JPH0718952B2 - Power control method for boiling water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial field of application) The present invention provides an emergency shutdown (hereinafter referred to as scrum) of a boiling water reactor when the core flow rate is rapidly reduced. The present invention relates to a power control method for a boiling water reactor, which maintains the health and safety of the plant as well as the health of the fuel.

(Prior Art) A boiling water reactor (hereinafter referred to as BWR) is generally configured as follows. That is, the reactor core in the reactor pressure vessel,
A steam separator, a steam dryer, and a coolant are contained, and the core is composed of a plurality of fuel assemblies, control rods, and the like. The coolant flows upward in the core, and at that time, the temperature rises due to the nuclear reaction heat of the core and becomes a two-phase flow state of water and steam. The coolant in the two-phase flow state is separated by the steam separator into water and steam. The steam thus separated is dried by a steam dryer to become dry steam, which is transferred to a turbine through a main steam pipe and used for power generation there. On the other hand, water is supplied to the lower part of the core in a state where it flows down to the downcomer section and is mixed with feedwater. After that, it again flows upward in the core. Repeat the same cycle below.

In such a configuration, core power control is performed by controlling the position of the control rod and controlling the flow rate of the coolant passing through the core. The control of the coolant flow rate is performed by a recirculation system. This recirculation system is composed of a jet pump installed inside the reactor pressure vessel, a recirculation pump installed outside the reactor pressure vessel, and a recirculation system piping arranged between these pumps. .

Further, in recent years, in order to simplify the structure of the recirculation system, a so-called internal pump type boiling water reactor in which a recirculation pump is installed in a reactor pressure vessel has been developed. This is a system in which multiple small capacity unit pumps (hereinafter referred to as internal pumps) are installed in a reactor pressure vessel so as to surround the core. The inertia constant of each of these internal pumps is very small. Therefore, when the internal pump is stopped for some reason, the coolant flow rate sharply decreases. On the other hand, the amount of heat generated in the core does not immediately follow the coolant flow rate, but there is a certain time delay. This time delay is generally about 6 seconds. Therefore, when the internal pump is stopped and the flow rate of the coolant sharply decreases, the ability to cool the fuel rods decreases, the boiling transition of the coolant around the fuel rods, and the fuel rods of the fuel assembly. Induces a temperature rise in the cladding of
A fuel rod damage accident is expected.

Therefore, in recent BWR plants, the heat output is calculated from the value of the neutron flux detected by the local power monitor in the core, and the reactor is scrammed when the calculated heat output exceeds a preset predetermined value. Control is in place. However, the above set values are determined assuming that the heat output suddenly increases during the rated operation of the reactor, and the protective function is expected even when the coolant suddenly decreases when the internal pump is stopped as described above. It was only possible under limited driving conditions.

(Problems to be Solved by the Invention) The present invention has been made on the basis of the above points, and an object of the present invention is to provide a fuel rod having a cooling capacity insufficient due to insufficient cooling capacity when the coolant flow rate in the core is rapidly reduced. An object of the present invention is to provide a power control method for a boiling water reactor, which makes it possible to prevent an increase in temperature.

[Configuration of Invention] (Means for Solving Problems) That is, the power control method for a boiling water reactor according to the present invention detects the temporal change rate of the deviation between the heat flux amount of the core and the coolant flow rate. It is determined whether or not the temporal change rate exceeds a preset set value, and a scrum signal is output when the temporal change rate exceeds a preset set value. It is what

(Operation) In other words, it monitors the coolant flow rate, detects that it has decreased sharply, and causes the reactor to scram, thereby preventing loss of fuel integrity due to insufficient cooling capacity. It is a thing.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 4. FIG. 1 is a configuration diagram in which the power control method according to the present embodiment is applied to a BWR plant, and a coolant 2 and a core 3 are housed in a reactor pressure vessel 1. Core 3 above
Is composed of a plurality of fuel assemblies (not shown), control rods and the like. The control rod is controlled by a control rod drive mechanism 4 installed below the control rod. That is, the control rod drive mechanism 4 controls the output of the core 3 by inserting and pulling out the control rod into and from the core 3. The control rod drive mechanism 4 is connected to the control rod position control device 5. Further, a plurality of internal pumps 6 are installed at the outer peripheral position of the core 3 at equal intervals in the circumferential direction (only two are shown in the drawing). Reference numeral 6a in the figure denotes a pump motor of the internal pump 6. In the case of recent large capacity plants,
For example, in the 1300 MWe class, about 10 internal pumps 6 and about 200 control rods are installed.

