JP2017191081A - Decontamination treatment method, and decontamination treatment device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a decontamination treatment method and a decontamination treatment device that can facilitate treatment of sludge precipitating in the bottom parts of storage tanks.SOLUTION: A decontamination treatment method comprises a step of stirring sludge in a storage tank and a step of sucking contents in the middle layer of the storage tank after the lapse of crude grain sedimentation time following the stirring step and transferring radiation-contaminated water containing sludge to an external decontamination facility. A decontamination treatment device is equipped with a circulating flow path 3 for discharging the radiation-contaminated water including sludge S sucked from the storage tank R into the storage tank and a transfer flow path 4 for transferring the radiation-contaminated water containing sludge sucked from the middle layer of the storage tank to an external decontamination facility U.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a decontamination processing method and a decontamination processing apparatus.

  Radioactive water containing radioactive materials such as radioactive cesium and radioactive strontium, such as circulating water for core cooling and wastewater at nuclear power plants after the accident (hereinafter referred to as nuclear power plants), remove radioactive materials to prevent environmental destruction Otherwise, it is not allowed to discharge.

  For this reason, in the nuclear power plant after the accident, the radioactive polluted water collected from the surplus reactor cooling water and the like is stored in a large number of storage tanks, and the amount thereof is increasing day by day. Many of these storage tanks are bolt-tight tanks formed by joining a plurality of plate-like members on-site by bolting. In such a bolt-clamped tank, leakage may occur due to deterioration of the packing or the like of the fastening portion between the members, so that the lifetime is considered to be several years.

  Therefore, it is necessary to replace such a bolted-type tank with a longer-life welding type tank formed by welding a plurality of plate-like members. That is, it is required to disassemble and dispose of a bolt-tight storage tank that stores radioactive contaminated water.

  As a method of dismantling radioactive contamination facilities such as storage tanks, a roof that covers the radioactive contamination facilities is constructed, and a crane that can be remotely operated is installed near the ceiling of the roof and this crane is used. Thus, it has been proposed to dismantle and decontaminate equipment without a person entering the indoor area (see, for example, JP-A-2013-181921). However, the construction of a roof requires a large cost, and a large number of storage tanks are densely arranged at the nuclear power plant after the accident, and the construction of the roof is technically extremely difficult.

  Therefore, in order to dismantle the storage tank installed at the nuclear power plant after the accident, it is first necessary to decontaminate the inside of the storage tank in order to prevent scattering of radioactive pollutants. The radioactively contaminated water stored in the storage tank is usually transferred to a decontamination facility by a pump and decontaminated. Specifically, by treating radioactive pollutants in radioactive polluted water with filter media and adsorbents, decontaminated water that does not contain suspended solids and radioactive pollutants is discharged, and filter media that contain radioactive pollutants at high concentrations and Dispose of the adsorbent as radioactive waste. By such decontamination treatment, the radioactive contaminated water can be converted into general waste, so that the amount of radioactive waste can be greatly reduced.

  However, sludge is precipitated at the bottom of the storage tank that stores radioactive contaminated water, and sludge remains at the bottom of the storage tank even after the radioactive contaminated water is discharged. Such sludge has a relatively high content of pollutants like the above-mentioned radioactive polluted water, and as such, is strictly handled such as sealing in a sealed container as radioactive waste, and the amount is relatively Many. Moreover, since the residual sludge in the storage tank has a high radiation dose, the activities of the workers in the storage tank are restricted, so it is not easy to carry out the storage tank, and its processing becomes a problem.

JP2013-181921A

  This invention is made | formed based on the above situations, and it is a subject to provide the decontamination processing method and decontamination processing apparatus which the process of the sludge settled in the bottom part of a storage tank becomes comparatively easy. To do.

  The invention made to solve the above-mentioned problems is a decontamination method for a storage tank in which radioactive polluted water is stored and sludge is settled at the bottom, the step of stirring the sludge in the storage tank, and the stirring And a step of sucking from the middle layer in the storage tank after the coarse sedimentation time has elapsed after the process and transferring radioactive contaminated water containing sludge to an external decontamination facility. It is.

