JP7441075B2 - Filter media for removing radioactive iodine and filters for removing radioactive iodine - Google Patents

Description

The present invention relates to a radioactive iodine removal filter for removing radioactive iodine in gas, and a radioactive iodine removal filter.

In fields where radioactive materials are handled , in order to prevent radioactive materials from being released into the atmosphere, radioactive iodine is removed from the gas using an adsorbent that has a carrier loaded with a drug that adsorbs radioactive iodine. There is.

Conventionally, a radioactive iodine adsorbent that adsorbs radioactive iodine contained in exhaust gas discharged from the ventilation air conditioning system of a nuclear-related facility, a gas introduction part having a gas introduction hole, and a pair of filter bodies that accommodate the radioactive iodine adsorbent are used. An iodine filter consisting of the following is known (Patent Document 1). In this iodine filter, the exhaust gas that has entered the iodine filter passes through the holes on one side of the filter body and comes into contact with the radioactive iodine adsorbent, and the radioactive iodine in the exhaust gas is adsorbed by the radioactive iodine adsorbent. . The exhaust gas passes through the gaps between the radioactive iodine adsorbents while coming into contact with the radioactive iodine adsorbents, passes through the holes on the other side of the filter body, and flows to the downstream side of the iodine filter.

International Publication No. 2011/158564

Incidentally, a filter containing an adsorbent that adsorbs radioactive iodine is sometimes used in combination with an air filter such as a HEPA filter that collects particulates in the gas. In recent years, from the viewpoint of energy saving, air filters are sometimes required to have small pressure loss even when used at a large air volume.
However, since the above-mentioned iodine filter is filled with a large amount of radioactive iodine adsorbent in order to increase the removal efficiency of radioactive iodine, the pressure loss is high and it is difficult to use it at a large air volume. Here, by arranging a plurality of iodine filters in parallel in the airflow direction, the processing area may be increased and pressure loss may be reduced. However, in this case, it is necessary to ensure a large flow path area in the portion where the iodine filter is arranged. Furthermore, it is necessary to change the design of the flow path for arranging the iodine filters in parallel.

An object of the present invention is to provide a radioactive iodine removal filter medium and a radioactive iodine removal filter in which pressure loss is reduced while ensuring radioactive iodine removal efficiency equivalent to that of conventional filters.

One aspect of the present invention is
A filter medium for removing radioactive iodine in the gas by passing gas therethrough,
two sheets of breathable sheet material spaced apart in the direction in which the gas passes;
a plurality of adsorbents arranged to be sandwiched between the breathable sheet materials,
Each of the adsorbents includes a drug with an adsorption function that adsorbs radioactive iodine, and carrier particles carrying the drug with an adsorption function on the surface,
90% by mass or more of the carrier particles have a particle size of 1 mm or less,
The carrier particles have first carrier particles and second carrier particles whose particle size peak in the particle size distribution is smaller than the first carrier particles,
The adsorbent is arranged between the breathable sheet material along the direction in which gas passes so as to form a second part and a first part in order from the upstream side,
The mass of the second carrier particles per unit area of a plane parallel to the air-permeable sheet material is larger in the second portion than in the first portion .

Preferably , the first carrier particles have a particle size of 0.180 to 0.355 mm, and the second carrier particles have a particle size of 0.150 to 0.250 mm .

It is preferable that the proportion of the drug with an adsorption function contained in the adsorbent is 7.5% by mass or less.

The drug with an adsorption function contains triethylenediamine,
It is preferable that the proportion of the triethylenediamine contained in the adsorbent is 4.0% by mass or less.

The basis weight of the filter medium is preferably 700 to 850 g/m ² .

Another aspect of the present invention is a radioactive iodine removal filter, comprising:
The filter medium;
It is characterized by comprising a frame surrounding the filter medium.

According to the radioactive iodine removal filter medium and the radioactive iodine removal filter of the above-described aspects, pressure loss is reduced while ensuring radioactive iodine removal efficiency equivalent to that of the conventional filter.

