JP4556007B2 - Radioactive element-containing waste adsorbent and radioactive element immobilization method - Google Patents

Description

The present invention includes an adsorbent that adsorbs and collects radioactive elements generated in a spent nuclear fuel reprocessing plant, and in particular, C-14, Cl-36, Se-79, Tc-99, I-129 and the like having a long half-life. The present invention relates to a method of adsorbing and collecting radioactive element-containing waste to obtain a solidified body suitable for final disposal.

Radioiodine, which is one of the volatile radionuclides generated when reprocessing spent nuclear fuel at a spent nuclear fuel reprocessing plant, will be contained in the offgas, so usually the offgas is washed with alkali, The radioactive iodine is absorbed and removed through an off-gas through a filter filled with an iodine adsorbent so as to eliminate the influence on the environment. However, the adsorption removal by the iodine adsorbent is becoming mainstream. The iodine adsorbent or the like that adsorbs and collects radioactive iodine is solidified as a radioactive iodine-containing waste and is subjected to final disposal.

In addition, Co-60 (cobalt 60), a system water containing radionuclides of nuclear power plants,
Cs-137 (cesium 137), Sr-90 (strontium 90), other high-activity cation species such as Fe (iron) and Ni (nickel), C-14 (carbon 14), Cl
-36 (chlorine 36), Se-79 (selenium 79), Tc-99 (technetium 99), I
-129 (iodine 129) -containing anionic species with low radioactivity intensity (H ¹⁴ CO ₃ ⁻ , ¹⁴ C
O ₂ ²⁻ , H ⁷⁹ SeO ₃ ⁻ , ⁷⁹ SeO ₄ ²⁻ , and ⁹⁹ TcO ₄ ⁻ are present). Particularly in the case of these radioactive anions, an ion exchange resin is used as the adsorbent because there is no appropriate mineral adsorbent. It is necessary to reduce the volume of the ion exchange resin at the time of disposal, and after the volume reduction treatment, the waste is treated as a cement solidified body or an asphalt solidified body.

Radioactive waste solidification methods include cement solidification (Patent Document 1), plastic solidification, asphalt solidification, metal solidification (Patent Documents 2 and 3), glass solidification (Patent Documents 4 and 5), and apatite solidification (Patent Document 6). Various methods have been proposed. Cement solidification, plastic solidification, and asphalt solidification have the advantage that the adsorbent can be sealed at a low temperature as it is, so that the treatment process is simple and the generation of secondary waste is small. However, cement, plastic, asphalt, and metal are generally deteriorated in materials from several decades to several hundred years, and iodine is encapsulated in a non-uniform manner, so that there is a risk that iodine will leach outside after deterioration of the material. On the other hand, the glass material is a dense material, and if iodine is dissolved, it can be expected that the leaching of iodine is suppressed to the extent of dissolution of the glass. In addition, apatite is a bone component, and since it has been demonstrated from dinosaur fossils and the like that it can be stably maintained for millions of years, it is suitable for a fixed material for stable long-term storage. It is believed that.

Japanese Patent Laid-Open No. 10-227895 Japanese Patent Laid-Open No. 10-62598 JP 2000-249792 A Japanese Patent Laid-Open No. 09-171096 JP 2001-116894 A JP 2001-91694 A

In general, the problems in solidifying radioactive iodine are that I-129 is a long half-life nuclide, so that stable confinement can be obtained over a long period of time, and trace amounts of radioactive elements, particularly volatile iodine, can be captured efficiently. It must be able to collect and adsorb. From the viewpoint of the immobilization methods that have been studied so far, a matrix immobilizing agent such as an apatite-based mineral is preferable in view of long-term stability. However, when the adsorbent is fixed in an apatite-based ceramic matrix, compared to a solidified body in which waste and glass are homogeneously melted, stress is concentrated in the adsorbent domain during solidification, which causes cracks in the solidified body. Occurs and becomes a problem.

