EP1393326A1

EP1393326A1 - Method for decontaminating surfaces or preventing contamination thereof by means of an exchange mechanism

Info

Publication number: EP1393326A1
Application number: EP02740826A
Authority: EP
Inventors: Franck Rouppert; Annie Rivoallan; Christophe Largeron
Original assignee: Commissariat a lEnergie Atomique CEA; Compagnie Generale des Matieres Nucleaires SA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA; Orano Demantelement SAS
Priority date: 2001-05-29
Filing date: 2002-05-28
Publication date: 2004-03-03
Also published as: FR2825507A1; JP2004533613A; US20040149319A1; CN1513186A; FR2825507B1; WO2002097824A1; RU2003137560A

Abstract

The invention relates to a method for decontaminating or preventing contamination of surfaces by an exchange mechanism. The method for decontaminating a surface contaminated by Z1 contaminants is characterized by bringing the surface into contact with an aqueous solution containing Z2 chemical species which can remove the Z1 contaminants by an exchange mechanism which is applied in the contaminated areas to the surface of the Z1 contaminants, so that the said Z1 contaminants are replaced by the chemical species Z2 in said areas and the aqueous solution containing the displaced Z1 contaminants is separated from the decontaminated surface.Said device is used to decontaminate stainless steel surfaces contaminated by caesium by means of Z2 chemical species made from Na+ ions.

Description

METHOD FOR DECO TAMINATION OR PREVENTION OF CONTAMINATION OF SURFACES BY AN EXCHANGE MECHANISM

DESCRIPTION

Technical area

The present invention relates to a method for decontaminating contaminated surfaces, in particular metal surfaces. It applies to the decontamination, maintenance and dismantling of installations in the nuclear industry.

STATE OF THE PRIOR ART The methods of decontamination of surfaces used in particular in the nuclear industry call on various mechanisms to ensure this decontamination.

Thus, processes using chemical extraction are known, processes based on the installation of surface coatings, laser decontamination processes, physical and mechanical processes, thermal processes, suction processes and washing. Among these methods, chemical extraction methods use erosion of the surface layers of the surface to be decontaminated to remove contamination from this surface. This can be obtained by acid erosion, dissolution in an acid medium or hydrogen peroxide, treatment with mixtures of alkaline salts, caustic treatments with using detergents, complexing with organic complexing agents, oxidation, redox and electro-polishing.

In all these processes, the objective is to achieve erosion of the surface layers of the surface to be decontaminated, for example oxide layers in the case of metals, which are the site of contamination.

In washing methods, hot water, steam, superheated steam or very high pressure water can be used to dissolve the contaminating chemical species and also to erode the layers surface of the surface to be decontaminated and thus eliminate contamination. The above-mentioned decontamination processes are described in the document: "Decommissioning Technology Descriptions: Decontamination", DOE, August 1, 2000 [1].

Most of the above methods are aggressive with respect to the surface to be treated and their objective is to remove the passive layer or the oxide layer, seat of contamination. They are therefore unsuitable for ensuring maintenance decontamination and are not suitable when the surface properties of the material to be decontaminated must be preserved. Thus, in the nuclear fuel cycle industry, we are faced with the problem of decontaminating oxidized and contaminated steels which may have very thick surface oxide layers, when using high temperature processes. The processes currently used to ensure this decontamination are washing with demineralized water under pressure and sandblasting, but these methods are not entirely satisfactory insofar as the level of residual contamination remains too close to the maximum permitted limits. In addition, these methods cause abrasion of the surface.

It would therefore be interesting to be able to have methods for decontaminating surfaces, in particular metal surfaces, making it possible: - to preserve the properties of the surface,

- use existing decontamination equipment,

- to be of low cost,

- to extend the life of equipment in the nuclear zone,

- to limit the doses integrated by the intervention personnel,

- lead to manageable effluents, and

- have a low impact on the environment.

Statement of the invention

The present invention specifically relates to a method for decontaminating a contaminated surface, which makes it possible to achieve the abovementioned objectives by using the complexing properties of this surface. According to the invention, the method for decontaminating a surface, in particular a metal surface, contaminated by contaminants Z _x is characterized in that the surface is brought into contact with an aqueous solution containing chemical species Z ₂ likely to displace contaminants Z _λ by a exchange mechanism at the binding sites on the surface of the Z _x contaminants so that the Z contaminants are replaced by the chemical species Z ₂ at these binding sites, and the aqueous solution containing the Z _x contaminants thus displaced is separated , from the decontaminated surface.

In this process, a mechanism is used to move the chemisorption equilibria of the contaminants Z _x in the desired direction, that is to say of the release of said contaminants in the aqueous solution. This is facilitated by the fact that one can use the physicochemical properties of. the surface to be decontaminated, in particular its complexing behavior with respect to the contaminants Z _x and the chemical species Z ₂ .

None of the methods of the prior art uses such properties to displace contaminants in an aqueous solution, without at the same time resorting to abrasion or degradation of the surface to be decontaminated.

