EP1776707B1 - Method and device for removing flammable gases from a sealed chamber and chamber equipped with one such device - Google Patents

Abstract

Methods and devices for removing inflammable gases produced by radiolysis in a closed chamber containing radioactive matters comprising organic compounds and possibly water, or radioactive matters in the presence of organic compounds and possibly water. Inside the chamber there may be placed a catalyst of at least one reaction for oxidizing the inflammable gases by oxygen contained in the chamber atmosphere, supported by an inert solid support; a catalyst of at least the reaction for oxidizing CO to CO2; possibly an oxygen source; and possibly a hygroscopic microporous inert solid support. Also, chambers for radioactive matters containing such devices.

Description

Technical area

The invention relates to a method and a device for eliminating flammable gases, such as hydrogen, in a closed chamber containing radioactive materials, in the presence of solid or liquid organic compounds and possibly water capable of producing such substances. gas, by radiolysis, or when the radioactive materials include such compounds and possibly water.

The invention also relates to a closed enclosure such as a container, tank, or container intended for the transport or storage of radioactive materials in the presence of organic compounds and possibly water, or comprising components of this type, said enclosure being equipped with such a flammable gas elimination device.

The invention can be used in any closed chamber in which are enclosed radioactive materials containing organic compounds and optionally water, or radioactive materials in the presence of organic compounds and optionally water. By way of non-limiting example, these radioactive materials may be technological waste coming from a manufacture or treatment of fuel elements for a nuclear reactor or from such a reactor.

State of the art

Nuclear installations such as fuel element fabrication workshops for nuclear reactors generate a certain amount of waste, known as "technological waste". This technological waste can include objects and materials of very different natures such as engine parts, filters, scrap metal, rubble, glasses, etc. It also contains organic materials based on cellulose. such as paper, wood, cotton, or in the form of plastics such as vinyl or polyurethane packaging covers, boots, gloves, and miscellaneous objects of polymeric materials. All this waste can also contain small amounts of liquids such as water and organic liquids (oils, hydrocarbons, etc ...). All these wastes in themselves constitute radioactive materials, because they are metal parts activated during their stay in the facilities, or organic or other materials contaminated with uranium or radioactive plutonium powder when they are used in these facilities. same facilities.

Technological waste is periodically evacuated to treatment and storage centers. Their routing to these sites then requires as many precautions as the transport of any other material radioactive. In particular, waste must be packaged and transported in containers meeting the requirements of the regulations for the transport of radioactive material on public roads.

In practice, transport is generally carried out by packaging the technological waste in containers such as drums, trash cans or cases and then placing these containers in containers.

The transport of technological waste poses a specific difficulty related to the nature of the materials transported. Indeed, as we saw earlier, this waste often contains solid organic materials or in the form of residual liquids, or a certain amount of water, contaminated with uranium or plutonium which gives these materials a radioactive nature. However, uranium and plutonium are emitters of α particles which have the particular property of dissociating organic molecules to release gaseous compounds such as carbon monoxide, carbon dioxide, oxygen and nitrogen, as well as flammable gases. This phenomenon, called "radiolysis", results in a dissociation of molecules of carbon and hydrogenated organic compounds such as those contained in plastics and hydrocarbons or by a dissociation of water molecules, with production of hydrogen.

The production of flammable gases, particularly hydrogen by radiolysis, mainly problems when technological waste is confined in a closed enclosure of relatively limited volume. In effect, the radiolysis gases are then released in a confined volume, so that a high concentration of flammable gases can be reached rapidly if the nature of the waste and the intensity of the radiation lead to a large production of these gases.

This problem is particularly critical during transport, because then a large number of waste containers are usually placed in the same container, in order to optimize the transport capacity. Indeed, this has the effect of reducing the free space available in the container for flammable gases that escape waste and containers.

It can also be seen that the waste containers often themselves have a certain tightness, because they are closed by crimped lids that can be provided with gaskets. In this case, the flammable gases preferentially accumulate in the residual free space existing inside each of the containers. Because these volumes are also very small, this can lead to significant concentrations of flammable gases in the packaging containers themselves.

In general, flammable gases produced by radiolysis are an explosive mixture when they are in contact with other gases such as air, when their concentration exceeds a certain value. limit, known as the "flammability threshold". The flammability threshold is variable depending on the nature of the flammable gas and the temperature and pressure conditions. In the case of hydrogen, the flammability threshold in air is around 4%. This means that when the concentration of hydrogen in the air exceeds this threshold, a source of heat or a spark may be sufficient to ignite the mixture or to produce a violent explosion in a confined space.

Various studies and observations have shown that the concentration of flammable gases such as hydrogen, produced by radiolysis in a closed chamber containing radioactive material containing hydrogenated components, can sometimes reach values of about 4% after a few days. This situation corresponds in particular to the case where technological waste emits intense α particles and contains numerous organic molecules.

