JP2002333498A - Method of decontaminating radioactive substance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decontaminate metal contaminated with a radioactive substance in a short time. SOLUTION: A decontaminant solution added with hydrogen peroxide to an aqueous organic acid solution is used when a base material of the contaminated metal contaminated with the radioactive substance is dissolved to remove contamination, by the aqueous organic solution. The radioactive substance is removed since the base material of the contaminated metal is dissolved when the decontaminant solution is made to contact with the contaminated metal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decontaminating radioactive materials for removing contamination from metals contaminated with radioactive materials generated during operation of nuclear facilities, periodic inspections, decommissioning, and the like.

[0002]

2. Description of the Related Art Conventionally, as a decontamination technique for removing contaminated radioactivity from radioactively contaminated metal, for example, Japanese Unexamined Patent Publication No.
JP-A-63-18799, JP-A-1-31300, JP-A-2
As described in JP-A-22597, chemical decontamination methods using an acidic decontamination solution have been developed in Japan and overseas. In these methods, a metal contaminated with radioactivity (contaminated metal) is dissolved in a sulfuric acid solution having a concentration of 5 wt% or more as a first solution.
After being immersed in a temperature condition of not less than ° C., it is immersed and dissolved in an aqueous solution obtained by adding an oxidizing metal salt to sulfuric acid as a second liquid.

There is also known a method of short-circuiting a part of a contaminated metal and a sacrificial anode or repeating a cycle in which a reduction voltage is applied for a certain period of time to stop the contaminated metal, thereby decontaminating the debris of the contaminated metal. ing.

Further, for example, Japanese Patent Application Laid-Open No. 60-235099 discloses that the surface of a device to be decontaminated with an organic acid solution having a concentration of 1% by weight or less is used for the purpose of system decontamination for maintaining the soundness of the device. A method for selectively dissolving only an oxide film, which is a source of contamination, is described.

Further, a decontamination method for dissolving a contaminated metal base material contaminated with a radioactive substance with an organic acid solution to remove the contamination is disclosed in, for example, JP-A-9-113690.
In this decontamination method, for example, when the material of the contaminated metal to be decontaminated is stainless steel, carbon steel having a potential more negative than the oxidation-reduction potential of the stainless steel is brought into contact with the material to dissolve and remove the base material of the stainless steel. Dye.

In this decontamination method, the dissolution rate of the stainless steel is improved by bringing carbon steel into contact with the stainless steel, and accordingly, the decontamination rate is improved. When the metal to be decontaminated is carbon steel, simply immersing carbon steel in an organic acid dissolves the metal base material and removes contamination.
In addition, since the used organic acid after the decontamination work can be decomposed by ultraviolet rays and hydrogen peroxide, the amount of secondary waste generated can be reduced.

[0007]

For example, formic acid (HCOO)
H) When an aqueous solution and sulfuric acid (H ₂ SO ₄ ), which is an inorganic acid, are used as decontamination solutions, both dissolve carbon steel by the oxidizing power of H ⁺ ions as shown in the following formula. ^{HCOOH = H + + HCOO - pKa} 3.55 H 2 SO 4 = 2H + + SO 4 2- pKa 1.99

Since formic acid is a weak acid as can be seen from the acid dissociation index (pKa), it has low corrosiveness to carbon steel. Therefore, a method of increasing the temperature and concentration of the formic acid aqueous solution to increase the dissolution rate of the metal base material and removing the radioactive substance in a short time has been adopted.

However, in order to raise the temperature of the formic acid aqueous solution, a heater facility is required, and a facility for condensing and recovering the evaporating decontamination liquid is required, so that the cost of the decontamination apparatus increases. In addition, when the temperature of the decontamination solution is high, it is necessary to sufficiently consider the safety of the decontamination worker. In addition, the formic acid aqueous solution can be decomposed into carbon dioxide and water to reduce the amount of secondary waste, but if the formic acid concentration is high, there is a problem that the decomposition treatment requires a long time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. Even if the concentration and the temperature of an organic acid are reduced, the dissolution rate of a contaminated metal base material is increased so that radioactive substances adhering to the contaminated metal are increased. It is an object of the present invention to provide a method for decontaminating a radioactive substance, which can remove odors in a short time and can efficiently decompose a used decontamination solution after decontamination.

