EP2188814B1 - Method for decontaminating surfaces, which have been contaminated with alpha emitters, of nuclear plants - Google Patents

Description

The invention relates to methods for the decontamination of surface contaminated with alpha emitters of nuclear facilities, such as decontamination of the surface of components of the coolant system of nuclear power plants, to which reference is made below by way of example.

During power plant operation, radioactive oxide layers are formed on the internal surfaces of components of the coolant system, such as pipes, pumps, steam generator pipes, which must be removed in the event of the decommissioning of a decommissioned nuclear power plant in order to reduce the radioactive radiation of the components to tolerable levels.

The removal of the oxide layer on component surfaces is effected, for example, by a two-stage decontamination process in which the oxide layer is pretreated in the first stage with a strong oxidizing agent such as potassium permanganate or permanganic acid and in a second step the oxide layer is dissolved with a cleaning solution containing one or more complexing acids. The spent cleaning solution containing the constituents of the oxide layer in dissolved form is either evaporated to a residual amount or passed through ion exchangers to remove the ionic form of the oxide layer from the cleaning solution. Exhausted ion exchange material and residual amount of cleaning solution remaining on evaporation be fed in an appropriate form an intermediate or final storage.

EP 753 196 B1 discloses a method for disposal of an aqueous cleaning solution previously used to remove ferritic deposits. The cleaning solution comprises an organic acid as well as ferritic deposits dissolved in the form of iron complexes. The disclosed process makes it possible to convert the entire organic acid into CO ₂ and water by means of a cyclic process. After carrying out this cyclic process, only a relatively small amount of iron salt remains in the solution, which can be removed with the aid of a cation exchanger. Since the cyclic process presupposes the presence of iron ions in the cleaning solution, they are supplied to the cleaning solution at the beginning, as far as they are not already present in sufficient concentration by the previous decontamination.

A similar purification process is disclosed US 4,729,855 A in which the inner surfaces of a nuclear reactor are first oxidized and then treated with the aid of an acidic cleaning solution. The radioactive metal ions go into solution and are then removed with the aid of an ion exchanger from the cleaning solution.

With such or a comparable decontamination carried out during routine inspection work on the coolant system, essentially only gamma nuclides such as Cr-51 and Co-60 are detected. These nuclides are to a large extent, for example incorporated in an oxide layer of a component, in the form of their oxides, these being from the active substances of conventional decontamination solutions, for example relatively easily dissolved by complexing acids. Also transuranic elements like Am-241 are partially solved.

However, on the surfaces decontaminated with, for example, one of the abovementioned cleaning methods, ie, surfaces free of oxide layers, oxide particles which are not visible to the naked eye and which contain alpha emitters or are bound to the alpha emitters remain. These particles only adhere loosely to the surface of the components previously freed of their oxide layer, so that the alpha emitters can be partially wiped off with a cloth, for example during a wipe test. In addition to these particles, gamma-emitting particles may also be present on a component surface.

When decommissioning a nuclear installation, the components of the coolant system should be recycled, which is only possible if the radioactivity measured on a component is below the specified limit values with regard to gamma and beta radiation as well as alpha radiation.

US 2003/0172959 A1 discloses a method for surface decontamination. The cleaning solution proposed for this purpose contains a surfactant as wetting agent and a ketoamine as active cleaning component. The cleaning solution may additionally be provided with an acid, for example oxalic acid. After surface treatment, the used cleaning solution is drained and collected in a suitable container. The spent cleaning solution is now an unspecified, commonplace Disposal procedures, such as evaporation available.

On this basis, it is the object of the invention to propose a method with which can be easily and effectively removed on surfaces of nuclear facilities, for example components of the coolant system of nuclear power alpha emitters, in particular, the radioactive residual waste should be kept as low as possible.

To achieve this object, the surfaces of the components are treated according to claim 1 with an aqueous solution containing oxalic acid in addition to a cationic and / or zwitterionic surfactant. Such treatment is carried out following a decontamination process which has at least partially removed oxide layers present on the surfaces. At least part of the aqueous solution is passed through an ion exchanger after being exposed to a surface.

After extensive experiments with a variety of substances were carried out without success, it was completely surprising that can be removed with a treatment according to the invention on surfaces adhering particles with alpha activity. The particles may consist of the oxide of an alpha emitter. It can also be other particles adhere to the surface of alpha emitters.

The existing example of stainless steel or a nickel-based alloy components of a nuclear plant, such as components of the coolant system, then do not need to be disposed of consuming, but can be reused become. The proportion of alpha-emitting nuclides (in the following abbreviated to alphanuclides) can be lowered so far that the surfaces have an activity of less than 0.1 Bq / cm ² . By reducing the alpha activity, the ratio of gamma to alpha decay results in values that often exceed a threshold of 10 significantly. If this limit value is detected for the components, their release, for example for recycling, requires only measurements of the gamma radiation, which is possible with simple devices and with little time expenditure. If a limit of 10 can not be established, time-consuming and complex measurements of the alpha radiation must be carried out and the personnel must be monitored in a time- and cost-intensive manner by precipitation analyzes for the incorporation of alphanuclides.

