JP2015225066A - Method for removing radioactive contaminant in radioactive contaminated water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently, inexpensively removing a radioactive contaminant, especially radioactive Sr in radioactive contaminated water.SOLUTION: The method for removing a radioactive contaminant from radioactive contaminated water comprises the steps of: adding silicate and/or borate to radioactive contaminated water including at least radioactive Sr to mix them; separating and collecting a radioactive Sr silicate and/or borate solid from the radioactive contaminated water after mixing by a centrifuge; and separating and collecting a fine radioactive Sr silicate and/or borate not separated and collected by the centrifuge, through a Max filter.

Description

  The present invention relates to a method for removing radioactive contaminants in radioactively contaminated water.

Examples of radioactively contaminated water include water containing radioactive particles generated from nuclear fuel rods, groundwater used to cool the heat generated from nuclear fuel, and contaminated seawater generated when contaminated cooling water flows in. . These radioactive polluted waters contain many suspended solids (SS) along with radioactive pollutants. As shown in Table 1 to be described later, many metal element oxides, silicates, and borates are insoluble or hardly soluble in water and become precipitates or SS. Radioactive suspended substances of these metal elements can usually be removed by membrane filtration, but the membrane is likely to deteriorate because the particles are fine and have radioactivity.
Radioactive Sr is ionized and dissolved in water, and is removed using an ion exchange resin or reverse osmosis membrane (RO membrane), converted to carbonate and filtered as an insoluble matter, or a coagulating precipitation agent Use a large amount to recover. However, in membrane filtration, the membrane quickly deteriorates due to SS coexisting in large quantities. Membrane exchange is a heavy burden in terms of time and money, and a lot of secondary waste is generated. There is a method using activated carbon, but the activated carbon itself must be replaced because the adsorption capacity is significantly reduced when SS or the like is deposited in the pores. Also, fine activated carbon is generated by collision between activated carbons caused by water flow, and another filtration device is required to remove the activated carbon.
As described above, in the prior art, in order to remove radioactive pollutants, it is necessary to use a multi-stage removal facility such as activated carbon, membrane filtration, or ion exchange resin.
Moreover, even if recent publications (such as Patent Documents 1 to 5) related to related technologies are viewed, a technology similar to the present invention is not disclosed.
Furthermore, sampling and analysis methods have been established for radioactive elements attached to solids, but analysis methods for radioactive elements present in a large amount of water have not yet been established.
The filtration device “Max filter” used in the present invention is a device according to a patent (see Patent Documents 6 to 7) of Monobe Engineering Co., Ltd. and has already been put into practical use.

JP 2014-029269 A JP 2014-016179 A JP, 2014-217686, A JP 2013-094694 A JP 2013-0776628 A Japanese Patent No. 3394490 Japanese Patent No. 4774223

  An object of the present invention is to provide a method for efficiently and inexpensively removing radioactive pollutants, particularly radioactive Sr, in radioactive polluted water.

The above problems are solved by the following <1> to <2> inventions.
<1> A method for removing radioactive Sr from radioactively contaminated water containing radioactive pollutants, comprising at least the following steps (1) to (3).
(1) Step of adding and mixing silicate and / or borate to radioactively contaminated water containing at least radioactive Sr (2) From the radioactively contaminated water after mixing, radioactive Sr silica Step of separating and collecting solid salt of acid salt and / or borate (3) Separating and collecting fine radioactive Sr silicate and / or borate that could not be separated and collected by a centrifuge with a Max filter Step <2> The radioactively contaminated water containing the radioactive pollutant according to <1>, wherein the silicate and / or borate mixed in the step (1) is a borosilicate composite salt Of removing radioactive Sr from the surface.

  According to the present invention, it is possible to provide a method for efficiently and inexpensively removing radioactive contaminants in radioactively contaminated water, particularly radioactive Sr. Further, the chemicals used are only inorganic metal salts used for the production of solids, and since a flocculant generally used for removal of SS and the like is not used, secondary waste can be reduced.

