JPH022982A - Nuclear reactor protecting method - Google Patents

Abstract

PURPOSE:To contrive the accuracy of a local output peaking protection by finding output peaking coefficient from the output of a predeterminated area and the average output of a nuclear reactor and modifying a scram preset value so as not to exceed the thermal limit of fuel. CONSTITUTION:An area output detecting circuit 21 average-operates the output of neutron detectors 3a-3d of the same area and the same power source to find the output of each local area. A total output detecting circuit 23 average- operates local area output to find total output signal value 31. Further high value selecting circuit 23 finds maximum output value 32 and both the values are divided by a divider 24 to find maximum output peaking coefficient. A function generator 25 modifies a scram preset value 24 according to the increase and the decrease of the output peaking coefficient by the thermal limit of fuel. When the preset value 34 is compared with a signal 31 by a comparator 28 and drops down to the same value of the signal 31, a scram signal is output to a nuclear reactor protecting device. Further, when the preset value 34 is lowered by multiplying the preset value 34 by a gain 29, compared with the signal 31 by the comparator 30 and drops down to the same value of the signal 31, a control rod pulling stop command is output.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a nuclear reactor in a nuclear power plant, and in particular detects and suppresses a rise in the local power peaking level of a nuclear reactor. The present invention relates to a nuclear reactor protection system and its device suitable for ensuring safety and plant operation reliability.

[Conventional technology]

In order to protect a nuclear reactor, it is necessary to constantly monitor the reactor output, detect abnormal transient changes or malfunctions that may impair the safety of the reactor, and appropriately operate the reactor shutdown system. By doing so, it is necessary to suppress the generation of excessive reactor power and prevent damage to the fuel cladding.

As described in JP-A-61-172096, conventional equipment divides a nuclear reactor into four regions in a cross pattern and monitors the output in each region to detect abnormalities in the power distribution between each region. It was detected and protective actions were taken to protect the reactor. That is, the thermal output of each region is determined by a region output monitoring device (hereinafter referred to as RPM) that calculates the average of local thermal neutron flux signals from neutron detectors in that region of the reactor, and this R
The PM was configured as shown below to perform protection operations.

(1) When monitoring four areas independently, each area is equipped with four RPMs (16 in total), and each two sets are configured to be independent from each other, so that the output signal of the RPM is set according to the predetermined scram setting. When the reactor protection system logic circuit exceeds the value and operates 1 out of 2 twice, a scram signal is sent. Thereby, a protective operation is performed against a partial increase in output within each region.

(2) When monitoring two areas independently, such as in the case of a loss of coolant accident, 1. The second area, or the third area. Four RPMs (total 8) are provided for each fourth region, and if the output signal of the RPM exceeds a predetermined scram setting value, 2 out of 4
When the reactor protection system logic circuit operates, a scram signal is sent. As a result, a protective operation is performed against a partial increase in output within the two regions.

Further, the scram setting value is fixed regardless of the distortion of the power distribution of the reactor.

[Problem to be solved by the invention]

The above conventional technology divides the reactor core into four regions, monitors the average value of the neutron flux level in each region, and
The structure was designed to protect the reactor and control output, and the scram setting value was fixed, so no consideration was given to protection against local output peaking. Therefore, there were the following problems.

(1) Even if the reactor output increases locally due to incorrect withdrawal of a control rod, the RPM output signal will not rise to the scram setting value because the neutron flux level is averaged.
Protective operation does not occur.

Therefore, there is a risk that the fuel cladding tube near the pulled out control rod may be damaged. During normal operation, the control rods are pulled out in a predetermined order to avoid misalignment of the control rods.
Considering operator error, single malfunction of equipment, etc., 1
It is also necessary to consider the phenomenon in which the control rods of books are continuously pulled out.

(2) When the control rod is pulled out, as the temperature near the control rod increases,
The moderator's neutron flux absorption cross section is reduced and a positive moderator temperature reactivity is introduced. As a result, in a nuclear reactor with a relatively small design margin for safety, even if other output coefficients are negative, an event occurs in which the output locally continues to increase after control rod withdrawal is stopped. Since the RPM output signal does not rise to the set value as in (1) above, there is a risk that the fuel cladding near the withdrawn control rod may be damaged. The figure shows the phenomenon in which the output locally increases due to moderator temperature reactivity.

