JP2001091694A - Fixing method for radioactive element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing method for a radioactive element stably fixing the radioactive element such as radioiodine. SOLUTION: In this fixing method, a radioactive element (e.g. iodine) is adsorbed inside a window part of zeolite [WmZnO2n: W=Na, Ca, K, Ba or Sr; Z=Si+Al (Si:Al; >1)], and the zeolite window part is blocked up by silica- coating the window part with a silicon alkoxide or the like to form a network of silica and reducing a window diameter. Thus, outward elution of the enclosed iodine is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for immobilizing a radioactive element for stably immobilizing a radioactive element such as radioactive iodine for a long period of time.

[0002]

BACKGROUND ART Disposal of radioactive iodine having a long half-life associated with reprocessing of nuclear fuel cannot be used to delay the transfer of nuclides into the biosphere by an engineered barrier agent or the like. It is the nuclide that has the largest contribution to the radiation dose. Various conventional iodine fixing methods have been proposed, such as a fixing method using silver iodide glass, a fixing method using cement, and a hydrothermal synthesis fixing method after adsorption onto an aluminosilicate.

[0003]

However, in the above-mentioned method for immobilizing and disposing of iodine, vitrification is a point of low leaching rate of glass, cement is a point of long-term soundness, and water is adsorbed to aluminosilicate. Methods such as thermosynthesis immobilization have been further studied in terms of low leaching rate of silica,
Each method was not satisfactory in terms of long-term stability and low dissolution in consideration of changes in the environment.

[0004]

Means for Solving the Problems The invention [Claim 1] of the present invention for solving the above problems, radioactive elements zeolite _{[W m Z n O 2n:} W = Na, Ca, K, Ba, Sr; Z
= Si + Al (Si: A;> 1)] in the window,
Closing the zeolite window with a silica network,
It is characterized in that the contained radioactive element is prevented from being eluted outside.

[0005] The invention of claim 2 is characterized in that in claim 1, the periphery of the radioactive element-fixed zeolite after the formation of the silica network is coated with a coating agent.

[0006] The invention of claim 3 is characterized in that in claim 2, the coating is fixed with a fixing agent.

The invention of claim 4 is characterized in that, in claim 2 or 3, the coating agent or the fixing agent is apatite or silica.

According to a fifth aspect of the present invention, in the first aspect, the formation of the silica network is performed by repeatedly subjecting a silicon alkoxide, a silane gas or the like to a temperature of 650 ° C. or less by a CVD method, and then hydrolyzing with water vapor to form a window. It is characterized by closing the part.

[0009] The invention of claim 6 is characterized in that, in claim 3 or 4, after being pressure-formed together with the binder, firing at a high temperature.

The invention of claim 7 is characterized in that, in any one of claims 1 to 6, the radioactive element is iodine and the zeolite is an A-type zeolite.

The invention according to claim 8 is characterized in that, in claim 7, the A-type zeolite is a CaA-type zeolite ion-exchanged with Ca ions.

The invention according to claim 9 is the invention according to claim 8, wherein a part of the Ca ions of the CaA-type zeolite is Ag.
It is a Ca-Ag-A type zeolite ion-exchanged with ions.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the method for immobilizing radioactive elements according to the present invention will be described below, but the present invention is not limited to these embodiments.

The present invention when processing a radioactive element (e.g., iodine), the radioactive elements zeolite [W _m Z _n O
_2n : W = Na, Ca, K, Ba, Sr; Z = Si + Al
(Si: A;> 1)], and the zeolite window is silica-coated with, for example, silicon alkoxide to form a silica network, thereby reducing the window diameter and closing the window. It suppresses elution to the outside. Further, the silica-coated radioactive element-containing zeolite is covered with a coating material such as apatite, which has a low solubility in an aqueous solution and is a stable mineral for a long time, so as to be fixed for a long time.

