JP6671856B2 - Treatment method for soil containing radioactive cesium - Google Patents

Description

  The present invention relates to a method for treating soil containing radioactive cesium.

  When a large-scale accident occurs at a nuclear power plant, there is a concern that a large amount of radionuclides will be scattered and cause environmental pollution. This environmental pollution covers a wide range of soils, trees, buildings, buildings, oceans, lake waters and the like.

Most of the radionuclides contained in the contaminated soil are ¹³⁴ Cs, ¹³⁷ Cs, and ⁹⁰ Sr. In particular, ¹³⁷ Cs has a long half-life of 30.2 years and is expected to have a long-term effect. . Therefore, establishment of a technology for removing radioactive cesium from soil, buildings, buildings, sludge, incinerated ash and the like containing radioactive cesium is being sought.

  Several proposals have been made regarding such removal of radioactive cesium. For example, an aqueous solution of nitric acid, sulfuric acid, hydrochloric acid, acetic acid, ammonium or the like is added to the soil to which cesium is attached, and the mixture is stirred to obtain a mixture. The resulting mixture is held at 60 to 90 ° C. for 1 to 6 hours to remove the mixture. A separation method has been proposed (for example, see Patent Document 1).

  In addition, the method includes a decontamination step of decontaminating the contaminated soil and separating the contaminated soil into a removed substance and a treated soil, and a plant removal step of separating the contaminated soil into a plant mixed with the contaminated soil and a plant-removed soil from which the plant has been removed. Soil decontamination methods have been proposed. In the soil decontamination method, it has been proposed to perform a step of mixing a contaminated soil with water to obtain a primary mixture and wet-classifying the obtained primary mixture prior to the plant removal step (for example, Patent Document 1). 2).

  In addition, a step of screening the contaminated soil while applying washing and vibration with water to sieve to a plurality of sizes, and then mixing the minimum-size soil component with the wastewater generated at the time of washing, and There is known a method for separating and removing contaminants having a step of precipitating (see, for example, Patent Document 3).

  However, in any of the above methods, for example, decontamination efficiency may be different for soils having different soil properties, and there is a concern that sufficient decontamination cannot be performed. In addition, there is a problem that the amount of generated waste may increase due to insufficient decontamination.

JP 2013-88362 A JP 2013-178177 A Patent No. 4970627

  The present invention has been made in order to solve the above-mentioned problems, and aims to provide a method for treating radioactive cesium-containing soil that can improve the decontamination efficiency and reduce the amount of waste generated. And

  One embodiment of the method for treating radioactive cesium-containing soil of the present invention is a method for treating radioactive cesium-containing soil for decontaminating radioactive cesium-containing soil, wherein the soil is converted into a plurality of types of soil having different specific gravities by heavy liquid. It is characterized by comprising a specific gravity separation step of separating and a cesium removal step of removing cesium contained in the soil separated by the specific gravity.

  According to the present invention, it is possible to provide a method for treating radioactive cesium-containing soil, which can improve the decontamination efficiency and reduce the amount of waste generated.

The flow figure showing roughly the processing method of the radioactive cesium containing soil of an embodiment. The graph which shows the relationship of specific gravity and radioactive cesium concentration distribution of the soil separated by specific gravity with heavy liquid. The graph which shows the relationship between specific gravity and the radioactive cesium removal rate at the time of performing the elution process using oxalic acid about the soil isolate | separated by specific gravity. The graph which shows the relationship between the particle size and the radioactive cesium concentration of the soil classified by wet classification. 7 is a graph showing the relationship between specific gravity and radioactive cesium concentration when soil having a particle size of less than 2 μm is separated by specific gravity using a heavy liquid. The graph which shows the specific gravity, the radiocesium removal rate, and the radioactive cesium density | concentration at the time of performing the elution process by the oxalic-acid aqueous solution, after performing the specific gravity separation of the soil with a particle diameter of less than 2 micrometers with a heavy liquid.