An object of the present invention is to provide a nuclear reactor protection system and its device that properly protects a nuclear reactor against local power peaking and has high safety and operational reliability.

[Means to solve the problem]

The above purpose is to subdivide the range monitored by RPM into areas where bypassing (not using) the neutron detector will have no adverse effects, creating a configuration in which output from each area can be obtained, and providing one of the following means, or a combination thereof. This is achieved by performing a protective operation.

(1) The relationship between the power peaking coefficient ((maximum reactor power)/(average pressure of the reactor)), which represents the distortion of the power distribution of the reactor, and the limit power is determined from the thermal limitations of the fuel as shown in Figure 3. It is predetermined as shown in .

In other words, by focusing on the fact that when the peaking coefficient is large, the reactor's limit output is small, and when the peaking coefficient is small, the reactor's limit output is large, the output for each region and the total output are determined, and the ratio of the two is taken to calculate the output for each region. Means for determining the output peaking coefficient for each output peaking coefficient, and means for increasing or decreasing the scram set value of the reactor in accordance with the coefficient, that is, the increase or decrease in strain are provided.

(2) After control rod withdrawal is stopped, the relationship between the local power increase due to moderator temperature reactivity and the power peaking coefficient is determined in advance from the reactor characteristics as shown in Figure 4. . In other words, when the output peaking coefficient is small, the increase in output when control rod withdrawal is stopped is small, but
Focusing on the fact that as the output peaking coefficient increases, the amount of increase in output after the control rod stops being pulled out increases, and the method for determining the output peaking coefficient and the increase or decrease in the coefficient, the output after the control rod stops being pulled out is calculated. A means will be provided to increase or decrease the reactor scram setting value in consideration of the increase.

[Effect]

The above technical means takes into account the power increase due to the thermal limitations of the fuel or the temperature reactivity of the moderator, and adjusts the scram setting value of the reactor according to the local power peaking coefficient, that is, the local distortion of the power distribution. change. Therefore, even if one control rod is continuously pulled out due to erroneous control, etc., and local output peaking occurs, the scram set value will decrease and the output peaking level will decrease, as shown in Figure 5. To ensure safety by reliably outputting a scram signal and performing a protective operation before the fuel reaches its allowable design limit.

In addition, the above technical means, in a control rod withdrawal event,
In order to avoid reactor scrams as much as possible and improve plant operational reliability, it is also applied as a means to change the control rod withdrawal stop level setting and early alarm setting according to the local output peaking coefficient. can do.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. In FIG. 2, a control rod control device 4 operates a control rod drive mechanism 5 to drive the control rods in the reactor 1 up and down to adjust the output of the reactor.

The control rods 2 are arranged almost uniformly within the reactor, and are inserted one by one or several at a time by manual operation by the operator.

Can be extracted. Further, the poison concentration control device 8 increases or decreases the poison concentration within the reactor via the moderator circulation loop 9, and adjusts the output of the reactor. Neutron detectors 3 in the nuclear reactor 1 are arranged substantially uniformly in the radial and axial directions of the reactor, and output signals proportional to the surrounding neutron flux. The power supply for this detector is equally divided into four systems A to D, similar to the reactor protection device 7.

The neutron flux measurement device 6 constantly monitors the output of the reactor based on the signal from the neutron detector 3, and when it detects abnormal transient changes or malfunctions that may impair the safety of the reactor, controls the control rods. A control extraction stop command is output to the device 5. In addition, in the reactor protection device 7, which outputs the scram signal, when the reactor protection system logic circuit is activated by the scram signal, the electromagnetic clutch in the control rod drive mechanism 5 is disengaged, and the control rod 2 is dropped. and scram the reactor.

Note that in order to accurately understand the power distribution of a nuclear reactor and improve safety, it is desirable to be able to monitor the power in as narrow a range as possible. In this embodiment, a configuration is adopted in which each area surrounded by four radial neutron detectors is monitored. This is a region that is as narrow as possible within a range where the allowable number of bypasses (non-use) of neutron detectors in one region is similar to a predetermined number, and the power supply configuration of the reactor protection system and It is determined by considering the logic circuit configuration, etc.

