JPS62144100A - Method of removing contaminant on surface contaminated by radioactive substance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Deposits containing secondary radioactive elements often form within the cooling systems of nuclear reactors. Removal of these radioactive element-containing deposits is necessary for the safe maintenance and repair of cooling systems. For example, this can be done by using an oxidizing solution of alkaline permanganate followed by a decontaminant solution of oxalic acid, citric acid, and ethylenediaminetetraacetic acid (EDTA). These solutions solubilize radioactive metal ions and other ions in the deposit. The solution is circulated between a cooling system and an ion exchange resin, where ions are removed from the solution.

Many effective contaminant removal and oxidizing solutions have been found that remove deposits easily, are inexpensive, and are easy to dispose of.
The volume of the object is small (e.g. used ion exchange resin)
Improved solutions are always needed. Also, since reactor downtime to remove deposits is very expensive, a solution that can clean the cooling system more quickly can save costs significantly.

Another problem in the nuclear crab industry is the disposal of steam generators at the end of their useful life.

Since steam generators have high radioactivity, it is necessary to construct a radioactive containment building around the steam generator to prevent radiation from escaping. Contaminant removal solutions have not been effective in reducing the radioactivity of the steam generators described above to levels that do not require radioactive containment buildings.

Still other problems in the nuclear crab industry arise during the mining, processing/reprocessing, and manufacturing of nuclear fuel. Not only will the equipment become contaminated with radioactivity, but the contaminants will also contain uranium and plutonium, which, if quickly and inexpensively recovered from the contaminated equipment, could be used in the production of nuclear fuel. .

Today we have found that a complex solution of ceric acid and inorganic acid is very effective for removing deposits from the cooling system of a nuclear reactor in a certain critical concentration range.

This solution is so effective, in fact, that it alone can remove at least 97% of the radioactivity in the cooling system, obviating the need for separate oxidizing and decontaminant solutions. Additionally, we have found that the solution can remove radioactivity from spent steam generator deposits to an extent that does not require storage of the used steam generator in specially constructed radioactive containment buildings. Furthermore, a used steam generator can be safely stored in the outside world by sealing its opening by welding.

We have also found that when circulating the ceric acid solution, the ceric 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 tower that removes radioactive metal cations such as cobalt. This is because the solution is able to oxidize and remove much more radioactive deposits than other methods. The solution also reduces the amount of radioactive waste that must be disposed of.

Furthermore, we have found that the uranium or plutonium present can be recovered by extraction from the ceric acid solution. In this way, small amounts of uranium or plutonium that would otherwise be lost and required to be disposed of as transuranium (TRU) radioactive waste can be recovered in gold quantities, making nuclear fuel Can be used for manufacturing.

Related US Pat. No. 4,162,229 discloses the use of cerium (IV) salts to remove contaminants from metal surfaces in nuclear reactors. Acids such as sulfuric or nitric acid may be present.

DESCRIPTION The principles of the present invention can be applied to the cooling system of any nuclear reactor, including pressurized water reactors, boiling water reactors, and gas cooled reactors. When contaminants are removed from the entire reactor, the operation of the reactor is first shut down. Shutdown means reducing the temperature of the coolant to 21-93°C (70-200°C). Next, ceric acid and inorganic acid are added directly to the cooling water. When removing contaminants from a portion of a cooling system, such as a steam generator, or other equipment contaminated with radioactivity, such as from nuclear fuel plant equipment, the equipment is drained and an aqueous solution is created by firstly This aqueous solution is circulated through the apparatus.

The ceric acid solution of the present invention is an aqueous solution comprising one or more of three types of ceric acid solutions and an inorganic acid that forms a complex compound with the ceric acid solution. The ceric acid used in this solution is tetrasulfatoceric acid [H=C
e(SO4)-2゛Usually called cerium sulfate]
, hexasulfamate ceric acid [H2Ce (S
O: + N I-12) a, usually called cerium sulfamate], hexaperchloradocerate [
H2Ce(CIo4)6, commonly referred to as cerium perchlorate] or mixtures thereof. Among the three types of ceric acids, tetrasulfatoceric acid is preferable because it requires less IK food.

