JPS60123795A - Open-close loop type seawater cooling system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a system that uses seawater as a cooling source in a nuclear power plant, and particularly relates to seawater pump tripping caused by an abnormal drop in seawater tide level during a nuclear reactor scram caused by an earthquake. The present invention relates to a seawater cooling system suitable for preventing nuclear reactors from occurring and safely shutting down a nuclear reactor without activating a main steam relief safety valve.

[Background of the invention]

A conventional circulating water system and residual heat removal system using seawater as a cooling source will be explained with reference to FIG.

The circulating water system basically has the function of condensing and cooling the steam after driving the turbine 6 in the condenser 3 and maintaining the inside of the condenser 3 at a high vacuum.

Seawater 11 taken in from the water intake port 4 is transferred into the condenser 3 by the circulation pump 1, and the steam is condensed and cooled to maintain the inside of the condenser 3 at a high vacuum. The seawater leaving the condenser 3 is sent to the water outlet 5.
It is released as is to rt6-.

If a reactor scram occurs in this state, the circulating water system will continue operating to cool the reactor water and reduce the internal pressure of reactor 7, while the residual heat removal system will operate in reactor shutdown cooling mode. The reactor 7 can be safely shut down without activating the safety relief valve 9.However, due to grounding and other reasons, the reactor scram and the abnormally low seawater level caused by the tsunami caused by the earthquake occur at the same time. If this occurs, the water level in the water intake tank 2 will drop, and the circulation pump 1 and the residual heat removal seawater pump 24 will trip due to the water level drop signal from the water intake tank water level detection device 15. When the circulation pump 1 trips, the degree of vacuum inside the condenser 3 decreases, and the degree of vacuum P
Turbine 6 trips at T smHg. When the degree of vacuum further decreases to PvmmHg, the main steam isolation valve 8 is closed to prevent contaminated steam from leaking from the turbine 6. In this case, since the reactor 7 is isolated, the internal pressure rises due to the decay heat of the fuel, the main steam relief safety valve 9 operates, and high-temperature steam is ejected into the pressure suppression chamber 10. It is cooled with water stored in the tank. However, since the residual heat removal seawater pump 24 trips and the heat exchanger 25 does not exchange heat, the water held in the pressure copper Ill chamber 10 has no cooling source, and the temperature continues to rise, exceeding its temperature limit value. It was loud.

If the temperature rises in this way, the filtration function will not work properly and the inside of the pressure suppression chamber 10 will be contaminated, so it is necessary to safely shut down the reactor without activating the main steam relief safety valve 9 as much as possible. This is desirable.

[Purpose of the invention]

The purpose of the present invention is to keep seawater within the system, maintain the degree of vacuum in the condenser, and maintain the vacuum level in the condenser even if a nuclear reactor scram caused by an earthquake and an abnormal seawater level drop caused by a tsunami caused by the earthquake occur simultaneously. To provide a seawater cooling system suitable for safely stopping a nuclear reactor while preventing a steam relief safety valve from operating.

[Summary of the invention]

The present invention takes seawater into the water intake tank from the water intake, supplies it to the circulating water system and residual heat removal system, cools the condenser, etc. through heat exchange, and discharges the used seawater from the water outlet to the sea. In the open-loop seawater cooling system that discharges water, a backflow prevention device is installed near the water intake of the water intake tank, and the piping from the circulating water system and residual heat removal system to the water outlet is branched by bypass piping to the water intake tank. A switching valve is installed to selectively discharge seawater to the water outlet and water intake tank, which prevents seawater from flowing backwards from the water intake when the seawater tide level is low due to an earthquake or tsunami. The existing seawater is circulated to the water tank to form a closed-loop seawater cooling system, while when the seawater level returns, the backflow prevention device is released and the switching valve is returned to its original state to create a normal open-loop system that uses fresh seawater. This provides an open/closed loop type seawater cooling system that returns to the type seawater cooling system.

Now, when a reactor scram occurs, the ratio of heat exchanged in the condenser (the amount of heat exchanged in the condenser at 100% reactor output Qo)
As shown in FIG.

Assuming that the time after which a reactor scram occurs and the seawater level abnormally drops due to a tsunami or the like is t seconds later, the heat exchange ratio in the condenser at that time is Q'' Q/Qo x 100 (chi).

FIG. 3 shows a condenser performance curve, in which the horizontal axis represents the heat exchange ratio within the condenser, and the vertical axis represents the degree of vacuum in the condenser.

