JPH1114795A - Decontaminating method for reactor pressure vessel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make chemical decontamination more efficient and reduce the exposure of a worker with radiation by carrying a decontaminating solution having a specified flow velocity near the bottom part within a reactor pressure vessel or the bottom part of a jet pump. SOLUTION: A lattice plate 8 and a reactor core support plate 11 are held by a shroud 29 to form a fuel assembly. A jet pump 4 is arranged between the shroud 29 and a pressure vessel 1 to forcedly circulate a coolant. A reactor recirculating system 2 is formed of a recirculating outlet nozzle 12, a recirculating piping 6, two recirculating outlet and inlet valves 14, a recirculating pump 3 and a recirculating inlet nozzle 13, which are situated in the outside of the pressure vessel 1. The circulating pump 3 is formed of a centrifugal pump whose pump shaft is sealed and cooled with the cooling water of the pump. In order to circulate a decontaminating solution within the pressure vessel 1, the recirculating system 2 is operated by the pump 3 to circulate the decontaminating solution, and a flow velocity of 1 cm/sec. is ensured near the bottom of the pressure vessel 1 and the bottom of the pump 3. The decontaminating solution is carried at this flow velocity and circulated, whereby chemical decontamination effect can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear power plant, in which a flow rate of a decontamination solution during a chemical decontamination operation in a reactor pressure vessel is secured, and a predetermined flow rate is controlled to efficiently perform chemical decontamination. The present invention relates to a decontamination method in a reactor pressure vessel that can be performed.

[0002]

2. Description of the Related Art In a nuclear power plant, radioactivity in reactor water is taken into an oxide film on the surface of equipment and piping and becomes a radiation source. Therefore, this radioactivity is removed before maintenance, inspection, and repair. Measures have been taken. One of them is a chemical decontamination technique.

[0003] Conventionally, this is performed for systems such as pump casings, impellers, valves, and other equipment alone, a reactor recirculation system, and a reactor purification system. However, when performing repair work inside the reactor pressure vessel, it may be necessary to rely on humans for parts that cannot be performed remotely and automatically, and decontamination is required throughout the reactor pressure vessel. .

[0004]

Conventionally, in the reactor pressure vessel, decontamination of the inner wall surface of the reactor pressure vessel and the like has been performed by a mechanical method such as jet cleaning, but the mechanical decontamination method is not so common. A high decontamination effect cannot be expected,
There is a problem that it takes a long time.

In order to effectively perform chemical decontamination in the reactor pressure vessel, not only is it immersed in the decontamination solution, but also the reactor is recirculated in order to secure the necessary reactor flow for decontamination. A method of operating a system (PLR) pump is conceivable.

In the latter case, if the recirculation pump is operated as usual with the fuel assemblies and control rods taken out of the reactor pressure vessel, an excessive flow rate will result. In addition, since it is necessary to supply seal water with the operation of the recirculation pump, it is necessary to take some measures to prevent overflow from the reactor pressure vessel. Furthermore, when the operation is performed with the water level kept low, the inner wall surface of the reactor pressure vessel cannot be decontaminated, so that there is a problem that the radiation dose in the reactor pressure vessel cannot be sufficiently reduced.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and operates a recirculation pump during chemical decontamination in a reactor pressure vessel to secure a flow rate necessary for decontamination and to soak the decontamination liquid. The decontamination solution is supplied to the inner surface of the reactor pressure vessel where there is no or short immersion time, and the flow rate of the decontamination solution in the reactor pressure vessel is controlled to a predetermined value to enable efficient chemical decontamination. It is an object of the present invention to provide a decontamination method and a decontamination apparatus in a reactor pressure vessel that can reduce radiation exposure of workers as a result.

[0008]

According to a first aspect of the present invention, there is provided a jet pump for forcibly circulating reactor water with a shroud installed in a reactor pressure vessel, and the reactor water from the jet pump is provided. A recirculation pipe and a recirculation pump that circulate through a recirculation system inlet nozzle and an outlet nozzle connected to a lower outer surface of the reactor pressure vessel are provided,
In the chemical decontamination method in a reactor pressure vessel for applying a flow to the decontamination liquid supplied into the reactor pressure vessel by driving the recirculation pump, the bottom in the reactor pressure vessel or the bottom of the jet pump The flow rate of the decontamination liquid flowing in the vicinity is selected to be at least 1 cm / sec, and the decontamination liquid is provided with a flow.