Schematic which shows an example of the system for enforcing the removal method of this invention. The photograph which shows the melt | dissolution volume reduction of the insoluble deposit collect | recovered from the seawater which carried out ND process. (A) Photograph of an insoluble precipitate obtained by treating seawater of Kunohama with ND. (B) Photograph of insoluble precipitate melted and vitrified by heating. A picture of ND being added to the sea of Kunohama. The photograph which shows a mode that a cloudy precipitate settles gradually after ND addition. Photographs before and after the ND addition of Kunohama seawater on a centrifuge. (A) shows a state before being applied to a centrifuge in which a cloudy precipitate is generated by addition of ND, and (B) shows a state separated from the precipitate after being centrifuged and the supernatant. The photograph of the state which attached diatomaceous earth to the Max filter using the water suspension of the diatomaceous earth powder of a filter medium. The photograph which shows a mode that the seawater of Kunohama after ND addition of FIG. 4 is filtered with the Max filter shown in FIG. A photograph of the simple Max filter device used in Reference Example 1. Photo of a small practical machine of Max filter system used in the demonstration test. A photograph of raw water, treated water, and a solid part subjected to dehydration treatment in Reference Example 2.

Hereinafter, the present invention will be described in detail.
The radioactive Sr is diffused as ions in the radioactively contaminated water and has a low concentration, so it is difficult to remove it as it is. Then, as a result of considering changing to an insoluble or hardly soluble solid substance and removing it, radioactive sr contained in radioactively contaminated water containing radioactive Sr and mixed with silicate and / or borate was insoluble or It turned out that it became silicic acid Sr and / or boric acid Sr (refer to the following Table 1) which are hardly soluble solids, and could be solidified and separated. As the silicate and borate, alkali metal salts are usually used.
In particular, borosilicate complex salt (ND) is preferable because it forms an insoluble precipitate even at a low concentration at room temperature. “ND” means a composite salt prepared by mixing silicate and borate and melting by heating, and is different from a mere mixture of silicate and borate. The mixing ratio of silicate and borate can be variously changed. As an example, the case of the Na salt used in the examples shows SiO ₂ : Na ₂ O: B ₂ O ₃ = 24.3 to 26. .8: 11.5 to 13.1: 0.7 (mass ratio). The reason why the numerical range is wide is that the crystal structure of the raw material has various types such as ortho and meta, and cannot be specified as one.

In addition, when ND is mixed with seawater, silicates of Mg and Ca are formed at the same time, but these form insoluble coprecipitates with silicic acid Sr and boric acid Sr, and precipitate silicic acid Sr and boric acid Sr. It functions as an auxiliary agent when removed separately.
However, in the case of radioactively contaminated water with little contamination of seawater, when adding ions of group 2 elements of the periodic table to the contaminated water and adding ND or the like, silicic acid Sr and / or boric acid Sr It is preferable to form an insoluble coprecipitate with the silicate and / or borate of the group 2 element of the periodic table and to increase the amount of solids so that radioactive Sr can be easily separated by precipitation.
Carbonic acid Sr has low solubility and is hardly soluble (see Table 1 below), but it is not easy to produce carbonic acid Sr in seawater due to the presence of other salts.

  Although radioactive Cs is a typical radioactive pollutant along with radioactive Sr, it is easy to adhere to the fine particles and is not in the state of Cs ions in the radioactively contaminated water. Therefore, the fine particles to which the radioactive Cs is attached may be removed. In fact, it floats in the radioactive polluted water together with other fine particles, or is mixed with mud and settles on the seabed or the like, and can be removed by using an appropriate filtration device. However, when processing a large amount of radioactively contaminated water, it is necessary to consider that the cost for replacing the filter can be reduced. In this regard, if a Max filter described later is used, radioactive Cs can be separated and removed efficiently and inexpensively.

