JP2016200490A - Method for reducing concentration of radioactive substances - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for reducing the concentration of radioactive substances, that reduces the concentration of tritium in the air of a space.SOLUTION: The method includes a step of reducing the mole fraction of water vapor in the air. This water vapor reducing step includes a step S1 of cooling the air in a space, a step S2 of exchanging the air in the space; and a step S3 of dehumidifying the intake air of an operator in the space.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a radioactive substance concentration reduction method.

  Contaminated water containing radioactive pollutants such as radioactive cesium and radioactive strontium, such as circulating water for core cooling and wastewater at the nuclear power plant after the accident, should be discharged unless the radioactive pollutants are removed to prevent environmental destruction. Is not allowed.

  For this reason, in the nuclear power plant after the accident, the contaminated water obtained by collecting the surplus reactor cooling water is stored in a large number of tanks. Many of these tanks are bolted tanks formed by joining a plurality of plate-like members on site by bolting.

  In such a bolt-clamped tank, leakage can occur due to deterioration of the packing or the like of the fastening portion between the members. Therefore, it is desired to replace the bolted tank with a welded tank that is formed by welding a plurality of plate-like members and has a longer life.

  However, the tanks that are frequently used at the nuclear power plant after the accident are not designed to discharge all of the stored liquid, and even after the stored contaminated water has been drained and purified, a certain amount of contamination will remain in the tank. Water remains. If the tank in which the contaminated water remains at the bottom in this way is left for a while, tritium may be contained in the radioactive substance released from the remaining contaminated water in the air in the internal space of the tank.

  Therefore, when attempting to dismantle such a tank, there is a risk that an operator may inhale the air in the tank and there is a risk of internal exposure. Tritium can be present in the air as tritium water vapor and is therefore difficult to remove with a dust mask.

  Similarly, when radioactive polluted water is present in the enclosed space of a building, the concentration of radioactive pollutants including tritium in the air increases, so workers can enter the work and dismantle the building. Not easy to do.

  As a method of dismantling radioactive contamination facilities such as tanks, a roof that covers the radioactive contamination facilities is constructed, and the radioactive contamination facilities are dismantled using a remotely operable crane installed inside the roof. In addition, a method has been proposed in which radioactive contaminants are prevented from being scattered to the outside by performing decontamination (see, for example, JP-A-2013-181921). However, the construction of a roof requires a large cost, and a large number of tanks are densely arranged in the nuclear power plant after the accident, so that the construction of the roof is extremely difficult technically.

JP2013-181921A

  In view of the above inconveniences, an object of the present invention is to provide a radioactive substance concentration reducing method capable of reducing the radioactive pollutant concentration of air in a space.

  The invention made to solve the above-mentioned problems is a method for reducing the concentration of radioactive material containing tritium in the air, comprising a step of reducing the mole fraction of water vapor in the air. This is a substance concentration reduction method.

  Tritium, which is an isotope of hydrogen, usually exists as tritium water in combination with oxygen, and exists in the space in a liquid or water vapor state mixed with normal water in which hydrogen and oxygen are combined. That is, there is a high possibility that tritium exists as tritium water vapor in the air in the space where contaminated water exists. On the other hand, in the radioactive substance concentration reduction method, by providing a step of reducing the mole fraction of water vapor in the air, tritium water vapor in air is the same as water vapor in normal water in which hydrogen and oxygen are combined. It can be reduced at a rate. For this reason, the radioactive substance concentration reduction method can reduce the radioactive pollutant concentration of air in the space.

  The air may be cooled in the water vapor reduction step. Thus, by cooling the air in the water vapor reduction step, the tritium water vapor can be condensed and the radioactive pollutant concentration in the air in the space can be easily and reliably reduced.

  The water vapor reduction step may include a step of discharging the air in the space and a step of passing the hygroscopic material-containing filter through the discharged air. In this way, the water vapor reduction step includes removing the air in the space and passing the hygroscopic material-containing filter through the discharged air to adsorb and remove tritium water in the air, The concentration of radioactive pollutants in air can be easily and reliably reduced.