[0011]

The invention according to claim 1 is
It is characterized by dissolving and decontaminating metals contaminated with radioactive materials using a decontamination agent obtained by adding hydrogen peroxide to an organic acid aqueous solution. According to the invention of claim 1, when hydrogen peroxide is added to an organic acid aqueous solution as a decontamination liquid, the dissolution rate of the contaminated metal base material is reduced as compared with a decontamination liquid of an aqueous solution in which formic acid is dissolved in water alone. Since it can be made larger, radioactive substances can be removed from contaminated metals in a short time.

According to a second aspect of the present invention, there is provided a decontamination step of dissolving and decontaminating a metal contaminated with a radioactive substance using a decontamination agent obtained by adding hydrogen peroxide to an organic acid aqueous solution. It has a concentration measuring step of measuring the concentrations of the organic acid and the hydrogen peroxide, and a replenishing step of replenishing the organic acid and the hydrogen peroxide according to the measured concentration in the concentration measuring step.

According to the second aspect of the present invention, the concentration of the organic acid and hydrogen peroxide gradually decreases with the dissolution of the base material of the contaminated metal. The concentration can be measured as needed, and an organic acid and hydrogen peroxide corresponding to the concentration decrease can be supplied in the reagent supply step. Thereby, stable decontamination performance can be obtained.

According to a third aspect of the present invention, there is provided a decomposing step of decomposing an organic acid in the decontamination waste liquid into carbon dioxide gas and water in the decontamination step, and adjusting the pH of the decomposition waste liquid after the decomposition step to neutral or alkaline. And a separation step of separating metal ions precipitated by the pH adjustment step.

According to the third aspect of the present invention, a large amount of carbon dioxide is generated by the decomposition of the organic acid from the metal surface, but the radioactive substance is removed for a short time by removing the oxide film by the bubbling effect of the carbon dioxide in the decomposition and separation step. Can be removed. Further, in the pH adjusting step, metal ions are precipitated as hydroxide, and the supernatant liquid can be discharged to a waste liquid treatment system.

The invention according to claim 4 is a decomposition step of decomposing an organic acid in the decontamination waste liquid after the decontamination step into carbon dioxide gas and water, and converting the metal ions in the decomposition waste liquid after this decomposition step into ions. And a separation step of separating with an exchange resin.

According to the fourth aspect of the invention, the metal ions in the decomposed waste liquid can be adsorbed on the cation exchange resin, and the separated treated waste liquid can be discharged to the waste liquid treatment system. Thereby, the treatment of the used decontamination waste liquid can be performed simultaneously with the decontamination treatment of the contaminated metal, and the organic carbon can be decomposed in a short time.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a first embodiment of a method for decontaminating radioactive substances according to the present invention will be described. FIG.
FIG. 4 is a process diagram for describing the present embodiment. In this embodiment, a mixed solution of an organic acid and hydrogen peroxide is used as the decontamination solution in the decontamination step 1 as shown in FIG. Then, when the contaminated metal 2a contaminated with the radioactive substance is brought into contact with the decontamination liquid, the base material of the contaminated metal 2a is dissolved and the radioactive substance is removed, and a clean surface is exposed on the metal 2b after decontamination. .

That is, radioactivity (X ₀ ), weight (W)
Is decontaminated in the decontamination step 1 by a decontamination liquid composed of a mixed aqueous solution of an organic acid and a hydrogen peroxide solution. In the decontamination solution, the amount of metal △ W eluted from the metal base material and the amount of radioactivity △ X are transferred, and the weight of the metal 2b after decontamination is W-
△ in W, radioactivity is X ₀ - △ decreases respectively X.

In the decontamination step 1, the concentrations of the organic acid and hydrogen peroxide in the decontamination solution decrease with the dissolution of the metal base material. After the measurement, the amounts of the organic acid and hydrogen peroxide corresponding to the decrease in the concentration are supplied from the reagent supply step 4 to the decontamination step 1.

The spent decontamination liquid in the decontamination step 1 is transferred to the decomposition processing step 5, where the organic acids in the used decontamination liquid are decomposed into carbon dioxide and water, and the organic carbon (TOC) is removed. ) The concentration drops below 10 ppm.