  Sludge is considered to contain radioactive contaminants in the surface layer portion within a certain range from the surface of the particle. Therefore, if the sludge has a large surface area relative to the volume and can remove relatively small fine particles that appear to have a large area containing radioactive contaminants, the remaining sludge has a small surface area relative to the volume. Therefore, the region containing almost no internal radioactive contaminant is mainly composed of relatively large coarse particles. As a result, the radioactive pollutant content per volume of the remaining sludge, that is, the contamination level (radiation dose) becomes relatively small, so it is considered that the sludge remaining in the storage tank can be treated relatively easily. . The decontamination processing method waits for a relatively large coarse particle in the sludge to settle after the agitation step in the transfer step, and then relatively small fine particles from the middle layer in the storage tank. Fine particles in the sludge can be removed by sucking the radioactive polluted water in which the particles are floating and transferring them to an external decontamination facility. For this reason, the decontamination processing method makes it relatively easy to process sludge that settles on the bottom of the storage tank after the decontamination processing. The “coarse particle settling time” is a time that is considered to be required for a particle having a particle size of 0.5 mm, preferably 0.3 mm, more preferably 0.2 mm to settle to a height at which it is not sucked from the water surface. The “particle diameter” is expressed by a sieve opening that does not allow particles to pass through.

  The said transfer process is good to be performed within the fine particle sedimentation time after a stirring process. In this way, the transfer step is performed within the fine particle settling time in which the relatively small fine particles in the sludge are almost settled, so that the fine particles in the sludge are relatively efficiently removed together with the radioactive contaminated water. Can be transferred to. Thereby, the process of the sludge which remains in the bottom part of a storage tank can be facilitated more reliably. The “fine particle sedimentation time” is a time that is considered to be required for sedimentation of particles having a particle diameter of 20 μm, preferably 40 μm, more preferably 50 μm to a height at which the particles are not sucked from the water surface.

  The agitation step may be performed by recirculation of radioactive contaminated water sucked together with sludge by a pump into a storage tank. In this way, the agitation step is performed by recirculation of the radioactively contaminated water sucked together with the sludge by the pump into the storage tank, so that the sludge can be agitated relatively easily. For this reason, the process of the sludge which remains in the bottom part of a storage tank can be facilitated more reliably.

  The suction of radioactive contaminated water in the agitation step and the suction of radioactive contaminated water in the transfer step may be performed by the same pump capable of moving up and down. Thus, the suction of radioactive contaminated water in the agitation step and the suction of radioactive contaminated water in the transfer step are performed by the same pump that can be raised and lowered, so that the bottom of the storage tank can be formed with a relatively simple device configuration. Processing of the remaining sludge can be facilitated.

  The height of the suction port for sucking the radioactive contaminated water in the transfer step is preferably from 1/4 to 3/4 of the average level of the radioactive contaminated water. Thus, when the height of the suction port for sucking the radioactive contaminated water in the transfer step is within the above range, the coarse particles settle with the radioactive contaminated water to a height at which they are not sucked by the pump, and then the fine particles are put into the pump. It is possible to ensure a relatively long time until the fine particles cannot be sucked (time until fine particle settling time elapses). Thereby, the process of the sludge which remains in the bottom part of a storage tank can be facilitated more reliably.

  The coarse particle settling time is {5 × (L−H)} [, where L [m] is the water level at the suction position of radioactive contaminated water, and H [m] is the height of the suction port for sucking radioactive contaminated water. s] or more and {20 × (L−H)} [s] or less is preferable. Thus, by setting the coarse particle settling time within the above range, the size of particles that can be transferred to the external decontamination equipment in the transfer step can be appropriately determined, and the load on the external decontamination equipment can be reduced. At the same time, the treatment of sludge remaining at the bottom of the storage tank can be facilitated more reliably.