The use of hexaperchloradoceric acid is limited to the treatment of used cooling system equipment due to the presence of chlorine in the acid solution. Hexaperchloradoceric acid can produce chlorides, which can cause stress+g corrosion cracking in 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 because ceric acid oxidizes organic acids, wasting ceric acid and increasing the amount of waste products to be treated. Inorganic acids that do not form complexes with ceric acid are not suitable because compounds that do not form complexes are very difficult to react with. Preferably, the inorganic acid should correspond to the ceric acid in solution. For example, sulfuric acid can be used when the ceric acid is tetrasulfatoceric acid, sulfamic acid can be used when the ceric acid is hexasulfamate ceric acid, and sulfamic acid can be used when the ceric acid is hexaperchloradoceric acid. perchloric acid can be used. The combination of ceric acid and inorganic acid mentioned above more easily forms complex compounds and monitors during disposal of the solution.
This means fewer types of ions need to be processed. Although the use of the corresponding acids is preferred, other inorganic acids which form complexes with ceric acids, such as nitric acid, can also be used.

We have found that in the absence of minimal amounts of inorganic acid and ceric acid, no complex is formed. That is, the concentration of ceric acid and inorganic acid in the solution is considered important to the effectiveness of the solution in performing contaminant removal on metal surfaces. The concentration of ceric acid in the solution is about 0.5~
3% (all percentages herein are weight percentages based on the weight of the solution). Ceric acid is 0.
Below 5%, there is virtually no contaminant removal effect, and there is no need for ceric acid to exceed 3%, which also increases waste volume without causing contaminant removal. Also, the presence of a larger amount of ceric acid requires the presence of a larger amount of inorganic acid, but the presence of a larger amount of inorganic acid will corrode the metal surface more. The concentration of inorganic acid in the solution is about 1-5%.

When less than 1% of inorganic acid is used, even higher concentrations of ceric acid are virtually ineffective in removing contaminants from metal surfaces. More than 5% inorganic acid corrodes metal surfaces too much and unnecessarily increases waste volume.

The temperature of the solution should be about 70-200°C.

We have found that virtually no contaminant removal occurs at temperatures as low as room temperature (ie, 20-25°C).

However, at temperatures above about 200'C, the solution becomes too corrosive to metal surfaces.

The ceric acid solution is circulated through the apparatus until the radioactivity level of the solution stabilizes. That is, the solution is circulated until the radioactivity level of the solution exiting the device is no longer higher than the radioactivity level 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 the ceric acid solution alone removes at least 97% of the radioactivity, some additional residual radioactivity can be removed by using 1D River Contaminant Removal Solution after using the ceric acid solution. A conventional contaminant removal solution is a mixture of a chelate compound such as ethylenediaminetetraacetic acid or nitrilotriacetic acid and a dihydric acid such as citric acid or oxalic acid. Conventional contaminant removal solutions are passed between the apparatus and the cation exchange resin column at temperatures of 70°C to about 200'C until the radioactivity level of the solution exiting the apparatus is no longer higher than the radioactivity level of the solution entering the apparatus. Circulate the solution. The device is then rinsed with deionized water and conventional contaminant removal operations are completed. The spent cerium solution can be purified using an anion exchange resin-cation exchange resin mixture or neutralized using hydroxide and evaporated to be disposed of as solid waste. Spent contaminant removal solutions can be purified using anion exchange resins or ion exchange resin mixtures.

Degradation of cerium(IV) in solution can be caused by replacing cerium(Ir[) with cerium(IV) before recycling the solution to contaminated equipment.
Prevention 11: can be achieved by oxidizing to V). This oxidation can be carried out, for example, by adding an oxidizing agent such as ozone or peroxide to the solution, or by electrolyzing the solution. The use of ozone is preferred because it has the highest oxidizing potential, is the most reactive oxidizing agent, 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, or the ozone can be formed electrically by the use of diaphragms at appropriate locations. (Also, the electrolysis used to form ozone can be used instead of a cation exchange tower to remove transition metals or other metals.) Cerium (I [[) to Cerium (IV) After oxidizing,
It is advantageous to pass the solution through a hydrogen-type cation exchange resin. Cation exchange resins remove radioactive metal ions such as iron, cobalt, and nickel that do not form complexes with cerium (IV) acid solutions. Cerium (III) does not form a strong anionic complex and is removed in the cation exchange tower, but cerium (■) forms a strong complex with an inorganic acid and passes through the cation exchange tower. Second, the oxidation of cerium (III) to cerium (IV) must be carried out before the solution is sent to the cation exchange column. The cation exchange column must be of the hydrogen type (ie, releases hydronium ions), and strong acid type ion exchange resins such as sulfonic acid type resins or chelating type resins are preferred. Contaminant metal ions are removed in the resin tower to liberate cerium complexes, which can be further used to remove contaminated metal ions from contaminated equipment.

