JPH0740073B2 - Automatic decompression system - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an automatic depressurization system for an emergency core cooling system in a boiling water nuclear power plant, and in particular, has been improved so as to automatically start even in the event of a water supply system accident. Regarding automatic decompression system.

[Technical background of the invention]

In general, boiling water nuclear power plants have various safety equipment.
It is equipped with three or three layers and has extremely high safety measures.

In the unlikely event of a serious accident or other abnormal situation in the nuclear power plant, fission products are released to the external environment,
Care is taken not to give the general public any risk of radiation exposure.

At the boiling water nuclear power plant, one of the above-mentioned safety facilities is an emergency core cooling system (ECCS).
em) and the reactor containment vessel.

The emergency core cooling system sufficiently cools the core even when a loss of coolant accident (LOCA: Loss of Coolant Accident) occurs due to breakage of the cooling water recirculation system piping, and the core fuel loaded in this core is It keeps the temperature below the specified value.

Further, the reactor containment vessel is to contain the fission products from the core fuel so as not to be released to the external environment even if the core is not sufficiently cooled.

Loss of coolant accident (LOCA), which is assumed to be the maximum accident in boiling water nuclear power plants, is a case where the recirculation piping is instantaneously completely broken during the operation of the reactor.

This recirculation pipe is used to generate a forced circulation flow in the coolant in the reactor pressure vessel by a recirculation pump to effectively cool the core fuel. However, assuming a case where this recirculation pipe is severely broken, the coolant in the reactor pressure vessel, which is maintained at a high pressure of about 70 atm during normal operation, suddenly flows out from this pipe breakage port. The water level in the inner part is lowered and the core is exposed. At the same time as this pipe rupture accident, various safety equipment is activated and the nuclear reaction of the reactor is stopped.

Then, the exposed core is rapidly melted due to the large amount of decay heat remaining due to the nuclear reaction of the fission products already loaded into the core during reactor operation, and a considerable amount of fission products is released from this core part. Will cause the worst situation.

The emergency core cooling system (ECCS) is a safety facility against accidents in which recirculation pipes break, and this cooling system is a high pressure core spray system.
It is composed of multiple low pressure core spray systems, low pressure water injection systems and automatic decompression systems.

The high-pressure core spray system introduces suppression pool water into the reactor pressure vessel by a high-pressure pump, sprays and directly injects a coolant into the nuclear fuel material of the core, re-submerges the core, and melts the core fuel. Is to prevent.

In this high-pressure core spray system, when the reactor water level is not recovered and the reactor pressure is sufficiently low, the suppression pool water is sprayed to the core portion by the low-pressure pump of the low-pressure core spray system. At the same time, the low-pressure water injection system is activated to inject the suppression pool water into the reactor water to submerge the reactor core.

The emergency core cooling system is provided with, for example, three pumps for the low-pressure water injection system and one pump for each of the high-pressure spray system and the low-pressure spray system. This means that the number of pumps in the emergency core cooling system required for large pipe breakage, where the recirculation pipes are completely broken instantaneously, is 1-2, so the pumps should be arranged with a sufficient margin. Is shown.

In this way, the current boiling water nuclear power plant has implemented sufficient safety measures against accidents and is equipped with multiple safety facilities.

By the way, when a coolant loss accident (hereinafter referred to as a small or medium size break accident) due to a local breakage that partially damages the recirculation pipe occurs, the above high pressure spray system operates immediately. , Low pressure system such as low pressure water injection system does not work.

That is, in the case of a small-to-medium-sized rupture accident, the rupture area of the recirculation pipe is small, and the rupture is in the liquid phase, so the reactor internal pressure is maintained at a high pressure. For this reason, the high-pressure pump operates, but the low-pressure pump cannot operate until the pressure in the reactor pressure vessel decreases.

The above-mentioned high-pressure pump has the characteristics shown by reference character A in FIG. 4, for example, and it is possible to inject the coolant at any reactor pressure of the reactor. On the other hand, the low-pressure pump has, for example, the characteristic shown by reference character B in FIG. 4, and it becomes almost impossible to inject the coolant when the reactor pressure is high.

Therefore, as shown in FIG. 4, when a small and medium pipe breakage accident occurs, the low-pressure pump cannot function and the low-pressure system such as the low-pressure water injection system having this low-pressure pump does not start. As a result, only one high-pressure spray system high-pressure pump operates.

Therefore, when a failure of this high-pressure pump is assumed, a safety means for forcibly reducing the pressure of this reactor is required. A safety measure for this is an automatic decompression system.

The automatic depressurization system is automatically started when the water level of the reactor pressure vessel cannot be recovered by the high pressure core spray system, and the internal pressure of the reactor pressure vessel is lowered, and the low pressure core spray system and the low pressure water injection system It enables the injection of coolant, releases the high-pressure steam in the reactor pressure vessel, opens the safety valve, and discharges it to the suppression pool to reduce the pressure.

