DE69613496T2 - Foam for decontamination containing ozone and method for decontamination with this foam - Google Patents

Description

The invention relates to an ozone-containing decontamination foam which can be used in particular to decontaminate or rinse the surface of objects, in particular relatively large or complexly shaped objects, especially hollow vessels. The invention also relates to a decontamination process which uses this foam.

The foam according to the invention is used in particular for the decontamination of equipment or circuits in the nuclear, chemical, agricultural, food or sanitary industries. It is particularly used for the decontamination of circuits or equipment made of copper, copper alloy, ordinary steel or iron, chromium and nickel alloys, such as stainless steel, in particular austenitic stainless steel, for example stainless steel 304L, or the alloy known under the brand name Inconel.

Chemical or radioactive decontamination of the surface of objects is generally carried out by wet means using chemical processes that use solutions containing decontamination reagents and generally have a pickling effect.

The decontamination techniques used with solutions can consist of washing, spraying or immersion, but when the pieces to be decontaminated are large in size, these techniques are unsuitable because they have the disadvantage of requiring large volumes of reagents and consequently leading to the formation of very large quantities of liquid active effluents.

To decontaminate such items using smaller volumes of reagent, foams containing decontamination solutions can be used.

Therefore, a decontamination foam is a microdispersion or a gas-liquid emulsion consisting of:

- a liquid phase forming the dispersion medium, which (which) contains the dispersing reagent(s) and the additive(s) required for foam formation, and

- a gaseous phase constituting the dispersed phase, which is generally formed by air, nitrogen or a neutral gas such as argon (EP 0 526 305 A).

In such a foam, the volume of the liquid phase can be small, since the foam generally contains 5 to 50 times more than liquid. Consequently, the liquid active effluents obtained after decontamination are significantly reduced.

In the specific case of radioactive decontamination of elements of the nuclear industry, it must be noted that the radioactive contamination decays into two types, regardless of whether it is fission products, activation products or actinides.

These two types of contamination are:

- surface radioactive contamination due to radioactive elements deposited on the surfaces of equipment and pipes and held in place by metal oxides also present on these surfaces, and

- deep radioactive contamination, due to a small fraction of the radioactive elements penetrating the metal of the equipment and pipes, i.e. into the microcracks and grain boundaries.

In the context of the maintenance of nuclear installations, it is generally sufficient to remove the surface radioactive contamination. However, a certain residual dose remains which irradiates the personnel working on the installation. In the context of the dismantling of nuclear installations, on the other hand, it is necessary to remove not only the surface metal oxides and their impurities, but also the metal (generally steel) that makes up the equipment and pipes, eroding it by a few micrometers in order to remove the radioactive contamination that has penetrated the microcracks in the metal. This reduces the dose absorbed by the personnel working on the installation to practically zero and also allows the steel to be reused and recycled after melting in a specialised steelworks.

A process for decontamination and removal of corrosion products deposited on the contaminated surfaces of a nuclear reactor is already known from the French patent application FR 2 454 159. According to this process, the surfaces to be treated are brought into contact with ozone in the form of water saturated with ozone or of water mist containing ozone. The ozone dissolves the chromium oxides in the treated steels and makes them sensitive to subsequent treatment with acid reagents.

This process is therefore selective in the dissolution of the superficial oxide, but gives a dissolution of the metal surface which is very low, even zero. In addition, it generates a large amount of liquid waste.

According to the article by J.R. COSTES and J.P. GAU- CHON entitled: "Foam Decontamination of Large Nuclear Components before Desmantling", Waste Management '92, Meeting in Tucson, 1992, 738-743, a decontamination foam is also known which is used to decontaminate hollow objects in the nuclear industry, in particular those made of steel and having complex shapes such as valves or heat exchangers. This foam has a liquid phase comprising at least one foaming agent and a mixture of acids or bases for decontamination and a gaseous phase containing air, nitrogen or argon. The application envisaged by this document concerns only ferritic steels.

A process for decontaminating the surfaces of the primary circuit of pressurized water reactors, namely the surfaces made of chromium-nickel-iron alloy, is also known from patent application WO 85/04279. This process consists in bringing the surfaces to be treated into contact with an acidic decontaminating solution containing cerium (IV) nitrate and chromic acid in the presence of ozone as an oxidizing agent. The ozone is preferably in the form of a saturated solution. It can also be used in the form of a two-phase gas-water mixture in which the ozone is in gaseous form, or in the form of oxygen enriched with ozone.

However, the attack on the metal is still very slow and very weak (of the order of a few micrometers in 48 hours), but it is precisely the erosion of the surface layer of the metal that allows good decontamination to be achieved. In addition, the volumes of reagent brought into play represent several times the volume of the circuit of the equipment to be decontaminated.