Curve inlet No. 2 is the condenser inlet seawater temperature θN.

This figure shows the relationship between the exchange heat ratio in the condenser and the degree of vacuum in the condenser when θ and θV are kept constant.

Here, the condenser inlet seawater temperatures θN, θ, θV are θN × θ
There is a relationship of θV.

Point E in the figure is at the condenser design point (condenser operating state when the reactor output is 100 cm), the condenser vacuum degree PM, and the condenser condenser inlet seawater temperature.

The point in the figure is the condenser vacuum degree Pv and the condenser inlet seawater temperature θV. At this point, the main steam isolation valve 8 is closed and the main steam relief safety valve 9 is activated.

Now, t seconds after a reactor scram occurs, when the water level in the water intake tank decreases due to a tsunami, etc., the water intake tank 2° circulation pump 1
．． The condenser 3 forms a closed loop and the circulation pump 1 continues to operate, but in this case - the closed loop seawater temperature increases and the condenser vacuum decreases.

In other words, the state of the condenser 3 reaches point G along the curve from point E to point G, and as the seawater circulation temperature rises, the state of the condenser 3 reaches point G along the straight line from point E to point G. is closed, and the main steam relief safety valve 9 is activated.

Figure 4 shows the water intake tank 2° circulation pump 1. The figure shows a condenser inlet temperature rise curve when the condenser 3 forms a closed loop.

This curve is expressed by differential equation (1).

However, C: heat capacity of the circulating water system θ: condenser inlet seawater temperature τ: time after the circulating water system forms a closed cycle k: heat transfer coefficient A: total surface area of the circulating water system θ0: ambient temperature (θ0 x θN) QQo :Condenser heat exchange amount From formula (1), the condenser inlet seawater temperature after one hour is expressed as follows, assuming that the ambient temperature θ0 is approximately equal to θN.

Next, if the period of a tsunami is tT, the tide level will be above the design level for tT/20, and if the backflow prevention device is released during this period, the seawater whose temperature has risen to a certain extent will be mixed with fresh outside seawater in the water intake tank. After sufficient mixing and cooling, the temperature of the seawater at the condenser inlet is
As shown in FIG. 5, it becomes a curve that moves up and down with a period t↑.

Now, if the pipe diameter of the circulating water system is d1 and the length is t, and the surface area and heat capacity of the condenser are ignored, kA = πdt ・River ・
(4) Since the longest tsunami cycle experienced so far was about 30 minutes for the Chile tsunami, taking a margin and setting the tsunami cycle to t T = 1 hour, however, k =
10 Lol/h 0cnr2d = 3.5 m Also, q Q o / k A = Q Qo / kπ
3x10 in d7, but 7 = 200 n1+ qQ
o = 6 X 10' kcal/h (1 series of exchange thermoelectricity) Therefore, θ-θN (3C).

When the condenser population temperature θV reaches 80 to 90C, the main steam isolation valve 8 closes, but the operating state of the condenser is on the curve H shown in Fig. 3, and the condenser vacuum degree is Pv. The temperature of the seawater will not exceed θV, and if the reactor can be operated in an open/close cycle that takes in fresh seawater, a cooling effect will be added and the reactor can be shut down safely.

[Embodiments of the invention]

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

During normal plant operation, seawater taken in through the water intake 4 is stored in the water intake tank 2.

Seawater in the water intake tank 2 is transferred to the υ condenser 3 by the circulation pump 1.

The seawater leaving the condenser 3 is directly discharged into the sea through the water outlet 5.

If an earthquake occurs in this state, the reactor 7 will scram, but the circulating water system continues to operate to remove the decay heat of the fuel, and as soon as the internal pressure of the reactor 7 drops to a low value,
The residual heat removal pump 23 and the residual heat removal seawater pump 24 are started, the reactor cooling mode operation is entered, and the reactor 7 is safely stopped without operating the main steam relief safety valve 9.

However, after an earthquake occurs, if a tsunami etc. occurs and the seawater tide level drops abnormally and falls below the design tide level, the water intake tank 2
Since the seawater inside will flow backwards, the detector 16 activates the backflow prevention device 17 to prevent the seawater from flowing backwards, and also opens the switching valve 13 of the bypass pipe 31 from closed to open the bypass pipe 3.
The switching valve 14 located between the connection point of water intake tank 2.1 and the water outlet 5 is turned from open to closed. Circulation pump 1. The condenser 3 forms a closed cycle, allowing continued operation.

