FR3080706A1 - PROCESS FOR TREATING RADIOACTIVE BITUMIN WASTE - Google Patents

Abstract

The present invention relates to a process for the treatment of radioactive bituminous waste, in particular radioactive sludge coated with bitumen (hereinafter referred to as radioactive bituminous waste) which is packaged in containers for example in metal drums, said process comprising at least one grinding step cryogenic waste in containers, for example in drums, the successive stages of incineration and then melting of the resulting ground material, in particular bituminated waste shards, and a step of forming a solid mass from the molten bath of waste in which the radioelements are incorporated and stabilized.

Description

Process for the treatment of radioactive bituminous waste

The present invention relates to a process for the treatment of radioactive bituminous waste, in particular radioactive sludge coated in bitumen (hereinafter referred to as radioactive bituminous waste) which is packaged in containers, for example in metal drums, said process comprising at least one grinding step. cryogenic of waste in containers, for example in drums, an optional step of separation of radioactive bituminous waste from their containers or pieces of containers, the successive stages of incineration and then fusion of the resulting ground material, or fragments of radioactive bituminous waste, and a step of forming a solid mass from the waste molten bath in which the radioelements are incorporated and stabilized.

The invention thus relates to the treatment of bituminized radioactive waste, said treatment leading to the neutralization of the chemical reactivity of bituminous radioactive waste, in response to the current needs of the nuclear energy sector in order to allow storage, in the very long term or definitive , of this waste, in particular in response to the request from the Nuclear Safety Authority (ASN) in France.

Bituminous radioactive waste results from the stabilization of sewage sludge from radioactive effluent treatment plants as by-products mainly from the operation of nuclear installations. This waste comes in containers, such as steel drums containing radioactive mud coated in bitumen. These barrels are stored as and when they are produced with a view to their final storage in the state in an operational storage facility; today, according to the opinion of ASN, storage in its original state is only possible if the concept of the storage facility is changed. Alternatively, a preliminary treatment intended to eliminate chemical reactivity would avoid the need to change the storage concept.

The radioactive waste proposed for treatment according to the present invention is classified into distinct types depending on whether it is radioactive waste of Medium Activity and Long Life (MAVL), or radioactive waste of Low Activity and Long Life (FAVL). ).

The present invention therefore relates to a process for the treatment of bituminous waste in containers, in particular in drums, by cryogenic grinding and if necessary separation of bituminous waste from their containers, followed by incineration and fusion of bituminous waste with or without the containers, the resulting product of which comprises the residues of treated waste which themselves constitute the main part of the matrix of said product, said process responding to the request for neutralization of the chemical reactivity, in particular as formulated by ASN. The MAVL product resulting from this process can then be permanently stored without risk, for example in the Industrial GEOIogical Storage Center (CIGEO), and the FAVL product in a future FAVL storage center.

In addition to its ability to ensure neutralization of chemical reactivity, the process has an advantage due to the reduction in volume of the bituminized radioactive waste, and therefore the reduction in storage space. The challenge of this storage is economically significant since it amounts to billions of euros in France.

To address this problem of the treatment of bituminous radioactive waste with a view to its final storage, in a study published as part of the National Plan for the Management of Radioactive Materials and Waste (PNGMDR) 2013-2015 under the title "Technico-economic evaluation of a process for the treatment of bituminous sludge mixes by incineration / vitrification ”, the possibility of incinerating and then vitrifying bituminous waste in barrels has been proposed. The proposed process would use a single incineration / vitrification oven, heated by a plasma torch.

Many technical obstacles have been identified in the study published regarding such a process, which include:

• the difficulty of feeding the oven and the need to provide a prior stage of cryogenic grinding (grinding at -60 ° C), up to a centimeter particle size, • the incomplete combustion of the bitumen which leaves a few percent of unburnt material, depositing on the internal walls of the furnace, • the difficult treatment of gases from combustion, • the uncertainties about the fate of radioelements during heat treatment, • the difficulty in obtaining a homogeneous vitrified final product, • control of the risk of criticality requiring frequent stopping of the oven.