However, it is common for a container to remain closed for much longer periods before being opened. There is a risk of accident, because a spark caused by shocks or friction can occur during transport in the enclosure of the container or in a container filled with waste. In such a case, the ignition or explosion may spread to the entire load of the container, which results in a risk of serious accident on public roads. A comparable risk exists if the container is caught in an accidental fire situation during transport. Moreover, the risk of accidents remains during the final operations of opening the container and unloading the containers, and when they are opened. Indeed, these operations require many manipulations, then potentially dangerous. It is therefore particularly important to take into account the risk of accumulation of flammable gases in any closed enclosure called to contain radioactive materials containing hydrogenated compounds.

A technique for removing flammable gases such as hydrogen located inside a closed enclosure such as a radioactive waste transport container is based essentially on the introduction into the chamber of a recombination catalyst oxygen and hydrogen in water (or catalytic recombinant hydrogen), in contact with which hydrogen combines with the oxygen present in the air cavity to form water according to the mechanism of catalytic oxidation of hydrogen.

Devices implementing this technique are described for example in the documents EP-A-0 383 153 and EP-A-0 660 335 .

The document EP-A-0 383 153 discloses a device for reducing the internal pressure in a radioactive waste storage container. This device comprises an enclosure placed in an opening of the wall or the lid of the nuclear waste storage container. The interior of this chamber receives a catalyst and has an opening in communication with the interior of the storage container in which is placed a sintered metal candle. The catalyst is separated from the outside by a wire cloth, a vapor permeable plate or a sintered metal cover.

The hydrogen that has formed in the storage vessel passes through the sintered metal candle and arrives on the catalyst where the hydrogen is oxidized into water by the oxygen in the air.

The catalyst used comprises a precious metal, for example palladium on an inert support, for example alumina.

In this document, an external source of oxygen constituted by the ambient air is used, which can not be envisaged for closed, hermetic enclosures or perfectly sealed packages.

The document EP-A-0 660 335 discloses a device for reducing overpressures in waste storage tanks, in particular radioactive waste generating hydrogen, wherein a hydrogen recombination catalyst with oxygen and a desiccant are placed in a closed envelope located inside the storage tank and in communication with its environment via a bursting disc.

Inside the casing are provided two vapor-permeable separation sheets below which two layers of desiccant are arranged.

In a first embodiment, above the separation sheet are two grids supporting the recombination catalyst.

In a second embodiment, above the separator sheet is a layer of an oxidizing agent which is held in place by a gas permeable separator sheet.

The desiccant is chosen for example from silica gel, molecular sieves, dehydrated complexing agents such as, for example, copper sulphate or hygroscopic chemicals such as calcium chloride, magnesium sulphate, or pentoxide. phosphorus, optionally on a support material.

The recombination catalyst is chosen in particular from platinum or palladium-coated catalysts. In this device, the recombiner becomes inoperative once all the oxygen in the chamber has been consumed.

It has furthermore been found that these devices and processes, which use catalytic hydrogen recombiners, have in common a reduced efficiency in the case in particular where the enclosure contained organic carbon compounds.

The document FR-A-2 061 573 discloses a gas purification process which uses the successive passage of the gas on two catalysts with oxygen supply from the outside.

There is therefore a need for a method and a device which makes it possible to eliminate flammable gases, and in particular hydrogen, in a closed chamber containing radioactive materials comprising organic compounds whatever the nature of these and their content. carbon, and possibly water.

There is also a need for a method and a device for the elimination of flammable gases in a closed chamber which make it possible to ensure the elimination of flammable gases, in particular hydrogen for a long period, or even a practically unlimited duration.

• en présence d'autres gaz de radiolyse tels que les oxydes comme le monoxyde de carbone et le dioxyde de carbone,
• sous irradiation et quelle que soit le type et l'intensité de cette radiation,
• à différentes températures, ces températures pouvant être négatives.

There is still a need for a method and apparatus for flammable gas removal in a closed enclosure that is simple, reliable, secure, easy to implement, does not involve expensive and time-consuming procedures, and provides a high degree of reliability. effective removal of flammable gases such as hydrogen under a wide variety of conditions, including but not limited to:

• in the presence of other radiolysis gases such as oxides such as carbon monoxide and carbon dioxide,
• under irradiation and whatever the type and intensity of this radiation,
• at different temperatures, these temperatures may be negative.

The object of the invention is to provide a method and a device that meet, among others, all the needs listed above.

The object of the invention is further to provide a method and a device which do not have the disadvantages, limitations, defects and disadvantages of the methods and devices of the prior art and which provide a solution to the problems posed by the methods and devices of the prior art. prior art, such as those described in the documents EP-A-0 383 153 and EP-A-0 660 335 .

1. a) un premier catalyseur d'au moins une réaction d'oxydation des gaz inflammables par de l'oxygène contenu dans l'atmosphère de l'enceinte, supporté par un support inerte solide;
2. b) un deuxième catalyseur d'au moins la réaction d'oxydation de CO en CO₂; qui est un récipient, réservoir ou conteneur, destiné au transport et/ou au stockage de matières radioactives, ladite enceinte fermée.

