JPS6214045B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a water supply control device for a nuclear reactor, and more particularly to a water supply control device suitable for controlling excessive rise in reactor water level when the recirculation flow rate of a nuclear reactor is significantly reduced.

Figure 1 shows the basic configuration of a nuclear reactor water supply control system. However, the conventional configuration excludes the portion surrounded by the broken line in FIG. Steam generated in the nuclear reactor is led to the main turbine 3 through the main steam pipe 2, generates mechanical energy, is condensed in the condenser 4, and returns to water. This water passes through the water supply pipe 5 and the water supply pump 6
The pressure is increased by the water, passes through the feed water heater 7 and the check valve 8, and returns to the reactor 1. There are usually multiple lines of the feed water pump 6, feed water heater 7, and check valve 8 shown in FIG. In addition, 18 is a recirculation pump, 19 is a main control valve, 20 is a bypass valve, and 21 is a relief valve.

Here, the method of controlling the water supply flow rate is as follows.
Low-pressure steam extracted from the main turbine 3 is supplied to a water supply turbine 16 via a turbine control valve 15 .
Since the water supply pump 6 is directly connected to the water supply turbine 16, by controlling the turbine control valve 15, the turbine rotational speed is changed and the water supply flow rate is adjusted.

The turbine control valve 15 is the turbine speed controller 14
The water level regulator 13 that controls this includes a water level detector 9 and a main steam flow rate detector 1.
0, three-element control is performed using three signals from the water supply flow rate detector 11. That is, the water supply flow rate is controlled by a signal obtained by adding a difference signal between the main steam flow rate and the water supply flow rate as a preceding signal to the water level deviation signal.

The purpose of this water supply control device is to control water level fluctuations that occur in the reactor 1 during power fluctuations such as those experienced during normal operation to within a target range.

Here, we will discuss the transient phenomenon that occurs when the recirculation flow rate of reactor cooling water (hereinafter simply referred to as recirculation flow rate) decreases significantly as a disturbance occurring in reactor 1, especially for boiling water reactors. explain.

In a boiling water reactor, steam bubbles exist in the cooling water that flows from the bottom to the top in the core (hereinafter referred to as core flow rate), and this void volume fraction remains at a constant value during steady operation due to the reactivity balance in the core. have here,
Decreasing the recirculation flow rate also reduces the core flow rate,
The number of voids in the reactor core increases transiently and the reactor water level increases, but on the other hand, the increase in void rate causes a negative reactivity feedback and the reactor power decreases.When the voids decrease again and the reactivity is balanced, the reactor power decreases. Settling. Power plants using boiling water reactors (hereinafter referred to as BWR plants) change their output due to fluctuations in recirculation flow rate in the normal operating range, but in response to the recently increasing demand for load-following operation of nuclear plants, Requires operation with large fluctuations in recirculation flow rate. FIG. 2 shows, as an example of the response of a BWR plant with a conventional feedwater control system, the responses of various parameters when the recirculation flow rate is significantly reduced in a ramp-like manner in response to a request for a large reduction in reactor output. The recirculation flow rate decreases with a slight response delay in response to the ramp-like decrease in the recirculation flow rate request signal, but as a result, the reactor power decreases and the main steam flow rate also decreases due to the above-mentioned in-core phenomenon. At this time, the feedwater flow rate is reduced by the three-element control due to the deviation between the main steam flow rate and the feedwater flow rate and the deviation between the reactor water level set value and the actual reactor water level, but the reactor water level is related to the amount of deviation between the main steam flow rate and the feedwater flow rate. Due to the increase in voids in the core, which does not occur, the feed water flow rate increases significantly even though the degree of decrease is greater than the main steam flow rate, and there is very little margin for the turbine trip set value at a high water level. This phenomenon of reactor water level rise occurs in almost the same way when the recirculation pump trips, but
In fact, the rate of decrease in the recirculation flow rate is much greater than during normal load following operation, and the amount of decrease is also significant, resulting in a greater rise in the reactor water level. If the turbine trip water level reaches the set value, the main turbine and generator will trip and the reactor will also scram, so there is no problem in terms of safety, but the plant operating rate will decrease and there will be economic loss. become.