Uranyl ion or prunyl ion (respectively U O
22+, believed to be PL1022+), uranium and plutonium also pass through the resin column to form anionic complexes with inorganic acid anions.

The uranium and plutonium can be recovered from the solution either continuously before recycling the solution to the contaminated equipment or after the equipment has been decontaminated. Recovery of uranium and plutonium can be performed by methods known in the industry. These methods use tributyl phosphate in decane or DEPA-TOPA [di(
2-ethylhexyl)phosphate-tri(n-octyl)phosphine oxide]. Uranium and plutonium can also be recovered using anion exchange resin columns (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, fewer metal ions will be present and less radiation, especially gamma rays. Therefore, it is preferable to pass through an ion exchange resin tower.

The present invention will be further explained by giving examples below.

1iifl For approximately 38m in length from the steam generator of a pressurized water reactor <1
．． Samples of tubing approximately 19 mm (3/4 inch) in diameter were cut in half lengthwise. The samples were loaded into beakers containing various contaminant removal solutions (in some experiments the solutions were circulated through the samples in the beakers). Treat individual samples with contaminant removal solution.

After treatment, the pollutant removal coefficient was measured. [The contaminant removal factor (DF) is the value obtained by dividing the radioactivity value (microcuries) before contaminant removal treatment by the radioactivity value (microcuries) after treatment. ] The table below describes the processing procedure for 11 different samples treated with various contaminant removal solutions at different times and temperatures. In the table, rCMJ is a commercially available contaminant removal solution that appears to contain 30% citric acid, 30% oxalic acid, 40% ethylenediaminetetraacetic acid, and an inhibitor believed to be thiourea.

rCAS-, is cerium ammonium sulfate, rC
A N J is cerium ammonium nitrate, rT
S CA J is tetrasulfatoceric acid.

lOOoC48°C111111095°C
100℃-pl+2.60.5XCM, 24 hours
0.1lKFeO 14 hours 0.5$CM, 23 hours
0.1$CAS, 6 hours 100°C48°C, pHlO
'95℃ 100℃, pl
+2.60.5ZCM, 4 hours 0.5$CAS, 6
Time 0.5gCM, 4 hours 100℃
100°C 100°C 7.5 hours, 100
℃ 6 hours, 100℃ 6 hours, 70℃ 100℃ 0.5KCM, 10 hours -1--1,05--1,
001,05100°C 0.5gCM, 12 hours ---1,05--1,
001,05100℃ 1.10 0.94 1.01
1.051.01 1.08 0.8
9 0.971.1+
0.95 1.34 0.8B
1.241.03 1.03 1.03
1.090.5$CN, 4
Time 1.05 1.22 1.03 60.
60 0.97 77.83100℃ 1.03 1.02 1.00
1.051.07 1.11 0.9
4 1.111.06
1.13 1.02
1.23 The above table shows that cerium ammonium nitrate is not effective in decontaminating samples.

It also shows that while tetrasulfatocerium is very effective, it is not effective at concentrations of 0.25% or less.

Example-? engineering Example 1 was repeated. The results obtained are shown in the table below.

□・:+, t:1, expansion (t゛' The above table shows that 1% sulfuric acid alone is not effective, 5% sulfuric acid is not effective at 22℃, but it is accompanied by considerable corrosion at 100℃. (A temperature of 100°C is also a high temperature in which the amount of material that can be obtained without boiling the solution is also high.) The above table also shows that the cerium ammonium nitrate-nitric acid solution is a contaminant in the sample. Furthermore, the combination of tetrasulfatoceric acid and sulfuric acid is also not effective at 20°C, but is very effective at 100°C, and the combination of sulfuric acid at a concentration of 5-6% is It was much more effective than sulfuric acid alone.

Claims (1)

[Claims] In a method for removing contaminants from a surface contaminated with radioactive materials, the surface is treated with a ceric acid selected from the group consisting of tetrasulfatoceric acid, hexasulfamate ceric acid, hexaperchlorate ceric acid, or a mixture thereof. A method for removing contaminants from a surface contaminated with radioactive substances, the method comprising contacting with an aqueous solution containing 1 to 5% of an inorganic acid that forms a complex compound with the ceric acid.