JPS601600B2 - Method of chemical decontamination of nuclear reactor structural parts - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the method for chemically decontaminating nuclear reactor structural parts described in Japanese Patent Application Laid-Open No. 11820/1982, "namely, a method for chemically decontaminating nuclear reactor structural parts using an aqueous alkaline permanganate solution at 85 to 1250C.
After an oxidation pretreatment for 5 to 20 hours, there is an intermediate wash with deionized water, followed by a suppressed citrate-oxalate decontamination aqueous solution with a pH value adjusted to about 3.5. 8
The present invention relates to an improvement in the method for chemically decontaminating structural parts, components, subsystems and complete systems, especially of water-cooled nuclear reactors, by soaking with 5 to 12,500 ml of water and then washing again with deionized water.

After a short period of operation in the primary circuit of a nuclear power plant, a dense oxide film forms over the entire surface due to corrosion of the structural materials.

This initially inert oxide film becomes contaminated during operation, even in components not located in the direct radioactive area of the core region. This is due to the formation of radioactive corrosion products. This process is continuous and results in an enrichment of nuclides with particularly long half-lives in the oxide film. This surface contamination must therefore be removed by suitable decontamination methods. This need becomes increasingly critical as operating times increase.
This is because there is a risk that workers will be exposed to increasingly high levels of radioactivity during maintenance inspections and repair work on equipment. Attempts have already been made to decontaminate contaminated surfaces with aqueous solutions of mineral and organic acids.

However, the results obtained thereby were not sufficient, especially since at the same time damage to the structural material itself occurred. Only this step-type AP public carbon method (alkaline permanganate-ammonium-citrate) has a good beam removal coefficient, but it also causes selective corrosion phenomena depending on the structural material. This results in unacceptable and severe corrosion of Motomura metals. Furthermore, in the case of the APAC method, the beam removal aqueous solution is suppressed by sulfur-containing substances. However, sulfur compounds must not be introduced into the primary circuit system of water-cooled reactors. This is because sulfur causes selective corrosion of the Ni alloy during long-term operation. In order to overcome this drawback, the method described at the beginning was developed.

This method has since been validated through tests and used in practice. The present invention continues this process with a citric acid/hydrogen peroxide solution containing suspendable solid particles.
A post-treatment of 2 to 8 hours at .about.80 oo is proposed and it has been found that an improvement and development of the method is thereby achieved. In order to facilitate understanding, the entire processing method will be described in detail below, including all the target parts of the previous invention.

The aqueous alkaline permanganate solution for acidic pretreatment contains 10 to 50 parts of caustic soda solution and 5 to 30 parts of potassium permanganate per 1000 parts of water.

In this preliminary oxidation, it is important that the treatment time be within 2 hours. This is because scale-soluble soft manganese (Mn)
This is because there is a risk that 02) will be multiplied. On the other hand, aqueous solution containing citrate is 2 parts per 1,000 parts of water.
The first three ingredients listed include a composition of a rusting agent and an organic acid. The beam removal coefficient is increased by these factors.The content of oxalic acid is especially important for the beam removal coefficient.
The value of 40 oxalic acid per 000 indicates the upper limit. If the oxalic acid concentration exceeds this value, there is a risk of oxalate formation on the surface of the component. Furthermore, there is no possibility that the beam removal coefficient can be further increased by increasing the oxalic acid value. When producing an aqueous solution for removing beams, it is important that citric acid should be included in the solution in a larger amount than oxalic acid.

This is because citric acid and ethylenediaminetetraacetic acid have the problem of stripping poorly soluble oxalate from the surface of the component. The ratio of citric acid/oxalic acid/ethylenediaminetetraacetic acid should therefore be 12.5:10:1. If ethylenediaminetetraacetic acid is not added, the amount of citric acid must be doubled.
In addition to the concentration of oxalic acid, the pH value also has a decisive influence on the beam removal coefficient. For decontamination treatment, it is important to constantly maintain the pH value at 3.5±0.5. When the pH value exceeds pH 4,
Decontamination effectiveness will decrease. On the other hand, pH below pH 3
At higher values, the risk of selectively damaging the substrate increases. Ammonia is used in the above-mentioned pH adjustment as is well known. Divalent and trivalent metal salts of organic acids are used to suppress aqueous decontamination solutions. The above-mentioned value of iron formate (m) 5 kg indicates the lower limit and must not be exceeded. If less than this amount of inhibitor is added, the substrate will corrode and the structural materials will be selectively damaged. Maintaining the processing temperature mentioned above is important for decontamination results.