  Another invention made in order to solve the above-mentioned problem is a decontamination treatment apparatus for a storage tank that stores radioactive contaminated water and in which sludge settles at the bottom, and stores radioactive contaminated water sucked together with the sludge from the storage tank. Decontamination of the storage tank, comprising: a circulation flow path for discharging the water into the storage tank; and a transfer flow path for transferring radioactive contaminated water containing sludge sucked from the middle layer in the storage tank to an external decontamination facility It is a processing device.

  The decontamination treatment apparatus transfers the radioactive contaminated water containing sludge sucked from the middle layer in the storage tank to the external decontamination equipment, and the circulation flow path for discharging the radioactive contaminated water sucked together with the sludge from the storage tank into the storage tank. By providing a transfer flow path, it is relatively easy to agitate sludge in the storage tank using the circulation flow path and transfer radioactive contaminated water containing sludge to the external decontamination equipment using the transfer flow path. Can be done. As a result, the decontamination treatment apparatus sucks the radioactive polluted water in which fine particles having a relatively large content of radioactive pollutants in the sludge are suspended and transfers them to the external decontamination equipment, so that the fine particles in the sludge The sludge remaining in the bottom of the storage tank can be relatively easily processed.

  As described above, the decontamination processing method and the decontamination processing apparatus of the present invention can relatively easily process the sludge that settles on the bottom of the storage tank.

It is a schematic diagram which shows the structure of the decontamination processing apparatus of 1st embodiment of this invention. It is a schematic diagram which shows one process of the decontamination process using the decontamination processing apparatus of FIG. FIG. 3 is a schematic diagram showing a step subsequent to FIG. 2. It is a time chart which shows the procedure of the decontamination process using the decontamination processing apparatus of FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

[First embodiment]
The decontamination processing apparatus 1 according to the first embodiment of the present invention shown in FIG. 1 is used for decontamination of a storage tank R that stores radioactive contaminated water and in which sludge (particulate matter) S is precipitated at the bottom. .

  The decontamination processing apparatus 1 includes a suction pump 2 that sucks radioactive contaminated water together with sludge from a storage tank R, and a circulation channel that discharges radioactive contaminated water containing sludge sucked by the suction pump 2 into the storage tank R. 3 and a transfer flow path 4 for transferring the radioactively contaminated water sucked by the suction pump 2 to the external decontamination equipment U outside the storage tank R. The circulation flow path 3 and the transfer flow path 4 are provided so as to be selectable by a switching valve 5 that can be remotely operated from the outside of the storage tank R. Moreover, it is preferable that the said decontamination processing apparatus 1 is further equipped with the elevator 6 which raises / lowers the suction pump 2. As shown in FIG.

<Storage tank>
The storage tank R to be decontaminated by the decontamination method is not particularly limited. For example, the storage tank R is used for storing radioactively contaminated water such as circulating water for core cooling and waste water in a nuclear power plant after an accident. A bolt-tightened tank formed by fastening bolts between flanges disposed on the peripheral edges of a plurality of plate members is assumed.

  Further, the storage tank R discharges the internal radioactive contaminated water before dismantling, but the entire amount cannot be discharged and the radioactive contaminated water inevitably remains. FIG. 1 shows the state of the storage tank R after discharging the radioactive contamination water inside from the discharge port.

  Although it does not specifically limit as a minimum of the average internal diameter of the storage tank R, 3 m is preferable and 5 m is more preferable. On the other hand, the upper limit of the average inner diameter of the storage tank R is preferably 25 m, and more preferably 20 m. When the average inner diameter of the storage tank R is less than the lower limit, the tank capacity cannot be increased beyond a predetermined level, and the number of storage tanks R for storing radioactive contaminated water may increase. Conversely, when the average inner diameter of the storage tank R exceeds the above upper limit, it becomes difficult to agitate the entire radioactive contaminated water inside the storage tank R, and the fine particles in the sludge are transferred to the external decontamination equipment together with the radioactive contaminated water. May be difficult. The “average inner diameter” means an average value of the minimum horizontal dimension inside the tank and the horizontal dimension orthogonal thereto.