Added to this is the strong aggressiveness of bitumen for the refractories of an incinerator or melting furnace. Finally, the factor for reducing the volume of the process, an important factor for the economic aspect (the less bulky the final product, the less costly the final storage), would be relatively low since by a factor of 2.

More particularly, when designing a process for the treatment of bituminous radioactive waste, the following parameters relating to the composition of this waste should be considered. As indicated above, bituminous radioactive waste is radioactive sludge coated in bitumen. In addition to the radioactive elements which it is necessary to ensure stabilization in the final product resulting from the treatment, the process used must take into account the composition of the sludge and the coating of bitumen. The final product must be chemically inert, stable and have a certain degree of homogeneity.

With regard to the difficulty in obtaining a homogeneous final product, a difficulty in mastering the chemistry of the molten bath was pointed out: the bitumen provides a reduction capacity, and the sulfates, as well as the nitrates, present in the sludges contained in bitumen, bring an oxidation capacity. If the bath is too reducing, metal sulfide inclusions are formed and if the bath is too oxidizing, a second liquid phase separates and floats on the bath, this phase concentrating the alkali and alkaline earth sulfates.

In addition, the composition of bituminous waste being very variable, since they contain more or less sulfates and nitrates, it is difficult to control the oxidizing or reducing nature of the bath, and to place oneself in the thin domain of redox producing a homogeneous bath.

Nevertheless, the inventors have admitted that the homogeneity of the final product which will be definitively stored in the installations provided for this purpose is not required as strictly as is chemical neutralization.

On the contrary, it is essential that the chemically neutralized (inert) final product is also solid and non-dispersible in the event of a fall during handling during storage.

In this respect, if the formation of metallic sulfides is not a nuisance, that leading to a separate phase of alkali and alkaline earth sulfates is. Indeed ¹³⁷ Cs, major radioelement of the radiological spectrum of bituminous waste, is an alkaline element, and ⁹⁰ Sr, the second major element is an alkaline earth. Radioactivity would therefore be concentrated in this phase, which is found to be soluble, therefore dispersible in the presence of water. This results in the need for the final product to be solid and non-dispersible.

Another risk is due to the fact that the drums containing the bituminous waste contain appreciable quantities of plutonium, plutonium constituting part of the radioactivity of the drums. When the plutonium accumulates in quantities greater than a few hundred grams in the treatment plant, which cannot be excluded a priori, there is a so-called criticality risk. A criticality phenomenon consists of the start of a nuclear chain reaction, which is out of control. It is a very serious accident, which can radiate strongly, up to lethal doses and beyond several hundred meters.

It is therefore crucial to fully control this risk of criticality. The only way to control the risk of criticality is to carry out a “material balance” by measuring the quantity of plutonium entering and plutonium leaving the process. Since the measurements are always carried out with a certain uncertainty, the installation must be stopped as soon as the cumulative uncertainties approach the "safe mass" of plutonium (mass below which there is no risk of criticality, for example 250 g) . The installation can only restart after complete cleaning, allowing the cumulative uncertainties to be reset to zero.

Methods for conditioning radioactive waste via their stabilization have been described in the state of the prior art.

For example, patent application WO2006 / 081441 describes a process for melting material to be treated in a container, then for vitrification of the molten material, which is then stored with or without the container. The material to be treated, including radioactive waste, is mixed with earthen materials or with earth in order to destroy organic contaminants and immobilize dangerous inorganic and radioactive materials, in a highly intact, vitrified and / or crystalline. Joule heating is used to form the molten material.

US Pat. No. 4,581,163 describes a process for treating weakly to moderately active radioactive waste, by vitrification (by melting them in glass) by means of electrodes to generate the melting temperature. The crucible, which is used as a container in which the waste is melted, serves as a container for the final storage of glassy products incorporating the waste. The electrodes used to generate the melting temperature to produce a glassy matrix remain in the glass material for storage.

In these methods, however, no incineration of the waste is carried out and the treated waste is simply included in a glassy matrix.