Below 85 qo, Fe, Cr, Ni oxides (spinel) are only incompletely and only very slowly dissolved by the alkaline permanganate solution.

Similarly, when the beam removal aqueous solution is lower than 8500, Fe, Cr and M
Dissolves oxides very slowly and incompletely. 1
00qo is the boiling temperature of water.

This temperature, and therefore the beam removal coefficient, can be increased by increasing the pressure, but it must not exceed a temperature of 125°C, because above this the organic components of the beam removal aqueous solution will decompose. Leg time should be strictly adhered to.

This is because if the treatment with the decontamination aqueous solution exceeds 20 hours, the particle limit region of the structural material may be attacked. The processing time depends on the material of construction and the type of contamination at the time. Generally speaking, 6 to 1 hours of beam stripping is sufficient to remove contamination. The suspension solution contains citric acid≧1.0%, hydrogen peroxide≧0.5%, perfluorocarboxylic acid 0.1-0.5%, and cellulose fiber 0.0% per 1000 parts of water.
Includes evenings 1-5.

In this beam removal post-treatment process, it is important to move the fiber suspension solution vigorously. This is because the fibers need to wipe the surface of the part. Vigorous movement of the solution can be effected by means of pumps or air blowing, methods known per se. The inert fiber material has the purpose of removing loosely adhered residual oxide films that still remain on the surface after the previous two-stage treatment by a simple mechanical scavenging action. Organic or inorganic fibers and pieces of cloth made of these fibers are used as inert agents. Sponge rubber bulbs are used instead of organic fiber materials in small tube systems and heat exchangers. It is preferable that this soft sphere has a diameter 0.1 to 0.3 times larger than the inner diameter of the pipe to be removed. The fiber material is 0.1 to 5.
It is necessary to maintain the above concentration at 0 pm. This is because if the concentration is lower than this, the scavenging effect is too low, and if the concentration is higher than this, the movement of the solution and the pumping ability are no longer guaranteed. At the same time during the beam removal post-treatment, in order to remove the slightly soluble iron oxalate (0) that may have occurred in the previous two-stage treatment by converting it into easily soluble iron oxalate (m), a suspension solution is used. Add hydrogen peroxide.

The risk of iron(0) oxalate occurring is particularly 13% and 17%.
%Cr steels and also in destabilized CrNi steels. However, since hydrogen peroxide simultaneously oxidizes oxalate to CO2, an organic carboxylic acid, dicarboxylic acid, oxycarboxylic acid, or hydroxycarboxylic acid is further added to the suspension solution to oxidize the liberated iron ions. . If these acids are not added, the iron will be newly hardened. Addition of a reticulating agent significantly lowers the surface tension of the suspension solution. This allows the fiber to sweep its surface intensely. The standard concentration of the silking agent added is the concentration specified by the manufacturer in each case. All organic reticulating agents which do not contain sulfur-containing compounds can be used in this case. In all two treatment steps, namely pre-oxidation treatment, decontamination treatment and post-beam removal treatment, it is important that these solutions are free of sulfur-containing compounds.

In the primary circuit of a nuclear reactor, the use of sulfur-containing substances is prohibited because, in the case of Ni alloys, nickel/sulfur compounds are formed which at high temperatures create brittle phases in the structural material. Moreover, in the steam generator of the primary circuit, various operating conditions can lead to the formation of polythionic acids which, even at room temperature, can cause internal crystalline corrosion in Inconel 600. The beam removal method described above has already been used in large-scale beam removal operations in nuclear power plants with very good results in practice.

Processed samples subjected to this beam stripping process showed, as a result of subsequent metallographic analysis, that the beam stripping process according to the invention never caused selective damage to these parts. Material losses were less than 0.1 French in all cases. Various examples are then detailed with tables showing the results of large-scale decontamination treatments as well as the parts tested. This table shows that in the decontamination treatment described above, highly alloyed C and Nr steels, Ni alloys and highly alloyed Cr steels can all be decontaminated with high beam removal coefficients without damaging the base material.

