JP2017181335A - Method for decontamination of radioactive cesium-containing material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently decontaminating, with a small amount of acid, the radioactive cesium of a radioactive cesium-containing material such as a soil contaminated by radioactive cesium.SOLUTION: The method includes: an acid cleaning step of sending a radioactive cesium-containing processing target material and an acid aqueous solution into a processing chamber and agitating and mixing the material and the solution for a predetermined time at a predetermined temperature; a first separation step of solid-liquid separating the mixture of the radioactive cesium-containing processing target material and the acid aqueous solution; a water cleaning step of adding water not containing radioactive cesium to the obtained first deposit and agitating and mixing the mixture for a predetermined time; a second separation step of solid-liquid separating a first deposit obtained in the water cleaning step and the mixture containing water; and a radioactive cesium adsorption step of circulating, for a predetermined time a second supernatant liquid between the chamber containing the second supernatant liquid and the container filled with a radioactive cesium adsorbent.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for decontaminating radioactive cesium-containing materials such as soil contaminated with radioactive cesium.

Waste discharged from facilities handling radioactive materials such as nuclear power plants contains radioactive materials. Among them, radioactive cesium has a long half-life of ¹³⁴ Cs for about 2 years and ¹³⁷ Cs for about 30 years. Therefore, it is necessary to pay close attention to its storage. In particular, a large amount of radioactive material has recently been released due to an accident at a nuclear power plant in Fukushima Prefecture, causing widespread pollution, and the treatment of radioactive cesium-containing materials such as soil from the contaminated area has also become a problem.

  By the way, fine particles of soil contaminated with radioactive substances are made of weathered biotite, vermiculite, illite and other minerals, and radioactive substances such as radioactive cesium are adsorbed.

  Therefore, a method for desorbing and decontaminating this radioactive substance using an acid has been developed.

  For example, in Patent Document 1, an aqueous solution selected from the group consisting of nitric acid, sulfuric acid, hydrochloric acid, ammonium acetate, and combinations of two or more thereof is added to the soil and / or soil components to which cesium is adhered, and the mixture is stirred. Removing cesium from the soil, including obtaining and desorbing the mixture at 60 to 90 ° C. for 1 to 6 hours, and removing the aqueous solution from the soil and / or soil components in the mixture A method is disclosed.

  Patent Document 2 discloses a soil decontamination apparatus for decontaminating soil containing radioactive cesium, containing the soil and an eluent containing an acid solution, and using the cesium in the soil as the eluent. A treatment tank to be eluted; an adsorption tower containing an adsorbent that adsorbs cesium in the eluent; a first pipe connecting the treatment tank and a water inlet of the adsorption tower; and a water outlet of the adsorption tower. And a second pump that connects the treatment tank, and a transfer pump that is interposed in the first pipe and circulates the eluent between the treatment tank and the adsorption tower. A soil decontamination device is disclosed.

JP 2013-88362 A Japanese Patent Laid-Open No. 2014-235011

  However, in the method of Patent Document 1 in which the soil on which the radioactive substance such as cesium is adsorbed is acid-washed, the acid contaminated with the radioactive substance remains in the solid residue when the solid-liquid separation is performed, and the concentration of the radioactive substance is reduced. It was necessary to perform acid cleaning many times in order to lower it.

  The method of Patent Document 2 is also a method of repeatedly washing with an acid as shown in Examples 1 to 3, and in Example 4, cesium is eluted with an acid using an adsorption tower. Although continuous operation is shown, the cesium elution rate was still 70%. Further, in Patent Document 2, the relationship between the elution rate of cesium and the pH of the eluent is also examined, and when the pH is 1.5, the elution rate is reduced to 40%. It is desirable.

  An object of the present invention is to provide a method for efficiently decontaminating radioactive cesium with a small amount of acid from a radioactive cesium-containing material such as soil contaminated with radioactive cesium.

