WO2015107049A1

WO2015107049A1 - Method for treating sodium-based residues using an aqueous foam

Info

Publication number: WO2015107049A1
Application number: PCT/EP2015/050502
Authority: WO
Inventors: Sylvain Faure; Vincent LERAT; Serge BRESSON
Original assignee: Commissariat à l'énergie atomique et aux énergies alternatives
Priority date: 2014-01-14
Filing date: 2015-01-13
Publication date: 2015-07-23
Also published as: FR3016297B1; FR3016297A1

Abstract

The present invention relates to a method for removing a sodium-based residue that consists of placing said sodium-based residue in contact with a gelled or viscose aqueous foam having an inert gas and a moisture content of 4 % to 20 %.

Description

PROCESS FOR TREATING SODIUM RESIDUES

USING AQUEOUS FOAM

DESCRIPTION

TECHNICAL AREA

The present invention relates to the treatment of sodium and in particular the treatment of installations contaminated with sodium. More particularly, the present invention applies to the decontamination / destruction of sodium during the operation / maintenance or dismantling of sodium-cooled nuclear reactors, associated research facilities or installations of the metal sodium industry.

Indeed, the present invention provides a method of treating sodium-based residues with aqueous foams, gelled or viscose, with controlled humidity and containing inert gas bubbles.

STATE OF THE PRIOR ART Sodium metal is a powerful chemical reducer, paramagnetic

(liquid between 98 ° C and 883 ° C under 1 atm) and has high thermal, electrical and acoustic conductivities. The viscosity of liquid sodium is close to water and its density slightly less than 1. In addition, unlike water, sodium attenuates neutrons very weakly and its activation under neutron flux is weak.

All of these characteristics predispose metallic sodium as the ideal heat carrier in the fast neutron reactor (NRR) pipeline. These reactors use fast neutrons as opposed to thermal neutrons. These fast neutrons with high kinetic energy have the advantage of fissioning fissile materials. The use of fast neutrons also limits sterile catches ie catches do not give rise to a new fission, which tends to improve the efficiency of the reactor. These reactors generally have a primary (radioactive) sodium circuit which transfers heat from the reactor to a secondary (little or no radioactive) sodium circuit which in turn transfers the heat to a water and vapor circuit (for example). intermediate of steam generators).

Provisions for the design and operation of this type of RNR are however necessary for the proper functioning (use of specific materials, purification of sodium in cold traps ...) but also to control the major risks associated with use of sodium.

In fact, sodium reacts strongly with oxygen above 125-

130 ° C (spontaneous inflammation). With high concentration hydrogen, hydrides are formed which are unstable. Sodium reacts with the humidity of the air. With liquid sodium i.e. at a temperature above 200 ° C, there is a risk of spontaneous ignition with release of sodium hydroxide and oxide. With water, the reaction is violent, highly exothermic (138 kJ / mol), explosive due to the release of hydrogen (explosive LEL in air 4%) and producing potentially corrosive soda. Sodium also reacts with alcohols and attacks the glass.

The maintenance and dismantling operations of "sodium equipment", especially in sodium-cooled RN Rs, therefore require specific "cleaning" processes. These processes must limit the chemical risks to the operators and possibly the risk of contamina- tion when the sodium is activated or contaminated.

Eighty percent of sodium treatment processes use water. The water used as a reagent transforms sodium into sodium hydroxide and hydrogen according to the following reaction (I):

Na + H ₂ 0 NaOH + Vi H ₂ 1 ^s + 138 KJ / mol (I)

The sodium / water reaction follows the Arrhenius law and is very exothermic. It gets very fast at the same time as the sodium temperature increases to a value above 600 ° C, there is appearance of a red flash. In addition, it produces Hydrogen, which can explode very easily in the air by energy input (shock or heat).

The various constraints due to this reaction are as follows: maintain a permanent control of the temperature since the reaction is very exothermic and thermosensitive;

limit the amount of water in direct contact with sodium, or the amount of sodium in contact with water and thus limit the energy released;

to limit the corrosion problems with hot soda and this, for a temperature higher than 230 ° C;

- control the percentage of hydrogen released, the ignition limit in the air being 4%.

Various methods of treating sodium using water are listed and discussed below.

Thus the process involving spraying liquid water is used mainly for small parts and containing a small amount of sodium. It consists of immersing or watering the room. A sun roof allows to evacuate the overpressure in case of runaway or explosion. Such a method is easy to use and economical but a risk of explosion and inflammation exists if the amount of sodium is too important. In addition, a large amount of water is needed.

The washing processes discussed below are mainly applied in wells in which large soda components are suspended.

