EP0355628B1 - Process for chemically decontaminating the surface of a metallic construction element of a nuclear power plant - Google Patents

Description

The invention relates to a method for chemical decontamination of the surface of a metallic component of a nuclear reactor plant.

In order to reduce the radiation exposure of the personnel during control, maintenance and repair measures on components and circulation systems in pressurized water reactor systems or in boiling water reactor systems, radioactive oxide layers must be removed from the surfaces of the components to be treated or tested. A suitable method with chemical decontamination is known for example from German patent specification 26 13 351. In the known method, decontamination takes place in two steps or stages. First, an oxidative treatment with alkaline permanganate solution is carried out as a first step. The second step involves contacting the components with a citrate oxalate solution, in which an essential component is oxalic acid.

All other known decontamination processes also run in two stages, with oxalic acid always being used to detach deposits, in particular to detach oxide deposits. Known decontamination processes see e.g. B. as a first stage oxidation with manganese acid (HMnO₄), with nitric acid (HNO₃) in conjunction with potassium permanganate (KMnO₄) or with sodium hydroxide (NaOH) in combination with potassium permanganate (KMnO₄) before. In the second stage, the oxides are then detached from the surface to be decontaminated; complexing organic acids are used as reducing agents, and often only oxalic acid is used. In all other known cases, a mixture of different acids is used used, in which oxalic acid always forms an essential component.

Processes for the chemical decontamination of surfaces of metallic components of nuclear reactor plants which do not require oxalic acid have hitherto not been known.

However, the use of oxalic acid in a decontamination process is disadvantageous for the success of the process. Oxalic acid, for example, causes an intergranular attack on sensitized materials, for example in the area of a weld. In addition, the use of oxalic acid in the presence of heavy metals leads to the precipitation of heavy metal oxalates. When components of a nuclear reactor plant are decontaminated, oxalates of manganese, cobalt, nickel and iron can fail. Since the metals mentioned contain radioactive isotopes, the precipitation of the oxalates leads to a new contamination of the surfaces of the components during the decontamination process. So there is a so-called recontamination. The likelihood of recontamination is particularly high if the components to be decontaminated are made of nickel-based alloys, such as Inconel 600.

The components and systems to be decontaminated in a nuclear reactor plant generally consist of different materials. As a result, different oxides can also be removed with the decontamination process. Compared to a specific decontamination process, each oxide type shows a specific dissolution behavior. A component, for example a pump housing, which consists of two different materials, such as a nickel-based material and an iron-based material, cannot be optimally decontaminated with any of the known decontamination processes, which always use oxalic acid, with a one-time sequence of the two cleaning steps. Rather, a separate, specific decontamination process is usually necessary for each material on the component.

From BE-A-689 498 a method for cleaning metallic surfaces is known, in which organic acids containing OH groups are used. Such an acid can be a mixture of citric acid and methyl tartronic acid.

The invention has for its object to provide a cost-effective method for chemical decontamination of the surface of a metallic component of a nuclear reactor plant, which excludes recontamination by precipitation, which does not attack sensitized materials, for example in the area of weld seams, and on all materials used for the metallic components to be decontaminated are usually achieved with an equivalent good decontamination success. With just one run of the method, components that are made up of several materials should also be able to be completely decontaminated.

The object is achieved according to the invention in that the surface is treated in a one-step process with an aqueous solution which is free from the carboxylic acid oxalic acid and contains another carboxylic acid which is converted into a further carboxylic acid by a chemical or thermal process.

This conversion can take place directly in the aqueous solution which is intended for the treatment of the surface. However, the conversion can also take place in a process step preceding the actual decontamination. With a conversion of a carboxylic acid into another carboxylic acid, the advantage is achieved that, starting from an inexpensive carboxylic acid, a carboxylic acid can be obtained which ensures very good decontamination success, but which would be difficult to obtain commercially since it is either not available or very expensive is.

The method according to the invention also has the advantage that recontamination is avoided. Heavy metal salts of carboxylic acids other than oxalic acid are much more soluble than oxalates. The fact that only oxalic acid takes the place of oxalic acid in the process according to the invention prevents recontamination of the surfaces. It is essential not only to use carboxylic acids other than oxalic acid, but also to completely dispense with the smallest amount of carboxylic acid oxalic acid in the aqueous solution. Carboxylic acids other than oxalic acid are able to dissolve iron oxides and also nickel oxides and, which is essential, to keep them in solution. These can then be easily removed. In addition, as confirmed by tests, the method according to the invention achieves the advantage that sensitized materials are not attacked intercrystalline.