  The present invention has been made to solve these problems. A radioactive cesium-containing material such as soil contaminated with radioactive cesium is washed once with an acid, and then filled with a radioactive cesium adsorbent. The present inventors have found that radioactive cesium can be efficiently decontaminated by washing with circulating water using a container.

That is, the present invention
An acid cleaning step in which a radioactive cesium-containing workpiece and an acid aqueous solution are fed into a treatment tank and stirred and mixed at a predetermined temperature for a predetermined time, and the radioactive cesium-containing workpiece and acid obtained in the acid cleaning step A first separation step of separating a mixture containing an aqueous solution into a first supernatant and a first precipitate; and the first supernatant from a tank containing the first supernatant and the first precipitate. After discharging, adding water not containing radioactive cesium, stirring and mixing for a predetermined time, and solid-liquid separation of the mixture containing the first precipitate obtained in the water washing step and the water, A second separation step for separating the second supernatant liquid and the second precipitate; a tank containing the second supernatant liquid and the second precipitate; and a container filled with a radioactive cesium adsorbent. A radiocesium adsorption process that circulates two supernatants for a predetermined time. A decontamination method of radioactive cesium-containing product, characterized in Rukoto,
After the completion of the radioactive cesium adsorption step, the water washing step, the second separation step, and the radioactive cesium adsorption step are repeated once or more for the second supernatant and the second precipitate. Decontamination method of radioactive cesium-containing material,
In the acid washing step, the liquid in the treatment tank is heated to 70 ° C. to 100 ° C., and the decontamination method for a radioactive cesium-containing material is provided.

  According to the present invention, the amount of acid used for decontamination can be greatly reduced, acid-resistant equipment can be reduced in size, and decontamination costs can be greatly reduced.

It is a figure which shows schematic structure of the experimental apparatus used in the Example of this invention. It is a graph which shows the change by the circulating water washing frequency of the cesium density | concentration of the liquid part obtained in the Example. Similarly, it is a graph showing a change in the adsorption rate of cesium adsorbed on zeolite.

  The type of radioactive cesium-containing material to be treated is not limited, and examples thereof include soil, vegetation, incineration ash, sewage sludge, construction waste, and the like contaminated with radioactive cesium. Among these methods, the method of the present invention is effective particularly for soil because the soil generally contains a mineral that adsorbs cesium.

Acid cleaning step This step is a step of adding an acid aqueous solution to a radioactive cesium-containing object to be processed, stirring and mixing the acid to wash the object to be processed, and eluting the radioactive cesium contained in the object to be processed into an acid aqueous solution. It is the process of making it transfer.

  The acid of the acid aqueous solution is capable of eluting cesium from the object to be treated, for example, hydrochloric acid, sulfuric acid, nitric acid, chloric acid such as perchloric acid, inorganic acid such as hydrofluoric acid, oxalic acid, malic acid, maleic acid, Organic acids such as fumaric acid, pyruvic acid, succinic acid, formic acid, citric acid, acetic acid, lactic acid and malonic acid can be used. The pH is 3 or less, preferably about 1-2. The amount of the acid aqueous solution used is sufficient to elute the radioactive cesium sufficiently, can flow the object to be treated, and further, about 70 to 90%, preferably about 80 to 90% of the acid aqueous solution after the acid cleaning is used as a supernatant. It is preferable that it is determined so that it can settle and separate from the workpiece.

  The treatment tank in which the radioactive cesium-containing material and the acid aqueous solution are charged to perform acid cleaning may have a normal cylindrical shape or box shape, and is equipped with a stirrer and a heating mechanism for stirring the inside. is there. The type of the stirrer is not particularly limited, but may be a propeller type or a paddle type.

  It is preferable that the acid cleaning is performed until the radioactive cesium contained in the object to be processed is sufficiently eluted and transferred to the aqueous acid solution to promote the elution. The preferred heating temperature is 70 to 100 ° C., more preferably 80 to 90 ° C., and the washing time may be about 10 to 120 minutes, usually about 10 to 30 minutes.