Therefore, the present invention is capable of efficiently adsorbing and collecting volatile iodine, radioactive anions in waste water, etc., and confining the adsorbent excellent in solidified crack resistance after solidification and the radioactive element-containing waste. It is an object of the present invention to provide a method for solidifying radioactive element-containing wastes that is stable and has long-term stability.

As a result of intensive studies to improve the above-mentioned problems, the present inventors have found that metal hydroxides strongly adsorb and collect volatile iodine and radioactive anions. By using an adsorbent made of this metal hydroxide in a spherical shape, defects such as cracks are less likely to occur in the solidification process, and a solidified product in which the radioactive waste-containing adsorbent is uniformly dispersed in the solidified matrix can be obtained. It became clear.

That is, this invention was solved by taking the structure shown by the following [1]-[14].
[1] Group 2, Group 4, Group 5, Group 6, Group 11, Group 12, Group 13 of Periodic Table of Elements
A radioactive element-containing waste adsorbent comprising a metal atom and a metal-containing spherical metal hydroxide selected from the group of metal atoms consisting of manganese, iron, cobalt, nickel, lead, and bismuth.

[2] The metal hydroxide contains aluminum hydroxide, magnesium hydroxide, iron hydroxide (I
The radioactive element-containing waste adsorbent comprising the spherical metal hydroxide according to [1], which is I), iron oxide (III) oxide, or iron (III) hydroxide.

[3] In [1] or [2], the average particle size of the spherical metal hydroxide is 1.0 μm to 20 μm.
A radioactive element-containing waste adsorbent comprising a spherical metal hydroxide, characterized by being 0 μm.

[4] Adsorption of radioactive element-containing waste comprising spherical metal hydroxide according to any one of [1] to [3], wherein the surface of the spherical metal hydroxide is hydrophobized. Agent.

[5] A radioactive element-containing waste adsorbent comprising a spherical metal hydroxide, wherein the hydrophobizing treatment according to [4] is performed with a silanizing agent.

[6] The radioactive element-containing waste adsorbent according to [5], wherein the silanizing agent is represented by the following formula:

R _4-n SiX _n n = 1, 2, 3 (R: a hydrocarbon group having 1 to 32 carbon atoms, or a part or all of the hydrogen atoms are substituted with fluorine atoms. (However, those having 1 carbon atom and n = 1 are excluded. X: an alkoxy group, a hydrogen atom, a hydroxyl group, a phenoxy group, or a diethylamino group.)

[7] A composite powder comprising a spherical metal hydroxide powder (A) in which a radioactive element is adsorbed on the spherical metal hydroxide, and a fixing agent (B) having corrosion resistance in the disposal environment of the solidified body. A method for immobilizing a radioactive element, characterized by firing at a predetermined temperature after pressure molding.

[8] The composition of the composite powder is such that the blending amount of the spherical metal hydroxide powder (A) adsorbing the radioactive element and the immobilizing agent (B) is (A) :( B) = 5: 95 to mass ratio. It is 60:40, The fixing method of the radioactive element as described in [7] characterized by the above-mentioned.

[9] In the method for immobilizing radioactive element-containing waste according to [8] or [9], a sintered body is obtained by subjecting a pressure-molded sample to microwave treatment. Immobilization method.

[10] In the method for immobilizing radioactive element-containing waste according to [7] or [8], a composite powder comprising a spherical metal hydroxide powder (A) adsorbed with a radioactive element and an immobilizing agent (B) is used. A method for immobilizing a radioactive element, comprising compressing with a predetermined pressing force to obtain a green compact, and applying a pulse voltage to the green compact to heat it to a predetermined temperature.

[11] A method for immobilizing a radioactive element, wherein the immobilizing agent according to any one of [7] to [10] is a calcium phosphate ceramic.

[12] A method for immobilizing a radioactive element, wherein the calcium phosphate ceramics according to [11] is hydroxyapatite and / or fluorapatite.

