EP1060476B1 - Method and installation for decontaminating metallic surfaces - Google Patents

Abstract

The invention concerns a method for decontaminating metallic surfaces, whereby the latter are treated with a cerium acid solution having valence 4<+>, the ceric ion being regenerated with ozone. The method is characterised in that the metallic surface is treated at a temperature ranging between 60 DEG C and 90 DEG C with an acid solution containing cerium with valence 4<+>, the cerium with valence 4<+> being continuously regenerated at substantially the same temperature as that of said solution, said regeneration being carried out by injecting ozone into the decontaminating solution in a gas-liquid contactor (2) of the static mixer type wherein the ozone and the cerium-based acid solution are transported in cocurrent.

Description

The subject of the present invention is a method for decontaminating metal surfaces, according to which the latter are treated with an acid solution of cerium with a valence of 4 ⁺ , the ceric ion being regenerated with ozone in such a way that continuous in a gas-liquid contactor at substantially the same temperature as that of the aforesaid solution.

The metal surfaces to be decontaminated may or may not be covered with a layer of oxides. Decontamination takes place by reaction of the cerium acid with the metal and / or the oxide layer covering the surface of this metal.

The metal surfaces in question may be contaminated with both natural and artificial radioisotopes and non-radioactive elements.

In the first case, the surfaces are part of metal parts that can come from nuclear reactors of different types, such as pressurized water reactors, boiling water reactors, gas-cooled reactors or others.

The radioactive contamination is either related to the activation of impurities by the reactor core and the deposit and fixation of this contamination on the metal walls, either caused by radioactive leaks at the fuel elements and the fission product deposition. and of fuels on the metal walls.

Parts contaminated with radioactive material may also come from nuclear fuel, spent fuel reprocessing facilities, nuclear waste facilities, low, medium and high activity laboratories handling radioactive elements, radioactive waste storage facilities, and any facility in which radioactive products are handled.

In the second case, the metal parts contaminated by non-radioactive elements can be contaminated either by the deposit or by the attachment of a contaminant on the metal or in the oxide layer present on the surface of the metal. The metals may have undergone oxidation at a temperature above room temperature, the layer of corrosion products formed having the particularity of strongly fixing the contaminants.

A known decontamination process used for the decontamination of metal surfaces of parts or equipment from pressurized water nuclear reactors, and in particular the decontamination of chromium oxide with a chromium-nickel-iron alloy, is described in US Pat. WO-A-85/04279.

This known method comprises treating the contaminated surfaces with an aqueous oxidizing agent having a pH of less than 7 and containing cerium nitrate, chromic acid and ozone at a temperature below 60 ° C and preferably below 25 ° C.

A similar process is described in WO-A-90/01774. The oxidation is carried out at low temperature in the presence of Ce ⁴⁺ ions, ozone and chromic acid but also perhalogenic acid and at a pH below 3.

In these known processes the oxidizing agent is an acidic solution of Ce ⁴⁺ saturated with ozone. This solution is sent co-currently in the system to be decontaminated until the total depletion of Ce ⁴⁺ / 0 ₃ before returning to the ozonation reactor and recovering its oxidizing potential ".

This known method is relatively slow, also because of the rather low temperature. A temperature above 60 ° C is strongly discouraged since a higher temperature would cause such a strong decomposition, for example of ozone, that the favorable effect of the increase of the reaction rate due to the rise in temperature would be canceled. In addition, at high temperature the solubility of the ozone in this reaction medium is very low.

EP-A-0.180.826 discloses a method as defined in the first paragraph. The treatment of the surface also takes place at low temperature, that is to say a temperature below 50 ° C. The ceric ion is regenerated with ozone in an injection column in which ozone is injected at the lower end while the solution with ceric ion flows up and down.

According to EP-A-0.134.664, metallic surfaces of a nuclear reactor are decontaminated first by a decontamination composition which is added to the refrigerant and then removed, and then, after cooling the refrigerant to 40 to 100 ° C. C, by a composition comprising ozone dissolved by spraying in water and a soluble compound of Ce ⁴ . The refrigerant containing this composition is circulated in the reactor cooling system and then heated to at least 100 ° C. Finally this refrigerant is passed through an exchanger comprising an anionic resin, its temperature is adjusted to 60 to 200 ° C and it is re-used.

KLEIN N. et al. "THOROUGH DECONTAMINATION OF METALLIC PIECES WITH THE CERIUM PROCESS", PROCEEDINGS OF THE INTERNATIONAL TOPICAL MEETING ON NUCLEAR AND HAZARDOUS WASTE MANAGEMENT (SPECTRM '96): VOLUME 3. ANON. LA GRANGE PARK, IL: AMERICAN NUCLEAR SOCIETY, 1996. P. 1728-1734, describes the use of the oxidative Ce ⁴⁺ at a temperature of 80 ° C to decontaminate metal surfaces and the regeneration of ceric ion with ozone. in counter-current gas-liquid contactor formed by a washing column.