This and other objects are achieved according to the invention by a process for removing flammable gases produced by radiolysis in a closed chamber and the flammable gases react with the oxygen contained in the atmosphere of the chamber, this oxidation reaction of flammable gases being catalyzed by the first catalyst containing radioactive materials comprising organic compounds and optionally water, or radioactive materials in the presence of organic compounds and optionally water, in which one places inside the enclosure:

1. a) a first catalyst of at least one oxidation reaction of the flammable gases by oxygen contained in the atmosphere of the chamber, supported by a solid inert support;
2. b) a second catalyst of at least the oxidation reaction of CO to CO ₂ ; which is a container, tank or container, intended for the transport and / or storage of radioactive materials, said enclosure closed.

The oxidation reaction of the flammable gases by the oxygen contained in the chamber atmosphere is generally, essentially, a reaction of oxidation of hydrogen into water.

Preferably, the catalyst a) is a catalyst for at least the oxidation reaction of hydrogen in water.

The catalyst a) supported by an inert solid support constitutes a first active product which allows the continuous removal of flammable gases, and in particular hydrogen, produced by radiolysis of the molecules, organic compounds and optionally water at room temperature. inside the enclosure.

This elimination is ensured by the oxidation reaction of the flammable gases with the oxygen contained in the chamber atmosphere, and in particular by the recombination reaction of the hydrogen with the oxygen of the chamber atmosphere. to give water.

Catalyst a) of this oxidation reaction, which is supported by an inert solid support, may be a precious metal which is advantageously selected from the group consisting of platinum, palladium, and rhodium.

The precious metal is present in an amount which is generally less than 0.1% by weight.

Either the catalyst a) of this oxidation reaction may be a rare earth, advantageously chosen from the group of lanthanides such as lanthanum.

The catalyst support a) is an inert solid support.

By inert support is meant a support which does not react chemically with the compounds in the enclosure, the atmosphere thereof, and the other active products.

Preferably, the catalyst support a) is a microporous solid inert carrier.

This microporous support is generally chosen from the optionally activated molecular sieves.

The term activated is a term commonly used in this field of the art, which means that the compound forming the molecular sieve such as alumina has undergone a particular thermal treatment in particular, to increase its specific surface area.

This molecular sieve is preferably a material selected from aluminas and activated aluminas.

The solid, inert, microporous support generally has a high specific surface area, ie a surface area generally of at least 200 m ² / g, preferably at least 300 m ² / g.

The catalyst b) comprises a second active product, it catalyses the oxidation reaction of CO to CO _2. Preferably, catalyst b) is a specific catalyst for the CO ₂ oxidation reaction in CO ₂ .

By specific is meant that the oxidation kinetics of CO ₂ CO ₂ catalysed by b) is much greater than that catalyzed by a).

Preferably, catalyst b) comprises a mixture of manganese dioxide MnO ₂ and copper oxide CuO.

The method according to the invention uses a combination of two active products, specific catalysts a) and b) which has never been described in the prior art as represented in particular by the documents EP-A-0 383 153 and EP-A-0 660 335 .

The process according to the invention essentially because of the use of such a specific combination of active products, catalysts a) and b), meets the needs and requirements listed above and provides a solution to the problems posed by the processes. of the prior art. In particular, the inventors have been able to demonstrate that the efficiency processes of the prior art are greatly reduced in the presence of other radiolysis gases such as carbon monoxide CO; this decrease in efficiency is explained by a poisoning of the oxidation catalyst of H ₂ (a)) such as palladium, by CO carbon monoxide.

Therefore, by associating with the oxidation catalyst flammable gases a catalyst b) the CO ₂ CO ₂ reaction, it is surprisingly possible to avoid the poisoning of the catalyst a) oxidation of flammable gases by CO . Catalyst b) ensures the continuous removal of carbon monoxide, by oxidation, to give carbon dioxide which causes no problem of poisoning catalyst a).

There was nothing to suggest in the prior art that the decrease in catalyst efficiency was basically due to the presence of CO in the chamber. There was no indication in the prior art that would have led the skilled person to associate with catalyst a), such as a recombiner, commonly used in this field of the art, a specific catalyst b).

The method according to the invention makes it possible to effectively remove, over a very long time, or even a virtually unlimited duration, flammable gases such as hydrogen, contained in the closed chamber. It retains a high degree of efficiency irrespective of the waste contained in the enclosure and in particular when the waste contains organic carbon and hydrogen compounds capable of releasing both CO and hydrogen. The process according to the invention functions perfectly in the presence of various radiolysis gases which are in addition to hydrogen, for example CO, CO ₂ , etc.

The process according to the invention likewise functions perfectly in a wide range of temperatures and in particular at negative temperatures and under irradiation whatever the nature of the latter.

Optionally, there is placed inside the enclosure, in addition to the two catalysts a) and b) which are always present, a source of oxygen c). This source of oxygen is a third optional active product that makes it possible to overcome the lack of oxygen, once all the oxygen initially present in the chamber has been consumed.

