KR20120045319A - Boron dilution accident alarm system using cusum(cumulative sum) control chart and method thereof - Google Patents

Abstract

PURPOSE: A boron dilution accident alarming system based on a cumulative sum control chart and a method for the same are provided to prevent the occurrence of accidents by generating alarming sounds. CONSTITUTION: An out of reactor neutron meter(10) detects neutron leaked from a nuclear reactor. A signal processing unit(20) is composed of a preamplifier, an amplifier, and a pulse classifier. The preamplifier supplies high voltages of 1,000V or more to the neutron meter in order to convert electric charge signals into voltage pulse signals. The preamplifier amplifies the voltage pulse signals in order to facilitate a signal processing process. A boron dilution accident alarming system is composed of a digital neutron counting rate converting unit(30), an alarming calculating unit(40), an alarming lamp, and an alarming speaker(50).

Description

Boron Dilution Accident Alarm System using CUSUM (Cumulative Sum) Control Chart and Method Thereof}

The present invention replaces the conventional method for monitoring the reach of the maneuvering area monitoring ratio for the detection of boron dilution accident, and continuously measures the neutron counting rate and uses the linear deviation pattern of the neutron counting rate using the accumulated deviation of the mean value. The present invention relates to a boron dilution accident alarm system and a method using a cumulative deviation control chart capable of detecting an increase early.

Since the boron control system of PWRs consists of numerous tanks and valves for injection of boron or pure water for fuel loading, reactivity control and emergency safety injection, there is always a possibility of malfunction due to mechanical damage and human error. It exists. In the last decades, much effort has been made to prevent and detect early boron dilution accidents, particularly by the US Nuclear Regulatory Commission (NRC) and the US utility. The Final Safety Analysis Report (FSAR) for PWRs specifies the installation and requirements for the boron dilution alarm system to alert operators to preliminary warnings to prevent critical reactor reach due to boron dilution. In addition, in order to secure conservative stop control capability, excessive boron concentration of reloading water is required. In some power plants, an online boron meter is installed to check the change in the concentration of boron at all times, but it cannot provide reliable information to the operator due to the problem of accuracy. Therefore, efforts have been made to incorporate measures in power plant monitoring procedures, such as locking certain valves or shortening concentration measurement cycles, which may cause boron dilution, during fuel loading, maintenance, repair and repair work in subcritical conditions. .

If the boron dilution accident occurs during fuel reloading (Mode 6), the boron dilution alarm system must raise an alarm so that the operator can take the necessary measures 30 minutes before the reactor reaches the threshold. In the standstill mode (Mode 5), the standstill mode (Mode 4) and the stand-by mode (Mode3), an alarm must be issued to the operator 15 minutes before the threshold is reached. Some power plants also introduce a boron dilution prevention system that automatically injects boron water when a boron dilution is detected.

The boron dilution accident is caused by mechanical failure or human error in the boron control system, or due to boron dilution in unplanned coolant, which increases the reactivity of the reactor, resulting in a continuous increase in output, resulting in a nuclear boiling escape (DNB). Or an accident where the fuel is damaged. When boron dilution occurs, as shown in FIG. 1, the output of the reactor increases linearly slowly in the low subcritical state due to its intrinsic core dynamics, and increases exponentially when it reaches near the critical point due to the continuous boron dilution. Because of this, it is difficult for the operator to take measures for power and responsiveness control in a short time. Here, FIG. 1 shows the change in output according to the approach of the critical state from the subcritical state.

The primary way to monitor boron dilution is to continuously monitor the neutron counting or reverse counting ratio (ICRR) of the starting channel, or monitor boron water concentration changes through boron water sample analysis.

A method of measuring the absolute criticality (keff) of a reactor is not possible with current technology. Therefore, in most nuclear power plants, neutrons leaking from the reactor are measured, and it is determined whether or not the critical vicinity is reached by using the reverse coefficient ratio (ICRR).

ICRR = C / C0

Here, C0 represents the neutron count rate at the reference time point, and C represents the neutron count rate at the current time point.

