JPS6244633B2 - - Google Patents

Description

Detailed Description of the Invention The present invention relates to a heavy water-moderated nuclear reactor that uses heavy water as a moderator and is equipped with a large number of control rods and a device for controlling the concentration of liquid poison in heavy water. This paper relates to a control rod and a method for controlling liquid poison concentration at low power to keep the coolant void reactivity coefficient below a certain value in order to improve control characteristics, and a device therefor.

We will discuss conventional methods of operating a heavy water reactor, especially how to operate the control rods at low power and how to control the liquid poison concentration.

In a heavy water-moderated boiling light water-cooled nuclear reactor, the change in reactivity associated with the change in the void volume fraction of the coolant, that is, the value of the coolant void coefficient, has a large influence on the control characteristics of the reactor. For example, if we consider the case where the void coefficient shows a positive value, if we increase the reactor power, we will increase the void volume fraction in the coolant, positive reactivity will be introduced, and the reactor power will become more and more This creates a vicious cycle of rising prices. In order to suppress this, the control rods must be operated skillfully. However, in the range of 5% to 20% of the furnace output, special care must be taken in operation. There are two reasons for this; the first is that the lower the output, the more positive the coolant void coefficient becomes. Figure 1 shows typical changes in coolant void reactivity in the equilibrium core of a 60 MWe class heavy water-moderated boiling light water-cooled reactor. The slope of this curve, that is, the change in reactivity per 1% change in void ratio (%ΔK/
K) is the coolant void coefficient. From the change in the slope of this curve, the smaller the void volume fraction, that is, the lower the furnace output, the larger the positive value of the void coefficient becomes. Therefore, the lower the power, the more positive the void coefficient, and the worse the control characteristics of the furnace.

Second, the lower the output, the greater the change in void volume fraction per change in output. FIG. 2 is a diagram showing the relationship between the steam weight fraction of the coolant and the void volume fraction.
Since steam weight rate and reactor power are directly proportional,
From FIG. 2, the lower the furnace power, the larger the change in void volume fraction per change in power. As a result, since the void coefficient is positive, the lower the power, the greater the positive reactivity is input, and the control characteristics of the furnace deteriorate.

For the two reasons explained above, it was necessary to be very careful when operating the reactor in the low power range, and it was necessary to insert and withdraw the control rods while monitoring the reactor power sufficiently. Therefore, at low power, there was a high possibility of scram, leading to a decrease in reactor operating efficiency.

The operating method of a raw water reactor in the prior art, especially the control rod operation method when increasing the reactor output, and the heavy water poison method will be briefly explained. FIG. 3 shows changes in the number of inserted control rods and the poison concentration in heavy water when the output increases in the conventional example. In this method, the output is increased by withdrawing the control rods until ~50% output, and from ~50% output to 100% output, heavy water poison (boron) is used to increase the output.
Increase output by removing concentration. but,
In this method, the boron concentration in heavy water is high up to low output (0% to 50%), and as a result, as shown in Figure 4, the higher the boron concentration, the larger the void coefficient becomes on the positive side. The void coefficient at low power increasingly shows a large positive value, and the control characteristics at low power deteriorate, leading to a decrease in the operating rate of the furnace and a decrease in the safety of the furnace.

The purpose of the present invention is to eliminate the drawbacks of the prior art described above and to provide a nuclear reactor capable of improving the reactor's operating rate and safety by improving the reactor self-control characteristics at low power. To provide driving methods and equipment.

The present invention is characterized by adopting the following reactor operation in order to improve the reactor self-control characteristics at low power. Since the cause of poor self-control characteristics at low output is the positive void coefficient, let's consider an operating method that makes the void coefficient negative.

