WO1995016997A1 - Method for decontaminating contaminated flexible plastic waste and plant therefor - Google Patents

Abstract

A method and a plant for decontaminating contaminated flexible plastic waste are disclosed. Surface components of the contaminated waste are selectively chemically attacked during a closed-circuit washing process, said components being the ones with the highest levels of contamination, and the reaction products are removed, whereafter the processed waste is recovered for recycling. Said plant (100) typically comprises a washing reactor (101), at least one washing tank (110) with a means (113) for supplying a strong base to be mixed with the washing solution, a rinsing tank (210), and filter units (130, 138, 139; 220, 227) in the recirculating circuits of the washing and rinsing circuits. Said filter units operate continuously to regenerate the washing and rinsing solutions flowing through the closed circuit, and contaminated particles are filtered out.

Description

 Method for decontaminating contaminated waste, made of flexible plastic, and installation for implementing said method,

The present invention relates to the decontamination of contaminated waste, made of flexible plastic, for subsequent recycling.

The contamination more specifically targeted is nuclear contamination.

It is well known that the nuclear industry is one of the industries in which safety regulations are the most draconian. The putting into practice of these instructions, in particular during the decontamination of slices of nuclear power plants that have been shut down, requires the use of a large amount of flexible plastic material, generally polyvinyl chloride, ethylvinyl acetate (EVA), or polyethylene. In the form of a flexible sheet, for example, polyvinyl chloride, EVA or polyethylene is used for packaging packages that can be radio-contaminated, for making tents or ventilated hoods for work in very radioactive areas, or for the making of various work clothes in the radioactive zone. The known techniques for recycling plastics include energy recycling by incineration, chemical recycling by pyrolysis, and thermomechanical recycling. In particular, during the recycling of thermoplastics by a thermomechanical process, a washing process is used. The washing process is then carried out in a conventional manner by stirring under water and surfactants in a tank provided with a flotation system for plastics denser than water (such as polyvinyl chloride), and the soiling is then separated from the plastic; for plastics less dense than water (such as EVA or polyethylene), an equivalent result is obtained by mechanical stirring in tanks fitted with blades.

This thermomechanical recycling technique with washing is unsatisfactory if we try to use it for inborn radio contact waste.

Indeed, the presence of effluents obliges to organize the protection of the personnel in charge of the implementation of the machine, and moreover the output is poor because one obtains a decontamination factor lying between 2 and 4 (it is recalled that this factor is a value commonly used in radiation protection, and it corresponds to the ratio between the initial activity and the final activity of one or more decontamination steps). Documents US-A-4,855,081 and US-A-4,855,080 also state a process for decontaminating radioactive plastics by dissolving in organic solvents, then liquid-liquid extraction of the decontaminants found in the organic solvent with water. Such a process is perhaps more efficient for decontamination, but it is extremely cumbersome and expensive to implement, in particular because of the large quantity of organic solvents used and the large number of stages of filters which are necessary. In addition, the final product has a very different constitution, because the fillers and additives remain in the filters during the overall filtering carried out.

This explains why the treatment commonly used for radio-contaminated plastics consists in compacting the products, followed by landfilling.

The state of the art is also illustrated by documents CS-B-257 964 and CS-B-257 961 (abbreviated in English edited by DATABASE INIS, Vienna, Austria) which describe a detergent composition and a washing process in view of the decontamination of plastic material, with, after washing, a chemical attack on the residual water.

For the technological background, we can finally cite the document DE-A-3733 551 which describes a process of chemical decontamination test of military vehicles or devices, using a washing solution containing chalk, a surfactant and a gelling agent.

The invention aims precisely to solve this problem, by designing a decontamination technique whose yield is high, both from the point of view of the decontamination factor and of the weight treated per unit of time, and which lends itself particularly well to the treatment of waste. contaminated with flexible plastic material for recycling.

The subject of the invention is therefore a process and a decontamination installation capable of treating at least 100 kg / h of contaminated waste in flexible plastic material, in particular radiocontaminated polyvinyl chloride, with a decontamination factor which can reach values of 8 to 10, and this without risk for the personnel concerned.

