JPH06265665A - Natural circulation type boiling water reactor - Google Patents

Abstract

PURPOSE:To obtain a stable flow rate of natural circulation at the time of starting under a low pressure. CONSTITUTION:In a reactor pressure vessel 1, a core 2, riser 3, a shroud, downcomer 5 and a lower plenum 7 are provided respectively. A drain piping 10 is connected to the bottom of the lower plenum 7. A heat exchanger 22 is fitted to the drain piping 10 through a valve 21. The fore end of an injection water piping 23 coming out from the heat exchanger 22 is disposed at the lower part of the downcomer 5. At the time of starting under a low pressure, reactor water 6 is sent into the heat exchanger 22 by opening the valve 21. The reactor water 6 heated by the heat exchanger 22 is returned into the lower plenum 7 from the injection water piping 23. Thereby the temperature of the lower plenum 7 is raised and a stable flow rate of natural circulation is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a natural circulation type boiling water nuclear reactor, and more particularly to a natural circulation type boiling water nuclear reactor capable of ensuring a stable natural circulation flow rate at low pressure startup.

[0002]

2. Description of the Related Art FIG. 4 shows a conventional natural circulation type boiling water nuclear reactor. In the figure, reference numeral 1 is a reactor pressure vessel. While being stored, a riser 3 is formed above the core 2.
A downcomer 5 is formed on the outer side of these through a cylindrical shroud 4. The reactor water 6 naturally circulates in the downcomer 5, the lower plenum 7, the core 2 and the riser 3, and the steam generated in the core 2 is sent to a gas turbine (not shown) through the main steam pipe 8 and the turbine. The steam after working in is returned to the inside of the reactor pressure vessel 1 through the water supply pipe 9 after the condensate.

At the bottom of the reactor pressure vessel 1, as shown in FIG. 4, is a drain pipe 10 which is a part of a coolant purification system.
Is provided, and the reactor water 6 extracted by the drain pipe 10 is sent to a heat exchanger 11 composed of an electric heater or the like, and the reactor water 6 heated here is supplied with a water injection pipe 12 and a water supply pipe. It is designed to be returned to the downcomer 5 via 9.

In the above structure, when the decay heat is not available at the time of start-up (heating and pressure rise of the reactor), the reactor water 6 is
The heat exchanger 11 sufficiently heats up to about 100 ° C. under atmospheric pressure. Then, after that, the control rod is pulled out to make the reactor critical, and the reactor is heated and pressurized by nuclear heating.

[0005]

FIG. 5 shows natural circulation characteristics in a natural circulation type boiling water reactor.
The natural circulation flow rate is determined by the density difference between the core / riser section and the downcomer section. Under high pressure conditions such as during normal operation, even with a small output, the natural circulation flow rate is large and it is found to be thermally stable. . On the other hand, the natural circulation characteristics at low pressure startup have not been evaluated in detail, but since the density ratio of water and steam is large at low pressure, the natural circulation flow rate is higher than at the rated output for the same output. It is considered that there is no problem that a sufficient natural circulation flow rate is not secured at low pressure startup. However, at low pressure startup,
It is important to ensure a stable and high natural circulation flow rate.

The pressure at the time of starting the reactor is about atmospheric pressure. Natural circulation reactors with large output are provided with risers with relatively large vertical dimensions in order to increase the natural circulation flow rate. The natural circulation flow rate is determined by the density difference between the core / riser section and the downcomer section as described above, and this density difference is due to the temperature difference or the void ratio difference. In order to increase the natural circulation flow rate, it is necessary to make the temperature or void ratio of the core / riser part larger than that of the downcomer part. Further, in the core / riser part, the natural circulation flow rate increases as the temperature or void rate increases toward the lower part. On the contrary, when the temperature of the downcomer part is higher than that of the core / riser part, the natural circulation flow rate decreases. That is, when the natural circulation reactor is started (near the atmospheric pressure), if the temperature distribution shown in FIG. 6 is achieved, a high flow rate of natural circulation is secured.

However, in the conventional natural circulation type nuclear reactor, since the reactor water 6 heated by the heat exchanger 11 is returned to the downcomer 5, the temperature of the downcomer 5 rises and a high flow rate of natural circulation occurs. Increasing the flow rate is difficult. Further, the high-temperature reactor water 6 that has passed through the heat exchanger 11 is injected from the water supply pipe 9, and since this injection position is a little lower than the normal water level, up to 100 ° C under atmospheric pressure. Only the reactor water 6 can be injected. Furthermore, since the water injection position is the upper part of the reactor pressure vessel 1, the natural circulation force is weak and a stable natural circulation flow cannot be expected. If sufficient natural circulation flow rate cannot be obtained at low pressure startup, low temperature reactor water 6
May flow into the core 2 to cause a change in reactivity, which may cause an unstable heat-hydraulic phenomenon, and temperature stratification may occur in the lower plenum 7 to make the thermal stress of the reactor pressure vessel 1 severe. is there.