Here, there are several hundred types of zeolites, but A-type zeolites, faujasite-type zeolites, and the like are mass-produced and inexpensive. For example, when treating iodine as a radioactive element, A-type zeolite ion-exchanged with Ca ions is particularly preferable. This is because iodine has a bimolecular structure, the shorter one is 4.3 mm from the van der Waals radius, the longer one is 6.96 mm, and iodine is not adsorbed unless the zeolite window diameter is 4.3 mm or more. This is because a CaA zeolite obtained by ion exchange with Ca ions controlled at 5 ° is preferable. The window diameter of the zeolite can be controlled by ion exchange. Usually, the window diameter of NaA-type zeolite in which Na ions are present in the window is 4 mm, but Na ions are dissolved in the solution.
When ion exchanged with a ion, CaA zeolite having a window diameter of 5 ° is obtained. Therefore, since the iodine molecular diameter is 4.3Å and does not adsorb on the NaA-type zeolite in which Na ions are arranged, the present invention uses CaA-type zeolite.

Here, the window diameter may be controlled by using divalent ions such as Mg ions, Sr ions and Br ions instead of Ca ions.

When a radioactive element having a large diameter other than iodine is included as a radioactive element, a faujasite-type zeolite having a window diameter of 9 ° can be used. Further, when a radioactive element having a small diameter other than iodine is included, an A-type zeolite having a small window diameter can be used.

Since iodine reacts with silver to form stable silver iodide, Ca-Ag-A-type zeolite is more preferably obtained by exchanging a part of Ca ions of CaA-type zeolite with Ag ions.

The Ag of the Ca-Ag-A type zeolite
The ion exchange is preferably performed at about several percent to about 60%. This is because if the ion exchange rate of Ag is high, the silver blocks the window, so that the adsorption amount of iodine decreases.

In order to remove the water of crystallization contained after the ion exchange, it is necessary to bake at 350 to 650 degrees. This is because if the temperature is less than 350 degrees, crystallization water is not sufficiently removed, while if the temperature exceeds 650 degrees, zeolite is destroyed, which is not preferable. The atmosphere gas for the heat treatment is not particularly limited, and may be either air or nitrogen.

A-type zeolite having adsorbed iodine has a strong binding force with water, and when it is put into an aqueous solution, iodine and water are exchanged and the adsorbed iodine is released. Therefore, the window cannot be closed by ion exchange in the liquid after iodine adsorption. .

Therefore, in order to prevent the iodine contained in the window of the zeolite A from being released again from the window, the CV
Method D (Chemical Vapor Depositi
on; chemical vapor deposition), and the window was closed with silica in the gas phase. Blocking condition by CVD melting is 650 ° C
It is as follows.

The blocking by silica is better at higher temperatures and the window is uniformly blocked, but when the temperature rises suddenly, the adsorbed iodine is released.
Therefore, it is desirable to gradually increase the temperature to 100 degrees, 150 degrees, 200 degrees, and 350 degrees.

Here, as a silica source for forming a silica network, TMOS (tetramethoxysilane),
Silicon alkoxide such as TEOS (tetraethoxysilane), silane gas, or the like can be used. Thereafter, it is hydrolyzed by contact with water vapor to form a silica network around the zeolite and reduce the window diameter.

A catalyst (eg, ammonia, acetic acid, hydrochloric acid, etc.) can be used as necessary for the steam to promote the hydrolysis reaction. This step is preferably performed several times due to the blockage of the window by the silica network.

After closing the window, apatite (Ca ₅
A coating agent such as (PO ₄ ) ₃ Z (Z = F, Cl, Br, OH, I) or silica is used to coat the periphery of the iodine-containing A-type zeolite. Here, apatite is a component of bone, and it has been proved from fossils of dinosaurs that it keeps its shape stably for several million years, and among the apatite minerals, fluorapatite (Ca) has the lowest solubility in water. ₅ (P
O ₄ ) 3F) is preferably used.