In other words, when the reactor reaches the threshold, the number of neutrons measured by the starting area neutron measuring instrument becomes infinitely large, and thus the ICRR becomes zero. Therefore, tracking and extrapolating ICRR curves according to time, control rod position, or boron concentration during the transition from the subcritical state to the critical state can infer future critical delivery time. Or supercritical monitoring of the reactor during the initial critical arrival. However, according to this method, the value of the ICRR varies depending on the setting of C0 (neutron count rate at the reference time point), and the value of the ICRR cannot be absolutely zero, and the linearity of the ICRR curve decreases as the threshold is reached. . Therefore, the NRC and the International Nuclear Power Organization (INPO) recommend that you take special care when taking critical approaches using ICRR.

At present, there are two methods of measuring boron concentration which are applied in nuclear power plants. The first method is a visual analysis method using a chemical test paper by taking a sample of coolant, and is used as an official method for measuring boron concentration in power plants due to its high accuracy. However, this first method has a disadvantage in that boron concentration cannot be measured continuously or frequently because a person must perform sampling / analysis / judgment. The second method is a method of measuring the boron concentration according to the number of neutrons measured while flowing coolant between the neutron source and the neutron measuring instrument, and has the advantage of continuously measuring the boron concentration. For this reason, this second method is used in the boron control system to instruct the boron meter in digital control as a digital number and provide it to the operator. However, this second method is affected by the coolant temperature and has a disadvantage in that the accuracy is somewhat reduced. Therefore, this second method is not used to measure the official boron concentration in power plants.

It will take at least 20 days from fuel reload (Mode6) to initial critical reach (Mode2) during the plant preventive shutdown, during which time it is impossible to monitor boron diluent accidents using the above method at least 15 minutes apart. can do. Therefore, in most pressurized water-type nuclear power plants, boron dilution alarm systems using the start channel signal of the off-neutron flux monitoring system are applied to detect inadvertent boron dilution accidents that may occur during the reactor shutdown subcritical mode.

As shown in Fig. 2, the boron dilution alarm system generates a neutron flux alarm when the neutron count rate signal measured by the start area neutron meter exceeds the start range monitoring ratio setting value. In this case, the start area monitoring ratio is defined as follows.

Starting area monitoring ratio = starting area signal (t) / starting area signal at the beginning of boron dilution

2 is a view for explaining the operating principle of the boron dilution alarm system.

The boron dilution alarm system consists of two independent multiple systems that use two channels of active area neutron meter signals to ensure integrity.

In a circuit for converting all neutron pulse signals to analog voltage signals, a base voltage is generated by the power supply and the semiconductor component, and a base signal is generated by alpha rays / gamma rays from the reactor structure or burnt nuclear fuel. The base voltage of the analog voltage signal used by the boron dilution alarm system has a problem of delaying the time required to recognize the boron dilution accident.

As shown in Figs. 3A and 3B, since the base voltage of the analog voltage signal has a very large signal-to-noise ratio at low output, the average time of 1 minute is insufficient to remove the probabilistic perturbation of the signal. Probabilistic perturbation of the signal causes the operator to ignore it or not trust it at all because it generates inaccurate and frequent alarms. In order to avoid this, if the set filter time is increased (for example, approximately 5 minutes), the boron dilution alarm system is quickly responsive and delays the time taken to recognize it after the accident. 3A and 3B show neutron counts in a subcritical state.

Existing boron dilution alarm system does not inform the driver of any accident until the boron dilution accident occurs and the output reaches the operating range monitoring ratio (2.5 ~ 4.0), although the output reaches the operating range monitoring ratio. Even though only the alarm signal is generated, it does not give a continuous trend of changes in reactor characteristics and outputs before and after the alarm. Therefore, the operator cannot be provided with prompt and clear criteria for preliminary detection and future actions.

The current boron dilution alarm system has adopted a more stringent requirement in setting the alarm conditions 30 minutes before the alarm 15 minutes before the threshold reached in the operating modes 3, 4, 5, and the like.