In patent application WO2006 / 081441, radioactive waste is conditioned in the form of synthetic rock. Said radioactive waste, the dry extract of which comprises at least 90% of compounds chosen from CaCCh, FejOa, SiCh, AI2O3 and B2O3, is subsequently melted and transformed into a product comprising a synthetic, vitreous or vitro-crystalline rock after cooling. The fact of using the waste itself as a matrix is described, but the waste to be treated does not correspond to the type of bituminous radioactive waste of the present invention.

Outside the field of radioactive waste, methods of treating waste by incineration and vitrification have already been proposed with a heating scheme different from the present invention.

Patent application FR 3 032 635 thus reports a process for destroying asbestos-containing waste in which the asbestos-containing waste is destroyed by incineration and by subsequent fusion by Joule effect in a bottom ash melting furnace to obtain the molten waste which is then molded in an ingot mold system to obtain ingots.

Patent application WO 95/04004 describes a plasma torch inerting process for solid products containing metals, in particular solid residues from the incineration of household waste, in which said residues are vitrified in an oven. The plasma torch is permanently immersed in said products and the molten products are removed from the molten bath continuously or semi-continuously and then are cooled by quenching.

Patent EP1137603 relates to a process for treating organic waste, divided solid and / or liquid, in a single reactor in which said waste is incinerated in the presence of oxygen on the surface of a bath of molten glass and vitrified in said bath. molten glass. An injection of oxygen into the molten bath is carried out so as to ensure complete combustion and to control the oxidation-reduction conditions of the bath.

Another method concerning the combustion and vitrification of waste is described in patent application WO 03/084883, in which a double heating means is suggested: oxygen plasma and induction, the plasma ensuring the combustion of waste and the start of the merger, induction supporting the merger.

The invention differs from the methods devised in the prior art for treating radioactive bituminous waste with a view to its long-term storage and at least partially remedies the drawbacks of the technology using the use of the directive plasma torch by the choice of the use of an electric oven advantageously allowing the heating by an electric arc or by the use of a plasma torch under conditions allowing the effective heating of the appropriate zones of the oven and by the implementation of '' a melting technique in a lost crucible furnace.

The present invention relates in this context to a process for the treatment of radioactive bituminous waste, said waste comprising or containing radioactive sludge coated in bitumen and being packaged in containers for example in metal drums, comprising an incineration step and a melting step said waste in a crucible allowing the formation of a solid mass obtained after solidification of the liquid bath defining a stable matrix within the product resulting from the treatment, which is intended to be packaged together with the crucible for final storage. The product designated by “solid mass” defines the “stable matrix” of the waste after treatment.

In the context of the invention, the expressions “product resulting from the treatment” or “final product” mean on the one hand, the liquid bath or molten bath, solidified after cooling and containing the sludge initially coated in bitumen and calcined during of the fusion, as well as the ashes of the bitumen and, if necessary, if they have been treated together, the fusion product of the containers, and on the other hand, the crucible containing the solidified liquid bath. Advantageously, the solid mass and the final product have a glass or ceramic structure. In a particular embodiment, the “product resulting from the treatment” or “final product” does not contain the fusion product of the bituminous waste containers, for example metal drums.

Said method advantageously employs a single electric oven with electric arc heating, Joule or plasma effect, the design of the oven allowing the choice between these heating modes or their combination according to the phase of the process. Alternatively two ovens can be combined, for the incineration operations on the one hand, and melting operations on the other hand, so as to allow the almost complete combustion of the bitumen of the bituminous radioactive waste, before the fusion. Thus, unlike some of the existing methods using vitrification in which the waste is incorporated into a glass matrix, the present invention uses the waste itself, with additives if necessary, to generate a chemically and physically stable matrix.

The proposed method advantageously makes it possible to achieve a volume reduction factor from starting radioactive bituminous waste of the order of 4 to 6.

It is specified that the intervals (open or closed) of values (in particular of temperatures or quantities and more generally of all types of quantities) expressly include the limits of said intervals in the absence of contrary indications. Similarly, the individual values are expressed with a variation margin of the order of%, for example a variation of 1 to 5%.

