JP2003149390A - Processing method for radioactivated graphite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently and stably processing radioactivated graphite. SOLUTION: This processing method of radioactivated graphite comprises (1) a process for pulverizing graphite waste, (2) a process for burning the pulverized graphite to turn it into carbon dioxide gas (a) or into carbon monoxide gas (b), (3) a process for recovering C-14 methane from the carbon dioxide gas (a) or the carbon monoxide gas (b) by reduction and isotope separation, and (4) a thermal decomposition process for thermally decomposing the C-14 methane to turn it into C-14 carbon; and (5) the reduction is executed in the presence of a noble metal catalyst.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating activated graphite generated in a graphite slowdown reactor with decommissioning.

[0002]

2. Description of the Related Art In a graphite slowdown reactor, thousands of tons of graphite are generated as decommissioned waste per reactor due to decommissioning. Naturally, metals and concrete other than graphite are also generated as waste from decommissioning, but these treatments are similar to those in the world's mainstream light water reactors, and there are relatively few problems. The following methods have been studied as a method for treating graphite following the decommissioning of the graphite slowdown reactor. (1) Fill in drums as it is. (2) After canned in a drum, the voids are filled with cement or the like and solidified. (3) Dispose in the ground. (4) Incinerate as it is or after crushing (gas treatment is required).

[0003]

[Problem to be Solved by the Invention] Graphite is 1% in graphite.
C-13, which is strongly contained, is activated by neutrons and contains radioactive C-14 with a half-life of 5,730.
For this reason, this activated graphite cannot be treated like general industrial waste, and various treatment methods have been investigated as described above. If the drum is packed or solidified without volume reduction treatment, the amount is enormous, which is several thousand tons per furnace, which causes a great problem in storage and storage. When incinerated, graphite (C) becomes carbon dioxide (CO
₂ ), the amount is larger than that of the original graphite, and carbon dioxide gas cannot be fixed by absorbing alkali (for example, calcium hydroxide (Ca (OH) ₂ ) is collected as calcium carbonate (CaCO ₃ )). , The amount will increase further. Further, it is not preferable to release carbon dioxide gas to the atmosphere because it may release harmful radioactive substances to the environment and may change the abundance ratio of natural C-14 in the natural world. The present invention has been proposed in view of such a situation, and gasifies activated graphite to efficiently generate C-
It is an object of the present invention to provide a method for separating 14.
Furthermore, the present invention provides a method for continuously and efficiently performing a reduction reaction required for gasification of graphite and isotope separation.

[0004]

The present invention is a method for treating activated graphite, which comprises (1) a crushing step for crushing graphite, (2) burning the crushed graphite, and a carbon dioxide gas (a ) Or an oxidation step using carbon monoxide gas (b),
(3) A separation step of reducing carbon dioxide gas (a) to methane gas, followed by a separation step of isotope-separating C-14 methane from methane gas, or (3 ′) a carbon monoxide gas (b) to C- After the separation step of isotope-separating 14 carbon monoxide, C-14 carbon monoxide is oxidized to carbon dioxide gas (c), and then reduced to C-14 methane, or (3 '') After the reduction step of reducing the carbon monoxide gas (b) to methane gas, the methane gas was converted into C-14.
C-14 carbon monoxide is reduced after the separation step for isotopically separating methane or the separation step for isotope separating C-14 carbon monoxide from (3 ′ ″) carbon monoxide gas (b), C
-14 methane reduction step, (4) C-14 methane is thermally decomposed to C-14 carbon,
(5) A treatment method in which reduction is carried out in the presence of a noble metal catalyst.

According to the first treatment system of the present invention, graphite is treated in the following steps. (Crushing step) Activated graphite is first crushed in order to improve handleability and facilitate oxidation in the next step. Crushing can be performed by a known method. (Oxidation step) Next, the crushed graphite is burned to form carbon dioxide gas (a). (Reduction step) Carbon dioxide gas (a) is reduced to methane gas. Such reduction reaction is represented by the following formula (I). CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O (I) Such a reaction is called the Sabatier first reaction. The reduction is carried out in the presence of a noble metal catalyst. Examples of the noble metal catalyst include platinum and ruthenium. The noble metal catalyst is preferably supported on a carrier such as alumina. The reduction reaction is preferably performed at about 250 to 350 ° C.

Prior to the reduction, carbon dioxide gas (a) may be subjected to a treatment selected from the group consisting of reoxidation, chemical absorption, physical absorption and combinations thereof to reduce carbon monoxide contained as an impurity. preferable. The combination can be a combination of at least two kinds of treatments. The (reoxidation) carbon dioxide gas (a) contains carbon monoxide, which poisons the reduction catalyst. In order to reduce the carbon monoxide, it is preferable to oxidize the carbon dioxide gas (a) again. At this time, it is preferable to carry out in the presence of a Hopcalite catalyst which is a mixture of manganese dioxide and copper oxide. Hopcalite catalyst is a catalyst that oxidizes carbon monoxide into carbon dioxide in the presence of oxygen at room temperature.

(Chemical absorption or physical absorption) The carbon dioxide gas (a) is brought into contact with an absorption liquid to selectively absorb the carbon dioxide, and then the absorption liquid is heated or decompressed to release the carbon dioxide to obtain high purity. It is preferable to use carbon dioxide gas. The heating is 110 ° C. or lower. Examples of the absorbing liquid include alkaline absorption, which is an aqueous solution of an alkaline inorganic salt in the case of chemical absorption, amine-based absorbing liquid, and water in the case of physical absorption. By such chemical absorption or physical absorption, the concentration of the impure gas (oxygen, carbon monoxide) in the carbon dioxide gas (a) can be reduced, and the reduction reaction of the above formula (I) can be carried out efficiently and stably. be able to. Any one of reoxidation, chemical absorption, and physical absorption may be performed, or a combination of these may be performed. The order of combination is not limited. That is, chemical absorption or physical absorption may be carried out after reoxidation, or reoxidation may be carried out after chemical absorption or physical absorption.

(Installation of reduction reactors in parallel) In the reduction reaction, it is preferable that a plurality of reduction reactors are installed in parallel so that at least one series is subjected to the reduction reaction and the other is subjected to the catalyst regeneration reaction. For example, two systems of carbon dioxide reduction reactors are installed, and while the reduction reaction is being performed on one side, the other is
The catalyst can be regenerated by the desorption reaction of carbon monoxide, which is a catalyst poison, and the carbon dioxide reduction treatment can be continuously performed by alternately repeating this. The desorption reaction of carbon monoxide can be carried out at a temperature of 400 to 600 ° C.

(Separation step) C-14 methane and C-12 methane are isotope separated from methane gas by a known method. Isotope separation can be performed by any method such as gas diffusion method, centrifugal separation method, distillation, chemical exchange method, adsorption method and the like. The form at the time of separation may be either gas-gas or gas-liquid. (Pyrolysis step) C-14 Methane is pyrolyzed to give solid C-
14 carbons. Such a reaction is represented by the following formula. CH
₄ → C + 2H ₂