According to the first aspect of the present invention, the flow velocity in the vicinity of the bottom of the reactor pressure vessel or the bottom of the jet pump, which is the decontamination liquid retention section or the low flow rate section in the reactor pressure vessel, is at least 1 cm / sec. Thereby, the decontamination efficiency in the part can be increased. The reason why the flow rate of the decontamination solution was selected to be at least 1 cm / sec is based on experimental results as shown in FIG.

[0010] The invention of claim 2 is characterized in that the recirculation pump is operated at a rotation speed of 20% to 30% of the rated operation to impart a flow to the decontamination liquid. According to the second aspect of the invention, in order to obtain a flow rate of at least 1 cm / sec, the recirculation pump is selected to have a rotation speed of 20 to 30% of the rated operation.
If the flow rate is less than 20%, a predetermined flow rate, that is, a flow rate of 1 cm / sec or more cannot be obtained, so that a flow required for decontamination cannot be secured. On the other hand, if it exceeds 30%, the sealing water of the recirculation pump will flow more than necessary and the decontamination solution will be diluted, which will not only reduce the decontamination efficiency, but also increase the flow rate of the reactor pressure vessel. It is impossible because it overflows from the.

According to a third aspect of the present invention, a temporary submersible pump is suspended at the bottom of the reactor pressure vessel or near the bottom of the jet pump installed in the reactor pressure vessel, and the decontamination is performed by driving the submersible pump. It is characterized in that a fluid is imparted to the liquid.

According to the third aspect of the present invention, by installing a temporary submersible pump, the bottom of the reactor pressure vessel or the bottom of the jet pump which is a decontamination liquid stagnation section or a low flow rate section in the reactor pressure vessel. The flow rate of the decontamination solution in the vicinity can be maintained at 1 cm / sec or more.

According to a fourth aspect of the present invention, a flow of the decontamination liquid at the bottom of the reactor pressure vessel is provided by capping the upper end opening of a plurality of control rod drive mechanism guide tubes penetrating from the bottom of the reactor pressure vessel. Is promoted.

According to the fourth aspect of the present invention, the control rod drive mechanism guide tube protruding from the bottom of the reactor pressure vessel is covered with a cap, thereby dispersing the flow of the decontamination liquid and allowing the decontamination liquid to flow inside the reactor pressure vessel. The condition of claim 1 can be achieved by increasing the flow rate at which decontamination efficiency can be uniformly obtained from the vicinity of the center to the bottom to improve the fluidity of the decontamination liquid.

[0015] The invention of claim 5 is characterized in that a rotating body is provided at the bottom of the reactor pressure vessel or at the bottom of the jet pump to promote agitation of the decontamination liquid at each of the bottoms.
According to the invention of claim 5, the decontamination liquid in the vicinity of the bottom of the reactor pressure vessel or the bottom of the jet pump, which is a decontamination liquid stagnation section or a low flow rate section in the reactor pressure vessel, is supplied to the temporary rotating body. The condition of claim 1 can be achieved by stirring by driving to secure the flow rate of the decontamination liquid.

A sixth aspect of the present invention is characterized in that a large-diameter temporary tank is installed at the center of the reactor pressure vessel to improve the flowability of the decontamination liquid at the bottom of the reactor pressure vessel. According to the invention of claim 6, the tank is installed as an obstacle in the reactor pressure vessel, and the bottom portion of the reactor pressure vessel which serves as a decontamination liquid stagnation section or a low flow rate section with a small number of rotations of the recirculation pump. Can improve the fluidity of the decontamination liquid, and can achieve the conditions of claim 1.

A seventh aspect of the present invention is characterized in that a mechanical decontamination function in the reactor pressure vessel is used together. According to the invention of claim 7, when a flow rate of 1 cm / sec or more cannot be secured at the bottom in the reactor pressure vessel or the jet pump bottom,
By using mechanical cleaning together, the decontamination effect can be enhanced. That is, radioactivity is not sufficiently dissolved in the decontamination agent,
The decontamination effect can be improved by discharging the sludge deposited outside the system in combination with mechanical decontamination.