  FIG. 1 is a flowchart schematically showing a method for treating radioactive cesium-containing soil in the present embodiment. The method of treating radioactive cesium-containing soil in the present embodiment is a method of removing and decontaminating cesium in radioactive cesium-containing soil S. The soil S may include sludge, sand, and the like.

  The method for treating radioactive cesium-containing soil shown in FIG. 1 includes a specific gravity separation step S1 of separating radioactive cesium-containing soil S at a specific gravity using a heavy liquid, and removing cesium contained in the specific gravity-separated soil S. And a cesium removal step S2. Further, the method for treating radioactive cesium-containing soil shown in FIG. 1 includes a classification step S3 for classifying the soil S into soil having a plurality of particle sizes before the specific gravity separation step S1. Here, the classification step S3 is not essential, and may be performed as needed.

  In the method for treating radioactive cesium-containing soil according to the present embodiment, in the specific gravity separation step S1, the heavy liquid L is used to separate the soil S into a plurality of types of soils having different specific gravities.

The heavy liquid L can be used without any particular limitation as long as the soil S can be separated from the specific gravity. As the heavy liquid L, a known liquid such as an organic liquid or a salt solution can be used. Examples of the organic liquid used as the heavy liquid L include acetone, bromoform, dibromomethane (CH ₂ Br ₂ ), tetrabromoethane (Br ₂ CHCHBr ₂ ), methyl iodide (CH ₃ I), and methylene iodide (CH ₂ I _2), and the like carbon tetrabromide (CBr _4).

When a salt solution is used as the heavy liquid L, the solvent is not particularly limited as long as it dissolves the salt, and an organic solvent such as ethanol, acetone, benzene, carbon tetrachloride, chloroform, water, or the like can be used. . Examples of the salt solution include tin tetrachloride (SnCl ₄ ), molybdenum hexafluoride (MoF ₆ ), antimony chloride (SbCl ₃ ), barium mercury tetrabromide (BaHgBr ₄ ), tin tetrabromide (SnBr ₄ ), and fluorine fluoride. Tungsten iodide (WF ₆ ), tin iodide (SnI ₄ ), osmium tetroxide (OsO ₄ ), thallium malonate, thallium formate (HCO ₂ Tl), sodium polytungstate (SPT), lithium heteropolytungstate (LST) , A solution of lithium metatungstate (LMT) or the like, a tool solution (aqueous solution of K ₂ HgI ₄ ), a crerich solution (a mixed aqueous solution of thallium formate and thallium malonate), a Klein solution (Cd (OH) ₂ .B ₂ O ₃₎ • 9WO aqueous solution).

  As the heavy liquid L, among the above, it is preferable to use acetone and bromoform from the viewpoint of availability and reduction of the amount of waste generated.

  Specific gravity separation using the heavy liquid L can be specifically performed, for example, as follows. The heavy liquid L is added to and mixed with the soil S, and the soil S is dispersed in the heavy liquid L. At this time, the liquid-solid ratio of the heavy liquid L to the soil S (heavy liquid L (mL) / soil S (g)) is preferably, for example, about 60 mL / g or more, and about 30 mL / g or more. Is also good. Thereafter, the heavy liquid L is allowed to stand still, and is separated into a sediment consisting of the soil S having a specific gravity equal to or higher than the specific gravity of the heavy liquid L, and a supernatant liquid containing the soil S having a lower specific gravity than the heavy liquid L. Thereafter, the heavy liquid L is removed from the soil S, which is the sediment, by a filter or the like as necessary. Similarly, the soil S contained in the supernatant liquid is separated from the heavy liquid L by using a filter or the like as necessary. Thereby, it can be separated into two types of soil S having different specific gravities. Furthermore, using a plurality of heavy liquids L having different specific gravities, in the same manner as described above, for each heavy liquid L, mixing the heavy liquid L and the soil S, separating the precipitated soil S and the soil S contained in the supernatant, and separating the separated soil S By repeating the operation of removing the heavy liquid L from S, it is possible to separate the soil into a plurality of types of soil S having different specific gravities.