Generally, the binding sites of the contaminants Z on the contaminated surface consist of functional groups F present on this surface. In this case, use is made of chemical species Z ₂ capable of reacting with these functional groups F, the chemical species Z ₂ being such that the equilibrium constant K ₂ of the reaction of these species with the groups F is greater than the constant equilibrium Ki of the reaction of contaminants Z _x with groups F. According to a variant of the invention, the physicochemical properties of a surface can be used to prevent contamination of this surface.

Also, the subject of the invention is also a method of treating a surface in order to prevent the fixing on this surface of contaminants Z _{X /} characterized in that the surface is brought into contact with an aqueous solution containing chemical species Z ₂ capable of occupying the sites for fixing the contaminants Z _x on this surface and which cannot then be displaced from these sites by the contaminants

Zi

When the binding sites of the contaminants Z are constituted by functional groups F present on this surface, chemical species Z _{2 are used} capable of reacting with these functional groups F, the chemical species Z ₂ being such that the equilibrium constant K ₂ of the reaction of these species with groups F is greater than the equilibrium constant Ki of the reaction of contaminants Z _x with groups F.

To implement the methods of the invention, it is therefore necessary to have elements on the complexing properties of the surface to be treated with respect to the Zl contaminants, and then to select the Z ₂ species which will be liable to displace the Z contaminants of the surface to be decontaminated.

When the fixing of the contaminants Z is carried out on sites constituted by functional groups F, the chemical species Z ₂ capable of reacting chemically with the functional groups F are first chosen, then the constant is determined. of equilibrium K of the reaction of contaminants Z with the groups F and the equilibrium constants ₂ of the chemical species Z ₂ with the groups F, in order to retain the chemical species Z ₂ for which K ₂ is greater than K.

When the chemical species Z ₂ capable of decontamination have been chosen, an aqueous solution containing these species is prepared at a concentration which can go as far as saturation in species Z ₂ of this solution.

The solution is then applied to the surface to be treated by any known means such as soaking, spraying with or without pressure, and circulation on the surface to be treated. The aqueous solution which has resulted in the desorbed contaminants Z is then separated from the decontaminated surface.

Preferably, the decontaminated surface is then rinsed with water and dried. The rinsing can be carried out with pure water and the drying can be carried out for example with compressed air.

The process of the invention can be used to decontaminate any material with complexing and / or adsorption properties, for example metals, metal alloys, plastics, polymers and even certain glasses.

It applies to the desorption of any contaminant fixed on the material by chemical or physicochemical reaction. As examples of contaminants capable of being displaced by this process, one can mention the metal ions, in particular the Cs ⁺ ions, Ag ⁺ , Pb ²⁺ , Cd ²⁺ , ...

When the contaminants Z _x are ions, chemical species Z _{2 are} also used which are ions, in order to displace the contaminants by an ion exchange mechanism.

For example, one can treat by the process of one invention a stainless steel metal surface contaminated with cesium or cobalt. In this case, the cesium binding sites are functional groups F constituted by CrO groups, and sodium ion Z ₂ can be used as chemical species because the equilibrium constants K _x and K ₂ of the following reactions: Cr-OH + Cs ⁺ ≈ ± Cr-O-Cs + H ⁺ (K _x )

Cr-OH + Na ⁺ ^ Cr-O-Na + H ⁺ (K ₂ ) for the formation of the compounds Cr-0-Cs and Cr-O-Na are such that K ₂ is greater than K _x .

Therefore, an exchange of cesium ions by sodium ions is obtained according to the following reaction scheme:

Cr-O-Cs + Na ⁺ → Cr-O-Na + Cs ⁺ This reaction shows that by incorporating a Na ⁺ ion into the decontaminating aqueous phase, the effectiveness of the decontamination of a surface with water can be improved.

Generally, the aqueous solution used for this decontamination is an aqueous solution of sodium ions. and / or potassium having a total concentration of Na ⁺ and / or K ⁺ ions of 10 ^"8 to 3 g / 1 approximately When the process in accordance with the invention for treating a surface is used to prevent the attachment of contaminants Z to this surface, the chemical species Z _{2 which} can be used are chosen in the same way, then a preparation is prepared. aqueous solution containing these chemical species Z ₂ and this solution is brought into contact with the surface to be treated in order to saturate the possible sites for fixing the contaminants Z _x with the chemical species Z _x . In the case of a stainless steel surface liable to be contaminated by cesium or cobalt, an aqueous sodium and / or potassium solution having a sodium and / or potassium concentration of 10 ⁸ to ^{8 can also be used.} 3 g / 1 to prevent the fixation of cesium or cobalt.

The process of the invention has many advantages. It is easy to implement and effective. It uses non-aggressive aqueous solutions with respect to the treated surfaces and has a selectivity with respect to the contaminants to be removed. Furthermore, it allows the surface properties of the treated material to be preserved; and it leads to effluents which are easy to manage because they can be evaporated and introduced for example into the vitrification circuit of installations for the reprocessing of spent nuclear fuels. In addition, it can be implemented in existing decontamination facilities.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of illustrative and not limiting, with reference to the accompanying drawing.

