JP7111447B2 - Method for producing radioactive iodine adsorbent - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a radioactive iodine adsorbent, and more particularly to a method for producing an iodine adsorbent with improved adsorption efficiency for gaseous iodine compounds.

Uranium ( ²³⁵ U) is a major element in nuclear fission reaction in the nuclear fuel used in general uranium reactors such as light water reactors. Examples of nuclides of radioactive substances generated by the nuclear fission reaction of uranium ( ²³⁵ U) include strontium ( ⁹⁰ Sr), iodine ( ¹³¹ I), cesium ( ¹³⁴ Cs, ¹³⁷ Cs) and the like.

Normally, uranium, which is a nuclear fuel, is stored inside the fuel rod, and the fissile material of the uranium inside does not diffuse to the outside. Here, after the nuclear fission of uranium has progressed to some extent, the fuel rods are unsealed for processing into MOX fuel or the like, disposal of nuclear fuel, or the like. There is also the risk of damage to the fuel rods themselves. In such cases, although work is carried out in a shielded environment in principle, there is a possibility that radioisotope nuclides may be dispersed to the outside in an emergency situation.

In radioisotopes produced by fission of uranium, iodine is known to exist as gaseous I ₂ or organic iodine compounds such as methyl iodide (CH ₃ I). Although iodine ( ¹³¹ I) has a half-life of about eight days, it decays into xenon (Xe) while emitting high-energy β-rays. Therefore, the problem of suppressing diffusion of radioactive iodine into the atmosphere is more important than other nuclides. In particular, it is necessary to pay attention to the ingestion of iodine by the human body, and it is clear from the fact that the intake of iodine preparations is recommended.

In nuclear power plants, nuclear related facilities (reprocessing facilities, storage facilities), etc., air conditioners are equipped with adsorbents to adsorb radioactive iodine contained in the atmospheric air in the work areas of facility workers. there is In addition to installation in such facilities, consideration is also being given to installing air conditioners in evacuation facilities for nearby residents. Therefore, the demand for radioactive iodine adsorbents is expected to increase in the future.

As conventional adsorbents for radioactive iodine, for example, activated carbon impregnated with potassium iodide, zeolite impregnated with silver, and the like are known. However, the adsorption performance of these adsorbents for iodine and iodine compounds was not always sufficient. Therefore, an adsorbent in which triethylenediamine and potassium iodide are impregnated on activated carbon has been proposed (see Patent Documents 1 and 2).

The radioactive iodine adsorbents typified by Patent Documents 1 and 2 are extremely suitable adsorbents because they are durable even under high temperature and high humidity conditions and have the ability to adsorb both iodine and iodine compounds. However, the impregnation component for exhibiting the adsorption performance of radioactive iodine is expensive, and reduction of the amount used has been studied.

Japanese Patent Application Laid-Open No. 2012-2606 JP 2015-45588 A

After that, the inventor conducted extensive research on the materials and manufacturing processes of the radioactive iodine adsorbent, and confirmed that sufficient adsorption efficiency can be exhibited even if the amount of impregnated substance is reduced compared to the conventional conditions.

The present invention has been proposed in view of the above situation, and has durability even under high temperature and high humidity, and is capable of adsorbing radioactive iodine from the gas to be treated containing both radioactive iodine and radioactive iodine compounds. and capable of reducing the amount of substances acting on adsorption.

A first invention is a method for producing a radioactive iodine adsorbent that adsorbs radioactive iodine from a gas to be treated containing radioactive iodine, comprising a cleaning step of obtaining a substrate activated carbon by washing activated carbon with an acid and drying it, and iodination. A first impregnating step of mixing an aqueous solution of potassium and the base activated carbon and drying to obtain a first impregnated activated carbon, and mixing an aqueous solution of triethylenediamine and the first impregnated activated carbon and drying to obtain a second impregnated activated carbon. and a second impregnation step, wherein the impregnation amount of the potassium iodide in the first impregnation step is 1.8 to 3 wt% of the weight of the radioactive iodine adsorbent, and the amount of triethylenediamine in the second impregnation step. It relates to a method for producing a radioactive iodine adsorbent, characterized in that the loading amount is 1.9 to 2.5% by weight of the radioactive iodine adsorbent.

A second invention relates to the method for producing a radioactive iodine adsorbent according to claim 1 , wherein the base material activated carbon has a BET specific surface area of 1173 to 1723 m ² /g.