  The specific gravity of the heavy liquid L can be appropriately selected according to the specific gravity of the soil S to be subjected to specific gravity separation. As the specific gravity of the heavy liquid L, for example, two or more specific gravities, preferably four or more, more preferably five or more specific gravities can be selected with the specific gravity of the whole soil S to be treated as a center. Specifically, for example, when the specific gravity is 2.0 g / mL, the specific gravity is 1.7 g / mL, 1.8 g / mL, 1.9 g / mL, 2.0 g / mL, 2.3 g / mL, and the like. Heavy liquid L having a specific gravity of 0.1 to 0.3 g / mL can be used. The specific gravity of the heavy liquid L can be adjusted by mixing two or more heavy liquids L. When a salt solution is used as the heavy liquid L, the specific gravity of the heavy liquid L can be adjusted by changing its concentration.

  The concentration of radioactive cesium in the soil S differs depending on the specific gravity of the soil S separated as described above. Therefore, the soil S separated by specific gravity can be classified into a soil S having a high radiocesium concentration and a soil S having a low radiocesium concentration. Therefore, for example, the following cesium removal step S2 can be performed except for the soil S whose radioactive cesium concentration is a concentration (for example, 8000 Bq / kg or less) that can be treated as normal waste, whereby the treatment of the soil S can be performed. It can be done efficiently.

  Further, among the soils S separated by the specific gravity separation, the soil S having a low radioactive cesium concentration tends to easily remove cesium, and the soil S having a high radioactive cesium concentration tends to be difficult to remove cesium. Therefore, after removing the soil S having a high radiocesium concentration that is difficult to remove cesium, and performing a cesium removal step S2 following the soil S having a low radiocesium concentration, the radioactive cesium can be efficiently removed. Can be.

  The reason why the radioactive cesium concentration and the easiness of removal of cesium differ in the soils S having different specific gravities is as follows. The soil S is composed of, for example, Si, Al, Fe, Ca, Na, K, Mg and the balance. These elements are usually present in the soil S as oxides or predetermined salts, and the rest includes organic substances produced by animals and plants. The soil S has a laminated structure composed of, for example, oxides of the above-described elements, and most of cesium exists in a form incorporated in the laminated structure. The soils S having different specific gravities have different soil properties, and therefore, the types and configurations of the components constituting the soil S, the structure of the soil S, and the like are different. It is considered that the amount of cesium taken into the soil S and the easiness of removal of the cesium taken into the soil S are different.

  After the specific gravity separation step S1, a washing step of washing the soil S subjected to the specific gravity separation may be performed as necessary. The washing step can be performed by mixing the soil S with the washing liquid, washing, and then removing the washing liquid from the soil S. As the cleaning liquid, an organic solvent such as ethanol, acetone, benzene, carbon tetrachloride, chloroform, water, or the like can be used. As the washing liquid, it is preferable to use the same organic solvent or water as the solvent of the heavy liquid L from the viewpoint of reducing the amount of generated waste, and among them, water is particularly preferable.

  Before the specific gravity separation step S1, a classification step S3 may be performed. In the classification step S3, the soil S is classified into soil S having a plurality of particle sizes by wet classification or the like. The soil S classified into a plurality of particle sizes in this way has a different radiocesium concentration depending on the particle size. Therefore, it is possible to sort the soil S having a high radioactive cesium concentration and the soil S having a low radioactive cesium concentration. Therefore, except for the soil S having a low radioactive cesium concentration, for example, the soil S having a radioactive cesium concentration that can be treated as normal waste, the specific gravity separation step S1 and the cesium removal step S2 can be performed, thereby removing the soil S. Dyeing can be performed efficiently.

  Next, in the cesium removal step S2, decontamination is performed by removing cesium contained in the soil S that has been subjected to the specific gravity separation. The method of removing cesium is not particularly limited as long as the method can reduce the radioactive cesium concentration of the soil S to, for example, a concentration that can be treated as normal waste.

