenomenon is whether it is related to the entire reactor core or if it is localized. A reactor output monitoring method in which it is determined whether the neutron is local, and if it is determined to be local, the location of the occurrence is determined, and the method uses multiple neutron detectors installed scattered within the reactor. The power spectral density function is determined by spectral analysis of the measurement signal from the Assuming that there is a peak of When it is determined that this is the case, the unstable local position is determined based on the absolute value and phase difference of the transfer function.