It is an external perspective view showing a radioactive iodine removal filter of this embodiment. It is a figure showing the cross section of the filter medium for radioactive iodine removal. It is a figure showing an example of the arrangement mode of an adsorption material. It is a figure explaining how to arrange an adsorbent.

The filter medium for removing radioactive iodine and the air filter for removing radioactive iodine of this embodiment will be described below. This embodiment includes various embodiments described below.

FIG. 1 is an external perspective view showing a radioactive iodine removal filter (hereinafter simply referred to as a filter) 1 of this embodiment.
The filter 1 includes a filter medium for removing radioactive iodine (hereinafter simply referred to as a filter medium) 2, a separator 3, and a frame 4.

The filter medium 2 is used to allow gas to pass through and remove radioactive iodine from the gas.
Radioactive iodine refers to ¹²⁹ I, ¹³¹ I, and ¹³³ I. In the following explanation, these are collectively expressed as I without distinguishing them from ¹²⁷ I, which is a non-radioactive iodine (stable isotope). Radioactive iodine is contained in simple iodine (I ₂ ) or in an iodine compound. Examples of iodine compounds include organic iodine compounds such as methyl iodide (CH ₃ I).

The filter medium 2 includes two breathable sheet materials 11 and 12 and a plurality of adsorbents.

The breathable sheet materials 11 and 12 are arranged at intervals in the direction in which gas passes. In this specification, the direction in which gas passes with respect to the filter medium 2 means the direction perpendicular to the air-permeable sheet materials 11 and 12 extending in a plane (the airflow direction in FIG. 3), and This is distinguished from the direction in which gas passes through filter 2 and the direction in which gas passes through filter 1 (X direction in FIG. 1). For the breathable sheet materials 11 and 12, a material is used that allows gas to pass through while holding the adsorbent therebetween. A preferred example of such a material is a fibrous body containing a plurality of fibers. Since the breathable sheet materials 11 and 12 made of fibers can be easily bent, the filter medium 2 can be easily processed such as pleating. Further, the filter medium 2 can be disposed of by incineration or the like. Preferred fibers for the fibrous body include, for example, glass fibers, organic fibers, or mixed fibers thereof. A preferable form of the fibrous body is, for example, a nonwoven fabric. A specific example of the fibrous body is a nonwoven fabric made of organic fibers bonded together with a binder. The same material can be used for the breathable sheet materials 11 and 12.

The adsorbent is a large number of particles arranged to be sandwiched between the breathable sheet materials 11 and 12. Each adsorbent includes a drug with an adsorption function (hereinafter simply referred to as a drug) and carrier particles 23, which are not shown.

The drug used is one that has the ability to adsorb radioactive iodine. Preferably, the drug includes at least triethylenediamine (TEDA). TEDA is a polyamine compound having two tertiary amino groups, and the tertiary amino group has the property of converting into quaternary ammonium and bonding with CH ₃ I, and CH ₃ I is adsorbed to the tertiary amino group. In this way, radioactive iodine contained in CH ₃ I can be adsorbed.

Preferably, the drug further contains at least one component of KI, I ₂ and Ag in addition to TEDA. When a drug contains KI or _I2 , radioactive iodine is isotopically exchanged with non-radioactive iodine of KI or _I2 , and the radioactive iodine can be adsorbed. Furthermore, when the drug contains Ag, radioactive iodine reacts with Ag to become AgI, which allows radioactive iodine to be adsorbed.

A drug is supported on the surface of the carrier particles 23. The material used for the carrier particles has a plurality of pores on its surface, such as activated carbon, alumina, zeolite, silica gel, activated clay, and the like. For example, activated carbon made from coconut shells is used. The carrier particles can be used alone or in combination of two or more types.