Since the pH of the decomposition effluent after the decomposition treatment in the decomposition step 5 is about 6, the pH of the decomposition effluent is adjusted to neutral 7 or more alkaline in the pH adjustment step 6 to remove metal ions. Precipitate as hydroxide, and then precipitate is separated and removed in a precipitate separation step 7, and the supernatant is discharged to a waste liquid treatment system for treatment. As the organic acid, for example, formic acid, oxalic acid, citric acid and the like are used.

Next, a second embodiment of the radioactive substance decontamination method according to the present invention will be described with reference to FIG. FIG. 2 is a process chart for explaining the second embodiment. This embodiment is the same as the above-described first embodiment up to the decomposition processing step 5, except that the elution metal separation step for eluting metal ions in the decomposition waste liquid discharged in the decomposition processing step 5 is performed. 8 is provided. This elution metal separation step 8
Thus, the metal ions in the waste liquid for decomposition treatment are adsorbed and separated by the mixed resin composed of the cation exchange resin and the anion exchange resin, and then discharged to the waste liquid treatment system for treatment.

Next, the effect of the decontamination liquid composed of the mixed aqueous solution of the organic acid and the hydrogen peroxide solution in the decontamination step 1 in the first and second embodiments will be described with reference to FIGS. 3 and 4. FIG. 3 shows carbon steel test pieces (material: SS41, size: 50) for an aqueous solution of formic acid alone and a mixed aqueous solution of formic acid and hydrogen peroxide when formic acid is used as an organic acid.
2 shows the results of an Arrhenius plot of the dependence of temperature upon immersion of the sample. The test condition is that the temperature is 20
8080 ° C., the concentration of formic acid was 1 mol·L ⁻¹ , and the concentration of hydrogen peroxide was 1/2 equivalent of the concentration of formic acid. The dissolution rate of carbon steel is 1 / l for both the formic acid alone aqueous solution and the mixed aqueous solution of formic acid and hydrogen peroxide.
A linear relationship was obtained for T.

FIG. 4 shows the dependence of formic acid concentration on a carbon steel test piece (material: SS41, size: 50 mm × 50 mm) immersed in an aqueous solution of formic acid alone and a mixed aqueous solution of formic acid and hydrogen peroxide. ing. The test conditions were as follows:
The concentration of L- ¹ and hydrogen peroxide was 1/2 equivalent of the concentration of formic acid, and the temperature was room temperature (about 20 ° C).

The dissolution rate of carbon steel was found to increase as the formic acid concentration increased in both the formic acid alone aqueous solution and the formic acid / hydrogen peroxide mixed aqueous solution. However, formic acid concentration
Compared with 1mo · L ^-1 , 1.5μm · h ^-1 for formic acid aqueous solution alone
The dissolution rate of the mixed aqueous solution of formic acid and hydrogen peroxide was about 10 times as high as that of.

As described above, it was recognized that the dissolution rate of the metal base material was improved when hydrogen peroxide was added to the organic acid aqueous solution. This dissolution of the base material with formic acid _{(HCOOH) (HCOOH) 2 +} Fe → Fe (HCOO) 2 + H 2 ↑ (1) Fe 2+ and (H ₂ O ₂₎ OH radical by reaction with (OH · )) H ₂ O ₂ + Fe ²⁺ → Fe ³⁺ + OH ⁻ + OH · (2) It is considered that the dissolution of the base material is promoted by the oxidizing power of the OH radical.

Since the dissolution rate of the metal base material can be increased by adding hydrogen peroxide to the formic acid aqueous solution as compared with the conventional aqueous solution of formic acid alone, the radioactive substance can be removed from the metal surface in a short time. it can. In addition, since a large amount of carbon dioxide gas, which indicates the decomposition of formic acid, is generated from the metal surface, the oxide film on the surface can be removed by the bubbling effect of carbon dioxide gas, and this effect can also remove radioactive substances in a short time. .