The principle of cold steam washing is to make cold steam by heating deionized water contained in the bottom of a well maintained under vacuum. In addition, the carbon dioxide contained in the well transforms sodium hydroxide into sodium bicarbonate. The process is not harmful to the environment and there is no risk of soda corrosion. The evacuation of the container may however pass air if the seal is not good and in fact produce a strong reaction with oxygen. Cutting off the water heating does not instantly stop steam production and thus stop the reaction with sodium. The technique of washing with hot steam is carried out under an inert atmosphere. After heating the well (150 ° C) in which the soda object is located, water is introduced to the bottom of the well and then heated to obtain steam at 105 ° C. The reaction is followed by the measurement of hydrogen produced. The component is then rinsed. This process has many advantages namely effectiveness of water vapor, speed, low production of effluents, accessibility of steam in difficult areas (holes, games, threads ...). In addition, such a process is more economical and easier to use than cold steam. However, in addition to the formation of rust, there is too much inertia of the dynamics of the process and the possibility of local overheating which lead to the risk of corrosion by soda. As for the cold steam wash, cutting off the water heating does not instantly stop steam production and thus stop the reaction with sodium. The presence of hydrogen is also a potential hazard.

A water atomization (diphasic mist) is performed cold on the soda component, positioned in its inert well. The reaction is controlled by stopping or modulating the flow of water. This easy process of implementation and economic has the advantage of a limited rise in temperature without risk of corrosion. However, its efficiency is lower compared to steam, especially in areas that are difficult to access. Moreover, the sodium present in the retention zones can be isolated by a film of soda which stops the reaction, the sodium then reacts violently during a final rinsing with water.

Another treatment method involving water is a vacuum wash. The C0 ₂ inerted well is evacuated to a pressure of about 400 mbar, to make cold vapor from the water injected at the bottom. Soda produced by the sodium-water reaction is converted with slower kinetics to sodium bicarbonate. This method, although having good efficiency and low temperatures of implementation, does not provide control of the reaction and requires depression in the well.

The CO2 sparge wash is performed in a well in which the soda component is located and a small amount of water is introduced in the bottom. Some gas carbonic is injected at this level to bubble and thus conveyed moisture that reacts with sodium. The water can be reheated to increase the dew point of the gas and thus raise the humidity of the gas. C0 ₂ also allows the conversion of sodium hydroxide to carbonate. The component can be rinsed and sometimes immersed before being dried. The process is controlled by the percentage of hydrogen produced, the temperature of the component and the pressure of the well. It is a safe and controllable, non-corrosive process in which C0 ₂ injection stoppage immediately stops the reaction. It does not remain less long, consumer of large quantities of gas, risky because of the hydrogen produced and only usable for components of simple geometries with little sodium.

The cold spray wash is carried out in a well with atomization of water, the supply of which is gradually increased by using CO ₂ and nitrogen (N ₂ ). The component is then sprayed with demineralized water, immersed and then dried under vacuum under nitrogen. This safe, controllable and non-corrosive process remains long, gas consuming and weakly effective in baffles and games.

WVN ("Water Vapor Nitrogen") in-situ washing processes are used on fixed installation parts and are intended to remove residual sodium with very low kinetics and a limited water supply. This is to circulate an inert gas with a very low dew point so as to transform the sodium very slowly without going through an aqueous sodium phase (diffusion regime). Prior thermal heating is possible if it is desired to decompose the hydrides. This process with preheating is used for the treatment of cold traps. Note that there is a variant with vacuum also for the treatment of cold traps. The main advantage of these processes is their implementation in situ. However, they present radiological risks for the operators, who must intervene on cold active capacities or traps, for the connection of the links of the treatment frames and the heating means.

There is also a treatment, in an inert autoclave reactor, of sodium ingots (a few hundred grams) immersed in water and producing very high pressures. This manageable treatment applies to sodium and its alloy with highly contaminated potassium (NaK) and can be used in cell. However, the quantities treated are low and a large peak of overpressure has already been observed (up to 25 bar for 400 g of Na).

The ELA process for "Active Washing Enclosure" is used to treat specific objects of dismantling, with polluted and very active (contaminated) sodium components, previously placed in a washing basket (perforated) in a dismantling cell. This process allows the treatment of sodium in all its forms by atomized water in a flow of inert gas. The reaction zone is cooled by a strong stream of inert gas which allows dilution of the hydrogen produced, the latter still constituting a risk. This process requires substantial installation with pressure equipment, cooling by the supply of large quantities of inert gas, prior cutting of equipment in "active" cells, careful preparation of baskets to ensure the treatment of areas difficult. accessible, to prevent water or soda from being retained on sodium retention.

The gas carbonation treatment consists of sweeping the container containing sodium with an inert gas (N ₂ ) carrying carbon dioxide and a small amount of water. The humidity of the gas is controlled to prevent condensation on the walls and thus a local water concentration. Sodium reacts with water to form soda and hydrogen, which is then transported by the gas. Finally, soda reacts with CO ₂ and generates sodium bicarbonate or sodium carbonate. Sodium carbonates are very advantageous because they are inert, stable and soluble in water.

NaOH + CO ₂ NaHCO ₃ (II)

2NaOH + CO ₂ Na ₂ CO ₃ + H ₂ O (III)

All the sodium can not be removed with this process, residual sodium remains inaccessible by carbonation, especially in the case of high thicknesses and particular geometries, which must undergo local treatments, the carbonate layer can create a barrier between sodium and water.

The Noah process makes it possible to carry out the hydrolysis of the liquid sodium sprayed in a reaction chamber in the presence of aqueous sodium hydroxide which constitutes a risk. This process makes it possible to treat large amounts of sodium but requires liquid sodium and purified, inert confinement.