When examining the properties of the void coefficient using a computer-based reactor physical analysis, we found that as the boron concentration in heavy water increases, the void coefficient tends to move toward a larger positive value, as shown in Figure 4. As shown in the figure, the void coefficient tends to move more toward the positive side as the number of control rods inserted (control rod reactivity value) decreases. The graph in Figure 4 is displayed with a boron concentration of 0 and the coordinate origin set to a void coefficient of 0,0 with a constant number of inserted control rods, and the graph in Figure 5 is displayed with a constant boron concentration. In this figure, the inserted control rod reactivity value 0 is displayed with the coordinates adjusted to the relative void reactivity coefficient 0,0, and especially in Figure 5, the reactivity value and the number of control rods inserted are shown. are in a proportionally corresponding relationship as shown on the horizontal axis of the graph in FIG. For this reason, at low output (0% to 50% output)
As shown in Figure 3, reducing the number of inserted control rods to about 7 while maintaining a high boron concentration as shown in Figure 3 is possible if the void coefficient is set in advance to be negative at extremely low output. This is dangerous because the void coefficient tends to move more towards the positive side, and if the void coefficient set in advance is close to the positive/negative boundary, it will shift significantly towards the positive side. For example, as in the conventional example of the line connecting the circles in Figure 7, ( ⁺⁵ to +2) Sexually unfavorable. For this reason, it is clear from the standpoint of reactor safety that it is preferable to lower the boron concentration during low power and to increase the number of control rods inserted as much as possible. From the above, we conclude that in order to operate with a more negative void coefficient at low output, it is desirable to reduce the boron concentration in heavy water as much as possible and instead insert as many control rods as possible. can.

FIG. 6 shows the operating method from reactor startup to 100% output according to the present invention. As can be seen from Figure 6, unlike the conventional operation method in Figure 3, the boron concentration in heavy water is removed at ~5% output (high temperature standby state) just before voids occur in the coolant, and instead, Insert multiple control rods. Thereafter, the control rods are gradually withdrawn and the output is increased until a suitable high output (~50% output) where self-control becomes better. By adopting this operating method, the void coefficient from 5% to 40% power becomes significantly negative compared to the conventional example, as shown in FIG. 7, and the reactor self-control characteristics at low power are improved. As a result, the availability and safety of the reactor are improved.

Next, a specific example will be described.

FIG. 8 is a sectional view of the heavy water-moderated nuclear reactor of the present invention. FIG. 9 is an embodiment of a heavy water poison concentration control device and a control rod insertion device at low output of a heavy water moderated nuclear reactor according to the present invention. First, the reactor is started by pulling out the starting control rod 2 shown in FIG. 9, and reaches a high temperature standby state at about 5% output, and the pressure of the coolant becomes the operating pressure and operating temperature. At outputs above this state, voids occur in the coolant, and if the void coefficient is positive, increasing the output in the low output range requires careful attention to the stability of the furnace. In the method according to the present invention, a stable increase in output is achieved by making the void coefficient negative by controlling the control rods and controlling the boron concentration in heavy water as shown in FIG. In order to easily operate the control rods and the boron concentration in heavy water, we devised a reactor control system having the configuration shown in Figure 9. The operating principle of the control device of the present invention will be explained using FIG.

When the void coefficient is positive in the power increase in the low power region, as shown in FIG.
A steam drum 11 is placed in contact with the fuel assembly 7 in the pressure pipe.
The water level in the steam drum 11, which is circulated to the reactor core side again by the pump 10, may fluctuate and exceed the drum water level fluctuation range set based on safety design etc., which may cause the reactor shutdown system to operate. , the furnace will stop, reducing the operating rate of the furnace. Therefore, in the apparatus of the present invention, the allowable steam drum water level fluctuation range (gives two set points of high water level L _H and low water level L _L ) is input to the steam drum water level fluctuation range setter 22, and the steam drum The allowable void coefficient calculator 23 calculates the allowable void coefficient using the following formula from the steam drum water level L _N just before the start of void generation (high temperature standby state) sent from the steam drum water level signal generator 21 that detects the water level in the steam drum. Calculated by.

