JPS5913719B2 - Residual heat removal system for fast breeder reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a residual heat removal system for removing residual heat and core decay heat after plant shutdown in a sodium-cooled fast breeder reactor.

Heat from the reactor is transferred from the primary main cooling system to the intermediate heat exchanger.
In a sodium-cooled fast breeder reactor that generates electricity by transmitting steam to the main cooler and generating steam using a steam generator, a residual heat removal system is installed to remove residual heat and core decay heat after reactor shutdown. Various designs have been considered, one of which is to provide a bypass flow path in the secondary main cooling system and cool the sodium with an air cooler.

Since this system has a simple configuration, it is considered to be a promising system as a residual heat removal system for future fast breeder reactors.

An example of a conventional system configuration using this system is shown in FIG.

In FIG. 1, details not related to the explanation are omitted.

During normal power operation, the heat of the reactor 1 is transferred to the primary main cooling system 2.
is transmitted to the secondary main cooling system via the intermediate heat exchanger 4,
The steam generator 7 generates steam to generate electricity.

At this time, the secondary main cooling system hot leg 5 and cold leg 6
The residual heat removal system consisting of the air cooler 14 and bypass piping 16 installed between A small flow rate of sodium is supplied by a small-diameter flow control valve 18 so as to cover the amount of heat.

In addition, in order to compensate for changes in sodium volume due to changes in the operating temperature of the secondary main cooling system, an overflow tank 10
Sodium is always pumped up at a constant flow rate by a pump 11 and returned to an overflow tank 10 via an overflow pipe 12.

If any abnormality occurs in the plant in the above system, the reactor will scram and power generation will stop, but the primary main circulation pump 3 and the secondary main circulation pump 8 will be activated to remove residual heat and decay heat of the core. Shifts to low flow rate operation using the pony motor.

At this time, the following problems arise regarding system operation.

That is, (1) water supply to the steam generator 7 is stopped due to a turbine trip accompanying a reactor scram, so in order to protect the steam generator 7, isolation valves 20 and 21 are installed at the inlet and outlet of the steam generator. It is necessary to immediately close the bypass pipe 16, open the valve 17 of the bypass pipe 16, and shift to cooling using the cooler 14.

However, it takes about one minute to start the blower 15 of the cooler, and in the case of loss of external power, it takes even more time to start the blower after waiting for the emergency diesel generator to start up. .

As a result, the high temperature sodium in the hot leg 5 of the secondary main cooling system directly flows into the cold leg 6, causing extreme thermal shock and thermal stress to the air cooler 14, secondary main circulation pump 8° piping, etc. The health of the equipment cannot be maintained.

(2) In order to avoid problem 1 above, in the system configuration example shown in Figure 1, cooling by the steam generator 7 is continued for several minutes until the blower 15 of the air cooler starts up, and the blower starts up. Thereafter, the sodium flow path is switched by a valve, and cooling is shifted to the air cooler 14.

However, in this case, 1) the steam generator is involved in core cooling after a plant accident, so strict design conditions are added as a safety facility for the reactor, and 2) water cannot be supplied to the steam generator even after a turbine trip. 3) The operation control of the entire system has become extremely complex, and the control equipment has also become excessively large.

(3) In addition, in order to improve the safety of nuclear reactors, reactor regulations require that even if all power sources except battery power are lost to the plant, the cooling function of the reactor core will be maintained for a certain period of time. Although requested by the authorities, in the example shown in Figure 1, such a function cannot be expected under conditions where the air cooler floor does not work.

Therefore, an object of the present invention is to provide a residual heat removal system for a fast breeder reactor that can improve the safe shutdown function of a plant in the event of an accident, alleviate thermal shock, and rationalize equipment.

A second example of system configuration employing the residual heat removal system according to the present invention is shown below.
As shown in FIG.

The present invention provides a hot leg 5 and a cold leg 6 of the secondary main cooling system.
An air cooler 14 is placed in a pipe 16 that communicates with the air cooler 14, and a tank 22 in which sodium is retained is provided upstream of the air cooler 14, and a pipe 25 that introduces sodium from the secondary main circulation pump 8 is connected to the tank 22. It is characterized by what it did.

The sodium retention tank 22 shall hold a volume of sodium that can provide at least a residence time longer than the time required to start the cooler blower 15 when sodium at the rated flow rate of the auxiliary core cooling system flows in, and if necessary, A buffer plate is provided in the tank to delay the time for high temperature sodium flowing from the secondary main cooling system hot leg 5 through the pipe 16 to reach the cooler 14.

Further, the tank 22 has a structure having a wide free liquid level as shown in FIG. 3, and can have a function of absorbing the volume change caused by the temperature change of sodium in the secondary main cooling system by the liquid level change.

Furthermore, it is generally useful to install the tank 22 to also serve as the conventional pump overflow column 9 shown in FIG.

In this case, the pipe 25 becomes an overflow pipe for the pump 8.

If it is not possible to obtain a direct bypass flow from the pump body to the piping 25 due to the type or arrangement of the secondary main circulation pump, the piping 25 may be branched from the discharge side of the pump and connected to the tank as shown in Figure 3. It will connect to 22.

In addition, the sodium retention tank 22 functions as a low-temperature sodium storage tank so that the core cooling function can be maintained for a certain period of time even when the cooling blower 15 is inoperable when the plant loses all power. Can be combined.

In this case, the sodium volume of tank 22 is determined from the time required for core cooling.

When the auxiliary core cooling system according to the present invention is adopted, isolation valves 20, 21 are installed at both or one of the inlets and outlets of the steam generator 7.