  In the water vapor reduction step, a hygroscopic material-containing filter may be provided in the intake path of the decontamination mask with respect to the decontamination mask of the worker in the space. In this way, in the water vapor reduction process, by providing a moisture absorbing material-containing filter in the intake passage of the decontamination mask for the worker's decontamination mask in the space, the tritium water in the air sucked by the operator By reducing the concentration of radioactive contaminants by adsorption and removal, radioactive contaminants can be prevented from being taken into the human body.

  The hygroscopic material may be zeolite or silica gel. Thus, when the hygroscopic material is zeolite or silica gel, the tritium concentration in the air can be reduced relatively easily and reliably.

  As described above, the radioactive substance concentration reducing method of the present invention can reduce the radioactive pollutant concentration of air in the space.

It is a flowchart which shows the procedure of the radioactive substance concentration reduction method of one Embodiment of this invention. It is a schematic diagram which illustrates the structure of the apparatus which performs the air replacement | exchange process of FIG.

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

[Radioactive substance concentration reduction method]
The radioactive substance concentration reducing method according to an embodiment of the present invention is a method of reducing the radioactive substance concentration including tritium in the air in a space separated from the outside.

  The method for reducing the concentration of radioactive material can be used to reduce the concentration of radioactive material so that there is no harm even if a person inhales air in the space.

  Examples of the space that reduces the concentration of radioactive material in the air by the method for reducing the concentration of radioactive material include an internal space of a tank, an indoor space of a building, and the like. Typically, contamination including radioactive material at a nuclear power plant after an accident. Examples include the internal space of a contaminated water tank that is used to store water and is dismantled over its lifetime.

  Although it does not specifically limit as said contaminated water tank, The bolt fastening type tank formed by fastening between the flanges arrange | positioned in the periphery of a several board | plate material with a volt | bolt is assumed. As such a bolt-clamping tank, for example, a tank formed of a general structural rolled steel material such as SS400 defined in JIS-G3101 (2010), and the inner surface thereof is subjected to rust-proof coating with, for example, tar epoxy resin or the like. There is.

  Although it does not specifically limit as a minimum of the average internal diameter of the said contaminated water tank, 3 m is preferable and 5 m is more preferable. On the other hand, the upper limit of the average inner diameter of the contaminated water tank is preferably 25 m, and more preferably 20 m. If the average inner diameter of the contaminated water tank is less than the above lower limit, the contaminated water tank can be moved into the sealed space and disassembled using a remotely operable device provided in the sealed space. There is a possibility that the merit of applying the substance concentration reduction method cannot be obtained. On the other hand, when the average inner diameter of the contaminated water tank exceeds the above upper limit, the volume of the internal space of the contaminated water tank increases, which may increase the cost of the radioactive substance concentration reducing method. 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 said contaminated water tank, 3 m is preferable and 5 m is more preferable. On the other hand, the upper limit of the average height of the contaminated water tank is preferably 30 m, and more preferably 25 m. When the average height of the contaminated water tank is less than the above lower limit, it is more efficient to move the contaminated water tank into the sealed space and seal the pollutant, and there is a merit of applying the radioactive substance concentration reduction method. May not be obtained. On the contrary, when the average height of the contaminated water tank exceeds the above upper limit, the volume of the internal space of the contaminated water tank increases, which may increase the cost of the radioactive substance concentration reducing method.

  The said radioactive substance concentration reduction method applied to space, such as the inside of the said contaminated water tank, is equipped with the water vapor reduction process which reduces the mole fraction of the water vapor in the air in this space.

<Water vapor reduction process>
As shown in FIG. 1, the water vapor reduction step includes a step of cooling the air in the space (step S1), a step of replacing the air in the space (step S2), and a step of dehumidifying the intake air of the worker in the space. (Step S3).

(Air cooling process)
The air cooling process of step S1 is a process of condensing water vapor in the air and cooling the absolute humidity by cooling the air in the space.

  As a method of cooling the air in the space, in addition to the method of directly cooling the air in the space, a method of cooling the outer wall of the space, a method of cooling the liquid water existing in the space, and a refrigerant in the space A method of charging can be used. The method of cooling the air in the space is preferably a method that does not increase the amount of contaminated water in order not to increase the load of the contaminated water treatment.

  In this way, by cooling the air in the space, water vapor in the air in the space can be condensed, and the mole fraction of water vapor in the air can be reduced. Thereby, the content of tritium water vapor in the air in the space, that is, the concentration of the radioactive substance in the air in the space can be reduced.