A further major advantage is the fact that the so-called "decontamination factor" when using the method according to the invention is significantly higher than with chemical decontamination with oxalic acid. The "decontamination factor" is the quotient of the dose rate of a component to be decontaminated before the treatment and the dose rate of the same component after the treatment. With the same acid concentration, the advantage of the method according to the invention is achieved that far higher decontamination factors are achieved than would be possible using oxalic acid, without the risk of recontamination due to the precipitation of previously detached radioactive nuclides on the cleaned metal surface. Since the method according to the invention can be used with the same success in all materials used in the nuclear field, components and systems consisting of several materials can advantageously also be decontaminated, such as a pump housing which partly consists of an iron-based material and partly of a nickel-based material. However, large decontamination factors are also achieved with the method according to the invention on components consisting of only one material. While only one decontent factor 140 could be achieved with the carboxylic acid oxalic acid in one test series under the same conditions, other carboxylic acids, namely dihydroxy tartaric acid in combination with pyridine-2,6-dicarboxylic acid, led to a decontent factor 650.

With the method according to the invention, surfaces of components which consist of a single material or even of several materials can thus be decontaminated better than was previously possible. In addition, there is no recontamination by precipitation. In addition, the resistance of sensitized materials, for example in the area of a weld, is not impaired. There is no intercrystalline attack.

Finally, the fact that the method according to the invention is a one-step process achieves the advantage that intermediate steps, such as rinsing steps, which would be necessary in a multi-step process, can be omitted. So you come with one short decontamination time.

The surface of the component to be decontaminated is treated, for example, with an aqueous solution which contains at least one ketocarboxylic acid. According to other examples, the solution can contain at least one hydroxycarboxylic acid or a mixture of at least one ketocarboxylic acid and at least one hydroxycarboxylic acid. A particularly suitable ketocarboxylic acid is mesoxalic acid. Particularly suitable hydroxycarboxylic acids are tartronic acid and dihydroxy tartaric acid.

With all of these carboxylic acids, the stated advantages of the process according to the invention are achieved particularly clearly. Glyoxylic acid and hydroxyacetic acid are also suitable for the process according to the invention.

Even after acidic preoxidation, better decontamination results are achieved than with oxalic acid with another carboxylic acid, for example with tartronic acid, mesoxalic acid and dihydroxy tartaric acid.

At least one complexing agent can advantageously be added to the aqueous solution. This significantly improves the decontamination effect of ketocarboxylic acids and hydroxycarboxylic acids.

A suitable complexing agent is a chelating agent, such as, for example, ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and nitrilotriacetic acid (NTA), or else a pyridinecarboxylic acid, such as, for example, 2-picolinic acid or dipicolinic acid.

A particularly good decontamination result is achieved, for example, after alkaline preoxidation with a ketocarboxylic acid or a hydroxycarboxylic acid, if this is combined with a pyridine carboxylic acid as a complexing agent. The decontamination factors then achieved are greater than 100. Decontamination factors up to 650 are achieved.

Tables 1 and 2 use examples for the decontamination of austenitic CrNi steel and for the decontamination of a nickel alloy to give decontamination factors that can be achieved when using the decontamination solutions according to the invention and for comparison when using oxalic acid.

To set a certain redox potential, the aqueous solution can contain, for example, hydrogen peroxide or hypophosphite. This advantageously increases the rate of dissolution of different oxide forms in the decontamination solution.

Tartronic acid can only be stored refrigerated at temperatures between 0 ° C and 4 ° C. In addition, tartronic acid is very expensive. It is therefore envisaged, for example, that a solution containing easily stored dihydroxy tartaric acid is brought into contact with the surface to be decontaminated, and that this solution is then heated to form tartronic acid. With certain materials, better decontamination is achieved with tartronic acid than with dihydroxy tartaric acid. The advantage is achieved that tartronic acid is produced directly in the decontamination solution from easily stored dihydroxy tartaric acid.

Of course, the tartronic acid can also be formed from dihydroxy tartaric acid in a process step preceding the decontamination. The tartronic acid thus formed is then used for decontamination.

In contrast to tartronic acid, dihydroxy tartaric acid is easy to store, but it is hardly available commercially. The dihydroxy tartaric acid is therefore preferably prepared from its salts, in particular from its sodium salt, which is readily and inexpensively available.

Mesoxalic acid can also be prepared from its salts, in particular from its sodium salt.

For example, the acids mentioned are prepared from their salts by ion exchange.

Mesoxalic acid can also be obtained from tartronic acid instead of from its salts. For this purpose, hydrogen peroxide is added to the aqueous decontamination solution, which contains tartronic acid, which can already be made from dihydroxy tartaric acid, which leads to the formation of mesoxalic acid from the tartronic acid. This has the advantage that the mesoxalic acid is also obtained from a salt of dihydroxy tartaric acid. The dihydroxy tartaric acid made from its salt is heated to give tartronic acid. This is then added with hydrogen peroxide, which leads to the formation of mesoxalic acid.