The object of the invention is to overcome the aforementioned drawbacks and to provide a method for decontaminating metal surfaces rapidly and effectively.

This object is achieved by the fact that the metal surface is treated, that is to say oxidized, at a temperature between 60 ° C. and 90 ° C. by an acid solution of cerium containing cerium at the valence 4 ⁺ . the cerium being regenerated by injection of ozone into the decontamination solution in a static mixer type gas-liquid contactor in which the ozone and the acidic solution based on cerium are transported in co-current.

It has been found that by in-situ regeneration which maintains a high concentration of Ce ⁴⁺ and the relatively high temperature of both oxidation and regeneration, a high rate of decontamination was achieved.

No. 4,162,229 discloses the treatment of contaminated surfaces with an aqueous solution based on a cerium (4 ⁺ ) salt at a temperature between 20 ° C. and 90 ° C. followed by the removal of the solution and No. 4,657,596 discloses the treatment of such surfaces with an aqueous solution containing ceric acid at temperatures between 70 ° C and 200 ° C. None of these documents describes a regeneration of cerium, which suggests that such eventual regeneration takes place in a separate step and at another temperature.

The invention also relates to a device particularly intended for carrying out the method according to the aforementioned invention.

The invention more particularly relates to a decontamination installation for metal surfaces, comprising a decontamination tank, and a gas-liquid contactor connected to an ozone production system, the tank and the contactor being mounted in a circulation loop of the decontamination solution.

Such an installation is known from EP-A-0.180.826. In this installation, the gas-liquid contactor is a conctactor with an injector or with an injection column.

According to the invention, the gas-liquid regeneration contactor is a gas-liquid contactor of the static mixer type.

For the sake of clarity, an exemplary embodiment of a method and a device for decontaminating metal surfaces according to the invention will be described below for illustrative and not restrictive, reference being made to the accompanying drawings which schematically show an installation according to the invention.

The installation represented in the figure essentially comprises a decontamination tank 1 filled with decontamination solution, a gas-liquid regeneration contactor 2 connected to an ozone production system 3, and a buffer tank 4, the decontamination tank 1. , the contactor 2 and the buffer tank 4 being mounted in the same circulation loop 5 of the decontamination solution.

The decontamination tank 1 is made of zirconium and has for example a content of about 2 m ³ . It is closed by a cover 6 on which ultrasound probes 7 are fixed.

This decontamination tank 1 is provided at its upper part with an overflow 8 connected to a discharge pipe 9 opening into the buffer tank 4 located below the decontamination tank 1 and heated by a heating system 10.

In a variant, this heating system 10 is not mounted in the buffer tank 4 but in the decontamination tank 1.

Above the overflow 8, the tank 1 has an exhaust duct 11 for gas opening into a gas treatment device 12 comprising in series a condenser 13, a deiciler 14 and a residual ozone destruction unit. 15. The condensates of the condenser 13 are collected in a tank 16 and returned via the conduit 17 to the exhaust duct 9 aforesaid.

Inside the decontamination tank 1 is a basket 18 for the parts to be decontaminated. Like the rest of the installation, it must be made of a material having a high corrosion resistance, although it may be lower than the resistance of the material of the tank 1. This basket 18 and the other components such as the buffer tank 4, loop 5 and contactor 2 can be made of titanium provided that the medium remains oxidizing or coated material such as enamelled materials or materials coated with a fluoropolymer coating.

The circulation loop 5 comprises, apart from the evacuation duct 9, also a suction duct 19 connected on the one hand to the bottom of the buffer tank 4 and on the other hand to a pump 20 and a discharge duct 21 between the pump 20 and the bottom of the decontamination tank 1, the gas-liquid contactor 2 being mounted in this duct 21.

A duct 22 comprising a valve 23 connects this discharge duct 21, just below the decontamination tank 1 with the buffer tank 4.

The ozone production system 3 is connected to the discharge pipe 21, between the pump 20 and the contactor 2, via a pipe 24.

This ozone production system 3 comprises an ozonizer 25 connected to an oxygen reservoir 26 by a line 27.

The gas-liquid contactor 2 is a co-current contactor formed by a column filled with packing elements providing a high exchange surface, more particularly a static mixer.

The buffer tank 4 is also mounted in a filtration loop 28 and comprises a suction duct 29 connected to the bottom of the buffer tank 4 and to a pump 30, and a discharge duct 31 between the pump 30 and the upper part of the tank. buffer 4, a valve 32, a filter 33, a second valve 34 and a third valve 35 being successively mounted in this discharge conduit 31.