  As a method for directly cooling the air in the space, for example, there is a method in which a radiator, that is, a heat exchanger capable of exchanging heat between the air and the refrigerant is arranged in the space, and the refrigerant is supplied to the radiator from the outside.

  Moreover, as a method of cooling the outer wall of a space, the method of watering the outer wall of a space is mentioned, for example. For example, a contaminated water tank that is installed outdoors and has a small amount of stored water after discharging the contaminated water inside has a considerably high internal temperature in summer. In such a case, by sprinkling water into the contaminated water tank, the temperature of the outer wall of the contaminated water tank, and hence the temperature of the contaminated water remaining inside and the temperature of the air in the space above it, can be reduced easily and inexpensively. Can be made.

  Moreover, as a method of cooling the liquid water existing in the space, for example, a method of immersing a heat exchanger in the contaminated water remaining in the contaminated water tank and supplying a refrigerant to the heat exchanger from the outside can be mentioned. Thus, the air on the water surface can be indirectly cooled by cooling the liquid water present in the space.

  Moreover, since the saturated vapor pressure of water falls by cooling the liquid water which exists in space, the partial pressure of the water vapor in the air in an evaporation equilibrium state with this can be lowered. Thereby, the molar fraction of water vapor in the air can be efficiently reduced. Thereby, the radioactive substance density | concentration in the air in space can be reduced efficiently.

  Further, if the temperature of the liquid water in the space is lowered in this way, the evaporation of the water can be suppressed, so that the time until the partial pressure of water vapor in the air rises again becomes longer. Thereby, the duration of the radioactive substance concentration reduction effect can be lengthened.

  Moreover, when using the method of throwing in a refrigerant | coolant in space, as a refrigerant | coolant thrown in in space, dry ice, liquid nitrogen etc. are mentioned, for example.

  If dry ice or liquid nitrogen is used as the refrigerant, the amount of contaminated water can be prevented from increasing because the refrigerant evaporates. However, when dry ice or liquid nitrogen is used, air containing radioactive materials in the space may leak out of the space due to the gas evaporated from these refrigerants, so that it is the same as the air replacement step in the space described later. In addition, it is preferable to take measures to prevent the radioactive material from diffusing.

  Thus, the molar fraction of water vapor in the air in the space can also be reduced by introducing the refrigerant into the space. Thereby, the radioactive substance density | concentration containing the tritium in the air in space can be reduced.

(Air replacement process)
The air replacement process of step S2 includes a process of discharging the air in the space and a process of passing the hygroscopic material-containing filter through the discharged air.

  In this air replacement step, the air in the contaminated water tank X is replaced by sucking outside air into the contaminated water tank X by discharging the air in the contaminated water tank X. Therefore, it is preferable to perform the air replacement step in a state where the upper lid of the contaminated water tank X is at least partially removed.

  For example, as shown in FIG. 2, the air replacement process includes a duct 1 having one end inserted into a space such as the inside of the contaminated water tank X, and a suction fan 2 that sucks out air in the space through the duct 1. The air exchange device having the hygroscopic material-containing filter 3 disposed in the duct 1 and adsorbing moisture in the air passing through the duct 1 can be used.

  The lower limit of the total amount of air sucked by the suction fan 2 is preferably 1 times the internal volume of the contaminated water tank X, more preferably 6 times, and even more preferably 10 times. On the other hand, the upper limit of the total amount of air sucked by the suction fan 2 is preferably 50 times the inner volume of the contaminated water tank X, and more preferably 30 times. When the total amount of air sucked by the suction fan 2 is less than the lower limit, the air in the contaminated water tank X cannot be sufficiently replaced, and the concentration of radioactive pollutants including tritium may not be sufficiently reduced. On the contrary, when the total amount of air sucked by the suction fan 2 exceeds the above upper limit, the time required for the air replacement process may be unnecessarily increased, and the consumption of the hygroscopic material of the hygroscopic material-containing filter 3 is unnecessarily increased. There is a fear.

  The hygroscopic material of the hygroscopic material-containing filter 3 is not particularly limited as long as it can adsorb water vapor, but zeolite and silica gel are preferable. By using zeolite or silica gel as the hygroscopic material, the concentration of radioactive pollutants including tritium in the air can be reduced relatively easily and reliably.