An eighth aspect of the present invention is characterized in that a decontamination liquid outlet and / or an inlet for promoting the flow of the decontamination liquid are provided at the bottom of the reactor pressure vessel or at the bottom of the jet pump.

According to the eighth aspect of the present invention, the flow rate of the decontamination liquid in the vicinity of the bottom of the reactor pressure vessel or the bottom of the jet pump, which is the detention section of the decontamination liquid in the reactor pressure vessel or the low flow rate section, is ensured. In order to achieve this, the condition of claim 1 can be achieved by providing a suction port and / or a discharge port for the decontamination liquid in the section.

A ninth aspect of the present invention is characterized in that a bubble generating port is provided at the bottom in the reactor pressure vessel or near the bottom of the jet pump to promote the flow of the decontamination liquid. According to the ninth aspect of the present invention, the decontamination liquid staying part in the reactor pressure vessel or the bottom part in the reactor pressure vessel that is a low flow rate part,
In order to secure the flow rate of the decontamination liquid near the bottom of the jet pump, a port for generating air bubbles is provided in the portion, and the flow of the decontamination liquid is promoted by the air bubbles, so that the condition of claim 1 can be achieved.

According to a tenth aspect of the present invention, a control rod driving mechanism guide tube penetrating through the bottom of the reactor pressure vessel is removed to form a discharge port for a decontamination liquid, thereby forming a bottom of the reactor pressure vessel. Characterized by promoting the flow of the decontamination liquid.

According to the tenth aspect of the present invention, the control rod drive mechanism is removed to improve the flowability of the decontamination liquid at the bottom of the reactor pressure vessel, which is a low flow rate portion as a discharge port for the decontamination liquid. Condition 1 can be achieved.

[0023] The invention of claim 11 is that the bottom drain line of the reactor pressure vessel is used to form a discharge port for the decontamination solution to promote the flow of the decontamination solution inside the reactor pressure vessel. Features.

According to the eleventh aspect of the present invention, the decontamination liquid is extracted from the bottom line of the reactor pressure vessel, and the flow of the decontamination liquid at the bottom inside the reactor pressure vessel is promoted, so that the condition of the first aspect can be achieved. .

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First and second embodiments of a method for decontaminating a reactor pressure vessel according to claims 1 and 2 of the present invention will be described with reference to FIGS. Figure 1 shows the first
7 shows experimental data of minimum fluidity (flow rate) required for decontamination in the decontamination method according to the embodiment. That is,
The vertical axis is the decontamination efficiency (DF), and the horizontal axis is the linear flow velocity (m / se).
c), and the required minimum flow rate is 1 cm from the experimental data in FIG.
/ Sec, it is recognized that the decontamination efficiency increases.

In order to achieve the required minimum flow velocity, an appropriate flow rate and water level can be secured by operating the reactor recirculation system at a recirculation pump flow rate of 20% to 30% of the rated value. Note that a reducing agent and an oxidizing agent are used as the decontamination liquid, and the components of these decontamination liquids are selected depending on the type of contaminant and the state of adhesion.

FIG. 2 shows a reactor pressure vessel and a recirculation system used for obtaining the experimental data of FIG. 1 for explaining the second embodiment, and a flow in the case of performing in-reactor chemical decontamination. In the figure, a dryer, a separator, a fuel assembly, a control rod, and the like have been removed from the reactor pressure vessel 1.

In order to obtain the flow of the decontamination liquid in the reactor pressure vessel 1, the reactor recirculation system 2 is circulated by the recirculation pump 3, and the bottom of the reactor pressure vessel 1 and the vicinity of the bottom of the jet pump 4 The flow rate of the decontamination liquid can be secured.

In FIG. 2, reference numeral 29 denotes a shroud which holds the upper lattice plate 8 and the core support plate 11 and wraps the entire fuel assembly (not shown) to form a core. The core support plate 11 holds a fuel assembly via a control rod guide tube of the control rod drive mechanism 5 and a fuel support. The jet pump 4 is disposed between the shroud 29 and the reactor pressure vessel 1 to forcibly circulate the coolant.