According to the second treatment system of the present invention, graphite is treated in the following steps. (Crushing step) Activated graphite is first crushed in order to improve handleability and facilitate oxidation in the next step. Crushing can be performed by a known method. (Oxidation step) Next, the crushed graphite is burned to form carbon monoxide gas (b). (Isotope separation) C-14 carbon monoxide and C-12 carbon monoxide are isotope separated from the carbon monoxide gas (b). After the (C-14 oxidation of carbon monoxide) separation step, C-1
4. Carbon monoxide is oxidized to form carbon dioxide gas (c). Such a reaction is represented by the following formula. CO + 1 / 2O ₂ → C
O ₂ (reduction step) Carbon dioxide gas (c) is reduced to C-14 methane. Such a reaction is represented by the following formula. CO ₂
+ 4H ₂ → CH ₄ + 2H ₂ O (Pyrolysis step) C-14 Methane is pyrolyzed and solid C-
14 carbons. Such a reaction is represented by CH ₄ → C + 2H ₂ . Also in the second treatment system, like the first treatment system, prior to the reduction reaction of carbon dioxide gas (c), impurities in carbon dioxide should be removed by reoxidation, chemical absorption, physical absorption, or the like. Is preferred. It is also preferable to install a plurality of reduction reactors in parallel so that at least one series is subjected to a reduction reaction and the other is subjected to a catalyst regeneration reaction.

According to the third treatment system of the present invention, graphite is treated in the following steps. (Crushing step) Activated graphite is first crushed in order to improve handleability and facilitate oxidation in the next step. (Oxidation step) Next, the crushed graphite is burned to form carbon monoxide gas (b). (Reduction step) Carbon monoxide gas (b) is reduced to methane gas. Such a reaction is represented by the following formula. CO + 3H
₂ → CH ₄ + H ₂ O (isotope separation) C-14 methane isotope separated from methane gas. (Pyrolysis step) C-14 Methane is pyrolyzed to give solid C-
14 carbons. Such a reaction is represented by the following formula. C
H ₄ → C + 2H ₂

According to the fourth treatment system of the present invention, graphite is treated in the following steps. (Crushing step) Activated graphite is first crushed in order to improve handleability and facilitate oxidation in the next step. (Oxidation step) Next, the crushed graphite is burned to form carbon monoxide gas (b). (Isotope separation) C-14 carbon monoxide is isotope separated from the carbon monoxide gas (b). (Reduction step) C-14 carbon monoxide is reduced to C-14 methane. (Pyrolysis step) C-14 Methane is pyrolyzed to give solid C-
14 carbons. Such a reaction is represented by the following formula. C
H ₄ → C + 2H ₂

[0013]

The present invention will be described below based on the embodiments.

Embodiment 1. The first treatment system of the graphite treatment method of the present invention is shown below.

[Chemical 1] The outline of the flow is as follows. First, graphite waste is crushed, and the crushed graphite is burned to carbon dioxide, and the carbon dioxide is reduced to methane, which is then condensed and isotope separated to thermally decompose C-14 methane to produce C-14. Use carbon.

Embodiment 2. In the second treatment system of the present invention, first, graphite waste is crushed, the crushed graphite is burned to carbon monoxide, carbon monoxide isotope separated, and C-14 carbon monoxide after separation is separated. It is oxidized to C-14 carbon dioxide, this C-14 carbon dioxide is reduced to C-14 methane, and then C-14 methane is thermally decomposed to C-14 carbon. The difference between the second processing system and the first processing system of the present invention is as follows. When burning crushed graphite, first
In the second treatment system, carbon monoxide is burned into carbon dioxide, whereas in the second treatment system, carbon monoxide is used. Further, in the first treatment system, carbon dioxide is reduced to methane for isotope separation, whereas in the second treatment system, isotope separation is performed in the state of carbon monoxide. In the second treatment system, after such isotope separation, carbon monoxide is oxidized to carbon dioxide. Subsequent reduction and thermal decomposition yields C-14 carbon.

Embodiment 3. In the treatment system of the present invention, as shown in the flow below, a carbon monoxide oxidizer can be provided in front of the carbon dioxide reduction reactor to perform reoxidation.

[Chemical 2]

Embodiment 4. In addition, as shown in the flow below, a carbon monoxide oxidizer is installed in front of the carbon dioxide reduction reactor, reoxidation is performed, and a carbon dioxide absorber and a diffuser are further installed to remove carbon dioxide by chemical absorption or physical absorption. It is possible to concentrate carbon (remove impurities).