  The hygroscopic material-containing filter 3 is preferably capable of removing dust floating in the air in the contaminated water tank X. Thereby, the diffusion of the radioactive substance floating in the air in the contaminated water tank X and the radioactive substance adhering to the floating dust to the outside of the contaminated water tank X can be prevented.

  Since this air replacement step has a step of discharging air, by sucking outside air into the space to supplement the discharged air, the mole fraction of water vapor in the space is reduced and tritium water vapor in the air is reduced. , That is, the radioactive substance concentration can be reduced.

  Moreover, since an air replacement | exchange process has a process which passes the hygroscopic material containing filter 3 to the air discharged | emitted from the inside of space, the air which removed tritium water with the hygroscopic material containing filter 3 is discharge | released outside. Thereby, it can prevent that the tritium water vapor | steam in the air in space diffuses outside.

(Intake dehumidification process)
In the intake air dehumidifying process in step S3, tritium water vapor in the air that the operator inhales into the body by breathing is intensively removed from the air in the space.

  This intake dehumidification step is performed by using a decontamination mask for an operator in the space and providing a hygroscopic material-containing filter in the intake passage of the decontamination mask.

  As the decontamination mask, a full-face mask is preferable.

  The hygroscopic material of the hygroscopic material-containing filter provided in the intake passage of the decontamination mask is not particularly limited as long as it can adsorb water vapor, but zeolite and silica gel are preferable. By using zeolite or silica gel as the hygroscopic material, the concentration of radioactive pollutants including tritium in the air can be reduced relatively easily and reliably.

[Contaminated water tank dismantling method]
Next, a method for dismantling the contaminated water tank using the radioactive substance concentration reducing method will be described.

  The method for disassembling the contaminated water tank includes a step of discharging the contaminated water stored in the contaminated water tank, a step of diluting the contaminated water remaining in the contaminated water tank after the contaminated water discharging step, and a low concentration in which the contaminated water is diluted. A step of discharging contaminated water, a step of removing water remaining in the contaminated water tank, a step of performing an air cooling step of the radioactive substance concentration reduction method, and a step of performing an air replacement step of the radioactive substance concentration reduction method And a step of dismantling the contaminated water tank while performing an intake air dehumidification step of the radioactive substance concentration reduction method.

<Contaminated water discharge process>
In the polluted water discharge process, the polluted water tank is discharged by general means such as outflow from a nozzle provided in the contaminated water tank, suction by a suction pump connected through a pipe inserted into the contaminated water tank, and the like. The polluted water stored in is discharged.

  This polluted water discharge step can be performed as a step of continuously supplying the polluted water discharged from the polluted water tank to the purification equipment to remove pollutants in the contaminated water, that is, a step for purifying the polluted water. is assumed.

<Contaminated water dilution process>
In the contaminated water dilution process, the contaminated water remaining after the contaminated water discharge process is diluted by supplying dilution water to the contaminated water tank.

  For example, most of the structures of contaminated water tanks used for storing contaminated water generated at the nuclear power plant after the accident are difficult to discharge the entire amount of stored contaminated water by general means. Therefore, by diluting the contaminated water remaining in the contaminated water tank in this contaminated water dilution step, the contaminant concentration is reduced to such an extent that an operator can work in the contaminated water tank.

  In the contaminated water dilution step, it is preferable to reduce the concentration of radioactive contaminants in the residual water in the contaminated water tank to 3,000 Bq / cc or less.

  In this contaminated water dilution step, it is preferable to introduce the diluted water so as to contact the entire inner surface of the side wall of the contaminated water tank, and it is more preferable to inject the diluted water so as to sequentially collide with the entire inner wall surface of the contaminated water tank. In this way, by bringing the dilution water into contact with the inner wall surface of the contaminated water tank, it is possible to wash away contaminants adhering to the inner surface of the side wall of the contaminated water tank.

  As a method of injecting the dilution water, it is preferable to use a three-dimensional cleaning nozzle that can inject while automatically changing the injection direction of the purified water by 360 ° according to the supply pressure of the purified water. As such a three-dimensional cleaning nozzle, a nozzle having a rotation mechanism for rotating the direction of the injection port about two axes orthogonal to each other is preferable.

