DECONTAMINATION PASTE AND METHOD FOR DECONTAMINATING A SUBSTRATE MADE OF A SOLID MATERIAL USING SAID PASTE

Abstract

A decontamination paste comprising at least one inorganic viscosifier selected from clays, at least one compound in the form of fibers and optionally further one or more optional components, the remainder being of solvent. A method for decontaminating a substrate made of a solid material using the paste, the substrate being contaminated by at least one contaminant species referred to as the labile contaminant species and/or by at least one contaminant species referred to as the surface contaminant species located on one of the surfaces thereof, and/or by at least one contaminant species referred to as the subsurface contaminating species located just below said surface, and/or by at least one contaminant species located under the surface in the depth of the substrate.

Claims

1. A decontamination paste comprising: at least one inorganic viscosifier selected from clays, said inorganic viscosifier representing from 20% to 70% by weight, preferably from 35% to 70% by weight, more preferably from 40% to 65% by weight, better from 45 to 55% by weight of the total weight of the paste, and said inorganic viscosifier being in the form of micron- and/or nano-sized particles; at least one compound in the form of fibres; optionally, furthermore, one or more component(s) selected from the following components: at least one surfactant; at least one active decontamination agent; at least one contaminant species extracting agent; at least one contaminant species chelating agent; at least one colouring agent; and the balance of solvent.

2. The decontamination paste according to claim 1, wherein said component(s) is (are) present in the following proportions: 0.1 to 8 or 10% by weight, preferably 0.1 to 5% by weight, more preferably 0.5 to 5% by weight, better 1 to 5% by weight, still better 1 to 3% by weight based on the weight of the paste, of the at least one compound in the form of fibres; optionally, 0.1 to 2% by weight, based on the weight of the paste, of the at least one surfactant; optionally, 0.1 to 10 mol/L of paste, preferably 0.5 to 10 mol/L of paste, more preferably 1 to 10 mol/L of paste, and better 3 to 6 mol/L of paste, of the at least one active decontamination agent; optionally, 0.1% to 5% by weight, based on the weight of the paste, of the at least one contaminant species extracting agent; optionally, from 0.1% to 5% by weight, based on the weight of the paste, of the at least one contaminant species chelating agent; optionally 0.01% to 10% by weight, preferably 0.1% to 5% by weight, based on the weight of the paste, of the at least one colouring agent; and the balance of solvent.

3. The decontamination paste according to claim 1, wherein the fibres are selected from the fibres of organic compounds such as cellulose fibres, and fibres of mineral compounds such as rock wool and glass wool.

4. The paste according to claim 1, wherein the active decontamination agent is selected from bases such as sodium hydroxide, potassium hydroxide, and mixtures thereof; acids such as nitric acid, phosphoric acid, hydrochloric acid, sulphuric acid, hydrogen oxalates such as sodium hydrogen oxalate, and mixtures thereof; oxidising agents such as peroxides, permanganates, persulphates, ozone, hypochlorites such as sodium hypochlorite, cerium IV salts, and mixtures thereof; quaternary ammonium salts such as hexadecylpyridinium (cetylpyridinium) salts, such as hexadecylpyridinium (cetylpyridinium) chloride; reducing agents; and mixtures thereof.

5. The paste according claim 1, wherein the surfactant is selected from non-ionic surfactants such as block copolymers, like ethylene oxide and propylene oxide block copolymers, and ethoxylated fatty acids; and mixtures thereof.

6. The paste according claim 1, wherein the contaminant species extracting agent is selected from inorganic adsorbents such as zeolites, clays, phosphates such as apatites, titanates such as sodium titanates, and ferrocyanides and ferricyanides.

7. The paste according to claim 1, wherein the contaminant species chelating agent is selected from n-octylphenyl-N,N-diisobutylcarbamoyl methylphosphine oxide (CMPO), tributyl phosphate (TBP), 1-hydroxyethane-1,1-disphosphonic acid (HEDPA), di-2-ethylhexylphosphoric acid (DHEPA), trioctylphosphine oxide (TOPO), diethylenetriamine pentaacetate (DTPA), primary, secondary and tertiary organic amines, cobalt dicarbollide, calixarenes, niobates, ammonium molybdophosphate (AMP), (trimethylpentyl)phosphinic acid (TPPA), and mixtures thereof.