The subject of the invention is a process for the treatment of radioactive bituminous waste, said waste to be treated being packaged in metal containers and containing radioactive sludge coated in bitumen, comprising the steps of:

at. Cooling of conditioned waste at a temperature between -200 ° C and -10 ° C, in particular at -20 ° C;

b. Cryogenic grinding of conditioned waste at a temperature between -200 ° C and -10 ° C, in particular -200 ° C to -20 ° C, in particular in the range of -30 ° C to -10 ° C, more particularly in the range of -22 ° C to -18 ° C, so as to crystallize the bitumen and make it brittle by forming a shred consisting of bituminous waste fragments and pieces of metal containers, said bituminous waste fragments being detached pieces of metal containers;

vs. In an electric oven, incineration and fusion of the ground material from step b or incineration and fusion of the bituminous waste fragments, the incineration and fusion being carried out in a crucible until a liquid bath is obtained, the pressure inside the oven being kept below atmospheric pressure;

d. Uncoupling of the crucible containing the liquid bath from above the furnace after incineration and melting according to step c and recovery of the crucible comprising the liquid bath;

e. Cooling of the liquid bath until a solid non-dispersible and chemically neutralized mass is obtained comprising the bituminous waste treated if necessary in combination with the fusion product of the containers; and

f. Packaging together of the solid mass at the end of step e and of the crucible containing it, in a storage container for final storage.

In the context of the invention, the term "bitumen" means a hydrophobic binder known in the prior art and widely used to encapsulate, coat, immobilize or solidify waste, in this case radioactive waste.

Bitumen is essentially (more than 90%) a mixture of heavy hydrocarbons; it also contains impurities such as oxidized hydrocarbons, and organic molecules with sulfur, nitrogen and phosphorus atoms.

By "radioactive waste" is meant waste comprising or containing sewage sludge from a radioactive effluent treatment station produced in nuclear installations. This sludge contains salts and water; coating them with bitumen causes most of the water to evaporate. The sewage sludge coated in bitumen generally comes from installations for the reprocessing of spent nuclear fuel: therefore, the radioactivity consists essentially of actinides (U, Pu, ...) and fission products. After ten years, only relatively long-lived radioactive elements remain, that is to say the majority cesium 137, strontium 90, and the isotopes of plutonium (238 to 242). This waste can be classified as FAVLor MAVL.

In the context of the invention, the expression "radioactive bituminous waste" or "bituminous waste" means radioactive waste encapsulated, coated or immobilized in a bitumen matrix. The term “bituminous waste” is equivalent to the terms “bituminous waste” and used interchangeably. The radioactive bituminous waste is prepared by mixing in a worm between the hot bitumen (160 ° C) and a liquid concentrate containing the radioactive salts. This preparation is carried out with a view to final storage in the state of the bituminous waste. In view of their treatment in the context of the invention, radioactive bituminous waste is presented in containers, such as steel drums.

In the context of the invention, the bituminous waste to be treated comprises bitumen, oleates, diatoms, water, salts, and tributyl phosphate (TBP), the salts being from the treatment of effluents contaminated with radioactive materials generated in particular in nuclear installations, in particular by coprecipitation with calcium carbonate, calcium phosphate, or barium sulfate. Therefore, the major constituents of these salts are calcium carbonate, calcium phosphate, or barium sulfate.

Said salts also include iron, copper, aluminum, magnesium and uranyl hydroxides, nickel and potassium ferrocyanide, sodium and magnesium nitrates, and sodium sulfate.

In a particular embodiment of the invention, the bituminized waste contains at least 30% by mass of bitumen in particular from 40 to 85%.

In a particular embodiment of the invention, the bituminous waste is conditioned in metal drums, for example in steel or stainless steel barrel, if necessary with an overcut, in particular in stainless steel, for example in 150 L metal drums at 250 L, in particular 217 at 225 L. In the context of the present description, when the barrel is involved, if it is contained in a barrel, the latter is considered to be an element of the barrel.

When bitumen drums are received in the installation for treatment, the Dose Equivalent Rate (DED, level of irradiation emitted by the barrel) is optionally checked. The fine radiological characterization (determination of the radioelements present and the quantity of each) has already been carried out upstream, before arrival on the installation, when the casks from the casemates are taken back where they were initially stored.