The formation of mesoxalic acid from tartronic acid and hydrogen peroxide can, for example, also take place in a separate container, after which the mesoxalic acid formed is introduced into the decontamination solution.

For decontamination with mesoxalic acid, a solution is brought into contact with the surfaces to be decontaminated, which contains dihydroxy tartaric acid, which is produced from an inexpensive salt of this acid. The solution is then heated to form tartronic acid. Hydrogen peroxide is then added to the solution to form mesoxalic acid from the tartronic acid.

In this way, mesoxalic acid is advantageously formed in the decontamination solution from an inexpensive substance, such as the sodium salt of dihydroxy tartaric acid.

Suitable acids to replace the oxalic acid are also hydroxyacetic acid and ketoacetic acid. Hydroxyacetic acid can be formed from tartronic acid by heating. Ketoacetic acid can be formed from either mesoxalic acid by heating or by adding hydrogen peroxide from hydroxyacetic acid.

The treatment of the surface with the aqueous decontamination solution can be preceded by an oxidation step which is carried out in an acidic or alkaline medium. This oxidation step takes place, for example, in the presence of permanganate. With this preliminary stage, the success of decontamination is increased. From case to case, the treatment of the surface with the aqueous decontamination solution can also be preceded by several oxidation steps, alternately in the acidic and in the alkaline medium.

The oxidation solutions present after the oxidation step, which contain, for example, permanganate, can be destroyed and neutralized with a supplied carboxylic acid, which can be part of the aqueous decontamination solution. For example, the acidic or alkaline oxidation solutions mentioned can be destroyed by mesoxalic acid or by tartronic acid. Oxalic acid is not required to reduce the permanganate.

After the surface of the metallic component has been treated, the decontamination solution, which may contain radioactive substances, is preferably fed to an evaporator. There the volume of the solution to be disposed of is reduced.

For example, the solution to be disposed of can also be fed to an ion exchanger in which radioactive ions are retained.

Dicarboxylic acids still contained in the solution to be disposed of are thermally broken down to monocarboxylic acids, for example. An evaporator is usually used for this.

If the system to be decontaminated has a closed circuit, the decontamination solution can be pumped around the system via a cleaning device to increase the decontamination effect during the treatment of the surface of the metallic component that is part of the system. Such a system can be the primary circuit, but also the auxiliary system of a nuclear reactor plant.

If a single component, such as a pump housing, is to be decontaminated, then this is inserted into a container of a decontamination system. In addition to the container, the decontamination system has a pump and a cleaning device which are connected by lines and form a circuit. The decontamination solution is pumped around in this system.

The cleaning device is, for example, an ion exchanger or a filter. For example, the cleaning device is arranged in a bypass line that is only opened during the decontamination process.

Suitable devices for carrying out the method according to the invention, such as the decontamination system mentioned, are known per se.

The method according to the invention for the chemical decontamination of surfaces has the particular advantage that a high decontamination factor is achieved without using oxalic acid can be. In addition, even heavy metal salts are kept in solution, which prevents recontamination of the surfaces with precipitated salts, which may contain radioactive isotopes. In addition, with the acids used according to the invention, there is no intercrystalline change in sensitized materials, for example in the welding area. Finally, the method according to the invention is characterized in that even components which consist of several different metals can be decontaminated with good success. The process according to the invention achieves equally good results for all materials used in nuclear reactor plants, for example for chromium-nickel steel, chromium steels and nickel alloys.

The production of individual acids which can be used according to the invention and which are used instead of oxalic acid is explained in more detail in some exemplary embodiments with reference to the drawing:
The drawing shows the conversion possibilities of the acids and their production from salts.

In the drawing, salts are symbolized as circles, acids as rectangles and conversion processes as arrows. Mesoxalic acid 3 is obtained from the sodium salt 1 of mesoxalic acid 2 by ion exchange. Analogously, 5 dihydroxy tartaric acid 6 is obtained from the sodium salt 4 of dihydroxy tartaric acid by ion exchange. Tartronic acid 8 is obtained from the dihydroxy tartaric acid 6 by thermal conversion 7. Mesoxalic acid 3 can be produced from tartronic acid 8 by reaction 9 with added hydrogen peroxide. Hydroxyacetic acid 11 can also be obtained from the tartronic acid 8 by thermal conversion 10. Ketoacetic acid 12 can be obtained from mesoxalic acid 3 by thermal conversion 14. This can also be prepared from hydroxyacetic acid 11 by reaction 13 with added hydrogen peroxide.

