JPH07119834B2 - Method for removing contaminants from surfaces contaminated with radioactive substances - Google Patents

Description

BACKGROUND OF THE INVENTION Radioactive element-containing deposits are often formed in the cooling system of nuclear reactors. It is necessary to remove these radioactive element-containing deposits in order to safely maintain and repair the cooling system. For example, this can be done by using an oxidizing solution of alkaline permanganate, followed by a decontamination solution of oxalic acid, citric acid, and ethylenediaminetetraacetic acid (EDTA). These solutions solubilize radioactive metal ions and other ions in the deposit. The solution circulates between the cooling system and the ion exchange resin, which removes ions from the solution.

Many effective decontamination solutions and oxidizing solutions have been found, but improved solutions that remove deposits easily, are inexpensive, and have low waste volumes (eg, used ion exchange resins). Is always required. Also, because the downtime of the reactor to remove deposits is very expensive, a solution that allows the cooling system to be cleaned more quickly can be significantly less expensive.

Another problem in the nuclear industry is the disposal of steam generators at the end of their useful life. Since the steam generator has high radioactivity, it is necessary to construct a radioactive containment building around the steam generator to prevent radiation escape. Decontamination solutions have not been effective in reducing the radioactivity of the steam generators described above to a level where radioactive containment buildings are not required.

Still other problems in the nuclear industry arise during the mining, processing / reprocessing and manufacturing of nuclear fuel. Not only is the device radioactively contaminated, but the pollutants contain uranium and plutonium, which can be used in the production of nuclear fuel if they can be quickly and cheaply recovered from the contaminated device. .

SUMMARY OF THE INVENTION We have found that complex solutions of ceric acid and inorganic acids are very effective at removing deposits from the cooling system of a reactor in a particular critical concentration range. This solution is very effective and, in fact, it alone can remove at least 97% of the radioactivity in the cooling system, eliminating the need for separate oxidation and decontamination solutions. Also,
We have found that the solution is capable of removing radioactivity from spent steam generator deposits to the extent that it does not require storage of the used steam generator in a specially constructed radioactive containment building; Moreover, the used steam generator can be safely stored in the outside world if its opening is sealed by welding.

Also, we have found that when the cerium acid solution is circulated, the cerium acid solution can be continuously regenerated by oxidizing the cerium. Furthermore, we have found that the radioactivity of the circulating solution can be reduced by passing the circulating solution through a hydrogen type cation exchange resin column that removes radioactive metal cations such as cobalt. This allows the solution to oxidize and remove much more radioactive deposits than other methods. The solution also reduces the amount of radioactive waste that must be treated.

Furthermore, we have found that the uranium or plutonium present can be recovered by extraction from the cerium acid solution. In this way, small amounts of uranium or plutonium that could otherwise be lost but still need to be treated as Transuranium (TRU) radioactive waste can now be recovered and used to produce nuclear fuel. Can be used.

RELATED TECHNOLOGY U.S. Pat. No. 4,162,229 discloses the use of cerium (IV) salts to remove contaminants on the metal surfaces of nuclear reactors. Acids such as sulfuric acid or nitric acid can be present.

DESCRIPTION OF THE INVENTION The principles of the present invention may be applied to any reactor cooling system including pressurized water reactors, boiling water reactors and gas cooled reactors. If the entire reactor is to be polluted, the reactor is first shut down. Shutdown means reducing the temperature of the coolant to 21-93 ° C (70-200 ° F). Next, the ceric acid and the inorganic acid are added directly to the cooling water. When removing pollutants from a part of other equipment that is radioactively contaminated, such as from cooling systems such as steam generators or nuclear fuel plant equipment, drain the equipment to create an aqueous solution, then This aqueous solution is circulated through the device.

The ceric acid solution of the present invention is an aqueous solution comprising one or more of three ceric acid solutions and an inorganic acid forming a complex compound with the ceric acid solution. The ceric acid used in this solution was tetrasulfatoceric acid [H ₄ Ce (S
O ₄ ) ₄ , usually called cerium sulfate], hexasulfamate cerium acid [H ₂ Ce (SO ₃ NH ₂ ) ₆ , usually called cerium sulfamate], hexaperchloratoceric acid [H ₂ Ce (ClO ₄ ) ₆ , commonly referred to as cerium perchlorate] or mixtures thereof. Of the three ceric acids, tetrasulfatoceric acid is preferable because it has less corrosion. The use of hexaperchloratoceric acid is limited to treating used cooling system equipment due to the presence of chlorine in the acid solution. Hexaperchloratoceric acid can form oxides, which chlorides can cause stress corrosion cracking of stainless steel.