In order to perform the immobilization, zeolite in which iodine is included, the window is closed with silica, and apatite are mixed, and further, silica sol, alumina sol, water glass, or an inorganic binder such as kaolin is mixed. After molding, the surroundings may be further solidified with apatite and an inorganic binder and fired at 350 degrees. The conditions for the pressure molding are as follows: 100 to 600 ° C. and 100 to 60 ° C.
It is preferred to be about 0 kg / cm ² .

Further, by using the above-mentioned coating agent such as apatite or silica to coat and fix around the zeolite solidified with apatite, it has become possible to stably fix iodine for a long period of time.

As described above, in the present invention, iodine is contained in the window of the zeolite adsorbent and the window is confined by the silica network to immobilize the iodine. In addition, after covering the window with this silica network, it is coated with a coating agent such as apatite, and then the coating is fixed with a fixing agent such as apatite, so that long-term stable iodine fixation is possible. become.

Next, the outline of the method for immobilizing iodine will be described with reference to FIG.

FIG. 1 is a process chart of an iodine fixing method.
FIG. 1A shows a conceptual diagram of a zeolite skeleton. As shown in FIG. 1A, the skeleton of the A-type zeolite 11 is S
Aluminum is arranged around Si-O-Al-O-Si- and silicon with i, Al, and O as components, and silicon atoms are arranged around oxygen atoms with oxygen around the aluminum atoms.
Forming a network. A silicon atom or an aluminum atom is at a vertex of the hexagon in FIG. 1A, and an oxygen atom is arranged on a line connecting the vertex. Silicon atoms and aluminum atoms are four-coordinated, and cations such as Na and Ca ions are arranged for charge compensation of the aluminum atoms. FIG.
In (a), no cation is shown.

FIG. 1B shows the concept of zeolite on which iodine 12 is adsorbed. Iodine contains several molecules in the window 13.

FIG. 1C is a view in which the zeolite window 13 is closed by the silica network 14 and the iodine 12 is confined. The formation of the silica network 14 forms an iodine holding material 15 in which the iodine 12 contained in the window 13 of the zeolite 11 is blocked. Since the iodine molecules are large, they cannot be emitted due to a slight reduction of the window portion 13.

FIG. 2 is a conceptual diagram in which iodine is contained and the periphery of the iodine holding material 15 whose window diameter is reduced by the silica network 13 is coated with a coating material. Here, apatite is used as the coating agent 16. Since the iodine does not penetrate into the apatite due to the coating of the apatite, even if iodine comes out of the zeolite, the apatite does not diffuse.
This prevents the zeolite holding iodine from coming into contact with groundwater.

Further, the coating material whose periphery is coated with apatite is further solidified by using apatite as the fixing agent 17. By doing so, the zeolite surrounding the zeolite is combined with the coating covering the apatite, and the zeolite holding iodine is further prevented from coming into contact with groundwater.

Instead of covering the periphery of the iodine holding material 15 with apatite, the iodine holding material 15 and the apatite are mixed and fired, and then the periphery thereof is covered with apatite and fixed. Good.

[0037]

EXAMPLES In order to demonstrate the effectiveness of the present invention, iodine was adsorbed on zeolite, the amount of adsorbed iodine was measured, the iodine-containing zeolite was coated with silica, the confinement performance was evaluated, the zeolite was immobilized with apatite, and iodine was eluted. The amount was measured.

EXAMPLE 1 FIG. 3 shows various zeolites.
Shows an apparatus for measuring the iodine adsorption amount. As shown in FIG. 3, the apparatus for measuring the amount of adsorbed iodine includes an iodine generating cylinder 21 containing iodine therein, an adsorption cylinder 22 for adsorbing generated iodine 12, and an iodine exhausted from the adsorption cylinder. It consists of a methanol tank 23, and a He cylinder 2
5 was introduced, passed through iodine of which temperature was adjusted in a water bath 25, and was introduced into zeolite in the adsorption column 22. The inside of the adsorption cylinder 22 is covered with silica wools 27, 27 before and after the zeolite 26. The flow rate of the He gas is controlled by a flow meter 28. Since iodine has a high vapor pressure, it sufficiently sublimates even at room temperature, and iodine having a near saturated vapor pressure was introduced into the zeolite. The iodine that could not be adsorbed by the zeolite was recovered in the methanol tank 23 at the outlet.
The iodine adsorption amount was determined by a gravimetric method.