A process similar to that described in WO 85/04279 is also known, but it does not use ozone and consists in adding Ce(IV) in the amount that it is consumed to form Ce(III). However, this leads to Apart from being expensive, this product tends to accumulate ions in the solution, which increase the volume of the dry residue. However, it is precisely this dry residue that, after treatment, contains all the initial contamination and is destined to be conditioned and stored for very long periods. Consequently, the more the volume of the dry residue is increased, the higher the storage costs are.

Finally, a process similar to that described in WO 85/04279 is known, but which does not use ozone and which also consists in electrolytically regenerating cerium(III) to cerium(IV). However, this solution is difficult to use in the treatment of ferrous alloys because it is then the regeneration reactions of Fe²⁺ to Fe³⁺ that predominate at the electrodes.

The invention therefore aims to remedy the disadvantages of the aforementioned previous technology.

For this purpose, the invention relates to a decontamination foam. According to the characteristics of the invention, this foam comprises:

- a) a liquid phase comprising a solution of at least one surfactant and a redox couple forming a decontamination agent, and

- (b) a gaseous phase dispersed in the liquid phase and comprising air enriched with ozone or oxygen enriched with ozone (preferably between 1 and 1 000 g of ozone per Nm3 of oxygen, or more preferably between 10 and 200 g of ozone per Nm3 of oxygen).

Advantageously, the redox couple forming a decontamination agent is chosen from the pairs Ce⁻⁺/Ce⁻³⁺, Co⁻⁺/Co⁻²⁺, Ag⁻⁺/Ag⁻, S⁻O⁻²⁻/SO⁻²⁻, V⁻⁺/V⁻³⁺ or Cr⁻⁺/Cr⁻³⁺ and combined with at least one acid. This acid makes it possible to solubilize the redox couple. It can be chosen from HCl, HNO⁻⁺, H⁻⁺SO⁻⁺, H⁻⁺PO⁻ or their mixtures or be a weak acid. This redox couple can also be MnO⁻⁺/MnO⁻ combined with at least one base, namely a strong base.

Thanks to the properties of the invention, the oxidizing effect of the ozone is increased due to the enormous increase in the liquid surface (in the form of fine lamellae) present in the foam bubbles and due to the considerable dissolution of the ozone inside this foam, while on the other hand the ozone has a very limited solubility in water.

The effect of the ozone-containing foam is particularly remarkable for the decontamination of the first oxide layer that forms on the surface of the copper alloys or iron, chromium and nickel alloys, in particular austenitic stainless steels. In fact, the liquid layers in contact with the metal surfaces to be treated are rapidly renewed due to the coalescence (degradation) of the bubbles and the drainage of the liquid phase on these surfaces, whereas if a liquid decontaminant solution is used and not a foam, the boundary layers hinder the deep action of the liquid phase. In addition, the use of a foam makes it possible to remove deposits and metallic particles of several mm³, which is not possible with a conventional liquid decontaminant solution. For this reason, the foam of the present invention is particularly suitable in cases where a strong and rapid action is required, for example for demolition and to "decategorize" the waste.

On the other hand, this decontamination foam is also particularly suitable for decontamination of the surface layer of a metal substrate. Indeed, the cerium (IV) initially introduced into the foam (for example) tends to decompose into cerium (III), but can then be converted back into cerium (IV) by the action of the ozone and can thus oxidize the chromium and make it soluble, which facilitates decontamination. For this reason, the invention is very economical in terms of cerium (or other redox element) and it is not necessary to add cerium again during the use of the foam.

The surfactant used in the liquid phase of the foam is preferably a betaine, in particular a sulphobetaine or an alkyl ether of an oligosaccharide, which have the advantage of being biodegradable. These two surfactants can also be used in a mixture.

In particular, one can use a sulfobetaine such as that sold by Seppic under the trade name Amonyl®.

Examples of alkyl ethers of oligosaccharides include those sold by Seppic under the trade name Oramix CG 110® and those sold by Rohm & Haas under the name Triton CG60®.

The combination of the two surfactants is also advantageous because it remains surfactant whatever the pH and is therefore equally suitable in neutral medium as well as in acidic or basic medium.

It was unexpectedly discovered that the surfactant used in the decontamination foam decomposes very slowly when it comes into contact with ozone. For this reason, it is sometimes necessary to add a little more surfactant to the decontamination foam during use. On the other hand, ozone has the great advantage of destroying the surfactants over a longer period, namely during the intermediate storage of the liquid waste obtained at the end of the decontamination before the liquid waste is discharged into the treatment plant. The foam-forming effect of this liquid waste is then considerably reduced.

In a general manner, the liquid phase consists of an aqueous solution containing 0.1 to 3 mol/l H₂SO₄ (or better 1 mol/l), 0.1 to 3 mol/l HNO₃ (or better 1 mol/l), 0.001 to 0.1 mol/l ceric sulfate (or better 0.005 mol/l) and 0.2 to 1% by weight of surfactant (or better 0.5%).