According to the method for producing a radioactive iodine adsorbent according to the first invention, a method for producing a radioactive iodine adsorbent that adsorbs radioactive iodine from a gas to be treated containing radioactive iodine, comprising: a first impregnation step of mixing an aqueous solution of potassium iodide and the base activated carbon and drying to obtain a first impregnated activated carbon; mixing an aqueous solution of triethylenediamine and the first impregnated activated carbon; , a second impregnation step of drying to obtain a second impregnated activated carbon, wherein the impregnation amount of the potassium iodide in the first impregnation step is 1.8 to 3% by weight of the radioactive iodine adsorbent; Since the amount of triethylenediamine impregnated in the second impregnation step is 1.9 to 2.5% by weight of the radioactive iodine adsorbent, potassium iodide, which has stable properties and is relatively easy to procure, is used. A simple radioactive iodine adsorbent that has high adsorption performance of radioactive iodine from the gas to be treated containing both radioactive iodine and radioactive iodine compounds, and that can reduce the amount of substances that act on adsorption. It can be obtained by a simple manufacturing method.

According to the method for producing a radioactive iodine adsorbent according to the second invention, in the first invention, the BET specific surface area of the base material activated carbon is 1173 to 1723 m ² /g, so the amount of chemicals per volume that affects the adsorption capacity It is possible to produce a radioactive iodine adsorbent that avoids deterioration in performance due to a decrease in breakthrough time due to a decrease in the impregnation amount and pore volume.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic process drawing explaining the manufacturing method of the radioactive iodine adsorbent of this invention.

The radioactive iodine adsorbent obtained by the production method of the present invention suppresses the diffusion of radioactive iodine into the atmosphere by adsorbing radioactive iodine from the gas to be treated containing radioactive iodine (mainly ¹³¹ I etc.). is an adsorbent for Iodine ( ¹³¹ I, etc.) produced by nuclear fission of uranium is highly reactive because it is a kind of halogen, and depending on temperature conditions, iodine (I ₂ ) exists as a gas. It is also known that ionized I 2 ⁻ reacts with other organic substances to react with methyl groups derived from organic substances to produce organic iodine compounds such as methyl iodide (CH ₃ I). A radioactive iodine adsorbent obtained by the production method of the present invention is proposed as an efficient adsorbent for the gas to be treated containing such radioactive iodine and iodine compounds in the gas.

Therefore, the radioactive iodine adsorbent will be described together with its manufacturing method in conjunction with the schematic process diagram of FIG. A substrate activated carbon is prepared as a main material of the radioactive iodine adsorbent. The substrate activated carbon is known activated carbon granules. Activated carbon is used to carbonize, burn, and burn crushed woody plant materials rich in cellulose, such as sawdust and shavings generated during lumber processing and processing, waste wood, thinned wood, waste bamboo, felled bamboo, and coconut shells. It is a carbide obtained through the activation of In addition to plant materials, used tires, charcoal of various resin products such as phenolic resin, etc. can also be added to the activated carbon. Therefore, the base material activated carbon can be procured relatively inexpensively and in large quantities.

Activated carbon is used as an adsorption and filtering material, and is highly safe, and can adsorb a wide range of substances due to the pores developed on the surface of the activated carbon. Therefore, the following impregnated substances adhere to the surface of the activated carbon and its pores, and the permeation performance of radioactive iodine and gas to be treated containing iodine compounds into the pores of the activated carbon is also high. The activated carbon is granular with a size of about 1 to 5 mm, and the size is irregular. The size and shape of the activated carbon can be arbitrarily controlled by pulverization, crushing, sieving, or the like. However, the radioactive iodine adsorbent is intended to adsorb a gas to be treated containing radioactive iodine. If the particle size is extremely small, pressure loss such as clogging increases, which may affect gas ventilation. However, if the gas to be treated can be pressurized and aerated, powdered activated carbon can also be selected.

As an index for evaluating the adsorption performance of activated carbon, the specific surface area (m ² /g) according to the BET method is used. As a general trend, as the specific surface area increases, the pores inside the activated carbon develop and the adsorption performance increases. However, depending on the target to be adsorbed, the size range of the pores to be developed is important, and simply increasing the specific surface area does not lead to an improvement in adsorption performance. In particular, the radioactive iodine adsorbent obtained by the production method of the present invention mainly adsorbs organic iodine compounds and the like. In this case, the activated carbon having a relatively large number of mesopores (pores in the range of approximately 2 to 50 nm) is considered preferable, considering the size of molecules to be adsorbed.