The zeolites used were faujasite zeolite, NaX zeolite having a silica-alumina ratio of 2.5, AgX zeolite obtained by exchanging silver ion thereof, Ca-Ag-A type zeolite obtained by ion exchange of CaA zeolite, and ALPO (aluminophosphoric acid). Salt) adsorbent, SAPO
(Silica aluminophosphate) and apatite. AL
The PO adsorbent and the SAPO adsorbent are zeolitic adsorbents having different crystal structures and having a composition of Al, P, and Si.

The ion exchange of CaA-type zeolite with silver is carried out by putting zeolite in purified water to form a slurry,
Silver nitrate is introduced and stirred for several hours. Next, it is filtered, washed, dried and calcined at a predetermined temperature to remove water of crystallization in the zeolite. The calcination was also performed on the non-ion-exchanged adsorbent. The adsorbent was formed into a mesh (14 to 60 mesh) so as to be uniform before firing.

There are two types of adsorption: chemical adsorption with strong binding force and physical adsorption with weak binding force. An agent having a large amount of chemisorption is desirable for immobilization. The chemical adsorption amount is an adsorption amount obtained by subtracting the physical adsorption amount from the iodine adsorption amount of a fresh product, and is stable iodine. The physical adsorption amount was obtained by purging zeolite sufficiently adsorbing iodine with helium gas and desorbing the iodine adsorption amount. The adsorption treatment is performed for 24 hours × 100 ml / min, and then performed with helium gas for 24 hours × 100 ml / m
Purged in. Table 1 shows the results of the iodine adsorption amount test. The amount of adsorption was determined by the following equation. Adsorption amount = [Iodine adsorption amount / Adsorbent amount (not including iodine)]

[0042]

[Table 1]

From Table 1, the following results were obtained. Apatite hardly adsorbed iodine. Therefore, it was confirmed that when the zeolite was immobilized, there was no iodine diffusion by the apatite. The ALPO adsorbent and the SAPO adsorbent also had a small amount of iodine adsorbed, and particularly low chemical adsorbed amount and lacked iodine stability. The AgX adsorbent obtained by silver ion exchange of the NaX type adsorbent reduced the ratio of the physical adsorption amount from 10% of the chemisorption amount to the order of 3% due to the ion exchange of Ag, and increased the stability of iodine. Since ion exchange of Ag by 100% increases the weight of zeolite by 1.5 times, the amount of chemisorption is reduced with respect to zeolite. For the A-type zeolite, the adsorption amount was determined for CaA-type zeolite and AgA-type zeolite obtained by subjecting CaA-type zeolite to Ag ion exchange. Several kinds of Ca-AgA-type zeolites were used for ion exchange and treatment methods. The silver ion exchange rate had an optimum value and the ion exchange rate of 20% showed the highest amount of chemical adsorption. The physical adsorption amount was hardly at 20% or more, and iodine was stabilized by Ag ion exchange. A large amount of silver makes the adsorbent expensive, so a smaller amount is better.
Considering stability, it was confirmed that the higher the ion exchange rate, the better, and the upper limit of the ion exchange rate should be about 60%. As long as the pretreatment method of zeolite was in the range of 350 ° to 650 °, the difference was not inferior, and the difference was not changed by firing with air and nitrogen.

