JPS6224759B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power control device for a nuclear reactor, particularly in a boiling water reactor (hereinafter referred to as a BWR reactor), which is used to control the abnormal power output of a nuclear reactor when the reactor steam load is cut off. This invention relates to a nuclear reactor output control device that prevents nuclear reactors from occurring and safely controls the reactor.

In a BWR reactor, reactor power is controlled by increasing or decreasing the recirculation flow rate and changing the amount of voids in the reactor core. That is, if the amount of voids in the reactor core increases, the effect of reactor water as a moderator will decrease, and the reactor output will decrease, and conversely, if the amount of voids decreases, the reactor output will increase. In a BWR reactor that uses this type of control, when the pressure is constant, the reactor output is self-regulating, but as the core pressure increases, the voids in the reactor collapse, resulting in a positive Reactivity is applied and reactor power increases. During abnormal transients with pressure increases, such as main steam load shedding, the output increases due to the increase in the positive reactivity of the reactor, and the increase in heat flux from the fuel rods causes the fuel rods to reach the tolerance level. The user will experience a change in output as described above, which may cause fuel damage. Damage to fuel rods is a problem from the standpoint of environmental safety and the availability of nuclear power plants. In current nuclear reactors, in the case of main steam system load shedding, etc., in order to rapidly apply negative reactivity in response to an abnormal increase in output, it is necessary to improve the control rod insertion characteristics and increase the recirculation flow rate. It has been considered to use a so-called recirculation pump trip system to provide rapid reduction. However, rapid insertion of control rods to improve their properties has limitations due to their mechanical operation, and methods for rapidly reducing recirculation flow rate reduce the flow rate due to the inertia of the fluid and pump. The time constant for this is as long as 5 to 6 seconds, and the combination of these methods is not necessarily sufficient to safely suppress the abnormal output rise due to this transient phenomenon.

The present invention has been made in view of the above-mentioned points, and its purpose is to, in addition to the conventional slam and recirculation pump trip system, A two-phase flow with the same composition as the high-temperature, high-pressure coolant is rapidly injected into the reactor core from outside the reactor, and as the amount of voids inside the reactor increases, negative reactivity is rapidly applied to safely shut down the reactor. The objective is to obtain an output control device that can

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a system using the output control device according to the present invention.
This is a schematic configuration diagram of a BWR reactor, main steam system, water supply system, and recirculation system. Steam generated in the reactor vessel 1 passes through a main steam pipe 3 and a main steam stop valve 4 and enters a steam turbine 5, which rotates a generator directly connected to the steam turbine 5 to generate electricity. Steam exiting the steam turbine 5 is condensed in a condenser 6, heated to a constant temperature in a feed water heater 7, and returned to the reactor via a feed water spargeer 9 provided in the reactor vessel 1 via a feed water pipe 8. It can be done. Normally, the output of a nuclear reactor is controlled by controlling the core flow rate by changing the speed of the recirculation pump 11 and the driving flow rate of the fluid flowing in the recirculation pipe 10 using the recirculation flow rate control device 13. On the other hand, to suppress the output increase in the event of an abnormality such as main steam load cutoff, the valve position of the main steam stop valve 4 is detected and the control rod drive mechanism 15 is operated by a signal output from the scram signal generation circuit 18. This is done by rapidly inserting the control rods 16 into the reactor core 2. Separately, the main steam stop valve position signal also causes the pump trip signal generation circuit 17 to generate a recirculation pump trip signal, which stops the pump motor 12 and rapidly reduces the core flow rate. Open the release valve 21,
A two-phase flow of saturated steam and water is rapidly injected from the pressure heater 20 in which high-temperature, high-pressure steam and water are kept through guide pipes 22B and 22C and into a fixed header 23 provided in the reactor core. In this way, the control device according to the present invention includes, in addition to the conventional control device, a pressurizing heater 20 that stores high-temperature, high-pressure steam and water, a quick-release valve 21 that is always closed, and a guide pipe 22. It is composed of an annular fixed header 23 having many holes 24 as shown in FIG. 2 as an example.
The pressure heater 20 includes a one-way valve 19 and a guide pipe 22.
It is connected to the water supply pipe 8 via A and is supplied with steam.

