JPH043837B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a nuclear reactor power control device, and particularly to a nuclear reactor power control device suitable as a protection system for a boiling water reactor.

[Background of the invention]

In a boiling water reactor power plant (BWR),
The main steam isolation valve full closure event causes the largest transient pressure rise, and is considered a severe transient event. Conventionally, in such a case, the reactor protection system is activated by operating a limit switch provided in the main steam isolation valve, and the reactor is preliminarily scrammed. By performing a scram in advance, the reactor pressure rise is suppressed and the integrity of the reactor or fuel is ensured.

However, assuming an event in which the reactor does not scram (ATWS) for some reason, the reactor pressure will continue to rise and, depending on the plant type, may exceed the pressure limits established for such ATWS events. There is a possibility that it will be put away or the limit will be reached.

One way to suppress such a pressure rise during ATWS is to trip the recirculation pump using an abnormally high reactor pressure signal or an abnormally low reactor water level signal (ATWS signal). However, in this method, since the phenomenon signal is used as the activation signal, the response is slow and a considerable pressure increase is unavoidable.

The above is the case when the main steam isolation valve is fully closed.
Consider an event in which one or two of the main steam isolation valves close due to failure or operator error. Conventionally, when such an event occurs at high power, the reactor protection system operates due to one of the following three processes and the plant trips.

1 Instantaneous increase in reactor power (APRM high scram) due to a direct pressure surge due to the valve closure.

2 Due to the closure of the same valve, the steam flow rate in other main steam piping increased, causing the main steam isolation valve to fully close (main steam isolation valve fully closed scram).

3 Due to the characteristics of the pressure regulator, closing the valve increases piping pressure loss, so the reactor pressure gradually increases (reactor pressure high scram).

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a reactor power control device that can suppress a rise in reactor pressure in a short time when a reactor scram is disabled when a main steam isolation valve is closed.

[Summary of the invention]

The features of the present invention include a first main steam isolation valve closure detection means for detecting closure of the main steam isolation valve, and a first main steam isolation valve closure detection means as a means for scramming the reactor when the main steam isolation valve is closed. The reactor protection means inputs the main steam isolation valve closing signal from the means to scram the reactor, and is used as a means to reduce the reactor output when the reactor does not scram for some reason when the main steam isolation valve is closed. , a second main steam isolation valve closure detection means for detecting closure of the main steam isolation valve, and a flow rate of coolant supplied to the core when the second main steam isolation valve closure detection means outputs a main steam isolation valve closure signal. and control means for tripping the pump that controls the pump.

[Embodiments of the invention]

The embodiment of the present invention focuses on the fact that the short-term pressure rise when the reactor protection system cannot perform scram when the main steam isolation valve is fully closed is particularly severe in the main steam isolation valve fully closed event. Focusing on the ability to alleviate events during normal scrams by closing valves, and to avoid plant trips when one or two of the same valves are closed, we developed a main steam isolation valve closure detector that is separate and independent from the main steam isolation valve closure detector in the reactor protection system. A new steam isolation valve closure detector was installed, and by using the detection signal from this detector to trip the pump that controls the reactor core flow rate, the reactor output was instantly reduced and the reactor pressure rise was suppressed. This is what I did.

A preferred embodiment of the present invention applied to a boiling water reactor will be described with reference to FIG. In this embodiment, the reactor output is controlled by changing the flow rate of cooling water flowing through the reactor core 2 (referred to as core flow rate) by controlling the rotation speed of a recirculation pump 6 installed in a recirculation pipe 5 provided in the reactor 1. There is. Four main steam pipes 3 are connected to the reactor 1
The main steam isolation valves 4, which are connected to the Install an isolation valve closure detector. Separating the existing detector and making it independent is based on the basic idea related to an impossible scram event (ATWS), which is caused by a failure of the reactor protection system detector.
This is to deal with ATWS. From this point of view, it is desirable that the detector used in this embodiment be a detector based on a principle different from existing detectors, but a level switch may also be used. In this case as well, a detector of a different type from existing ones should be used to avoid common cause failures. The closure signal emitted from the main steam isolation valve closure detector is all connected under an OR condition and used as a trip signal for the recirculation pump 6 in order to obtain the three effects described below. In Figure 1, the main steam isolation valve 4 is
Since 12 are installed, there are 12 closure signals output from the main steam isolation valve closure detector when the main steam isolation valve 4 is closed. The aforementioned recirculation pump 6
The trip signal is input to a field circuit breaker 8 connected to a drive motor 7 connected to a recirculation pump 6 and a recirculation MG set 9, and by shutting off the field circuit breaker 8, recirculation is stopped. Trip the pump 6. When the recirculation pump 6 trips, the recirculation piping 5, which is the driving fluid for the jet pump 11,
The amount of cooling water spouted from the open end of the pipe will run out. Therefore, the amount of coolant introduced into the core 2 through the jet pump 11 is reduced, and only coolant is supplied to the core 2 through natural circulation. For this reason, the reactor output is significantly reduced and the amount of steam generated is reduced, so the pressure inside the reactor 1 is reduced. The reactor continues to operate at low power. Depending on the type of boiling water reactor plant, there may be no field breaker 8, but in that case, it is necessary to newly provide the field breaker 8. Although not shown in the drawings, this embodiment is a nuclear reactor that operates by inputting a main steam isolation valve closing signal output from each level switch when all of the level switches and main steam isolation valves mentioned above are closed. Has a protective system. The reactor is scrammed by activation of the reactor protection system. Specifically, activation of the reactor protection system rapidly inserts all control rods into the reactor core and scrams the reactor.