The OH radical shown in the formula (2) is known as an oxidizing agent for decomposing organic acids, and formic acid is decomposed by the reaction shown in the formula (3). HCOOH + 2OH ・ → CO ₂ ↑ + 2H ₂ O （３）

Also in this test, the concentration of organic carbon in the aqueous solution was measured during the test to confirm the decomposition of formic acid. In the formic acid alone aqueous solution, the organic carbon concentration after 20 minutes was reduced by about 4% when the formic acid concentration was 5 wt%. On the other hand, the mixed aqueous solution of formic acid and hydrogen peroxide decreased by about 25%.

As described above, the concentration of formic acid and hydrogen peroxide gradually decreases with the dissolution of carbon steel. Therefore, it is necessary to measure these concentrations as needed during decontamination, and to supply formic acid and hydrogen peroxide corresponding to the concentration decrease. Thereby, stable decontamination performance can be obtained.

Next, a method for decomposing an organic acid which is a used decontamination liquid after decontamination in the decomposing step 5 shown in FIGS. 1 and 2 will be described with reference to FIG. Fig. 5 shows the results of a decomposition test of a formic acid aqueous solution (formic acid concentration: 1 mol·L ^-1 ). The plots marked with ◇ indicate that 150 ppm of iron was dissolved in the waste liquid by the conventional decomposition method. Shows the result of adding 1.5 times equivalent of.

On the other hand, the plots marked with 、, □ and Δ show the decomposition treatment method according to the present invention, in which a carbon steel test piece was immersed in the decontamination waste liquid, and at the same time, 1.5 times equivalent of hydrogen peroxide was added to the formic acid concentration. The results are shown. ○ indicates test temperature 30 ℃, □ indicates 50
° C and Δ indicate results at 80 ° C. As can be seen from the test results, the decomposition method of the present invention is compared with the conventional decomposition treatment method.
The TOC concentration can be decomposed in a short time to the target concentration (10 ppm or less). The decomposition time at room temperature of the present invention was reduced to about half of the conventional one. At 80 ° C., the processing time is 1/10 of 30 ° C.

As described above, in the conventional decomposition processing method,
Since Fe ²⁺ in the waste liquid of the decontamination solution was oxidized to Fe ³⁺ by hydrogen peroxide, the concentration of Fe ²⁺ required for decomposition decreased, and the decomposition reaction took a long time. On the other hand, in the present invention, a contaminated metal made of carbon steel is immersed in the decontamination waste liquid, Fe ²⁺ is constantly supplied to the waste liquid according to the above formula (1), and OH radicals are increased according to the formula (2). As a result, formic acid (TOC) can be decomposed in a short time.

Most of the liquid waste liquid treatment systems of nuclear power plants have a standard for accepting the concentration of organic carbon in the waste liquid of 10 ppm or less, so that the decontamination waste liquid containing organic acids is discharged to the liquid waste treatment system. In order to do so, the decomposition treatment of the organic acid is indispensable. However, since the decomposition process is not related to the original decontamination process, it is desirable that the decomposition process can be performed in as short a time as possible. Therefore, by applying the decomposition treatment step 8 of the present invention, the decomposition treatment can be performed simultaneously with the decontamination treatment, so that the total number of decontamination treatment steps can be significantly reduced.

Next, a method of treating the waste liquid after the decomposition treatment of the organic acid in the decomposition treatment step 5 will be described. The pH of the waste liquid from the decomposition treatment in the decomposition treatment step 5 is about 6, and iron ions eluted from the contaminated metal and Co ions as radionuclides are dissolved therein. In this state, the salt concentration and the radioactive substance concentration in the waste liquid for decomposition treatment are high, so that the waste liquid cannot be discharged to the existing waste liquid treatment system.

In view of the above, as shown in FIG. 1, the decomposed effluent which has been decomposed as shown in FIG. , Co ions, etc.) as a hydroxide. Thereafter, the sludge precipitated in the precipitate separation step 7 for separating the eluted metal is settled or separated by a filter, and the supernatant is discharged to a waste liquid treatment system.

Next, a third embodiment of the method for decontaminating radioactive materials according to the present invention will be described. In the present embodiment, the decomposition treatment waste liquid decomposed in the decomposition treatment step 5 in FIGS. 1 and 2 is passed through a mixed resin of a cation exchange resin and an anion exchange resin to mix metal ions in the decomposition treatment waste liquid. There is a method of causing the resin to adsorb.