90% by mass or more of the carrier particles 23 have a particle size of 1 mm or less. The particle diameter of the carrier particles here means the individual particle diameter of the carrier particles 23, and is distinguished from the average particle diameter calculated according to JIS K1474. For example, when the particle size of 90% by mass or more of a predetermined number (for example, 50 to 500) of carrier particles randomly selected from the carrier particles 23 serving as an adsorbent is 1 mm or less, the above-mentioned 90% by mass or more of the carrier particles 23 corresponds to a case where the particle size is 1 mm or less.

In the above-mentioned iodine filter, in order to improve the removal efficiency of radioactive iodine, it is effective to lengthen the time that gas passes through the filter body containing the adsorbent, and to bring the gas into contact with more adsorbents. However, if the thickness of the filter body is increased in order to prolong the time for gas to pass through the filter body, pressure loss tends to increase. Therefore, it is difficult to use it with a large air volume (for example, 50 m ³ /min).
According to the study of the present inventor, by using carrier particles adjusted to satisfy the above-mentioned condition regarding particle size, that is, 90% by mass or more of the carrier particles 23 have a particle size of 1 mm or less, gas can be It was revealed that even if the thickness of the filter medium in the direction of passage is thin, the removal efficiency of radioactive iodine can be maintained at the same level as conventional methods. Moreover, the thickness of the filter medium in the direction through which gas passes is thinner than that of the filter body, and the area of the filter medium can be increased by applying pleats, so that pressure loss can be reduced. That is, according to the filter medium 2 of this embodiment, pressure loss is reduced while ensuring radioactive iodine removal efficiency equivalent to that of the conventional filter. Such a filter medium can be used with a large air volume, so even if it is used in combination with a high air volume type HEPA (High Efficiency Particulate Air) filter, for example, pressure loss is unlikely to increase. Therefore, it is not necessary to arrange a plurality of filters 1 including filter media 2 in parallel in the air flow direction in the middle of the flow path so that pressure loss is reduced by increasing the filter media area.

Among the radioactive iodine removal efficiency of the filter medium 2, the radioactive iodine removal efficiency in the organic iodine compound (hereinafter referred to as organic iodine removal efficiency) is preferably 80% or more, more preferably 90% or more. , 99% or more is particularly preferred. Further, the removal efficiency of simple iodine is preferably 95% or more, more preferably 98% or more, more preferably 99% or more, and particularly preferably 99.5% or more.

The pressure loss of the filter medium 2 is less than 50 Pa, preferably less than 30 Pa, and more preferably less than 20 Pa at a wind speed of 5.3 cm/sec.

Returning to the explanation of the filter 1, the filter medium 2 in the example shown in FIG. 2 is pleated and has a zigzag shape in which mountain folds and valley folds are alternately repeated. FIG. 2 is a diagram showing a cross section of the filter medium 2 taken along a plane orthogonal to the Z direction in FIG. In FIG. 2, illustration of the adsorbent is omitted.

The separator 3 is inserted into each of the spaces inside the mountain-folded and valley-folded portions of the filter medium 2, and maintains the spacing between adjacent pleats. The separator 3 is formed by corrugating a thin plate-like member made of, for example, paper, PET, polystyrene (PS), aluminum, or stainless steel into a corrugated shape. In the example shown in FIG. 1, the separator 3 is arranged so that the fold extends in the direction in which gas passes through the filter 1 (airflow direction, X direction).

The frame body 4 is a member that circumferentially surrounds the filter medium. In the example shown in FIG. 1, the frame 4 has a rectangular outer shape when viewed in the airflow direction, and includes an upper surface portion 41, a lower surface portion 43, and side portions 45, 47 extending along each of the four sides. have. The frame body 4 is produced, for example, by combining a plurality of plate materials corresponding to each of these parts 41, 43, 45, and 47. The material of the plate material is, for example, aluminum, aluminum alloy, stainless steel, or wood. The stainless steel may be plated with zinc or aluminum. Further, the plate material may include a film formed by alumite treatment, chromate treatment, or the like.
The space between the upper surface portion 41 and the lower surface portion 43 and the filter medium 2 is sealed with a sealing material such as urethane resin.