Therefore, based on the verification of Examples described later, the specific surface area of activated carbon by the BET method when adsorbing an organic iodine compound such as methyl iodide is derived to be 1900 m ² /g or less. When the specific surface area by the BET method exceeds 1900 m ² /g, the performance is deteriorated due to the reduction of the breakthrough time of methyl iodide. Furthermore, the upper limit of the specific surface area is defined as 1750 m ² /g or less, further 1550 m ² /g or less, more preferably 1400 m ² /g or less. The lower limit of the specific surface area is preferably 1100 m ² /g. An increase in the specific surface area leads to a decrease in the amount of attached chemicals per volume and a pore volume that affect the adsorption capacity, resulting in a decrease in breakthrough time as an adsorbent.

First, the activated carbon is immersed in a weakly acidic solution such as dilute hydrochloric acid and boiled in the same weakly acidic solution. Then, after washing with water and drying, the substrate activated carbon to be supplied to the next step is obtained (“washing step”). In order to finish at a low cost, many raw materials derived from natural products are used for activated carbon, as mentioned above. Therefore, the quality variation is not completely eliminated. Also, if organic substances and salts are present on the surface and in the pores of the activated carbon, it is conceivable that the following impregnation of alkali metal iodides and triethylenediamine may be inhibited. Therefore, when activated carbon is boiled in a weakly acidic solution such as dilute hydrochloric acid, organic substances are decomposed and salts can be dissolved. As a result, it is possible to stabilize the quality in advance. In particular, triethylenediamine exhibits basicity. Therefore, it is considered preferable to reduce the basic groups on the surface of the activated carbon by acid washing.

Next, an aqueous solution of alkali metal iodide is prepared, and the aqueous solution and the base activated carbon are mixed and then dried to obtain the first impregnated activated carbon (“first impregnated step”). Alkali metal iodides are specifically potassium iodide (KI) or sodium iodide (NaI). In the case of other alkali metals, the amount used increases due to the relationship with the equivalent (the number of grams per mole), and the price becomes higher than that of potassium iodide. Potassium iodide is preferably used because its properties are stable and it is relatively easy to procure.

The action of radioactive iodine adsorption by alkali metal iodides (potassium iodide or sodium iodide) is thought to be as follows. When the stable isotope iodine ( ¹²⁷ I ₂ ) on the adsorbent side comes into contact with radioactive iodine ( ¹³¹ I ₂ etc.) in the gas to be treated, iodine nuclide exchange occurs. In this regard, for example, reference is made to the reaction of formula (i). Formula (i) is an example of potassium iodide "KI" and "I ^* " denotes a radioactive isotope of iodine. As a result, radioactive iodine in the gas to be treated is reduced compared to before contact. Therefore, the alkali metal iodide acts to remove radioactive organic iodine compounds such as radioactive iodine ( ¹³¹ I ₂ and the like) and radioactive methyl iodide (CH ₃ ¹³¹ I and the like) present alone in the gas to be treated.

Potassium iodide or sodium iodide is dissolved in water, and then thoroughly mixed with the aqueous solution by spray coating or immersion onto the substrate activated carbon. The concentration of the alkali metal iodide aqueous solution is adjusted in consideration of the final loading amount. After that, it is dried at a temperature and for a time sufficient to evaporate the moisture. Since the alkali metal iodide is a salt, it is possible to raise the drying temperature. However, since it is necessary to avoid thermal decomposition of the base material, the heating and drying conditions are generally 100 to 200°C.

The amount of alkali metal iodide to be impregnated in the first impregnation step is controlled to 1 to 3% by weight, more preferably 1.5 to 3% by weight, of the weight of the final radioactive iodine adsorbent. If the weight ratio per unit radioactive iodine adsorbent is less than 1% by weight, the amount of alkali metal iodide (potassium iodide, etc.) decreases and the adsorption effect decreases. And when it exceeds 1.5% by weight, the effect is enhanced. Regarding the upper limit, even if the weight ratio exceeds 3% by weight, the improvement in the adsorption effect reaches a plateau, and it is considered excessive from the point of view of the effect. Therefore, the upper limit is defined as 3% by weight for the purpose of suppressing the amount used.

Subsequently, an aqueous solution of triethylenediamine is prepared. The triethylenediamine aqueous solution and the first impregnated activated carbon are mixed and then dried to obtain the second impregnated activated carbon (“second impregnation step”). Triethylenediamine has a structure represented by formula (f) and is also called 1,4-diazabicyclo[2.2.2]octane. Triethylenediamine has two tertiary amines in its molecule. Moreover, the bond around the nitrogen atom is a back-tied structure. For this reason, the lone electron pair becomes a region that is less susceptible to steric hindrance and rich in nucleophilicity. Therefore, it is considered that a nucleophilic reaction tends to occur with methyl iodide (CH ₃ ¹³¹ I) or the like containing radioactive iodine in the gas to be treated, and a quaternary ammonium salt is generated. The reaction is described, for example, as the mechanism of formula (ii).