[13] The method for immobilizing a radioactive element according to any one of [7] to [12], wherein a predetermined pressing force applied to the composite powder is 5 MPa or more and 100 MPa or less.

[14] The method for immobilizing a radioactive element according to any one of [7] to [13], wherein the sintering temperature of the composite powder is 700 ° C. or higher and 1200 ° C. or lower.

Hereinafter, the configuration of the present invention will be described in more detail. The spherical metal hydroxide suitably used in the present invention can be obtained by a method such as spray drying. What is used for the fixation process of the radioactive element of this invention has an average particle diameter of the range of 1-200 micrometers.

The metal hydroxide in the present invention is a substance that can be easily synthesized by a neutralization reaction with alkali at a relatively low temperature, precipitation from a supersaturated aqueous solution, hydrolysis of a metal alkoxide, and the like. These metal hydroxides are Group 2, Group 4, Group 5, Group 6, Group 11, Group 12 of the Periodic Table of Elements.
A metal-containing spherical metal hydroxide selected from the group of metal atoms consisting of Group, Group 13 metal atoms, and manganese, iron, cobalt, nickel, lead, and bismuth. Specifically, group 2 is beryllium, magnesium, calcium, strontium, barium, radium,
Group 3 is scandium, yttrium, lanthanoid, actinoid, Group 4 is titanium,
Zirconium, hafnium, Group 5 is vanadium, niobium, tantalum, Group 6 is chromium,
Molybdenum, tungsten, Group 11 is copper, gold, Group 12 is zinc, cadmium, Group 13 is aluminum, gallium, indium, thallium, and manganese, iron, cobalt, nickel, lead, and bismuth.

As a specific example of the metal hydroxide comprising a more preferable metal atom among the metal atoms,
Magnesium hydroxide, calcium hydroxide (II), strontium hydroxide, barium hydroxide, titanium oxide hydrate, vanadium hydroxide (III), copper hydroxide (II), gold hydroxide (III), zinc hydroxide, Cadmium hydroxide, Gibbsite α-Al (OH) ₃ , Vialite β-Al (
OH) ₃ , boehmite α-AlO (OH), aluminum hydroxide represented by diaspore β-AlO (OH), gallium hydroxide (III), indium hydroxide (III), thallium hydroxide (I), hydroxylated Thallium (III), manganese hydroxide (II), manganese oxide hydroxide (III
), Iron hydroxide (II), goethite α-FeO (OH), akaganeite β-FeO (OH
), Lepidocrocite γ-FeO (OH), amorphous iron hydroxides such as iron oxide (III) oxide represented by limonite δ-FeO (OH), iron hydroxide (III), Schwertmannite , Cobalt hydroxide, cobalt oxide (III) hydroxide, nickel hydroxide, lead (II) hydroxide, bismuth oxide hydrate and the like. Of these, magnesium hydroxide, aluminum hydroxide, iron hydroxide (II), iron oxide hydroxide (III), and iron hydroxide (III) are particularly preferable.

In order to obtain the spherical metal hydroxide of the present invention, a method by a spray drying method is suitable. By using this, a spherical metal hydroxide having a simple and relatively uniform particle diameter can be obtained. That is, it is a method in which a metal hydroxide is dispersed in an aqueous solvent to form a gel, and then the dispersion is spray-dried. In preparing the gel, the concentration of the metal hydroxide is preferably 20% by weight or less, more preferably 1 to 10% by weight. If the concentration exceeds 20% by weight, the gel viscosity is high, it is difficult to feed the liquid to the spray nozzle during spray drying, and the nozzle is clogged.

For spray drying, general spray drying methods such as a disk type, a pressure nozzle type, and a two-fluid nozzle type can be applied. In both cases, the inlet air temperature during spraying is 300 for metal hydroxide.
A temperature range of about 100 to 300 ° C. can be set since it is sufficiently thermally stable up to about ° C. The particle size of the granular metal hydroxide thus obtained is 1 to 200 μm. The spherical metal hydroxide is classified and used by an ordinary dry classification method as necessary.