As described above, in a low reactor subcritical state, a linear tendency pattern is obtained in which the output of the reactor gradually increases linearly at the beginning of the boron dilution accident. In addition, at low power, alarms are frequently generated due to the probabilistic perturbation of the neutron counting signal, which causes the operator to ignore or distrust them at all. Existing boron dilution alarm system, but can detect the state of the change in the neutron count rate is obvious, such as (2) Up Shift Pattern and (3) Down Shift Pattern of Figure 4, Early detection of the change in the neutron count rate having a linear trend pattern such as (4) Upward Trend Pattern and (5) Downward Trend Pattern of FIG. No alarm signal is given to the operator until the neutron counting rate reaches the maneuvering zone monitoring rate, which does not provide the operator with prompt and clear criteria for prior detection and future action. 4 shows the type of neutron count rate pattern in the subcritical state.

The present invention replaces the conventional method for monitoring the arrival of the monitoring range for the boron dilution accident, and continuously measures the neutron count rate and uses a cumulative sum of the deviations from the mean value of the neutron count rate. It allows for early detection of increases.

An object of the present invention is to provide a boron dilution accident alarm system and method capable of measuring neutron counts continuously and detecting an increase in neutron counts having a linear tendency pattern by using a cumulative deviation from an average value.

To this end, the present invention is an equipment for measuring neutrons leaking out of a reactor in proportion to the output of the reactor by using an neutron measuring instrument and a signal processing device.

A digital neutron count rate conversion device for measuring the TTL signal from the signal processing device and converting the number of measured TTL signals per second into a digital form and outputting the digital neutron counting factor;

An alarm computing device that accumulates and adds a deviation between the count rate of the number of measured TTL signals per second and the set average count rate converted by the digital neutron count rate converter, and outputs an operation signal when the cumulative sum exceeds the set limit value;

Provided is a boron dilution accident alarm system using a cumulative deviation sum management chart having an alarm means for generating a visual and auditory signal to recognize an inadvertent boron dilution accident upon receiving an operation signal from the alarm computing device.

Further, in the above system, the signal processing apparatus,

A preamplifier for providing a high voltage to the neutron measuring instrument and converting a charge signal into a voltage pulse signal;

An amplifier for amplifying the voltage pulse signal to facilitate signal processing;

And a pulse selector for removing a pulse signal below a voltage set to remove gamma and alpha line noise included in the voltage pulse signal, and generating a TTL signal corresponding to the passed pulse.

In the above system, the high voltage provided to the neutron meter is a voltage of 1,000 V or more.

According to the boron dilution accident alarm system using the cumulative deviation sum control chart of the present invention, when the neutron count rate signal exceeds the set range of the monitoring range, the alarm for the boron dilution accident is generated earlier than the conventional method of generating the neutron flux alarm. In addition, it is possible to prevent accidents such as continuous increase in output due to the increase in the reactivity of the reactor due to boron dilution in the coolant, and to improve the reliability of the boron dilution alarm system by eliminating frequent alarms. The boron dilution alarm is irrelevant to the sensitivity, allowing the selection of off-site instruments of various types.

Furthermore, the present task is to convert boron dilution which monitors boron dilution accident by converting the measured pulse signal into digital neutron counting rate in relation to nuclear reactor nuclear reactor monitoring and neutron counting rate measurement. Provide incident monitoring.

In addition, the present invention, as a method for detecting a boron dilution accident using a cumulative sum control chart,

1) measuring the neutron count rate y (t) from the activation channel;

2) calculating the deviation C _t by subtracting the reference neutron count rate k from the measured neutron count rate y (t),

3) accumulated deviation up to the previous step

Adding a new deviation to the, and

4) The present invention provides a method for detecting boron dilution accident using a cumulative sum control chart comprising determining whether or not the accumulated deviation value is out of a set range and detecting a boron dilution accident.

The present invention provides an alarm for a boron dilution accident earlier than the conventional method of generating a neutron flux alarm when the neutron count rate signal exceeds a starting range monitoring ratio setting value.

-By increasing the reactivity of the reactor due to boron dilution in the coolant, which is controlled by mechanical defects or human error of the boron control system or unplanned coolant, it is possible to prevent accidents in which the output increases continuously.

-It can improve the reliability of boron dilution alarm system by eliminating frequent alarms caused by stochastic perturbation of neutron count signal at low power.

-By generating boron dilution alarm irrespective of the sensitivity of the off-road instrument, it is possible to select the off-road instrument of various models.