After dissolving triethylenediamine in water, it is thoroughly mixed with the aqueous solution by spray coating or immersion onto the substrate activated carbon. The concentration of the triethylenediamine aqueous solution is also adjusted in consideration of the final loading amount. After that, it is dried at a temperature and for a time sufficient to evaporate the moisture. However, triethylenediamine is thermally decomposed at a high temperature exceeding 100° C., and the attached amount is reduced. Therefore, in order to avoid thermal decomposition, drying is performed at a temperature of approximately 80° C. or less, preferably 70° C. or less. Therefore, the high-temperature drying described in the first impregnation step cannot be performed. Therefore, for the convenience of drying, when triethylenediamine is dissolved in water, the amount of water used is such that the first impregnated activated carbon is moistened in consideration of low-temperature drying.

The amount of triethylenediamine impregnated in the second impregnation step is controlled to 0.5 to 2.5% by weight of the finally produced radioactive iodine adsorbent. The upper limit of the attachment amount is based on the verification of Examples described later. Even if the impregnation amount exceeds 3% by weight, the performance improvement effect corresponding to the excess amount is small. This is because the adsorption result was good even in the case of further reducing the amount. As for the lower limit, it is considered that the effect is poor unless the loading amount is approximately 0.5% by weight. Therefore, in order to further enhance the effect, it is desired that the amount of addition is 1.7% by weight or more. Therefore, the loading of triethylenediamine is defined as in the range of 0.5 to 2.5 wt%, more preferably in the range of 1.7 to 2.5 wt%. After finishing the second attachment step, the "radioactive iodine adsorbent" is completed.

In addition to having such extremely simple properties, the amount of impregnated components was suppressed more than in conventional adsorbents. Therefore, the radioactive iodine adsorbent is filled as a filter medium for gas, for example, in dust collectors, air purifiers, air conditioners, and suitable intake ports for outside air, storage portions, and the like (not shown). These devices are installed in nuclear power plants, nuclear fuel reprocessing facilities, nuclear fuel disposal and storage facilities, and even evacuation facilities outside these facilities. In particular, even if the amount of impregnated components to activated carbon is suppressed, the same effect can be expected, so there are advantages such as a reduction in the amount used and an extension of the period of use.

[raw materials used]
The inventor used the following raw materials to prepare a radioactive iodine adsorbent.
- Activated carbon Coconut shell activated carbon "HC-20" (particle size: 1.18-2.36 mm) manufactured by Tsurumi Coal Co., Ltd. was used (activated carbon 1).
- Alkali metal iodide Potassium iodide (manufactured by Wako Pure Chemical Industries, Ltd.) was used as the alkali metal iodide.
· Triethylenediamine Triethylenediamine manufactured by Kanto Kagaku Shiki Co., Ltd. was used.

[Preparation of radioactive iodine adsorbent (I)]
The inventor produced four kinds of radioactive iodine adsorbents while changing the amount of triethylenediamine impregnated (Prototype Examples 1 to 4). First, 120 mL of 1N hydrochloric acid was added to 3 L of water to prepare a diluted hydrochloric acid solution, and 1500 g of activated carbon was added thereto. Here, the activated carbon was boiled for about 30 minutes. After boiling, the activated carbon was washed with running water. Washing with running water was continued until the pH of the wash water was in the range of 6-8. After washing with water, it was dried at 115° C. for 16 hours to obtain a substrate activated carbon.

A potassium iodide aqueous solution was prepared by dissolving 8 g of potassium iodide in 300 mL of water. An aqueous solution of potassium iodide was added to 400 g of the aforementioned base material activated carbon, and the two were thoroughly mixed so that they were familiar. Thereafter, the potassium iodide impregnated activated carbon (first impregnated activated carbon) was obtained at 115° C. for 16 hours.

12 g of triethylenediamine (prototype 1), 20 g of triethylenediamine (prototype 2), 28 g of triethylenediamine (prototype 3), and 10.8 g of triethylenediamine (prototype 4) were dissolved in 67 mL of water to prepare a triethylenediamine aqueous solution of each example. did. An aqueous solution of triethylenediamine was sprayed onto the aforementioned potassium iodide-impregnated activated carbon, and the mixture was sufficiently mixed so that the two were familiar. Subsequently, it was dried while maintaining a temperature of 80°C or less, mainly around 70°C. Drying was continued until a weight change of approximately 2% was confirmed before and after drying. Thus, the radioactive iodine adsorbents of Prototype Examples 1 to 4 were produced.