The spherical metal hydroxide of the present invention has an average particle size of 1 to 200 μm and a particularly preferred range of 2 to 100 μm in the particle size measurement method by electron microscopy. From the viewpoint of the filling property to the solidified body and crack resistance, the particle size of the particles to be used has a certain preferable range, and the above range is a preferable range in that the pressure can be applied uniformly.

In this case, the particle size can be measured by a sedimentation weight method, a centrifugal sedimentation light transmission method, a laser diffraction / light scattering method, or the like. Since there is a possibility that aggregation in the fluid phase cannot be discriminated, it is directly observed with a transmission electron microscope or scanning electron microscope, the average value of the major and minor diameters of individual particles is obtained, and the log normal distribution of each fraction based on the number It is preferable to determine the average particle size from

In the case of removing radioactive anions contained in water, the adsorbent is required to have water resistance. The spherical metal hydroxide of the present invention has excellent water resistance, but the water resistance can be improved by further surface treatment.

As the surface treatment agent, a silanizing agent may be used, but a silanizing agent represented by the following formula is particularly preferable.
R _4-n SiX _n n = 1, 2, 3
(R is a hydrocarbon group having 1 to 32 carbon atoms, or part or all of the hydrogen atoms are replaced by fluorine atoms, provided that the number of carbon atoms is 1 and n = 1. X:
An alkoxy group, a hydrogen atom, a hydroxyl group, a phenoxy group, and a diethylamino group. )

Examples of this silanizing agent include 3,3,3-trifluoropropylmethoxysilane, n-
Octadecyltriethoxysilane, n-octadecyltrimethoxysilane, n-octadecylsilane, n-octylmethyldimethoxysilane, n-octylsilane, n-octyltriethoxysilane, n-butyltrimethoxysilane, n-propyltrimethoxysilane, Ethyltriethoxysilane, ethyltrimethoxysilane, methyltrimethoxysilane,
Examples include dimethyldimethoxysilane, diethyldiethoxysilane, and n-octadecyldimethylmethoxysilane.

Next, a method for fixing the spherical metal hydroxide adsorbed with the radioactive element in the solidified body will be described. First, the spherical metal hydroxide (A) adsorbed with the radioactive element and the fixing agent (B) having corrosion resistance in the disposal environment of the solidified body are mixed in a predetermined amount, the mixed powder is filled in the mold, and compressed at a predetermined pressure To obtain a sintered body. This sintering process can be performed with the pressure maintained or released.

The compounding ratio of the metal hydroxide powder (A) adsorbing these radioactive elements and the immobilizing agent (B) is in the range of (A) :( B) = 5: 95 to 60:40, preferably by mass ratio. 10: 90-4
The range is 0:60, more preferably 10:90 to 30:70. If the blending amount of the component (A) is less than 5% by mass, the cost for waste treatment will be affected. If the blending amount of the component (A) exceeds 60% by mass, the component (B) that is the matrix will decrease, resulting in radioactive. There is a possibility that the element-containing waste cannot be completely fixed.

Examples of the fixing agent (B) of the present invention include calcium phosphate compounds such as hydroxyapatite, fluorapatite, carbonate apatite, and the like, dicalcium phosphate, tricalcium phosphate, tetracalcium phosphate, and eight phosphate phosphates. Calcium and the like,
One or more of these can be used in combination. As a solidifying agent for immobilizing a radioactive element, it is preferable to use fluorapatite (Ca ₅ (PO ₄ ) 3F) having the lowest solubility in water among apatites.

Examples of the method for immobilizing radioactive element-containing waste described in the present invention include hot press sintering (HP), hot isostatic sintering (HIP), thermal plasma sintering, in which a predetermined pressure and a predetermined temperature are simultaneously applied. Examples include sintering, spark plasma sintering (SPS), atmospheric pressure sintering (PLS) in which a predetermined temperature is applied after forming a molded body at a predetermined pressure, microwave sintering, millimeter wave sintering, and high-frequency plasma sintering. it can.