As a result of the treatment mentioned, the particles in question adhering to a component surface pass into the solution, but also gamma activity present on the surface is reduced. To reduce the volume, the aqueous solution is passed through a cation exchanger before it is optimally subjected to a further treatment, for example by evaporation, or reused. Quite surprisingly, it could be found that the bound to the surfactants alpha activity passes to the ion exchanger. By this measure, it is possible to reduce the remaining waste at the end of the cleaning, which must be fed in an appropriate form an end or intermediate storage to a very low level. The oxalic acid remains in the solution and can, for example, with the EP 0 753 196 Procedures are removed.

Why the combination of oxalic acid and surfactant causes the detachment of the particles from a component surface is not known. It was also surprising that both when using cationic and zwitterionic surfactants, the particles containing alpha-nuclide are bound to cation exchange resins and therefore can be removed from the solution in a simple manner.

The process according to the invention is carried out following a generally known decontamination process aimed at the removal of oxide layers, wherein it is particularly advantageous if oxalic acid is already used in this process. The addition of oxalic acid is then unnecessary or at least only to a lesser extent required. To remove the particles dissolved out of the oxide layers, mainly metal ions, at least part of the solution is passed through an ion exchanger, preferably via a cation exchanger.

The appropriate oxalic acid concentration depends i.a. However, depending on the nature and thickness of the oxide layers to be removed, it should in any case be at least 250 ppm. The upper limit of the oxalic acid concentration is 15,000 ppm. Other levels are hardly associated with a significant effect.

A surfactant which is particularly suitable for carrying out the process according to the invention must, on the one hand, be effective in cooperation with oxalic acid with regard to the detachment of particles from surfaces. On the other hand, it must mediate the binding of the particles to a cation exchanger, so that they can be removed from the solution at least in part.

This is the case for primary amines of the general formula R-NH ₂ having an aliphatic radical of 8 to 24 C atoms, in which case hexadecylamine is to be emphasized as being particularly suitable. Among the zwitterionic surfactants, amino acids of the general formula HOOC-R-NH ₂ having an aliphatic radical of 4 to 24 C atoms and N-oxides of the general formula R 1 - (NO) (R ₂ ) (R 3) have been found, in the latter Case as aliphatic radicals R1 having 4 to 24 carbon atoms and R2 and R3 each having 1 to 10 carbon atoms are present. Particular preference is given to N-oxides in which R1 comprises 12 to 24 C atoms and R2 or R3 comprises 1 to 3 C atoms. Among this group of surfactants, dimethyl-octadecylamine-N-oxide has emerged as the best-acting surfactant.

In all cases, a treatment at elevated temperature of more than 30 ° C is appropriate. In view of the efficiency and the duration of the treatment, a temperature of at least 50 ° C is preferred, the upper limit being 200 ° C.

The concentration of the particular surfactant used depends i.a. its chemical structure and the resulting effectiveness, as well as the type and thickness of the oxide coatings. A wide range of applications covering the concentration range is between 50 ppm and 3000 ppm.

To test the effectiveness of individual surfactants, tests were carried out with original samples from different systems, for example tubes of the primary circuit and of steam generators. The surfaces were coated with an oxide layer of incorporated or adherent activity during power plant operation. For removing the oxide layer she was patented accordingly EP 0160831 first treated oxidatively and then detached from the sample surfaces using an oxalic acid-containing decontamination solution. Surfaces treated in this or similar manner appear metallic bright with the naked eye. However, non-visible particles adhere to them, especially the particles comprising the above-mentioned alphanuclides.

To remove them, the samples were placed in a container with an aqueous solution containing a surfactant at a concentration of at least 150 ppm, 350 ppm and 2000 ppm, respectively, and oxalic acid in a concentration above 50 ppm. The samples were treated for between 5 and 40 hours at an elevated temperature of 50 ° C to 200 ° C.

The effect of the measure was checked by observing two typical representatives of the total nuclides present on the surface, namely americium-241 (alpha emitter) and cobalt-60 (gamma emitter). Americium was chosen because it can be determined by relatively simple means via gamma radiation accompanying its alpha decay. In each case, measurements were carried out in the untreated state (there is an oxide layer produced in power plant operation), after the oxide layer has been removed and after treatment with a solution containing a surfactant and oxalic acid and the ratio of the gamma activity to the alpha activity calculated.