Therefore, in part, various proposals have been made to solve this problem. That is, Japanese Patent Publication No.
There is one disclosed in Japanese Patent No. 1159. this is,
A heat exchanger is incorporated in the reactor pressure vessel, and heat from a boiler provided outside the reactor pressure vessel is supplied to the reactor cooling water in advance through the heat exchanger at startup.
However, even with this method, since only the reactor water in the upper part of the downcomer is warmed, the driving force for natural circulation cannot be obtained, and it is difficult to secure a high natural circulation flow rate.

An example of the natural circulation type nuclear reactor is disclosed in Japanese Patent Laid-Open No. 59-217188. This is to install a heat exchanger in the lower part of the core in the reactor pressure vessel to shorten the time required for startup and facilitate control at startup. However, although this method easily obtains a high natural circulation flow rate for warming the lower plenum, a large number of control rod guide tubes and their drive mechanisms are provided in the lower plenum, so that the location of the heat exchanger is It is difficult to secure.

Further, as an example of a boiling water reactor pressure vessel, there is one disclosed in Japanese Patent Laid-Open No. 4-188096. This is because in order to make the temperature distribution in the reactor pressure vessel as uniform as possible at startup, the high temperature coolant above the reactor core in the pressure vessel is taken out from the coolant purification system piping, and the pressure vessel passes through the bottom drain piping. Water is injected from the bottom of the reactor pressure vessel. However, this method also has two problems. That is, since the high temperature coolant does not exist in the upper part of the core before the criticality of the nuclear reactor, high temperature cooling water cannot be injected into the lower plenum, and a high natural circulation flow rate cannot be secured. Further, since the flow of the normal coolant purification system is in the direction of extracting the coolant from the bottom of the pressure vessel, it is necessary to change the pump characteristics of the coolant purification system when such a method is adopted. .

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a natural circulation type boiling water reactor capable of ensuring a stable natural circulation flow rate at low pressure startup.

[0012]

As a means for achieving the above object, the present invention is to connect a heat exchanger to a drain pipe at the bottom of a reactor pressure vessel via a valve and to provide an outlet pipe from the heat exchanger. Open the tip of the valve near the bottom of the downcomer, and open the valve when the low pressure starts,
It is characterized in that high temperature water is injected into the lower plenum.

Further, in the present invention, it is preferable to include a controller for adjusting the output of the heat exchanger according to the difference between the saturation temperature in the lower plenum and the temperature in the lower plenum.

[0014]

In the natural circulation type boiling water reactor according to the present invention, the valve is opened when the low pressure is started, and the reactor water in the lower plenum is extracted through the drain pipe. Then, the high temperature water heated by the heat exchanger is injected into the lower plenum from a position near the bottom of the downcomer.

As described above, the natural circulation flow rate is determined by the density difference between the inside of the shroud such as the core / riser section and the outside of the shroud which is the downcomer. That is, the higher the temperature inside the shroud and the more steam is generated inside the shroud, the more the natural circulation flow rate increases.

In the present invention, since the high temperature water is directly injected into the lower plenum, the lower plenum temperature can be raised without raising the temperature of the downcomer, and under atmospheric pressure, a riser having a large vertical dimension can be used. The natural head used in the natural circulation reactor has a large head, and the saturation temperature differs by about 25 ° C inside the reactor pressure vessel, so it is possible to inject higher temperature hot water and increase the natural circulation driving force. it can. As a result, a stable natural circulation flow is secured even before the reactor is started, and the subcooling degree at the core inlet is small even in a very small output area after the reactor is started, so steam is likely to be generated in the core. Becomes
Further, the natural circulation driving force increases and the natural circulation flow rate increases.

Further, in the present invention, by providing a controller for adjusting the output of the heat exchanger according to the difference between the saturation temperature in the lower plenum and the temperature in the lower plenum, the heat exchanger is adjusted in accordance with the lower plenum temperature. It becomes possible to adjust the output.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a natural circulation type boiling water reactor according to an embodiment of the present invention. In the figure, reference numeral 1 is a reactor pressure vessel, and the inside of this reactor pressure vessel 1 is shown. , Core 2
And the riser 3 is installed above the core 2.
Are formed, and downcomers 5 are formed on the outer sides of these through a cylindrical shroud 4. The reactor water 6 naturally circulates in the downcomer 5, the lower plenum 7, the core 2 and the riser 3, and the steam generated in the core 2 is sent to a turbine (not shown) via the main steam pipe 8 and works at the turbine. The steam after the operation is returned to the inside of the reactor pressure vessel 1 as water supply through the water supply pipe 9 after the condensate.