  Moreover, as a minimum of the average height of the storage tank R, 3 m is preferable and 5 m is more preferable. On the other hand, the upper limit of the average height of the storage tank R is preferably 25 m, and more preferably 20 m. When the average height of the storage tank R is less than the lower limit, the installation area of the storage tank R becomes larger than the capacity capable of storing radioactively contaminated water, so that the storage efficiency with respect to the installation area may be reduced. On the other hand, when the average height of the storage tank R exceeds the upper limit, decontamination of the inner wall surface of the storage tank R may not be easy.

  Further, the radioactively contaminated water stored in such a storage tank R may contain any radioactive contaminants, but is typically radioactively contaminated water containing radioactive strontium.

  The radioactive contaminated water containing radioactive strontium is primarily treated and temporarily stored in the storage tank R by, for example, SARRY (Simple Radioactive Water Retrieve and Recovery System) at the nuclear power plant after the accident. And the like.

  Although it does not specifically limit as contaminant content of the radioactive contamination water stored by the storage tank R, For example, you may be 500 Bq / cc or more and 500,000 Bq / cc or less.

<Suction pump>
The suction pump 2 is disposed in the storage tank R by insertion from the opening above the storage tank R. For example, the suction pump 2 is suspended by a wire or the like. As this suction pump 2, it is preferable to use a submersible pump. Among them, a known low water level drainage submersible pump that is placed on the bottom of the storage tank R and sucks and sends out surrounding water from the lower part thereof is particularly preferably used. Thus, by using the submersible pump for low water level drainage, sludge precipitated on the bottom surface of the storage tank R can be sucked and efficiently stirred.

  The lower limit of the width and depth of the suction pump 2 is preferably 40 cm, and more preferably 50 cm. On the other hand, the upper limit of the width and depth of the suction pump 2 is preferably 80 cm, and more preferably 70 cm. When the width or depth of the suction pump 2 is less than the above lower limit, there is a possibility that the ability to transfer radioactive contaminated water below the storage tank R to the external decontamination equipment U through the opening above the storage tank R may not be obtained. is there. On the contrary, when the width or depth of the suction pump 2 exceeds the upper limit, the suction pump 2 may not be inserted from the upper opening of the storage tank R.

The lower limit of the wicking capability of the suction pump 2, preferably from 10 m ³ / time, 12m ³ / time is more preferable. On the other hand, the upper limit of the wicking ability, preferably 20 m ³ / time, 15 m ³ / time is more preferable. If the suction capacity is less than the lower limit, it may take too much time to purify the radioactive contaminated water, or sludge that settles at the bottom of the storage tank R due to the water flow of the radioactive contaminated water decontaminated by the decontamination treatment device 1. May not be suspended in radioactive contaminants. On the contrary, when the suction capacity exceeds the upper limit, the suction pump 2 becomes too large, and there is a possibility that the suction pump 2 cannot be inserted from the opening above the storage tank R.

<Flow path>
The circulation flow path 3 and the transfer flow path 4 are formed from, for example, pipes, hoses, combinations thereof, and the like, and are connected to the discharge side of the suction pump 2 via the switching valve 5.

  In order to allow the sludge S that settles on the entire bottom surface of the storage tank R to rise efficiently, the circulation channel 3 may be disposed so as to be movable at the position of the outlet opening thereof, and radioactive polluted water is added to the outlet side end thereof. You may have the water injection module (not shown) which has the nozzle which injects and can change the direction.

  The water injection module that changes the direction of the nozzle includes, for example, a nozzle that can rotate around a central axis that is different from the direction of the fountain, a turbine that is driven by the flow of water supplied to the nozzle, and the driving of the turbine. What has a drive mechanism which rotates a nozzle with force can be used.

  As the switching valve 5, one three-way valve or a pair of two-way valves having an actuator that can be operated from the outside of the storage tank R can be used.

