JPH09230088A - Stop method for boiling water nuclear power plant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the dose rate of a residual heat removing system by starting the operation of a residual heat removing system within a range of specified reactor water temperature and a reactor internal pressure gauge indicated value. SOLUTION: With the start of operation for stopping a plant, a control rod is inserted in fuel to stop nuclear reaction. The calorific value from fuel is decreased, and at the stage where the steam generation quantity is decreased and it is difficult to keep rated reactor pressure, power generation is stopped. When the reactor water temperature reaches below about 130 deg.C (reactor internal pressure gauge indicated value 2kg/cm<2> ), warming operation is started. At the stage where a difference between the reactor water temperature and the residual heat removing system outlet temperature is about 50 deg.C or less, the warming operation is ended. After that, reator water is passed through the residual heat removing system, and a nuclear reactor accessary cooling water is introduced to a residual heat removing system heat exchanger 23 through a piping 29 to cool the reactor water in the residual heat removing system. Thus, by the operation of setting the operation start temperature of the residual heat removing system to about 130 deg.C (reactor internal pressure gauge indicated value 2kg/cm<2> ) or less, it is possible to reduce adhesion of ion-like reactive matter to the residual heat removing system piping.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces the amount of radioactive substances accumulated on the surface of a structural material in contact with reactor water by maintaining a low concentration of radioactive water in the reactor water in a boiling water nuclear power plant. Related to the technology for reducing the radiation dose received by workers who perform regular inspections.

[0002]

2. Description of the Related Art FIG. 2 shows the structure of a boiling water nuclear power plant.

The steam generated by the combustion of the nuclear fuel loaded in the nuclear reactor 1 is introduced into the high pressure turbine 3 and the low pressure turbine 4 through the main steam pipe 2 to perform work. After the work is completed, the steam is condensed in the condenser 5, passes through the condenser pipe 6, passes through the condensate pump 7, the condensate filter 8, the condensate desalinator 9, and the condensate booster pump 10 to the water supply system. Be guided. In the water supply system, the water is passed through the water supply pipe 11 and the water supply pump 12, heated by the water supply heater 13, and then returned to the reactor again.

On the other hand, the reactor water circulates in a two-line recirculation system composed of a recirculation pump 14 and a recirculation pipe 15.
Further, a part of the reactor water is guided to the reactor purification facility 18 through the reactor purification system pump 16 and the reactor recirculation system pipe 17 and purified. Then, it joins the water supply system piping 11 and returns to the nuclear reactor. Meanwhile, the reactor decontamination equipment filtration desalination device 18
Since the powder ion exchange resin is used, the reactor water is cooled by the reactor cleaning system heat exchanger 19 because it is necessary to cool the reactor water to about 50 ° C. In addition, impurities flowing from the feedwater are concentrated by boiling in the nuclear reactor, and a part of the impurities is removed by a filtration and desalination apparatus of the reactor purification system, so that the reactor water is kept clean.

Furthermore, when the output of the nuclear power plant is reduced to stop the cold temperature, a control rod is inserted between the fuels to stop the nuclear reaction of the fuels, and the residual heat of the fuels is removed as vaporization heat by evaporating the reactor water. At the same time, the reactor water is guided to the residual heat removal system that branches from the reactor recirculation system and heat-exchanged to cool it. The plant shutdown operation of a boiling water nuclear power plant follows the procedure below.

(1) The rated reactor water temperature is 280 ° C., and control rods are inserted in that state to stop the nuclear reaction of fuel.

(2) From 280 ° C to 15 at the same time as the operation of (1)
Since the reactor pressure is high and the amount of steam generated is large up to about 0 ° C., steam is sent from the main steam line 2 to the condenser to cool the reactor by heat of vaporization. After the turbine is disconnected, steam is sent from the turbine bypass line 20 to the condenser for cooling.

(3) Before starting the operation of the residual heat removal system, warming is performed to gradually raise the temperature of the system and bring it close to the reactor water temperature in order to alleviate thermal stress of equipment and piping and maintain soundness. Drive.