The reactor recirculation system 2 includes a recirculation outlet nozzle 12, a recirculation pipe 6, two recirculation inlet / outlet valves 14, a recirculation pump 3, and a recirculation inlet nozzle 13 which are maintained outside the reactor pressure vessel 1. It consists of The recirculation pump 3 is, for example, a vertical single-stage mixed flow spiral pump. The pump shaft is sealed with a two-stage mechanical seal, and the shaft seal is cooled by flowing cooling water by a pump provided on the pump shaft. Reference numeral 7 denotes a water supply nozzle for supplying cooling water into the reactor pressure vessel 1;
Denotes a core spray line, and 10 denotes a water supply purger.

According to the present embodiment, the reactor pressure vessel 1
In order to obtain the flow of the decontamination liquid in the reactor, the decontamination liquid is circulated through the reactor recirculation system 2 by operating the recirculation pump 3, and the flow velocity near the bottom of the reactor pressure vessel 1 and the bottom of the jet pump 4 is reduced. At least 1 cm / sec can be secured.

Hereinafter, third to eleventh embodiments of the decontamination method in the reactor pressure vessel according to the third to eleventh aspects of the present invention will be described with reference to FIGS. 3 to 11, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description of the overlapping parts will be omitted. In each embodiment, only the points different from the second embodiment will be described. .

That is, the third embodiment differs from the second embodiment in that the reactor pressure vessel 1 shown in FIG.
Is that a temporary submersible pump 15 is installed at the bottom of the. With this temporary pump 15, a local flow can be created and the decontamination efficiency can be improved. That is, a fountain-shaped flow can be formed as shown by B in FIG. 3 by driving the temporary submersible pump 15 where a linear flow usually shown by A in FIG. 3 is obtained. Can be achieved.

In the fourth embodiment, as shown in FIG. 4, the guide tube of the control rod driving mechanism 5 is covered with a CRD guide tube cap 24 to control the flow of the decontaminant at the bottom in the reactor pressure vessel 1. Has been promoted. According to the present embodiment, since the guide tube is closed by covering the cap 24,
The decontamination liquid does not flow into the guide tube, and the decontamination liquid at the bottom in the reactor pressure vessel 1 can be maintained at a predetermined flow rate.

In the fifth embodiment, as shown in FIG. 5, a furnace bottom stirrer 18 is provided at the bottom of the reactor pressure vessel 1 and at the bottom of the jet pump 4 by being suspended by a suspension wire 30. It is in. The stirrer 18 can be provided with a temporary rotating body, and the stirrer 18 can promote the stirring of the decontamination liquid at the bottom of the reactor pressure vessel 1 and the bottom of the jet pump 4 to secure a predetermined flow rate.

In the sixth embodiment, as shown in FIG. 6, a temporary tank 21 is provided at the center of the reactor pressure vessel 1 to make the fluidity of the decontamination liquid at the bottom of the reactor pressure vessel 1 lower. Has been improved. With the provision of the temporary tank 21, the amount of the decontamination liquid can be reduced, and the flow rate at which the decontamination liquid at the bottom of the reactor pressure vessel 1 is improved and the decontamination efficiency is obtained uniformly. It is easier to keep

In the seventh embodiment, as shown in FIG. 7, a suction cleaning device 26 is provided as a mechanical cleaning mechanism at the bottom of the reactor pressure vessel 1 and near the jet pump 4,
A temporary pump 27 and a filter 28 were connected to the suction cleaning device 26 by a suction pipe 31, and an outlet side of the filter 28 was connected by a return pipe 32, thereby increasing the decontamination efficiency by circulating the decontamination liquid. It is in.

According to the present embodiment, the reactor pressure vessel 1
When a flow rate of 1 cm / sec cannot be secured at the bottom of the jet pump 4 or near the bottom of the jet pump 4, suction cleaning after chemical decontamination is performed, and the decontamination effect can be enhanced by using such a mechanical cleaning mechanism together. . As the mechanical cleaning mechanism, jet cleaning, sprinkling cleaning, brush cleaning, ultrasonic cleaning and the like can be arbitrarily selected in addition to suction cleaning.