Given the risk of explosion, it is excluded to introduce the container or complete barrel of bituminous waste into the oven. It is also excluded to heat the bitumen to make it flow in the oven for the following reasons:

• on the one hand, it would be necessary to clean the barrel, for example with an appropriate solvent, solvent to be heat treated then like the bitumen itself: the installation would be made more complex, and its dimensioning should be increased;

• heating the barrel, if it exceeded 120 ° C, would risk generating exothermic reactions leading to trigger the combustion of the waste outside the oven, or even an explosion. On the other hand, if the heating step were carried out at a temperature below 120 ° C., the bitumen would risk not being sufficiently liquid to flow by gravity.

Thus, according to step b of the process of the invention, the containers or drums comprising the bituminous waste undergo cryogenic grinding at a temperature between -200 ° C and -10 ° C, so as to crystallize the bitumen and make it brittle to divide it into the ground form. The temperature at which the cryogenic grinding stage is carried out can be adjusted according to the technique used. Preferably, the temperature range is from -30 ° C to -10 ° C, more particularly from -22 ° C to -18 ° C.

In a particular embodiment of the invention, the bituminous waste packaged in metal containers or drums, and if necessary the overkill, are subjected to a preliminary cooling step at a temperature between -200 ° C and -10 ° C , in particular at -20 ° C or between -22 ° C and -18 ° C, before the cryogenic grinding stage. By way of example, in particular to take account of the requirements for barrel inventory in the installation, two technical solutions can be proposed.

In the event of a small authorized inventory (in the case of an installation classified for environmental protection (ICPE)), rapid cooling can be carried out by immersing the containers in liquid nitrogen at -200 ° C. In this case, the outside of the barrel is at -200 ° C, and the core at less than -10 ° C. The immersion time is adjusted according to this objective. In the event of a high authorized inventory (in the case of a basic nuclear installation (BNI)), it is possible to cool in an upstream storage, at 20 ° C, for several days, the temperature in the barrel thus being homogeneous.

In the context of the invention, the term “ground material” therefore signifies the bituminous waste, its container or barrel, and if necessary the overdust, the whole divided into pieces resulting from the cryogenic grinding step. The "ground material" is made up of bits of bituminous waste and pieces of metal containers, said bits of bituminous waste being detached from pieces of metal containers.

At the end of step b, the particle size of the ground material is between 20 and 300 mm, preferably between 50 and 100 mm. The criterion defining the particle size is the capacity of the oven to absorb a brutal supply of heat: a single block of crushed bitumen which would burn instantly (in a pessimistic hypothesis) should not increase the oven temperature by more than 200 to 300 ° C .

In a particular embodiment of the invention, the particle size of the bituminous waste fragments is equal to or less than 50 mm and the particle size of the pieces of metal containers is greater than 50 mm, preferably between 60 mm and 300 mm, more particularly the pieces have an average size of about 200 mm. Preferably, the particle size of the bituminous waste fragments is between 20 mm and 50 mm and is less than the particle size of the pieces of container as described above.

In the context of the present invention, for the purpose of the cryogenic grinding described, the containers or drums containing the bituminous waste are immersed in liquid nitrogen to be cooled. The temperature applied during this step is cold enough for the bitumen to be crystalline. The temperature during grinding is advantageously maintained in the range of -20 ° C to -60 ° C, for example around -40 ± 5 ° C. The grinding brings mechanical energy to the bituminized waste, and therefore heats it up a bit. If necessary, the crusher itself is cooled, which avoids heating the bituminized waste. The salts and metals of the containers (barrel, overkill) are crystalline at room temperature and remain so when cooled. In any event, it is known that steel or stainless steel grinds without particular difficulty, and that the problem does not arise for the salts, which are in divided form before any grinding. A rapid temperature drop is required to avoid excessive upstream storage. In the context of the invention, the drums containing the bituminous waste are ground as is, without opening, the metal of the barrel and the bituminous waste together. In the case where the barrel is conditioned in stainless steel barrel, grinding is carried out with the barrel, barrel and bituminous waste together in a single operation.