This source of oxygen can be either in gaseous form or in solid form.

In the case where the oxygen source is in solid form, it is generally chosen from solid peroxides. These compounds release oxygen in the presence of water which is for example the water formed during the oxidation of hydrogen by the catalyst a). These solid peroxides are generally selected from alkali and alkaline earth metal peroxides and mixtures thereof such as calcium peroxide, barium peroxide, sodium peroxide, potassium peroxide, magnesium peroxide and mixtures thereof.

In the case where the oxygen source is in gaseous form, it is generally formed in replacing all or part of the atmosphere of the enclosure with pure oxygen.

Optionally, a microporous hygroscopic inert solid support d) is also placed inside the enclosure.

The hygroscopic microporous inert solid support constitutes a fourth optional active product, making it possible to continuously ensure the lowering of the hygrometric degree of the atmosphere of the enclosure, by adsorption of water. Depending on the temperature, the amount of water removed generally represents from 15% to 30% of the weight of the microporous hygroscopic support. The residual moisture inside the chamber is thus kept at a low value, for example less than 10% (humidity level) until saturation of said support. This allows in particular to capture the free water produced by the oxidation reaction, in particular hydrogen, catalyzed by the catalyst a) and which would not have been absorbed in the micropores of the solid support, such as alumina, catalyst a) which can generally absorb up to 30% of its mass in water.

The hygroscopic microporous support is preferably chosen from molecular sieves.

Advantageously, the molecular sieve of the microporous inert solid support d) is made of a material chosen from silico-aluminate type materials, for example of formula Na ₁₂ [(AℓO ₂ ) ₁₂ (SiO ₂ ) _12] XH ₂ O, with X up to 27, or 28.5% by weight of the anhydrous product.

The hygroscopic microporous support generally has a high specific surface area, i.e. at least 200 m ² / g, preferably at least 300 m ² / g.

It should be noted that this fourth active product is particularly present in the case where the third active product is constituted by a source of gaseous oxygen. Indeed, in this case, the presence of water that would not have been absorbed by the catalyst support a) such as alumina, is not necessary to generate oxygen, contrary to the case where the Oxygen source consists of a solid peroxide that releases oxygen only in the presence of water.

Preferably, the solid inert microporous support supporting the catalyst a); catalyst b); and optionally the oxygen source c) and the microporous hygroscopic support d) are in the form of discrete elements, particles, such as for example crystals, beads or granules optionally forming a powder.

Thus, in a preferred embodiment of the invention, the solid inert carrier, preferably microporous, supporting the catalyst a); catalyst b); and the hygroscopic microporous support d), if any, are fractionated into discrete elements, such as for example crystals, beads or granules, having an envelope diameter generally of between about 2 mm and about 20 mm.

The expression "envelope diameter" designates the diameter of a fictional sphere constituting the envelope of said element.

The active product c) is advantageously in a finely divided form such as a powder.

In general, the active products a), b) and optionally c) and d) are placed, mixed or separately, in at least one container, at least partially permeable, such as a textile envelope, a strainer, a wire mesh , or a container pierced with holes, such as a cartridge.

Preferably, the active products a) and b) are mixed.

On the other hand, active products c) and d) must be separated.

For example, each of the active products can be dispersed between two screens in the form of superposed layers or form a single layer with a mixture of the two mandatory active products a) and b), each of the optional active products c) and d) being separately packaged. for example in the form of separate layers.

Several containers such as cartridges can be placed inside the same closed chamber in order to increase the exchange surface.

The mass ratio between the catalyst b) and the catalyst a) is generally from 1/1 to 1/10, preferably from 1/2 to 1/4, this mass ratio being generally given for a ratio of the flow rate of CO on the flow rate of H ₂ generally about 1:11.

1. a) un catalyseur d'au moins une réaction d'oxydation des gaz inflammables par de l'oxygène contenu dans l'atmosphère de l'enceinte, supporté par un support inerte solide ;
2. b) un catalyseur d'au moins la réaction d'oxydation de CO en CO₂ ;

éventuellement une source d'oxygène c) ; éventuellement un support solide inerte microporeux hygroscopique d) ; a), b), c) et d) étant tels que définis plus haut.The invention also relates to a device for the elimination of flammable gases produced by radiolysis in a closed chamber which is a container, tank or container, intended for the transport and / or storage of radioactive materials, said sealed enclosure containing radioactive materials comprising: compounds organic and optionally water, or radioactive materials in the presence of organic compounds and optionally water, comprising:

1. a) a catalyst for at least one oxidation reaction of the flammable gases by oxygen contained in the atmosphere of the enclosure, supported by a solid inert support;
2. b) a catalyst of at least the oxidation reaction of CO to CO ₂ ;

possibly a source of oxygen c); optionally a microporous inert solid support hygroscopic d); a), b), c) and d) being as defined above.