Next, the effects of this embodiment will be explained. This embodiment provides the following three effects.

1 This is a countermeasure against ATWS in BTR.

2 Mitigation of events caused by reactor pressure rise during scram with all main steam isolation valves closed.

3. Avoid plant trips by closing one or two main steam isolation valves at high output.

The effects of this embodiment described above will be specifically explained below.

1 ATWS countermeasures in BWR Limiting conditions in the event of an anti-scram event (ATWS) are determined by requirements from the reactor pressure vessel integrity and containment vessel integrity. Of these,
The latter, containment health, relates to long-term events and is highly dependent on the corresponding operational actions after an ATWS occurs (in particular, SLCS activation, etc.).

What we are focusing on here is the former, the health of the reactor pressure vessel. This concerns pressure transients in the early events after ATWS occurs. Currently, a value of 1.2 times or 1.15 times the pressure vessel design pressure, which is a condition for determining accident conditions, is applied as a limit condition regarding this integrity. In plant dynamic characteristic analysis,
Assuming an ATWS, it was found that the reactor pressure exceeds the above-mentioned limit condition or reaches close to the limit condition during an ATWS caused by all main steam isolation valves being closed. In the event that the main steam isolation valves are fully closed and scram is not possible, the steam generated in the reactor cannot be used in the turbine line or the bypass line, so it is transferred to the condensate water, which is the largest heat sink. All steam is discharged from a safety relief valve (not shown) installed in the main steam line 3 into a suppression pool (not shown). When such an event occurs during rated operation of a nuclear reactor, the reactor pressure increases, the voids in the reactor core collapse, the reactor output increases, and the amount of steam generated increases, resulting in a vicious cycle that causes the reactor pressure to rise. Continue. In such a case, according to the results of plant dynamic characteristic analysis, the plant
Depending on the type, reactor power may continue to rise and exceed the limit, or may rise close to the limit. In this way, it can be said that the full valve closure event of the main steam isolation valve is the most severe event in ATWS, even in the sense that the condenser, which is the largest heat sink, cannot be used at all, and countermeasures must be taken against this event. can be said to have taken measures against BWR. In fact, due to other initiating events
It is unlikely that ATWS will cause the reactor pressure to exceed the limit in a short period of time. Note that fully closing the main steam isolation valve means that the condenser, which is the largest heat sink, cannot be used at all, and all main steam piping (4
Book) 3 is to be blocked. That is, fully closing the main steam isolation valve means that at least one main steam isolation valve is closed in each of the four main steam pipes.

Measures to deal with such ATWS include:
A method has been considered in which the recirculation pump is tripped by a so-called ATWS signal indicating abnormally high reactor pressure or abnormally low reactor water level to reduce reactor output. However, since this method uses the phenomenon signal as the cause signal, a considerable increase in reactor pressure, etc. is unavoidable, and depending on the case (or plant), the limit may be exceeded.

As mentioned above, this example is based on the fact that the only ATWS-induced event in which the reactor pressure exceeds the limit within a short period of time is due to the main steam isolation valve being fully closed. This study focuses on the fact that if the reactor output is lowered in advance using the steam isolation valve closing signal, the rise in reactor pressure can be significantly reduced. In other words, since the recirculation pump trip signal is generated by the main steam isolation valve closing signal at almost the same time as the original scram signal, even if a scram is not possible, the core flow rate will immediately decrease with the coastdown characteristic caused by the recirculation pump trip. ,
Due to the increase in voids, the reactor power decreases, and together with the decrease in the amount of generated steam and the decrease in the recirculation pump discharge pressure, the increase in reactor pressure is significantly suppressed. FIG. 2 shows a comparison of the changes in the reactor core flow rate and reactor pressure at the time of this event according to the conventional example described above and the changes according to this embodiment.

When a main steam isolation valve fully closed event occurs, the main steam isolation valve closure signal detected by the level switch of the reactor protection system is sent to the reactor protection system to detect the main steam isolation valve closure signal, which is installed separately from the level switch. The closure signals output from the closure detectors are respectively input to the field interrupters 8. If no scram disable event occurs, the reactor is scrammed by actuation of the reactor protection system and the recirculation pump 6 is tripped by shutting off the field interrupter 8. When a scram impossible event occurs, the recirculation pump 6 is tripped by shutting off the field circuit breaker 8, and as a matter of course, the reactor protection system does not perform a reactor scram.

