JPS62228980A - Safety monitor device for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention is intended to predict nuclear and thermal-hydraulic instability in the core of a nuclear reactor such as a boiling water reactor, and to take appropriate measures. The present invention relates to a nuclear reactor stability monitoring device that provides an operating method.

(Prior art) It is generally known that nuclear or thermo-hydraulic instability is likely to be induced in boiling water reactors at low flow rates and high outputs, such as during natural circulation of coolant. .

Nuclear instability is so-called core stability, and is caused by nuclear feedback effects due to void reactivity effects within the reactor core.As the coolant flow rate decreases, the reactor power It is known that the higher the value, the more likely it is to occur and the more unstable it becomes.

On the other hand, thermo-hydraulic stability is generally known as channel stability, and cooling is caused by changes in pressure loss in the gas-liquid two-phase flow due to increases and decreases in voids in the coolant flow path formed in the reactor core. This refers to instability in the flow of material that causes vibrations. Regarding thermo-hydraulic stability, ie channel stability, as well as core stability, the lower the coolant flow rate and the higher the reactor power, the more likely it is to occur and become unstable.

The mechanism of generating core and channel stability is very complex, but in current boiling water reactors, the thermal-hydraulic conditions and nuclear characteristics of the core are manually calculated, and various core parameters are calculated manually. The impact on stability has been considered and reflected in the design. It is known that the lower the coolant flow rate and the higher the reactor output, the more unstable the reactor core cold flow or reactor power (thermal output) becomes, so a deterioration in stability is predicted. In other words, in current boiling water reactors, sufficient attention is paid to operation at low flow rates and high outputs.

By the way, core and channel stability, which is the stability of a nuclear reactor, is determined not only by the core flow of coolant and the reactor output, but also by nuclear characteristics such as void reactivity coefficient, the shape of the fuel channel, and the radial and axial directions of the core. It depends on many parameters such as the power distribution of the fuel and the heat transfer characteristics of the fuel rods. In current nuclear reactor design, these parameters are analyzed in advance using values that conservatively and strictly estimate stability, taking into account fluctuations associated with core combustion. From this analysis value, we predict the region of reactor stability that is poor, and pay attention to operations that approach this region.

Figure 6 shows the power flow m control curve of a boiling water reactor. ,
This is a region where stability is predicted to deteriorate based on the analysis values described above, and care must be taken when operating the reactor in this region.

However, the driving caution area C determined in this way is
It is based on conservative predictions for various parameters, that is, strict predictions that predict the stability of the reactor, and even if the reactor operation reaches the cautionary region in reality, it will immediately become unstable. It's not a translation.

Considering the stability of a nuclear reactor, conservatively predicting an operational caution area will actually hinder the flexibility of reactor operation. There was a problem in that the degree of freedom in furnace operation was restricted.

(Problem to be solved by the invention) The operational caution areas for maintaining the stability of the nuclear reactor have been predicted in an excessively conservative manner and have been predicted in a strict and conservative manner, eliminating the flexibility of the reactor operation. It was becoming an obstacle.

The present invention has been made in consideration of the above-mentioned circumstances, and therefore, from the viewpoint of nuclear reactor stability and its operability, it is possible to improve the stability of a nuclear reactor by providing an appropriate operational caution area in order to maintain the stability of the nuclear reactor. The purpose is to provide a monitoring device.

[Structure of the invention]

(Means for Solving the Problems) The stability monitoring device for a nuclear reactor according to the present invention combines a stability monitoring device that judges the stability of a nuclear reactor with a control rod drive control system that controls the operation of control rods, and a nuclear reactor stability monitoring device that determines the stability of a nuclear reactor. The stability monitoring device is connected to a recirculation flow rate control system that controls the drive of the recirculation pump of the reactor recirculation system, and the stability monitoring device monitors the distortion of the axial power distribution using the core flow rate of coolant and the reactor power as parameters. At the same time, this axial power distribution distortion tolerance value is compared with the actually measured axial power distribution distortion degree of the reactor core, and the control rod drive control system J3 and the recirculation flow rate control system are operated. It is controlled.

(Operation) The present invention considers the axial power distribution of the reactor core, in addition to the conventional core flow rate and reactor power, as parameters that adversely affect the stability of the reactor (core stability and channel stability). Instead of operating the reactor while paying attention to the stability of the reactor by controlling only the core flow m and the reactor power as in the past, the axial power distribution of the reactor core is monitored using a stability monitoring device. The measured degree of distortion of the axial power distribution is compared with the allowable value of the degree of distortion of the axial power distribution set using the core flow G and reactor output as parameters, and the reactor is designed so that the stability of the reactor does not deteriorate. It is designed to allow you to drive.

(Example) Hereinafter, an example of the present invention will be described with reference to the accompanying drawings.