The result of tests conducted with hexadecylamine (A) as a cationic surfactant and dimethyl octadecylamine N-oxide (B) as a zwitterionic surfactant is shown in Table 1 below removable. Table 2 gives the decontamination factors obtained in the tests. <b><u> Table 1: </ u></b> test surfactant ppm (mg / l) nuclide Untreated [Bq / cm ² ] after separation Ox.schicht [Bq / cm ² ] after treatment with surfactant / oxalic acid (10000 ppm) [Bq / cm ² ] 1 A 350 Co-60 Am-241 ratio 4,4E + 05 4,1E + 03 106 3,4E + 00 4,4E + 00 8 9.8E-01 2.4E-03 408 2 A 150 Co-60 Am-241 ratio 6,5E + 06 3E + 03 2000 1,5E + 04 8,5E + 0 1760 6,4E + 03 6,2E-01 10330 3 B 2000 Co-60 Am-241 ratio 4.1E + 04 1.9E + 02 215 2,5E + 02 2,3E + 00 109 8.5E + 01 8.1E-02 1050 after H2C204 DF according to ALPHA DF Co-60 50 - 1,000 300 - 90,000 Am-241 50 - 1,000 1,000 -350,000

In a conventional oxide dissolution, a ratio of gamma to alpha activity (Co-60 / Am-241) is often found to be smaller than the corresponding ratio for the as yet untreated components or their surfaces, ie the relative proportion of alpha emitters has increased. If, on the other hand, the method according to the invention is used-in the case of the tests in question, it was carried out after conventional oxide detachment-a large proportion of the alphanuclides present on the component surface are removed, which is reflected in the gamma / alpha ratio. This increased by a factor of about 500 in test 1, by a factor of about 6 in test 2 and by a factor of about 10 in test 3. The conventional treatment ends in metallically bright surfaces, but often have intolerable residual activity which could not be further reduced by known chemical processes. Subsequent surfactant treatment of the type proposed can drastically reduce the residual activities or decontamination factors (DF) by 6 to 90 fold for Co-60 and 20 to 350 fold for Am-241.

A solution with activity detached from the sample surface was contacted with a cation exchange resin, then mechanically filtered, and then the percent distribution of activity on the exchange resin, filtrate, and filter residue was determined. As a result, it was found that about 95% of the Co-60 and 100% of the Am-241 become bound to the cation exchange resin. Most of the activity can thus be removed from the solution and bonded to the cation exchange resin. This can be supplied to the usual disposal route.

The remaining solution can be further treated by destroying the oxalic acid present in it under the action of UV radiation and then for further purification over a Solid bed is passed. The solution can then be reused or evaporated for disposal.

Claims (13)

1. Method for decontaminating nuclear plant surfaces contaminated with alpha emitters, said method being carried out after a decontamination method in which oxide layers present on the surfaces have been at least partially removed, the surfaces being treated with an aqueous solution containing oxalic acid and a zwitterionic and/or cationic surfactant, wherein an amino acid of general formula HOOC-R-NH₂ or an N-oxide of general formula R1-(NO)(R2)(R3) with an aliphatic radical R1 containing 4 to 24 C atoms and aliphatic radicals R2, R3 each containing 1 to 10 C atoms are used as a zwitterionic surfactant, and a primary amine of general formula R-NH₂ is used as a cationic surfactant, wherein at least some of the solution is fed through an ion exchanger once it has acted on a surface.
2. Method according to claim 1, characterised by the use of a cation exchanger.
3. Method according to one of the preceding claims, characterised by an oxalic acid concentration of more than 250 ppm.
4. Method according to claim 3, characterised by an oxalic acid concentration of 250 ppm to 15,000 ppm.
5. Method according to one of the preceding claims, characterised in that said method is carried out after decontamination of a surface, an oxide layer present on the surface being removed using a cleaning solution containing oxalic acid.
6. Method according to one of the preceding claims, characterised by the use of an amine with an aliphatic radical R containing 8 to 24 C atoms.
7. Method according to one of the preceding claims, characterised by the use of an amino acid with an aliphatic radical R containing 4 to 24 C atoms.
8. Method according to one of the preceding claims, characterised in that it is carried out using dimethyl amine octadecyl N-oxide.
9. Method according to one of the preceding claims, characterised in that a surfactant concentration of more than 20 ppm is used.
10. Method according to claim 9, characterised by a surfactant concentration of 50 ppm to 5,000 ppm.
11. Method according to one of the preceding claims, characterised in that it is carried out at a temperature of more than 30°C.
12. Method according to claim 11, characterised by a temperature of 60 °C to 100 °C.
13. Method according to claim 1, characterised by an N-oxide having a radical R1 of 12 to 24 C atoms and radicals R2 to R3 each containing 1 to 3 C atoms.