<Elevator>
The elevator 6 changes the height of the suction port of the suction pump 2. As the elevator 6, for example, a mechanical device having a motor, a pneumatic device using an air cylinder, a hydraulic device using a hydraulic cylinder, or the like can be used.

  The height of the suction port of the suction pump 2 by the elevator 6 may be adjusted by moving the suction pump 2 up and down, and a suction pipe that can be bent is provided in the suction pump 2 so that the suction pipe is bent. The suction port may be raised and lowered without moving the lifting pump 2. Moreover, when raising / lowering the suction pump 2, the elevator 6 may suspend the suction pump 2 via a wire etc., for example, but the suction pump 2 is suspended by the pipe etc. which comprise the transfer flow path 4. It may be used as a member for lowering.

  The decontamination processing apparatus 1 operates the switching valve 5 and the elevator 6 so that one suction pump 2 sucks radioactive polluted water together with sludge S precipitated from the bottom in the storage tank R to circulate the flow path 3. And a pump for discharging radioactive contaminated water together with the sludge S floating from the middle layer in the storage tank R and transferring it to the external decontamination equipment U through the transfer channel 4. In this way, by providing one suction pump 2 so as to be movable up and down, the decontamination processing apparatus 1 can facilitate the treatment of sludge remaining at the bottom of the storage tank with a relatively simple apparatus configuration. .

<External decontamination equipment>
The external decontamination equipment U is not limited as long as it can decontaminate radioactively contaminated water. However, a water receiving tank, a supply pump, a plurality of filter units, a plurality of primary adsorption towers, a plurality of secondary adsorption towers, a cushion tank, and Those equipped with a delivery pump are preferably used.

  The water receiving tank temporarily receives radioactive contaminated water discharged from the storage tank R. The supply pump pumps water from the water receiving tank to the filter unit. The filter unit filters radioactive contaminated water supplied from a supply pump. In the primary adsorption tower, radioactive polluted water filtered by the filter unit is passed and adsorbs strontium contained in the water. The secondary adsorption tower adsorbs other radioactive contaminants contained in the water that has passed through the primary adsorption tower. The cushion tank temporarily receives water that has passed through the secondary adsorption tower, that is, purified water. The delivery pump discharges purified water from the cushion tank.

  These water receiving tank, supply pump, filter unit, primary adsorption tower, secondary adsorption tower, cushion tank and delivery pump are preferably arranged on one or more movable platforms. As a gantry on which these components are arranged, for example, it is preferable that the gantry is configured to be movable by a forklift or a unic car, or has a wheel and is movable by self-propelling or towing. Thereby, since the external decontamination equipment U can be arrange | positioned in the vicinity of the storage tank R, purification | cleaning of radioactive contaminated water becomes easy.

  In addition, when the external decontamination equipment U includes a plurality of filter units, primary adsorption towers, and secondary adsorption towers, water can be selectively passed to any of the units, and when the limit of processing capacity is reached, other A unit that has reached the limit of its processing capacity can be exchanged for a new one while passing water through the unit. Therefore, when the external decontamination equipment U includes a plurality of filter units, primary adsorption towers, and secondary adsorption towers, the decontamination process can be performed continuously and reliably.

[Decontamination method]
The decontamination processing method concerning this invention can be performed using the decontamination processing apparatus 1 of FIG. The decontamination processing method includes a step of stirring the sludge S in the storage tank R, and a radioactive contaminated water containing the sludge S that is sucked from the middle layer in the storage tank R after the coarse sedimentation time Tr has elapsed after the stirring step. And a step of transferring to the external decontamination equipment U.

<Stirring step>
By stirring the radioactively contaminated water in the storage tank R, the sludge S precipitated at the bottom of the storage tank R is floated and dispersed as evenly as possible in the radioactively contaminated water as shown in FIG.