<Solubility of metal oxides, metal silicates, and metal borates in water>

An example of a system for carrying out the removal method of the present invention will be described with reference to the schematic diagram shown in FIG.
This example includes a Max filter 1 for separating and collecting radioactive Cs, an ND aqueous solution tank, an in-line mixing for ND mixing, a centrifuge, and a Max filter 2 for separating and collecting radioactive Sr. Actually, in addition to these devices, a pump that pumps seawater and sends it to the Max filter 1, a pump that discharges the purified seawater from the Max filter 2, and samples the purified seawater to measure the concentration of radioactive pollutants Although there are various well-known devices that are naturally necessary in this type of system, such as a device to be used, they are omitted here.
And by connecting the above apparatus with a pipe and designing it so that it can be operated almost fully automatically, radioactive pollutants can be continuously separated and removed.
Although FIG. 1 shows the case where both radioactive Cs and radioactive Sr are removed, when the radioactive Cs is not a removal target, it is not necessary to provide the Max filter 1.
Moreover, although the example of seawater was shown in FIG. 1, it is the same also with other radioactively contaminated water.

As the operation, the contaminated seawater was sent to the Max filter 1 to remove radioactive Cs, and then the radioactive Sr solid produced by mixing with ND in-line was solid-liquid separated with a centrifuge and could not be separated. After removing the fine solids with the Max filter 2, it may be returned to the sea. The solid material of radioactive Sr has a relatively large specific gravity of 2.5 or more, and the specific gravity of seawater is 1.03. Therefore, it can usually be removed by a centrifuge, but solid-liquid separation is not possible when the operation is performed in a short time. Since it may be sufficient, the removal operation by the Max filter 2 is necessary. Also, the radioactive Sr solid is designed to be automatically discharged from the centrifuge.
Although ND is used in FIG. 1, silicate or borate may be used instead of ND. In either case, the chemicals to be used are only inorganic metal salts, and since a flocculant generally used for removing SS and the like is not used, the generation of secondary waste can be reduced.

The Max filter is a filtration device according to Patent Document 6 and is registered with the Ministry of Land, Infrastructure, Transport and Tourism (Registration No. KT-06004, OS-12005-A), and is a “Chiba Monozukuri Certified Product” in Chiba Prefecture. (Certification number 7). It is also used for muddy water treatment at construction sites, factory wastewater treatment, groundwater purification, etc., and the filtrate is discharged into rivers.
The Max filter is also called a spring filter, which is a filter in which a specially shaped coil spring and fine diatomaceous earth are fixed as a filtering material. By using this system, the target liquid is continuously sucked and filtered. SS of 0.1 μm or more can be filtered out. 0.1 μm or more is a size that can remove bacteria and microorganisms by filtration, and the turbidity of the filtrate is 0 to 3 NTU. In addition, a centrifuge for collecting the solid matter filtered by backwashing the filter is incorporated, and continuous filtration and recovery operations are possible.

The Max filter is made of stainless steel or resin and can be selected according to the properties of the liquid to be filtered. The size of the Max filter can be appropriately changed. For example, a cylindrical spring filter having a length of 17.5 cm and a diameter of 1.5 cm is used by attaching 5.0 g of diatomaceous earth or the like. The size of the substance to be filtered can be slightly changed depending on the particle size of diatomaceous earth.
The flow rate of the liquid that can be filtered varies depending on the properties of the liquid, but in the case of a liquid such as seawater, a flow rate of 5.5 liters / minute can be obtained with one small filter. And the filter capacity can also be increased by changing the length and number of Max filters, and in fact, a filter of 1 to 150 tons per hour is often used.
Furthermore, if acidic liquid, seawater, liquid with a high salt concentration, etc. are filtered, rust and corrosion deterioration may occur even in stainless steel products. Therefore, plastic may be used as the material of the spring filter.