8. The paste according to claim 1, wherein the colouring agent is selected from organic dyes and mineral pigments, preferably micronized, such as metal(s) and/or metalloid(s) oxides, metal(s) and/or metalloid(s) hydroxides, metal(s) and/or metalloid(s) oxyhydroxides, metal(s) ferrocyanides and ferricyanides, metal(s) aluminates, and mixtures thereof.

9. The paste according to claim 1, wherein the solvent is selected from water, organic solvents and mixtures thereof.

10. The paste according to claim which is supersaturated with solvent.

11. A method for decontaminating a substrate made of a solid material, said substrate being contaminated by at least one contaminant species called a labile contaminant species and/or by at least one contaminant species called a surface contaminant species located on one of its surfaces, and/or by at least one contaminant species called a subsurface contaminant species located just below said surface, and/or by at least one contaminant species located below said surface in the depth of the substrate, wherein at least one cycle is carried out, comprising the following successive steps of: a) applying the paste according to claim 1 on said surface; b) maintaining the paste on the surface at least for a sufficient time for the paste to destroy and/or inactivate and/or absorb and/or solubilise the contaminant species, and for the paste to dry and form a dry and solid residue containing said contaminant species; c) removing the dry and solid residue containing said contaminant species.

12. The method according to claim 11, wherein the substrate is a porous substrate, preferably a porous mineral substrate.

13. The method according to claim 11, wherein the solid material is selected from metals and metal alloys such as stainless steel, painted steels, aluminium and lead; polymers such as plastic materials or rubbers such as poly(vinyl chloride)s or PVCs, polypropylenes or PPs, polyethylenes or PEs, especially high density polyethylenes or HDPEs, poly(methyl methacrylate)s or PMMAs, poly(vinylidene fluoride)s or PVDFs, polycarbonates or PCs; glasses; cements and cementitious materials; mortars and concretes; plasters; bricks; natural or artificial stone; ceramics.

14. The method according to claim 11, wherein the contaminant species is selected from chemical, biological, nuclear or radioactive contaminant species.

15. The method according to claim 14, wherein the contaminant species is a biological species selected from bacteria, fungi, yeasts, viruses, toxins, spores, prions, and protozoa.

16. The method according claim 11, wherein the paste is applied on the surface in an amount of 2,000 g to 50,000 g of paste per m.sup.2 of surface, preferably 5,000 g to 10,000 g of paste per m.sup.2 of surface, which corresponds to approximately a paste thickness of 2 to 50 mm per m.sup.2 of surface, preferably 5 to 10 mm of paste per m.sup.2 of surface.

17. The method according to claim 11, wherein during step b), drying is carried out at a temperature of 1° C. to 50° C., preferably 15° C. to 25° C., and under a relative humidity of 20% to 80%, preferably 20% to 70%.

18. The method according to claim 11, wherein the paste is maintained on the surface for a period of 2 to 72 hours, preferably 2 to 48 hours.

19. The method according claim 11, wherein the dry and solid residue is in the form of one or more piece(s), each of said pieces having a size greater than or equal to 1 cm, preferably greater than or equal to 2 cm, more preferably greater than or equal to 5 cm.

20. The method according claim 11, wherein the dry solid residue is removed from the solid surface by a mechanical process such as brushing.

21. The method according to claim 11, wherein the cycle described is repeated 1 to 10 times by using the same paste in all cycles or by using different pastes in one or more cycle(s).

22. The method according to claim 11, wherein the paste applied during step a) is a paste supersaturated with solvent.

23. A decontamination paste consisting of: at least one inorganic viscosifier selected from clays, said inorganic viscosifier representing from 20% to 70% by weight, preferably from 35% to 70% by weight, more preferably from 40% to 65% by weight, better from 45 to 55% by weight of the total weight of the paste, and said inorganic viscosifier being in the form of micron- and/or nano-sized particles; at least one compound in the form of fibres; optionally, furthermore, one or more component(s) selected from the following components: at least one surfactant; at least one active decontamination agent; at least one contaminant species extracting agent; at least one contaminant species chelating agent; at least one colouring agent; and the balance of solvent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0222] FIG. 1 is a photograph of the Paste-1 prepared in Example 1, in a container.