Sintering by microwave heating is preferable in that it can be densified at a lower temperature and in a shorter time than conventional heating. The mechanism of heating and sintering by microwaves cannot be said to be fully understood, but at present, a dense sintered body is formed by effects such as promotion of diffusion of heating, internal heating, and surface activation. Can be obtained.

Furthermore, since a sintered body can be obtained in a relatively short time in terms of excellent thermal efficiency due to a direct heat generation method by discharge, discharge plasma sintering (SPS) is particularly preferable. The electric discharge sintering apparatus compresses the powder to form a green compact, and applies a pulse voltage to the green compact to reduce the Joule heat generated in the powder and the electric field diffusion effect generated between the powder particles. It is used to promote sintering.

The pressurizing condition in the method for immobilizing radioactive element-containing waste described in the present invention is 5 MPa or more and 10
It is preferably 0 MPa or less. More preferably, it is 10 or more and 80 MPa or less. 5
If the pressure is less than MPa, a dense sintered body cannot be obtained, and gaps and voids may be formed between the solidifying agent particles. If the pressure is 100 MPa or more, the sintered body breaks due to stress concentration on the adsorbent particles. There is a fear.

The heating condition in the method for immobilizing radioactive element-containing waste described in the present invention is a sintering temperature of 700.
It is at least 1200 ° C and at most 1200 ° C. More preferably, it is 800 degreeC or more and 1000 degrees C or less. 70
If the temperature is less than 0 ° C, the immobilizing agent (B) is not sufficiently sintered, and the radioactive element-containing waste cannot be stably immobilized for a long time. If the temperature exceeds 1200 ° C, the radioactive element is decomposed and released in the heating process of two components. There is a risk that. In order to prevent oxidation during sintering of the green compact, the sintering atmosphere may be a vacuum, or an argon atmosphere or the like may be used instead of the vacuum.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited at all by these Examples. In addition, spherical metal hydroxide and fluorapatite (FAp) use CuKα rays generated at 40 kV / 40 mA using a RINT2200 by X-ray powder diffraction (manufactured by Rigaku Corporation), and a divergence slit angle is 1 degree. , Divergence length limiting slit 10mm, scattering slit 1.25mm, light receiving slit 0.3mm, scan speed 2 ° / min, sampling width 0.02 °) and morphological observation (field emission scanning electron microscope HITACHI
S-5000, acceleration voltage 10 kV).

Boehmite α-AlO (OH) was prepared as a metal hydroxide as an adsorbent. For the preparation of boehmite, aluminum chloride hexahydrate (AlCl ₃ .6H ₂ O) reagent special grade Wako Pure Chemical Industries, Ltd. and sodium hydroxide (NaOH) reagent special grade Wako Pure Chemical Industries, Ltd. were used. For the preparation of FAp, calcium carbonate reagent (99.99%) manufactured by Wako Pure Chemical Industries, Ltd., phosphate reagent manufactured by Wako Pure Chemical Industries, Ltd., and hydrofluoric acid reagent manufactured by Wako Pure Chemical Industries, Ltd. were used.

Synthesis of Spherical Boehmite (α-AlO (OH)) Powder Adsorbent While stirring 0.02M AlCl ₂ aqueous solution at room temperature, 0.1M NaOH was added dropwise at a rate of 1
. The gel sample was dropped at 7 mL / min to gel, and the gel sample was washed with distilled water and then hydrothermally treated at 100 ° C. for 24 hours. The obtained sample was boehmite (α-AlO (OH)). FIG. 1 shows an XRD pattern of boehmite. A diffraction peak of only boehmite was shown, and no impurities were observed.