  In this agitation step, the suction pump 2 is disposed near the bottom of the storage tank R. Typically, the suction pump 2 is landed on the bottom of the storage tank R and settles from the bottom of the storage tank R. The radioactive contaminated water can be sucked together with the sludge S, and the radioactive contaminated water can be circulated into the storage tank R through the circulation channel 3.

  In addition, in this agitation process, aside from the decontamination processing device 1, a device that sucks and injects radioactive contaminated water inside the storage tank R using, for example, an agitation device having a propeller and a pump different from the suction pump 2. Etc. may be provided.

<Transfer process>
In the transfer step, as shown in FIG. 3, after the coarse sedimentation time Tr has passed after the above-described stirring step, the suction pump 2 sucks radioactive contaminated water from the middle layer in the storage tank R, and The radioactively contaminated water containing the sludge S soared in the stirring process is transferred to the external decontamination equipment U through the transfer flow path 4.

  In the case of using the decontamination treatment apparatus 1 of FIG. 1, in this transfer step, the suction pump 2 arranged near the bottom of the storage tank R in the agitation step is raised by the elevator 6 and arranged in the middle layer of the radioactive contaminated water. To do. That is, in the decontamination processing method, the suction of radioactive contaminated water in the stirring process and the suction of radioactive contaminated water in the transfer process are performed by the same suction pump 2 that can be moved up and down. In this way, by using one suction pump 2 as a single unit, it is possible to facilitate the treatment of sludge remaining at the bottom of the storage tank with a relatively simple device configuration.

(Coarse grain settling time)
As the coarse particle settling time Tr, the time required for the coarse particles having a relatively large particle diameter existing on the water surface at the end of the stirring step in the sludge S to settle to the height of the suction port of the suction pump 2 is selected. The That is, in the decontamination processing method, after the agitation step, the coarse sedimentation time Tr elapses, the coarse particles having a relatively large particle size settle in the sludge S, and the fine particles having a relatively small particle size are radioactively contaminated. The radioactively contaminated water containing the fine particles is sucked by the suction pump 2 while floating in water.

  The sludge S is considered to contain radioactive contaminants in the surface layer portion of the particles. Specifically, it is considered that the sludge S particles contain radioactive contaminants within a range of several tens of μm from the surface. For this reason, if the diameter of the particle | grains of the sludge S is 200 micrometers or more, it can be said that a radioactive contamination substance does not contain in the center, but a contamination level is comparatively low.

  Therefore, if the fine particles in the sludge S can be removed, the remaining sludge S is mainly composed of coarse particles having a relatively small contaminant content, so the overall contaminant content (contamination level) is compared. Become smaller. For this reason, by removing the fine particles in the sludge S, the contamination level of the sludge S finally remaining in the storage tank R is reduced to such an extent that it can be treated relatively easily as general waste. It is thought that you can.

  Moreover, it is thought that the material of the sludge S is mainly composed of iron oxide regardless of the particle size, and the specific gravity is substantially constant. Therefore, the sludge S can be separated into coarse particles and fine particles by sedimentation separation.

  Therefore, the decontamination processing method waits for the coarse particle settling time Tr to elapse after the stirring step in the transfer step, and waits for the coarse particles in the sludge S to settle. The fine particles in the sludge S are removed by sucking the radioactive polluted water in which small fine particles are floating and transferring them to the external decontamination equipment U. For this reason, in the decontamination processing method, the sludge S that settles at the bottom of the storage tank R after the decontamination processing is mainly composed of coarse particles having a relatively small content of radioactive contaminants, so that the processing is relatively easy. It becomes.