In the eighth embodiment, as shown in FIG. 8, in order to promote the flow of the decontamination liquid at the bottom of the reactor pressure vessel 1 and at the bottom of the jet pump 4, a decontamination liquid inlet / outlet 16 is provided. The decontamination liquid circulation line 17 is provided.

According to the present embodiment, the circulation pump 17 is connected to the circulation line 17, and the decontamination liquid can be circulated by the circulation pump 33 to obtain a predetermined flow rate. That is, since the flow is formed by providing the decontamination liquid inlet / outlet 16 at a portion where the flow velocity is poor, decontamination can be performed more effectively.

In the ninth embodiment, as shown in FIG. 9, in order to promote the flow of the decontamination liquid to the bottom in the reactor pressure vessel 1 and the bottom of the jet pump according to the eighth embodiment. In addition, a bubble generating port 19, a bubble generating device 20, and a bubble supply pipe 34 are provided. According to the present embodiment, by sending bubbles into the decontamination liquid from the bubble generation port 19, the flow of the decontamination liquid can be promoted by the stirring effect of the bubbles.

In the tenth embodiment, as shown in FIG. 10, the control rod drive mechanism 5 in the reactor pressure vessel 1 is removed, and a temporary ring header 22 from the CRD is attached as a decontamination liquid outlet. The decontamination device 23 is provided on the temporary ring header 22 via an example of one end of the connection line 35.
The other end of the connection line 35 opens into the reactor pressure vessel 1. According to the present embodiment, the flow of the decontamination liquid at the bottom of the reactor pressure vessel 1 can be promoted by providing the outlet for the decontamination liquid.

The eleventh embodiment is similar to the tenth embodiment. An RPV drain line 25 is provided at the bottom of the reactor pressure vessel 1 as a bottom line to form a discharge port for a decontamination liquid. The purpose is to promote the flow of the decontamination liquid at the bottom of the reactor pressure vessel 1. That is, the reactor pressure vessel 1
The RPV drain line 25 penetrating through the bottom of is connected to a connection line constituting a circulation line provided with the decontamination device 23.

According to the present embodiment, the reactor pressure vessel 1
By promoting the flow of the decontamination liquid in the inside, a uniform flow velocity at the bottom can be obtained, and the improvement of the decontamination efficiency of the entire bottom can be enhanced.

[0045]

According to the present invention, the decontamination liquid is caused to flow at a flow rate of at least 1 cm / sec to the bottom of the reactor pressure vessel or the vicinity of the bottom of the jet pump, which is a narrow overflow portion in the reactor pressure vessel. By imparting fluidity, the chemical decontamination effect can be improved.

Further, in order to obtain a necessary flow rate of at least 1 cm / sec at all the decontamination target sites for eliminating the liquid detained portion of the decontamination liquid, an existing or temporary pump or a circulation line may be used, or a combination thereof. As a result, the flow rate of the decontamination solution can be controlled to a predetermined value, and a sufficient chemical decontamination effect can be obtained.

Further, when a sufficient flow rate cannot be obtained, the decontamination effect can be enhanced by the combined use of mechanical cleaning, whereby any repair work in the reactor pressure vessel can be carried out, and furthermore, a large radiation exposure can be achieved. Reduction can be achieved.

[Brief description of the drawings]

FIG. 1 is a curve diagram showing experimental data for explaining a first embodiment of a decontamination method in a reactor pressure vessel according to the present invention.

FIG. 2 is a schematic sectional view showing a second embodiment of the decontamination method in the reactor pressure vessel according to the present invention.

FIG. 3 is a schematic sectional view showing a third embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 4 is a schematic sectional view showing a fourth embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 5 is a schematic sectional view showing a fifth embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 6 is a schematic sectional view showing a sixth embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 7 is a schematic sectional view showing a seventh embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 8 is a schematic sectional view showing an eighth embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 9 is a schematic sectional view showing a ninth embodiment of the method for decontaminating a reactor pressure vessel according to the present invention.

FIG. 10 is a schematic sectional view showing a decontamination method in a reactor pressure vessel according to a tenth embodiment of the present invention.