A flow rate detector 8 is installed below the reactor pressure vessel 1 via an instrumentation pipe 7. The flow rate detector 8 detects the coolant flow rate. A local output monitor 9 is installed in the core 3, and the local monitor 9 detects the neutron flux in each part of the core 3. The output of the local output monitor 9 is input to the heat flux calculator 10 and converted into the heat flux signal W ₀ by the heat flux calculator 10. This heat flux signal
W ₀ is input to the converter 12 of the deviation calculator 11. The deviation calculator 11 includes this converter 12, another converter 13 and a subtractor 14.
It consists of and. The heat flux calculator 10 has a function of converting a neutron flux indicating the degree of nuclear reaction of fuel into an actual amount of heat, and is usually configured with a primary delay of about 6 seconds in consideration of the heat transfer time constant of fuel. Has been done. On the other hand, the flow rate signal Q ₀ which is the detection signal of the flow rate detector 8 is also the deviation calculator.
It is input to the converter 13 of 11. In the deviation calculator 11, the heat flux signal Q ₀ and the flow rate signal W ₀ are standardized by the converters 12 and 13 as values with the amount during rated operation as 100%. These standardized signals W and Q are obtained by the above subtractor 14
Is subtracted (W-Q) and output to the differentiator 15. The differentiator 15 differentiates the above (W−Q) with respect to time to calculate a temporal change rate, and outputs it as a signal R to the determiner 16. The determiner 16 compares the input signal R with a preset value S and outputs a scrum signal S16 to the control rod position control device 5 when the signal R is larger than S. The above set value S
Is set to, for example, (30% / sec) with a allowance for the temporal change rate (50% / sec) at which the coolant 2 may actually transition to boiling transition.

The operation will be described based on the above configuration. First, when the plant is in the normal operation state, the heat flux W and the flow rate Q are
It is in the state shown in the figure. FIG. 2 is a characteristic diagram showing the relationship between the flow rate Q on the horizontal axis and the heat flux W on the vertical axis. As shown in FIG. 2, the distribution state of the control rod positions is a parameter. Is in a substantially linear proportional state. Even when the flow rate is adjusted, the rate of change is about several percent / second, and the rate of change in the value of (WQ) with time, in other words, the input signal R to the judging device 16 is sufficiently small. is there.
Therefore, the signal R never exceeds the set value S.

Next, a case will be described in which the coolant flow rate sharply decreases due to the stop of the internal pump 6. Ie the first 10
In the 0% operation state, when all the internal pumps 6 are stopped at time 0, the flow rate Q sharply decreases as shown in FIG. It will be about 20% of the circulating flow rate. On the other hand, the heat flux W does not decrease sharply like the above-mentioned flow rate Q because of the heat transfer time constant, but decreases relatively gently. At this time, the judging device
The input signal R to 16 immediately increases and exceeds the set value S as shown in FIG. 3 (B). Result judge 16
The scrum signal s16 is output. The scrum signal S16
As a result, the control rod is urgently inserted into the core 3, and as a result, a negative reactivity is added to the core 3 and the gradually decreasing heat flux W rapidly decreases as shown in FIG. 3 (A). To go.
Further, in such a process, the temperature (T) of the cladding of the fuel rod becomes as shown in FIG. 3 (C). That is, the temperature T rapidly increases at time 0 along with the rapid decrease of the coolant flow rate. However, after that, the rising speed slows down due to the effect of scrum, and after reaching the maximum temperature in a few seconds, it is cooled. On the other hand, as shown by the alternate long and short dash line in FIGS. 3 (A) and 3 (C), in the conventional case, the heat flux W does not sharply decrease, and the temperature T of the cladding increases without being suppressed. I will do it.

Next, a case where the internal pump 6 is stopped in the other operating state than the normal operating state will be described with reference to FIG. At the initial stage, as shown in FIG.
The heat flux W is 100% and the flow rate Q is 80%.
When some of the internal pumps 6 stop, the flow rate Q sharply decreases. That is, the flow rate Q rapidly decreases at time (0) and reaches about 40% in about 1 second. On the other hand, the heat flux W starts to decrease gradually. As a result, the output signal R to the determiner 16 rapidly increases at time 0 and exceeds the set value S. Then, the scrum signal S16 is output. The effect of the scrum appears after 1 second as the scrum, the heat flux W sharply decreases as shown in FIG. 4 (A), and the cladding temperature T of the fuel rod of the fuel rod as shown in FIG. 4 (C) decreases. The rise is suppressed. On the other hand, if the present embodiment is not applied under the same conditions, the decrease of the heat flux W is gradual, as shown by the alternate long and short dash line in FIGS. 4 (A) and 4 (C). The tube temperature T continues to rise and rises to about 1200 ° C.

According to this embodiment, the following effects can be obtained. That is, even if all or some of the internal pumps 6 are stopped and the flow rate of the coolant suddenly decreases, the rate of change in the deviation between the heat flux W and the coolant flow rate Q (signal Since R) is monitored and the scrum signal S16 is output when it exceeds the set value S, it is possible to suppress the rise of the heat flux and to decrease it rapidly. Therefore, an increase in the temperature T of the cladding of the fuel rod can be effectively suppressed and can be suppressed to about 650 ° C. Therefore, it is possible to prevent various problems due to the temperature rise of the cladding tube, and also to maintain the soundness of the entire plant.