The automatic decompression system is designed to be automatically started by an automatic start signal, but in the past, this automatic start signal was formed by the start signal forming circuit shown in FIG.

That is, as shown in FIG. 5, the conventional start signal forming circuit has a low reactor water level signal 1, a high drywell pressure signal 2 and an OR signal.
The output of the gate 3 was input to the AND gate 4. Further, a core spray pump operation signal 5 and a low pressure water injection pump operation signal 6 are input to the OR gate 3. The output side of the AND gate 4 is connected to a delay device for giving a delay of a predetermined time to the output signal of the AND gate 4, for example, a timer 7. The output signal of the timer 7 is an automatic depressurization system start signal (hereinafter referred to as ADS). A signal (abbreviated as a start signal) 8 is sent to a main steam relief safety valve (not shown).

Then, the loss of coolant accident (LOC
A) When it occurs, the main steam relief safety valve is automatically opened,
High-pressure steam in the reactor pressure vessel is discharged to the suppression pool to reduce the pressure. If the loss of coolant accident is caused by a small-to-medium-sized breakage accident in the recirculation pipe, the core part is cooled by the low-pressure core spray system and the low-pressure water injection system even if failures of the high-pressure core spray system occur. , And flooded, and the safety of the reactor is maintained.

[Problems of background technology]

However, when the loss of water supply is assumed, the above-described conventional start signal forming circuit poses the following problems.

In other words, assuming a water supply accident in which the water supply system that supplies water to the reactor pressure vessel fails for some reason and water supply to the reactor pressure vessel stops, the reactor pressure at the beginning of the accident Since the steam flows out of the vessel through the main steam pipe, the reactor water level decreases relatively slowly. When the decrease in the reactor water level reaches a predetermined water level, for example, level -2, the reactor pressure vessel loses its steam escape place and becomes an even higher pressure state because the main steam pipe is fully closed.

When this pressure rises to a predetermined high pressure, the main steam relief safety valve operates to release the high-pressure steam to the suppression pool installed in the lower part of the reactor containment vessel, thereby reducing the pressure in the reactor pressure vessel. When the pressure is reduced to a predetermined low pressure, this automatic relief safety valve closes, and the pressure inside the reactor pressure vessel repeatedly rises and falls in a cycle. In any case, since the reactor pressure vessel is maintained in a considerably high pressure state, the coolant injection by the low pressure system such as the low pressure water injection system of the emergency core cooling system cannot operate.

On the other hand, each time the main steam relief safety valve is activated, the steam in the reactor pressure vessel is discharged into the suppression pool, so the reactor water level decreases. If this state is left unattended, the core will be exposed, and in the worst case, a core melting accident will occur. Originally, when the reactor pressure vessel reached an abnormally high pressure, the high pressure spray system was activated to inject the coolant, and the above-mentioned core melting was avoided, but this high pressure spray system also fails here. We will assume a very severe case again.

Furthermore, in the event of a water supply accident, the pressure detector that detects the pressure of the coolant from the reactor pressure vessel to the reactor containment vessel cannot detect the abnormally high pressure outside the reactor containment vessel. , The high pressure signal in the dry well is not sent.

Therefore, the automatic start signal is not transmitted, and the automatic depressurization system relies on the manual start of the operator.

Therefore, in the unlikely event of a water loss accident and a high-pressure spray accident, the operator is exposed to extremely high mental stress, and in such a severe situation, the operator manually activates the automatic decompression system. Must.

From the viewpoint of taking thorough safety measures, it is strongly desired to automatically start the automatic depressurization system even under such extremely severe conditions.

[Object of the Invention]

The present invention has been made in view of the above-mentioned circumstances, and even in the case of an extremely severe situation in which a main steam pipe breakage accident and a high-pressure core spray system accident doubly occur, the emergency core cooling system is automatically operated. The depressurization system can be activated automatically, and the low pressure system can reliably inject the coolant into the reactor pressure vessel whose pressure has been reduced by this automatic depressurization system. It is an object of the present invention to provide an automatic decompression system that can be prevented.

[Outline of Invention]

The present invention reduces the pressure in the reactor pressure vessel, in an automatic depressurization system that enables the injection of coolant into the reactor pressure vessel by a low pressure core spray system or a low pressure water injection system, in the reactor pressure vessel Both the reactor water level low signal, which was obtained when the reactor water level fell below the prescribed water level, and both pump operation signals, which were obtained when the low-pressure core spray system and the low-pressure injection system were started, were both received. At the same time, the reactor water level low signal, one of the both pump operation signals, and the drywell pressure high signal obtained when the pressure in the drywell of the reactor containment vessel is increased to a high pressure higher than a predetermined pressure. It is characterized in that when both are received, they are automatically activated after a predetermined time.