First Separation Step This step is a step of separating a part of the acid aqueous solution as a first supernatant liquid from the object to be treated after acid washing, and a flocculant is added to promote sedimentation and increase the amount of the supernatant liquid. It is desirable to add. When a combination of an inorganic flocculant and a polymer flocculant is used as the flocculant, the inorganic flocculant promotes aggregation by controlling the flock charge, and the polymer flocculant crosslinks to increase the floc. This is preferable because it causes precipitation and aggregation. Examples of inorganic flocculants include polyaluminum chloride (PAC) and aluminum sulfate. Examples of polymer flocculants include polyacrylamide polymer flocculants. The addition amount is preferably about 100 to 5,000 ppm for the inorganic flocculant and about 1 to 1,000 ppm for the polymer flocculant.

  The flocculant may be added while it is hot. After the addition, the flocculant is gently stirred and allowed to stand to precipitate the material to be treated. The standing time is usually about 5 to 20 minutes.

  As long as there is no special circumstance, the treatment tank can be used as it is as the tank for performing the first separation step.

Water washing step In this step, after discharging the first supernatant liquid from the tank in which the first separation step was performed, water containing no radioactive cesium was added to the remaining first precipitate, followed by stirring and mixing. This is a step of washing one precipitate with water.

  The water that does not contain radioactive cesium is preferably one that does not contain a substance that adsorbs to the adsorbent in preference to cesium in the subsequent radioactive cesium adsorption step, and it is preferable to use soft water, pure water, distilled water, or the like. This water replaces the discharged first supernatant.

  As washing conditions, stirring may be performed at room temperature for about 5 to 60 minutes, but when the supplied water is warm water or hot water, the temperature may be as it is.

  The discharged first supernatant liquid can be reused when processing radioactive cesium-containing materials by sending it to a container filled with radioactive cesium adsorbent, removing cesium, and storing it. it can. When removing cesium, the first supernatant may be circulated.

Second Separation Step This step is because if the suspension of the object to be processed after washing with water is sent to the container filled with the radioactive cesium adsorbent as it is, the object to be processed will flow into this container in a large amount. This is a step for allowing the suspension to settle and sending only the supernatant to the adsorbent container.

  As the flocculant, the inorganic flocculant and the polymer flocculant used in the first separation step can be used.

  The sedimentation time may usually be about 5 to 20 minutes.

Radiocesium adsorption process In this process, the second supernatant liquid formed by sedimentation in the second separation process is circulated through a container filled with the radioactive cesium adsorbent, and the radioactive cesium in the second supernatant liquid is adsorbed. It is the process of making it adsorb | suck and remove.

  As the adsorbent, zeolite, ferrocyanide, silicotitanic acid, cation exchange resin and the like can be used. There are synthetic products and natural products, and there are X-type, Y-type, A-type, mordenite-type, chapasite-type, etc., but any zeolite can be used as long as it can adsorb cesium. Ferrocyanides include complex salts of various metals such as potassium ferrocyanide and sodium ferrocyanide, but can be used regardless of the type. Silicotitanic acid may also be a salt.

  The container filled with the adsorbent may be either a tower or a tank.

  In order to carry out this adsorption process, a circulation line is formed between the tank used in the second separation process and the container filled with the adsorbent. This circulation line is usually provided with a liquid feed pump, and it is also preferable to provide a filter for removing suspended matters such as the object to be processed contained in the second supernatant liquid.

  Circulation is performed until the decrease in the radioactive cesium concentration of the supernatant liquid is substantially stagnant, and this circulation time is usually about 5 to 120 minutes.

  After the circulation is complete, drain the second supernatant and store it.

  On the other hand, the second precipitate, which is the remaining object to be treated, is washed with water that does not contain new radioactive cesium, and is repeatedly washed with water until the radioactive cesium concentration becomes 100 becquerels / kg or less. The number of repetitions depends on the concentration of radioactive cesium contained in the object to be processed, but may be about 2 to 10 times.