(4) At a temperature of 150 ° C. or lower, the cooling efficiency due to the heat of vaporization decreases, so the residual heat removal system piping 21 and the residual heat removal system pump 22 branched from the reactor recirculation system.
The reactor water is guided to the residual heat removal system heat exchanger 23 for cooling.

Further, the cooling rate of the reactor water is adjusted by controlling the flow rate of water flowing through the residual heat removal system heat exchanger 23 and the bypass piping 24 of the residual heat removal system heat exchanger.

Next, the application status of the dose reduction technology in a boiling water nuclear power plant will be described.

Generally, corrosion products such as metal ion components and insoluble components (clads) are eluted little by little from the surfaces of constituent materials such as pipes, pumps and heat exchangers of boiling water nuclear power plants. Corrosion products eluted from the material flow into the reactor with cooling water and adhere to the fuel surface. Corrosion products adhering to the surface of the fuel are irradiated with neutrons generated by the combustion of the nuclear fuel and become radioactive substances with a long half-life such as Co-58 or Co-60. Some of the corrosion products that have become radioactive due to being attached to the fuel surface are eluted or desorbed again into the reactor water, and the reactor coolant recirculation system that circulates the reactor water or the reactor water. It adheres to and accumulates on the inner surface of the equipment and pipes of the reactor coolant purification system that purifies the impurities of, and has a radiation dose rate.

Here, in order to suppress the dose of the residual heat removal system, the publication K.OTOHA, et. Al., International Conf.
erence on Water Chemistry of Nuclear Reactor Syste
ms, 5, 315-316, 1989, clad containing radioactive material adhering to the fuel surface by reducing the rate of change of reactor water (solid corrosion products that do not dissolve in reactor water It is reported that it is effective to lower the concentration of the form.

Further, as shown in Japanese Patent Application Laid-Open No. 7-157618, in the temperature range of about 150 ° C., which is the operating temperature of the residual heat removing system, corrosion of carbon steel, which is a constituent material of the residual heat removing system, occurs Since radioactive substances in the form of (components dissolved in reactor water) are taken in, a method has been proposed in which the system is pre-oxidized to suppress the incorporation of radioactivity.

[0015]

As described above, in order to suppress the increase in the dose of the residual heat removal system, the concentration of the clad component is suppressed when the plant is stopped and the intake of the ionic component radioactive substance into the oxide film is suppressed. It is effective to do both at the same time. On the other hand, in order to improve the operation rate of the plant, it is desirable to improve the efficiency and shorten the period of the periodic inspection period, and to reduce the dose in consideration of the shortening of the plant shutdown period.

Therefore, the present inventors first examined the operation method for maintaining the dose rate of the residual heat removal system. Specifically, a test was conducted on the behavior of incorporating radioactive substances into the carbon steel pipe forming the residual heat removal system. The test results conducted by the present inventors are shown below.

The adhesion behavior of Co to carbon steel used in the residual heat removal system was carried out using a Co-58 tracer. As the test conditions, test water containing Co-58 was passed through the carbon steel pipe, and the dose rate change on the pipe surface was measured to assume the amount of Co adhering to the inner surface of the pipe. The temperature of the test water passing through the inner surface of the pipe was lowered at a temperature lowering rate of about 15 ° C./h with an upper limit of 150 ° C. which is the operating temperature of the residual heat removal system.

[0018]

The results of the test are shown in FIG.
As shown in FIG. 1, the dose rate change on the surface of the carbon steel pipe while the test water temperature was decreasing from 150 ° C. at a constant temperature decrease rate was saturated when it reached about 130 ° C. This phenomenon is
It shows that the corrosion of carbon steel was reduced at a temperature of 130 ° C. or lower, and as a result, the radioactive material stopped adhering and progressing. Based on these results, the operating temperature of the residual heat removal system was set at 150 ° C (conventional furnace pressure gauge: approx. 4 kg / cm).
² ) to approx. 130 ° C (pressure reading in furnace: approx. 2 kg / c
By setting it to be m ² ) or less, it is possible to suppress the adhesion of radioactivity to the inner surface of the residual heat removal system pipe, and it is possible to keep the dose rate of the plant low. The pressure in the furnace shown in the present invention means not the absolute pressure but the value indicated by the pressure gauge.

On the other hand, it is necessary to perform a warming operation before starting the operation of the residual heat removal system, which is performed by passing the reactor water to the pressure suppression chamber at atmospheric pressure by the pressure in the furnace.

If the reactor pressure becomes too low, the reactor water will pass through the residual heat removal system for a long time, or if the reactor water temperature is 100 ° C. (reactor pressure gauge reading: 0 kg / cm ² ) or less. You will not be able to water. Therefore, in order to shorten the stop time, in order to perform the warming operation and then cool the reactor water, it is necessary to set the lower limit pressure and temperature for passing the reactor water through the residual heat removal system. . Specifically, considering the flow rate required for warming operation and the system pressure loss of the residual heat removal system, the indicated value of the in-core pressure gauge is 0.
It is necessary to secure a pressure of 5 kg / cm ² (reactor water temperature of about 110 ° C) or higher. From the viewpoint of suppressing the radioactivity from adhering to the inner surface of the pipe and ensuring the reactor pressure necessary to perform warming operation, the reactor water temperature is 130 ° C to 110 ° C (reactor pressure gauge reading 2
It is considered effective to start the operation of the residual heat removal system within the range of up to 0.5 kg / cm ² .

Further, in order to start the operation of the residual heat removal system within the range of the reactor water temperature of 130 ° C. to 110 ° C. (reactor pressure of ^{2 to} 0.5 kg / cm ² ), the reactor water is kept at a temperature of 130 ° C. or less. It is necessary to reduce the vapor rapidly by the method of guiding the steam to the condenser, while suppressing the separation of the clad adhering to the fuel surface. In order to rapidly reduce the reactor water temperature in the range of 150 ° C. or lower, it becomes necessary to open the turbine bypass valve opening more than ever. On the other hand, the operation of opening the turbine bypass valve to a large degree may cause large fluctuations in the steam flow rate, increase clad delamination on the fuel surface, and enter the system during residual heat removal system operation, reducing the dose reduction effect. have. In order to reduce the fluctuation range of the steam flow rate and to stabilize the reactor water low acceleration, it is effective to automatically control the turbine bypass valve opening based on the information of the cooling rate of the reactor water temperature.

In order to reduce the dose of the residual heat removal system and to perform it within a certain range of the plant shutdown period, the reactor water temperature should be 1
30 ℃ -115 ℃ (In-furnace pressure gauge reading 2-0.5kg / c
It is effective to start the operation of the residual heat removal system in the range of m ² ) and reduce the amount of ionic component radioactive substances adhering to the inner surface of the residual heat removal system piping. In addition, the method of blowing steam from the turbine bypass system to the condenser up to the reactor water temperature of 130 ° C or lower automatically adjusts the opening of the turbine bypass valve based on the data of the cooling rate of the reactor water temperature. It is possible to keep the dose rate of the residual heat removal system lower than the current level by suppressing the radiation of the clad component radioactivity by suppressing the radiation of the clad component and by suppressing the amount of the radioactive material of the clad component into the residual heat removal system. .

In the automatic control of the turbine bypass valve, the rate of change is calculated from the data of the reactor water temperature. If the rate of change is smaller than the plan, the opening of the turbine bypass valve is opened at regular intervals, while the rate of change of the reactor water temperature is If it is larger than the plan, it is possible to add an automatic control device that repeatedly performs the operation of closing the opening of the turbine bypass valve at regular intervals.

By applying the present invention, the dose rate of the residual heat removal system can be kept low, and by adding an automatic control device for the turbine bypass valve, it is possible to contribute to the reduction of the stop time.

[0025]

Embodiments of the present invention will be described below.

Reactor water temperature 130 ° C. (Reactor pressure gauge reading: 2
FIG. 3 shows an example of a method of cooling using a residual heat removal system after kg / cm ² ).

When the plant stop operation is started, the control rod is inserted between the fuels to stop the nuclear reaction. Further, the reactor water is sent as steam from the main steam pipe 2 to the condenser 5 via the high-pressure turbine 3 and the low-pressure turbine 4 while the heat generation from the fuel in the initial stage of the stop operation continues, as in the operation. When the amount of heat generated from the fuel is small and it becomes difficult to reduce the amount of steam generated and maintain the rated furnace pressure, the turbine is disconnected and power generation is stopped.

The steam thereafter is the turbine bypass line 2
It is sent from 0 to a condenser and condensed. At this time, the rate of change of the reactor water temperature changes depending on the amount of steam sent from the turbine bypass line to the condenser.

Further, the amount of steam is adjusted by adjusting the opening of the turbine bypass valve 26. In order to keep the rate of decrease of the reactor water temperature constant, the operation of gradually increasing the opening of the turbine bypass valve must be performed carefully.
Since the change in reactor pressure due to the decrease in temperature gradually decreases, it is necessary to gradually increase the rate of change in the degree of opening of the turbine bypass valve in order to keep the rate of decrease in reactor water temperature constant. Yes Requires skilled operator.

The reactor water temperature is about 130 ° C. (reactor pressure gauge reading:
Start warming operation when it reaches 2kg / cm ² ) or less. In the warming operation, the reactor water is guided from the inlet pipe of the residual heat removal system, the valve 33 is opened in the pressure suppression chamber 30 through the residual heat removal system circulation pump 20 and the heat exchanger 23, and the reactor water is blown from the pipe 31. To do. The warming operation is ended when the difference between the reactor water temperature and the residual heat removal system outlet temperature becomes approximately 50 ° C. or less. After that, the reactor water is passed through the residual heat removal system, and the reactor auxiliary cooling water is introduced into the residual heat removal system heat exchanger 23 through the pipe 29 to cool the reactor water. The operation start temperature of this residual heat removal system is about 130
It becomes possible to reduce the adherence of ionic radioactive substances to the residual heat removal system piping by the operation below ℃ (reactor pressure gauge indicated value: 2 kg / cm ² ).

In the second embodiment, the operation start temperature of the residual heat removal system of the first embodiment is about 130.degree. C. (internal pressure gauge indicated value: 2 kg).
/ Cm ² ) or less to suppress the adhesion of the ionic component radioactive material, and also to reduce the adhesion of the clad component radioactive material to the residual heat removal system by automatically controlling the opening of the turbine bypass valve. (See Figure 4).

The start of automatic control of the turbine bypass valve is
This is performed by providing a switching switch 37 on the conventional manual turbine bypass valve operation signal line 34 and switching to a signal from the reactor water temperature change rate controller.

In the turbine bypass valve automatic control method, the indicated value of the reactor water temperature is called into the temperature change controller 36, and the reactor water temperature change rate (dT / dt) is calculated at regular time intervals. When the reactor water temperature change rate is smaller than a preset change rate, a signal for increasing the opening degree of the turbine bypass valve by a certain amount is sent. On the contrary, when the rate of change of the reactor water temperature is higher than the preset rate of change, a signal for reducing the opening of the turbine bypass valve by a fixed amount is sent. By performing this operation continuously at regular intervals, it becomes possible to control the rate of decrease of the reactor water temperature at a constant rate. This operation is continued until the reactor water temperature becomes approximately 130 ° C. (reactor pressure gauge indicated value: 2 kg / cm ² ) or less, and then cooling operation in the residual heat removal system is performed. This makes it possible to suppress not only the adhesion of the radioactive substance of the ionic component but also the adhesion of the radioactive substance concentration of the clad component.

In the third embodiment, a control method different from that of the second embodiment is used to automatically control the opening of the turbine bypass valve to reduce the adhesion of the clad component radioactive material to the residual heat removal system. An example will be shown (see FIG. 5).

The start of automatic control of the turbine bypass valve is
This is performed by providing a switching switch 37 on the conventional manual turbine bypass valve operation signal line 34 and switching to a signal from the reactor water temperature change rate controller.

In the method of automatically controlling the turbine bypass valve, the temperature change with respect to the stop operation time is input to the reactor water temperature controller in advance. The difference between the temperature change and the actual reactor water temperature with respect to the stop operation time is sequentially measured. When the actual reactor water temperature is higher than the planned reactor water temperature as shown in (1) of FIG. 5, a signal for increasing the opening degree of the turbine bypass valve by a certain amount is sent. On the contrary, when the actual reactor water temperature is lower than the planned reactor water temperature as shown in (2) of FIG. 5, a signal for reducing the opening degree of the turbine bypass valve by a certain amount is sent. By performing this operation continuously at regular intervals, it becomes possible to control the rate of decrease of the reactor water temperature at a constant rate.

This operation is continued until the reactor water temperature becomes about 130 ° C. (reactor pressure gauge indicated value: 2 kg / cm ² ) or less, after which the cooling operation in the residual heat removal system is performed. This makes it possible to suppress not only the adhesion of the radioactive substance of the ionic component but also the adhesion of the radioactive substance concentration of the clad component.

[0038]

By applying the method shown in the present invention, the dose of the residual heat removing system can be further reduced, and the dose rate received by workers such as inspection workers in the plant can be reduced.

The enhancement of the maintenance and management technology for such a nuclear power plant can improve the reliability of the plant and contribute to a stable power supply.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the test results of the relationship between radioactivity adhesion to a carbon steel oxide film and temperature.

FIG. 2 is a system diagram showing a configuration of a boiling water nuclear power plant.

FIG. 3 is a system diagram of an example of an operation in which the operation start temperature of the residual heat removal system is lowered.

FIG. 4 is a system diagram showing an embodiment in which the opening of a turbine bypass valve is automatically controlled.

FIG. 5 is a system diagram showing an embodiment for automatically controlling the opening of a turbine bypass valve.

[Explanation of symbols]

1: Nuclear reactor, 8: Condensate filtration unit, 9: Condensate desalination unit, 1
8 ... Reactor purification system filtration desalination apparatus, 20 ... Residual heat removal system pump, 21 ... Residual heat removal system piping, 23 ... Residual heat removal system heat exchanger, 26 ... Turbine bypass valve, 35 ... Reactor water thermometer, 36 ... Reactor water temperature change controller, 37 ... Turbine bypass valve signal switching switch.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsumi Ohsumi 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd., Hitachi Works

Claims (2)

[Claims] 1. In a reactor shutdown operation of a boiling water nuclear power plant, the operation start temperature of the cooling operation mode when the residual heat removal system is stopped is set to a reactor water temperature of 130 ° C. to 110 ° C. or a reactor pressure gauge reading of 2 kg. / Cm ² to 0.5 kg / cm ^{2 A} method for shutting down a boiling water nuclear power plant characterized by being set to 0.5 kg / cm ² . 2. In a reactor shutdown operation of a boiling water nuclear power plant, the operation start temperature of the cooling operation mode when the residual heat removal system is stopped is set to a reactor water temperature of 130 ° C. to 110 ° C. or a reactor pressure gauge reading of 2 kg. / Cm ² to 0.5 kg / cm ² and lowering the reactor water temperature to 130 ° C or the reactor pressure gauge reading 2 kg / cm ^{2 according} to the preset rate of change in reactor temperature. A method for shutting down a boiling water nuclear power plant, which comprises performing a method of sending steam from a turbine bypass system to a condenser by automatically controlling the opening degree of a valve of the system to cool the system.