JP2003307594A - Method for separating/recovering radioactive nuclide carbon 14 adsorbed on graphite structure material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of separating/recovering safely only<SP>14</SP>C by oxidizing collectively a graphite structure including<SP>14</SP>C in a nuclear reactor, while a processing/disposal method of a radioactive graphite structure is now under study wherein the whole structure is burned in a high temperature region and<SP>14</SP>C is collected by an absorbent such as zeolite. <P>SOLUTION: In this method, the radioactive graphite is oxidized by air at a temperature below 600°C, to thereby generate a gaseous mixture of<SP>14</SP>CO<SB>2</SB>and<SP>14</SP>CO. After converting<SP>14</SP>CO into<SP>14</SP>CO<SB>2</SB>,<SP>14</SP>CO<SB>2</SB>is selectively separated and recovered, to thereby separate/recover the radioactive nuclide carbon 14 adsorbed on the graphite structure material. In this case, the air oxidation of the radioactive graphite can be heated in a pressure vessel in the nuclear reactor. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

Technical Field of the Invention ¹⁴ C generated from a nuclear facility
, Which has a long half-life and is a nuclide that affects the health of living organisms, may diffuse into the environment even if it is released locally and may cause human-bombed damage on a global scale. There is. Therefore, research on recovery, treatment, transportation and treatment of these radionuclides which become gaseous and affect the living body is a highly important field.

[0002] The present invention Magnox reactors, Advanced gas-cooled reactor, ¹⁴ contained in the graphite structure with the decommissioning of HTGR
It is a method of separating and recovering C by air oxidation.

[0003]

2. Description of the Related Art Radioactive nuclides such as carbon 14 ( ¹⁴ C) having an extremely long half-life are generated in graphite materials used for core structures of magnox furnaces, improved gas cooling furnaces, and high temperature gas reactors. Graphite materials are used as structures such as moderators and reflectors with complicated shapes, but since they are porous materials, ¹⁴ C is adsorbed even inside the graphite parts, making separation and removal extremely difficult. Has become.

There are about 110 or so nuclear reactors in the world that use a large amount of graphite material as a large structure.
At present, the method of treating and disposing of radioactive graphite structures is not yet clear. In addition, the method for recovering and removing ¹⁴ C contained in the graphite structure accompanying the decommissioning of the gas-cooled reactor is currently in the research stage in each country, and a definite method has not been determined.

[0005]

At present, ¹⁴ C burns graphite to convert it into a chemical form of ¹⁴ CO ₂ , and then insoluble Ca
A method of generating and immobilizing ¹⁴ CO ₃ and Ba ¹⁴ CO ₃ (precipitate) and a method of absorbing and separating into an absorbent such as zeolite have been studied.

However, in the combustion method, a high temperature oxidizer having a temperature of 800 ° C. or higher is newly required to gasify the entire graphite structure. Among radionuclides other than ¹⁴ C contained in graphite, nuclides having a high melting point are simultaneously generated in sludge, and nuclides having a low melting point are simultaneously generated in combustion gas. Equipment for collection is required. Furthermore, the current situation is that research has not advanced to the point where the separated and recovered carbon-14 can be stably stored.

The radioactive graphite structure treatment / disposal method currently being investigated is ¹⁴ C by burning all in a high temperature range.
There is a method of collecting carbon with an absorbent such as zeolite, but by applying the present invention, only ¹⁴ C can be separated and recovered by a collective oxidation treatment of a graphite structure containing ¹⁴ C in a waste nuclear reactor. it can.

[0008]

In the present invention, the radioactive graphite structure is subjected to air oxidation at a temperature of 600 ° C. or less to selectively separate and recover only ¹⁴ C, and to carry out the oxidation treatment of the present invention. It is possible to safely separate and collect all at once by carrying out in an abandoned nuclear reactor.