It is more particularly a process for decontaminating contaminated waste, made of flexible plastic, characterized in that the contaminated waste is subjected, during a washing process in a closed circuit, to a selective chemical attack by certain agents constituents present on the surface and which mainly support contamination, and elimination of the reaction products, after which the treated waste is recovered for recycling.

Selective chemical attack, whose action combines with that of washing, is very effective for the decontamination of polyvinyl chloride, EVA or polyethylene waste, or more generally plastic waste consisting of a polymer and processing agents (plasticizers, stabilizers, ...).

In particular, the selective chemical attack can be mainly directed against the plasticizing agents present on the surface of the contaminated waste, and this selective chemical attack is for example carried out in adding a strong base to the washing water, in particular sodium hydroxide or potassium hydroxide in aqueous solution, in such a way that the saponification reaction then carried out concurrently with the action of the washing achieves the desired decontamination.

Such selective chemical attack on the surface is particularly effective for flexible polyvinyl chloride waste, because the quantity of plasticizing agents is very large, whence it results from migration phenomena due to concentration gradients, which phenomena are further accentuated when the material wears out. By chemically treating the surface layer of each individual waste, the contaminating particles are detached on the surface, thus achieving very effective decontamination thanks to the saponification of the plasticizers. In fact, since the phenomenon of exudation is probably in the majority, a mono- or di-olecular layer of plasticizers is formed on the surface of the waste. These plasticizers being mainly made up of phthalic acid esters, saponification combined with the action of washing makes it possible to eliminate contamination very completely.

Preferably, the selective chemical attack is carried out at a temperature below 70 ° C, and preferably between 40 ° C and 60 ° C. It thus succeeds in accelerating the reaction and therefore increasing the yield in significant proportions.

It may also be advantageous to provide for the presence of a surfactant, preferably a low foaming agent, which makes it possible to suspend the products formed.

Advantageously, the washing process in a closed circuit comprises at least one washing cycle with thermomechanical action during which the selective chemical attack is carried out, then at least one rinsing cycle with mechanical action, with emptying cycles. during which the washing or rinsing effluents are respectively filtered. Preferably then, respective washing and rinsing solutions will be used for the washing and rinsing cycles which are continuously regenerated with retention by filtration of the contaminated particles.

The invention also relates to a decontamination installation intended for the implementation of the above-mentioned process, this installation being remarkable in that it comprises:

- a washing reactor;

- at least one washing tank containing a washing solution, connected to the washing reactor by a washing circuit including a supply circuit and a return circuit, said washing tank being equipped with a supply means d 'a strong base which is mixed with the washing solution contained therein;

- a rinsing tank containing a rinsing solution, connected to the washing reactor by an associated rinsing circuit including a supply circuit and a return circuit;

- filtration means mounted on the return circuits of the washing and rinsing circuits, said means operating continuously to regenerate the respective washing and rinsing solutions which circulate in a closed circuit, with retention by filtration of the contaminated particles.

According to a particular embodiment, the means for supplying a strong base is arranged to simultaneously supply a surfactant.

Preferably, the or each washing tank is thermostatically controlled, so as to send a washing solution heated and maintained at a predetermined temperature into the associated supply circuit. It may be interesting to plan two washing tanks each equipped with its strong base supply means, and optionally also with surfactant, independent filtration means being additionally mounted on each respective return circuit. Advantageously also, to facilitate the installation of the installation and to reduce its size on the ground, it is intended that the installation includes a decontamination module on which are mounted the filtration means as well as the pumping means associated with the circuits of washing and rinsing, with the corresponding portions of said washing and rinsing circuits, said module being equipped with connections at the periphery to make all the connections leading to the washing reactor and to the washing and rinsing tanks. Other characteristics and advantages of the invention will appear more clearly in the light of the description which follows and of the appended drawings, relating to a particular embodiment, with reference to the figures in which: - Figure 1 illustrates schematically a decontamination installation in accordance with the invention, implementing the method of the invention, the installation here comprising a single washing tank and a rinsing tank; - Figure 2 illustrates a variant with two washing tanks, with a duplication of the strong base supply means and filtration means associated with the respective return circuits;

- Figure 3 schematically illustrates a functional decontamination module on which are mounted the filtration and pumping means of the installation of Figure 2_, this module forming a unit ready to be connected to the washing reactor and the tanks washing and rinsing; - Figure 4 schematically illustrates a sub- assembly associated with the industrial water supply, this sub-assembly possibly being able to be mounted on the aforementioned module.

FIG. 1 illustrates a decontamination system 100 in accordance with the invention, comprising a washing reactor 101, a washing tank 110 here unique, containing a washing solution, this tank being connected to the washing reactor 101 by a circuit associated washing circuit including a supply circuit 111 and a return circuit 112, and a rinsing tank 210 containing a rinsing solution, this latter tank also being connected to the washing reactor 101 by an associated rinsing circuit including a circuit d supply 211 and a return circuit 212.

The washing reactor 101 can be either a reactor designed solely for this use, in which the contaminated waste or crushed waste, made of flexible plastic material, is packaged in metal baskets (in which case a wringer is associated with this reactor), or an industrial washing machine in which these comminuted products are packaged in nets, and which has a high wiping power. The drum of an industrial washing machine constituting the washing reactor 101 has been shown diagrammatically at 102. Two feed inlets 103, 104 are respectively associated with the supply of the washing solution from the tank. 110, and of the rinsing solution coming from the tank 210, and two drain outlets 105, 106 are respectively associated with the return of the washing and rinsing solutions to the corresponding tanks. One could for example use a washing machine for industrial laundry, with a capacity of 150 kg, of which one can program a large number of parameters. Its speed of rotation in rapid spin (of the order of 720 revolutions / minute) allows to get out of the shredded material having between 5 and 10% of humidity, thus avoiding the use of a wringer. Both drain outlets are programmed in such a way that the washing solution and the rinsing solution always take the same route. The drum 102 of the washing reactor 101 will preferably be made of stainless steel, so as to support the use of a strong base such as an aqueous solution of soda or potash. Two solenoid valves 107, 108 control the supply of washing solution or rinsing solution from the respective tanks 110, 210, and two solenoid valves 109, 142 control the emptying for the corresponding washing or rinsing cycles. Preferably, the above-mentioned four solenoid valves 107, 108, 109, 142 will be internal to the washing reactor 101.

The washing tank 110, preferably made of stainless steel, is provided with a plunging resistance heating system 114, temperature controlled by self-regulating regulators 115. Sensors 116 and 117 are also provided, respectively in the form of a temperature probe giving an indicated and regulated temperature, and a contact detector giving a transmitted temperature. The heating system makes it possible to carry out a selective chemical attack by the washing solution charged with a strong base brought to a temperature which will generally be less than 70 ° C., and preferably between 40 ° C. and 60 ° C. Two sensors 118 have also been provided which are high level and low level detectors for the washing solution contained in the tank 110.

In accordance with an essential aspect of the invention, there is provided a means 113 for supplying a strong base which is mixed with the washing solution which is contained in the tank 110. This means of supply can be a simple introduction circuit, or a circuit comprising an external pump (not shown here). The means 113 for supplying a strong base can also be arranged to simultaneously supply a surfactant, and preferably a surfactant. non-foaming or little foaming. We can for example use as surfactant the product sold by the company Henkel under the brand Texapon. In practice, the corresponding reagents will be introduced (concentrated soda solution and any surfactant) before filling the washing tank 110, so as not to exceed the high level.

In the lower part of the washing tank 110, there are three outlets equipped with associated valves 119, 120, 127. The valve 127 is here associated with a circuit 128 which corresponds to the feeding of a centrifugal pump 135 associated with the return of the solution of washing. The valve 120 makes it possible to bring the washing solution containing the desired quantity of strong base, possibly added with surfactant, to a pump 125, which can also be centrifugal. On the discharge side of this pump 125, the circuit is split in two, after a purge valve 121, with a portion of circuit 124 on which an electromagnetic flowmeter 126 is installed, preferably giving the indicated flow and the totalized flow, this portion of circuit leading to the supply solenoid valve 107 already mentioned. The other portion of the circuit 122 is a recirculation portion, on which is installed a guillotine valve 123 which makes it possible to adjust the feed rates of the washing reactor 101 in washing solution.

For the return of the washing solution from the corresponding drain outlet 105 of the reactor 101 and

The associated solenoid valve 109, filtration means 130, 138, 139 are provided mounted on the return circuit 112, said means operate continuously to regenerate the washing solution which circulates in a closed circuit, with retention by filtration of the particles contaminated. The return circuit comprises a valve 129 leading to a first filter 130, which can be a basket filter making it possible to protect the associated centrifugal pump 135 provided downstream of said filter. The porosity of this basket filter will for example be of the order of lOOμ, which allows effective protection of the pump 135 for the case for example where a net containing the contaminated waste would break. With this protective filter 130 is associated a sensor 131 of high level and of low level, and two sensors 132, 133 which carry out the control of a pneumatic solenoid valve 134 mounted downstream of the filter, upstream of the pump 135. The circuit 128 permanently provides a booster supply for the pump 135. When the pump 135 is discharged, there is an electromagnetic flowmeter 136 which makes it possible to totalize the volume of solution passing through the filters 138 and 139 located downstream, to provide for a possible replacement of the washing solution. A guillotine valve 137 is arranged upstream of the two filters 138 and 139, and each of these filters is equipped with a differential pressure gauge 140 (for example from O to 4.10 ⁵ Pa) making it possible to measure the pressure difference upstream and downstream filtration cartridges to determine their clogging.

In practice, it will be possible to use cartridge filters, with for example a porosity of 20 μm for the first filter 138, and of 5 μm for the second filter 139. It will be possible, for example, to use cartridges composed of microfibers of molded polypropylene. A valve 141 finally leads to the return provided in the upper part of the washing tank 110.

There is thus a washing circuit equipped, on its return line, with filtration means operating continuously to regenerate the washing solution circulating in a closed circuit, with retention by filtration of contaminated particles.

For the constantly regenerated circulation of the rinsing solution contained in the associated tank 210, the supply 211 and return 212 circuits are designed to similar to those just described for the circulation and return of the washing solution. The rinsing solution coming from the tank 210 thus arrives by a valve 214 in suction from a centrifugal pump 215. At the discharge of this pump 215, adjustable in flow rate by an associated valve 217, there is a purge with its valve 216, and a supply circuit 211 on which a float flow meter 218 is provided, arriving at the supply solenoid valve 108. For the return of the rinsing solution, the same means are found as for the return of the washing solution, with a basket filter 220, and a single cartridge filter 227. These means thus successively comprise a valve 219, a sensor 221 of high and low level, a solenoid valve 224 controlled by sensors 222 and 223, a centrifugal pump 225 with, at the discharge thereof, a flow control valve 226 leading to a cartridge filter 227, the porosity of which may be of the order of 1 μm. The filter 227 is also equipped with a differential pressure gauge 228, and an outlet valve 229 leading to the return to the tank 210. The pneumatic solenoid valve 224, controlled by its two level sensors located on the filter 220 allows to prevent the pump 225 from being primed. A valve 230 provided in the lower part of the tank 210 corresponds to the feeding circuit of the pump 225. The valve 226 finally makes it possible to adjust the inlet flow rate in the filter 227. A general drain valve 213 and also provided in the lower part of tank 210.

Finally, provision is made for a circuit for filling the tanks 110 and 210. In order to prevent water containing sludge or any other impurities from circulating in the installation, industrial water filtration is provided upstream of these tanks. The industrial water inlet is shown diagrammatically at 50, and the valve 51 thus leads to a cartridge filter 52, the porosity of which may be of the order of 1 μm, and this filtered water is directed by valves 54 and 55 to the respective washing tanks 110 and rinsing tanks 220. The water flow rate is measured by a flow meter 53.