2 Mitigation of events caused by pressure rise during main steam isolation valve full closure scram In Section 1, we explained the steam isolation valve full closure event during unable to scram (ATWS). Among these events, the main steam isolation valve full closure event is the most severe event from the perspective of pressure rise. The scram signal is generated by the main steam isolation valve close detection limit switch.
Another method is to perform a recirculation pump runback using a recirculation flow rate control system using a scram signal (scram pilot valve excitation signal) in order to suppress an abnormal drop in the reactor water level during scram. Suppression of reactor water level decline by the above method is as follows:
The drop in reactor water level due to the disappearance of poids due to control rod insertion is compensated for by the increase in poids due to the reduction in core flow rate due to recirculation pump runback, and the reduction in reactor water level is suppressed. Effective for pressure increase events such as total closure events. However, even with this method, the reactor water level drops considerably, sometimes to the point where it drops close to the emergency high-pressure reactor water injection system start-up set water level (L ₂ ). When this embodiment is applied, the coast down time constant of the core flow rate by the recirculation pump 6 is small, the flow rate in the recirculation pipe 5 decreases to 0, and the recirculation pump trip is triggered by the main steam isolation valve closing signal. Immediate implementation of this will further increase the effect of suppressing the drop in the reactor water level, and will also have the effect of suppressing the pressure rise during an event.

3. Avoidance of plant trips by closing one or two main steam isolation valves during high output. Preventing one or two main steam isolation valves from closing due to a failure or operator error during rated reactor operation. Assuming that the plant trips due to one of the following three processes.

1 The reactor pressure suddenly rose due to the closure of the main steam isolation valve, and the reactor pressure became instantly high, leading to an APRM high scram.

2 Due to the closure of the main steam isolation valve, the main steam piping becomes disconnected, the flow rate of other main steam piping increases, a high steam flow signal is generated, the valves are all closed, and a scram occurs.

3 Due to the characteristic of the pressure regulator, which controls main steam pressure at a constant level, the reactor pressure gradually increases due to the apparent increase in piping pressure loss due to the closure of the main steam isolation valve.
High reactor pressure leads to scram.

Even in the above case, if the reactor output is lowered in advance by the main steam isolation valve closing signal, all of the above three processes can be avoided, and a plant trip can be avoided.

In the embodiment of the present invention shown in Fig. 1, in order to expect the three effects mentioned above, the main steam isolation valve closing signal is connected with OR logic to become the recirculation pump trip signal, but this logic is used to protect the current reactor. It can also be the same as the logic of the system. However, in this case, the third effect among the aforementioned effects cannot be expected.

It is also possible to combine the embodiment of FIG. 1 with the conventional recirculation pump trip caused by an abnormally high reactor pressure signal. By doing this,
Assuming ATWS, in case of a particularly severe event of pressure rise, such as due to the main steam isolation valve being fully closed,
With the function of the embodiment shown in Fig. 1, the output is reduced in advance with the main steam isolation valve closing signal to alleviate the pressure event, and even in ATWS caused by other causes, the pressure event is further alleviated by reducing the output due to an abnormal rise in pressure. becomes possible.

One of the above effects which is the main aim of this embodiment
Boiling Water Nuclear Power Plant (BWR)
Let me explain once again the significance of this as a countermeasure against ATWS.

As explained earlier, abnormal non-scram events (ATWS) are a common problem in nuclear power plants, but here we are targeting boiling water nuclear power plants (BWR). Limiting conditions are assumed for safety when ATWS occurs, but reactor pressure Examples have been taken as a measure to comply with container restriction conditions.

The above pressure limits must be followed equally for ATWS events, but it is important to note that for pressure events the limits can only be exceeded during ATWS due to main steam isolation valves being fully closed, and countermeasures must be taken against the main event. If you do this, you will be taking measures against BWR (for the pressure increase problem).

In particular, in the case of this example, the pressure event is severe.
By reducing the reactor output in advance and suppressing the pressure rise in response to a main steam isolation valve full-valve closure event that could result in ATWS, it is possible to prevent the restriction from being observed even if the output is reduced based on the phenomenon signal. Even if a large plant were to arise, it would be sufficient to cope with the situation.

〔Effect of the invention〕

According to the present invention, when the reactor scram is not possible when the main steam isolation valve is closed, the reactor power can be rapidly reduced to a low output, and the reactor pressure increase due to the main steam isolation valve closure can be suppressed in a short time. can.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram illustrating an embodiment of the present invention, and FIG. 2 is an explanatory diagram comparing the phenomenon when the conventional example and the present invention are applied at the time of ATWS due to full closure of the main steam isolation valve. 1... Nuclear reactor, 2... Reactor core, 3... Main steam piping, 4...
Main steam isolation valve, 6... recirculation pump, 8... field circuit breaker.

Claims (1)

[Claims] 1. A first main steam isolation valve closure detection means for detecting closure of a main steam isolation valve provided in a main steam pipe connected to the reactor, and a main steam isolation valve from the first main steam isolation valve closure detection means. a reactor protection means that inputs a closure signal and scrams the reactor; a pump that controls the flow rate of coolant supplied to the core in the reactor; and a second main steam isolation device that detects closure of the main steam isolation valve. A nuclear reactor power control system comprising: valve closure detection means; and control means for tripping the pump when the second main steam isolation valve closure detection means outputs a main steam isolation valve closure signal.