The filter 33 is short-circuited by a duct 36 with a valve 37.

Between the valves 34 and 35, a duct 38 is connected to the duct 31. This duct 38 comprises a valve 39 and is connected to an effluent storage tank 40.

The exhaust duct 11 is connected by a duct 41 to the buffer tank 4.

To decontaminate parts contaminated for example by radioactive elements and from a nuclear reactor, these parts are placed in the basket 18 which is immersed in the decontamination solution in the decontamination tank 1.

These parts are covered with corrosion products and in particular with a layer with a high content of chromium oxides, chromium being present at the 3 ⁺ valence.

After closure of the lid 6, the decontamination solution, heated to a temperature between 60 ° C and 90 ° C and preferably at a temperature between 80 ° C and 85 ° C, for example at 82 ° C, in the buffer tank 4, is transferred by the pump 20 from the latter into the reservoir of decontamination 1.

In the variant where the heating system 10 is in the decontamination tank 1, the solution is heated to the aforesaid temperature in the latter tank.

This decontamination solution is an acid solution of cerium sulphate thus containing Ce ⁴⁺ . The principle of decontamination is based on the oxidative nature of this ⁴⁺ / Ce ³⁺ couple. When this solution is brought into contact with steels, it leads to their corrosion by oxidation reactions of metals and oxides.

In order to minimize the consumption of cerium IV and to ensure maximum stability to the solution, the electrolyte must be chosen carefully. The most suitable electrolyte according to the invention is sulfuric acid, although nitric acid is also usable.

The total concentration of cerium is between 0.1 and 50 g / l and preferably between 1 and 15 g / l, for example of the order of 0.05 M and the concentration of sulfuric acid between 10 ^-1 and 2 M, preferably between 1 and 2 M, for example 1 M.

The aforementioned decontamination solution circulates continuously through the circulation loop 5, that is to say, the solution overflowing by the overflow 8 returns to the buffer tank 4, from where it is pumped via the conduit of suction 19 by the pump 20.

Then this solution is discharged through the contactor 2 where it is regenerated by means of ozone of the ozone production system 3 and, before returning via the conduit 21 to the decontamination tank 1.

During the oxidation of the contaminated parts in the decontamination tank 1, the cerium 4 ⁺ is in fact consumed and converted into Ce ³⁺ . In contactor 2, this cerium is oxidized to bring it back to the 4 ⁺ valence by the reaction in an acid medium:

O ₃ + 2Ce ³⁺ + 2H ⁺ = 2Ce ⁴⁺ + O ₂ + H ₂ O

The oxygen of the reservoir 26 is charged with ozone, for example with a concentration of 5 to 500 g of ozone per m ³ , in the ozonizer 25, and is injected towards the line 27 at the bottom of the contactor 2.

The ratio of ceric sulfate (Ce ⁴⁺ ) to cerous sulphate (Ce ³⁺ ) is between 20 and 0.1 and preferably between 3 and 0.5. The ratio Ce ⁴⁺ / Ce ³⁺ must be maintained at a value greater than or equal to 1 to ensure sufficient attack speed.

The ozone flow rate is adjusted according to the particular application and is essentially a function of the treated surface, the etching rate of the material of the parts to be decontaminated and the regeneration efficiency. This flow rate is normally between 0.1 and 1 kg O ₃ / h for 20 m ² of treated surface.

The oxygen charged with residual ozone leaving the exhaust pipe 11 is first cooled in the condenser 13 to condense the acid vapors that are evacuated via the tank 16 to the buffer tank 4 via the ducts 17 and 9. The gases leaving the condenser 13 are removed from the liquid aerosols in the stripper 14 after which the residual ozone is destroyed in the unit 15.

The flow rate of the solution pumped by the pump 20 depends on the particular application but is generally between 10 and 100 renewals of the contents of the decontamination tank 1.

As the solution passes continuously in the same circulation loop 5 and therefore the overflowing solution of the decontamination tank 1 is sent to the contactor 2, through the buffer tank 4 which is maintained at the relatively high temperature above, both the decontamination as the regeneration take place at this same rather high temperature.

During the decontamination of parts covered by a layer of oxides, a part of the oxides is detached without dissolving.

For this reason, the solution is filtered after decontamination. The valves 32, 34 and 35 are open and the pump 30 is turned on. The solution is pumped from the buffer tank 4 and discharged through the filter 33 to this buffer tank 4.

The filtration rate is normally from 1 to 10 renewals of the contents of the tank 4 per hour.

To accelerate the decontamination process, the ultrasound probes 7 plunging into the bath of the tank 1 can emit ultrasound. These ultrasounds accelerate the kinetics of the process and can achieve either lower residual contamination levels, or achieve identical efficiencies in shorter times.