An example of the dimensions of the filter 1 is 610 mm in length (Z direction in FIG. 1) x 610 mm in width (Y direction in FIG. 1) x 292 mm in depth (X direction in FIG. 1). The number of pleats in the filter medium 2 is not particularly limited, but is, for example, 30 to 50.

The structure of the filter 1 is not limited to the separator type shown in FIG. 1, but may be a well-known V-bank type as shown in FIGS. 1 and 4 of Japanese Patent Application No. 2014-013675, for example. An example of the dimensions of a V-bank type filter is 610 mm long x 610 mm wide x 292 mm deep. The number of V-shaped filter pack pairs (bank number) is not particularly limited, but is, for example, 3 to 7.

According to one embodiment, the carrier particles 23 preferably include two or more types of carrier particles having different particle size peaks in the particle size distribution. The term "seed" as used herein refers to each species separated according to differences in particle size distribution. This makes it possible to achieve both the effect of improving radioactive iodine removal efficiency and the effect of reducing pressure loss at a high level. Since the filter medium 2 contains many small-diameter carrier particles 23 under the above-mentioned conditions, the amount of the drug contained in the entire adsorbent is large, and the surface area of the carrier particles 23 is large. Therefore, both organic iodine removal efficiency and inorganic iodine removal efficiency can be effectively improved. Furthermore, by including many carrier particles 23 with small particle diameters, the thickness of the filter medium 2 can be reduced. On the other hand, if the particle size of the carrier particles 23 is too small, the gaps between the carrier particles 23 will become small, which may increase pressure loss. Furthermore, if the particle size of the carrier particles 23 is too small, the carrier particles 23 may pass through the air-permeable sheet materials 11 and 12 and leak to the outside of the filter medium 2, which may result in a decrease in radioactive iodine removal efficiency. There is. Therefore, by using a combination of a plurality of types of carrier particles 23 having different particle size distributions, it is possible to suppress an increase in pressure loss while maintaining a high removal efficiency of radioactive iodine. Carrier particles having a large particle size increase the gap between the particles, thereby increasing the effect of suppressing an increase in pressure loss. Moreover, carrier particles having a large particle size are difficult to pass through the air-permeable sheet materials 11 and 12 and are difficult to leak to the outside of the filter medium 2. Furthermore, by using carrier particles with a large particle size, the amount of carrier particles with a small particle size can be reduced, so that the carrier particles 23a with a small particle size pass through the air permeable sheet materials 11 and 12 and are exposed to the outside of the filter medium 2. There is no need to prevent it from leaking. For example, there is no need to increase the basis weight of the breathable sheet materials 11 and 12. In order to enhance such an effect, it is further preferable that two or more peaks exist in the particle size distribution of the entire carrier particle 23, which is obtained by adding up the particle size distribution of each carrier particle.

As the two or more types of carrier particles 23, for example, two types of carrier particles are used: carrier particles with a small particle size peak in the particle size distribution (second carrier particles) and carrier particles with a large particle size peak (first carrier particles). The compounding ratio in this case is preferably 4:1 to 1:1, more preferably 3:1 to 1:1 in terms of mass ratio, from the viewpoint of improving radioactive iodine removal efficiency. Furthermore, from the viewpoint of suppressing leakage of the carrier particles through the breathable sheet materials 11 and 12, the above-mentioned blending ratio is preferably 5:1 to 2:1, more preferably 4:1 to 1.5: It is 1. Examples of preferred combinations of particle size distributions of the two types of carrier particles include 0.180 to 0.355 mm and 0.150 to 0.250 mm.