Other processes use alcohol rather than water. The reaction of sodium with alcohol is much slower than with water and does not produce soda, but is also exothermic:

Na + C2H5OH NaOC ₂ H ₅ + ½ H ₂ † (IV)

The choice was made of ethyl alcohol to have a reasonable reaction speed because the lower the molecular weight of the alcohol, the faster it reacts.

The percentage of water contained in the alcohol must be less than 4% by volume because the reaction between water and sodium can cause a fire on contact with alcohol. The process is expensive and not totally effective for the sodium present in gaps. It is considered dangerous because of the instability of the reaction. In addition, effluents with an organic base are difficult to treat.

Other methods exist such as burning, vacuum distillation or conventional physical treatments. These are limited in industrial use because relatively tedious. They involve, for example, sanding, blasting, evaporation or scraping.

However, none of the processes presented above are polyvalent and can not meet the entire need for the treatment of sodium. Expected performance requirements vary depending on the nature or amount of sodium to be treated. The choice of a process must also take into account the ultimate waste management in terms of treatment chain. In addition, most of these processes are expensive, and the investments are often significant following the safety analysis of the process.

The inventors have set themselves the goal of developing a process for the treatment of sodium, easy to implement and this, whatever the element to be treated, requiring neither structure nor expensive reagent and low risk for the environment and operators.

DISCLOSURE OF THE INVENTION The objects set and still others are achieved by the invention which proposes a method of treating sodium-based residues with controlled humidity aqueous foams containing inert gas bubbles.

The method of the invention by foam filling or immersion of soda pieces is a new way compared to existing methods and offers a solution to all or part of the disadvantages of these methods.

Indeed, the process of the invention is polyvalent because it makes it possible to treat radiocontaminated or non-radiocontaminated sodium residues in the form of fragments or deposits on particularly complex-shaped metal parts (cold traps) in two configurations that are (i ) the treatment of soda-lined or fragment-based sodium residues or liquid residues in a foam bath contained in open or closed process vessels and (ii) the direct filling of the sodium-containing vessels or pipes with the foam .

In the two configurations mentioned above, the sodium is destroyed by the water of the foam and converted into sodium hydroxide and hydrogen. The gases escaping from the foam are conventionally controlled by dilution or washing.

In addition, the moisture-controlled foam controls the supply of water on sodium, the resulting rise in temperature and the kinetics of destruction of the corresponding sodium. In other words, the foam bath used limits the sprays of sodium hydroxide observed in the other washing processes with water and protects the installations and the operators of the explosion in case of runaway of the water. sodium-water reaction. It also makes it possible to treat substantial thicknesses in contrast to the gaseous process (carbonation). Thus, the method of the invention deployable in the field and inexpensive (water, inert gas, gelling agents / viscosants and surfactants) is more secure vis-à-vis:

- the risk of hydrogen explosion thanks to the confining properties of the foam which absorbs the shock waves in the event of an explosion and

- The thermal runaway of the sodium / water reaction due to the insulating properties of the foam, the walls of the reactor remaining cold.

Finally, the process of the invention produces small amounts of dilute, liquid, pumpable soda which can be treated in a dedicated installation. In addition, if radiological contamination is present, it is trapped in the water films of the foam and is concentrated in a small amount of diluted sodium hydroxide after destabilization of the foam by drainage. It is evacuated by simply pumping the drained liquid from the foam.

More particularly, the present invention provides a method for removing a sodium-based residue comprising contacting said sodium-based residue with an aqueous, gelled or viscose, inert gas foam having a moisture content of from 4 to 20%.

In the context of the present invention, the term "eliminate a sodium-based residue", converting said sodium-based residue to at least one other preferred compound and in a controlled manner. This elimination can be partial or total. Advantageously, the conversion of the sodium-based residue follows the reaction (I) previously explained. More preferably, when the elimination implemented in the context of the process according to the invention is total, the preferred compounds obtained comprise in particular sodium hydroxide and hydrogen. Note that other compounds can also be obtained during this conversion and this, depending on the nature of the sodium-based residue to be treated.

Indeed, in the context of the present invention, a sodium-based residue may be any type of residue comprising sodium and typically met in RNR type reactors, associated research units and / or in the metal sodium industry. Advantageously, the sodium-based residue as treated in the context of the present invention is chosen from the group consisting of a metal sodium residue, a sodium oxide residue (Na ₂ O), a hydride residue of sodium (NaH), a residue of an alloy containing sodium as a residue of a sodium and potassium eutectic alloy (NaK) or a mixture thereof. The sodium-based residue can be contaminated (by different radioelements) and sometimes activated (radiologically). Alternatively, the sodium-based residue is conventional (in the residue sense neither contaminated nor activated).

In other words, the present invention provides a method for removing a sodium-based residue comprising contacting said sodium-based residue with an aqueous, gelled or viscose, inert gas foam having a moisture content of between 4 and 20%, said sodium-based residue being selected from the group consisting of a metal sodium residue, a sodium oxide residue (Na ₂ O), a sodium hydride residue (NaH), a residue of a sodium-containing alloy as a residue of a sodium and potassium eutectic alloy (NaK) or a mixture thereof.