Any inorganic acid or mixture of inorganic acids that can form a complex with ceric acid in solution can be used. The acid used must be an inorganic acid in order to oxidize the organic acid, waste the ceric acid and increase the amount of waste products to be treated. Inorganic acids that do not form complex compounds with ceric acid are not suitable because compounds that do not form complex compounds are very difficult to react. Suitably, the inorganic acid should correspond to the ceric acid in solution. For example, when the cerium acid is tetrasulfatocerium acid, sulfuric acid can be used, when the cerium acid is hexasulfamate cerium acid, sulfamic acid can be used, and when the cerium acid is hexaperchloratocerium acid, Can use perchloric acid. The combination of ceric acid and inorganic acid described above more easily forms complex compounds, which can be monitored during the disposal of the solution,
This means less ion species have to be processed. The use of the corresponding acid is preferred, but other inorganic acids which form a complex with ceric acid, such as nitric acid, can also be used.

We have found that complex compounds do not form in the absence of minimal amounts of inorganic and ceric acids. That is, the concentrations of ceric acid and inorganic acid in the solution are considered to be important to the effectiveness of the solution in removing contaminants on the metal surface. The concentration of ceric acid in the solution is about 0.5 ~
It should be 3% (herein, all% are weight% based on the weight of the solution). Ceric acid is 0.5
If it is less than%, there is substantially no pollutant removal effect, there is no need for ceric acid to exceed 3%, and further, the volume of waste is increased without causing pollutant removal. Further, if a larger amount of cerium acid is present, it is necessary to allow a larger amount of inorganic acid to be present, but if a larger amount of inorganic acid is present, the metal surface will be more corroded. The concentration of inorganic acid in the solution is about 1-5%. If less than 1% of inorganic acid is used, it is virtually ineffective in removing contaminants on the metal surface, even at higher concentrations of ceric acid. If the inorganic acid exceeds 5%, it corrodes the metal surface too much and unnecessarily increases the waste volume.

The temperature of the solution should be about 70-200 ° C. We have found that at low temperatures, room temperature (ie 20-25 ° C), virtually no contaminant removal occurs. However, at temperatures above about 200 ° C, the solution becomes too corrosive to metal surfaces.

The ceric acid solution is circulated through the device until the activity level of the solution stabilizes. That is, the solution is circulated until the activity level of the solution exiting the device is no higher than that of the solution entering the device. The device is then drained and rinsed with deionized water, preferably at a temperature of about 70-200 ° C.

Although at least 97% of the radioactivity is removed by the ceric acid solution alone, some of the remaining radioactivity can be removed by using a conventional contaminant removal solution after using the ceric acid solution. A conventional decontamination solution is a mixture of chelating compounds such as xylenediaminetetraacetic acid or nitrilotriacetic acid and organic acids such as citric acid or oxalic acid. Conventional decontamination solution is 70 ° C to approx.
The solution is circulated between the device and the cation exchange resin column at a temperature of 200 ° C. until the activity level of the solution leaving the device is no higher than that of the solution entering the device.
The device is then rinsed with deionized water and the conventional decontamination operation is complete. The spent cerium solution can be clarified using an anion exchange resin-cation exchange resin mixture or neutralized using hydroxide and evaporated to a solid waste. The used decontamination solution can be cleaned with an anion exchange resin or a mixture of ion exchange resins.

Reduction of cerium (IV) in solution can be prevented by oxidizing cerium (III) to cerium (IV) before recycling the solution to the contaminated equipment. This oxidation can be carried out, for example, by adding an oxidant such as ozone or peroxide to the solution or by electrolyzing the solution. The use of ozone is preferred as it has the highest potential oxidizing capacity, is the most reactive oxidant and is easy to add to the solution. Addition of ozone to the solution is preferably carried out by bubbling (sparging) ozone into the solution, and
Ozone can be formed electrically by using a diaphragm in place. (Alternatively, the electrolysis used to form ozone can be used in place of the cation exchange column to remove transition metals or other metals.) Oxidation of cerium (III) to cerium (IV) Afterwards, it is advantageous to pass the solution through a cation exchange resin in the hydrogen form. Cation exchange resins remove radioactive metal ions that do not complex with cerium (IV) acid solutions such as iron, cobalt and nickel. Cerium (III) does not form a strong anionic complex compound and is removed by the cation exchange tower, but cerium (IV) forms a strong complex compound with an inorganic acid and passes through the cation exchange tower. , Cerium (II
Oxidation of I) to cerium (IV) must be done before sending the solution to the cation exchange column. The cation exchange column must be in the hydrogen form (ie releasing hydronium ions) and strong acid ion exchange resins such as sulphonic acid type resins or chelated type resins are preferred. In the resin tower, the contaminated metal ions are removed to liberate the cerium complex compound, and the liberated cerium complex compound can be used to further remove the contaminated metal ions from the contaminated device.