(Diluted water)
The dilution water may be water having a low pollutant concentration, but purified water that has been supplied from the contaminated water tank to the purification facility in the contaminated water discharge step and from which the contaminant has been removed can be used. The contaminant concentration of the dilution water may be, for example, from the ND level (several Bq / cc) to 500 Bq / cc, depending on the capacity of the purification facility. Thus, by using the water which purified contaminated water as dilution water, the pollutant density | concentration of the stored water in a contaminated water tank can be reduced, without increasing the total amount of contaminated water.

(Purification equipment)
As purification equipment for obtaining purified water used as dilution water, it is preferable to use one provided with a filter unit, a primary adsorption tower, and a secondary adsorption tower.

  The filter unit filters out suspended matter and oil in the contaminated water. Thereby, the capability fall by the clogging of the adsorption agent of a primary adsorption tower and a secondary adsorption tower is suppressed, and adsorption capacity is fully exhibited.

  It is preferable that the plurality of filter units are connected in parallel so that a plurality of the filter units can selectively pass water, and are arranged so that the entire filter unit that is not allowed to pass water or a filter inside thereof can be replaced.

  As the filter disposed in the filter unit, for example, a membrane filter having an average opening diameter of about 0.2 μm is preferably used.

  Furthermore, the filter unit preferably has an activated carbon adsorbent layer on the downstream side of the membrane filter. When the filter unit has the activated carbon adsorbent layer, clogging of the adsorbent in the primary adsorption tower and the secondary adsorption tower can be more reliably prevented.

  The primary adsorption tower is filled with an adsorbent that selectively adsorbs strontium.

  It is preferable that a plurality of primary adsorption towers are provided. All of the primary adsorption towers are connected in series so that water can pass through all but excluding any one, and the separated primary adsorption towers are replaced with new ones. The new primary adsorption tower after replacement may be piped so that it can be connected to the most downstream side. Thereby, it is possible to purify continuously by sequentially replacing the primary adsorption tower with breakthrough, that is, the adsorption capacity being saturated, with a new one.

  As an adsorbent that selectively adsorbs strontium packed in the primary adsorption tower 14, for example, it has a film that does not transmit calcium and magnesium but selectively transmits strontium, and adsorbs strontium. A capsule-like adsorbent having an inorganic material inside can be used.

  Examples of the membrane that selectively permeates strontium include a calcium alginate membrane. Examples of the inorganic material that adsorbs strontium include A-type zeolite and X-type zeolite. Such an adsorbent for strontium is preferably supported on a porous body that also functions as a filter medium for filtering out suspended substances and oil. Examples of such a carrier include activated carbon and zeolite.

  The secondary adsorption tower is filled with an adsorbent that adsorbs a radioactive substance other than strontium.

  It is preferable that a plurality of secondary adsorption towers are provided. All of the secondary adsorption towers are connected in series so that water can pass through all but excluding any one, and the separated secondary adsorption towers are replaced with new ones. The new secondary adsorption tower after replacement may be piped so that it can be connected to the most downstream side. Thereby, it is possible to purify continuously by sequentially replacing the secondary adsorption tower with breakthrough, that is, the adsorption capacity being saturated, with a new one.

  Examples of the adsorbent packed in the secondary adsorption tower include an adsorbent composed of titanium oxide carrying cobalt ferrocyanide or iron ferrocyanide, an adsorbent composed of inorganic carbon and alumina (removal target: many Element), an adsorbent composed of a cesium oxide-based inorganic material (removal object: Sb, Se, Te, iodic acid), an adsorbent composed of activated carbon supporting iodine (removal object: iodine), and tannin. Adsorbent composed of activated carbon (removal object: transuranium element (U, np, Pu, Am, Cm)), adsorbent composed of activated carbon carrying reduced iron (removal object: Sb, Se, Te, many Element), adsorbent composed of aluminum-containing inorganic carbon material (removal object: Sb, Se, Te, Tc), various chelating agents (eg, DDTC, oxine, DTPA, cuperon, etc.) That the absorbent consists of activated carbon (removal target: transuranium (U, np, Pu, Am, Cm)) include those containing, and the like. Here, the removal target is “multi-element”: Ag, Cd, Eu, Mn, Co, Y, Ru, Ce, Te, ni, Zn, Rh, nd, Sn, Sb, Tc, Pr, Sm, Gd, It means that part or all of V and transuranium elements (U, np, Pu, Am, Cm) are to be removed.