The sodium-based residue as treated in the context of the present invention may be in various forms. Thus, it may be in the form of a deposit, in the form of a fragment or in liquid form.

In the context of a deposit, the latter may have a very variable maximum thickness, especially between 0.1 mm and 30 cm, in particular between 1 mm and 15 cm and, in particular, between 2 mm and 5 cm. Such a deposit can also fill a section of equipment or a section of pipes. Typically, such a deposit can be obtained after aerosolization of sodium on surfaces such as cold zones in the gas phase.

In the context of a fragment, the latter may have a very variable size and shape. Advantageously, the maximum size that can present a fragment of sodium-based residue is between 0.1 cm and 60 cm, especially between 1 cm and 40 cm and, in particular, between 2 cm and 30 cm. The sodium-based residue may also be in liquid form at room temperature; this is particularly the case for the eutectic alloy of NaK contained in certain RN R equipment such as cold traps and other equipment (detector, visus, bubbler ...).

Thus, in most cases, the sodium-based residue as treated in the context of the present invention is a solid residue.

The treatment method according to the present invention uses an aqueous foam, gelled or viscose, inert gas and whose moisture is between 4 and 20%. "Inert gas, gelled or viscous foam" means advantageously a foam consisting of a dispersion of inert gas bubbles in an aqueous foaming solution comprising:

from 0.2 to 2% by weight of a foaming organic surfactant or of a mixture of foaming organic surfactants relative to the total weight of the solution, and

from 0.05 to 0.5% by weight of an organic gelling or viscosifying agent or of a mixture of organic gelling or viscosifying agents relative to the total weight of the solution.

By virtue of its composition, the treatment foam used in the context of the process according to the present invention has the advantages of the controlled-life-time foams conventionally used in the radioactive decontamination treatment (see, for this purpose, the international application WO 2004 / 008463 on behalf of CEA and COGEMA, published on 22 January 2004).

By "inert gas" is meant a hydrogen-inert gas which is evolved during the reaction of sodium with the water of the foaming aqueous solution. In other words, the gas used to generate the foam used in the process according to the invention must be an oxygen-free gas. Advantageously, such a gas is selected from the group consisting of helium, argon and nitrogen. In fact, the foams used in the context of the present invention are distinguished from the foams described in the present invention. International Application WO 2004/008463 which are prepared with air and thus with an oxygen-containing gas.

In one embodiment of the present invention, the foaming aqueous solution consists only of (i) 0.2 to 2% by weight of a foaming organic surfactant or a mixture of foaming surfactants relative to to the total weight of the solution, (ii) from 0.05 to 0.5% by weight of an organic gelling or viscosifying agent or a mixture of organic gelling or viscosifying agents with respect to the total weight of the solution and (iii) water.

Note that the foaming aqueous solution used to prepare the aqueous foam, gelled or viscose, inert gas and controlled humidity of the invention contains no chelating agent or complexing agent of the diphosphonic acid type such as, for example, HEDPA (acid 1-hydroxyethane-1,1-diphosphonic), or organic solvent.

By "water" is meant both tap water, industrial water, deionized water, and distilled water. Advantageously, the aqueous, gelled or viscose, inert gas and controlled humidity foam of the invention is a neutral foam, ie a foam having a pH of between 6.5 and 7.5.

The foam as implemented in the context of the present invention has a moisture of between 4 and 20%. As a reminder, a foam is often characterized by its defined expansion, under normal conditions of temperature and pressure, by the following relation (V):

F = (Volgaz + Vol | liquid) / Vol | liquid = Volmousse / Volliquide (V)

Therefore, the moisture of a foam corresponds to the inverse of its expansion and therefore is defined by the Voliiquide / VoLousse ratio.

The treatment foams prepared in the context of the invention, like the foams described in the international application WO 2004/008463, allow savings on the liquid effluents generated after treatment and total drainage of the foam. The foams used in the context of the present invention have an abundance of between 5 and 25 and, advantageously, of between 7 and 20, which corresponds to a liquid fraction or moisture of the foam of between 4 and 20% and advantageously between 5 and 14%. Note that the volume of liquid (Volhquide) in the above ratios corresponds to the volumes of the various compounds initially mixed to prepare the foaming aqueous solution and, in particular, to the sum of the volume of the surfactant (s) (s) ) foaming organic (s), the volume of the gelling agent (s) or viscosity (s), organic (s) and the volume of water.

The treatment foams prepared in the context of the invention make it possible, for example, to treat in filling large volumes containing sodium residues as defined above, of 100 m ³ , with 5 m ³ to 14 m ³ of liquid. In fact, few liquid effluents to be treated later are produced. This leads to a simplification in terms of the global sodium treatment sector.

Finally, as presented in the experimental section below, the inventors were able to observe three types of treatment depending on the humidity of the foam used:

for an initially slightly damp foam, ie having a humidity of less than or equal to 8%, in particular between 4 and 8%, in particular between 5 and 7% or alternatively between 4 and 6.5%, it has been observed a slow destruction of sodium in a few hours at a temperature in the foam between 100 ° C and 170 ° C. The gases released, such as hydrogen, water vapor and sodium vapors, are first trapped in the foam in the form of pockets without risk of explosion within the foam. Then they are evacuated as bubbles or small fumaroles at the top of the foam and can be recovered by gas treatment systems;

for an initially moister foam, ie having a humidity greater than or equal to 12%, especially between 12 and 20% and in particular between 12 and 15%, it was observed a rapid destruction in a few minutes with a temperature in the foam above 300 ° C. The reaction is more violent but suffocated by the absorption properties of the shock waves and confinement of the foam; and for a foam with an intermediate humidity, ie having a humidity of between 8 and 12%, it has been observed a destruction of sodium at a moderate speed in a few tens of minutes.