Uranyl ion or Purnyl ion (U respectively)
O ₂ ²⁺ , which is believed to be PuO ₂ ²⁺ ) forms an anionic complex with the inorganic acid anion, so uranium and plutonium also pass through the resin column. Uranium and plutonium can be recovered from the solution either continuously before recycling the solution to the contaminated device or after the device has been decontaminated. Recovery of uranium and plutonium can be done by methods known in the art. These methods include solvent extraction into an organic solvent containing an extractant such as tributyl phosphate in decane or DEPA-TOPA [di (2-ethylhexyl) phosphate-tri (n-octyl) phosphine oxide] in kerosene. . Uranium and plutonium can also be recovered by using an anion exchange resin column (chromatographic separation). Solvent extraction can be performed before sending the aqueous solution to the cation-exchange resin tower, but if extraction is performed after passing through the cation-exchange resin tower, there will be less metal ions present and less radiation, especially gamma rays. After passing through the ion exchange resin tower, it is preferable.

The present invention will be further described below with reference to examples.

Example 1 A sample of a tube having a length of about 38 mm (1.5 inches) and a diameter of about 19 mm (3/4 inches) from a steam generator of a pressurized water reactor was longitudinally cut in half. The sample was loaded into a beaker containing various decontamination solutions (in some experiments the solution was circulated to the sample in the beaker). Decontamination factors were measured after treating each sample with the decontamination solution. [The pollutant removal factor (DF) is the value obtained by dividing the radioactivity value (microcurie) before the pollutant removal treatment by the radioactivity value (microcurie) after the treatment. ] The table below shows treatment with various decontamination solutions at different times and temperatures.
The procedure for processing different samples is described. In the table, "CM" is 30% citric acid, 30% oxalic acid, ethylenediaminetetraacetic acid
A commercial decontamination solution that appears to contain 40% and an inhibitor believed to be thiourea.

"CAS" is cerium ammonium sulphate, "CAN" is cerium ammonium nitrate and "TSCA" is tetrasulfatocerium acid.

The above table shows that cerium ammonium nitrate is not effective in performing sample decontamination. It also shows that tetrasulfatoceric acid is very effective at high concentrations, but not at concentrations below 0.25%.

Example 2 Example 1 was repeated. The results obtained are shown in the table below.

The above table shows that 1% sulfuric acid alone is not effective and 5% sulfuric acid is not effective at 22 ° C, but is effective at 100 ° C with considerable corrosion. (A temperature of 100 ° C. is virtually the highest temperature that can be obtained without boiling the solution). Also, the above table shows cerium ammonium nitrate-
It shows that the nitric acid solution is not effective for effecting decontamination of the sample. Also, the combined use of tetrasulfatoceric acid and sulfuric acid is not effective at 20 ° C, but is very effective at 100 ° C, and the combined use of sulfuric acid at a concentration of 5 to 6% is much more effective than that of sulfuric acid alone. .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Robert William White Frontenac Rhode, New Kensington, Pennsylvania, USA 103 (56) References Japanese Patent Publication 3-10920 (JP, B2)

Claims (12)

[Claims] 1. A method for removing contaminants from a surface contaminated with a radioactive material, the surface being selected from the group consisting of tetrasulfatoceric acid, hexasulfatoceric acid, hexaperchloratoceric acid or mixtures thereof. Contacted with an aqueous solution containing 0.5 to 3% of ceric acid and 1 to 5% of an inorganic acid forming a complex compound with the ceric acid, and the aqueous solution is continuously circulated on the surface, in which case the A method for removing contaminants from a surface contaminated with radioactive material, which comprises oxidizing cerium (III) to cerium (IV). 2. The method according to claim 1, wherein the ceric acid is tetrasulfatoceric acid. 3. The method according to claim 1, wherein when the ceric acid is tetrasulfatoceric acid, the inorganic acid is sulfuric acid. 4. The method according to claim 1, wherein when the ceric acid is hexasulfatoceric acid, the inorganic acid is sulfamic acid. 5. The method according to claim 1, wherein when the cerium acid is hexaperchloratocerium acid, the inorganic acid is perchloric acid. 6. The method according to claim 1, wherein the temperature of the aqueous solution is 7 to 200 ° C. 7. The surface of claim 1, wherein the surface is a metal surface in a spent steam generator from a pressurized water reactor.
Method described in section. 8. The method of claim 1 including the additional step of contacting the surface with a cerium acid solution and then contacting the surface with a contaminant removal solution containing an organic acid and a chelate. 9. The method according to claim 1, wherein the cerium (III) is oxidized with ozone. 10. The cerium (III) is converted to cerium (IV)
The method according to claim 1, wherein the aqueous solution is passed through a hydrogen-type cation exchange column after being oxidized to form. 11. The method according to claim 10, wherein the cation exchange column is a strong acid cation exchange column. 12. The method according to claim 10, wherein before or after passing the aqueous solution through the column, the aqueous solution contains uranyl ions and / or purnyl ions, and these ions are extracted with an organic solvent.