For example, the iron ions in the waste liquid of the decomposition treatment are Fe ³⁺
And is adsorbed on the cation exchange resin.
The removal waste liquid obtained by separating and removing metal ions from the decomposition treatment waste liquid can be discharged to a waste liquid treatment system for processing.

According to the present embodiment, since no manual operation is required as compared with the pH adjustment step 6 and the elution metal separation step 8, it is possible to reduce the exposure of workers. However, since a large amount of used ion exchange resin is generated,
It is desirable to incinerate the ion exchange resin.

[0041]

According to the present invention, the dissolving speed of the contaminated metal base material is higher than that of the decontamination solution of the aqueous solution of formic acid alone, so that the radioactive substance attached to the contaminated metal can be removed in a short time.

[Brief description of the drawings]

FIG. 1 is a process diagram for explaining a first embodiment of a radioactive substance decontamination method according to the present invention.

FIG. 2 is a process diagram for explaining a first embodiment of the radioactive substance decontamination method according to the present invention.

FIG. 3 is a characteristic diagram showing temperature dependence of a carbon steel dissolution rate in a decontamination step in the first and second embodiments according to the present invention.

FIG. 4 is a characteristic diagram showing the formic acid concentration dependence of the dissolution rate of carbon steel in the decontamination step in the first and second embodiments according to the present invention.

FIG. 5 is a characteristic diagram showing a change over time of formic acid decomposition in a decontamination step in the first and second embodiments of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Decontamination process, 2a ... Contaminated metal before decontamination, 2b ... Metal after decontamination, 3 ... Concentration measurement process, 4 ... Reagent supply process, 5 ... Decomposition treatment process, 6 ... pH adjustment process, 7 ... Precipitate Separation process, 8
... Eluted metal separation process.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C02F 1/64 C02F 1/64 Z 1/72 1/72 Z (72) Inventor Yumi Yaita Kawasaki-shi, Kanagawa 2-1, Ukishima-cho, Kawasaki-ku, Japan Toshiba Hamakawasaki Plant (72) Inventor Masato Murai 8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture In-house Toshiba Yokohama Office (72) Inventor Tetsuo Goto Yokohama, Kanagawa Prefecture 8, Shinsugita-cho, Isogo-ku, Japan Toshiba Yokohama Office (72) Inventor Tetsu Hiraki, 66-2 Horikawa-cho, Saisaki-ku, Kawasaki-shi, Kanagawa Prefecture Toshiba Engineering Corporation (72) Inventor Yoshinari Takamatsu, Kawasaki-shi, Kanagawa Prefecture 66-2 Horikawa-cho, Sachi-ku Toshiba Engineering Corporation F term (reference) 4D025 AA09 AB22 BA08 BA13 DA10 4D038 AA08 AB66 AB67 AB79 BB13 BB17 BB18 4 D050 AA12 AB16 BB09 BD06 CA08 CA15 CA16

Claims (4)

[Claims] 1. A method for decontaminating a radioactive substance, comprising dissolving and decontaminating a metal contaminated with the radioactive substance using a decontaminant obtained by adding hydrogen peroxide to an organic acid aqueous solution. 2. A decontamination step of dissolving and decontaminating a metal contaminated with a radioactive substance using a decontamination agent obtained by adding hydrogen peroxide to an aqueous solution of an organic acid; A method for decontaminating a radioactive substance, comprising: a concentration measuring step of measuring the concentration of hydrogen oxide; and a replenishing step of replenishing an organic acid and hydrogen peroxide according to the measured concentration in the concentration measuring step. 3. A decomposing step of decomposing an organic acid in the decontamination waste liquid in the decontamination step into carbon dioxide gas and water, and a pH adjusting the pH of the decomposed waste liquid after the decomposition step to neutral or alkaline.
3. The method for decontaminating a radioactive substance according to claim 2, comprising: an adjusting step; and a separating step of separating metal ions precipitated in the pH adjusting step. 4. A decomposing step of decomposing an organic acid in the decontamination waste liquid after the decontamination step into carbon dioxide gas and water, and separating the metal ions in the decomposed waste liquid after the decomposition step by using an ion exchange resin. 3. A method for decontaminating radioactive substances according to claim 2, comprising the steps of: