JPH05256991A - Activation method for natural circulation type reactor - Google Patents

Abstract

PURPOSE:To prevent the generation of instability during a startup by withdrawing control rods after degassing operation for startup of natural circulation state, rising the temperature and pressure by nuclear heating and obtaining the rated power. CONSTITUTION:A steam condenser 7 is evacuated with a vacuum pump 11 and the inside of the reactor is degassed by opening the main steam drain valve 9 with the main steam isolation valve 8 closed. At this moment, the incore pressure is lowered to approximately 0.3 atmosphere. Then control rods 3 are withdrawn to start nuclear heating. The power is suppressed to 1% of the rated power. As the cooling water temperature at this state is the room temperature and the core inlet subcooling is ca. 50 deg.C, the geysering does not occur. This is because coolant heatup by the nuclear heating still does not begin at this moment. Therefore, it is not necessary after this time to keep the power at 1% of the rated power for escaping the geysering and so the reactor power can be maximized within the coolant heatup rate of 55 deg.C/hr and the startup time is shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a startup method after a cold shutdown of a nuclear reactor, and more particularly to a startup method suitable for ensuring thermal-hydraulic stability of a nuclear reactor in a natural circulation state at startup. ..

[0002]

2. Description of the Related Art Generally, a boiling water reactor has a characteristic that a range (stability margin) in which the reactor can be stably operated during natural circulation is narrow.

For this reason, it is necessary to fully consider the stability margin when starting up the reactor. FIG. 6 shows the relationship between the reactor power and the core flow rate and the unstable region based on the conventional knowledge. It can be seen that the range in which the reactor can be operated stably during natural circulation is narrow.

As a method of increasing the stability margin, there is a method of forcibly circulating the cooling water of the reactor by a recirculation pump. As shown in FIG. 6, the forced circulation reactor uses a method of increasing the output while circulating the cooling water at a rated 20% pump speed. By such a method, it is possible to make the stability margin larger at the time of startup than at the time of operating in the natural circulation state.

The conventional reactor start-up method described above considers only the instability phenomenon known so far. However, it became clear that when the reactor is in a natural circulation state with void formation in the core, an instability other than the above-mentioned instability exists. Therefore, there is a problem that a sufficient stability margin cannot be secured at the time of startup in a nuclear reactor that is started by natural circulation.

A conventional technique for preventing the occurrence of the unstable phenomenon peculiar to the natural circulation is, as described in JP-A-59-143997, for heat supply at the time of periodic inspection at the time of starting the reactor. By supplying the heat from the boiler to the cooling water in the reactor pressure vessel, the temperature of the cooling water is raised to reduce subcooling and then nuclear heating is started to prevent deterioration of stability. As described in JP-A-60-69598, the subcooling of cooling water at the core inlet was set to a range smaller than the critical subcooling degree at which instability occurs by the heat exchanger installed in the lower plenum of the reactor. There is a system that can start the reactor while ensuring the stability of the reactor by starting the power increase later.

[0007]

In both of the above-mentioned conventional techniques, nuclear heating is not used to raise the temperature of the cooling water, but an external heating source is used. Therefore, a heat exchanger and a heat supply system are provided inside or outside the containment vessel or the pressure vessel. Since it is provided to raise the temperature of the cooling water, a group of pipes for heat supply and a control system for setting the sub-cooling are required, which complicates the structure of the reactor and reduces the economic efficiency of construction. there were.

It is an object of the present invention to provide an optimum starting method for preventing the occurrence of an unstable phenomenon peculiar to starting a natural circulation reactor.

[0009]

SUMMARY OF THE INVENTION The above-mentioned object is to perform a degassing operation in a conventional boiling water reactor for the purpose of removing dissolved oxygen in cooling water before the reactor is started by a natural circulation type. It is also used in nuclear reactors. It is achieved by performing deaeration operation at startup in the natural circulation state, then pulling out the control rod to heat up and boost by nuclear heating to obtain the rated output.

[0010]

In the present invention, in addition to the conventionally known unstable mode of the reactor (conventional unstable mode), a newly found unstable mode (hereinafter, the new unstable mode is referred to as Geisering) ) Is taken into consideration so that the reactor can be stably started in any case.

The mechanism of geysering generation, which is the motive of the present invention, and its characteristics will be described below in relation to the conventional unstable mode.

FIG. 7 shows a natural circulation circuit having a heating portion and a cooling portion. The result of the experiment using the natural circulation circuit is shown in FIG.
FIG. 8 shows the conditions under which geysering occurs using the amount of heat generation and subcooling. The conventional unstable mode is also observed in the figure. It can be seen that the conventional unstable mode occurs under the conditions of low subcooling and high power, whereas, on the contrary, Geisering occurs under the conditions of high subcooling and low power. The former occurs when the core outlet quality is high, and the latter conversely occurs when the quality is low. The mechanism is explained as follows.

The natural circulation flow rate Q is mainly the buoyancy force F caused by the density difference of the coolant in the ascending flow path 15 and the descending flow path 16 in the natural circulation circuit as shown in FIG.
5 and the friction loss ΔP of the two-phase flow.
Buoyancy F is a function F (α) of void ratio α, and friction loss ΔP
Is considered to be a function ΔP (χ, Q) of the quality χ and the flow rate Q. On the other hand, the void ratio and the quality in the two-phase flow generally have the relationship shown in FIG. Now, consider the response of the system when the flow disturbance δQ occurs in the natural circulation circuit shown in FIG. 7.

Conventional unstable mode: As shown in FIG. 9, in the high quality region (χ> 0.10), the variation ∂α / ∂χ of the void ratio with respect to the quality variation is small. Therefore, the variation of the void fraction with respect to the variation δQ of the same order as δχ ∂α /
Since ∂χ is also small, it is considered that the buoyancy is not affected by the variation amount δQ (δF = 0). On the other hand, the friction loss ΔP is given by the following equation.

[0015]

[Equation 1]

Φ ² (χ): Two-phase flow multiplication coefficient Therefore, the variation amount δΔP with respect to δQ is

[0017]

[Equation 2]

Is given by

As shown in FIG. 10, the outlet quality variation δ
The phase of χ is inverted with respect to the flow rate fluctuation δQ ∂χ
/ ∂Q <0. Further, since it is known that ∂φ ² (x) / ∂χ> 0, the first term on the right side of the equation (2) has a negative sign and the second term has a positive sign.

Therefore, under the condition that the left side of the equation (2) is <0 (established under the high quality condition), the friction loss is reduced with respect to the positive flow rate fluctuation, so that the flow rate fluctuation is promoted and instability occurs. ..

As described above, the conventional unstable mode is caused by the hydraulic negative resistance.

Geisering: In FIG. 10, ∂α / ∂χ in the low quality region shows a quality variation δχ, and therefore also a variation greater than the order of the flow rate variation δQ. Therefore, in this region, contrary to the conventional unstable mode, the fluctuation δF of the driving force (buoyancy) dominates the phenomenon. That is, the outlet quality decreases δ with respect to the positive flow rate variation δQ.
When χ occurs, a rapid decrease δα in the void rate occurs from FIG. 9. Therefore, as soon as the action of the disturbance δQ ends, the flow rate suddenly decreases and bumping occurs. The increase in voids due to boiling is the driving force δ
Since it acts in the direction of increasing F, the flow rate increases again and returns to the initial state. This cycle continues and becomes unstable. The change over time in the flow rate Q when gyseling instability occurs exhibits a waveform as shown in FIG.

Further, from the experiment using the natural circulation circuit, it was found that the generation range of Geisering is uniquely determined by the outlet quality of the ascending flow path 15, as shown in FIG. Furthermore, it was also clarified that Geysering does not occur in the non-boiling state. The quality at which gyseling occurs will be called the marginal quality Xc.

FIG. 13 shows the theoretically obtained result of the Geisering generation region in the natural circulation reactor based on the experiment using the natural circulation circuit. The geysering occurrence region is the limit quality curve 17X = Xc and the boiling start curve 1
It can be represented as a region surrounded by 8X = 0. From the figure, it can be seen that there is a local minimum value of subcooling in the Geisering occurrence region.

A feature of the operation method according to the present invention is that the reactor power and the core inlet subcooling are in a range below the geysering generation region shown in FIG. 13 at the time of starting the reactor in order to avoid geysering. Is.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a system diagram of a natural circulation type nuclear reactor 1 and its surroundings.

The system around the reactor is a turbine 6 rotated by steam from the reactor 1, a condenser 7 for condensing steam after driving the turbine, and a vacuum for the condenser 7 when the reactor is started. And a gas exhaust tube 12 for exhausting the air sucked by the vacuum pump to the atmosphere.

In the conventional boiling water reactor, deaeration operation is carried out before starting the reactor in order to remove dissolved oxygen in the cooling water. In the nuclear reactor starting method of the present invention, the degassing operation is performed before the nuclear reactor is started. As a result, in addition to removing dissolved oxygen in cooling water, which is the purpose of degassing operation,
It is possible to avoid the occurrence of geissering peculiar to starting at the natural circulation state.

The starting procedure of the nuclear reactor of the present invention will be described below.