Next, the operation of the neutron flux meter i1+1J device 6 will be explained with reference to FIG. The area output detection circuit 21 is in the same area and
The outputs of the neutron detectors 3a to 3d of the same power source are averaged to determine the output of each local area. The total output detection circuit 23 averages the local region outputs to obtain an average output value 31. Further, the high value selection circuit 22 calculates the maximum output value 32 and divides both by 1 by the divider 24, and the maximum output peaking coefficient 33
seek.

The function generator 25 changes the scram set value in accordance with an increase or decrease in the output peaking coefficient based on the relationship shown in FIG. 4 due to thermal limitations of the fuel. The relationship generator 26 is based on the relationship shown in FIG. The scram set value will be lowered by the amount of output increase due to later moderator temperature reactivity. This scram setting value 34 is compared with the total output signal 31 by the comparator 28, and the total output signal 3
When the value drops to the same value as 1, the reactor protection device 7 outputs a scram signal as shown in FIG.

Also, the scram set value 34 is multiplied by the gain 29, the set value is lowered, and the total output signal 34 is compared with the comparator 30, and when this set value has decreased to the same value as the total output signal 34, a control rod withdrawal stop command is issued. It is configured to output. This results in
According to this embodiment, local output peaking can be accurately detected, control rod withdrawal can be stopped or scram operation can be performed, and the operational reliability and safety of the plant can be improved. Significantly improved.

〔Effect of the invention〕

According to the present invention, there are the following effects.

(1) Accurately detect local power peaking in the reactor and perform scram according to the peaking coefficient.

By adding the function to change the set value, even if a local power peak occurs, it can be reliably scrammed before the fuel reaches its allowable design limit, significantly improving safety.

(2) By adding a function that accurately detects local power peaking in the reactor and changes the control rod withdrawal stop level according to the peaking coefficient, even if local peaking occurs, By shutting down the control rods and keeping them at a low level, reducing the probability of scram occurrence, the operational reliability of the plant was significantly improved.

(3) Changing the scram set value etc. according to the output peaking coefficient takes into account both the thermal limitations of the fuel and the influence of moderator temperature reactivity, so it has a wide range of applications in atomic couplers.

[Brief explanation of the drawing]

Figures 1 and 2 are detailed explanatory diagrams of the present invention, Figure 3 is an explanatory diagram of the relationship between the power peaking coefficient and the reactor output, and Figure 4 is a diagram showing the relationship between the power peaking coefficient and the output increase value after the control rods are stopped. FIG. 5 is a detailed explanatory diagram of the present invention, and FIG. 6 is an explanatory diagram of the phenomenon of output increase due to moderator temperature reactivity. 21... Area output detection circuit, 22... High value selection circuit, 23... Full output detection circuit, 24... Divider, 25
...Function generator, 26...Function generator, 27...
Decelerator, 28... Comparator, 29... Gain, 30.
...Comparator, 31...Full output signal, 32...Maximum area output signal, 33...Output peaking coefficient, 34...
・Scrum setting values. 2nd cause 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. In a nuclear reactor in which the output of the reactor is controlled by adjusting the position of control rods in the reactor core, the output peaking coefficient, that is, the output An atomic bomb characterized by having means for determining distortion of the distribution, and having means for changing the scram set value of the reactor in response to a change in the output peaking coefficient so as not to exceed the thermal limit of the fuel. Furnace protection method. 2. It is characterized by having means for changing the set value of the control rod withdrawal stop level or the alarm level so that the output in a predetermined area of the reactor does not exceed the scram set value in claim 1. Reactor protection method. 3. In a nuclear reactor where the output of the reactor is controlled by adjusting the position of control rods in the reactor core and adjusting the concentration of liquid poison contained in the moderator, the output in a predetermined range and the means for determining the output peaking coefficient, that is, the distortion of the output distribution from the average output of
A nuclear reactor protection system characterized by having either or both of the following means (a) and (b). (a) Means for changing the scram set value of the reactor in response to the change in the power peaking coefficient so as not to exceed the thermal limit of the fuel. (b) Means for changing the scram set value in accordance with the output peaking coefficient, taking into account the increase in output after control rod withdrawal stops due to moderator temperature reactivity. 4. It is characterized by having means for changing the set value of the control rod withdrawal stop level or the alarm level so that the output in a predetermined area of the reactor does not exceed the scram set value in claim 3. Reactor protection method.