[0223] FIG. 2 is a photograph of Paste-2 prepared in Example 1, in a container.

[0224] FIGS. 3A, 3B, and 3C are photographs showing a layer, deposit, of Paste-3 (FIG. 3A), a layer, deposit, of Paste-1 (FIG. 3B), and a layer, deposit, of Paste-4 (FIG. 3C) which are deposited onto a vertical mortar wall surface. The thickness of the paste layers deposited is 10 mm.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0225] The paste according to the invention may be easily prepared at room temperature.

[0226] For example, the paste according to the invention may be prepared by adding preferably gradually, (successively in any order, and/or simultaneously) the inorganic viscosifier(s), and the compound in the form of fibres to the solvent such as water, preferably deionised water, or to a mixture of the solvent and one or more components selected from components already listed above, namely: a surfactant, an active decontamination agent, a contaminant species extracting agent, a contaminant species chelating agent, and a colouring agent.

[0227] This mixing may be achieved by mechanical stirring, for example by means of a mechanical stirrer equipped with a three-blade propeller. The rotational speed is, for example, 200 rpm, and the stirring period of time is, for example, from 3 to 5 minutes.

[0228] The addition of the inorganic viscosifier(s) and the compound in the form of fibres to the solvent or to the mixture of the solvent and the above-mentioned component(s) may be carried out by simply pouring the viscosifier(s) and the compound in the form of fibres into said mixture, successively in any order or simultaneously. Upon adding the inorganic viscosifier(s) and/or the compound in the form of fibres, the mixture containing the solvent, this or these inorganic viscosifier(s), and/or the compound in the form of fibres and optionally the above-mentioned component(s) is generally kept under mechanical stirring.

[0229] This stirring may be, for example, achieved by means of a mechanical stirrer equipped with a three-blade propeller.

[0230] The stirring speed is generally increased gradually as the viscosity of the solution increases, eventually reaching a stirring speed of, for example, between 400 and 600 rpm when all of the inorganic viscosifier(s) and of the compound in the form of fibres have been added, without any splashing having occurred.

[0231] After the end of addition of the inorganic mineral viscosifier(s) and of the compound in the form of fibres, the stirring is still maintained, for example for 2 to 5 minutes, so as to obtain a perfectly homogeneous paste.

[0232] The paste thus prepared is then left to rest for at least one hour before being used.

[0233] It is obvious that other protocols for preparing pastes according to the invention may be implemented with an addition of the paste components in a different order from that mentioned above and/or with simultaneous addition of several components.

[0234] It is to be noted that the optional surfactant of the paste according to the invention favourably and significantly influences the rheological properties of the paste according to the invention. This surfactant especially avoids spreading or running risks during the treatment of vertical surfaces and ceilings.

[0235] The paste according to the invention thus prepared is then applied on the solid surface to be decontaminated of a substrate made of a solid material, in other words on the surface which has been exposed to contamination, for example biological contamination. This contamination has already been described above. In particular, the biological contamination may consist of one or more of the biological species already defined above.

[0236] As has been already indicated above, the active decontamination agent, for example the biological active decontamination agent, is selected according to the contaminant species, for example the biological species to be removed, eliminated, destroyed, or inactivated.

[0237] With the possible exception of alloys of light metals, such as aluminium, in the case where basic or acidic pastes are implemented, there is no limitation as to the material that constitutes the substrate to be decontaminated, indeed the paste according to the invention makes it possible to treat all kinds of materials, even fragile ones, without any damage.

[0238] The paste according to the invention generally does not generate any chemical, mechanical or physical deterioration, erosion, attack of the treated substrate.

[0239] However, in the case of sub-surface decontamination operations, the corrosion of the substrate is controlled over a few μm, as with suctionable gels.

[0240] The paste according to the invention is therefore in no way detrimental to the integrity of the treated substrates and even allows their reuse. Thus, sensitive materials such as military equipments are preserved and can be reused after their decontamination, while monuments treated with the paste according to the invention are not degraded at all and have their visual and structural integrity maintained.