(∂k/∂v) _L = α×Max (|L _N −L _H |or|L _N −L _L |) ...(1) Here, α is the drum water level fluctuation according to the plant dynamic characteristic analysis. It can be easily obtained by calculating and arranging the relationship between the width and the void coefficient. Also, (∂k/∂v) _L means the allowable void coefficient.

This allowable void coefficient is determined by the control rod insertion amount calculator 24.
and the signal L _N1 of FIGS. 4 and 5 and 21
and L _N2 , (∂k/∂v) _N = β | (L _N1 −L _H2 ) | ...(2) Also calculate the void coefficient actual value (∂k/∂v) _N Calculate the number of control rods and poison concentration so that the allowable void coefficient (∂k/∂v) _L is obtained.

Here, β is a constant calculated by plant dynamic characteristic analysis, and L _N1 and L _N2 are steam drum water levels at different outputs. In addition, when calculating the number of inserted control rods and the poison concentration, the current number of control rods sent from the control rod insertion position signal generator 19 and the current poison concentration sent from the poison concentration measuring device 30 are used. It is used when using FIGS. 4 and 5. The determined number of control rods to be inserted is sent to the control rod drive signal generator 25, which sends an instruction to insert the control rods according to the memorized insertion order to the control rod drive device 20, and the predetermined number of control rods to be inserted is sent to the control rod drive device 20. The control rod is inserted into the reactor until the end. On the other hand, the determined poison concentration is also sent from 24 to the poison removal signal generator 26. Then, the amount of heavy water that passes through the poison removal tower 13 provided in the flow path in which the heavy water 8 in the reactor core 5 is circulated by the pump 15 to reach the concentration is calculated, and the removal tower operation time is calculated from there. , by sending the signal to the poison removal tower flow rate regulator 27 and opening the poison removal valve 28 only for the necessary time, the heavy water 8 is transferred to the poison removal tower 1.
3 to remove poison, and the removed heavy water flows into the reactor core 5.

Next, when entering high-output operation from approximately 50% output to 100% output, control rods 2 and 3 are removed and the number of control rods inserted is reduced from the previous low output, following the graph curve in Figure 6. Instead, the poison injection valve 29 is opened and a liquid containing dissolved poison is injected from the poison dissolution tank into the flow path where the heavy water 8 circulates, increasing the poison concentration. Thereafter, as the output changes to 100%, the poison concentration in the heavy water is gradually reduced using the poison removal tower 13 as in the conventional manner, and the output reaches 100%.

By using the control device of the present invention based on the operating principle as described above, operation as shown in FIG. 6 becomes possible.

Here, the output when the recirculation flow rate is varied by ~2 times at 40% output when operating according to the conventional operation method shown in Figure 3 and when using the operation method according to the present invention shown in Figure 6. Calculating the fluctuations and the fluctuations in the steam drum water level, the results are shown in Figure 10, and it is clear that the fluctuations in the furnace output and the fluctuations in the steam drum water level are smaller when the operating method according to the present invention is adopted. Recognize.

As described above, by adopting the operating method and device of the present invention, the void coefficient is improved, and as a result, fluctuations in reactor output when power increases and fluctuations in steam drum water level are reduced, resulting in a stable nuclear reactor. Control becomes possible, improving furnace operation rate and safety.

The effects produced by the heavy water reactor operating method of the present invention are summarized below.

(1) By improving the void coefficient, output fluctuations become smaller, furnace control becomes easier, and furnace operation rate improves.

(2) By improving the void coefficient, fluctuations in the steam drum water level will be reduced and the safety of the furnace will be improved.