The above-mentioned phenomenon occurs when the recirculation flow rate decreases, but almost the opposite phenomenon occurs when the recirculation flow rate increases. That is, when the recirculation flow rate increases, the core voids decrease and the reactor power increases, and as the recirculation flow rate becomes constant, the core voids increase again and the reactor power stabilizes when it increases. At this time, the reactor water level decreases due to a transient decrease in voids. However, the difference between the normal water point and the low water scram set point is set to be relatively larger than the difference between the normal water point and the high water turbine trip set point, so it is considered less dangerous than when the recirculation flow is reduced. I can say that lowering the water level is not desirable.

The present invention solves the above-mentioned disadvantages, improves the reactor water level control characteristics when the recirculation flow rate fluctuates significantly, and provides water supply control that prevents the occurrence of turbine trips and reactor scrams due to large deviations from the normal operating water level. The goal is to provide equipment.

The key point of the present invention is to add the change in the reactor water level corresponding to the amount of change in voids in the core estimated from the rate of change in the recirculation flow rate to the reactor water level set point and modify the actual reactor water level request signal. The objective is to produce a rapid response in the feedwater flow rate to maintain the reactor water level at the normal water level.

A specific embodiment of the present invention is implemented by adding a reactor water level set point regulator 17, which is surrounded by a dashed line, to the conventional nuclear reactor water control system shown in FIG.
The recirculation flow rate detector 22 detects the recirculation flow rate, and the reactor water level set point regulator 17 calculates and outputs the rate of change of this recirculation flow rate. Since it is known that the amount of increase in voids in the reactor core during transient periods is approximately proportional to the rate of decrease in recirculation flow rate, the output of the reactor water level set point regulator 17 adjusts the amount of change in the reactor water level that occurs when the recirculation flow rate fluctuates. This is the estimated value, and the output value that is the result of comparing it with the output of the reactor water level setting device 12 becomes the actual reactor water level setting request signal that has been modified to take into account and compensate for the disturbance that occurs in the reactor water level. .

FIG. 3 shows a more specific embodiment of the present invention,
An embodiment is shown which is technically developed in all its details, including the reactor water level set point regulator 17. This embodiment is basically the same as that in FIG. 1, but there are some differences. The first is that there are four main steam pipes 2, and therefore the main steam flow rate for all four main steam pipes is being determined, and the second is that the water supply pipe 5
There are two water supply pipes, and therefore the water supply flow rate for all two water supply pipes is calculated. In addition, although not shown, there may be three series of water level detectors 9. In this case, one of the two streams will be selectively used.

Now, in FIG. 3, the main steam flow rate detector 10 for one pipe detects the flow rate as a pressure ΔP (actually a current I corresponding to ΔP), and this is passed through the square root calculator 23. Therefore, pressure-flow rate conversion is performed to detect the main steam flow rate. The main steam flow rate for one pipe thus obtained is input to the adder 25. The main steam flow rates for the remaining three main steam pipes obtained in the same manner as above are also applied to the adder 25 as an input 24. As a result, the total main steam flow rate is obtained as the output of the adder 25.

On the other hand, the water supply flow rate detector 1 is connected to the water supply pipe 5.
1, a pressure corresponding to the water supply flow rate is obtained, and by applying this pressure to the square root calculator 26, pressure-flow rate conversion is performed to obtain the water supply flow rate flowing in the water supply pipe. In the adder 28, the water supply flow rate 27 regarding the remaining water supply pipes obtained in the same manner as above is applied together with the output of the square root calculator 26, and the adder 28
Now, by adding both, the total amount of water supply flow rate is calculated.

Adder 29 calculates the deviation of the outputs of both adders 25 and 28. The deviation between these outputs becomes one input of the adder 30. The output of the water level detector 9 is the other input to this adder 30, and the adder 30 calculates the deviation between the two. The output of this adder 30 becomes an element for controlling three elements (water level, flowing water flow rate, and main steam flow rate). The switch 33 is a selection switch for inputting the detected water level 32 of the water level detector 9 or the three-element output 31 of the adder 30, and when the output is low such as at startup, the output 32
is selected, and output 31 is selected during steady operation. The signal obtained via switch 33 is input to adder 41. The other input of this adder 41 is the signal 37 obtained via the reactor water level set point regulator 17, which is the key point of the present invention. The calculation result of the adder 41 is subjected to PI calculation by the PI calculation unit 42, and is outputted as a water supply flow rate request signal.