[0241] This substrate material may therefore be selected from, for example, metals and alloys such as stainless steel, aluminium and lead; polymers such as plastic materials or rubbers, among which PVCs, PPs, PEs, especially HDPEs, PMMAs, PVDFs and PCs may be mentioned; glasses; cements and cementitious materials; mortars and concretes; plasters; bricks; natural or artificial stone; ceramics.

[0242] In all cases, whatever the material, the decontamination efficiency of the paste according to the invention is significant.

[0243] The treated surface may be painted or unpainted.

[0244] The efficiency of the treatment with the paste according to the invention is generally significant, including on substrates contaminated to a depth of several millimetres.

[0245] There are also no limitations as to the shape, geometry and size of the substrate and surface to be decontaminated, the paste according to the invention and the method implementing it allow the treatment of large surfaces with complex geometries, for example with hollows, angles and recesses.

[0246] The paste according to the invention ensures the effective treatment not only of substrates with horizontal surfaces such as floors, but also of substrates with vertical surfaces such as walls, or inclined or overhanging surfaces such as ceilings.

[0247] In comparison with decontamination methods, for example biological decontamination methods, which implement liquids such as solutions, the decontamination method according to the invention, implementing a paste, is particularly advantageous for the treatment of large-surface area, non-transportable materials and located outdoors. Indeed, the method according to the invention, because it uses a paste, allows in situ decontamination by avoiding the spreading of chemical solutions in the environment and the dispersion of contaminant species.

[0248] The paste according to the invention may be applied and spread on the surface to be treated by all the application methods known to the man skilled in the art.

[0249] Conventional methods are manual application, for example with a trowel, or application using a spraying machine in the manner of a mortar or coating.

[0250] The sufficiently short viscosity restoration, recovery time of the pastes according to the invention allows the applied pastes to adhere to all surfaces, for example to walls.

[0251] The amount of paste deposited onto the surface to be treated is generally 2,000 to 50,000 g/m.sup.2, preferably 5,000 to 10,000 g/m.sup.2. The amount of paste deposited per unit area and, accordingly, the thickness of the deposited paste influences the drying rate.

[0252] Thus, when a layer of paste with a thickness of 2 mm to 10 mm is deposited or sprayed on the surface of the substrate to be treated, the effective contact time between the paste and the materials is then equivalent to its drying time, period of time during which the active ingredient contained in the paste will interact with the contamination.

[0253] Further, it has been surprisingly shown that the amount of paste deposited—this paste further containing a specific viscosifier selected from clays—when it is within the ranges mentioned above and in particular when it is greater than or equal to 2,000 g/m.sup.2 and especially in the range of 5,000 to 10,000 g/m.sup.2, which corresponds to a minimum thickness of deposited paste, for example greater than or equal to 2,000 μm (2 mm) for an amount of deposited paste greater than or equal to 2,000 g/m.sup.2, makes it possible, after drying of the paste, to obtain a dry and solid residue in the form of one or more large piece(s) (the size being defined by the largest dimension of the piece or pieces), each of the pieces having a size greater than or equal to 1 cm, preferably greater than or equal to 2 cm, and more preferably greater than or equal to 5 cm.

[0254] The amount of deposited paste, and therefore the thickness of deposited paste, preferably greater than or equal to 2,000 g/m.sup.2 that is 2,000 μm, is, together with the specific nature of the viscosifier used in the paste according to the invention (see above), the fundamental parameter which influences the size of the dry residue formed after drying of the paste and which thus ensures that a dry and solid residue in the form of one or more large pieces, each of the pieces having a size greater than or equal to 1 cm, and not millimetre-sized dry residues or powdery residues, are formed. The dry and solid residue in the form of one or more large pieces obtained is easily removed by a mechanical method.

[0255] However, it is also to be noted that if the paste contains a surfactant at a low concentration, typically 0.1% to 2% of the total weight of the paste, then drying of the paste is improved and leads to an increased ability of the dry residues to be detached from the support.