Example of Invention

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

FIG. 1 shows the configuration of a startup signal forming circuit according to an embodiment of the present invention, in which a high drywell signal 11, a low reactor water level signal 12, and an output signal from an OR gate 13 are combined with a first AND gate. It has 14 inputs.

The OR gate 13 receives the low-pressure core spray pump operation signal 15 and the low-pressure water injection pump operation signal 16 as input, and the reactor water level low signal
12 and the output signal from the OR gate 13 are input to the second AND gate 17.

The first and second AND gates 14 and 17 have a first timer 18 and a second AND gate, respectively.
Connect each timer 19 and ADS from each timer 18,19
It is designed to output a start signal 20. Both of these timers 18 and 19 delay the output of the ADS start signal 20 by a predetermined time t ₁ or t ₂ to determine whether or not the ADS start signal 20 is an erroneous signal and give the plant operator a judgment time. Second timer
The delay time t _{2 of} 19 is set to 12 minutes. This is because the delay time t ₁ of the first timer 18 is set to 2 minutes and 10 minutes of the special operation time of the plant operation is added to this, and in the current design code, the start delay of the automatic decompression system is 12 minutes at maximum. Because it is settled to.

The delay time t ₁ is as short as 2 minutes because the first timer 18
Is activated in the event of loss of coolant, in this case,
This is because the reactor water level in the reactor pressure vessel drops sharply, and the automatic depressurization system must be started immediately.

On the other hand, in the event of an accident in the water supply system, the reactor water level in the reactor pressure vessel drops relatively slowly, so it is not necessary to activate the automatic depressurization system immediately, so the delay time t ₂ of the second timer 19 is set relatively low. Settling for a long time.

The dry well pressure high signal 11 is a signal output from a pressure detector (not shown) that detects when the pressure in the dry well of the reactor containment vessel (not shown) rises to a predetermined high pressure, The steam pipe or the recirculation system pipe broke in the drywell of the containment vessel,
Output when there is a loss of coolant accident that causes coolant to flow out.

The reactor water level low signal 12 is a signal output from a reactor water level detector (not shown) that detects when the reactor water level in the reactor pressure vessel (not shown) falls below a predetermined level, and the loss of coolant accident described above occurred. In addition to the above, it is also output when water supply is stopped due to an accident in the water supply system that supplies water to the reactor pressure vessel.

Further, the pump operation signals 15 and 16 for both the low pressure core spray system and the low pressure water injection system are signals that are output when both pumps are activated in response to the low reactor water level signal 12, respectively. This is a signal indicating that the water injection system can operate.

Next, the operation of this embodiment will be described.

First, the case where a water supply accident occurs will be described.

In this case, since the steam generated in the reactor pressure vessel is continuously supplied to the turbine (not shown) through the main steam pipe, when the water supply to the reactor pressure vessel is stopped, the reactor water level in the reactor pressure vessel becomes relatively slow. Fall to.

When the reactor water level drops below a predetermined water level, a reactor water level low signal 12 is output from a reactor water level detector (not shown).

The reactor water level low signal 12 is given to the low pressure core spray system, the low pressure water injection system, and the start signal forming circuit of the automatic depressurization system shown in FIG. As a result, the low pressure core spray system and low pressure water injection system pumps are activated, and pump operation signals 15 and 16 are applied to the OR gate 13 of the activation signal forming circuit. Even if one of the two pumps does not operate, the OR condition of the OR gate 13 can be satisfied.

Therefore, the second AND gate 17 of the activation signal forming circuit of the automatic depressurization system receives both the reactor water level low signal 12 and the output signal from the OR gate 13, satisfies the AND condition, and activates the second timer 19, ADS start signal 20 after delay time t ₂
Is output.

The ADS activation signal 20 is given to the main steam relief safety valve (not shown) of the automatic depressurization system, which opens it, releases the high pressure steam in the reactor pressure vessel to the suppression pool, lowers the reactor pressure, and low pressure core spray. Allows coolant injection into the reactor pressure vessel by the system and low pressure water injection system. That is, even when the high pressure core spray system is not driven,
The reactor water level can be restored.

When the delay time t ₂ of the second timer 19 is set to 12 minutes,
Fluctuations in the reactor water level from when the reactor water level begins to decrease to before and after the automatic depressurization system is automatically activated, and the maximum temperature of the cladding tube of the fuel loaded in the core are shown in FIGS. 2 and 3, respectively. As is clear from this figure, after the automatic depressurization system is started, the reactor water level is restored to the almost original state, the fuel cladding temperature is sufficiently suppressed below the limit value, and the integrity of the core is maintained.