Example 2 A silica network was formed. As the zeolite, a Ca-Ag-A type zeolite (ion exchange rate: 20%) was used. FIG. 4 shows a diagram of the silica coat test apparatus. As shown in FIG. 4, the silica coat test device includes a first container 31 containing TMOS as a silica agent and a second container 31 containing ammonia water (catalyst).
Are provided in parallel with each other, and gas is sequentially introduced into a coating cylinder 34 in which zeolite 33 adsorbed with iodine is placed. In FIG. 4, reference numeral 36 denotes a detector. First, helium gas is introduced from a helium cylinder 35 through an inlet, and ammonia water, TMOS, and helium gas are caused to flow into the iodine-containing zeolite 33 through independent lines. Ammonia water flows as saturated steam by bubbling. Tetramethylorthosilicate (TMOS) as a silica source was bubbled to remove supersaturated vapor and introduced. In addition, a water bath was used to stabilize the amount introduced. Ca-Ag- adsorbing iodine
A-type zeolite (ion exchange rate: 20%) was maintained at 100 degrees, and was repeatedly flowed several times while confirming the gas species in the order of TMOS, helium gas, aqueous ammonia, and helium gas with the detector 36. The evaluation was performed by baking at 350 ° C. to remove water, and evaluated from the iodine content relative to the number of coatings. The content was measured by wet analysis. The confinement condition can be from room temperature to 450 ° C., but at room temperature, it is considered that dew may condense on zeolite.

FIG. 5 shows the relationship between the number of coatings and the iodine content. The content of iodine reached 40% after 6 or more coats, and most of the adsorbed iodine was confined. Therefore, the number of times necessary for confinement is six or more.

Example 3 The zeolite was Ca-Ag-A
A zeolite (ion exchange rate 20%) was used. In the examples, the iodine-adsorbed zeolite was immobilized with apatite after silica coating. Apatite synthesis method A 0.1-1.0 mol aqueous solution of calcium nitrate was mixed with an aqueous solution of ethylenediaminetetraacetic acid tetrasodium having a concentration adjusted such that the molar concentration of ethylenediaminetetraacetic acid tetrasodium was 1.1 with respect to the molar concentration of calcium nitrate. In addition,
Make a calcium chelate complex solution. Ca /
An aqueous solution of disodium hydrogen phosphate adjusted so that the P molar ratio is 1.67 is further added in the same amount.
The pH is adjusted to 10 using an N sodium hydroxide aqueous solution. Sodium fluoride was added as a fluorine source at 1/5 mol of calcium nitrate, the mixture was heated to 90 ° C., hydrogen peroxide was added at 5 times mol of tetrasodium ethylenediaminetetraacetate, and the mixture was aged for about 6 hours to form apatite. Obtained. Using the obtained fluorapatite, a zeolite containing iodine and further closing the window with silica was mixed, further mixed with an inorganic binder, and press-molded (100 kg / cm ² ). Harden around with binder, 3
It was baked at 50 degrees. Furthermore, the area around the zeolite was surrounded and fixed by this apatite. The obtained sample was immersed in purified water and allowed to stand for one month, and the iodine in the water was analyzed. However, since no iodine was recognized, it was confirmed that there was no diffusion or elution of iodine into the solution.

[0047]

As described above, according to the present invention, according to the invention of [Claim 1], invention [Claim 1], radioactive elements zeolite _{[W m Z n O 2n:} W = Na, Ca, K , Ba, S
r; Z = Si + Al (Si: A;> 1)], the zeolite window is closed by a silica network, and the elution of the contained radioactive element to the outside is suppressed. It can be fixed stably for a long period of time.

According to the second aspect of the present invention, since the surroundings of the radioactive element-fixed zeolite after the formation of the silica network are covered with the coating agent, for example, even if the coating is buried underground, it is protected from groundwater. Can be.

According to the third aspect of the present invention, since the coating is fixed with a fixing agent, the coating is further solidified, so that the contact with the groundwater can be delayed when the coating is buried in the ground.

According to the fourth aspect of the present invention, since the coating agent or the fixing agent is apatite or silica, the radioactive element can be stably solidified for a long period of time without any permanent deterioration. And its elution is also prevented.

According to a fifth aspect of the present invention, the formation of the silica network is performed by repeatedly subjecting a silicon alkoxide, a silane gas or the like to a temperature of 650 ° C. or less by a CVD method.
Thereafter, the window is closed by hydrolysis with steam, so that the window diameter of the zeolite is reliably closed.