FIG. 2 shows a boiling water reactor 1 equipped with a reactor stability monitoring device according to the present invention.
A reactor pressure vessel 2 accommodates a reactor core 3 configured by loading a large number of four-piece fuel assemblies (not shown). Control rods 4 are provided in the reactor core 3 so as to be freely removable and removable by a control rod drive control system (control rod drive enclosure and control rod drive hydraulic device) 5 . Coolant 6 guided inside the reactor core 3
is forced to circulate through the reactor recirculation system 7.

The reactor recirculation system 7 includes a recirculation loop piping 8 , and a recirculation pump 9 is provided on the recirculation loop piping 8 .

When the recirculating water discharged from the recirculating pump 9 is injected from the jet pump 10, it is guided from the lower part of the reactor core to the reactor core 4, drawing in surrounding water. The reactor recirculation system 7 may include an in-reactor recirculation pump (internal pump) in the downcomer portion of the reactor pressure vessel 2 instead of the external loop system.

The recirculation pump 9 of the reactor recirculation system 7 has its pump speed controlled by a recirculation outflow control system 12 . This recirculation vlr
? A control signal is input to the i-control system 12 and the control rod drive control system 5 from the stability monitoring device 15. The stability monitoring device 15 inputs the core flow rate signal @A of the coolant flowing through the reactor core 3, the reactor output signal B1, and the reactor axial power distribution signal C, and controls the reactor based on these input signals. The stability (core stability and channel stability) is evaluated, and if instability of the reactor is predicted, an alarm is issued by an alarm device (not shown), and the control rod drive control system 5 and recirculation outflow control system 12 outputs a control rod withdrawal prevention signal and a recirculation flow rate reduction prevention signal E, respectively.

By the way, the nuclear reactor stability monitoring device 15 monitors the axial power distribution of the nuclear reactor core to monitor the stability of the nuclear reactor. This stability monitoring device 15 uses a simple index that indicates the formation of a bottom peak in the axial power distribution of the nuclear reactor. This index includes the axial power distribution distortion tolerance f, which is preset with the core flow rate A and reactor power B as parameters, and the axial power distribution distortion degree γ, which is obtained through actual calculations. .

Then, when the actual degree of distortion γ of the axial output distribution is within a range that does not exceed the preset tolerance value f of the degree of distortion of the axial output distribution, this is judged by a judgment device such as a small computer, and as shown in FIG. If the reactor continues to operate as shown in Figure 2, and if exceeded, the alarm system will issue an alarm indicating deterioration of reactor stability, or the control rod drive control system 5 will receive a signal to prevent control system withdrawal or recirculation outflow. The control system 12 outputs a recirculation flow rate reduction prevention signal E to maintain the stability of the reactor.

Further, the degree of distortion γ of the axial power distribution of the nuclear reactor is determined as follows.

For example, as shown in Figure 3, the core of a nuclear reactor is axially (
This method divides the axial output distribution into multiple nodes (in the height direction) and uses the axial output distribution calculation results from the process meter to determine the output integral amount up to a certain height of the axial output distribution. The degree of distortion γ of the output distribution can be determined. When the power distribution is expressed by dividing the reactor core 3 into 24 nodes in the axial direction, this degree of distortion γ is expressed as an integral amount of the height from the lower end of the core in the axial direction to the 3rd to 5th nodes.
m: 3 to 5 nodes P(i): Relative output at J3 to i-node. this(
It has been known from conventional analysis and experience that the degree of distortion γ of the power distribution in the io direction according to equation 1) can relatively well represent the degree of distortion of the power distribution in the axial direction of the core.

On the other hand, as a method for more easily determining the degree of distortion γ of the axial power distribution of the nuclear reactor, it is possible to use the LPRM signal. In general, the LPRM of a boiling water reactor (BWR)
(Local power range monitoring system) A from below in the axial direction of the core,
A neutron detector is placed at each level of B, C and D,
Local output is determined by this neutron detector, and the degree of distortion γ of the axial output distribution can be determined from the output signal from this neutron detector. That is, the degree of distortion γ of the axial output distribution is as follows: (2) However, P(i): is expressed as a value at the i (A, B, C, D) level of LPRM.

In addition, the allowable distortion degree f of the axial power distribution of the reactor is determined by preliminary analysis at various operating points a3 of the reactor power Q and core flow mW, and to what extent the axial power distribution can be distorted. It is set whether or not it is permissible, and this can be expressed by a function f (Q, W) using reactor power Q and core flow rate W as parameters.

In other words, f=f (Q, W) (3
).

After that, the skewness γ of the axial power distribution of the reactor is compared with the allowable skewness value of the axial power distribution. Calculation of the allowable value f can be easily performed using a small calculator.

Next, the relationship between the axial power distribution of a nuclear reactor and the stability of the reactor will be explained.

The axial power distribution in the reactor core, especially the axial distribution along the axis of the fuel assembly, affects reactor stability, as well as reactor power and core flow, as well as core stability and channel stability. has a major impact on

Generally, the axial power distribution of a nuclear reactor is distorted downward by lυ, and the further the power peaking moves downward (bottom peaking), the worse the stability of the reactor becomes. This is because the more the axial power distribution is distorted downward, the more the point where voids start to occur moves downwards, the more voids are generated, and the nuclear and thermo-hydraulic feedback effects due to changes in voids become stronger. It is.