[Chemical 3]

Embodiment 5. As shown in the following flow, two reduction reactors (A and B) are installed, one is used for the reduction reaction, and the other is used to regenerate the catalyst, which are alternately repeated. You can also

[Chemical 4]

Sixth Embodiment In the treatment system of the present invention, a plurality of the above-mentioned modes of reoxidation, chemical absorption, physical absorption, and parallel installation of reduction reactors can be combined.

[0020]

As described above, according to the present invention, activated graphite can be treated efficiently and stably. Further, according to the present invention, since carbon monoxide, which poisons the carbon dioxide reduction catalyst, can be reduced, the reduction reaction can be carried out efficiently and stably, and as a result, activated radioactivity can be obtained. The graphite can be treated efficiently and stably. Further, according to the present invention, carbon dioxide can be continuously reduced by providing a plurality of reducing devices in parallel.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideo Saiki             1-1 1-1 Wadasaki-cho, Hyogo-ku, Kobe-shi, Hyogo             No. Mitsubishi Heavy Industries, Ltd.Kobe Shipyard F-term (reference) 4G046 CA01 CC06 CC08 CC10

Claims (7)

[Claims] 1. A method for treating activated graphite, comprising:
(1) Crushing step of crushing graphite, (2) Oxidation step of burning crushed graphite to carbon dioxide gas (a) or carbon monoxide gas (b), (3) Reduction of carbon dioxide gas (a) A separation step of isolating C-14 methane from the methane gas after the reduction step of converting to methane gas, (4)
A treatment method comprising a thermal decomposition step of thermally decomposing C-14 methane to C-14 carbon, and (5) performing reduction in the presence of a noble metal catalyst. 2. A method of treating activated graphite, comprising:
(1) Crushing step of crushing graphite, (2) Oxidation step of burning crushed graphite to form carbon dioxide gas (a) or carbon monoxide gas (b), (3 ′) carbon monoxide gas (b)
From the C-14 carbon monoxide isotope-separated, the C-14 carbon monoxide is oxidized to form carbon dioxide gas (c).
And a reduction step of reducing to C-14 methane,
(4) A treatment method comprising a thermal decomposition step of thermally decomposing C-14 methane to C-14 carbon, and (5) performing reduction in the presence of a noble metal catalyst. 3. A method of treating activated graphite, comprising:
(1) Crushing step of crushing graphite, (2) Oxidation step of burning crushed graphite to carbon dioxide gas (a) or carbon monoxide gas (b), (3 '') carbon monoxide gas (b) )
After the reduction step of reducing methane to methane gas, a separation step of isotopically separating C-14 methane from methane gas,
(4) A treatment method comprising a thermal decomposition step of thermally decomposing C-14 methane to C-14 carbon, and (5) performing reduction in the presence of a noble metal catalyst. 4. A method of treating activated graphite, comprising:
(1) Crushing step of crushing graphite, (2) Oxidation step of burning crushed graphite to form carbon dioxide gas (a) or carbon monoxide gas (b), (3 ″ ′) carbon monoxide gas ( After the separation step of isotopically separating C-14 carbon monoxide from b), a reduction step of reducing C-14 carbon monoxide to C-14 methane, (4) pyrolyzing C-14 methane to C
A treatment method comprising a thermal decomposition step using -14 carbon, wherein (5) reduction is carried out in the presence of a precious metal catalyst. 5. Prior to reduction, carbon dioxide gas (a) or (c) is subjected to a treatment selected from the group consisting of reoxidation, chemical absorption, physical absorption and a combination thereof, and the monoxide contained as an impurity. The method according to claim 1 or 2, wherein carbon is reduced. 6. The method according to claim 1, wherein a plurality of reduction reactors are installed in parallel, at least one series is subjected to a reduction reaction, and the other is subjected to a catalyst regeneration reaction. 7. The method according to claim 1, wherein the noble metal catalyst is a ruthenium-based catalyst.