For the first filling of the tanks 110 and 210, it will naturally be necessary to follow a precise procedure making it possible to avoid accidents of false operations. Thus, for filling the tank 110, the valves 119, 120, 127 will be closed successively, then the water supply valve 55 will be closed, and the valve 54 will be opened; after checking the state of the filter cartridges 52, the valve 51 will be opened, this opening being maintained until the high level indicated by the detector 118 is reached. For the rinsing tank 210 , we will proceed in the same way, successively closing the valve 54 and opening the valve 55; after checking the state of the filter cartridges 52, the valve 51 will be opened until the high level of the tank 210 is indicated by the associated level detector 231. When the high level is reached in a tank, the corresponding valve 54 or 55 is closed, and lastly the valve 51 for the water inlet.

The installation 100 thus equipped allows the implementation of the process for decontaminating contaminated waste in flexible plastic material according to the invention, this process being remarkable in that the contaminated waste is subjected, during a washing process in a closed circuit, to a selective chemical attack on certain constituent agents present on the surface and which mainly support contamination, and to an elimination of the reaction products, after which the treated waste is recovered to be recycled. This elimination results from the combined action of chemical attack and washing with stirring (mechanical or thermomechanical action), possibly with the use of a surfactant. In an app- In particular, the selective chemical attack will be mainly directed against the plasticizing agents present on the surface of the contaminated waste. This selective chemical attack is carried out by adding a strong base to the washing water, by the above-mentioned means 113, in particular sodium hydroxide or potassium hydroxide in aqueous solution. The saponification reaction then carried out together with the action of washing achieves the desired decontamination. The contaminated waste can be placed in the drum 102 of the washing reactor 101 either in its natural form, or even in the form of small pellets obtained by prior shredding of the flexible waste.

The use of flexible sheet-shaped waste is particularly effective in the context of the invention, since this waste has a large specific surface, so that the selective chemical attack is very particularly effective, which makes it possible to obtain a decontamination factor commonly reaching values of 8 to 10. Contaminated particles detached by friction during washing, by mechanical or thermomechanical action, can be isolated by the surfactant which prevents redeposition. The use of a surfactant is certainly not compulsory, but it is nevertheless advantageous insofar as it promotes the saponification reaction, because the balance of the reaction is shifted in the direction of saponification and not 1 ' esterification. It thus succeeds in controlling the reaction perfectly, and in saponifying the organic esters, which are generally phthalates, forming almost all of the plasticizers used in the contaminated waste which are made of flexible plastic. It should be noted that the selective chemical attack is a surface attack, so that the waste does not undergo any attack in its mass, which makes it possible to preserve the other additives already present in the plastic. It is thus possible ultimately to recover perfectly recyclable treated waste, for example directly usable in an injection unit. The closed circuit washing process comprises in practice at least one washing cycle with thermomechanical action during which the selective chemical attack is carried out, then at least one rinsing cycle with mechanical action, with emptying cycles at during which the washing or rinsing effluents are respectively filtered. For the washing and rinsing cycles, respective washing and rinsing solutions are thus used which are continuously regenerated, with retention by filtration of the contaminated particles. In FIG. 2, a variant of the decontamination installation which has just been described has been illustrated, in which a second washing tank 310 is provided, operating independently from the first tank 110, thanks to its supply circuit 311 and return 312 associated. Identical components are thus associated with this second washing tank 310, and the same references have been used as for the components fitted to washing tank 110 increased by 200. The installation 100 is further supplemented by three-way valves tracks 150, 151, 152, 153 which make it possible to choose an operation on one or the other of the two washing tanks 110 and 310. A third valve 56 is associated with the supply of filtered water to this second washing tank 310. Such a unit is capable of processing at least

100 kg of plastic waste per hour.

. Thus, the washing reactor 101 can be operated independently with its filtration means, with its two filtration loops and its two supply loops on which the reactor, according to that it is in the washing cycle or in the rinsing cycle, draws, by means of the solenoid valves controlled by said reactor, the volume of solution it needs. When emptying, the washing or rinsing effluents are sent to the respective filtration circuits.

In the case of FIG. 2, the installation comprises two washing tanks 110, 310 each equipped with its means 113, 313 for supplying a strong base, possibly also with surfactant, and independent filtration means mounted on each respective return circuit 112, 312.

To reduce the footprint of the instal¬ lation, and facilitate its transport, it is also provided according to the invention that the installation 100 includes a decontamination module on which are mounted the filtration means and the means of pumping associated with the washing and rinsing circuits, with the corresponding portions of said washing and rinsing circuits. Such an advantageous option is illustrated in FIG. 3, where there is such a decontamination module 400. The module illustrated here corresponds to the installation of FIG. 2, with two washing tanks. We thus find in Figure 3 the various components described above, with, at the periphery, arrows diagramming the inlets and outlets associated with the washing reactor 101, and with the three tanks 110, 210, 310. The module 400 is thus fitted with periphery connections to make all the connections leading to the washing reactor 101 and to the three washing and rinsing tanks 110, 210, 310. For the connections to the washing reactor 101, the references of the solenoid valves 107 and 108 for supplying the washing and rinsing solutions are indicated in brackets, and the solenoid valves 109 and 142 for the return of the washing solutions. and rinsing, to recall that these four solenoid valves are preferably integrated in the reactor washing 101. The module 400 also includes an electrical control cabinet shown diagrammatically by a rectangle 401.

In Figure 4, there is illustrated a water supply unit 402 associated with the three tanks, with the three valves 54, 55, 56 associated with the water supply to each of the three tanks 110, 210, 310. This supply unit can naturally be integrated into the module 400, or even be arranged separately, it being understood that said unit is not affected by contamination. As an indication, such a functional decontamination module 400 will represent a footprint of approximately two meters by three meters, with a height of the order of 1.2 meters. The different components arranged on the module 400 will not be described again, because we find the arrangement already described with reference to Figures 1 and 2. Preferably, all the connection inlets and outlets as well as the connections between the various components will have the same nominal diameter, for example 50 mm. When the pumps fitted to the installation are centrifugal pumps, it will be necessary to ensure that these pumps are always loaded to have a permanent booster, by placing said pumps at a level lower than the level of the liquid contained in the tank. or the corresponding reactor. For example, the two washing tanks 110 and 310 could be placed approximately three meters from the ground.

This also implies, always for safety reasons, compliance with very strict instructions, even for routine acts such as changing filters. For filters 138 and 139 for example, the loading of cartridges must be carried out according to the following procedure

: at the end of a treatment cycle (washes + rinses),

• the washing reactor 101 and the pump 135 are de-energized, then the valves 137 and 141 are closed. Then, the operator wearing suitable gloves and glasses can open the filters 138 and 139, and change the filter cartridges, checking the state of the filter housing gasket. After closing the casings and blocking the cover closing nuts, it is then sufficient to reopen the valves 137 and 141, and to re-energize the pump 135 and the washing reactor 101. The same procedure will be used for the change of the filter cartridges 338 and 339. As regards the cartridge filter 227 associated with the return of the rinsing solution, the procedure will be the same, with the washing reactor 101 and the pump 225 being switched off, then a closure of valves 226 and 229; then the operator can change the filter cartridges, and check the condition of the filter housing gasket, then, after reclosing and locking the nuts, open valves 226 and 229 again, and switch on the pump again 225 and the washing reactor 101.

The fact of having two washing tanks will allow a more elaborate operation, with the use of a second type of selective chemical attack, in accordance with a washing making it possible for example to attack the material stabilizing agents. plastic, or pre-rinse. If one works with an alternative work, one could possibly envisage a second decontamination module to have one module per application.

It goes without saying that the components of the installation described above have been mentioned only by way of example. In particular, centrifugal pumps can be replaced by positive displacement pumps, for example Moineau pumps.

It is thus possible to design a technique for high efficiency ^• decontamination, both from the perspective of the decontamination factor of the Treaty weight per unit time, which lends itself particularly well to the treatment of contaminated waste, in flexible plastic material, with a view to their recycling. The decontamination installation such as that which has just been described, is capable of treating at least 100 kg per hour of radio-contaminated waste, made of flexible plastic material, in particular EVA, polyvinylchloride, or polyethylene with a decontamination factor that can reach values of 8 to 10, and this without risk for the personnel concerned. The selective destruction of the components carrying contamination makes it possible to practically not alter the essential functional properties of the product. Finally, the containment associated with the retention of contamination provides optimal security for the personnel assigned to the operation of the installation.

The invention is not limited to the embodiment which has just been described, but on the contrary encompasses any variant incorporating, with equivalent means, the essential characteristics set out above.

Claims

CLAIMS 1. Process for decontamination of contaminated waste, made of flexible plastic, characterized in that the contaminated waste is subjected, during a washing process in a closed circuit, to a selective chemical attack by certain constituent agents present on the surface and which mainly support contamination, and elimination of the reaction products, after which the treated waste is recovered for recycling. 2. Method according to claim 1, characterized in that the selective chemical attack is mainly directed against the plasticizing agents present on the surface of the contaminated waste. 3. Method according to claim 2, characterized in that the selective chemical attack is carried out by adding a strong base to the washing water, in particular sodium hydroxide or potassium hydroxide in aqueous solution, so that the reaction of saponification then implemented concurrently with the action of washing achieves the desired decontamination. 4. Method according to claim 3, characterized in that the selective chemical attack is carried out at a temperature below 70 ° C, and preferably between 40 ° C and 60 ° C. 5. Method according to claim 3 or reven¬ dication 4, characterized in that the selective chemical attack is carried out in the presence of a surfactant, and preferably a low foaming surfactant. 6. Method according to one of claims 1 to 5, characterized in that the closed-circuit washing process comprises at least one washing cycle with thermomechanical action, during which the selective chemical attack is carried out, then at minus one rinse cycle with mechanical action, with drain cycles during which the washing or rinsing effluents are respectively filtered. 7. Method according to claim 6, characterized in that one uses, for the washing and rinsing cycles, respective washing and rinsing solutions which are continuously regenerated with retention by filtration of the contaminated particles. 8. Decontamination installation intended for the implementation of a method according to any one of claims 1 to 7, characterized in that it comprises: - a washing reactor (101); - at least one washing tank (110; 310) containing a washing solution, connected to the washing reactor (101) by a washing circuit including a supply circuit (111; 311) and a circuit return (112; 312), said washing tank being equipped with means (113; 313) for supplying a strong base which is mixed with the washing solution contained therein; - a rinsing tank (210) containing a rinsing solution, connected to the washing reactor (101) by an associated rinsing circuit including a supply circuit (211) and a return circuit (212); - filtration means (130; 138, 139; 338, 339; 220, 227) mounted on the return circuits (112; 212; 312) of the washing and rinsing circuits, said means operating continuously to regenerate the solutions respective washing and rinsing cycles in closed circuit, with retention by filtration of contaminated particles. 9. Installation according to claim 8, characterized in that the means (113; 313) for supplying a strong base is arranged to simultaneously supply a surfactant. . 10. Installation according to claim 8 or claim 9, characterized in that the or each washing tub (110; 310) is thermostatically controlled, so as to send a solution of washing heated and maintained at a predetermined temperature. 11. Installation according to one of claims 8 to 10, characterized in that it comprises two washing tanks (110; 310) each equipped with its means (113; 313) for supplying a strong base, and possibly also for surfactant, independent filtration means (138, 139, - 338, 339) being further mounted on each respective return circuit (112; 312). 12. Installation according to one of claims 8 to 11, characterized in that it comprises a decontamination module (400) on which are mounted the filtration means (130, 138, 139, 220, 227, 338, 339) as well as the pumping means (125, 135, 215, 225) associated with the washing and rinsing circuits, with the corresponding portions of said washing and rinsing circuits, said module being equipped with periphery connections to make all the connections leading to the washing reactor (101) and to the washing and rinsing tanks (110, 310; 210).