Spray dryer (DL-4) while stirring boehmite suspension having a concentration of 5.7% by weight.
1, Yamato Science) drying temperature 180 ° C, spraying pressure 0.16MPa, spraying speed about 150m
Spherical particles were obtained by spray drying at L / min. The result of the observation image of boehmite true spherical powder by a scanning electron microscope (SEM) is shown in FIG. Boehmite true spherical powder having a particle size of 2 to 10 μm was obtained. 100 average particles were selected from the obtained photographic image, and the average particle size obtained from the diameter of the particle image using a scale was 4.1 μm.

Using this spherical boehmite (5 g) as an iodine adsorbent, an inner diameter of 15 mmΦ and a length of 200 m
A column was prepared by separating the silica tube from m at the bottom with silica wool and packed with I ₂ (3 g) / adsorbent (5 g). The iodine adsorption condition was that He gas was flowed at a flow rate of 1 cc / min, iodine was supplied to the support agent side, and reacted at room temperature for 72 hours. Excess iodine was collected in a trap connected with multiple ethanol. Iodine in the adsorbent was adsorbed to 2.06 mmol / g with respect to 1 g of the adsorbent as a result of quantification by ion chromatography after alkali dissolution.

(Comparative Example 1)
As a result of an experiment of iodine adsorption under the same conditions using Ca-type zeolite A (Ca-LTA) as a comparative sample, the iodine adsorption amount was 0.35 mmol / g. Ca-LTA was prepared by ion-exchange Na zeolite A (reagent manufactured by Wako Pure Chemical Industries, Ltd.) in CaCl _2.

Fluoroapatite (FAp) powder used as a solidifying agent was synthesized as follows.
The calcium carbonate powder was heated at 1050 ° C. for 3 hours, cooled to 280 ° C., and hydrolyzed with distilled water to prepare an aqueous calcium hydroxide solution. After bubbling with nitrogen for 24 hours, a mixed solution of phosphoric acid and hydrofluoric acid was gradually added dropwise, and ammonia water was added to maintain the pH at 7.5 or higher. The obtained suspension of FAp was filtered and washed, and again dispersed in distilled water (FAp / water = 3% by weight).
In the same manner as in Production Example 1, spherical particles were obtained by spray drying. After that, calcined at 800 ° C. for 3 hours was used as a matrix powder for composite sintering.

FIG. 3 shows an XRD pattern of FAp fired at 1200 ° C. A diffraction peak of only FAp was shown, and no impurity was observed. FIG. 4 shows spherical FAp calcined at 800 ° C. after spray drying.
The SEM image of powder is shown. Spherical secondary particles having a diameter of 5 to 20 μm composed of needle-like crystals having a major axis of about 100 nm and a minor axis of about 20 nm were observed.

Spherical boehmite synthesized in Example 1 and adsorbed with iodine and the above spherical FAp in a mass ratio of 1
The mixture was mixed at 5:85 (mass%) and filled into a carbon die (manufactured by Sumitomo Co., Ltd.) having an outer diameter of 70 mmΦ, an inner diameter of 20 mmΦ, and a thickness of 10 mm. Pulse energizing pressurizer (SPS-1030,
A sintered body was produced under the conditions of a pressure of 50 MPa, a temperature of 1000 ° C., and a holding time of 10 minutes. As a result, the sintered body was free from cracks and cracks, and a uniform solidified body was obtained.

Spherical boehmite synthesized in Example 1 and adsorbed with iodine and the above spherical FAp in a mass ratio of 3
A sintered body was prepared in the same manner as in Example 1 except that mixing was performed at 0:70 (mass%). As a result, the sintered body was free from cracks and cracks, and a uniform solidified body was obtained.

(Comparative Example 2)
A sintered body was prepared in the same manner as in Example 2 except that the boehmite powder before spray drying in the synthesis step of Example 1 was used as it was without adsorbing iodine. As a result, the obtained sintered body was cracked.