  The removal of cesium in the soil S in the cesium removal step S2 can be performed using, for example, an eluent as follows. First, the soil S containing cesium is immersed in the eluent. The amount of the eluent at this time is preferably 50 (mL / g) or more in terms of a liquid-solid ratio represented by eluent (mL) / soil S (g) from the viewpoint of promoting elution. The eluent is then agitated as needed. Thereby, the cesium contained in the soil S is eluted in the eluent.

  When performing elution, it is preferable to heat the eluent. The heating temperature is preferably from 60 ° C to 100 ° C, more preferably from 90 ° C to 100 ° C, and particularly preferably about 95 ° C. Thereby, elution is promoted and cesium can be efficiently eluted.

  Any eluent can be used without particular limitation as long as it dissolves the cesium component in the soil S. As the eluent, for example, an aqueous solution of an acid such as hydrochloric acid, sulfuric acid, nitric acid, and oxalic acid can be used. Further, an aqueous solution of the above-mentioned acid salt may be used as the eluent. The concentration of the eluent can be appropriately set according to the type of the soil S. As the eluent, one type may be used alone, or two or more types may be used in combination.

  As the eluent, among the above, it is preferable to use an aqueous solution of oxalic acid from the viewpoint of high elution performance. When an oxalic acid aqueous solution is used as the eluent, the concentration thereof is preferably 0.05 mol / L or more and 1.0 mol / L or less in that the components constituting the soil S are dissolved to promote elution. More preferably, it is not less than 0.5 mol / L and not more than 1.0 mol / L.

  The eluted soil S is separated from the eluent by a filter or the like. Thereafter, washing with water or the like is performed as necessary. Thus, decontaminated soil S can be obtained.

  As described above, according to the method for treating radioactive cesium-containing soil of the present embodiment, by sorting the radioactive cesium-containing soil into a plurality of types of soils having different specific gravities, prior to the cesium removal treatment, the soil to be treated is selected. be able to. Thereby, the cesium removal efficiency can be improved, and the amount of waste generated can be reduced.

(Example 1)
In this example, the relationship between the concentration of radioactive cesium in soil separated from the specific gravity by heavy liquid and the specific gravity was examined.

(Adjustment of heavy liquid)
Acetone and bromoform were mixed to prepare heavy liquids having the following specific gravities. For example, a heavy solution having a specific gravity of 1.7 g / mL was prepared by mixing 11.8 g of acetone and 39.2 g of bromoform. A heavy solution having another specific gravity was prepared by mixing acetone and bromoform in the following amounts.
Specific gravity 1.7 g / mL: acetone 11.8 g, bromoform 39.2 g
Specific gravity 1.8 g / mL: acetone 10.5 g, bromoform 43.6 g
Specific gravity 1.9 g / mL: acetone 9.2 g, bromoform 47.9 g
Specific gravity 2.0 g / mL: acetone 7.8 g, bromoform 52.4 g
Specific gravity 2.3 g / mL: acetone 4 g, bromoform 69 g

(Specific gravity separation)
First, 1 g of radioactive cesium-containing soil (30 mL / g of liquid-solid ratio) was added to 30 mL of a heavy liquid having a specific gravity of 1.7 g / mL, and the mixture was shaken gently. . Next, the stirred heavy liquid was allowed to stand at room temperature for 1.5 hours. Thereby, the soil having a specific gravity equal to or higher than the specific gravity of the heavy liquid was settled in the heavy liquid. Then, the settled soil was separated by collecting the supernatant of the heavy liquid. The sedimented soil was separated from heavy liquid by a filter, and the separated soil was washed with water and dried. Similarly, the soil contained in the supernatant was separated from the heavy solution, washed with water, and then dried. The sedimented soil was classified as a soil having a specific gravity of 1.7 g / mL or more, and the soil contained in the supernatant was classified as a soil having a specific gravity of less than 1.7 g / mL.

  Next, 30 mL of a heavy liquid having a specific gravity of 1.8 g / mL was added to the soil having a specific gravity of 1.7 g / mL or more, and the same operation as described above was performed. The soil settled in the heavy liquid having a specific gravity of 1.8 g / mL was classified as a soil having a specific gravity of 1.8 g / mL or more, and the soil contained in the supernatant was classified as a soil having a specific gravity of less than 1.8 g / mL.