As described above, when the carrier particles 23 have the first carrier particles and the second carrier particles whose particle size peak in the particle size distribution is smaller than that of the first carrier particles, the carrier particles 23 can be adsorbed. As shown in FIG. 3, the material includes a second portion 25b and a first portion 25a between the breathable sheet materials 11 and 12 in order from the upstream side along the direction in which gas passes through the filter medium 2. The mass of the second carrier particles 23b per unit area of a plane parallel to the breathable sheet materials 11, 12 may be larger in the second portion 25b than in the first portion 25a . preferable. FIG. 3 is an enlarged view of a portion surrounded by a broken line circle in FIG. 2, showing an example of the arrangement of adsorbents. In FIG. 3, carrier particles carrying a drug are shown with carrier particle symbols attached thereto for easy explanation. By arranging the adsorbent as described above, it is possible to prevent the adsorbent made of the second carrier particles 23b from passing through the breathable sheet material 12 on the downstream side and leaking out. Note that the first portion 25a and the second portion 25b may be separated from each other in the direction in which the gas passes, or may be in contact with each other, as shown in the figure.

Such an arrangement of adsorbents can be obtained, for example, by performing the method shown in FIG. 4. FIG. 4 is a diagram illustrating how to arrange the adsorbent. In the method shown in FIG. 4, while the sheet material that will become the breathable sheet material 12 is transported in one direction (to the right in FIG. 4), two hoppers that are arranged along the transport direction and accommodate raw material particles that will become the carrier particles are used. 31 and 32 onto the sheet material, a layer of second carrier particles 23b is formed on top of a layer of first carrier particles 23a. A mesh with a large opening is attached to the bottom of the hopper 31 on the upstream side, and a mesh with a small opening is attached to the bottom of the downstream hopper 32. Two types of carrier particles having different values can be dropped. In order to adhere the particles to each other and the particles to the sheet material, a powdered hot melt resin is uniformly sprinkled on the adsorbent and fixed by heat.

The proportion of the drug contained in the adsorbent is preferably 7.5% by mass or less. The proportion of the drug contained in the adsorbent means the total amount of the drug supported on all the adsorbents included in the filter medium 2. If the amount of the drug is limited in this way, it is possible to secure a large surface area of the part of the carrier particles 23 that does not carry a drug, so that while achieving high organic iodine removal efficiency by the drug, the corresponding part of the carrier particles 23 can be High inorganic iodine removal efficiency can be achieved by the surface. It is desirable that the content is 1.0% by mass or more, and the above ratio is, for example, 3% by mass.

When the drug contains TEDA, the proportion of TEDA contained in the adsorbent is preferably 4% by mass or less. Thereby, high removal efficiency of inorganic iodine can be achieved while achieving high removal efficiency of methyl iodide in particular.

As described above, when the carrier particles 23 include the first carrier particles 23a and the second carrier particles 23b, according to one embodiment, the second carrier particles 23b with respect to the mass of the second carrier particles 23b The mass ratio (average value) of the drug supported on the first carrier particles 23a is preferably larger than the ratio (average value) of the mass of the drug supported on the first carrier particles 23a to the mass of the first carrier particles 23a. The second carrier particles 23b have a smaller particle size and a larger specific surface area than the first carrier particles 23a. Therefore, even if the amount of drug supported on the second carrier particles 23b is larger than the amount of drug supported on the first carrier particles 23a, the surface area of the portion of the second carrier particles 23b that does not carry a drug is increased. Can be secured.

As described above, when the carrier particles 23 include the first carrier particles 23a and the second carrier particles 23b, and the particle size distribution is 0.180 to 0.355 mm and 0.150 to 0.250 mm, respectively. The following ranges are exemplified as specific surface area, packing density, and iodine adsorption performance.
-First carrier particles 23a
Specific surface area: 800-1600m ² /g
Packing density: 0.450-0.60g/ml
-Second carrier particles 23b
Specific surface area: 1000-1800m ² /g
Packing density: 0.50-0.60g/ml
When the specific surface area, packing density, and iodine adsorption performance of the first carrier particles 23a and the second carrier particles 23b satisfy the above ranges, organic iodine removal efficiency and inorganic iodine removal efficiency can be improved in a well-balanced manner. The specific surface area, packing density, and iodine adsorption performance are determined in accordance with JIS K1474.