[Physical property measurement]
<Physical property values regarding pores of activated carbon>
The physical properties of both the substrate activated carbon and the radioactive iodine adsorbent to which both substances were impregnated were measured. The results are shown in Table 1. Among the measurement items, packing density (g / mL), loss on drying (%), benzene adsorption capacity (%), residue on ignition (%), particle size distribution (%), hardness (%), and ignition point (°C ) was measured according to JIS K 1474 (2014).

The specific surface area (m ² /g) is obtained by measuring the nitrogen adsorption isotherm at 77 K using an automatic specific surface area / pore size distribution measuring device "BELSORP-miniII" manufactured by Microtrack Bell Co., Ltd., and determined by the BET method. (BET specific surface area). In addition, the pore volume described later was also measured by the same device.

<Amount of potassium iodide impregnated>
The amount of potassium iodide actually impregnated into the radioactive iodine adsorbent of each prototype was measured. When extracting potassium iodide from the radioactive iodine adsorbent, 200 mL of ion-exchanged water was used as a solvent for 10 g of the radioactive iodine adsorbent, and extraction was performed over 24 hours using a Soxhlet extractor. An extract solution was collected, and ion-exchanged water was added to dilute it to prepare an extract solution with a total volume of 250 mL.

The extraction solution and ion-exchanged water were added to a 100 mL separatory funnel to bring the total volume to 10 mL. 5 mL of 2N sulfuric acid and 2 mL of 30% hydrogen peroxide water were added thereto, and the mixture was allowed to stand for 5 minutes. Subsequently, 20 mL of chloroform was added, and after shaking for 1 minute, the chloroform layer separated into the lower layer was collected. After adding 20 mL of chloroform to the separatory funnel again and shaking for 1 minute, the chloroform layer separated into the lower layer was collected. Separately, chloroform was added to the recovered chloroform layer to prepare a sample solution of 50 mL in total.

The sample liquid was transferred to a quartz cell (optical path length 1 cm), and absorbance at a wavelength of 510 nm was measured using an absorptiometer (Hitachi High-Tech Science Co., Ltd., U-2001, double beam spectrophotometer). A calibration curve was prepared with a solution of potassium iodide having a known concentration in advance to determine the concentration. Then, the impregnated amount (% by weight) per unit weight of activated carbon was calculated from the amount of potassium iodide (impregnated amount) of the substrate activated carbon.

<Amount of triethylenediamine impregnated>
The amount of triethylenediamine actually attached to the radioactive iodine adsorbent of each prototype was measured. When extracting triethylenediamine from the radioactive iodine adsorbent, 200 mL of methanol was used as a solvent with respect to 10 g of the radioactive iodine adsorbent, and extraction was performed over 24 hours using a Soxhlet extractor. The extract was collected, and methanol was added to dilute the extract to prepare an extract solution of 250 mL in total.

Using gas chromatography (G-3900, manufactured by Hitachi High-Tech Science Co., Ltd.), the amount of triethylenediamine in the aforementioned methanol extraction solution was measured. Nitrogen was used as a carrier gas, and a column (Unisole 10T+KOH manufactured by GL Sciences Inc.) and a detector (FID) were used. A calibration curve is created by filling a gas chromatograph with a triethylenediamine solution of a known concentration in advance, and the amount is measured by comparing the peak areas on the detection chart. The attachment amount (% by weight) per unit was calculated.

<Measurement of adsorption amount of radioactive iodine>
In measuring the adsorption amount of radioactive iodine, the performance of the radioactive iodine adsorbent of each prototype was evaluated based on the adsorption amount of radioactive methyl iodide. Therefore, using radioactive methyl iodide containing a radioactive isotope of iodine, a reliability test of its removal efficiency was conducted (unit: %). The contents of the test conformed to ASTM D3803-91.

Moist air (pressure: about 1 atm, temperature: 30.0°C, relative humidity: about 95%) was passed through the radioactive iodine adsorbent of each prototype for 16 hours to saturate it with water. Subsequently, moist air (pressure: about 1 atm, temperature: 30.0° C., relative humidity: about 95%) was passed through for 120 minutes. Then, humid air (pressure: about 1 atm, temperature: 30.0°C, relative humidity: about 95%) containing methyl iodide (mass concentration: 1.75 mg/m ³ ) was passed for 60 minutes to emit ¹³¹ I gamma rays. The radioactivity intensity was measured, the transmittance was determined, and the radioactive iodine removal efficiency was determined.