  The same operation is performed in the order of smaller specific gravity using the above-mentioned heavy liquid of each specific gravity, and the soil is subjected to specific gravity of less than 1.7 g / mL, specific gravity of 1.7 g / mL or more and less than 2.0 g / mL, and specific gravity of 2.0 g. / ML or more and less than 2.3 g / mL, and divided into four types of soils having a specific gravity of 2.3 g / mL or more.

  After the specific gravity separation, the radioactive cesium concentration in the soil of each specific gravity was measured using a NaI (sodium iodide) scintillation detector. Using the measurement results, the distribution of the radioactive cesium concentration was calculated as a percentage (%) of the radioactive cesium concentration (Bq / kg) of the soil of each specific gravity with respect to the radioactive cesium concentration (Bq / kg) of the whole soil. The results are shown in the graph of FIG. 2 with the specific gravity of the soil as the horizontal axis and the radioactive cesium concentration distribution as the vertical axis.

  As shown in FIG. 2, the soil separated by specific gravity has different concentrations of radioactive cesium depending on the specific gravity, and the soil with high radioactive cesium concentration (specific gravity of 1.7 g / mL or more and less than 2.0 g / mL) and low soil (Other than the above). Therefore, if the soil to be treated is sampled, the relationship between the specific gravity and the radioactive cesium concentration distribution is measured in advance, and the cesium removal process is performed on the soil with a high radioactive cesium concentration distribution, the soil can be decontaminated efficiently. You can see that it can be done.

  Next, an elution treatment using oxalic acid was performed on the soil of each specific gravity separated as described above, and the relationship between the specific gravity and the removal rate of radioactive cesium was examined.

  The elution treatment was performed as follows. A 0.5 mol / L oxalic acid aqueous solution was used as an eluent. The oxalic acid aqueous solution was heated to 95 ° C., and then the soil was added so as to have a liquid-solid ratio of 50 mL / g. The eluent was heated with stirring to maintain the temperature above 90 ° C. for about 1 hour. Thereafter, the soil and the eluent were separated, and the radioactive cesium concentration in the separated soil was measured.

  From the measurement results of the radioactive cesium concentration before and after elution, the radioactive cesium removal rate (= ｛(radioactive cesium concentration in soil before elution-radioactive cesium concentration in soil after elution) / (radioactive cesium concentration in soil before elution) )｝ × 100 (%)) was calculated. The results are shown in the graph of FIG. 3 with the specific gravity on the horizontal axis and the radioactive cesium removal rate on the vertical axis.

  According to FIG. 3, the soil with a high radioactive cesium concentration before elution (specific gravity of 1.7 g / mL or more and less than 2.0 g / mL) is difficult to remove radioactive cesium by elution, and the soil with low radioactive cesium concentration before elution (see above). It can be seen that the radioactive cesium is easily removed by elution. Therefore, it can be seen that the removal of radioactive cesium can be performed efficiently by performing the cesium removal step after removing the soil having a low concentration of radioactive cesium.

(Example 2)
The soil containing radioactive cesium was wet classified and classified into four types of soil having a particle size of less than 2 μm, 2 μm or more and less than 20 μm, 20 μm or more and less than 212 μm, or 212 μm or more and less than 1000 μm. The radioactive cesium concentration in the soil of each particle size after the classification was measured using a NaI scintillation detector. The results are shown in the graph of FIG. 4 with the particle diameter on the horizontal axis and the radioactive cesium concentration on the vertical axis.

  From FIG. 4, it was found that the radioactive cesium concentration in the soil differs depending on the particle size. In the soil used in this example, since the radioactive cesium concentration was 8000 Bq / kg or less at a particle size of 2 μm or more, the soil having a particle size of 2 μm or more is treated as ordinary waste without performing a specific gravity separation step and a cesium removal step. We can see that we can do it.