If the filtration is continued, the filtration capacity is reduced due to clogging, which can be detected by changes in flow rate and pressure. If it drops significantly, the used filter medium is peeled off by reverse cleaning and a new filter medium is automatically attached. Since the new filter medium is only thinly coated on the surface of the spring filter (uses 5.0 g of filter medium with a surface area of 82 cm ² ), secondary waste is significantly reduced and is inexpensive.
The filtration and the collection of the filter medium are automatically performed by the pressure difference during filtration. If a large amount of sludge is contained in the radioactively contaminated water, the filter medium may be clogged at an early stage. It is preferable to make it longer. The filter medium peeled and removed by back washing is continuously centrifuged and dewatered and collected. Centrifuges and pumps are maintained once a year. The main objects of maintenance are motors and lubricants.

In the in-line mixing, the flow rate of the added ND aqueous solution is adjusted by a pump corresponding to the flow rate of the filtrate. In this process, maintenance of the pump and flow sensor is performed once a year. Precipitates generated by in-line mixing are applied to a centrifuge for solid-liquid separation, but the centrifuge is operated continuously, so maintenance is performed once a year.
Fine precipitates that cannot be collected by the centrifuge are collected by filtration using a Max filter.

Equipment generated as secondary waste is treated as follows.
The pre-filter material to be installed at the intake of radioactive polluted water is made of combustible material and is burned to reduce the volume after replacement. The filter medium used for the Max filter is recovered by dehydration and dehydrated, freeze-dried, and stored in a radiation shielding container in the absence of moisture. The amount generated is about the same as the amount of filter media used.
In the case of seawater, the main compounds recovered in quantity as silicates and borates are Mg silicate and Ca silicate, and the amount does not increase from the amount contained in seawater. Therefore, the amount is about 7 g per liter of seawater. These salts are separated by a centrifuge, dehydrated, and continuously stored in a collection container. The recovered salts can be melted and vitrified as a mixture, and can be reduced in volume as a solid that does not scatter even if radioactive Sr is contained, and can be stored in a radiation shielding storage container. This volume reduction can be performed at 900 ° C. for 30 minutes, for example. The change is shown in FIG. (A) is a photograph of an insoluble precipitate obtained by treating and recovering Kunohama's seawater with ND, and (B) is a photograph of an insoluble precipitate that has been melted and vitrified by heating.

As shown in Examples 1 to 4 described later, radioactive Sr reacts with ND to become insoluble or hardly soluble silicic acid Sr and / or boric acid Sr, and can be easily removed by a centrifuge and a Max filter. . The target value is set to a legal concentration limit value of 30 Bq / L or less.
As shown in Reference Examples 1 and 2 described later, even contaminated water containing a high concentration of radioactive Cs can be separated and removed by the Max filter to below the detection limit. The target values are set to legal density limit values where Cs134 is 60 Bq / L or less and Cs137 is 90 Bq / L or less.

EXAMPLES Hereinafter, although an Example and a reference example are shown and this invention is demonstrated further more concretely, this invention is not limited at all by these Examples.
The ND used in the examples was prepared by mixing water glass (Na silicate aqueous solution) and borax (Na borate) and dissolving by heating at 60 ° C. to 80 ° C. (Si and Na And a blend ratio of B to SiO ₂ , Na ₂ O, and B ₂ O ₃ (mass ratio) in liquid form.
Composition ratio: SiO ₂ : Na ₂ O: B ₂ O ₃ = about 25.5: about 12.3: about 0.7

Example 1
The state when ND is mixed with seawater of Kunohama, Iwaki City, Fukushima Prefecture and filtered through a Max filter is shown in FIGS.
FIG. 3 is a photograph of a state where 0.016% by volume of ND is added to the seawater of Kunohama, and a cloudy precipitate is generated simultaneously with the addition.
FIG. 4 is a photograph showing how the cloudy precipitate gradually settles after addition of ND.
FIG. 5 is a photograph showing a state before and after the ND-added Kunohama seawater is applied to a centrifuge (manufactured by ASONE: ASONE C-12B). (A) shows a state before being applied to a centrifuge in which a cloudy precipitate is generated by addition of ND, and (B) shows a state separated from the precipitate after being centrifuged and the supernatant.
FIG. 6 is a photograph of a state in which diatomaceous earth is mounted on a Max filter using an aqueous suspension of diatomaceous earth powder as a filter medium. The filtrate was not cloudy and the turbidity was 0.1 NTU.
FIG. 7 is a photograph of the state in which the supernatant of FIG. 5 (B) is being filtered by the Max filter of FIG. 6, and the filtered seawater comes out at the top of the apparatus, but it is transparent without white turbidity. . The turbidity of the filtrate was 1.5 NTU. The turbidity of the liquid before filtration was 1730 NTU.