[Results and discussion of preparation (I) of radioactive iodine adsorbent]
According to Prototype Examples 1, 2, and 3, the impregnated amount of potassium iodide (alkali metal iodide) is almost the same, and the impregnated amount of triethylenediamine increases in order. However, looking at the final removal efficiency of radioactive iodine, the difference in numerical values is as small as the difference in the amount of triethylenediamine impregnated. For this reason, there is room for reducing the amount of triethylenediamine to be impregnated. In particular, considering that even an impregnated amount of less than 3.0% by weight contributes to the adsorption of radioactive iodine, the preferable impregnated amount of triethylenediamine can be derived as a range of 0.5 to 2.5% by weight. can. Regarding the impregnated amount of potassium iodide, it can be considered that a range of approximately 1 to 3% by weight is appropriate, based on the measured values in each trial production example.

Prototype 4 is an example in which activated carbon, which is different from Prototypes 1 to 3, is used as a base material. Therefore, it is considered that the difference in the physical properties of the activated carbon used as the base material has an effect. A comparison of Prototype Examples 1 and 4 shows that the amount of potassium iodide impregnated is the same or slightly greater. Moreover, the amount of triethylenediamine impregnated is also increasing. However, the removal efficiency of radioactive iodine decreased slightly. Although this action has not been completely elucidated, it is probably closely related to the amount of surface acidic groups, hydrophilic groups, etc. present on the surface of activated carbon.

[Preparation of radioactive iodine adsorbent (II)]
From the above-mentioned "Preparation of radioactive iodine adsorbent (I)", it was possible to grasp the basic physical properties and the impregnation amount when producing the radioactive iodine adsorbent. Therefore, the range of impregnated amounts of potassium iodide and triethylenediamine was examined in more detail, and the range of specific surface area suitable for the base material activated carbon was also examined. Prototypes 11 to 14 are produced by changing the amount of triethylenediamine impregnated, Prototypes 15 to 17 are produced by changing the amount of impregnated potassium iodide, and Prototypes 18 to 22 are produced by changing the specific surface area.

In evaluating the different BET specific surface areas, the following activated carbons 2 to 8 were also used. The BET specific surface area of each activated carbon was measured in the same manner as described above.
When producing activated carbons 2 to 8, a common coconut shell activated carbon was prepared, and activated carbons with different specific surface areas were obtained by changing the activation conditions (time). All of the activated carbons were adjusted to have the same particle size distribution within the range of 1.18 to 2.36 mm.
(Activated carbon 2): Specific surface area: 1173 m ² /g
(Activated carbon 3): Specific surface area: 1223 m ² /g
(Activated carbon 4): Specific surface area: 1394 m ² /g
(Activated carbon 5): Specific surface area: 1519 m ² /g
(Activated carbon 6): Specific surface area: 1725 m ² /g
(Activated carbon 7): Specific surface area: 1552 m ² /g
(Activated carbon 8): Specific surface area: 1723 m ² /g

<Production of Prototype Examples 11 to 14>
When producing Prototype Examples 11 to 14 in which the amount of triethylenediamine impregnated was changed, 4.0 g (Prototype Example 11), 6.0 g (Prototype Example 12), and 8.8 g (Prototype Example 13) of triethylenediamine dissolved in 67 mL of water were used. ), and 9.6 g (prototype example 14), and the amounts were increased in order to prepare triethylenediamine aqueous solutions, respectively. The amount of potassium iodide impregnated in Prototype Examples 11 to 14 was the same as that in Prototype Examples 1 to 4 described above. In Prototype Examples 11 to 14, the same activated carbon 1 as in Prototype Examples 1 to 4 was used as the base activated carbon, and the same preparation was carried out.

<Production of Prototype Examples 15 to 17>
When producing Prototype Examples 15 to 17 in which the impregnated amount of potassium iodide was changed, 12.0 g (Prototype Example 15), 20.0 g (Prototype Example 16), 28.0 g (Prototype Example 17), increasing the amount in order to prepare an aqueous solution of potassium iodide. The amount of triethylenediamine impregnated was the same for all, and a solution was prepared by dissolving 10.8 g of triethylenediamine in 67 mL of water. In Prototype Examples 15 to 17, the same activated carbon 1 as in Prototype Examples 1 to 4 was used as the base activated carbon, and the same preparation was carried out.