[0256] The paste is then maintained on the surface to be treated for as long as it takes to dry. During this drying step, which can be considered as the active phase of the method according to the invention, the solvent contained in the paste, that is generally the water contained in the paste, evaporates until a dry and solid residue is obtained.

[0257] The drying time depends on the composition of the paste in the concentration ranges of its constituents given above, but also, as has been already set out, on the amount of paste deposited per unit area, that is the thickness of the deposited paste.

[0258] The drying time also depends on the weather conditions, that is the temperature and the relative humidity of the atmosphere in which the surface of the substrate made of a solid material is situated.

[0259] The method according to the invention may be carried out under extremely wide weather conditions, namely at a temperature T of 1° C. to 50° C. and a relative humidity RH of 20% to 80%.

[0260] The drying f time of the paste according to the invention is therefore generally 1 hour to 48 hours at a temperature T from 1° C. to 50° C. and a relative humidity RH from 20% to 80%.

[0261] It is to be noted that the formulation of the paste according to the invention especially when it contains surfactants such as “Pluronics®” generally ensures a drying time which is substantially equivalent to the contact time (between the decontamination agent, such as a biocidal agent, and the contaminant species, for example the biological, especially biotoxic species to be removed, eliminated) which is necessary, required to inactivate and/or absorb the contaminant species polluting the substrate material, and/or to sufficiently carry out surface erosion reactions of the material.

[0262] In other words, the formulation of the paste ensures a drying time which is no other than the inactivation time of the contaminant species, for example biological species, which is compatible with the inhibition kinetics of the contamination, for example biological contamination.

[0263] Or the formulation of the paste ensures a drying time which is no other than the time required for erosion reactions enabling a contaminated surface layer of the material to be removed.

[0264] In the case of radioactive contaminant species, the contamination is removed by dissolving the irradiating deposits or by corrosion of the contamination carrying materials. There is thus a real transfer of the nuclear contamination to the dry solid residue.

[0265] The surface area of the mineral filler generally used, which is generally from 50 m.sup.2/g to 300 m.sup.2/g, preferably 100 m.sup.2/g, and the absorption capacity of the paste according to the invention make it possible to trap the labile (surface) and fixed contamination of the material constituting the surface to be treated.

[0266] If necessary, the contaminant species, for example the biological contaminant species, are inactivated in the pasty phase. After drying the paste, the contamination, for example the inactivated biological contamination, is removed upon recovering the dry paste residue described below.

[0267] At the end of drying of the paste, the dry paste forms a dry residue that fractures little or not at all in contrast to the dry residue of suctionable gels. This dry residue comprises one or more large size piece(s). The dry residue may contain the inactivated contaminant species.

[0268] The dry residue obtained at the end of drying of the paste has a low adhesion to the surface of the decontaminated material. Therefore, the dry residue obtained after drying the paste can be easily recovered by simple mechanical methods such as brushing. However, the dry residue may also be discharged by a gas jet, for example a compressed air jet.

[0269] Thus, no rinsing with a liquid is generally necessary, and the method according to the invention does not generate any secondary liquid effluent.

[0270] However, it is possible, although not preferred, and if desired, to remove the dry residue by means of a liquid jet.

[0271] The method according to the invention thus achieves, firstly, a significant saving in chemical reagents in comparison with a decontamination method by washing with a solution. Secondly, because a waste in the form of a dry residue is obtained that can be easily mechanically recovered, a rinsing operation with water or a liquid, which is generally necessary to remove traces of chemical agents from the part, is generally avoided. This obviously results in a reduction in the amount of effluents produced, but also in a significant simplification in terms of waste treatment and disposal.

[0272] Due to the predominantly mineral composition of the paste according to the invention and to the small amount of waste produced, the dry waste can be stored or directed to a discharge channel (“outlet”) without prior treatment.

[0273] At the end of the method according to the invention, a solid waste is recovered in the form of one or more large size piece(s) of dry paste, which may be packaged as such, directly packaged, resulting, as already indicated above, in a significant reduction in the amount of effluents produced as well as in a significant simplification in terms of waste treatment channel and outlet.