The Sr concentration of the seawater of Kunohama and seawater to which ND was added was measured by ICP-OES method using an inductively coupled plasma optical emission spectrometer (manufactured by Hitachi High-Tech Science Co., Ltd .: SPS5100). The measurement results and Sr removal rate are shown in Table 2.

Example 2
The Sr concentration of the filtrate was measured after the same operation as in Example 1 except that the amount of ND added was changed to 0.16% by volume. The measurement results and Sr removal rate are shown in Table 2.

Examples 3-4
Sr concentration was measured after performing the same operation except that the seawater of Kunohama in Examples 1 and 2 was changed to artificial seawater having the following composition. The measurement results and Sr removal rate are shown in Table 2.
<Composition of artificial seawater>
The artificial seawater used is obtained by dissolving the following components in 50 L of water.
・ NaCl: 1425 g
MgSO ₄ 7H ₂ O: 341 g
MgCl ₂ 6H ₂ O: 258 g
・ CaCl ₂ 2H ₂ O: 73.5 g
・ KCl: 36.25 g
SrCl ₂ 6H ₂ O: 1.2 g
・ NaBr: 4.2 g
・ H ₃ BO ₃ : 1.37 g
-NaF: 143.5 mg
・ KI: 3.95mg

If the radioactive Sr is present in seawater at a high concentration of 1000 Bq / L, the legal concentration limit value (30 Bq / L or less) will be satisfied if the amount is 1/33 or less.
On the other hand, in the Sr removal test shown in Table 2 above, the Sr removal rate is 98% or more for artificial seawater, 98.5% or more for seawater of Kunohama, and the Sr concentration is 1/50 or less for artificial seawater. It is 1/67 or less in the seawater of Kunohama. Therefore, even if the radioactive Sr of raw water is 1000 Bq / L, it can be reduced to 20 Bq / L or less with artificial seawater and 15 Bq / L or less with seawater at Kunohama, and the legal concentration limit can be satisfied.
In addition, when 0.16% by volume of the ND solution was used, Sr was less than the detection limit of 0.1 ppm. This is an insoluble precipitate generated from artificial seawater or Mg or Ca present in a large amount in seawater. This is presumably because it became an auxiliary agent for separating and removing the insoluble precipitate of Sr.

Example 5
Although the case where ND was mixed with seawater was shown in the said Examples 1-2, it replaces with ND here and the test example at the time of mixing silicate Na, water glass, and borax is shown.
The materials used for the test are as follows. In addition, the density | concentration of Sr aqueous solution was 0.09 mM according to the density | concentration contained in general seawater. Moreover, the seawater of the same Kunohama as Example 1 was used for seawater, and the same thing as Example 3 was used for artificial seawater.

Sr aqueous solution (0.09 mM as Sr ²⁺ )
・ ND aqueous solution (430 mM as SiO ₂ )
・ Water glass aqueous solution (553 mM as SiO ₂ )
-Na silicate aqueous solution (254 mM as SiO ₂ )
・ Boston aqueous solution (105 mM as boric acid)

40 mL of each of the above Sr aqueous solution, artificial seawater, and seawater were prepared, 0.4 mL of ND aqueous solution, water glass aqueous solution, Na silicate aqueous solution and borax aqueous solution were added and mixed and stirred, and then the turbidity was measured. In the case other than the borax aqueous solution, the amount of precipitates generated was large, so the measurement was performed after diluting 100 times with deionized water.
The results are shown in Table 3. The unit of numerical values in the table is “NTU”.
As shown in Table 3, precipitates were formed for all Sr aqueous solutions, but it was found that the ND aqueous solution had the most precipitates and was preferred.