<Residual water removal step>
In the residual water removal step, the water remaining in the contaminated water tank is removed by, for example, suction using a low water level submersible pump. This residual water removal process removes residual water that could interfere with the dismantling of the contaminated water tank or leak to the surroundings due to dismantling, allowing the operator to perform dismantling work in the contaminated water tank .

  The low water level submersible pump used in the residual water removal process is, for example, a well-known low water level drainage pump widely used in civil engineering work sites, that is, it is placed on the bottom of a contaminated water tank, and is placed from the bottom to the surroundings. Mention may be made of pumps capable of sucking and delivering water.

  In the residual water removal step, the amount of contaminated water remaining in the contaminated water tank may be further reduced by combining a step of absorbing the contaminated water with, for example, a waste cloth.

<Air cooling process>
In the air cooling step, the tritium water vapor in the contaminated water tank is condensed by performing the air cooling step of the above-described radioactive substance concentration reducing method, thereby reducing the concentration of radioactive material in the air in the internal space of the contaminated water tank.

<Air replacement process>
In the air replacement step, the radioactive material concentration in the air in the contaminated water tank is further reduced by performing the air replacement step of the above-described radioactive material concentration reduction method.

<Tank dismantling process>
In the tank dismantling step, the tank is dismantled while further reducing the concentration of radioactive material in the air in the contaminated water tank that the operator sucks by performing the intake air dehumidifying step of the above-described radioactive material concentration reducing method. Thereby, the wall of a contaminated water tank is divided | segmented into a some board piece, and carrying out from a contaminated water tank installation place is made easy.

  As a method of dividing the wall of the contaminated water tank, a bolt is removed when the contaminated water tank is a bolted tank, and a cutting method using a band saw or the like is used when the contaminated water tank is a welded tank. Is mentioned.

  Further, when dividing the wall of the contaminated water tank, air in the vicinity of the divided portion where the work is performed may be sucked, and the air may be discharged after dust is removed by a filter or the like, for example. Thereby, scattering of contaminants can be prevented.

[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.

  In the radioactive substance concentration reduction method, the water vapor reduction step is not limited as long as it can reduce the mole fraction of water vapor, and any one of the air cooling step, the air replacement step and the intake air dehumidification step, or other methods. It is only necessary to have at least one step of reducing the mole fraction of water vapor.

In the air replacement step of the radioactive substance concentration reduction method, a refrigeration dehumidifier may be provided on the upstream side of the hygroscopic material-containing filter. Thus, the combined use of the refrigeration dehumidifier can extend the life of the hygroscopic material.
Further, when a manhole is installed near the lower part of the tank, it is possible to exchange air from the manhole.

  Further, in the air replacement step of the radioactive substance concentration reducing method, the duct may be inserted from a manhole provided on the side surface of the tank, in addition to being inserted from the upper part of the tank.

  The radioactive substance concentration reducing method according to the present invention can be widely applied to reduce the concentration of pollutants in the air, but in particular, contaminated water disposed for the purpose of storing contaminated water generated at the nuclear power plant after the accident. It can be suitably used to dismantle tanks and contaminated buildings.

DESCRIPTION OF SYMBOLS 1 Duct 2 Suction fan 3 Hygroscopic material containing filter S1 Air cooling process S2 Air replacement process S3 Intake dehumidification process X Contaminated water tank

Claims (5)

A method for reducing the concentration of radioactive material containing tritium in air,
A method for reducing the concentration of radioactive substances, comprising the step of reducing the mole fraction of water vapor in the air.   The radioactive substance concentration reducing method according to claim 1, wherein the air is cooled in the water vapor reduction step. The water vapor reduction step
Exhausting the air in the space;
The method of reducing a radioactive substance concentration according to claim 1, further comprising: passing the exhausted air through a hygroscopic material-containing filter.   The radioactive substance according to claim 1, wherein a decontamination mask is used for an operator in the space in the water vapor reduction step, and a hygroscopic material-containing filter is provided in an intake passage of the decontamination mask. Concentration reduction method.   The radioactive substance concentration reducing method according to claim 3 or 4, wherein the hygroscopic material is zeolite or silica gel.

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