Next, the reactor water level set point regulator 17 will be described. The regulator 17 includes a differential calculator 34 and a signal limiter 35. The differential calculator 34 takes in the recirculation flow rate signal 38, performs incomplete differential calculations, and outputs the result, and the signal limiter 35 cuts out minute signals in steady operating conditions, and also sets upper and lower limit signal limits to cut out excessive signals. Execute and output. here,
Appropriate values can be obtained for the time constant T and gain K of the differential calculator 34 and the signal cut setting value of the signal limiter 35 through simulation analysis and actual machine testing. The output of the signal limiter 35 is added to the normal water level setting signal 39 in an adder 36 and output as a corrected water level setting signal 37. The corrected water level setting signal 37 is set to 1 by the adder 41 of the water level controller 13.
During element control, a 1-element control signal equal to the actual water level detection signal is compared with a 3-element control signal equal to the actual water level detection signal plus the main steam flow rate and feed water flow rate difference signal during 3-element control, and the PI calculator 42
After proportional/integral calculations are performed, it is output as a water supply flow rate request signal.

FIG. 4 is a diagram showing an embodiment in which the reactor water level set point regulator 17 is realized by an electronic circuit. Differential calculator 34
are the DC amplifier A ₀ , the feedback resistor R _f , the resistor R ₁ ,
It consists of a capacitor C ₁ for differentiation, and a resistor R ₀ whose one end is grounded and the other end is connected to the ground input of the amplifier A ₀ . The signal limiter 35 has an input resistance
R ₂ , resistor R ₃ , limit Zener diodes Z ₁ , Z ₂ provided between the connection point of resistors R ₂ and R ₃ and ground, diodes D ₁ , D ₂ , and output of resistor R ₃ It consists of Zener diodes Z ₃ and Z ₄ provided on the sides. Further, the adder 36 includes a DC amplifier A ₀₁ , input resistors R ₄ , R ₅ , R ₀₁ , and a feedback resistor R _f1 .
In the above circuit configuration, R _f /R ₁ =K, C ₁ R ₁ =T, and R ₄ =R ₅ =R _f0 . Here, K is a gain and T is a time constant, which correspond to K and T in the transfer function KTS/1+TS of the differential calculator 34 described in FIG. Since the explanation of the operation of the above circuit configuration is simple, it will be omitted.

The above regulator 17 is just one example; for example, the time constant T and gain K may be optimally changed depending on the reactor output state using a computer program, or the function shape of the signal limiter may be continuously set. It is possible.

FIG. 5 shows the response when the recirculation flow rate is significantly changed according to the embodiment described above. Due to the ramp-like reduction request of the recirculation flow rate request signal, the recirculation flow rate follows and decreases with a slight delay, but since the corrected reactor water level setting signal 37 incorporates the rate of change of the recirculation flow rate decrease, the recirculation flow rate decreases as shown in the figure. It is decreasing temporarily. As a result,
A rapid and large reduction in feed water flow rate is achieved, and therefore the rise in reactor water level is significantly smaller than the response of the conventional control system shown in FIG. Figure 5 shows the response of each parameter when the recirculation flow rate is requested to decrease in a ramp-like manner, that is, when the plant output is requested to decrease in a ramp-like manner. Good reactor water level control characteristics can be achieved even under disturbances that involve large fluctuations in the circulating flow rate.

According to the present invention, the operability of plant operation is improved and unnecessary print trips are avoided, so that it can contribute to improving the operating rate.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of the reactor water supply control system, Figure 2
The figure is a response diagram of main parameters when the recirculation flow rate is significantly reduced according to the conventional technology, Figure 3 is a detailed configuration diagram of the feed water control device of the present invention, and Figure 4 is the reactor water level attached to the feed water control device of the present invention. FIG. 5, which is an embodiment of the electronic circuit of the set point regulator, is a response diagram of the main parameters when the recirculation flow rate is significantly reduced in a nuclear reactor using the feed water control device according to the present invention. DESCRIPTION OF SYMBOLS 1... Nuclear reactor, 13... Water level controller, 17... Reactor water level setter regulator, 22... Recirculation flow rate detector.

Claims (1)

[Scope of Claims] 1. In a reactor water supply control device equipped with a water level controller for water level adjustment and configured to control the water supply to the reactor of a nuclear power plant, means for capturing the recirculation flow rate and detecting its rate of change over time; and an input for adjusting the water level regulator to control the rise or fall of the reactor water level set point depending on the polarity of the rate of change. A water supply control device for a nuclear reactor, comprising means for taking in a water level set point according to the rate of change as an element; 2. The water level regulator incorporates, in addition to the water level setting point, three elements consisting of the main steam flow rate, feed water flow rate, and reactor water level, or one element of the reactor water level as an input element for adjustment. Water supply control device.