<Production of Prototype Examples 18 to 22>
Prototype Examples 18 to 22 used activated carbon with different specific surface areas as the base activated carbon. Prototype Example 18 used “activated carbon 2”, Prototype Example 19 used “activated carbon 3”, Prototype Example 20 used “activated carbon 4”, Prototype Example 21 used “activated carbon 5”, and Prototype Example 22 used “activated carbon 6”. In producing Prototype Examples 18 to 22, the amount of triethylenediamine impregnated was the same for all, and a solution was prepared by dissolving 10.8 g of triethylenediamine in 67 mL of water. The amount of potassium iodide impregnated in Prototype Examples 18 to 22 was the same as that in Prototype Examples 1 to 4 described above. The preparation of activated carbon was the same as described above.

The manufacturing method and the analysis method of Prototype Examples 11 to 22 were the same as those of Prototype Examples 1 to 4 described above. The results of Prototype Examples 11-22 are shown in Tables 2-5. However, in the preparation (II), the "measurement of radioactive iodine removal efficiency" was omitted for the sake of simplification, and the breakthrough time of methyl iodide was measured. In addition, in order to grasp the properties of the substrate activated carbon, the total pore volume (cm ³ /g) and each pore diameter section (“less than 2 nm”, “2 to 4 nm”, “4 to 10 nm”, “10 to 50 nm”, “50 nm or more”) were also measured.

<Measurement of breakthrough time>
Methyl iodide was diluted with air to adjust the vent gas concentration to 20 ppm and passed through the column packed with each prototype. The column had an inner diameter of 2.0 cm, a bed height of 5.0 cm, and a packing volume of 15.7 mL. As ventilation conditions, the air volume is 3770 mL/min, the LV is 0.20 m/sec, the SV is 14408 h ⁻¹ , the direction is down flow, the temperature is 29.6 to 30.0° C., and the relative humidity is 86.0 to 96.0° C. A condition of 0% was used. Then, the concentration at the time of breakthrough was set to 2 ppm and measured with a detector tube. The breakthrough time was defined as the time from the start of aeration (20 ppm) until the concentration exceeded the breakthrough point (2 ppm).

[Results and discussion of preparation (II) of radioactive iodine adsorbent]
<1. Amount of triethylenediamine impregnated>
From Prototype Examples 11 to 14, the breakthrough time is increased in proportion to the amount of triethylenediamine impregnated, and the performance as an adsorbent is improved. Here, considering the performance required of the radioactive iodine adsorbent, the range exceeding the breakthrough time of 240 min is very good, and sufficient performance has already been demonstrated. The breakthrough time of 240 min was adopted as a standard because it is a standard value for existing adsorbents. Therefore, the upper limit of the amount of triethylenediamine impregnated is set at 2.5% by weight. The lower limit is 0.5% by weight, preferably 1.2% by weight, more preferably 1.5% by weight, from the viewpoint of exhibiting performance. Further, when the lower limit is specified from the amount of impregnation exceeding the breakthrough time of 240 minutes, it is 1.7% by weight.

<2. Impregnation amount of potassium iodide>
From Prototype Examples 15 to 17, the breakthrough time decreased in inverse proportion to the increase in the amount of potassium iodide impregnated. In Prototype Example 17, when the amount of potassium iodide impregnated was 6.1% by weight, the breakthrough time was less than 240 minutes. Possibly, physical obstacles of potassium iodide inhibited the adsorption to the adsorbent of the prototype example. The role of potassium iodide, which is an impregnated component of the substrate activated carbon, is considered not as adsorption of methyl iodide itself, but rather as substitution of iodine atoms (see formula (i) above). Therefore, the amount of potassium iodide impregnated and the evaluation of methyl iodide adsorption should be considered separately. Therefore, the upper limit of the impregnated amount of potassium iodide exceeding the breakthrough time of 240 min is 5% by weight, and the preferable upper limit is 3% by weight, as a range for both suppressing interference during adsorption and promoting substitution.

<3. BET specific surface area of substrate activated carbon>
From the relationship between the BET specific surface area and the breakthrough time in Prototype Examples 18 to 22, when the BET specific surface area of Prototype Example 22 exceeded 1700 m ² /g, the breakthrough time decreased to 240 minutes. In addition, when the BET specific surface area increased up to Prototype Example 21, it decreased more gradually than the other Prototype Examples. Furthermore, focusing on the pore volume for each section in Table 4, in Prototype Example 22 having the largest BET specific surface area, the range of mesopores decreased and the range of micropores increased. From this point, it can be expected that the difference in size between the methyl iodide molecule and the pores affects the adsorption efficiency. Therefore, considering the BET specific surface area in addition to the pore size distribution, the upper limit is defined as 1900 m ² /g or less, more preferably 1850 m ² /g or less, more preferably 1750 m ² /g or less. The lower limit of the specific surface area is derived to be 1100 m ² /g based on Prototype Example 18, preferably 1200 to 1300 m ² /g from Prototype Examples 19 and 20, more preferably 1400 to 1500 m ² /g from Prototype Examples 20 and 21. be able to.