The basis weight of the filter medium 2 is preferably 700 to 850 g/m ² . According to the filter medium 2 that satisfies such a basis weight, the effect of suppressing the leakage of the carrier particles 23 through the air-permeable sheet materials 11 and 12 is large, without impairing the above-mentioned effect of reducing pressure loss. For the same reason, the thickness of each of the breathable sheet materials 11 and 12 is preferably 0.15 mm.

The thickness of the adsorbent layer between the breathable sheet materials 11 and 12 in the direction in which gas passes is preferably 1.0 to 2.5 mm. By satisfying such a thickness, it is possible to obtain the effect of making it easier to pleat the filter medium 2 without impairing the above-mentioned effect of improving the removal efficiency of radioactive iodine. The thickness is not particularly limited, but is preferably 1.5 to 2.0 mm. The thickness in the above range is thinner than the thickness of the adsorbent housed in the filter body of the iodine filter, for example, about 1/5 of the thickness, but as mentioned above, the adsorbent material containing many small carrier particles , the above-mentioned effect of improving the removal efficiency of radioactive iodine can be obtained.

The filter medium for removing radioactive iodine and the filter for removing radioactive iodine of this embodiment can be suitably used in places where gas containing radioactive iodine can be generated or handled. Examples of such places include RI facilities such as research institutes, hospitals, and factories that handle RI (Radioisotopes). The radioactive iodine removal filter and the radioactive iodine removal filter of this embodiment are installed in the middle of the gas flow path of the ventilation equipment or air conditioning equipment of these facilities.

(Experiment example)
In order to examine the effects of the present invention, filter bodies of conventional examples shown in Table 1 and below, and filter media of Comparative Examples and Examples 1 to 3 were prepared, and the organic iodine removal efficiency, inorganic iodine removal efficiency, and pressure loss were measured. was evaluated.

In conventional examples, as carrier particles with a particle size of 1 mm or less, 1% by mass or less of the following first carrier particles carrying a drug, 55% by mass or more of carrier particles with a particle diameter of 1.1 to 1.8 mm, An adsorbent in which TEDA was supported on carrier particles ("other carrier particles" shown in Table 1) was used. Instead of sandwiching the adsorbent between air-permeable sheet materials, the above-mentioned iodine filter containing an adsorbent inside a rectangular parallelepiped box was used instead of the filter medium. Note that the thickness of the adsorbent housed in the filter body along the direction in which gas passes through the filter body was 7.5 mm.

The filter media used in Comparative Examples and Examples 1 to 3 were produced in the following manner.
For the first carrier particles and the second carrier particles, activated carbon raw material particles produced from coconut shells were classified using a sieve mesh specified in JIS Z8801-1:2006. Classification was performed by selecting particles that passed through a sieve mesh corresponding to the upper limit of the particle size range shown below and did not pass through a sieve mesh corresponding to the lower limit value. Note that two or more peaks were present in the particle size distribution of the entire carrier particles, which is obtained by adding up the particle size distributions of the first and second carrier particles.
・First carrier particles: particle size 0.180 to 0.355 mm
・Second carrier particles: particle size 0.150 to 0.250 mm

Each of the classified carrier particles was impregnated with a TEDA solution and dried, and then impregnated with a KI solution and dried to produce an adsorbent. The amounts of drugs supported on the first carrier particles and the second carrier particles are as follows. The amount of drug is calculated based on the amount supported on each carrier particle.
- First carrier particles: TEDA 2-2.5% by mass, KI 1.5-2% by mass
-Second carrier particles: TEDA 3.5-4% by mass, KI 2-2.5% by mass

Hot-melt resin powder was sprinkled onto the adsorbent thus obtained, and the adsorbent was sandwiched between two nonwoven fabrics having a thickness of 0.15 mm and a basis weight of 30 g/m ² to form a breathable sheet material, thereby producing a filter medium.