The present invention relates to a spherical metal hydroxide excellent in adsorption and collection of iodine or anionic radioactive element-containing waste and a method for immobilizing the radioactive element. Spherical metal hydroxide is suitable for adsorption and collection of iodine in gas and other low-level radioactive anions in water, and it is mixed with calcium phosphate matrix, compressed, and heat treated. The adsorbent adsorbing the radioactive element-containing waste can be disposed of in a long-term stable state.

X-ray diffraction pattern of boehmite. SEM image substituted for drawing of spray-dried spherical boehmite powder. X-ray diffraction pattern of FAp baked at 1200 ° C. SEM image substituted for drawing of FAp powder calcined at 800 ° C. by spray drying.

Claims (14)

Beryllium, magnesium, calcium, strontium, barium, radium, scandium, yttrium, lanthanoid, actinoid, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, copper, gold, zinc, cadmium, aluminum, gallium, A radioactive element-containing waste adsorbent comprising a metal-containing spherical metal hydroxide selected from the group of metal atoms comprising indium, thallium, manganese, iron, cobalt, nickel, lead, and bismuth . The spherical shape according to claim 1, wherein the metal hydroxide is aluminum hydroxide, magnesium hydroxide, iron hydroxide (II), iron oxide hydroxide (III), or iron hydroxide (III). Radioactive element-containing waste adsorbent made of metal hydroxide. 3. The radioactive element-containing waste adsorbent according to claim 1, wherein the spherical metal hydroxide has an average particle size of 1.0 to 200 [mu] m. The radioactive element-containing waste adsorbent comprising the spherical metal hydroxide according to any one of claims 1 to 3, wherein the surface of the spherical metal hydroxide is subjected to a hydrophobic treatment. A radioactive element-containing waste adsorbent comprising spherical metal hydroxides, wherein the hydrophobizing treatment according to claim 4 is performed with a silanizing agent. The radioactive element-containing waste adsorbent according to claim 5, wherein the silanizing agent is represented by the following general formula (c).
R _4-n SiX _n n = 1, 2, 3
(Here, R is a hydrocarbon group having 1 to 32 carbon atoms, or a part or all of the hydrogen atoms are substituted with fluorine atoms. Provided that the number of carbon atoms is 1 and n = 1. X is an alkoxy group, a hydrogen atom, a hydroxyl group, a phenoxy group, or a diethylamino group.) The spherical metal hydroxide is adsorbed with a radioactive element, the composite powder composed of the spherical metal hydroxide powder (A) and the fixing agent (B) adsorbed with the radioactive element is pressure-molded, and then fired at a predetermined temperature. A method for immobilizing a radioactive element using the adsorbent according to any one of claims 1 to 6, wherein: The composition of the composite powder is such that the blending amount of the spherical metal hydroxide powder (A) adsorbed with the radioactive element and the immobilizing agent (B) is (A) :( B) = 5: 95-60 The method for immobilizing a radioactive element according to claim 7, wherein: The method for immobilizing radioactive element-containing waste according to claim 7 or 8, wherein the sintered compact is obtained by subjecting the pressure-molded green compact to microwave treatment. . The method for immobilizing radioactive element-containing waste according to claim 7 or 8, wherein the composite powder comprising the spherical metal hydroxide powder (A) adsorbed with the radioactive element and the immobilizing agent (B) is predetermined. A method of immobilizing a radioactive element, comprising: compressing with a pressing force of 1 to form a green compact, and applying a pulse voltage to the green compact to heat to a predetermined temperature. A method for immobilizing a radioactive element, wherein the immobilizing agent according to any one of claims 7 to 10 is a calcium phosphate ceramic. The method for immobilizing a radioactive element, wherein the calcium phosphate ceramic according to claim 11 is hydroxyapatite and / or fluorapatite. The method for immobilizing a radioactive element according to any one of claims 7 to 12, wherein a predetermined pressing force applied to the composite powder is 5 MPa or more and 100 MPa or less. The method for immobilizing a radioactive element according to any one of claims 7 to 13, wherein the sintering temperature of the composite powder is 700 ° C or higher and 1200 ° C or lower.

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