These three ranges of moisture can be advantageously and indifferently used to treat sodium-based residues according to the invention. Those skilled in the art will be able to choose the most suitable range depending mainly on the nature of the sodium-based residue, and this without any particular inventive effort in view of the teaching of the present invention.

Thus, in a more particular embodiment, the process according to the present invention is carried out using an aqueous, gelled or viscose foam with an inert gas and having a humidity greater than 10% i.e. having an abundance of less than 10.

The foaming aqueous solution constituting the foam used in the context of the present invention comprises at least one foaming organic surfactant. By "organic surfactant" is meant an organic molecule comprising a lipophilic part (apolar) and a hydrophilic part (polar). By "foaming organic surfactant" is meant an organic surfactant as defined above, further having a hydrophilic / lipophilic balance (or HLB for "Hydrophilic-Lipophilic Balance") of between 3 and 8. As a reminder, the HLB value of a The surfactant can be easily obtained by means of the Davies formula ("A quantitative kinetic theory of emulsion type", "Physical Chemistry of the Emulsifying Agent", "Gas / Liquid and Liquid / Liquid Interface" Proceedings of the International Congress of Surface Activity (1957). : 426-438) and HLB tables for different chemical groups, available to those skilled in the art.

More particularly, the aqueous foaming solution constituting the foam used in the context of the present invention may comprise a single foaming organic surfactant or a mixture of at least two foaming organic surfactants chosen from nonionic foaming surfactants. , anionic or cationic foaming surfactants, amphoteric surfactants, Bolaform type surfactants, Gemini type surfactants and polymeric surfactants.

Advantageously, the aqueous foaming solution used in the context of the present invention comprises a single surfactant or a mixture of at least two surfactants chosen from nonionic foaming surfactants and amphoteric surfactants. In other words, this foaming aqueous solution may contain at least one nonionic foaming surfactant and / or at least one amphoteric surfactant. In this advantageous embodiment, the aqueous foaming solution used in the context of the present invention does not comprise cationic surfactant.

In a first variant of the present invention, the surfactant used is a foaming nonionic surfactant. Nonionic (or neutral) surfactants are compounds whose surface-active properties, in particular hydrophilicity, are provided by unfilled functional groups such as an alcohol, an ether, an ester or an amide, containing heteroatoms such as than nitrogen or oxygen. Because of the low hydrophilic contribution of these functions, the nonionic surfactant compounds are most often polyfunctional. As foaming nonionic surfactants, it is possible to use the foaming nonionic surfactants described in the international application WO 2004/008463. Such a surfactant is, for example, selected from the family of alkylpolyglucosides or alkylpolyetherglucosides, natural derivatives of glucose and biodegradable. These are, for example, the "ORAMIX CG-110" from the company SEPPIC, or the "Glucopon 215 CS" from the company COGNIS.

In a second variant of the present invention, the surfactant used is an amphoteric surfactant. Amphoteric surfactants are compounds that act as either an acid or a base depending on the medium in which they are placed. As amphoteric surfactants, it is possible to use disodium lauroamphodiacetate; betaines or sulfobetaines such as alkylamidopropylbetaine, laurylhydroxysulfobetaine, alkylamidopropylhydroxy-sulfobetaines, such as "AMONYL 675 SB" marketed by the company SEPPIC; where the amine-oxides such as "AROMOX MCD-W", cocodimethylamine oxide marketed by the company Akzo Nobel.

In the foaming aqueous solution constituting the foam used in the context of the present invention, the surfactant or the mixture of at least two surfactants is present in a proportion of from 0.2 to 2% by weight, in particular from 0 to , 4 to 1.5% by weight and, in particular, from 0.6 to 1% by weight relative to the total weight of the solution.

Finally, the foaming solution constituting the foam used in the context of the present invention comprises, in addition to the surfactant (s) mentioned above, an organic gelling or viscosifying agent or a mixture of at least two organic gelling or viscosifying agents in a content of between 0.05% and 0.5% by weight and in particular between 0.1% and 0.4% by weight relative to the total weight of the solution.

Advantageously, such an organic gelling agent is a biodegradable and pseudo-plastic agent allowing the foam to be easily sprayable and to have a shelf life of between 10 min and 6 h.

This (or these) agent (s) gelling (s) or viscosant (s) is (are), more particularly, selected from water-soluble polymers, hydrocolloids or heteropolysaccharides such as, for example, the family of polymers polyglucosides with branched trisaccharide chains, such as Rhodopol 23 xanthan gum marketed by Rhodia. It may also be chosen from cellulose derivatives such as CarboxyMethylCellulose or polysaccharides containing glucose as the only monomer, for example Amigel marketed by Alban Muller International.