Next, a case where a coolant loss accident occurs due to pipe breakage will be described.

In this case, when the coolant is discharged from the breakage port of the pipe into the drywell of the reactor containment vessel and the pressure in the drywell rises and exceeds a predetermined high pressure, the pressure detector that detects this raises the drywell pressure. The high signal 11 is applied to the first AND gate 14 of the activation signal forming circuit of the automatic pressure reducing system. Since the dry well pressure high signal 11 alone does not satisfy the AND condition of the first AND gate 14, the first timer 18 does not start.

When the coolant continued to flow out from the pipe breakage port, the reactor water level in the reactor pressure vessel dropped sharply below the predetermined water level, and when the reactor water level low signal 12 was output from the reactor water level detector, this atom The reactor water level low signal 12 is the first AND gate 14 of the start signal forming circuit.
And low pressure core spray and low pressure water injection to start both pumps.

As a result, the pump operating signal 1
5, 16 are given to the OR gate 13 of the activation signal forming circuit, and the output signal from the OR gate 13 is given to the first and second AND gates 14, 17. Since the first and second AND gates 14 and 17 satisfy the respective AND conditions, the first and second timers 18 and 19 are activated respectively.

The delay time t ₁ of the first timer 18 is 2 minutes, and the second timer 19
Since the delay time is less than 12 minutes, the first timer 18 quickly times up after the delay time t _{1 of} 2 minutes, and gives the ADS start signal 20 to the main steam relief safety valve (not shown) to open it, High-pressure steam in the furnace pressure vessel is released to the suppression pool.

This lowers the reactor pressure and enables low pressure core spray and low pressure water injection into the reactor pressure vessel for coolant.

Therefore, according to the present embodiment, even when the high pressure core spray system is out of order, the automatic depressurization system can be reliably and automatically started not only in the case of the coolant loss accident but also in the case of the water supply system accident. it can.

As a result, a water supply system accident and a high-pressure core spray system accident should occur, and even if the plant operator is under extreme mental stress, the plant operator does not need to manually start the automatic decompression system. Therefore, the intervention of human error such as erroneous determination and erroneous operation associated with manual activation can be prevented in advance.

〔The invention's effect〕

As described above, the present invention receives both the reactor water level low signal and one of the pump operation signals of both the low pressure core spray system and the low pressure water injection system, and these signals. And the drywell pressure high signal are both received, the automatic decompression system is automatically activated.

Therefore, even when the high pressure core spray system is out of order, the present invention can be applied not only when multiple coolant loss accidents occur, but also when multiple water supply system accidents occur. , Automatic decompression system can be started automatically.

As a result, even in the unlikely event that the accidents of the high-pressure core spray system and the water supply system overlap each other, the automatic decompression type can be automatically started without depending on the plant operator. For this reason, the intervention of human error by the plant operator under an extremely tense state can be prevented in advance.

[Brief description of drawings]

FIG. 1 is a block diagram of a start signal forming circuit according to an embodiment of the present invention, and FIG. 2 is a graph showing an example of changes in reactor water level for explaining the operation of the embodiment shown in FIG. FIG. 4 is a graph showing an example of changes in cladding maximum temperature for explaining the effect of the embodiment shown in FIG. 1, FIG. 4 is a graph showing pump characteristics of a general high pressure core spray system and a low pressure system, 5
The figure is a conventional start signal forming circuit diagram. 1 ... Reactor water level low signal, 2 ... Drywell pressure high signal, 3 ... OR circuit, 4 ... AND circuit, 5 ... Low-pressure core spray pump operation signal, 6 ... Low-pressure injection pump operation signal, 7 …… Timer, 11 …… Drywell pressure high signal,
12 …… Low reactor water level signal, 13 …… OR gate, 14 …… First A
ND gate, 15 ... Low-pressure core spray pump operation signal, 16
...... Low-pressure water injection pump operation signal, 17 …… Second AND gate, 18
...... First timer, 19 ...... Second timer, 20 ...... ADS activation signal.

Claims (1)

[Claims] Claim: What is claimed is: 1. An automatic depressurization system for reducing pressure in a reactor pressure vessel to allow injection of a coolant into the reactor pressure vessel by a low pressure core spray system or a low pressure water injection system. Receiving either one of the reactor water level low signal, which is obtained when the reactor water level drops below a prescribed level, and both pump operation signals, which are both obtained when the low pressure core spray system and the low pressure water injection system are activated. And when
Receiving both the reactor water level low signal and one of the both pump operation signals, and the drywell high pressure signal obtained when the pressure inside the drywell of the reactor containment vessel is raised to a high pressure higher than a predetermined pressure. The automatic depressurization system is characterized in that when it is turned on, it automatically starts after a predetermined time.