[0274] Moreover, in the nuclear field, the fact that the solid dry residue does not have to be retreated before the waste is packaged is a considerable advantage; it allows the use of high performance active agents prohibited to date in decontamination liquids due to the operating constraints of the liquid effluent treatment plants (LETP).

[0275] The paste may therefore contain powerful oxidising agents such as cerium IV, which may very easily be regenerated from electrolysis of cerium III.

[0276] As an example, in the common case where 2,000 grams of paste are applied per m.sup.2 of treated area, the dry waste weight produced is less than 1,400 grams per m.sup.2.

EXAMPLES

Example 1

[0277] In this example, the preparation of two surface, subsurface and in depth, deep, decontamination pastes called “Paste-1”, and “Paste-2” according to the invention is described.

[0278] Paste-1 is a paste with the following composition: [0279] 0.8% by weight (based on the total weight of the paste) of Cellulose BC 1000 (fibre size of about 700 μm) marketed by Arbocel®; [0280] 49.6% by weight (based on the total weight of the paste) of kaolinite marketed by Sigma-Aldrich®; and [0281] 49.6% by weight (based on the total weight of the paste) of deionised water.

[0282] Paste-2 is a paste with the following composition: [0283] 8% by weight (based on the total weight of the paste) of Cellulose BC 1000 (fibre size of about 700 μm) marketed by Arbocel®; [0284] 20% by weight (based on the total weight of the paste) of kaolinite marketed by Sigma-Aldrich®; and [0285] 72% by weight (based on the total weight of the paste) of deionised water.

[0286] The synthesis protocol is similar for both pastes: [0287] Deionised water is first weighed in an adapted container. [0288] Kaolinite and cellulose are then added progressively to the water under stirring using a three-blade mechanical stirrer, until a homogeneous mixture is obtained with no lumps.

[0289] The pastes thus formed are finally kept under stirring for a few minutes.

[0290] FIG. 1 is a photograph of the prepared Paste-1.

[0291] FIG. 2 is a photograph of the prepared Paste-2.

[0292] Each of the two pastes has a malleable structure that can be deposited onto a surface, such as a wall of a facility for example, manually or using a spraying machine, in the manner of a mortar or coating.

Example 2

[0293] In this example, the efficiency of a paste according to the invention, namely the Paste-1 prepared in Example 1, for surface, subsurface and in depth, deep, decontamination of a porous material is demonstrated.

[0294] More precisely, in this example, the decontamination of a porous material consisting of a stack of glass beads, which is deeply contaminated by .sup.133Cs, is studied using a decontaminating paste according to the invention.

[0295] The decontaminating paste according to the invention used in this example is the Paste-1 prepared in Example 1.

[0296] The porous material used is a stack of glass beads, with sizes between 45 μm and 90 μm, which is prepared in a round crystallising dish with a diameter of 9.1 cm.

[0297] The stack of glass beads thus made, which is 2 cm high, is soaked and saturated with 43.42 mL of an aqueous CsNO.sub.3 solution with a CsNO.sub.3 concentration of 0.016 M. 93 mg of .sup.133Cs are present in the stack of glass beads.

[0298] A 2 cm layer of Paste-1 is then placed on the stack of glass beads saturated with the solution. The whole is then allowed to dry under ambient conditions. After one week of drying, the residual solid waste from the drying of the Paste-1 is separated from the stack of glass beads. The glass beads are then recovered and washed with a 0.1 M NaOH solution. The wash solution is finally analysed by atomic absorption spectroscopy in order to determine the amount of Cs remaining in the stack of glass beads, and thus to study the efficiency of the method. Analysis of the wash solution reveals the presence of 63.2 mg of Cs. The decontamination step thus allowed the recovery of 68% of the contaminants—that is .sup.133Cs—present in the stack of glass beads.

[0299] The Paste-1 dried and absorbed the aqueous solution by capillary action. The contaminants—that is .sup.133Cs—were thus absorbed from the porous material to the Paste-1 by advection.

[0300] By virtue of this example, it is clearly demonstrated that the “re-absorbent” pastes according to the invention have the capacity to decontaminate contaminated porous materials contaminated at the same time in surface, subsurface and in depth.