Example 6
Turbidity was measured in the same manner as in Example 5 except that the concentration of the Sr aqueous solution was changed to 0.9 mM that is 10 times that of Reference Example 3 and 9 mM that was 100 times that of Reference Example 3. In the case of other than the borax aqueous solution, the amount of precipitate generated is large, so in the case of 0.9 mM, it was diluted 10 times with deionized water, and in the case of 9 mM, it was diluted 100 times with deionized water. .
The results are shown in Table 4. The unit of numerical values in the table is “NTU”.

Reference example 1
<Radioactive Cs removal test using Max filter>
With the cooperation of the Fukushima Prefecture Land Improvement Business Association and the Minami Tohoku Reconstruction Research Institute, we conducted a removal test of radioactive pollutants in agricultural reservoirs using the simple Max filter device shown in FIG.
From the water in the reservoir, Cs134: 3.5 to 5.7 Bq / L, Cs137: 4.4 to 7.9 Bq / L, and from the mud at the bottom of the lake (dry mud) kg of radioactivity was detected.
The sample made of the mixture of water and mud was applied to a Max filter, and then the radioactivity of the filtrate was measured. As a result, it was revealed that radioactive Cs was not detected from the filtrate, and radioactive Cs could be separated and removed quickly.

Reference example 2
Based on the results of Reference Example 1, a small practical machine of the Max filter system shown in FIG. 9 was produced, and a river water and riverbed sludge filtration demonstration test was conducted in the vicinity of the Estado River estuary in Namie Town, Futaba County, Fukushima Prefecture. The outline of this demonstration test is as follows.
・ Sludge and river water were collected by using a submersible pump at the bank of Kashido River.
-The collected raw water was stirred to make sludge float and supplied to the Max filter.
-For the Max filter, diatomaceous earth as a filtering material was previously attached to the filter, and filtered at a flow rate of 80 L / min. The filter was programmed to operate automatically, such as by sensing the pressure of the filter part and the flow rate of the treated raw water with a sensor, and if it showed a certain value, the filter part was backwashed or a new filter medium was installed.
・ The filtered river water was stored in the treated water tank.
-Backwashed sludge, SS and filter media were solid-liquid separated with a centrifuge and dehydrated.
-The liquid part after solid-liquid separation was returned to the raw water tank and filtered again.
-The dehydrated solid part was discharged into a container installed at the bottom of the centrifuge.

A photograph of the raw water, treated water, and dehydrated solid part is shown in FIG. In FIG. 10, the upper left is the raw water, the upper right is the treated water, and the lower is the dehydrated solid part.
In addition, Table 5 shows the measurement results of these radioactivity.
The filtration capacity of the Max filter can be easily expanded by increasing the number of spring filters and increasing the filtrate water flow rate.
Therefore, considering the properties of the treated water, a practical machine with a throughput of about 500 tons per hour is produced if the viscosity of the liquid is not so high and the treatment of contaminated water does not contain a large amount of sludge. It is possible that a large amount of contaminated water can be treated in a short period of time.

Claims (2)

A method for removing radioactive Sr from radioactively contaminated water containing radioactive pollutants, characterized by comprising at least the following steps (1) to (3).
(1) Step of adding and mixing silicate and / or borate to radioactively contaminated water containing at least radioactive Sr (2) From the radioactively contaminated water after mixing, radioactive Sr silica Step of separating and collecting solid salt of acid salt and / or borate (3) Separating and collecting fine radioactive Sr silicate and / or borate that could not be separated and collected by a centrifuge with a Max filter Process   The silicate and / or borate mixed in the step (1) is a borosilicate complex salt, wherein radioactive Sr is contained in radioactive contaminated water containing radioactive contaminants according to claim 1. How to remove.