The invention also relates to a decontamination process consisting in bringing contaminated surfaces into contact with the aforementioned foam. This operation is generally carried out under ambient pressure and at a temperature of between 10 and 70°C, preferably between 20 and 30°C. Indeed, the reactivity of the foam increases with temperature, but above 70°C the foam tends to coalesce.

Advantageously, before the decontamination step, at least one degreasing step is carried out using a degreasing foam whose gaseous phase is air and whose liquid phase is an aqueous solution of at least one surfactant, a base and a viscosity-increasing agent, and after the decontamination step has been carried out, a rinsing step is carried out at least once using a rinsing foam whose gaseous phase is air and whose liquid phase is an aqueous solution of at least one surfactant.

In addition, after decontamination and when the aqueous phase of the decontamination foam contains an acid, the degreasing, decontamination and rinsing foams can be mixed to form a neutral effluent in which the residual ozone oxidizes the surfactant.

Finally, preferably after decontamination, any decontamination foam used is recovered, filtered and fed into the decontamination process and the gaseous phase of the decontamination foam is treated on a catalyst to destroy the remaining ozone before the gaseous phase is released into the atmosphere.

The invention will be better understood from reading the following description of an embodiment of the invention, given by way of illustration, not by way of limitation, said description being made with reference to the accompanying single figure which represents a plant for producing the foam used in carrying out the decontamination process according to the examples below.

This figure shows that the system comprises a container for decontamination solution 1, which is connected to a foam generator 3 via a line 5 equipped with a filter 7 and a circulation pump 9.

The foam generator 3 is supplied with gaseous phase from an ozone generator 11 via a line 13. A line 15 connects the foam generator 3 with the heat exchanger 17 to be cleaned. This will be described in more detail later.

In this generator, the foam is in contact with the walls to be decontaminated, then this foam exits the heat exchanger through an upper opening 19, while the liquid (coalesced foam) flows out the bottom 21 of the heat exchanger and is removed into the solution tank 1 through a line 23 provided with a filter 25 and a circulation pump 27.

The upper opening 19 is connected to the solution tank 1 by a line 29. The gaseous phase separated in the solution tank 1 is filtered through high efficiency filters 31 and then treated on a catalyst 32 before being discharged into the atmosphere by means of an exhauster 33.

The following examples illustrate the preparation and use of a foam according to the invention.

The decontamination tests were carried out on two decommissioned tubular heat exchangers 171 and 1711, which originate from the nuclear industry. These heat exchangers have the following characteristics:

- Material: stainless steel 304L,

- Dimensions of the casing: length 6.5 m, diameter 0.5 m,

- Surface of the tubes: 214 m²,

- the primary fluid circulated on the calandria side without flow restrictors. It is therefore the space between the tubes that was contaminated with beta and gamma radioelements and that is the subject of decontamination.

- the secondary fluid circulated on the tube side in three passes.

To carry out these tests, the insulation plates for the inlet and outlet of the primary fluid were replaced by plates equipped with a tap for the introduction of foam and a tap for the suction of liquid. The two heat exchangers were placed on slightly inclined planes.

In the following two examples, the decontamination procedure consisted of carrying out four phases of approximately 4 hours each:

- first phase: this is a degreasing phase, carried out using a basic degreasing foam, the liquid phase of which contains 250 l of water, 40 kg of NaOH and 0.5% by weight of surfactants.

- second phase: this is a phase of dissolution of the oxides and etching of the metal, carried out using an acid foam, the liquid phase of which contains 250 l of water, 16 kg of 92% sulphuric acid, 46 kg of 62% nitric acid, 6 kg of cerium (IV) sulphate and 0.5% by weight of surfactants.

- third phase: it is identical to the second.

- fourth phase: this is a rinsing phase using a rinsing foam made up of 250 l of water and 0.5% by weight of surfactants.

In the two examples mentioned above, the foam was formed in a mixer 3 by means of a hydropneumatic pump which simultaneously injected the liquid phase at 500 l/h and the gaseous phase at 3 to 4 Nm³/h. The pressure of the gas allows the foam to fill the heat exchanger from below. This foam has a humidity of around 15 vol.%. In addition, the pressure of this foam also causes the foam to overflow at the top of the heat exchanger via the return connection to the container for decontamination solution 1. The returning foam is partially destroyed in the container 1 by reversing the atomization of liquid and by centrifugation. In addition, the liquid emanating from this foam after coalescence serves to generate a new foam in the mixer 3. Part of the foam can itself be returned as such via the hydropneumatic pump. The liquids are temperature of 20ºC.

example 1

In this example, the decontaminant is used for heat exchanger No. 17I using aerated foam.

Example 2

Decontamination of heat exchanger No. 17II is carried out using foam containing ozone-enriched oxygen (100 g O₃ per Nm³ O₂) in the presence of cerium(IV).