Next, a second embodiment will be described with reference to FIGS. 5 and 6. This embodiment is intended to monitor and control only the coolant flow rate by utilizing the slow change of the heat flux W. FIG. 5 is a configuration diagram showing a state in which the present embodiment is applied to a BWR plant. The same parts as those in the first embodiment are designated by the same reference numerals and detailed description thereof will be omitted. The flow rate signal Q ₀ from the flow rate detector 8 is input to the converter 17. The input flow rate signal Q ₀ is converted by the converter 17 and output to the differentiator 15 as a standardized signal Q. The differentiator 15 time-differentiates the input signal Q to calculate a temporal decrease rate and outputs it as a signal R ₁ to the determiner 16. The determiner 16 compares the input signal R ₁ with a preset value S ₁ and if the signal R ₁ exceeds the preset value S ₁ , sends a scrum signal S 16 to the control rod position control device 5. Output. The set value S ₁ is set to be several percent / second larger than the set value S in the first embodiment. This is for controlling without considering the heat flux W.

The operation will be described based on the above configuration. First, the case where the reactor is in a normal operation state will be described. As shown in FIG. 6 (A), the coolant flow rate Q and the heat flux W at the initial stage are
It is 100%, and it is assumed that all the internal pumps 6 are stopped at time (0) in this state. The coolant flow rate Q sharply decreases to about 20% in about 1 second. At this time, the determiner 16
The input signal R ₁ to the signal sharply increases and exceeds the set value S ₁ as shown in FIG. 6 (B). As a result, the scrum signal S16 is output to the control rod position control device 5. At this time, the above input signal
The time required for R ₁ to reach the set value S ₁ is substantially the same as in the case of the first embodiment, as shown in FIG. 6 (B). This is because the set value S ₁ is a value that takes into account the decrease in the heat flux W with respect to the set value S. Due to the output of the scrum signal S16, the heat flux W starts to rapidly decrease from about 1 second as shown in FIG. 6 (A). At that time, the temperature T of the cladding of the fuel rod changes as shown in FIG. 6 (C). That is, the flow rate of the coolant rapidly increases as the coolant flow rate rapidly decreases at time 0. However, after that, the rising speed becomes pure due to the effect of scrum, and after reaching the maximum temperature in a few seconds, it is cooled.

Further, the above-mentioned action is similar not only during normal operation but also when the internal pump 6 is stopped under other operating conditions,
By outputting a scram signal S16 when it exceeds the set value S _1, rapidly reduces the heat flux W, it is possible to effectively suppress an increase in the temperature T of the cladding of the fuel rods.

The present invention is not limited to the above-mentioned two embodiments, and for example, the lack of cooling capacity may be detected by the amount of decrease of the coolant within a certain period of time. Alternatively, a rapid decrease in the coolant flow rate may be detected by monitoring the rotation speed of the internal pump.

[Effects of the Invention] As described in detail above, according to the power control method for a boiling water reactor according to the present invention, even if the coolant flow rate suddenly decreases, the rise in heat flux is regulated to reduce the fuel consumption. It is possible to suppress the temperature rise of the rod cladding tube. Therefore, not only the soundness of the fuel can be maintained, but also the soundness of the plant can be maintained and the safety can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 to FIG. 4 are views showing an embodiment of the present invention.
Fig. 2 is a block diagram showing the implementation of the output control method, Fig. 2 is a characteristic diagram showing the relationship between the coolant flow rate and heat flux, and Fig. 3 is the coolant flow rate and heat flux, input signal to the judgment device, and coating. Fig. 4 is a characteristic diagram showing the time change of the pipe temperature, Fig. 4 is a characteristic diagram showing the time change of the coolant flow rate and heat flux in different states, the input signal to the judging device, and the cladding pipe temperature, Fig. 5 and Fig. 6 is the second
FIG. 5 is a configuration diagram showing the implementation of the output control method, and FIG. 6 is a characteristic showing the time change of the coolant flow rate and heat flux, the input signal to the judging device, and the cladding temperature. It is a figure. 1 ... Reactor pressure vessel, 2 ... Coolant, 3 ... Reactor core, 4
...... Control rod drive mechanism, 5 ...... Control rod position control device, 6 ...
… Internal pump, 8 …… Flow rate detector, 9 …… Local output monitor, 10 …… Heat flux calculator, 11 …… Deviation calculator,
15 ... Differentiator, 16 ... Judge.

Claims (1)

[Claims] 1. In a boiling water reactor, a temporal change rate of a deviation between a heat flux amount of a core and a coolant flow rate is detected, and whether or not the temporal change rate exceeds a preset value. The method for controlling the output of a boiling water reactor is characterized in that a scrum signal is output when the temporal change rate exceeds a preset set value.