Claims (30)

1. Process for the chemical decontamination of the surface of a metal component of a nuclear reactor plant, characterised in that the surface is treated in a single-stage process with an aqueous solution which is free from the carboxylic acid oxalic acid and contains another carboxylic acid (3, 6, 8, 11, 12) which is converted by a chemical or heat process (7, 9, 10, 13, 14) into a further carboxylic acid (3, 8, 11, 12).
2. Process according to claim 1, characterised in that the aqueous solution contains a ketocarboxylic acid (3).
3. Process according to claim 2, characterised in that the aqueous solution contains mesoxalic acid (3).
4. Process according to one of claims 1 to 3, characterised in that the aqueous solution contains a hydroxycarboxylic acid (6, 8, 11, 12).
5. Process according to claim 4, characterised in that the aqueous solution contains dihydroxytartaric acid (6).
6. Process according to one of claims 4 or 5, characterised in that the aqueous solution contains tartronic acid (8).
7. Process according to one of claims 1 to 6, characterised in that the aqueous solution contains a complexing agent.
8. Process according to claim 7, characterised in that the aqueous solution contains a pyridine carboxylic acid as the complexing agent.
9. Process according to one of claims 7 or 8, characterised in that the aqueous solution contains ethylene diamine tetra acetic acid (EDTA) as the complexing agent.
10. Process according to one of claims 1 to 9, characterised in that the aqueous solution contains hydrogen peroxide or hypophosphite.
11. Process according to one of claims 5 to 10, characterised in that an aqueous solution containing dihydroxytartaric acid (6) is brought into contact with the surface, and in that the solution is then heated to form tartronic acid (8).
12. Process according to one of claims 6 to 11, characterised in that the aqueous solution contains tartronic acid (8) which has been formed previously from dihydroxytartaric acid (6) by heating.
13. Process according to one of claims 5 to 12, characterised in that dihydroxytartaric acid (6) is produced from one of its salts, in particular from its sodium salt (4).
14. Process according to one of claims 3 to 13, characterised in that mesoxalic acid (3) is produced from one of its salts, in particular from its sodium salt (1).
15. Process according to claims 13 or 14, characterised in that the acid (3, 6) is produced from one of its salts (1, 4) by ion exchange.
16. Process according to one of claims 6 to 15, characterised in that an aqueous solution containing tartronic acid (8) is brought into contact with the surfaces, and in that hydrogen peroxide is added (9) to the solution to form mesoxalic acid (3).
17. Process according to one of claims 3 to 16, characterised in that the aqueous solution contains mesoxalic acid (3) which has been previously formed by a reaction (9) of tartronic acid (8) and hydrogen peroxide.
18. Process according to one of claims 5 to 17, characterised in that an aqueous solution containing dihydroxytartaric acid (6) is brought into contact with the surfaces, in that the aqueous solution is then heated (7) to form tartronic acid (8), and in that hydrogen peroxide is then added (9) to the solution to form mesoxalic acid (3).
19. Process according to one of claims 1 to 18, characterised in that an oxidation step, carried out in an acidic or alkaline medium, precedes the treatment of the surface with the aqueous solution.
20. Process according to claim 19, characterised in that the oxidation step is carried out in the presence of permanganate.
21. Process according to one of claims 19 or 20, characterised in that several oxidation steps are carried out alternately in an acidic and alkaline medium before treatment of the surfaces with the aqueous solution.
22. Process according to one of claims 19 to 21, characterised in that when the oxidation step has been carried out the oxidation solution is destroyed and neutralised with a carboxylic acid (3, 6, 8, 11, 12), which is a component of the aqueous decontamination solution.
23. Process according to one of claims 1 to 22, characterised in that solution containing radioactive materials is delivered to an evaporator after treatment of the surface of the metal component.
24. Process according to one of claims 1 to 22, characterised in that solution containing radioactive substances is delivered to an ion exchanger after treatment of the surface of the metal component.
25. Process according to claim 23, characterised in that dicarboxylic acid (3, 6, 8) contained in the solution is thermally broken down in the evaporator to form monocarboxylic acid (11, 12).
26. Process according to one of claims 1 to 25, characterised in that during the treatment of the surface of the metal component, which is a component of the system to be decontaminated, the aqueous solution is recirculated in the system through a cleaning device.
27. Process according to one of claims 1 to 25, characterised in that during the treatment of the surface of the metal component, which is put in a container of a decontamination system, the aqueous solution is recirculated in the decontamination system through a cleaning device.
28. Process according to one of claims 26 or 27, characterised in that the cleaning device is an ion exchanger.
29. Process according to one of claims 26 to 28, characterised in that the cleaning device is a filter.
30. Process according to one of claims 26 to 29, characterised in that the cleaning device is arranged in a by-pass line of the system or of the decontamination system.

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