  In the case of soil having a particle diameter of less than 2 μm, the density exceeded 8000 Bq / kg, so that a specific gravity separation step and a cesium removal step using an eluent (aqueous oxalic acid solution) were performed. In the specific gravity separation step, four types of heavy liquids were prepared from 1.7 g / ml to 2.0 g / mL in 0.1 mg / mL increments by mixing acetone and bromoform. As a result, soil having a particle size of less than 2 μm can be used to convert soil having a specific gravity of less than 1.7 g / mL, a specific gravity of 1.7 g / mL or more and less than 1.8 g / mL, a specific gravity of 1.8 g / mL or more and less than 1.9 g / mL, or a specific gravity of 1. The specific gravity was separated into four types of soil of 9 g / mL or more and less than 2.0 g / mL.

  After the specific gravity separation, the soil of each specific gravity was subjected to a washing step of washing the attached heavy liquid with water, and after drying, the radioactive cesium concentration in the soil at each specific gravity was measured using a NaI scintillation detector. The results are shown in FIG. 5 with the specific gravity on the horizontal axis and the radioactive cesium concentration on the vertical axis. From FIG. 5, it can be seen that the radioactive cesium concentration in the soil is different in the soils having different specific gravities.

  Next, a cesium removal step was performed on the soil of each specific gravity by eluting with a 0.5 mol / L oxalic acid aqueous solution at 95 ° C. for 1 hour, as in Example 1. For the soil after elution, the radioactive cesium concentration in the soil was measured using a NaI scintillation detector, and the radioactive cesium removal rate was calculated as described above.

  FIG. 6 shows the measurement results of the radioactive cesium concentration in the soil after elution and the radioactive cesium removal rate, the radioactive cesium concentration and the radioactive cesium removal rate in the soil after elution, the vertical axis, and the specific gravity of the soil, horizontal axis. As shown. In FIG. 6, a white square indicates the radioactive cesium concentration, and a black circle indicates the radioactive cesium removal rate. From FIG. 6, the soil with a high specific gravity of 1.7 g / mL or more and less than 1.8 g / mL before the elution has a low radiocesium removal rate and a high radioactive cesium concentration in the soil, but has a high specific gravity of other soils. It can be seen that the concentration of radioactive cesium in the soil was reduced to a level (8000 Bq / Kg) or less that can be treated as ordinary waste.

  Although some embodiments of the present invention have been described above, these embodiments are shown as examples and are not intended to limit the scope of the invention. These new embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and their equivalents.

  L: heavy liquid, S: soil, S1: specific gravity separation step, S2: cesium removal step, S3: classification step.

Claims (6)

A method for treating radioactive cesium-containing soil for decontaminating soil containing radioactive cesium,
A specific gravity separation step of separating the soil into a plurality of types of soil having different specific gravities by heavy liquid,
E Bei and cesium removal step of removing the cesium contained in the gravity separation soil,
The method for treating radioactive cesium-containing soil, wherein the heavy liquid is at least one selected from acetone and bromoform . In the cesium removal step, said specific gravity separated soil is contacted with solvent syneresis radioactive cesium according to claim 1, wherein the cesium contained in the soil is eluted in the eluent, characterized be removed How to treat soil containing soil. Further comprising a cleaning step of washing the front Symbol soil that is gravity separation in the gravity separation step,
3. The method for treating radioactive cesium-containing soil according to claim 1, wherein the washed soil is subjected to the cesium removal step.   The radioactive cesium-containing soil according to any one of claims 1 to 3, wherein a soil having a predetermined specific gravity with a high radiocesium concentration is selected from the soil separated from the specific gravity and subjected to the cesium removal step. Processing method. Further, a classification step of classifying the soil into a plurality of types of soil having different particle sizes,
The method for treating radioactive cesium-containing soil according to any one of claims 1 to 4, wherein the classified soil is subjected to the specific gravity separation step.   The method for treating radioactive cesium-containing soil according to claim 5, wherein a soil having a predetermined specific gravity having a high radiocesium concentration is selected from the classified soil and subjected to the specific gravity separation step.

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