  As the lower limit of the coarse particle settling time Tr [s], the water level at the suction position of the radioactive contaminated water by the suction pump 2 in a plan view is L [m], and the height of the suction port for sucking the radioactive contaminated water is H. [M] is preferably {5 × (L−H)}, and more preferably {7 × (L−H)}. On the other hand, the upper limit of the coarse particle settling time Tr is preferably {20 × (L−H)}, and more preferably {15 × (L−H)}. When the coarse particle settling time Tr is less than the above lower limit, the suction pump 2 sucks coarse particles in the sludge S, so that the load on the filter unit of the external decontamination equipment U becomes large and is treated as radioactive waste. There is a possibility that the amount of used filter units to be discarded becomes unnecessarily large. On the contrary, when the coarse particle settling time Tr exceeds the above upper limit, the rate of fine particles settling below the suction port of the suction pump 2 becomes too large, and the contaminants of the sludge S remaining in the storage tank R There is a possibility that the content cannot be sufficiently reduced.

  This transfer step may be performed continuously within a range not exceeding the fine particle settling time Ts after the stirring step is completed. In the decontamination processing method, when the duration of the transfer process is limited as described above, the radioactively contaminated water in the storage tank R is gradually removed by repeatedly performing the stirring process and the transfer process as shown in FIG. Decrease.

(Fine granule settling time)
As the fine particle settling time Ts, the time required for the fine particles having a relatively small particle diameter existing on the water surface at the end of the agitation process in the sludge S to settle to the height of the suction port of the suction pump 2 is selected. The That is, in the decontamination treatment method, fine sedimentation time Ts elapses after the stirring step, fine particles having a relatively small particle size settle in the sludge S, and the radioactive contamination water sucked by the suction pump 2 is collected. When it is determined that the fine particles are substantially not contained, the transfer process is ended, and the stirring process is started again.

  The lower limit of the fine particle settling time Ts [s] is preferably {150 × (L−H)}, and more preferably {180 × (L−H)}. On the other hand, as the upper limit of the fine particle settling time Ts, {1150 × (L−H)} is preferable, and {800 × (L−H)} is more preferable. When the fine particle settling time Ts is less than the above lower limit, there is a possibility that the time required for the decontamination process may be unnecessarily increased due to an increase in the number of repetitions of the stirring process and the transfer process due to a decrease in the duration of the transfer process. . Conversely, when the fine particle settling time Ts exceeds the above upper limit, the radioactive contamination water in the storage tank R disappears before discharging the fine particles having a relatively large pollutant content. May not be sufficiently reduced.

  The lower limit of the suction port height H of the suction pump 2 in this transfer step is preferably 1/4 of the level L of radioactive contaminated water in the storage tank R at the suction position in plan view, more preferably 1/3. preferable. On the other hand, the upper limit of the suction port height H of the suction pump 2 in the transfer step is preferably 3/4 of the level L of radioactively contaminated water in the storage tank R, and more preferably 2/3. When the height H of the suction port of the suction pump 2 in the transfer process is less than the lower limit, the sludge S that has settled at the bottom of the storage tank R may be sucked. On the contrary, when the height H of the suction port of the suction pump 2 in the transfer process exceeds the above upper limit, there is a risk that the separation of coarse particles and fine particles in the sludge becomes insufficient, and the radioactive particles together with the fine particles are radioactive. There is a possibility that the decontamination process becomes inefficient because the time during which the contaminated water can be discharged becomes short.

  The height H of the suction port of the suction pump 2 in the transfer process may be constant from the water level L. Thereby, the coarse particle settling time Tr and the fine particle settling time Ts in the decontamination processing method can be set to fixed values, respectively, and the decontamination processing method can be further simplified. In particular, when the agitation process is not performed using the suction pump 2, the suction pump 2 has a high suction port height by, for example, suspending the suction pump 2 from a float that floats on the surface of radioactive contaminated water. The difference between the height H and the water level L can be kept constant. Even when the agitation process is performed using the suction pump 2, the difference between the height H of the suction port of the suction pump 2 and the water level L can be obtained by measuring the water level L of the radioactively contaminated water with a level sensor or the like. Can be kept constant.