On the other hand, in boiling water reactors, the fuel heating section of the reactor core is divided into 24 nodes along the axial direction, and the axial power distribution is determined from the output of each node to analyze the stability of the reactor. There is. For example, Figure 4 shows the relationship between reactor stability and peak node position, and from this figure,
As the nodes in the axial direction move upward, it can be seen that the reduction ratio becomes smaller.The reduction ratio, which is an index of reactor stability, is
g: As shown in Figure 55, the ratio of the first amplitude X and the second amplitude x 2 of the response to the step disturbance is x 27 x 1,
The larger the attenuation ratio, the worse the attenuation, and it can be said that the stability of the reactor is not good.

Therefore, as shown in Figure 4, there is a close relationship between the bottom peaking of the axial power distribution and the deterioration of reactor stability. Ji is connected to monitoring the stability of the nuclear reactor.

Incidentally, the axial power distribution of a nuclear reactor changes depending on the design of the nuclear fuel in the reactor core, the insertion angle of the control shaft, the burnup, etc. In conventional nuclear reactor plants, reactor stability analysis is performed assuming the most severe power distribution, taking into account nuclear fuel design, control rod insertion amount, burnup, etc., and predicting areas where stability will deteriorate. Ta. However, the stability monitoring device according to the present invention uses realistic values that have been actually measured for the axial power distribution of the reactor 4, so that the stability prediction of the reactor can be made realistically. As a result, there is no need to set operational caution areas in anticipation of excessively strict maintainability, and the quick drying caution area, which requires attention to reactor stability, is significantly reduced, making it possible to improve reactor operability. can.

〔Effect of the invention〕

As described above, in the present invention, the axial power distribution of the reactor core is constantly monitored during the operation of the nuclear reactor, and the reactor can be operated in such a way that the stability of the reactor is not compromised. It is no longer necessary to set a strict operational caution area for reactor stability in advance as in the past, and it is possible to significantly reduce or eliminate the stability caution area, so the operating range of a nuclear reactor can be It becomes wider, the degree of driving margin (degree of freedom) increases, and driving performance can be improved.

[Brief explanation of drawings]

FIG. 1 shows approach 1! which shows an embodiment of the nuclear reactor stability monitoring device according to the present invention! - Fig. 2 is a diagram showing a boiling water reactor plant equipped with the above-mentioned stability monitoring device, Fig. 3 is a diagram showing the relationship between the axial position of the LPRM and the axial power distribution, and Fig. 4 is a diagram showing the axial power distribution of the LPRM. A graph showing the general relationship between the axial node position of the reactor core and the width reduction ratio. Figure 5 is an explanatory diagram of the width reduction ratio with respect to reactor stability. Figure 6 is the power flow of a boiling water reactor. It is a figure showing a control curve and a driving caution area. 1... Boiling water reactor, 2... Reactor pressure vessel, 3
... Reactor core, 4... Control rod, 5... Control rod drive control system, 7... Reactor recirculation system, 9... Recirculation pump,
12... Recirculation flow rate control system, 15... Stability monitoring device, A... Core outflow signal, B... Reactor output signal,
C: Axial power distribution signal, D: Control rod withdrawal prevention signal, E: Recirculation flow ■reduction prevention signal. Representative patent attorney Nori Chika Ken Yudo Mitsumata Hirofumi No. I Fig. 1000 2 Fig. LPRM Hitsuji 3 Inyou 4- Kano Yubara 5 Fig. F Jino (2, Flow rate ('/a)
100 Figure 6

Claims (1)

[Claims] 1. A stability monitoring device that determines the stability of the reactor, a control rod drive control system that controls the operation of control rods, and a recirculation system that controls the drive of a recirculation pump in the reactor recirculation system. The stability monitoring device is connected to the flow rate control system, and sets the allowable skewness of the axial power distribution using the coolant core flow rate and the reactor output as parameters, and also sets the allowable axial power distribution distortion. A stability monitoring device for a nuclear reactor, characterized in that the control rod drive control system and the recirculation flow rate control system are controlled by comparing the value with the actually measured axial power distribution distortion degree of the reactor core. 2. When the axial power distribution distortion exceeds the axial power distribution distortion tolerance, the stability monitoring device sends a control rod withdrawal prevention signal and a recirculation flow rate reduction signal to the control rod drive control system and operating flow control system. The nuclear reactor stability monitoring device according to claim 1, wherein the device is configured to output each blocking signal. 3. The stability monitoring device is connected to an alarm device, and the alarm device is set to output an alarm signal when the axial output distribution distortion degree exceeds the axial output distribution distortion degree tolerance value. A stability monitoring device for a nuclear reactor according to paragraph 1.