Thus the residence time of the parts to be decontaminated in the decontamination tank 1 can be reduced to 1 to 8 hours, depending on the particular application.

After this treatment, the solution in the decontamination tank 1 is transferred to the buffer tank 4 after opening the valve 23 and the basket 18 is removed from the tank 1, drained and transferred to a rinsing tank.

The cleaning of the parts in the rinsing tank is achieved by preferably using an ultrasonic cleaning combined with a closed circuit filtration of the rinsing solution.

After this cleaning, the basket 18 is removed from the rinsing tank and drained and the pieces are removed from the basket 18 and checked.

Depending on the level of residual contamination, these parts are either discharged as non-radioactive waste, recycled for a second pass through the decontamination device, disposed of as radioactive waste or discharged to a metal waste melting facility.

When the radiological activity or concentration of dissolved metal in the decontamination solution exceeds a certain value, the solution is transferred from the buffer tank 4 via line 38 in the liquid effluent storage tank 40 by opening valve 39.

The device described above can be used to decontaminate the equipment on site. It is sufficient to connect the circulation loop 5 via a pump and temporary conduits to this equipment.

The solution circulating in the loop 5, the oxidation and the regeneration take place at the same time and continuously, at the same rather high temperature. Despite the high temperature, the regeneration efficiency, that is the ratio between the amount of ozone used and the amount of ozone produced, is high. The rate of destruction of the ozone and the activation energy of the oxidation reaction are therefore sufficiently low.

The contactor 2 allows optimal extraction of ozone from the gas phase, that is to say oxygen or air, and a sufficient contact time between the gas loaded with ozone and the solution.

It is obvious that many modifications can be made to the examples described above, without departing from the scope of the invention.

Claims (14)

1. Method for decontamination of metallic surfaces according to which these surfaces are treated using an acid solution containing cerium with a valency of 4⁺, the ceric ion being continuously regenerated with ozone in a gas - liquid contactor (2) at substantially the same temperature as the above mentioned solution, characterised in that the metallic surface is treated at a temperature of between 60°C and 90°C by the acid solution containing cerium with a valency of 4⁺, the cerium with a valency of 4⁺ being regenerated by the injection of ozone into the decontamination solution in a static mixer type gas - liquid contactor (2) in which ozone and the cerium based acid solution are transported in co current flow.
2. Method set forth in claim 1, characterised in that the acid solution containing cerium includes sulphuric acid as an acid medium, preferably with a concentration of between 10^-1 and 2 M.
3. Method set forth in either claim 1 or 2, characterised in that the cerium with a valency of 4⁺ originates from cerium sulphate, preferably with a concentration of between 0.1 and 50 g/l of the solution.
4. Method set forth in one of the above claims, characterised in that the pieces to be decontaminated are subjected to ultrasonic waves in the solution.
5. Method set forth in one of the above claims, characterised in that during decontamination, the solution circulates through a closed loop (5) composed of a decontamination tank (1) where decontamination takes place, a buffer tank (4) and a gas - liquid contactor (2) at which ozone is injected for regeneration, connected through pipes (9, 19, 21).
6. Method set forth in one of the above claims, characterised in that after decontamination, the solution is filtered.
7. Installation for decontamination of metallic surfaces, including a decontamination tank (1) and a gas - liquid contactor connected to an ozone production system (3), the tank (1) and the contactor (2) being installed in a decontamination solution circulation loop (5), characterised in that the regeneration gas - liquid contactor (2) is of the static mixer type.
8. Installation set forth in claim 7, characterised in that it includes a buffer tank (4) connected to an overflow (8) of the decontamination tank (1), the input of the contactor (2) being connected through a pipe (19) to the buffer tank (4) and the output of the contactor (2) being connected through the pipe (21) to the decontamination tank (1), a pump (20) being installed in the first pipe (19), the ozone production system (3) also being connected to this pipe (19) between this pump (20) and the contactor (2), the above mentioned pipes (19 and 21) forming part of the circulation loop (5).
9. Installation set forth in claim 8, characterised in that the heating means (10) are installed in the buffer tank (4).
10. Installation set forth in claim 8, characterised in that heating means (10) are installed in the decontamination tank (1).
11. Installation set forth in any one of claims 7 to 10, characterised in that at least one ultrasound probe (7) is installed on the decontamination tank (1).
12. Installation set forth in any one of claims 7 to 10, characterised in that the contactor (2) is a static mixer including a column filled with coating elements.
13. Installation set forth in either claim 8 or 9, characterised in that it includes a filtration circuit comprising a filter (33) connected to the buffer tank (4).
14. Installation set forth in any one of claims 7 to 13, characterised in that the decontamination tank (1) is made of zirconium.

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