During the basic and acid phases, the dose rate is measured at the bottom, middle and top of the heat exchanger at the start and at the end of the process (after 4 hours). In addition, 50 ml liquid samples are taken either from the foam return path (line 29) or from the liquid return path (line 23) 1 hour, 2 hours and 3 hours after the start of the process. In order to follow the progress of the decontamination, the dose rate of these samples is measured, as is their total iron content, which shows the dissolution of the metallic layer. Finally, the contamination eliminated and the quantities of ozone and cerium consumed are calculated.

The results are shown in Table 1 below.

In example 1, the added amounts of cerium(IV) are consumed evenly by the oxidation of the metals, which requires continuous additions of up to 16 kg. The decontamination is good, as judged by the reduction in dose rates (16 uGy/h) and the radioactivity removed. The amount of liquid waste water is small: 1.2 m³. The decontamination factor is 40.

In example 2, better results are obtained than in example 1. In fact, a smaller quantity of cerium (IV) is used (6 kg) and this product is expensive. Each of the two acid phases continues the decontamination. The quantity of iron dissolved can exceed by 100% that dissolved in example 1. Finally, and this is the most important result, the residual dose rate of the heat exchanger is low (< 6 uGy/h). The quantity of liquid waste is also low: 1.2 m³. The decontamination factor is 160. TABLE 1

SO: irrelevant

Claims (15)

1. Ozone-containing decontamination foam, characterised in that it comprises: a) a liquid phase comprising a solution of at least one wetting agent and an oxidoreduction pair forming a decontaminant, and (b) a gas phase dispersed in the liquid phase containing ozone-enriched air or ozone-enriched oxygen. 2. Decontamination foam according to claim 1, characterized in that the gas phase contains between 1 and 1000 g of ozone per Nm³ of oxygen, preferably between 10 and 200 g of ozone per Nm³ of oxygen. 3. Decontamination foam according to claim 1, characterized in that the decontamination agent-forming oxidoreduction pair is selected from the pairs Ce⁴⁺/Ce³⁺, Co³⁺/Co²⁺, V²⁺/V³⁺, Ag²⁺/Ag⁺, S₂O/SO or Cr⁴⁺/Cr³⁺ and that at least one acid is involved. 4. Decontamination foam according to claim 3, characterized in that the acid is selected from HCl, HNO₃, H₃PO₄, H₃PO₄ or their mixtures. 5. Decontamination foam according to claim 1, characterized in that the decontamination agent-forming oxidoreduction couple is MnO/MnO₂, and in that at least one strong base is involved. 6. Decontamination foam according to claim 1, characterized in that the wetting agent is chosen between a betaine, an oligosaccharide alkyl ether or their mixtures. 7. Decontamination foam according to claim 3, characterized in that the liquid phase is formed by an aqueous solution comprising: - 0.1 to 3 mol/l sulphuric acid, - 0.1 to 3 mol/l nitric acid, - 0.001 to 0.1 mol/l cerium sulfate, - 0.2 to 1 wt.% wetting agent. 8. Decontamination foam according to claim 7, characterized in that the liquid phase is formed by an aqueous solution comprising approximately 1 mol/l sulfuric acid, 1 mol/l nitric acid, 0.005 mol/l ceric sulfate and 0.5 wt.% wetting agent. 9. Decontamination process, characterized in that it consists in bringing a surface to be decontaminated into contact with the decontamination foam according to one of the preceding claims at a temperature of between 10 and 70ºC, preferably between 20 and 30ºC, 10. Decontamination method according to claim 9, characterized in that after carrying out the decontamination step, at least one rinsing step is carried out using a rinsing foam whose gas phase is air and whose liquid phase is an aqueous solution of at least one wetting agent. 11. Decontamination process according to claim 9 or 10, characterized in that before the decontamination step, at least one degreasing step is carried out using a degreasing foam whose gas phase is air and whose liquid phase is an aqueous solution of at least one wetting agent, a base and a viscosity agent. 12. Decontamination process according to claim 11, characterized in that after decontamination and when the aqueous phase of the decontamination foam contains an acid, the degreasing, decontamination and rinsing foams are mixed to form a neutral effluent in which residual ozone oxidizes the wetting agent. 13. Decontamination process according to claim 9, 10 or 11, characterized in that after decontamination, any used decontamination foam is recovered and returned to the decontamination process. 14. Decontamination method according to claim 9, 10 or 11, characterized in that after the decontamination step, the gas phase of the decontamination foam is treated in a catalyst in order to destroy the residual ozone before the gas phase is discharged into the atmosphere. 15. Decontamination method according to one of claims 9 to 14, characterized in that it is used for cleaning contaminated surfaces made of copper, copper alloy, steel or an alloy of iron, chromium and nickel such as stainless steel.