[Brief explanation of the drawing]

Figure 1 is a general example showing the change in coolant void reactivity in a heavy water-moderated boiling light water-cooled reactor, Figure 2 is a diagram showing the relationship between the steam weight fraction of the coolant and the void volume fraction, and Figure 3 is a conventional example. Fig. 4 shows the relationship between the boron concentration in heavy water and the void coefficient, and Fig. 5 shows how to control the insertion control Figure 6 is a diagram showing the relationship between the number of rods and the void coefficient. Figure 6 is a diagram showing the method of controlling the boron concentration in heavy water and the number of control rods inserted when increasing the output of the heavy water reactor of the present invention. Figure 7 is a diagram showing the relationship between the conventional and the present invention. A diagram showing a comparison of the relationship between void coefficient and output depending on the operating method, Figure 8 is a cross-sectional view of the reactor core to explain the operating method of the heavy water reactor of the present invention, and Figure 9 is a diagram showing the operating method of the present invention. Figure 10 is a diagram for explaining the operating principle of the control device to achieve this, and shows the fluctuations in the reactor output and steam drum water level when operating with the conventional method and the method of the present invention and varying the recirculation flow rate. FIG. 2... Starting control rod, 11... Steam drum, 1
3... Poison removal tower, 17... Control rod drive signal generator, 19... Control rod insertion position signal generator, 2
0...Control rod drive device, 21...Steam drum water level signal generator, 22...Steam drum water level dynamic range setting device, 23...Allowable void coefficient calculator, 24...Control rod insertion amount calculator, 25... ...Control rod drive signal generator, 26...Poison removal signal generator, 27...
Poison removal tower flow rate regulator, 28...Poison removal valve, 30...Poison concentration measuring device, 31...Poid coefficient calculator.

Claims (1)

[Scope of Claims] 1. When the heavy water reactor is started and passes from a high temperature standby state to a low power operating state and enters a high power operating state, the high power operating state lowers the poison concentration in the heavy water of the heavy water reactor to the above level. In the method of operating a heavy water reactor, the heavy water reactor is operated in a state in which heavy water is gradually reduced by passing through a poison removal tower and the number of control rods inserted in the heavy water reactor is smaller than the number inserted in the low power operating state. In a standby state, the heavy water is passed through the poison removal tower to reduce the poison concentration in the heavy water, and instead, the control rods of the heavy water reactor are inserted into the reactor core to reduce reactor coolant void reactivity during low power operation. A method for operating a heavy water reactor, characterized in that the coefficient is set to a negative state, and after the low power operation period, poison is injected into the heavy water to once increase the poison concentration in the heavy water and the high power operation state is entered. . 2. A cooling water circulation channel that circulates reactor cooling water to the reactor core via a steam drum, a heavy water circulation channel that circulates heavy water in the reactor, and a connection to the heavy water circulation channel via a flow rate adjustment valve. In the heavy water reactor, the heavy water reactor is composed of a poison removal tower having a structure including a poison removal tower, a poison injection means connected to the heavy water circulation flow path, and a plurality of control rods that are freely insertable into the core of the nuclear reactor by a plurality of control rod drive devices. a steam drum water level signal generator that detects the water level of the steam drum; an allowable void coefficient calculator that receives an output signal from the steam drum water level signal generator to calculate an allowable void coefficient; a void coefficient calculator that calculates the measured value of the void coefficient based on the output signal of the above-mentioned void coefficient calculator, and receives the output results of the allowable void coefficient calculator and the void coefficient calculator to determine the number of inserted control rods and the poison concentration that become the allowable void coefficient. a control rod insertion amount calculator for calculating, and a control rod drive that receives a signal of the number of control rods to be inserted from the control rod insertion amount calculator and outputs an instruction signal for inserting the control rods into the control rod insertion number core. a signal generator; a poison removal signal generator that receives the poison concentration signal from the control rod insertion amount calculator and calculates the opening operation time of the flow rate adjustment valve necessary to reach the poison concentration; The control rod drive device accepts the output result of the control rod drive signal generator and inserts the control rods into the reactor core in proportion to the number of control rods to be inserted; An operating device for a heavy water reactor, comprising a reactor control device having a poison removal tower flow rate regulator that opens the flow rate regulating valve for operating time.