Finally, the invention also relates to a closed enclosure, which is a container, tank or container, intended for the transport and / or storage of radioactive materials, said closed enclosure being capable of containing radioactive materials comprising organic compounds and optionally water, or radioactive materials in the presence of organic compounds and possibly water, capable of producing flammable gases, by radiolysis, said enclosure further containing at least one flammable gas elimination device as defined above.

• La figure 1 est un graphique qui donne la teneur en hydrogène (en % en volume), dans l'enceinte, mesurée par chromatographie, en fonction du temps (t en heures) lors de l'essai réalisé dans l'exemple.

The invention will be better understood on reading the following detailed description given by way of nonlimiting illustration with reference to the accompanying drawings, in which:

• The figure 1 is a graph that gives the hydrogen content (in% by volume) in the chamber, measured by chromatography, as a function of time (t in hours) during the test carried out in the example.

Detailed description of the invention

The invention applies to any closed enclosure, in which are placed radioactive materials which comprise organic compounds and possibly water, or radioactive materials which are in the presence of organic compounds and optionally water.

For the purposes of the invention, the term "organic compound" means a compound comprising at least one carbon atom, at least one hydrogen atom and optionally at least one other atom chosen, for example, from nitrogen, sulfur or phosphorus atoms. oxygen, and halogens.

This enclosure may have any shape and dimensions, and a level of sealing greater or less, without departing from the scope of the invention. It may especially be a container such as a drum or a container of cylindrical or parallelepiped shape. Moreover, the enclosure can be indifferently for the transport, storage or treatment of the radioactive material concerned.

Furthermore, the radioactive materials placed in the closed chamber may be constituted by all the radioactive materials comprising organic compounds and possibly water, or by all the radioactive materials in the presence of organic compounds and possibly water.

In general, the invention applies more particularly to the case where said compounds Organic compounds are compounds that in addition to hydrogen emit, produce CO and CO ₂ such as certain plastics. Indeed, it has been demonstrated according to the invention that the CO poisons the catalyst a) and can be effectively removed by the catalyst b) to preserve the effectiveness of the catalyst a).

By way of non-limiting example, the radioactive materials may be constituted by technological waste from a plant for the treatment or manufacture of nuclear fuel elements. As already mentioned, such wastes are contaminated with plutonium or radioactive uranium and may contain a certain fraction of water or solid or liquid organic materials such as cellulosic materials, plastics or hydrocarbons. .

According to the invention, at least two active products are placed inside the closed chamber containing the radioactive materials.

One of these active products, hereinafter referred to as "active product A", is designed to remove, by continuous catalytic oxidation by the oxygen present in the chamber, flammable gases, such as hydrogen, produced by radiolysis in the atmosphere of the enclosure, under the effect of radiation emitted by the radioactive isotopes present in said materials.

The second active product, hereinafter referred to as "active product B", is an active product designed to continuously oxidise carbon monoxide and form CO ₂ .

To these two active products A and B may optionally be associated with one or two other active products.

These two other optional active products comprise a third product, hereinafter referred to as "active product C", designed to constitute a source of O ₂ , making it possible to overcome the lack of oxygen once all the oxygen initially present in the pregnant was consumed; and a fourth product, hereinafter referred to as "active product D", consisting of an active water-absorbing product.

Active product A comprises a solid, preferably microporous, inert support which supports a precious metal (impregnated with a precious metal) such as palladium, platinum or rhodium. Alternatively, the solid inert carrier, preferably microporous, may also support a rare earth (be impregnated with a rare earth) advantageously chosen from the group of lanthanides, such as lanthanum.

The active product D is constituted by a microporous hygroscopic support.

The inert solid support of the active product A, when this support is microporous, and the hygroscopic microporous support of the optional active product D, are generally both constituted by a molecular sieve which has a large developed surface defined by a specific surface, for example greater than equal to 200, even 300 m ² / g.

Thus, when the microporous support of the active product A is impregnated with a precious metal or a rare earth, it has a very high reaction surface. important for the oxidation of flammable gases produced by radiolysis in the atmosphere of the enclosure and more particularly hydrogen.

In the active product A, the precious metal or the rare earth constitutes a catalyst for the continuous oxidation reaction of hydrogen by the oxygen contained in the chamber.

Generally, the presence of less than 0.1% by weight of precious metal in the microporous catalyst support provides the desired effect.

The preferred microporous inert support of the active product A and the microporous hygroscopic support of the active product D, if any, are generally constituted as indicated above by a molecular sieve preferably chosen with regard to the microporous support of the active product D in the group of silica-silicones. aluminates, of formula Na ₁₂ [(AℓO ₂ ) ₁₂ (SiO ₂ ) ₁₂ ] XH ₂ , where X can reach 27, which represents 28.5% of the anhydrous product, and as regards the microporous support of the active product A among the aluminas preferably activated.

In the active product A, the large specific surface area of the preferred microporous support makes it possible to use at best the catalytic action of the precious metal or the rare earth. Indeed, a large reaction surface is produced on a support material, using little catalytic compound and in reduced volumes.