Wall S 'in* Ship's flea [Homabi also staff q ・ Member's Atsushi < ■ Turtle A q K ・Tori gmi SI stacked seven tons ﾆ hair ☆ Cod cake three meals [Kanuka 3. In the large-scale decontamination treatment at the nuclear power plants listed in the tables above, there are obstacles during repair work. It was necessary to decontaminate part of the primary circuit to remove radioactive contamination.

This beam removal process decontaminated all parts of the circuit as well as the parts area. Parts that could be easily dismantled were disposed of by immersing them in a bathtub. Areas of the primary circuit that could not be disassembled were separated by isolation devices, and the solution was energized using an external beam-stripping circulation device. It has been shown that when a three-stage beam removal treatment is carried out, the third treatment step can increase the decontamination factor by an additional 5 to 1 pep depending on the construction material and the type of contamination targeted. Since the used decontamination aqueous solution itself is radioactive, the radioactivity must be attenuated.

In this case it is important to obtain a significant reduction in volume. In the present invention, both aqueous solutions, the oxidizing aqueous solution and the beam removal aqueous solution, are mixed, whereby oxalic acid is oxidized to C02 and KMn04 is reduced to Mn. When the mixing ratio is 1:1, the aqueous solution pretreated in this manner is concentrated by about 80% without salt precipitation due to evaporation.
In this case, chemical and physical methods known per se can be used for this concentration process up to final storage. Therefore, according to the method based on the present invention, not only can nuclear reactor structural parts contaminated with radioactivity be thoroughly removed without having a substantial negative effect on the base material, but also the used aqueous solution can be relatively removed. Can be easily concentrated.

Claims (1)

[Claims] 1. 85° to 125°C with alkaline permanganate solution
After pre-oxidation treatment for about 1 hour with
This is followed by immersion in a suppressed citrate-oxalate decontamination solution with a pH adjusted to about 3.5 for 5 to 20 hours, also at 85° to 125°C, and then again in deionized water. A method for chemically decontaminating nuclear reactor structural parts by cleaning, followed by post-treatment with a citric acid/hydrogen peroxide solution containing suspendable solid particles at 20-80°C for 2-8 hours. Characteristic method for chemically decontaminating nuclear reactor structural parts. 2. The method according to claim 1, characterized in that an alkaline potassium permanganate solution having the following composition is used: 10 to 50 g of caustic soda solution, 5 to 30 g of potassium permanganate, and 1000 ml of water. 3. Claim 1, characterized in that the permanganic acid solution is alkalized using an alkali metal hydroxide.
The method described in section. 4 The decontamination solution contains the following substances per 1000 ml of water: citric acid 25-50 g oxalic acid 20-40 g ethylenediaminetetraacetic acid 2-4 g iron (III) formate ≧5 g and ammonia to adjust the pH value to 3.5. A method according to claim 1, characterized in that the method comprises: 5. Process according to claim 1, characterized in that all solutions used are free of sulfur and sulfur-containing compounds. 6. The method according to claim 1, characterized in that the decontamination solution is inhibited by divalent metal salts and trivalent metal salts of organic acids. 7. The method according to claim 4, characterized in that the ratio of oxalic acid/citric acid/ethylenediaminetetraacetic acid in the decontamination solution is 10:12.5:1. 8. The method according to claim 1, characterized in that the pH value in the contaminated solution is always maintained at 3.5±0.5. 9. The suspension solution is characterized in that it contains the following substances: citric acid ≧1.0g hydrogen peroxide ≧0.5g perfluorocarboxylic acid 0.1-0.5g cellulose fiber 0.1-5g H_2O 1000ml A method according to claim 1. 10 The following dimensions length as an inert substance in suspension solution:
2. A method according to claim 1, characterized in that fibers made of an organic or inorganic substance are used, having a diameter of 0.5 to 15 mm, a density of ≧1 g/cm^3, and a density of ≧1 g/cm^3. 11. The method according to claim 1, characterized in that cloth pieces with a size of 0.2 to 4 cm^2 are used in the suspension solution. 12 Foamed organic material in suspension solution, e.g.
2. The method according to claim 1, characterized in that sponge rubber balls of mm are used. 13. Process according to claim 1, characterized in that organic carboxylic acids, dicarboxylic acids and hydroxycarboxylic acids are used as acids in suspension. 14. Process according to claim 1, characterized in that a conventional commercially available organic substance is used as relaxant in the suspension solution.