Note that the amount of carrier particles shown in Table 1 represents the mass of the carrier particles carrying the drug per unit area of a plane parallel to the breathable sheet material.

In the comparative example, in addition to the first carrier particles, the drug was supported on carrier particles with a particle size of 1.18 to 1.70 mm that were prepared using the same steps as the first carrier particles except for classification. I used the one that had been prepared.

Organic iodine removal efficiency and inorganic iodine removal efficiency were measured using a measurement method based on the test "Test method for atomic grade activated carbon" in accordance with the spirit of ASTM D-3803.

The pressure loss of the filter medium 2 was measured in accordance with JIS B9927 using a differential pressure gauge at a wind speed of 5.3 cm/sec.

As a result, when the organic iodine removal efficiency was 80% or more and the inorganic iodine removal efficiency was 95% or more, it was evaluated that the removal efficiency was improved to a level equal to or higher than that of the conventional method. Moreover, when the pressure loss was 30 Pa or less, it was evaluated that the pressure loss was reduced.

From a comparison between Examples 1 to 3 and Comparative Examples, it was found that by having the particle size of 90% by mass or more of the carrier particles be 1 mm or less, pressure loss was reduced while ensuring radioactive iodine removal efficiency equivalent to or higher than conventional methods. I can see that.
From the comparison between Example 1 and Examples 2 and 3, when the carrier particles of the adsorbent include first and second carrier particles whose particle size peaks in the particle size distribution are different from each other, the removal efficiency of radioactive iodine is It can be seen that both the effect of increasing the pressure loss and the effect of reducing the pressure loss can be achieved at a high level.
Note that when the filter media of Examples 2 and 3 were subjected to pleat processing, the processing of Example 2 was better.

Although the radioactive iodine removal filter and the radioactive iodine removal filter of the present invention have been described in detail above, the air filter of the present invention is not limited to the above embodiments and examples, and within the scope of the gist of the present invention, Of course, various improvements and changes may be made.

1 Air filter 2 Filter medium 3 Separator 4 Frames 11, 12 Air-permeable sheet material 23 Carrier particles 23a First carrier particles 23b Second carrier particles 25a First portion 25b Second portions 31, 32 Hopper

Claims (6)

A filter medium for removing radioactive iodine in the gas by passing gas therethrough,
two sheets of breathable sheet material spaced apart in the direction in which the gas passes;
a plurality of adsorbents arranged to be sandwiched between the breathable sheet materials,
Each of the adsorbents includes a drug with an adsorption function that adsorbs radioactive iodine, and carrier particles carrying the drug with an adsorption function on the surface,
90% by mass or more of the carrier particles have a particle size of 1 mm or less,
The carrier particles have first carrier particles and second carrier particles whose particle size peak in the particle size distribution is smaller than the first carrier particles,
The adsorbent is arranged between the breathable sheet material along the direction in which gas passes so as to form a second part and a first part in order from the upstream side,
A filter medium for removing radioactive iodine, characterized in that the mass of the second carrier particles per unit area of a plane parallel to the air-permeable sheet material is greater in the second portion than in the first portion. . The radioactive iodine removal method according to claim 1, wherein the first carrier particles have a particle size of 0.180 to 0.355 mm, and the second carrier particles have a particle size of 0.150 to 0.250 mm. filter medium. The filter medium for removing radioactive iodine according to claim 1 or 2 , wherein the proportion of the drug with an adsorption function contained in the adsorbent is 7.5% by mass or less. The drug with an adsorption function contains triethylenediamine,
The filter medium for removing radioactive iodine according to claim 3 , wherein the proportion of the triethylenediamine contained in the adsorbent is 4.0% by mass or less. The filter material for removing radioactive iodine according to any one of claims 1 to 4 , wherein the filter material has a basis weight of 700 to 850 g/m ² . The filter medium according to any one of claims 1 to 5 ,
A radioactive iodine removal filter comprising: a frame surrounding the filter medium.

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