That is, the dry oxidation by air oxidation of the present invention is
By using¹⁴Selective separation of C radionuclides
It can take such an air oxidation method, oxidation temperature, during oxidation
The processing conditions such as the space are materials derived from the manufacturing conditions of the graphite material.
Irradiated graphite is not dependent on the intrinsic fine pore structure.
Since it is more easily oxidized than the irradiated one, it is approximately 600
It can oxidize uniformly at temperatures below ℃
Therefore, only carbon-14 nuclide ¹⁴CO₂To separate into the air layer
be able to.

At the same time, since the whole core graphite structure can be collectively oxidized in a very tightly sealed structure called a nuclear reactor, by applying the method of the present invention, selective separation and recovery of ¹⁴ C radionuclides can be achieved. In addition, it can contribute to the treatment / disposal of radioactive graphite structures and the development of stable storage methods.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, ¹⁴ C alone is selectively separated and recovered by air-oxidizing a radioactive graphite structure at a temperature of 600 ° C. or lower,
By carrying out the treatment of the present invention in a waste nuclear reactor, it is possible to safely separate and collect all at once, and the details are as follows.

(A) In the present invention, a radioactive graphite structure is used.
Air oxidizes at temperatures below 00 ° C. The degree of oxidation is much smaller than that of the combustion method, and radionuclides on the surface of graphite and on the surface of internal pores such as ¹⁴ C are separated and recovered by selective oxidation.

(B) Considering that the radioactive graphite structure exists in the nuclear reactor, the present invention is applied to carry out the batch treatment using the existing facility. That is, disposing and disposing of 1,000 tons or more of graphite structure while disassembling it not only requires time and cost, but also due to the characteristics of the graphite material, which tends to become fine powder, contamination of the surrounding environment and exposure of workers. There is a large possibility of. Therefore, in order to apply the present invention in an abandoned nuclear reactor, engineering studies including peripheral equipment are necessary.
The inside of the pressure vessel is a region that is shielded from the outside world, and is an optimal place for handling radioactive substances.

By the way, the radionuclide existing in the irradiated graphite is mainly a nuclide produced by activation of impurities contained in the graphite material. Graphite is the most suitable place to confine these, and the radionuclides inside are stably present by covering the surface of the graphite block with a phosphate compound. However, it is necessary to remove only ¹⁴ C, which has a long half-life and is a nuclide that affects the health of living organisms.

In addition, under the present circumstances where a stable storage method for the separated and recovered ¹⁴ C is not established, it is stored in an alkaline aqueous solution rather than absorbed by an absorbent such as zeolite.
^It is considered a good idea to wait for the research and development of ¹⁴ C stabilization treatment.

When carrying out the air oxidation treatment at the laboratory level in the present invention, as shown in FIG. 1, irradiation graphite 6 is placed in a combustion furnace 3 and air (Air) is supplied to a filter 1 and a flow meter 2. Then, it is supplied to a combustion furnace and the irradiated graphite is air-oxidized at about 600 ° C. A gas containing carbon oxides generated in the combustion furnace is introduced into the platinum catalyst of the carbon dioxide gas conversion furnace 4 to convert the carbon oxides into ¹⁴ CO ₂ . The conversion processing gas is H ₂ SO ₄ , K
¹³⁴ C in collection vessels 8 and 9 containing MnO4
s, ¹³⁷ Cs, etc. are collected and then introduced into the collection containers 10 and 11 containing the aqueous sodium hydroxide solution to obtain ¹⁴ CO
₂ is absorbed. The subsequent gas is sucked by the suction pump 13 through the activated carbon absorption container 12 and discharged into the atmosphere.

Further, when the present invention performs the air oxidation treatment in a waste nuclear reactor, as shown in FIG. 2, a heating element is provided in a control rod hole provided in the irradiation graphite block in the pressure vessel of the nuclear reactor. Insert and heat. The air flow is supplied into the pressure vessel and the cooling gas inflow holes of the irradiation graphite block through the inflow tube. Generated by heating irradiated graphite in the presence of air
A mixed gas of ¹⁴ CO ₂ and ¹⁴ CO is introduced into a platinum catalyst furnace and converted into ¹⁴ CO ₂ , then introduced into an absorption cylinder and absorbed in an aqueous sodium hydroxide solution.