In the process according to the invention, the contacting between the sodium-based residue and the aqueous, gelled or viscose foam, with an inert gas and with a humidity of between 4 and 20% can be carried out in different ways. In a 1 ^st embodiment, contacting comprises immersing the sodium-based residue as defined above in a bath of aqueous foam, gelling or viscosée, inert gas and moisture as defined above, of between 4 and 20%. ¹ this embodiment can be used for sodium residues on easily movable parts, for the sodium residue in the form of fragments or residues to sodium in liquid form. In the latter case, the residue based on liquid sodium and in particular the eutectic NaK alloy can be poured into a foam bath.

The foam bath ¹ used in this embodiment is previously generated at the contacting and, in open or closed vessels. The foam bath is advantageously in the form of a foam column also called "foam well" or "wash well". Typically, this foam column has a total height of at least 120 cm and in particular at least 2 m. Parts contaminated with sodium-based residues or fragment-based sodium residues are placed in the core of the foam in the column at an immersion depth of at least 60 cm, including at least 1 m with respect to the surface of the foam column and at a height of at least 60 cm and in particular at least 1 m from the bottom of the foam column.

In a 2 ^nd embodiment, the bringing into contact consists of filling ("in static") structures containing at least one sodium-based residue as previously defined with an aqueous, gelled or viscose foam, with inert gas and with humidity between 4 and 20% as previously defined or to circulate ("dynamic") in such structures, said foam. These structures are chosen in the group consisting of tanks, pipes and enclosures. This 2 ^nd embodiment is particularly suitable for the treatment of elements contaminated with sodium residues and difficult to access. Foam limits dead liquid volumes, occupying the entire space and wetting all surfaces such as pipes, cooling coils and other equipment in the middle or in the tank.

Whatever the embodiment used to make the contact between the sodium-based residue and the treatment foam, the latter may be readily prepared at room temperature (ie at a temperature of the order of 23 ° C ± 5 ° C) by techniques known to those skilled in the art. The first step of this preparation process consists in mixing together the water, the surfactant (s) surfactant (s) foaming (s) and the (s) gelling agent (s) or viscose (s) ( s), organic (s), before generation of the foam. This mixing can be done by adding the components at once, in groups or one after the other. The second step of this preparation process consists in generating the foam. This step can be carried out by any prior art foam generation system known to those skilled in the art. This is any device ensuring the gas-liquid mixture, in particular by mechanical stirring, by bubbling, by static mixer containing balls or not, devices described in the patent application FR-A-2,817,170, or any other device including nozzle systems or venturi for large flows generally between 1 and 1000 m ³ / h.

After total drainage in a few hours, and especially after 1 to 10 hours, at a temperature of between 20 ° C. and 50 ° C., said drainage being controlled by means of the gelling agent (s) or viscosity agent (s), organic (s) present (s) in the controlled humidity aqueous foam as defined above, the latter quickly transforms the soda compounds in liquid soda without explosion thanks to the bubbles of inert gas. The resulting soda is found in a reduced volume of liquid effluent to be treated.

The method according to the present invention is particularly suitable for the treatment of particularly metallic surfaces, soiled by soda compounds, radioactive or not. It is particularly interesting in the treatment of nuclear installations of large volumes, complex geometry or inaccessible for which an economy of chemical reagents used and liquid effluents is necessary. For example, the process according to the invention makes it possible to decontaminate sodium inside the sodium-containing RNR tanks having a volume of between 100 and 500 m ³ . Therefore, it finds an interest in the dismantling and / or maintenance of fast nuclear reactors, past, present and future. The invention applies to eliminate, safely and economically, the sodium residues or its compounds encountered in these facilities. Thus, the invention applies to the decontamination / destruction of sodium during the operation / maintenance or dismantling of sodium-cooled nuclear reactors or associated research facilities or facilities using sodium or its eutectic NaK.

The invention applies in particular to "sodium installations" in operation or dismantling constituted by circuits which have been in contact with sodium derivatives and, in the context of circulation, storage, trapping, sweeping, etc ... These facilities mainly include:

capacities (tanks, tanks, etc.) having residual sodium in the form of films and / or retentions which could be treated by filling with the foam of the invention (foam bath);

equipment (cold trapping knitt pads, bubblers, plugging indicators, valves, etc.) with complex geometry that contain deposits or sodium in their internal structure and which can be treated by immersion in the foam of the invention in a dedicated tank and

large components (pumps and primary exchangers for example) which can be treated by immersion in dedicated foam-filled washing wells of the invention.

Apart from the installations of the RNRs using sodium as coolant, this compound is found in test loops of the nuclear research centers but also in the solar power plants and in the metal sodium industry. The method according to the present invention applies in fact to the decontamination / destruction of sodium during operation / maintenance or dismantling of all these activities.

Other features and advantages of the present invention will become apparent on reading the examples given below by way of illustration and not limitation and with reference to the appended figures. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A and 1B are presentations of experimental equipment implemented. More particularly, Figure 1A is a schematic view of the installation seen from above and Figure 1B is a schematic representation of the various elements present in the beaker in which the treating foam is generated.