-It will be able to secure technological advantage over other foreign competitors, such as satisfying the alarm generating condition that an alarm should be generated 30 minutes before the threshold is reached in the operation modes 3, 4, 5, and 6 required when exporting nuclear power plants overseas. It is expected.

1 is a view showing a change in output according to the approach from the critical state to the critical state.
2 is a view for explaining the principle of operation of the boron dilution alarm system.
3A and 3B are diagrams showing neutron counts in a subcritical state.
4 is a diagram showing the types of neutron count rate patterns in the subcritical state.
5 is a view showing a data fluctuation situation when a boron dilution accident occurs.
6 is a view showing the cumulative deviation change when the boron dilution accident occurs.
Figure 7 shows a boron dilution accident alarm system using the cumulative deviation sum management chart according to an embodiment of the present invention.

Hereinafter, the boron dilution accident alarm system and method using the cumulative deviation sum management chart according to the present invention will be described in detail with reference to the accompanying drawings.

Boron contained in the coolant is excellent in absorbing neutrons, and is used for the management of excess reaction during normal operation, and for suppressing nuclear fission chain reaction during nuclear reactor shutdown. Representative means for maintaining the subcriticality of the reactor at plant shutdown is the insertion of stop rods or excessive boron injection. In particular, during preventive shutdown for fuel reloading, the reactor is subcritical by only high concentrations of boron (2,500 ppm or more) without the insertion of stop rods. If a boron dilution accident occurs that the operator does not notice, the neutron absorption capacity is reduced by boron, and the output of the reactor gradually increases linearly. Starting channel non-neutron meters installed outside the reactor measure the number of neutrons leaking in proportion to the reactor output and display them in the main control room. At low subcritical states, the neutron counts vary widely from the mean and the probability of perturbation is very large. This means that the measured neutron count always fluctuates even if the reactor remains in the same state.

If a boron dilution accident occurs, it increases in a linear trend pattern. If y (t) is defined as the neutron count at time t, if there is no change in boron concentration, y (t) has an error x (t) with respect to the reference neutron count k, and x (t) is the standard normal distribution N. According to (0, 1). The data y (t) in a state where there is a linear tendency pattern due to boron dilution accident changes as shown in equation (1).

Here, sigma _x represents the standard deviation of the generated data, m is the slope of the linear trend pattern, and positive (negative) represents the upward (down) trend. Here, the larger the m, the greater the degree of boron dilution. In addition, t ₀ represents a linear trend starting point, and t represents a neutron count rate measurement time.

In the present invention, a cumulative sum management chart is used for early detection of boron dilution accident. The measuring method of the present invention is as follows.

1) Measure the neutron count rate y (t) from the activation channel.

2) Calculate the deviation C _t by subtracting the reference neutron count rate k from y (t).

3) accumulated deviation up to the previous step

Add a new deviation to

4) Detects boron dilution accident by judging whether the deviation value accumulated so far is out of the set section.

As an embodiment of the present invention, the cumulative deviation sum is managed as shown in FIG. 5 in the above order. If the reference neutron count rate is 10 cps (counts / second) and the boron dilution accident is started at 20 steps, it is assumed that the neutron count rate changes as shown in Equation (1). To generate. 5 shows data fluctuations when a boron dilution accident occurs.

Next, a change in cumulative deviation sum when a boron dilution accident occurs will be described with reference to FIG. 6.

6 is a view showing the cumulative deviation change when the boron dilution accident occurs.

When the data as shown in Fig. 6 is applied to the conventional method, the measured neutron count rate never exceeds the maneuvering area monitoring ratio (2.5 to 4.0) so that no alarm is generated. On the other hand, if the data as shown in Figure 6 is applied to the present invention, the boron dilution accident can be detected in only 27 steps, as shown by the red line graph. As described above, the boron dilution alarm system to which the present invention is applied has an advantage of detecting a boron dilution accident earlier than the conventional method even when the process average has a small variation.

Next, the boron dilution accident alarm system using the cumulative deviation control chart will be described with reference to FIG. 7.

7 is a view showing a boron dilution accident alarm system using the cumulative deviation sum management chart according to an embodiment of the present invention.