Bituminous waste at a temperature of -10 ° C or below behaves mechanically like glass and, after grinding, is in the form of splinters, with a maximum size of around 50 mm, accompanied by a fine powder with a particle size less than 1 mm. The metal containers, in particular steel, are shredded rather than crushed, and are in the form of pieces of a size larger than that of the fragments of bituminous waste, in particular of the order of 200 mm.

In one embodiment of the invention, the grinding of conditioned waste from a barrel lasts a maximum of 5 min. In a particular embodiment of the invention, the grinding takes less than 15 s. Cryogenic grinding makes it possible to detach or take off bituminous waste from metal containers, in particular steel.

In an alternative embodiment of the invention, the ground material is separated on the one hand into bits of bituminous waste and on the other hand into pieces of metal containers before the incineration and melting step. This step of separating the bituminous waste fragments and the pieces of metal containers is carried out by placing under the mill a grid made up of steel bars spaced apart on the order of the maximum size of the bituminous waste fragments, in particular of the order of 50 mm, and arranged obliquely, so that the bituminous waste fragments fall through and the pieces of metal containers slide sideways (Figure 4). Advantageously, the grid is cooled to the temperature of the grinder, in particular between -200 ° C and -10 ° C, in particular in the range from -30 ° C to -10 ° C, more particularly in the range from - 22 ° C to -18 ° C. In this particular embodiment, the bituminous waste fragments are collected to be treated separately from the pieces of metal containers.

Metal containers or drums are not inherently radioactive unless some traces of bituminized waste remain associated with the pieces of metal containers. The pieces of metal containers at the end of the step of separating the fragments of bituminous waste and pieces of metal containers are therefore a priori less radioactive than bituminous waste, and can therefore follow a less expensive waste pathway: if the waste bituminous materials are classified MAVL, metal containers are FAVL, or even “very low activity” (TFA), and if bituminous waste is classified FAVL, metal containers are TFA.

TFA classified metal containers can be placed in TFA final storage, that is to say simply packaged as is, for example in drums or in a large solid bulk container (FIBC). FAVL classified metal containers can be stored in a FAVL storage center, simply packaged as is, for example in drums or metal boxes.

Advantageously, the metal containers, in particular of steel, which are separated from the bituminized waste after cryogenic grinding are melted in the same processing installation as that of the bituminous waste to produce ingots classified FAVL or TFA. The melting of the pieces of metal containers reduces the occupied volume by a factor of 5 to 10. The melting of the metal containers is carried out using the oven for the melting / incineration of bituminized waste. The melting of metal containers, in particular steel, can be carried out in lost crucibles, such as the melting / incineration of bituminized waste, or in a specific metal crucible, in particular steel, or in a refractory ceramic crucible specific to steel smelting campaigns, which is tiltable or bung, depending on the technologies in common use in steelworks.

According to one embodiment of the invention, during the incineration in step c, an electric arc furnace is used for initiating the incineration of the ground material or bituminous waste fragments and then to form a liquid bath by fusion after the incineration of the ground material or respectively of the bituminous waste fragments.

This type of furnace is commonly used in metal, ferrous and non-ferrous foundries, as well as in the refractory industry. Temperatures of almost 2700 ° C are commonly reached in the refractory industry, metals usually do not require such temperatures.

An electric arc furnace in principle has three graphite electrodes, one per phase of the power supply, supplied with alternating current (50 Hz from the network) at voltages of the order of 30 kV, voltages supplied by an electric transformer ( usually supplied with medium voltage, 5000 or 6000 V). This voltage of 30 kV generates an electric arc between the electrodes, heating the interior of the oven by radiation.

The advantages of this type of electric arc heating are:

its economic aspect even for large powers used to avoid or limit unburnt and, the isotropic nature of the heater: unlike plasma technology which is very directive (a plasma torch heats especially in the direction of the flame), the heater in arc mode is isotropic: consequently the sky of the furnace is as much heated as the material in the crucible. This makes it possible to limit or avoid the unburnt which comes in part from the directivity in the case of heating by means of the plasma torch, especially with regard to the deposits on the walls of the oven.