Next, the operation of the control device according to the present invention will be explained. The fixed header 23 for two-phase flow jetting of steam and water installed at the bottom of the reactor core is intended to suppress abnormal power output of the reactor and safely control the reactor when the reactor load is cut off. be. That is, when the load is cut off, the main steam stop valve 4 is rapidly closed, so the reactor pressure rises rapidly, the voids in the reactor core collapse and decrease, the reactivity of the reactor rises rapidly, and the reactor output also rises rapidly. At this time, in response to the closing signal of the main steam stop valve 4, a scram signal and a recirculation pump trip signal are issued, the reactor is scrammed, and the recirculation pump 11 is stopped with a certain time constant. According to the present invention, when this recirculation pump trip signal is generated, the quick release valve 21 is opened by this signal, and the recirculation pump is passed through the guide pipes 22B and 22C to the fixed header 23 provided at the lower part of the reactor core.
By ejecting a two-phase flow of high-temperature, high-pressure steam and water and generating voids within the reactor core, negative reactivity due to the voids can be applied and abnormal increases in reactor output can be suppressed. Note that the above-mentioned high-temperature, high-pressure steam and water need to be pressurized to at least the set pressure at the time of reactor steam load cutoff (for example, the set pressure of the main steam relief safety valve). This is because in the event of a reactor steam load shedding accident, the main steam relief safety valve opens when the pressure inside the reactor vessel exceeds the set pressure, so it corresponds to the maximum pressure inside the reactor vessel.

Here, a feature of the present invention is that a two-phase flow of steam and water is ejected. Devices for injecting air, steam, gas (He, _N2 , etc.) from the lower part of the reactor core have been considered in the past, but according to the present invention, when the main steam is shut off, as an auxiliary function of the scram and recirculation pump trip, 2 to 3 times immediately after main steam shutoff
Inject a two-phase flow of steam and water for only seconds. That is, it is not preferable to inject air or gas from outside the reactor core because it further intensifies the rise in reactor pressure when the main steam is shut off. Furthermore, the method of injecting only steam will eventually lead to an increase in reactor pressure unless the temperature and pressure of the steam to be injected are controlled. Therefore,
Injecting a two-phase flow of steam and water is the most practical. Moreover, after the load is cut off, the reactor is 1~
The scram effect is sufficiently effective within 2 seconds, and it is only necessary to inject the two-phase flow of steam and water for 2 to 3 seconds immediately after load shedding with this device. Therefore, the heating pressurizer described above may be of a capacity capable of injecting a two-phase flow of steam and water into the reactor core for 2 to 3 seconds. Next, although the two-phase flow of steam and water is injected from a fixed header provided at the bottom of the reactor core as an example, other locations may be used. However, considering that the steady-state void distribution of a BWR reactor is as shown in Figure 3, it is thought that injecting from the bottom of the core will most effectively distribute the voids throughout the core. Further, the shape of the fixed header at the lower part of the core does not necessarily have to be annular unless there are other structural restrictions. The inside of the guide pipe between the quick release valve and the fixed header is filled with reactor water during normal operation, but the distance between the quick release valve and the fixed header is approximately
10 m, and when comparing the amount of water and steam during two-phase flow injection with the amount of reactor water in the guide tube described above, this amount does not pose any problem for the operation and effects of the present invention.

Next, the effects of the present invention will be explained in comparison with a conventional control method that does not use the present invention. Figures 4 and 5 show changes in neutron flux and transient changes in surface heat flux from the fuel rods when load shedding occurs in comparison with the conventional system. The solid line represents the present invention, and the broken line represents the conventional example. According to the present invention, the two-phase flow of steam and water injected in a short time before the scram effect of the reactor takes effect during load shedding becomes a void in the reactor core, suppressing the reactor output, Suppressing the abnormal output rise of the reactor during load shedding. Additionally, the two-phase flow of steam and water is set at a temperature that will condense within the reactor, minimizing the rise in reactor pressure. Thus, the nuclear reactor output control device according to the present invention can safely and effectively suppress an increase in the reactor output during abnormalities such as load shedding of the reactor.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the output control device of the present invention, FIGS. 2a and 2b are a plan view and a sectional view showing an embodiment of a header provided at the bottom of the core, and FIG.
Figures 4 and 5, which show examples of general void distribution in a steady state of a BWR type nuclear reactor, show the case of load shedding using the conventional method and the one with the control device according to the present invention. FIG. 3 is a diagram showing changes in reactor neutron flux and surface heat flux. 19... One-way valve, 20... Pressure heater, 21... Quick release valve, 22A, 22B, 22C... Guide pipe, 2
3...Hetzder.

Claims (1)

[Scope of Claims] 1. A fixed header that is disposed inside a reactor pressure vessel and injects a two-phase flow having the same composition as the reactor coolant into the reactor core, and a quick release valve provided in the fixed header. A pressure heater that stores high-temperature, high-pressure steam and water that is pressurized to a pressure higher than the set pressure at the time of reactor steam load cutoff, and the quick release valve is opened when the reactor steam load is cut off. An output control device for a nuclear reactor, comprising a control device. 2. Claim 1, wherein the fixed header is arranged at a lower part of the nuclear reactor core.
The power control device for the nuclear reactor described in Section 3. 3. Claim 1, wherein the fixed header is a ring having a hole bored in its inner peripheral surface.
The power control device for a nuclear reactor according to item 1 or 2.