  When the difference between the suction port height of the suction pump 2 and the water level (LH) is constant, the lower limit of the difference between the suction port height and the water level is preferably 10 cm, and more preferably 20 cm. On the other hand, the upper limit of the difference between the height of the suction port and the water level is preferably 70 cm, and more preferably 50 cm. If the difference between the height of the suction port and the water level is less than the lower limit, the fine particle settling time cannot be increased, and it is necessary to repeat the agitation step and the transfer step many times, which may be inefficient. On the contrary, when the difference between the height of the suction port and the water level exceeds the upper limit, the sludge S finally remaining at the bottom of the storage tank R when the water level L of the radioactively contaminated water in the storage tank R is low. There is a risk that the content of radioactive pollutants cannot be reduced sufficiently.

  In the decontamination processing method, since a difference is required between the suction port height H and the water level L of the suction pump 2 as described above, after the processing by the decontamination processing method, A certain amount of radioactive polluted water remains. For this reason, after repeating a stirring process and a transfer process, the suction pump 2 is arrange | positioned in the bottom part of the storage tank R, and the process of transferring radioactive contaminated water to the external decontamination equipment U through the transfer flow path 4 is provided. Is preferred.

<Advantages>
Since the decontamination processing apparatus and the decontamination processing method can selectively remove relatively small particles in the sludge S as described above, most of the sludge S finally remaining in the storage tank R. Can be made into coarse particles having a relatively large particle size and a relatively small content of radioactive contaminants. As a result, the sludge S finally remaining in the storage tank R can be treated relatively easily as general waste.

[Other Embodiments]
The said embodiment does not limit the structure of this invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description and common general knowledge of the present specification, and they are all interpreted as belonging to the scope of the present invention. Should.

  The decontamination processing method may be performed without using the decontamination processing apparatus of the present invention. That is, for example, the stirring process may be performed using a stirrer that stirs sludge with a propeller or the like, and the transfer process may be performed using an apparatus that does not have a circulation channel.

  The decontamination processing method can be suitably used particularly for processing a storage tank disposed in a nuclear power plant after an accident.

DESCRIPTION OF SYMBOLS 1 Decontamination processing apparatus 2 Suction pump 3 Circulation flow path 4 Transfer flow path 5 Switching valve 6 Elevator R Storage tank S Sludge Tr Coarse sedimentation time Ts Fine sedimentation time U External decontamination equipment

Claims (7)

A decontamination method for a storage tank in which radioactive contaminated water is stored and sludge settles at the bottom,
Stirring the sludge in the storage tank;
Decontamination of the storage tank, comprising the step of sucking from the middle layer in the storage tank after the coarse sedimentation time has passed after the agitation step and transferring radioactive contaminated water containing sludge to an external decontamination facility Processing method.   The decontamination processing method for a storage tank according to claim 1, wherein the transfer step is performed within a fine particle settling time after the stirring step.   The decontamination processing method for a storage tank according to claim 1 or 2, wherein the stirring step is performed by recirculation of radioactive contaminated water sucked together with sludge by a pump into the storage tank.   The decontamination processing method for a storage tank according to claim 3, wherein the suction of the radioactive contaminated water in the stirring step and the suction of the radioactive contaminated water in the transfer step are performed by the same pump capable of moving up and down.   The storage according to any one of claims 1 to 4, wherein a height of a suction port for sucking the radioactive contaminated water in the transfer step is not less than 1/4 and not more than 3/4 of an average level of the radioactive contaminated water. Decontamination method for tanks.   The coarse particle settling time is {5 × (L−H)} [s, where L [m] is the water level at the suction position of radioactive contaminated water, and H [m] is the height of the suction port for sucking radioactive contaminated water. The above is {20 × (LH)} [s] or less. The decontamination processing method for a storage tank according to any one of claims 1 to 5. A decontamination treatment device for a storage tank that stores radioactive polluted water and sludge settles at the bottom,
A circulation channel for discharging radioactive contaminated water sucked together with sludge from the storage tank into the storage tank;
A decontamination treatment apparatus for a storage tank, comprising: a transfer flow path for transferring radioactive contaminated water containing sludge sucked from the middle layer in the storage tank to an external decontamination facility.

Images