In contact with the microporous inert support supporting the catalyst (impregnated with the catalyst), the hydrogen combines with the oxygen of the enclosure, to form water. The water thus formed is trapped and fixed deep in the micropores of the preferred support of the product A, by molecular capillarity. By way of example, such a support can absorb up to 30% of its mass of water.

The excess water, not absorbed by the microporous support of the active product A, is optionally trapped in the micropores of the microporous hygroscopic support forming the active product D.

In this way, any formation of free water is avoided, which could be decomposed again by radiolysis, restoring some of the hydrogen removed. Indeed, the water trapped deep in microporous media is less subject to the effects of radiation emitted into the atmosphere of the chamber than if it were open water. Or, if the active product D is not present, the excess free water not absorbed by the catalyst support a) can then react with an active product C consisting of a solid peroxide to give a release of oxygen.

Furthermore, it should be noted that the oxidation process thus implemented works perfectly because the atmosphere of the enclosure can not reach a high humidity level. More precisely, the effectiveness of the active product D makes it possible to guarantee a relative humidity of the enclosure environment of less than about 10%. This ensures a maximum yield of the oxidation reactions using the active products A and B.

The active product B which is necessarily placed inside the enclosure, comprises a mixture metal oxides, preferably in the form of granules which allows the CO to be continuously removed by oxidation to CO ₂ .

A preferred product comprises a mixture of manganese dioxide MnO ₂ and copper oxide CuO.

The mixture of manganese dioxide MnO ₂ and copper oxide CuO generally represents about 80% of the weight of product B (generally about 66% MnO ₂ and 14% CuO).

This active product B plays a particularly important role, when the gases in the chamber contain CO. Indeed, and without wishing to be bound by any theory, it was then demonstrated that the active sites of the active product A are blocked by the CO because the CO molecule is more cumbersome than the H ₂ molecule. It is therefore the CO that is then converted preferentially by the catalyst a) and not hydrogen. In other words, the hydrogen is not recombined because the CO blocks the active sites of the catalyst a).

If, in addition to the catalyst a), a catalyst b) is placed inside the enclosure, the CO _{2 is} oxidized much faster than the catalyst a). The active sites of the catalyst a) are then more available to ensure the oxidation of flammable gases and in particular that of hydrogen.

A catalyst which can be used as the active product B is the product sold under the name Carulite ^® by ZANDER society. This is a mixture comprising CuO and MnO ₂ which Specifically catalyzes the oxidation reaction of CO to CO ₂ .

The Carulite ^® catalyzes this reaction, for example at a speed ten times greater, than does the catalyst), and therefore the catalyst a) still available for the oxidation reaction of combustible gases and in particular for the reaction oxidation of hydrogen into water.

The mass ratio of the active product B of the active product A is generally from 1/1 to 1/10, preferably from 1/2 to 1/4. This ratio is generally determined for a ratio of CO flow rate to H ₂ flow rate generally about 1/11; this flow ratio is that usually produced by technological waste.

Active product C which is optional is defined as a source of oxygen. This source of oxygen is generally either in gaseous form or in solid form.

In the latter case, it is usually a solid compound of the peroxide family that releases oxygen in the presence of water. This water is generally the water formed during the oxidation of hydrogen by the catalyst a) and which has not been absorbed by the solid inert carrier preferably microporous catalyst a).

Therefore, when the active product C is such a solid peroxide, it is preferable not to use active product D so that water remains available to react with the peroxide and release the oxygen.

The solid peroxide is generally selected from alkali and alkaline earth metal peroxides such as calcium peroxides, barium, sodium, potassium, magnesium and mixtures thereof.

The source of oxygen in solid form is initially introduced into the chamber when a deficit of oxygen is expected.

In order to be easily used and packaged in the chamber, the active products A, B, C, D are generally in the form of discrete elements or particles, such as granules, beads, crystals. Thus, the microporous supports of the active products A, and optionally D, are advantageously fractionated into small elements, particles, such as granules, beads or crystals.

More specifically, each of the elements of the microporous supports preferably has a shell diameter of between about 2 mm and about 20 mm. Each of said microporous support elements supports a (is impregnated with) a precious metal in the case of the active product A. The active product B is already generally in a fractionated form for example, namely generally in the form of granules of oxides MnO ₂ and CuO.

When it is present the active product C, if it is a solid product is generally in the form of powder.

Fractionation of the microporous supports (active products A and D) and the character already The fraction of the active product B optionally allows, as will be described more precisely below, to easily package at least one of the active products in various types of containers before placing them inside the enclosure. This fractionation also makes it possible to use the properties of the microporous support with maximum efficiency, by further increasing the oxidation surfaces of the support of the active product A. In fact, when the hydrogen diffuses into the small elements forming the Microporous catalytic supports, it comes to oxidize around the surfaces of all these elements. In other words, the total oxidation surface corresponds to the sum of all the surfaces of the elements forming the support, which is much larger than the external surface of the overall volume occupied by said elements.

The same reasoning applies to the splitting of the supports of the active product D, which increases the water absorption surfaces.