FIG. 11 is a schematic sectional view showing an eleventh embodiment of the decontamination method in the reactor pressure vessel according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor pressure vessel, 2 ... Reactor recirculation system, 3 ... Recirculation pump, 4 ... Jet pump, 5 ... Control rod drive mechanism (C
RD), 6: Recirculation pipe, 7: Water supply nozzle, 8: Upper lattice plate, 9: Core spray line, 10: Water supply sparger,
11 core support plate, 12 recirculation outlet nozzle, 13 recirculation inlet nozzle, 14 recirculation inlet / outlet valve, 15 temporary water pump,
16 ... decontamination liquid inlet / outlet, 17 ... decontamination liquid circulation line, 18 ... furnace bottom stirrer, 19 ... bubble generation port, 20 ... bubble generation device, 21 ... temporary tank, 22 ... temporary ring header from CRD, 23 ... Decontamination device, 24 CRD guide tube cap, 25 RPV drain line, 26 Suction cleaning device, 27 Temporary pump, 28 Filter, 29 Shroud, 30 Hanging wire, 31 Suction pipe,
32… Return piping, 33… Circulation pump, 24… Bubble supply piping, 35
... connecting line.

Claims (11)

[Claims] 1. A jet pump for forcibly circulating reactor water between a shroud installed in a reactor pressure vessel is provided, and reactor water from the jet pump is connected to a lower outer surface of the reactor pressure vessel. A recirculation pipe and a recirculation pump for circulating through a recirculation system inlet nozzle and an outlet nozzle are provided, and a reactor pressure for applying a flow to the decontamination liquid supplied into the reactor pressure vessel by driving the recirculation pump. In the method of chemical decontamination in a vessel, the flow rate of the decontamination liquid flowing at the bottom in the reactor pressure vessel or near the bottom of the jet pump is selected to be at least 1 cm / sec to impart a flow to the decontamination liquid. A decontamination method in a reactor pressure vessel, characterized by the following. 2. The reactor pressure vessel according to claim 1, wherein the recirculation pump is operated at a rotation speed of 20% to 30% of a rated operation to impart a flow to the decontamination liquid. Decontamination method. 3. A temporary submersible pump is suspended from the bottom of the reactor pressure vessel or near the bottom of a jet pump installed in the reactor pressure vessel, and the decontamination liquid is given a flow by driving the submersible pump. The method for decontamination in a reactor pressure vessel according to claim 1, wherein 4. A cap is placed on an upper end opening of a plurality of control rod drive mechanism guide tubes penetrating from the bottom inside the reactor pressure vessel to disperse and flow the decontamination liquid at the bottom of the reactor pressure vessel. The method for decontamination in a reactor pressure vessel according to claim 1, wherein the decontamination is promoted. 5. The reactor pressure vessel according to claim 1, wherein a rotating body is provided at the bottom of the reactor pressure vessel or at the bottom of the jet pump to promote agitation of the decontamination liquid at each of the bottoms. Decontamination method inside. 6. The reactor according to claim 1, wherein a large-diameter temporary tank is provided at a central portion in the reactor pressure vessel to improve the flowability of the decontamination liquid at the bottom of the reactor pressure vessel. Decontamination method in pressure vessel. 7. The method for decontamination in a reactor pressure vessel according to claim 1, wherein a mechanical decontamination function in the reactor pressure vessel is used together. 8. The reactor according to claim 1, wherein a decontamination liquid outlet and / or an inlet for facilitating the flow of the decontamination liquid are provided at the bottom of the reactor pressure vessel or at the bottom of the jet pump. Decontamination method in pressure vessel. 9. The reactor pressure vessel according to claim 1, wherein a bubble generating port is provided at the bottom of the reactor pressure vessel or near the bottom of the jet pump to promote the flow of the decontamination liquid. Decontamination method. 10. A control rod drive mechanism guide tube which penetrates the bottom of the reactor pressure vessel and is provided in a standing state is formed to form a discharge port for the decontamination solution, and the decontamination solution at the bottom inside the reactor pressure vessel is formed. The method for decontamination in a reactor pressure vessel according to claim 1, wherein the flow is promoted. 11. A decontamination liquid discharge port is formed by using a bottom drain line of a reactor pressure vessel to promote the flow of the decontamination liquid at the bottom inside the reactor pressure vessel. 2. The method for decontamination in a reactor pressure vessel according to claim 1.