In addition, when restarting the output operation of the plant, in order to match the sodium temperature around the isolated steam generator with that of the other secondary main cooling system parts, the steam generator is branched from the discharge side of the secondary main circulation pump 8. A branch pipe 23 and a valve 24 connected to the upstream side of the outlet side isolation valve 21 are provided.

In the system configuration shown in FIG. 2 or 3, during power operation of the plant, the valve 17 of the auxiliary core cooling system is closed, and the sodium retention tank 22 and cooler 14 have two
Cold sodium from the secondary main circulation pump 8 is circulated through piping 25.

This circulation amount is about 5 inches of the flow rate of the secondary main cooling system,
Since the temperature drop due to natural heat radiation of the cooler 14 is extremely small, it is not necessary to control the sodium flow rate using the small diameter valve 18 as shown in the example of FIG.

Note that thermal shock in the piping from the valve 17 to the tank 22 shown in FIG. 2 can be dealt with by making the valve 1T a valve that has a structure that allows a small amount of leakage even when it is fully closed.

In the event of an abnormality in the plant, the isolation valves 20 and 21 of the steam generator 7 are closed and the valve 17 is opened simultaneously with the reactor scram.

At the same time, the secondary main circulation pump 8 shifts to low-speed operation using the pony motor, so the low-temperature sodium flowing into the tank 22 from the piping 25 is rapidly reduced, and the high-temperature sodium flows into the tank 22 through the valve 17.

However, since the tank 22 has a longer sodium residence time than the startup time of the cooler blower 15, high-temperature sodium does not flow into the cooler 14 before the blower is started, and the cooler, secondary main circulation pump, etc. No thermal shock will occur.

Moreover, the sodium temperature at the outlet of the cooler can be easily controlled.

On the other hand, the steam generator 7 also does not require operation after the reactor scram, so the design conditions for the steam generator are relaxed, and the design requirements for the steam generator 7 are relaxed, and many times are required to continue water supply after a turbine trip, which was required in the conventional example. Significant streamlining can be achieved, such as eliminating the need for equipment and eliminating difficulties associated with operation control.

In addition, as shown in Fig. 3, if the tank 22 is designed to absorb changes in the volume of the secondary main cooling system due to changes in the liquid level, as shown in Fig. 2, the overflow tank is similar to the conventional design example shown in Fig. 1. 10. Sump pump 11. It becomes possible to remove equipment such as the overflow pipe 12.

Furthermore, if the tank 22 is designed to also serve as the pump overflow column 9 of the conventional example shown in FIG. 1, the pump overflow column shown in FIG. 3 can be removed.

In addition, if the sodium capacity of the tank 22 is increased to have the function of a low-temperature sodium storage tank, even if the cooler blower 15 does not operate due to a total power loss, the intermediate heat exchanger will remain in operation for a certain period of time. 4 can be supplied with low temperature sodium, ensuring cooling of the reactor core.

In this case, the primary main cooling system and the secondary main cooling system are operated by natural circulation or by low-speed operation of the pump motor by an uninterruptible power source (battery).

As shown above, the present invention can extremely effectively solve the conventional problems and achieve even greater rationalization of equipment.

Next, when restarting the output operation of the plant, open the valve 24 of the branch pipe 23 shown in FIG. 3, open the steam generator isolation valve 20, push out the sodium around the steam generator, and circulate it through the cooler 14. This makes it possible to easily equalize the sodium temperature in the entire system and prepare for resumption of output operation.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a conventional fast breeder reactor residual heat removal system;
The figure is a schematic diagram of a fast breeder reactor residual heat removal system of the present invention, and FIG. 3 is a schematic diagram showing a modification of the fast breeder reactor residual heat removal system of the present invention. 1... Nuclear reactor, 2... Primary main cooling system,
3...Primary main circulation pump, 4...Intermediate heat exchanger, 5...Secondary main cooling system hot leg, 6
...Secondary main cooling system cold leg, 7...
・Steam generator, 8... Secondary main circulation pump, 9.
...Pump overflow tank column, 10.
... Overflow tank, 11 ... Sump pump, 12 ... Overflow piping, 1
3... Dump tank, 14... Air cooler, 15... Blower, 16... Auxiliary core cooling system piping, 17... Flow control valve, 18.
... Small mouth system flow control valve, 20 ... Steam generator inlet isolation valve, 21 ... Steam generator outlet isolation valve, 22 ... Sodium retention tank, 23...
...Piping, 24...Valve.

Claims (1)

[Scope of Claims] The steam generating system is connected between a hot leg and a cold leg of a secondary main cooling system having a 12th main circulation pump, a steam generator, and a steam generator isolation valve for isolating the steam generator. In a fast breeder reactor residual heat removal system equipped with a bypass passage that bypasses the reactor, a flow rate control valve, a sodium retention tank, and an air cooler are connected in series to the bypass passage in order from the upstream side, and the secondary A residual heat removal system for a fast breeder reactor, characterized in that a branch pipe branched from the discharge side of a main circulation pump is connected to the sodium retention tank. 2. The sodium retention tank has a shape such that the coolant contained therein has a wide free liquid surface, and the volume change due to temperature change of the coolant in the secondary main cooling system is absorbed into the sodium retention tank. 2. The residual heat removal system for a fast breeder reactor according to claim 1, wherein the residual heat removal system is adapted to absorb changes in the liquid level of the coolant, and is also used as a receiving tank for the overflow of the secondary main circulation pump.