Therefore, the fractionation of the supports of the active product A and possibly D into small elements, the fractionality of the active products B and C and the use of microporous materials with a large specific surface area, combine to provide the process according to the invention. a great efficiency. Hydrogen is efficiently oxidized over large areas, as is CO, and the water formed is trapped deep within the small elements, due to the capillary properties of the microporous materials, particularly the carrier material of the active product. D.

In a preferred embodiment of the invention, the microporous support of the active products A is activated alumina Aℓ ₂ O ₃ , packaged in small granules. Activated alumina Aℓ ₂ O ₃ is a body with a large specific surface area greater than 200 m ² per gram, or even 300 m ² / g. For best results, the alumina granules have an envelope diameter of a few millimeters, preferably between about 2 mm and about 20 mm. In the case of the carrier of the active product A, the granules are poorly impregnated with precious metal (less than 0.1% by weight) or rare earth.

Under these conditions, a quantity of granules impregnated with the activated product A corresponding to 1 liter by volume or about 800 g by weight is sufficient to remove more than 400 liters of hydrogen in the free atmosphere of a closed chamber.

In a particular application relating to the transport of radioactive materials, these are generally packaged in containers such as drums stowed inside the container. The active products are then advantageously placed inside these containers. This eliminates hydrogen directly where it is produced. Only a very small fraction of the hydrogen will then escape from the containers and will diffuse into the free volume of the container, where it will be removed by the active products, also arranged in small quantities in this free volume.

If the containers are waterproof, we can choose to have the active products in quantity sufficient only inside these receptacles. Indeed, the concentration of hydrogen in the atmosphere of the container will then always be insignificant since the hydrogen is removed in the containers and diffuse very little in the enclosure of the container.

It should be noted that the introduction of the active products in the containers makes it possible to continue to prevent the accumulation of hydrogen after their final unloading. In addition, if the containers are to be stored on site for a long time, the active products may possibly be renewed to ensure the elimination of hydrogen continuously on the storage site. In other words, the use of the method according to the invention is not limited to the elimination of flammable gases produced in a closed enclosure during transport.

In conclusion, the method according to the invention is particularly simple to use in combination with different types of enclosures containing radioactive materials comprising organic components and possibly water. The manipulations required to place active products inside the enclosure are particularly simple and quick to perform. The elimination of flammable gases produced by radiolysis inside the enclosure is effectively ensured. In addition, the transport and storage times can be managed in a very flexible way since it is sufficient to introduce inside the enclosure quantities of active products appropriate for the duration of transport and / or storage envisaged.

The invention will now be described with reference to the following example, given by way of illustration and not limitation.

Example

• produit actif a) : alumine (support solide inerte microporeux) imprégnée de palladium (catalyseur) sous forme de billes de 3 mm et dont la surface spécifique est égale à 300 m²/gr ;
• produit actif b) : granules dont la composition chimique est la suivante : 65% de MnO₂, 13% de CuO, 9% d'Al₂O₃ et environ 10% d'H₂O. La dimension des granules est comprise entre 1 et 2 mm.

In this example, the process of the invention is illustrated by using the following active products a) and b):

• active product a): alumina (microporous inert solid support) impregnated with palladium (catalyst) in the form of 3 mm beads and whose specific surface area is equal to 300 m ² / gr;
• active product b): granules whose chemical composition is as follows: 65% of MnO ₂ , 13% of CuO, 9% of Al ₂ O ₃ and approximately 10% of H ₂ O. The size of the granules is between 1 and 2 mm.

The test was conducted without active products c) and d).

The test is performed as follows:

A quantity of active product a) described above equal to 25 grams and an amount of active product b) described above is placed in a 20 liter chamber (Tedlar bag) containing 600 ml of hydrogen and 53 ml of carbon monoxide. above equal to 12.5 g (the products are packaged separately). The initial hydrogen concentration is of the order of 5.6%.

A mixture H ₂ / CO is injected continuously with the following flow rates: 5.6 ml / h for the monoxide of carbon and 65 ml / h for hydrogen is a flow ratio H ₂ / CO equal to 11.6.

This report is representative of the ratio of H ₂ and CO flows generated in a package containing compacted waste from the reprocessing of irradiated fuels (the average composition of which is 90% of hulls and end pieces and 10% of technological waste); the flow rates of hydrogen and carbon monoxide being respectively equal to 2 liters / hour and 0.18 liters / hour.

The test lasted 95 hours (until the oxygen in the chamber was exhausted). The hydrogen content in the chamber is measured throughout the test by chromatography. This content remains below 1% (by volume) throughout the test as shown by the evolution curve of the H ₂ content as a function of time (hours) given on the figure 1 .