Figure 2 shows the evolution of the temperature in the beaker within the foam as a function of time for the treatment of a whole piece of sodium of 10 g with a foam at 8% humidity.

Figure 3 shows the evolution of the temperature in the beaker within the foam as a function of time for the treatment of 10 g large pieces of sodium with a foam at 8% humidity.

Figure 4 shows the evolution as a function of time of the temperature at the center of the foam and at the edge of the latter during the treatment of a section of stainless steel pipe of diameter 50 mm full of sodium with a foam at 8% d 'humidity.

Figure 5 shows the evolution of the temperature within the foam during the treatment of a section of stainless steel pipe of 50 mm diameter full of sodium with a 12% humidity foam, this evolution is followed not only in function of time but also according to the height positioning of the basket in which the section is placed.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

I. Formulation used for the foam.

The composition of the foaming solution during the experiments presented below is as follows:

7 g / l of GLUCOPON 215 Cs foaming surfactant,

- 1 to 3 g / 1 of XANTHANE to modulate the life of the foam,

- demineralized water, and

- nitrogen. II. Operative protocol and schematics.

As shown diagrammatically in FIG. 1A, the foaming solutions 1 are prepared in the laboratory and directly transported in 10 liter jerricans at the treatment site (rigid plexiglass lock 2) to be connected to the foam generator 3 (liquid pump suction). .

The gas injected into the foam generator is nitrogen (two bottles 200 bar 4), the air having been proscribed for the test (risk of presence of oxygen which could react with the hydrogen resulting from the reaction) .

The controlled expansion foam is generated then fills a plastic container of 50 liters (or 30 liters) type beakers open and graduated to the liter 5, in which is carried out the treatment of different soda waste.

A drip tray system 6 and an effluent disposal system 7 make it possible to manage the overflowing foam of the beaker 5 and a camera 8 is placed so as to record the experiments.

At the beaker 5 (Figure 1B), a safety grid 9 placed in the latter avoids the direct contact of sodium with the drained liquid.

A wire mesh basket 10 receiving the sodium sample is suspended from a hoisting system 11 comprising an electronic dynamometer connected to the ceiling of the airlock with the aid of a pulley system, the descent or ascent maneuver being carried out at the outside of the airlock. The basket gradually descended into the foam bath to a height sufficient to leave a "mattress" of foam below and above.

A temperature probe 12 and, incidentally, a pH probe 13 immersed in the foam bath make it possible to follow respectively the evolution of the temperature and that of the pH during experiments.

In addition, various sodium samples have been prepared. It is sodium alone in mass or chips or sections of metal piping containing sodium (film, retention or completely filled) by limiting the mass of sodium, the quantities being of the order of ten or hundred grams. During a test, several measures have been planned:

- the temperature: a temperature reading of the sodium and also of the metal container, when it is present, is carried out by remote display outside the airlock;

the volume of foam: the graduation of the beaker makes it possible to visualize the evolution of H ₂ and of vapor by difference with the initial volume; and

- the mass: a balance placed under the beaker makes it possible to quantify the loss of mass of the sodium.

III. Examples of treatment.

111.1. Slow treatment on a piece of sodium of 10 g.

This treatment was performed with a 6% moisture foam in which is immersed an entire piece of 10 g sodium.

Total treatment time: 2 hours.

Maximum temperature reached in the heart of sodium: 160 ° C after a few minutes.

Conclusion: Slow reaction without sudden rise in temperature or smoke release. The foam formed has a very satisfactory behavior over time and the metallic sodium has completely disappeared.

111.2. Optimal treatment on a piece of sodium of 10 g.

This treatment was carried out under the same conditions as that of Example III.1 with the exception of the use of a wetter foam: humidity of 8%.

Total processing time: 50 minutes.

Temperature reached in mitigate: after a gradual rise (slow dissolution), a temperature of 177 ° C is reached with large releases of bubbles and steam fumaroles of the geyser type but without foam projection. A peak temperature at 350 ° C is noted before seeing a production of fine bubbles associated with a drop in temperature (Figure 2). Phenomena observed: A self-healing of the foam (this one is reformed around the sodium) with a form of beat marking the alternation between the contact of the film of water with the sodium and the production of small peaks of temperature. Small fumaroles like geysers are observed. These are cracks in the foam, giving off a stream of vapor (water or soda) marking a reaction within the rather large foam.

Conclusion: Faster kinetics are observed. The reaction is contained within the foam. A moisture of 8% of the foam is very satisfactory for the destruction of sodium.

111.3. Fast treatment on large pieces of sodium.

Large pieces of 10 g sodium were treated with a foam having a moisture content of 8%.

Total treatment time: 6 minutes.

Phenomena observed: A very strong reaction must have occurred after 3 minutes in the foam with a temperature measured at more than 600 ° C and a strong smoke release. Treatment of a sodium fire (a few grams): As part of this test, the basket was returned to the open air while there was still sodium. A start of fire occurred following contact with the air. This fire was immediately extinguished by plunging the basket back into the foam bath in which the treatment could be completed (Figure 3).

111.4. Treatment of a pipe section contaminated by sodium deposits.

A section of stainless steel tubing with a diameter of 50 mm has sodium retentions with a thickness of up to 5 mm over the entire internal surface and has been treated with a foam with 8% moisture content.