According to the invention of claim 6, in claim 3 or 4, solidification is ensured because the mixture is press-molded together with the binder and then fired at a high temperature.

According to the invention of claim 7, since the radioactive element is iodine and the zeolite is A-type zeolite, iodine can be stably immobilized for a long period of time.

In the invention of claim 8, since the A-type zeolite is a CaA-type zeolite ion-exchanged with Ca ions, the incorporation of iodine in the zeolite window is ensured.

The invention of claim 9 is a Ca-Ag-A type zeolite obtained by ion-exchanging a part of the Ca ions of the above CaA type zeolite with Ag ions, so that silver iodide is formed and further stabilized. Inclusion is possible.

[Brief description of the drawings]

FIG. 1 is a process chart of an iodine immobilization method.

FIG. 2 is a conceptual diagram in which a type A zeolite containing iodine is covered with a coating agent.

FIG. 3 is a schematic diagram of an apparatus for measuring an iodine adsorption amount.

FIG. 4 is a schematic diagram of a silica coat test apparatus diagram.

FIG. 5 is a diagram showing the results of a silica coating test.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 A-type zeolite 12 Iodine 13 Window part 14 Silica network 15 Apatite 21 Iodine generation cylinder 22 Adsorption cylinder 23 Methanol tank 24 He cylinder 25 Water bath 31 First vessel 32 Second vessel 33 Iodine adsorption zeolite 34 Coating cylinder 35 He Cylinder

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Tsutaya 5-717-1, Fukabori-cho, Nagasaki-shi, Nagasaki Sansei Heavy Industries Co., Ltd. Nagasaki Research Laboratory (72) Inventor Kiyomichi Katsura 2-5-2 Marunouchi, Chiyoda-ku, Tokyo No. 1 Mitsuhishi Heavy Industries, Ltd. (72) Inventor Ichiro Yanagisawa 1-1-1, Wadazaki-cho, Hyogo-ku, Kobe-shi, Hyogo Pref. Mitsubishi Heavy Industries, Ltd.Kobe Shipyard (72) Inventor Nobuki Oka Fukahori-cho, Nagasaki City, Nagasaki Prefecture 5-7-17-1 Mitsuhishi Heavy Industries, Ltd., Nagasaki Research Laboratory (72) Inventor Akinori Yasutake 5-717-1, Fukahori-cho, Nagasaki City, Nagasaki Pref.

Claims (9)

[Claims] 1. The method of claim 1 wherein the radioactive element is zeolite [W _m Z
_n O _2n : W = Na, Ca, K, Ba, Sr; Z = Si +
Al (Si: A;> 1)] is fixed in the window, and the zeolite window is closed by a silica network to suppress the elution of the contained radioactive element to the outside. Method. 2. The method for immobilizing a radioactive element according to claim 1, further comprising coating the periphery of the radioactive element-immobilized zeolite after forming the silica network with a coating agent. 3. The method according to claim 2, wherein the coating is fixed with a fixing agent. 4. The method for fixing a radioactive element according to claim 2, wherein the coating agent or the fixing agent is apatite or silica. 5. The method according to claim 1, wherein the silica network is formed by repeating silicon alkoxide, silane gas, or the like at a temperature of 650 ° C. or less by a CVD method, and then hydrolyzing with water vapor to close the window. Method of fixing radioactive elements. 6. The method for immobilizing a radioactive element according to claim 3 or 4, wherein the composition is press-molded together with the binder and then fired at a high temperature. 7. The method for immobilizing a radioactive element according to claim 1, wherein the radioactive element is iodine, and the zeolite is an A-type zeolite. 8. The method according to claim 7, wherein the A-type zeolite is ion-exchanged with Ca ions.
A method for immobilizing a radioactive element, which is an A-type zeolite. 9. The method for immobilizing a radioactive element according to claim 8, wherein the CaA-type zeolite is a Ca-Ag-A-type zeolite obtained by partially exchanging Ca ions with Ag ions.