(A) Outside neutron measuring instrument (10)

The off-neutron neutron measuring instrument 10 is a device for measuring neutrons leaking out of the reactor in proportion to the output of the reactor. The neutron reacts with the measuring instrument to generate charge pulses.

(B) Signal processing device (20)

The signal processing device 20 is composed of a preamplifier, an amplifier, and a pulse selector. The preamplifier provides the neutron meter with a high voltage of more than 1,000V to convert the charge signal into a voltage pulse signal. The amplifier amplifies the voltage pulse signal to facilitate signal processing. The pulse selector removes a pulse signal below a voltage set to remove gamma and alpha line noise included in the signal, and generates a TTL signal corresponding to the passed pulse.

As shown in Figure 7, the boron dilution accident alarm system using the cumulative deviation sum management chart according to an embodiment of the present invention, the digital neutron count rate conversion device 30, the alarm calculation device 40 and the alarm lamp and the alarm speaker 50 It consists of.

(C) digital neutron counting rate converter 30

The digital neutron count rate converter 30 measures the TTL signal from the signal processing device 20, converts the number of TTL signals measured per second into a digital form, and sends them to the alarm computing device 40.

(D) Alarm calculation device (40)

The alarm calculating device 40 accumulates and adds a deviation between the count rate of the number of measured TTL signals per second and the set average count rate, which are converted by the digital neutron count rate converter 30, and when the cumulative sum exceeds the set limit value, the operation signal. Outputs the alarm light and the alarm speaker 50 to operate.

(E) Alarm lamps and loudspeakers (50)

The alarm lamp and the loudspeaker 50 receive the operation signal from the alarm computing device 40 and generate visual and audio signals to recognize the inadvertent boron dilution accident to the reactor operator.

According to the boron dilution accident alarm system using the cumulative deviation sum control chart of the present invention configured as described above, the neutron count rate is continuously measured and the increase in the neutron count rate having a linear trend pattern can be detected early by using the deviation accumulation with the mean value. Can be.

10 --- neutron measuring instrument
20 --- Signal Processing Equipment
30 --- digital neutron counting rate inverter
40 --- Alarm Unit
50 --- alarm lights and alarm speakers

Claims (5)

Equipment for measuring neutrons leaking out of the reactor in proportion to the output of the reactor by using an neutron measuring instrument and a signal processor.
A digital neutron count rate conversion device for measuring the TTL signal from the signal processing device and converting the number of measured TTL signals per second into a digital form and outputting the digital neutron counting factor;
An alarm computing device that accumulates and adds a deviation between the count rate of the number of measured TTL signals per second and the set average count rate converted by the digital neutron count rate converter, and outputs an operation signal when the cumulative sum exceeds the set limit value;
And an alarm means for generating a visual and auditory signal to recognize an inadvertent boron dilution accident upon receiving an operation signal from the alarm calculation device.
The method of claim 1, wherein the signal processing device,
A preamplifier for providing a high voltage to the neutron measuring instrument and converting a charge signal into a voltage pulse signal;
An amplifier for amplifying the voltage pulse signal to facilitate signal processing;
Using a cumulative deviation control chart, characterized in that it comprises a pulse selector for removing a pulse signal below a voltage set to remove gamma and alpha line noise included in the voltage pulse signal, and generating a TTL signal corresponding to the passed pulse. Boron dilution accident alarm system.
The boron dilution accident alarm system using a cumulative deviation control chart of claim 2, wherein the high voltage provided to the neutron meter is 1,000 V or more.
Monitoring of boron dilution accidents characterized by converting the measured pulse signal into digital neutron counting rate and monitoring boron dilution accident using the digital signal and alarm operation condition Way.
As a method for detecting a boron dilution accident using a cumulative sum control chart,
1) measuring the neutron count rate y (t) from the activation channel;
2) calculating the deviation C _t by subtracting the reference neutron count rate k from the measured neutron count rate y (t),
3) accumulated deviation up to the previous step

Adding a new deviation to the, and
4) A method for detecting a boron dilution accident using a cumulative sum control chart, characterized in that it comprises the step of detecting whether or not the accumulated deviation value is out of the set interval.

Images