Claims (27)

1. Method for removing inflammable gases produced by radiolysis in a closed chamber which is a receptacle, tank or container, suitable for transporting and/or storing radioactive matters, said closed chamber containing radioactive matters comprising organic compounds and possibly water, or radioactive matters in the presence of organic compounds and possibly water, in which the following are placed inside the chamber:

   a) a first catalyst of at least one reaction for oxidizing the inflammable gases by oxygen contained in the chamber atmosphere, supported by an inert solid support;

   b) a second catalyst of at least the reaction for oxidizing CO to CO₂;

   and the inflammable gases react with oxygen contained in the chamber atmosphere, said oxidation reaction being catalyzed by the first catalyst.
2. Method according to Claim 1, in which the catalyst a) is a catalyst of at least the reaction for oxidizing hydrogen to water.
3. Method according to either of the preceding claims, in which the catalyst a) is a precious metal selected from the group consisting of platinum, palladium and rhodium.
4. Method according to Claim 3, in which the inert solid support of the catalyst a) supports less than 0.1% by weight of precious metal.
5. Method according to any one of Claims 1 to 3 in which the catalyst a) is a rare earth, selected from the lanthanide group.
6. Method according to any one of the preceding claims, in which the inert solid support of the catalyst a) is a microporous inert solid support.
7. Method according to Claim 6, in which the microporous inert solid support is selected from molecular sieves, possibly activated.
8. Method according to Claim 7, in which the molecular sieve is made of a material selected from aluminas and activated aluminas.
9. Method according to any one of Claims 6 to 8, in which the microporous inert solid support has a specific surface area of at least 200 m²/g, and preferably of at least 300 m²/g.
10. Method according to any one of the preceding claims, in which the catalyst b) is a catalyst specific of the reaction for oxidizing CO to CO₂.
11. Method according to any one of the preceding claims, in which the catalyst b) comprises a mixture of manganese dioxide MnO₂ and copper oxide CuO.
12. Method according to any one of the preceding claims, in which the mass ratio of catalyst b) to catalyst a) is from 1/1 to 1/10, and preferably from 1/2 to 1/4.
13. Method according to any one of the preceding claims, in which the following is also placed inside the chamber:

    c) an oxygen source.
14. Method according to Claim 13, in which the oxygen source is in solid form or in gaseous form.
15. Method according to Claim 14, in which the oxygen source c) is a solid source selected from solid peroxides.
16. Method according to Claim 15, in which said solid peroxides are selected from peroxides of alkali and alkaline earth metals and mixtures thereof, such as peroxides of calcium, barium, sodium, potassium, magnesium, and mixtures thereof.
17. Method according to Claim 14, in which the oxygen source is a gaseous source formed by replacing all or part of the chamber atmosphere by pure oxygen.
18. Method according to any one of the preceding claims, in which a hygroscopic microporous support d) is also placed inside the chamber.
19. Method according to Claim 18, in which the hygroscopic microporous support is selected from molecular sieves.
20. Method according to Claim 19, in which the molecular sieve is made of a material selected from materials of the aluminosilicate type (for example with the formula Na₁₂[(AℓO₂)₁₂(SiO₂)₁₂] X H₂O, where X is up to 27, or 28.5% by weight of the anhydrous product.
21. Method according to any one of Claims 18 to 20, in which the hygroscopic microporous support a) has a specific surface area of at least 200 m²/g, and preferably at least 300 m²/g.
22. Method according to any one of Claims 6 to 21, in which the microporous inert solid support supporting the catalyst a), the catalyst b), and the optional hygroscopic microporous support d), are fractionated into discrete elements, such as crystals, beads or granules.
23. Method according to Claim 22, in which said discrete elements have an envelope diameter of between about 2 mm and about 20 mm.
24. Method according to either of Claims 22 and 23, in which at least one of the active products a), b) and possibly c) and d) is placed, mixed or separately, in at least one receptacle that is at least partially permeable, such as a textile envelope, a strainer, a metal grid or a perforated receptacle.
25. Method according to Claim 24, in which the active products a) and b) are mixed, and the active products c) and d) are separate.
26. Device for removing inflammable gases produced by radiolysis in a closed chamber which is a receptacle, tank or container, suitable for transporting and/or storing radioactive matters, said closed chamber containing radioactive matters comprising organic compounds and possibly water, or radioactive matters in the presence of organic compounds and possibly water, said device comprising:

    - a) a catalyst of at least one reaction for oxidizing the inflammable gases by oxygen contained in the chamber atmosphere, supported by an inert solid support,

    - b) a catalyst of at least the reaction for oxidizing CO to CO₂,

    - c) possibly an oxygen source;

    - d) possibly a hygroscopic microporous inert solid support;

    in which the catalysts a) and b) are such as defined in any one of Claims 1 to 25, the oxygen source c) is such as defined in any one of Claims 13 to 25, and the hygroscopic microporous inert solid support d) is such as defined in any one of Claims 18 to 25.
27. Closed chamber, which is a receptacle, tank or container, suitable for transporting and/or storing radioactive matters, said closed chamber being capable of containing radioactive matters comprising organic compounds and possibly water, or radioactive matters in the presence of organic compounds and possibly water, capable of producing inflammable gases, by radiolysis, said chamber further containing at least one device for removing inflammable gases as defined in Claim 26.

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