[Preparation of radioactive iodine adsorbent (III)]
Considering the results of the production of radioactive iodine adsorbents (I and II), a more optimal range of impregnation amount and specific surface area was determined. Therefore, the radioactive iodine adsorption effect of the radioactive iodine adsorbents (prototype examples 31 to 35) having specifications satisfying the above range was verified.

<Production of Prototype Examples 31 and 32>
In producing Prototype Examples 31 and 32, triethylenediamine aqueous solutions were prepared by adding 4.0 g (Prototype Example 31) and 10.8 g (Prototype Example 32) of triethylenediamine dissolved in 67 mL of water. The amounts of potassium iodide impregnated in Prototype Examples 31 and 32 were the same as those in Prototype Examples 1 to 4 described above. In Prototype Examples 31 and 32, the same activated carbon 1 as in Prototype Examples 1 to 4 was used as the base activated carbon, and the same preparation was carried out.

<Production of Prototype Example 33>
In preparing Prototype Example 33, an aqueous potassium iodide solution was prepared with 12.0 g of potassium iodide dissolved in 300 mL of water (Prototype Example 15). As for the amount of triethylenediamine to be impregnated, a solution was prepared by dissolving 10.8 g of triethylenediamine in 67 mL of water. In Prototype Example 33, the same activated carbon 1 as in Prototype Examples 1 to 4 was used as the base activated carbon, and the same preparation was carried out.

<Production of Prototype Examples 34 and 35>
In producing Prototype Examples 34 and 35, Prototype Example 34 used “activated carbon 7” and Prototype Example 35 used “activated carbon 8”. In producing Prototype Examples 34 and 35, the amount of triethylenediamine impregnated was the same, and a solution was prepared by dissolving 10.8 g of triethylenediamine in 67 mL of water. The amounts of potassium iodide impregnated in Prototype Examples 34 and 35 were the same as those in Prototype Examples 1 to 4 described above. The preparation of activated carbon was the same as described above.

The manufacturing method and the analysis method of Prototype Examples 31 to 35 were the same as those of Prototype Examples 1 to 4 described above. The results of Prototype Examples 31-35 are shown in Tables 6 and 7. The amount of radioactive iodine adsorbed in Prototype Examples 31 to 35 was measured under common conditions in accordance with ASTM D3803-91 described in the above-mentioned preparation of radioactive iodine adsorbent (I).

[Results and discussion of preparation (III) of radioactive iodine adsorbent]
From the results of the removal efficiency of radioactive iodine when using the radioactive iodine adsorbents of Prototype Examples 31 to 35, the actual removal performance of radioactive iodine generally matched the production of the radioactive iodine adsorbents (I and II). Therefore, the amounts of potassium iodide (alkali metal iodide) and triethylenediamine impregnated on the base material activated carbon are appropriate. Also, the suitability of the specific surface area of the substrate activated carbon was confirmed.

The radioactive iodine adsorbent obtained by the production method of the present invention exhibits good adsorption performance while reducing the amount of impregnated substance to be supported on the activated carbon compared to existing products, and can be produced at a lower cost. It acts favorably in substitution for goods. Moreover, according to the manufacturing method of the radioactive iodine adsorbent, the performance of the impregnated substance can be more easily brought out, so that the effect can be maintained even if the mass of the impregnated substance is reduced.

Claims (2)

A method for producing a radioactive iodine adsorbent that adsorbs radioactive iodine from a gas to be treated containing radioactive iodine,
a washing step of acid-washing the activated carbon and drying it to obtain a substrate activated carbon;
a first impregnation step of mixing an aqueous solution of potassium iodide and the base activated carbon and drying to obtain a first impregnated activated carbon;
a second impregnation step of mixing an aqueous solution of triethylenediamine and the first impregnated activated carbon and drying to obtain a second impregnated activated carbon;
The amount of potassium iodide impregnated in the first impregnation step is 1.8 to 3 wt% of the weight of the radioactive iodine adsorbent,
A method for producing a radioactive iodine adsorbent, wherein the amount of triethylenediamine impregnated in the second impregnation step is 1.9 to 2.5% by weight of the radioactive iodine adsorbent. 2. The method for producing a radioactive iodine adsorbent according to claim 1 , wherein the BET specific surface area of the substrate activated carbon is 1173-1723 m ² /g.

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