The treatment was carried out in 22 minutes with a peak temperature at 500 ° C without remarkable phenomenon. No degradation was observed on the stainless steel. I II .5. Treatment of a pipe section filled with sodium.

A. Treatment with 8% moisture foam.

A 50 mm diameter stainless steel pipe section full of sodium (49 g) was treated with 8% moisture foam.

Duration of treatment: 40 minutes

Phenomena observed: It was noted a gradual rise in temperature up to a plateau of 100 ° C (superficial melting of concomitant sodium boiling water film) and then up to 200 ° C before a rise to at 270 ° C (at the center of the foam) and fumaroles. No degradation was observed on the stainless steel (Figure 4).

Conclusion: Total reaction that demonstrates the draining and confining nature of the foam (valve effect depending on the height of the "mattress"). In addition, felling of soda vapors occurs during their ascent through the preferential paths (pockets).

B. Treatment with 12% moisture foam.

This treatment was carried out under the same conditions as that of Example I II .5. With the exception of the use of a wetter foam: humidity of 12%.

Total treatment time: about 1 hour.

Phenomena observed: In the median position, the reaction is progressive. In the low position, the reaction becomes more violent (fumaroles and smoke, temperature up to 350-400 ° C) but still confined. The positioning in height of the basket thus becomes one of the control parameters of the sodium-water reaction. No degradation was observed on stainless steel (Figure 5).

Conclusion: This example provides evidence of rapid treatment within the confining environment of the foam.

IV. Conclusion of the tests.

The foam statically destroys both sodium in the form of fragments (velocity influenced by the developed surface), 5 mm deposits (tube) or full patties. Sodium retentions simulated by solid tubes are also treatable.

The foam works slowly at 6% (one hour for 10 g in one piece) and starts to be faster at 8% humidity (10 to 50 g in a few minutes) and even faster at 12% humidity ( a few tens of seconds).

The calories released by the thermal reaction are used to heat the water of the foam. There is usually a plateau of a few minutes around 100 ° C testifying to the coexistence of two phenomena: heat absorbed for superficial melting of sodium and vaporization of water foams films.

All the gases released remain confined initially according to the amount of initial foam. The gases are progressively released in the form of bubbles on the surface of the foam or intermittent fumaroles between 100 ° C and 160 ° C.

It is recommended to initially sink the sod waste under a foam mattress of more than 60 cm high (waste at least 60 cm deep in the heart of the foam and 60 cm above the low level). The rising level of foam translates the production of confined gases: hydrogen, water vapor and sodium vapor within the foam.

The foam cools the reaction zone by adding water. For rapid treatments with a wet foam (> 12%), the confinement of the violent reaction is ensured, the foam playing its role of attenuation of shock waves and capture of aerosol droplets.

The walls of the treatment tank remain cold (waste-wall distances greater than 30 cm).

Control of the end of the reaction corresponds to a stabilized level of the foam, the absence of bubbles or fumaroles on the surface of the foam volume and the decrease in temperature. There is no more production of hydrogen or soda.

Claims

1) A process for removing a sodium-based residue comprising contacting said sodium-based residue with an aqueous, gelled or viscose, inert gas foam having a moisture content of between 4 and 20%,

said sodium-based residue being selected from the group consisting of a metal sodium residue, a sodium oxide residue (Na ₂ O), a sodium hydride residue (NaH), a residue of an alloy containing sodium or a mixture thereof. 2) Process according to claim 1, characterized in that said sodium-based residue is in the form of a deposit, in the form of a fragment or in liquid form.

3) Process according to claim 1 or 2, characterized in that said foam consists of a dispersion of inert gas bubbles in an aqueous foaming solution comprising:

4) Process according to any one of claims 1 to 3, characterized in that said inert gas is selected from the group consisting of helium, argon and nitrogen.

5) Method according to any one of the preceding claims, characterized in that the moisture of said foam is between 5 and 14%. 6) Process according to any one of claims 3 to 5, characterized in that said foaming aqueous solution comprises a single surfactant or a mixture of at least two surfactants selected from nonionic foaming surfactants and surfactants amphoteric.

7) Process according to any one of claims 3 to 6, characterized in that said foaming aqueous solution comprises an organic gelling agent or viscosifying agent or a mixture of organic gelling or viscosifying agents chosen from water-soluble polymers, hydrocolloids, heteropolysaccharides, cellulose derivatives and polysaccharides containing glucose as the sole monomer.

8) Process according to any one of the preceding claims, characterized in that said contacting comprises immersing the sodium-based residue in an aqueous bath, gelled or viscose, inert gas and moisture between 4 and 20%.

9) Process according to any one of claims 1 to 7, characterized in that said contacting comprises filling structures containing at least one residue based on sodium with an aqueous foam, gelled or viscose, inert gas and humidity of between 4 and 20% or to circulate, in such structures, said foam.

10) Method according to any one of the preceding claims, characterized in that said method is applicable to the decontamination / destruction of sodium during operation / maintenance or dismantling

- sodium-cooled nuclear reactors or associated research facilities;

- nuclear research centers;

- in solar power plants and

- in the metal sodium industry.