In this case, the circulating water system has a period of Even if a tsunami of tT occurs, if the backflow prevention device 17 is opened and closed at intervals of t-172, fresh seawater will be taken in as explained in Fig. 5. Water source degree is θV
is not exceeded, and the main steam relief safety valve 9 does not operate.

Similarly, the residual heat release system includes a bypass pipe 32, a switching valve 33.
34 was installed, and by closing the switching valve 34 and opening the switching valve 33 when the water level dropped, it was possible to operate even if a tsunami occurred, and the reactor pressure decreased to below a certain pressure. At this point, the reactor shutdown cooling mode operation is entered, the reactor core is cooled, and the reactor 7 can be safely shut down.

FIG. 7 shows an example of the backflow prevention device 17. In the figure, the backflow prevention device 17 is a lift trR installed in the building 20.
It consists of A19 and a gate 22 which is raised and lowered by this. The gate 22 is closed when the tide level is below the design tide level, and the gate 22 is opened when the tide level recovers.
6, the circulating water system and the seawater cooling system of the residual heat removal system form an open/close cycle.

According to this example, even if a reactor scram occurs due to an earthquake or the like during normal reactor operation, and the seawater level drops abnormally during a tsunami, the circulating water system and residual heat removal system can be operated. The nuclear reactor can be safely stopped without operating the Ap1 main steam relief safety valve 9.

Although a preferred embodiment is shown here that is equipped with a tide gauge, the present invention can be carried out even without a tide gauge. This is because the period of a tsunami is much longer than that of a normal wave, and there is plenty of time to manually turn on the gates or switch on the switching valve after the tide level is expected to drop after an earthquake occurs. It is.

Alternatively, the signal from the water intake tank water level detection device 15 may be used as 7F'u.

〔Effect of the invention〕

According to the present invention, even if a reactor scram caused by an earthquake and a tsunami caused by an earthquake occur at the same time as an abnormal seawater tide level drop phenomenon, seawater is kept in the system and the degree of vacuum in the condenser is maintained. An open/close loop seawater cooling system suitable for safely shutting down a nuclear reactor while preventing the main steam relief valve from operating is obtained.

[Brief explanation of drawings]

Figure 1 is a -flJ system diagram of a conventional open-loop seawater cooling system, Figure 2 is a diagram showing the transition of the exchange heat ratio after reactor scram, and Figure 3 is a condenser curve diagram. , Fig. 4 is a diagram showing the transition of the condenser inlet seawater temperature after the circulating water system forms a closed loop, and Fig. 5 shows the condensate water temperature after the circulating water system forms an open/closed loop according to the present invention. FIG. 6 is a diagram showing a change in seawater temperature at the vessel inlet, FIG. 6 is a system diagram of an embodiment of the present invention, and FIG. 7 is a diagram showing a backflow prevention device, etc. 15 circulation pump, 2... water intake tank, 3... condenser, 4
...Water intake, 5...Water outlet, 6...Turbine, 7
...°Reactor, 8...Main steam isolation valve, 9...Main steam relief safety valve, 10-...Pressure suppression chamber, 13.14...
・Switching valve, 15.6° water intake tank water level detection device, 16...
Tide gauge, 17... Backflow prevention device, 19... Elevator,
20... Building, 22... Gate, 23... Residual heat removal pump, 24... Residual heat removal seawater pump, 25...
...Heat exchanger, 31.32...Bypass piping, 33.
34...Switching valve. Agent Patent Attorney Tatsuyuki Unuma 2nd Eye Kaya 3rd Eye Suction 7, oo-One exchange U outside! Hibi No. 4 - Mekyo 5' solid

Claims (1)

[Claims] 1. Seawater is taken into the water intake tank from the water intake, supplied to the circulating water system, residual heat removal system, etc., cools the condenser etc. through heat exchange, and used seawater is discharged into the sea from the water outlet. In a loop type seawater cooling system, a backflow prevention device is installed near the water intake of the water intake tank, and the piping from the circulating water system and residual heat removal system to the water outlet is branched and connected to the water intake tank. A means for selectively discharging seawater to the cooling system is installed, and when the seawater level drops due to a tsunami caused by an earthquake, etc., the seawater that was in the cooling system is circulated to the water tank while preventing seawater from flowing backwards from the water intake. An open-closed loop seawater cooling system that forms a closed-loop seawater cooling system, but when the seawater tide level recovers, the backflow prevention device, etc. is released and the system returns to a normal open-loop seawater cooling system that uses fresh seawater. .