Brief description of the drawing

FIG. 1 is a graph illustrating the evolution of the decontamination factor of a metal surface contaminated with cesium ions, as a function of the sodium content of the aqueous solution used for this decontamination.

Detailed description of an embodiment

The following describes the use of the process of the invention to decontaminate a. metal surface in stainless steel contaminated by cesium, using Na ⁺ ions to displace Cs ⁺ ions from the surface by the ion exchange mechanism described above.

For this operation, an aqueous solution of NaOH is used and the influence of the sodium concentration on the decontamination factor FD is studied.

Several samples of stainless steel contaminated with cesium are used and they are soaked for 24 hours in aqueous NaOH solutions having increasing sodium concentrations, ranging from 5.10 ^"9 mol.L ^{" 1} to 2.10 ^"5 mol / L ^{" 1} .

In each case, the cesium concentration of the aqueous solution is determined after soaking to calculate the decontamination factor FD which corresponds to the ratio between the amounts of contaminant removed by the test solution and those removed by pure water. The results obtained are shown in the appended FIG. 1 which illustrates the evolution of FD as a function of the sodium concentration (in mol.L ^"1 ).

In this figure, the starting point of the curve refers to demineralized water. Thus, it can be seen that by adding a small quantity of sodium ions to demineralized water, the efficiency of cesium decontamination of the stainless steel surface is multiplied by a factor greater than 60. Thus, the process of the invention makes it possible to obtain the removal of volatile cesium-type contamination. No change in the surface properties of the material is observed.

When this process is used in the nuclear fuel cycle industry, it becomes possible to lower the residual surface contamination at the end of treatment far below the accepted standards and therefore:

- be limited to a single decontamination operation, and

- reduce the operating costs of the plant.

REFERENCE CITED

[1] Decommissioning Technology Descriptions Decontamination ", DOE, August 1, 2000

Claims

1. Process for decontaminating a surface contaminated by contaminants Z, characterized in that the surface is brought into contact with an aqueous solution containing chemical species Z ₂ capable of displacing the contaminants Z by an exchange mechanism level of the attachment sites on the surface of the contaminants Z _x so that the contaminants Z _x are replaced by the chemical species Z ₂ on these attachment sites, and the aqueous solution containing the contaminants Z _x thus displaced is separated from the surface decontaminated.

2. Method according to claim 1, in which the attachment sites of the contaminants Z on the contaminated surface being constituted by functional groups F present on this surface, chemical species Z ₂ capable of reacting with these functional groups F are used, the chemical species Z ₂ being such that the equilibrium constant K ₂ of the reaction of these species with the F groups is greater than the equilibrium constant K of the reaction of the contaminants Zi with the F groups.

3. Method according to any one of claims 1 and 2, in which the decontaminated surface is then subjected to rinsing with water and drying.

4. Method according to any one of claims 1 to 3, in which the contaminants Z _x and the chemical species Z ₂ are ions.

5. Method according to any one of claims 1 to 4, in which the surface is a metallic surface.

6. A method according to claim 5, wherein the metal surface is stainless steel.

7. Method according to claim 6, in which the functional groups F are groups. CrO, the contaminants Z _x are cesium or cobalt ions and the chemical species Z ₂ are sodium and/or potassium ions.

8. Method according to claim 7, in which the aqueous solution is a solution of Na ⁺ and/or K ⁺ ions having a total sodium and/or potassium concentration of 10 ^"8 to 3 g/1.

9. Process for treating a surface in order to prevent the fixation on this surface of contaminants Zi, characterized in that the surface is brought into contact with an aqueous solution containing chemical species Z ₂ capable of occupying the sites of fixation of the Z contaminants on this surface and cannot then be displaced from these sites by the Z contaminants.

10. Method according to claim 9, in which the attachment sites of the contaminants Z being constituted by functional groups F present on this surface, chemical species Z ₂ capable of reacting with these functional groups F are used, the chemical species Z ₂ being such that the equilibrium constant K ₂ of the reaction of these species with the F groups is greater than the equilibrium constant K _x of the reaction of the contaminants Z _x with the F groups.

11. Method according to any one of claims 9 and 10, in which the contaminants Z _x and the chemical species Z ₂ are ions.

12. Method according to any one of claims 9 to 11, in which the surface is a metallic surface.

13. Method according to claim 12, wherein the metal surface is made of stainless steel.

1. Process according to claim 13, in which the functional groups F are CrO groups, the contaminants Z are cesium or cobalt ions and the chemical species Z ₂ are sodium and/or potassium ions.

15. Method according to claim 14, in which the aqueous solution is a solution of Na ⁺ and/or K ⁺ ions having a sodium and/or potassium concentration of

10 ^~8 to 3 g/1.