Procedure 1: Reactor degassing The condenser 7 is evacuated by the vacuum pump 11, and the main steam drain valve 9 is opened while the main steam isolation valve 8 is closed to degas the reactor.

At this time, the reactor pressure is lowered to about 0.3 atm.

FIG. 3 shows changes in reactor pressure, cooling water temperature, and pressure in the condenser 7. First, start the vacuum pump 5,
By evacuating the condenser 7, the pressure of the condenser 7 becomes 0.
The pressure drops below 1 atm. After this state is reached, the main steam drain valve 3 is opened to degas the inside of the reactor, and the pressure is reduced to about 0.3 atm. In the process,
Dissolved oxygen in the cooling water in the reactor is removed.

After the reactor pressure of about 0.3 atm is established, the procedure proceeds to step 2.

Procedure 2: Start of nuclear heating After reducing the reactor pressure to about 0.3 atm by degassing operation, the control rod 3 is pulled out and nuclear heating is started. The output should be kept to about 1% of the rated output.

The state at this time will be described in comparison with the Geisering occurrence region of FIG. The saturation temperature at a reactor pressure of 0.3 atm is about 70 ° C. On the other hand, the temperature of the cooling water is about 20 ° C. because it is room temperature before the reactor is started. Therefore, the core inlet sub-cooling is about 50 ° C (= 70 ° C-
20 ° C). It can be seen that the state where the output is about 1% and the core inlet subcooling is about 50 ° C. is the left side of the Geisering occurrence region in FIG. 5, that is, the state where Geisering does not occur. The reason for this is that boiling of the coolant due to nuclear heating has not started.

Step 3: Temperature rising pressure: Cooling water temperature of the reactor 1 is about 20 ° C., output is about 1%.
When the nuclear heating is continued at, the cooling water is gradually heated and the temperature is raised to the saturation temperature of about 70 ° C at the reactor pressure of about 0.3 atm. At this point, the boiling of the cooling water is started, and natural circulation is started by the buoyancy caused by the density difference of the cooling water between the chimney portion 4 and the downcomer portion 5 in the reactor as shown in FIG. The temperature of the cooling water circulated by natural circulation is
Since the saturation temperature is reached, the core cooling subcooling can be made almost zero. It can be seen that the state of the core inlet sub-cooling 0 is below the Geisering occurrence region in FIG. 5, that is, the state where Geisering does not occur.

As the cooling water boils and steam is generated, the reactor pressure rises, and the temperature of the cooling water rises at the saturation temperature of the pressure at each time point as the pressure rises. At this time,
The reactor power is controlled so that the rate of increase in cooling water temperature is 55 ° C / hr or less. The cooling water temperature rise rate is 55 ° C /
The reason for setting the time to be equal to or shorter than hr is to suppress thermal fatigue of the pressure vessel due to start / stop, and this is the same as the standard used in the conventional thermal power boiler and the like. While the temperature inside the reactor is being raised and boosted, the cooling water temperature is at the saturation temperature, so that the core inlet subcooling 0 can be maintained. If the core inlet sub-cooling is 0, it is below the Geisering generation region in FIG. 5, so there is no concern that Geysering will occur during the process of temperature rise and pressure increase.

The temperature and pressure inside the nuclear reactor are continuously raised by nuclear heating to obtain the rated state for both temperature and pressure. Step 4: Power increase After ensuring the rated conditions for both temperature and pressure inside the reactor, pull out the control rod 3 to increase the reactor power to the rated value. As shown in FIG. 5, the geysering generation region when the reactor pressure is increased moves upward as shown in FIG. When the reactor power is increased, the cooling water that becomes steam and is sent to the turbine is replenished by the feed water, so that the cooling water temperature of the downcomer section 5 in the reactor becomes the saturation temperature or less, so that the core inlet subcooling is To rise. However, as described above, since the pressure has reached the rated value, there is a large margin until the occurrence of Geisering, and the core inlet subcooling can maintain the lower side of the Geisering occurrence region in FIG. That is, it is possible to avoid the occurrence of geissering. The start-up operation is completed when the reactor power reaches the rated value.

FIG. 4 shows changes in the reactor pressure, temperature, and power from procedure 1 to procedure 4 described above.

Here, the role of the deaeration operation in the procedure 1 for avoiding the gyseling will be explained. When the reactor is started up once, cooled down, and restarted, the cooling water in the core is being heated by the decay heat of the core. Therefore, the saturation at the reactor pressure of about 0.3 atm reached by the degassing operation. The temperature has been raised to about the temperature.
Therefore, when the reactor pressure is reduced to about 0.3 atm by the degassing operation, the cooling water in the core is in a state of reduced pressure boiling. The state where the cooling water temperature in the nuclear reactor is equal to the saturation temperature is the state of the core inlet subcooling 0.