By "electric arc" is thus meant an arc which heats isotropically. Although an electric arc is in fact a plasma (ie gas in highly ionized form) or at least contains a plasma, the terms "electric arc furnace" are used here in order to make the difference between the arc which is isotropic, and plasma, generated by a plasma torch, which is directive.

In the context of the invention, the term "oven sky" means the upper part of the oven which is at least above the height of the crucible introduced into the oven.

When the crucible is introduced into the furnace, it is empty. During the incineration and fusion stage, the crucible is filled with the incineration and fusion products from each charge injected with ground material or bituminous waste fragments.

In a particular embodiment of the invention, step c of the incineration and of the fusion comprises steps of:

i. Injection of a charge of the ground material or of a charge of the bituminous waste fragments into the furnace provided with an empty crucible or a crucible partially filled at the end of a previous treatment cycle for radioactive bituminous waste;

/ 7. By means of the oven electrodes, initiation of the incineration of the ground material or bituminous waste fragments by heating, by electric arc or by plasma, in the presence of oxidizing gas and further combustion of the bitumen by adding the ground material or fragments of bituminous waste fragments to be treated in controlled quantities by adjusting the oxidizing gas so as to maintain the temperature in a range of 1100 to 1800 ° C. to form a liquid bath resulting from the fusion and comprising the salts contained in the radioactive sludges released by the combustion of bitumen and molten and / or thermally dissociated as well as, where appropriate, the product of the melting of the containers, optionally by providing additional heating by electric arc or by plasma, iii. If necessary, repeat step / 7 until the volume capacity limit of the crucible liquid bath is reached;

iv. Optionally electric arc or plasma heating to treat any residue from the ground material or bituminous waste fragments contained in the oven and,

v. Optionally heating the liquid bath, by Joule effect or by electric arc or by plasma, to a temperature in the range of 1300 to 1800 ° C, in particular to 1600 ° C, so as to carry out the thermal dissociation of alkali and alkaline sulfates - earths contained in radioactive sludge.

In a particular embodiment of the invention, the electrodes of the furnace are motorized according to a vertical movement, in particular with a view to the production by Joule effect of step v, which makes it possible to:

• Compensate for their erosion by the regular supply of new graphite, in the form of electrode sections added at the top of the electrode;

• Allow operation by Joule effect when liquid is present in the crucible: to do this, immerse the electrodes in the bath. This type of heating is more efficient since 100% of the power is transferred to the molten bath;

• Authorize the withdrawal of the electrodes to separate the lower part of the oven (the crucible) from the sky, in a horizontal movement.

The electrodes are partially reassembled outside the oven (above the oven) while part remains inside the oven (under the roof of the oven). The part of the electrodes outside the oven can be housed in a ventilated space confined above the oven, in which the vacuum is lower than inside the oven. If necessary, several confined spaces are arranged around the furnace with successive levels of depression, which become progressively lower as the distance from the furnace increases and the risk of dissemination of radioactive materials decreases.

According to another embodiment of the invention, a plasma heating technology is used as an alternative to the arc furnace. Such technology also uses solid, consumable graphite electrodes, in general two electrodes, the electric current is direct, the voltage relatively low (a few hundred volts), and the current relatively large (depending on the desired power). This technology is usable, because it is inexpensive and robust, but less suitable, because the plasma is directive.

In step / of the method according to the invention, a charge of the ground material or a charge of the bituminous waste fragments is injected in an amount of 10 to 200 kg by gravity: the ground material or the bituminous waste fragments originating from the crusher falls (s) directly into a hopper serving as an oven airlock: when the quantity of ground material or bituminous waste fragments in the hopper corresponds to what you want to inject (10 to 200 kg), the mill is stopped, the top of the hopper is closed, and the bottom is opened, so that the selected quantity falls into the oven.