That is, in the nuclear reactor, control rod holes, cooling gas inflow holes, etc. are opened in the graphite block of the core. Therefore, when the expiration date of the reactor became decommissioning past, when ¹⁴ C-graphite structure contaminated with ¹⁴ C provided on the core of its decommissioning separated and recovered by air oxidation, the A heating element (kanthal heating element, etc.) is inserted into the control rod hole, inflow hole, etc. through the pressure vessel wall of the furnace to heat the graphite structure of the core. In that case, many heating elements are loaded so that a temperature distribution as uniform as possible can be obtained. In this case, the heating element is a sheath heater because graphite is a good electric material. Further, since the graphite material is a good electric body as used in the electrode, it is possible to raise the temperature by directly energizing.

[0019]

Example 1 Air-oxidation treatment at a laboratory level As shown in FIG. 1, irradiation graphite on an alumina boat 5 was oxidized at 585 ° C. in an air flow having a flow rate of about 600 ml / mm and generated. The mixed gas of ¹⁴ CO ₂ and ¹⁴ CO thus prepared was converted into ¹⁴ CO ₂ through platinum catalyst 6 (680 ° C.), recovered in sodium hydroxide aqueous solutions 10 and 11, and the radioactivity concentration of carbon 14 was analyzed by a liquid scintillation counter. did.

By the air oxidation treatment for 5 hours, 33% of the amount of ¹⁴ C activity existing in the irradiated graphite and 42% by the oxidation treatment for 10 hours could be selectively separated and removed. When compared in terms of ¹⁴ C concentration (Bq / g) per 1 g of irradiated graphite, about 90% could be removed.

Also, other than ¹⁴ C, ¹³⁴ C having a relatively low melting point
There was no contamination in the H ₂ SO ₄ and KMnO ₄ absorption column installed to separately collect radionuclides such as s and ¹³⁷ Cs. As described above, it was experimentally proved that only the ¹⁴ C site was selectively oxidized by the air oxidation treatment and could be separately collected with other radionuclides.

Example 2 Air Oxidation Treatment in Nuclear Reactor As shown in FIG. 2, graphite was prepared by loading a heating element into an irradiation hole or a control rod hole in a graphite block by utilizing peripheral equipment of the reactor. The temperature of the whole block is heated in the range of 550 to 585 ° C. At that time, oxidation is performed while feeding air at a flow rate of 300 to 600 ml / min. Generated ¹⁴ CO ₂ , ¹⁴ C
O is converted to ¹⁴ CO ₂ in a platinum catalyst furnace and absorbed in an aqueous sodium hydroxide solution. The radioactivity concentration of carbon-14 absorbed in the aqueous sodium hydroxide solution was analyzed by a liquid scintillation counter.

[0023]

EFFECTS OF THE INVENTION The present invention brings about the following remarkable effects peculiar to the present invention. (B) By experimentally confirming that ¹⁴ C concentration can be reduced by air oxidation of irradiated graphite at about 600 ° C,
The radioactive graphite structure can be collectively processed in the reactor.

(B) According to the present invention, the radioactive graphite structure can be treated as low-level waste by effectively and selectively separating and removing ¹⁴ C nuclide. (C) The fact that only ¹⁴ C can be reliably recovered can contribute to the development of stabilization and safe storage methods using polymer membranes and hollow membranes.

[Brief description of drawings]

FIG. 1 is a diagram showing the air oxidation treatment of the present invention performed at a laboratory level.

FIG. 2 is a diagram showing an air oxidation treatment of the present invention performed in a nuclear reactor.

Claims (2)

[Claims] By air oxidation 1. A temperature of the radioactive graphite 600 ° C. or less, to generate a mixed gas of ¹⁴ CO ₂ and ¹⁴ CO, converts the ¹⁴ CO to ¹⁴ CO _2, ¹⁴
A method for separating and recovering radionuclide carbon 14 adsorbed on a graphite structure material, characterized by selectively separating and recovering CO ₂ . 2. The method according to claim 1, wherein the air oxidation of radioactive graphite is performed by heating in a pressure vessel of a waste nuclear reactor.