  The adsorbent is replaced with a new adsorbent when the adsorption capacity is reduced. The extracted adsorbent adsorbing radioactive cesium is stored.

  A schematic configuration of the experimental apparatus used in the example is shown in FIG.

  In this apparatus, a treatment tank 1 having a stirrer 2 placed in a hot water bath 3, a tube pump 4 for extracting liquid in the treatment tank 1, a differential pressure gauge 5, a filter 6, and a container 7 filled with an adsorbent are connected in series. It consists of the circulation line 8 arrange | positioned in this. Zeolite was used as the adsorbent.

  Vermiculite adsorbing 41.321 mg / g of cesium was used as model soil.

  15 g of vermiculite (cesium adsorption amount 619.815 mg) was put into a treatment tank 1 containing 300 mL of an 8% aqueous oxalic acid solution of pH 1, heated to 80 ° C. in a water bath 3 while stirring, and kept at 80 ° C. for 60 minutes. Stir and wash. As a flocculant, polyaluminum chloride (PAC) as an inorganic flocculant and a diaphragm flocculant as a polymer flocculant are added, and the mixture is allowed to stand for 5 minutes for sedimentation separation. And 200 mL of the first supernatant was discharged.

  Furthermore, 300 mL of distilled water was added to the first precipitate and stirred, and then allowed to stand for 5 minutes, followed by sedimentation separation to separate into a second supernatant and a second precipitate. Next, the tube pump 4 was operated, and the second supernatant was circulated at room temperature for 10 minutes at a rate of 100 mL / min. When the radioactive cesium concentration of the second supernatant liquid did not decrease, the circulation was completed, and the inside of the treatment tank 1 was stirred and allowed to stand for 5 minutes to perform sedimentation separation, and 200 mL of the second supernatant liquid was discharged. This circulation washing operation was repeated four times.

  Table 1 shows the results of measuring the cesium concentration by diluting the supernatant solution 100 times each time, and calculating the elution rate and the amount of residual adsorption in vermiculite.

  FIG. 2 shows the change in the cesium concentration of the liquid portion due to the number of circulation washings, and FIG. 3 shows the change in the adsorption rate of the cesium adsorbed on the zeolite according to the number of circulation washings.

  Since contaminants such as soil containing radioactive cesium can be efficiently decontaminated by the present invention, it can be widely used for decontamination of radioactive cesium contaminants.

1 Treatment Tank 2 Stirrer 3 Hot Water Bath 4 Tube Pump 5 Differential Pressure Gauge 6 Filter 7 Adsorbent Container 8 Circulation Line

Claims (3)

  An acid cleaning step in which a radioactive cesium-containing workpiece and an acid aqueous solution are fed into a treatment tank and stirred and mixed at a predetermined temperature for a predetermined time, and the radioactive cesium-containing workpiece and acid obtained in the acid cleaning step A first separation step of separating a mixture containing an aqueous solution into a first supernatant and a first precipitate; and the first supernatant from a tank containing the first supernatant and the first precipitate. After discharging, adding water not containing radioactive cesium, stirring and mixing for a predetermined time, and solid-liquid separation of the mixture containing the first precipitate obtained in the water washing step and the water, A second separation step for separating the second supernatant liquid and the second precipitate; a tank containing the second supernatant liquid and the second precipitate; and a container filled with a radioactive cesium adsorbent. A radiocesium adsorption process that circulates two supernatants for a predetermined time. Decontamination method of radioactive cesium-containing product, characterized in Rukoto.   After completion of the radioactive cesium adsorption step, the water washing step, the second separation step, and the radioactive cesium adsorption step are repeated once or more for the second supernatant and the second precipitate. Item 2. A method for decontaminating a radioactive cesium-containing material according to Item 1.   The method for decontaminating radioactive cesium-containing materials according to claim 1 or 2, wherein in the acid cleaning step, the liquid in the treatment tank is heated to 70 ° C to 100 ° C.

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