That is, in the case where the once-started reactor is cold-started and then restarted, the core inlet subcooling can be made zero by degassing. By setting the core inlet sub-cooling to 0, it is possible to maintain the state below the Geisering generation region in FIG. 5, so it is possible to avoid the occurrence of Geisering regardless of the reactor output.

Therefore, after the procedure 2, it is not necessary to keep the reactor output at about 1% of the rated value for the purpose of avoiding geysering, and the reactor output is kept within the range in which the temperature rise rate of the cooling water does not exceed 55 ° C / hr. Since it can be maximized, startup time is shortened.

Of course, even if the core inlet subcooling cannot be set to 0 due to degassing, the core inlet subcooling can be set to 0 in step 3. Therefore, after this, the reactor power is increased by increasing the cooling water temperature. Temperature ratio is 55 ° C /
It may be raised within the range not exceeding hr, but at least step 2
It is necessary to keep the reactor power at about 1% until the control rod is pulled out in step 3 and the core inlet subcooling becomes 0 in step 3.

FIG. 5 shows the reactor power and the trajectory of the core inlet sub-cooling when the reactor is started according to this embodiment in comparison with the Geisering generation region. When starting the reactor for the first time, degassing the reactor core inlet subcooling to 0
It is not possible to set the reactor power to about 1% of the rated value.
Depending on the degree, the operating state shifts from the point A to the point B on the locus AB. While the reactor is being heated and boosted, the operating state is at point B until the temperature and pressure reach the rated value.

During this period, the reactor pressure rises, so that the geysering generation region moves upward in FIG. In the process of increasing the reactor power to the rated value, the operating state shifts from the point B to the point C on the locus BC.

When the reactor is restarted after cold shutdown, since the cooling water is heated by the decay heat of the core, the subcooling of the core inlet can be made almost zero by deaerating the core. The heating can start from point B. That is, since the transition process of the trajectory AB can be omitted by degassing, it is possible to shorten the time required for starting while avoiding geysering. Further, it is also possible to simultaneously remove dissolved oxygen in the coolant, which is the purpose of degassing in a boiling water reactor.

[0047]

EFFECTS OF THE INVENTION According to the present invention, by carrying out deaeration which has been conventionally carried out in a boiling water reactor, in a natural circulation reactor, not only removal of dissolved oxygen in cooling water but also natural circulation It is possible to prevent the occurrence of specific gusseling at the time of starting up and reduce the time required for starting up.

[Brief description of drawings]

FIG. 1 is a system diagram of a natural circulation reactor showing an embodiment of the present invention.

FIG. 2 is an explanatory view showing a structure of a natural circulation reactor.

FIG. 3 is a characteristic diagram showing a reactor degassing operation state according to the present invention.

FIG. 4 is a characteristic diagram showing a reactor starting state according to the present invention.

FIG. 5 is a characteristic diagram showing a reactor start-up curve according to the present invention in comparison with a Geisering occurrence region.

FIG. 6 is a graph showing an operation curve in the relation between reactor power and core flow rate.

FIG. 7 is an explanatory diagram showing natural circulation of a nuclear reactor.

FIG. 8 is a characteristic diagram in which a conventional unstable mode and a region where geysering is generated are plotted and displayed.

FIG. 9 is a characteristic diagram showing the relationship between the outlet quality and the void rate.

FIG. 10 is a characteristic diagram showing the relationship between the circulation flow rate and the outlet quality.

FIG. 11 is a characteristic diagram showing the relationship of the flow rate with the passage of time in the gyseling state.

FIG. 12 is a characteristic diagram plotting the relationship between outlet quality and flow rate amplitude.

FIG. 13 is a characteristic diagram showing a region where geysering occurs.

[Explanation of symbols]

1 ... Natural circulation type reactor, 2 ... Reactor core, 3 ... Control rod, 4 ... Chimney part, 5 ... Downcomer part, 6 ... Turbine, 7 ... Condenser, 8 ... Main steam isolation valve, 9 ... Main steam drain valve 10 ... Turbine bypass valve, 11 ... Vacuum pump, 12 ... Exhaust pipe.

Claims (1)

[Claims] Claim: What is claimed is: 1. When starting a natural circulation reactor in a cold shutdown state, the core inlet subcooling is determined by the reactor power and the core inlet subcooling by degassing the inside of the reactor. A method for starting a natural circulation reactor, which comprises starting to increase the output after setting to 1.