At the bottom of the reactor pressure vessel 1, as shown in FIG. 1, is a drain pipe 10 which is a part of a coolant purification system.
The drain pipe 10 is provided with a valve 2
1 to which a heat exchanger 22 is connected, and the heat exchanger 22
The tip of the outlet water injection pipe 23 is open at the bottom of the downcomer 5. Then, the reactor water 6 heated by the heat exchanger 22 is directly injected into the lower plenum 7 from the bottom of the downcomer 5.

As shown in FIG. 1, a pressure gauge 24 and a thermometer 25 are provided in the lower plenum 7, and output signals from these are input to a controller 26, and the controller 26 outputs them. The control signal from the valve 21
Is controlled and the output of the heat exchanger 22 is controlled.

That is, the controller 26 uses the pressure gauge 24
The saturation temperature in the lower plenum 7 is calculated based on the output signal from the thermometer 25, and the difference between the calculation result and the output signal from the thermometer 25 is calculated. The output is adjusted by the difference signal.

Next, the operation of this embodiment will be described.

At the time of starting at low pressure, the valve 21 is opened to send the reactor water 6 to the heat exchanger 22, and the high temperature water heated by the heat exchanger 22 is returned from the lower part of the downcomer 5 to the lower plenum 7.

As an example, FIG. 2 shows changes in the output of the heat exchanger 22, changes in the temperature of the thermometer 25, and changes in the natural circulation flow rate when the reactor is started (temperature rise and pressure increase) from atmospheric pressure and reactor water of 30 ° C.

Initially, the temperature of the coolant in the reactor pressure vessel 1 is uniform and 30 ° C. Since the pressure of the steam dome portion is atmospheric pressure, the saturation temperature of the lower plenum 7 is 125 ° C. considering the head head. The signal from the controller 26 opens the valve 21 and activates the heat exchanger 22. Then, the controller 26 first adjusts the output of the heat exchanger 22 so that the temperature of the lower plenum 7 becomes 125 ° C. When the reactor water 6 heated by the heat exchanger 22 is injected into the lower part of the downcomer 5 by the water injection pipe 23, the coolant temperature in the lower plenum 7 becomes 30 ° C. or higher, and a natural circulation flow occurs. The high-temperature water is supplied to the lower plenum 7 from the bottom of the downcomer 5, so that the natural circulation flow rate increases as the temperature of the lower plenum 7 increases. And
As the temperature of the lower plenum 7 increases, the output signal value of the thermometer 25 increases and the controller 26 keeps the output of the heat exchanger 22 low.

When the natural circulation flow rate is sufficiently secured in this way, the control rod is pulled out and the output increases.
As a result, the temperatures of the core 2 and the riser 3 are further increased by the outputs of the heat exchanger 22 and the core 2, and the natural circulation flow rate is increased to provide a stable flow.

Therefore, as shown in FIG. 3, at the time of starting at a low pressure, the core inlet subcooling degree can be lowered, and even if the output is low, steam is easily generated in the core, the natural circulation driving force increases, and Stable natural circulation flow rate can be obtained. As a result, it is possible to prevent the occurrence of an unstable flow phenomenon that tends to occur at low pressure and low output, and to prevent stratification of the lower plenum temperature.

[0029]

As described above, according to the present invention, the valve is opened at the time of starting the low pressure, and the high temperature water heated by the heat exchanger is
Since it is injected into the lower plenum from a position near the bottom of the downcomer, it is possible to secure a stable natural circulation flow rate at low pressure startup.

Further, in the present invention, by providing a controller for adjusting the output of the heat exchanger according to the difference between the saturation temperature in the lower plenum and the temperature in the lower plenum, the heat exchanger is adjusted according to the lower plenum temperature. The output can be adjusted.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing a natural circulation type boiling water reactor according to an embodiment of the present invention.

FIG. 2 is a graph showing changes in heat exchanger output, lower plenum temperature, and natural circulation flow rate at startup.

FIG. 3 is a graph showing the effect of the present invention.

FIG. 4 is a configuration diagram showing a conventional natural circulation type boiling water reactor.

FIG. 5 is a graph showing the natural circulation characteristics of the natural circulation reactor.

FIG. 6 is an explanatory diagram showing a temperature distribution at a high natural flow rate.

[Explanation of symbols]

 1 Reactor pressure vessel 2 Core 3 Riser 4 Shroud 5 Downcomer 6 Reactor water 7 Lower plenum 21 Valve 22 Heat exchanger 23 Water injection pipe 24 Pressure gauge 25 Thermometer 26 Controller

Claims (2)

[Claims] 1. A heat exchanger is connected to a drain pipe at the bottom of a reactor pressure vessel via a valve, and a tip of an outlet pipe from the heat exchanger is opened to a position near a bottom of a downcomer, and the valve is , A natural circulation type boiling water reactor characterized by opening at low pressure startup and injecting high temperature water into the lower plenum. 2. The natural circulation type boiling according to claim 1, further comprising a controller that adjusts the output of the heat exchanger according to the difference between the saturation temperature in the lower plenum and the temperature in the lower plenum. Water reactor.