Example 3

[0301] In this example, the fixing of a contaminant in the final solid waste obtained after decontamination with a paste according to the invention is studied. It is demonstrated here that the clay is essential to both acts as a viscosifier and also as a contamination fixer. More precisely, in this example, the ability of the pastes according to the invention to fix a contaminant within the dry paste obtained after a decontamination operation is demonstrated.

[0302] For this, 1 g of kaolinite marketed by Sigma-Aldrich® has been soaked with 4 mL of a Cs solution. Different samples were made with different Cs concentrations, namely: 10.sup.−1M, 10.sup.−2M, 10.sup.−3M, and 10.sup.−5M. The samples have been allowed to dry until evaporation and then suspended in 100 mL under stirring for 24 h. After these 24 h, the solution has been filtered and the Cs content has been analysed by atomic absorption spectroscopy and then compared to the amount of Cs introduced into the gram of kaolinite. The Cs content retained by the clays has finally been calculated and expressed in %. The results are gathered in Table 1 below.

TABLE-US-00001 TABLE 1 Cs retention by clay (kaolinite) Cs concentration of the m.sub.Cs in H.sub.2O % Cs soaking solution after suspending retention by the clay 10.sup.−1M  4.4.10.sup.−2 g  13% 10.sup.−2M 3.97.10.sup.−3 g  22% 10.sup.−3M 2.84.10.sup.−4 g  47% 10.sup.−5M Below the detection limit ≈100% of the apparatus

[0303] The percent of Cs retained by the clay depends on the initial amount of Cs added, which may be explained by saturation of the fixing sites.

[0304] The test carried out with a Cs concentration of 10.sup.−5 M may be considered as the most representative of nuclear contamination because of the small amount of contaminant present. In this case, it is observed that almost all the contaminants (that is Cs) can be fixed by the clay.

[0305] In conclusion, in the case of a paste used for a Cs decontamination operation, the clay proves to be indispensable to act both as a viscosifier and a contamination fixer.

Example 4

[0306] In this example, it is demonstrated that the pastes according to the invention can hold on a vertical wall even if they are applied with a significant thickness, by virtue of the presence of a compound in the form of fibres.

[0307] Two new pastes have then been prepared (according to the same protocol as described in example 1): [0308] a “Paste-3”, not in accordance with the invention, the composition of the which is as follows: 50% by weight of kaolinite marketed by Sigma-Aldrich, and 50% by weight of deionised water. [0309] a “Paste-4”, according to the invention, the composition of which is as follows: 2.4% by weight of Cellulose BC 1000 marketed by Arbocel®; 48.8% by weight of kaolinite marketed by Sigma-Aldrich; and 48.8% by weight of deionised water.

[0310] FIGS. 3A, 3B, and 3C show a layer, deposit, of Paste-3 (FIG. 3A), a layer, deposit, of Paste-1 (described in Example 1) (FIG. 3B) and a layer, deposit, of Paste-4 (FIG. 3C) deposited onto a vertical wall surface made of mortar. The thickness of the deposited layers of paste is 10 mm.

[0311] It is observed in FIGS. 3A, 3B, and 3C, that in the absence of cellulose fibres, the paste (in this case Paste-3, not in accordance with the invention) does not hold on a vertical surface with a large thickness. The addition of cellulose fibres in small amounts (Paste-1 and Paste-4 according to the invention) enables the paste to hold on a vertical wall, even when a large thickness of paste, especially of at least 10 mm, is deposited.

[0312] Moreover, at the end of drying of Paste-1 and Paste-4, according to the invention, a non-powdery solid waste with a similar size to that which has been deposited is obtained and no fracture is observed in this solid waste.

[0313] In other words, the non-powdery solid waste has the same centimetre size as the wet paste deposit, which demonstrates that the presence of fibres does not negatively impact the size of the final waste.

REFERENCES

[0314] [1] US-A1-2012/0121459 [0315] [2] WO-A2-2007/039598 [0316] [3] WO-A2-2004/008463 [0317] [4] EP-A2-0642846 [0318] [5] WO-A1-2010/037809 [0319] [6] U.S. Pat. No. 7,737,320 [0320] [7] FR-A1-2 967 422