According to another embodiment of the invention, a horizontal conveyor is interposed between the outlet of the crusher and the hopper / airlock supplying the oven: it is thus more freely possible to locate the crusher (at -60 ° C) relative to the oven ( at 1600 ° C), and authorize the crusher to feed the furnace almost continuously. The ground material or bituminous waste fragments fall (s) on the conveyor, which acts as a buffer between the crusher and the hopper, and feeds the hopper only when necessary.

In a particular embodiment of the invention, once the combustion of the bitumen has been started, at around 300 ° C., the process operates in step / 7 as much as possible in self-heat: the bitumen indeed has a very high calorific value, its combustion maintaining a high temperature, for example from 1100 to 1800 ° C in the oven.

The bitumen undergoes combustion while the salts of the radioactive mud coated in the bitumen undergo fusion and / or thermal dissociation. Thus, the combustion of bitumen from bituminous waste releases the salts contained in the mud of bituminous waste. Certain salts, for example sulfates, nitrates, carbonates and hydroxides, undergo thermal dissociation producing oxides of these. Salts such as phosphates are not thermally dissociated.

In a preferred embodiment of the invention, the ground material or bits of bituminous waste is (are) added to the oven in successive controlled quantities so as to maintain the temperature in the range of 1100 ° C. to 1800 ° C until molten material is obtained in the crucible, at least part of the salts contained in the radioactive bituminous waste undergoing thermal dissociation. The temperature of the liquid bath therefore causes thermal dissociation of at least a portion of the salts, for example sulfates, nitrates, carbonates and hydroxides, to produce oxides.

In addition, oleates and tributyl phosphate (TBP) also undergo combustion.

In a particular embodiment of the invention, the temperature of the resulting liquid bath is maintained in a range from 1100 ° C. to 1800 ° C. by providing additional heating by electric arc or by plasma.

Each quantity of ground material or bituminous waste fragments injected into the furnace undergoes two successive stages, incineration, then fusion. In general, taking into account the mode of supplying the furnace with ground material or bituminous waste fragments, the two stages of incineration and melting take place at least partially simultaneously in a crucible, consisting of both the incineration of the ground material or fragments of freshly introduced bituminous waste (s), and in the melting of the residue of the ground material or fragments of bituminous waste previously introduced and already incinerated.

In a preferred embodiment of the invention, the supply of oxidizing gas is adjusted so as also to control the temperature of the oven, in the temperature range indicated. The gas supply is continuous, at a rate (kg / h) adjusted to control combustion and the oven temperature. The flow rate is determined as a function of the combustion stoichiometry of the bitumen. To ensure complete or almost complete combustion, we generally introduce an excess of 1.5 to 4 times of the oxidizing gas compared to the stoichiometry (the minimum quantity of oxidizing gas which in theory allows the complete combustion of the fuel present in radioactive waste ). According to a particular embodiment, the stoichiometry of oxidizing gas is doubled to optimize the reaction and reduce the unburnt.

In a particular embodiment of the invention, the oxidizing gas is air.

In another particular embodiment of the invention, the oxidizing gas is air enriched in oxygen, for example air comprising 20 to 99.9%, in particular 95% (v / v) of oxygen . The oxidizing gas can be 100% oxygen.

The decision to use oxygen-enriched air as an oxidizing gas may depend on the size of the furnace which is used to treat bituminous waste, in particular packaged in containers or drums and, where appropriate, in drums.

The use of oxygen-enriched air advantageously allows a lower production of nitrogen oxides (NOx), which subsequently facilitates the treatment of the fumes resulting from the incineration of bituminous waste: part of the NOx is produced during the combustion, by oxidation of nitrogen in the air (when air is used as an oxidizer). Enriching the air with oxygen makes it possible to considerably reduce the NOx, which therefore only comes from the nitrogen contained in the bituminized waste, in particular in the form of nitrates and cyanides. In addition, the combustion of the bitumen and the oxidation of the barrels on the surface using the air enriched with oxygen is more complete and the operation of the furnace in self-combustion is easier.

In addition, the use of oxygen-enriched air makes it possible to divide the flow of smoke to be treated by a factor of approximately 4.

In return, due to the high reactivity of oxygen, the control of the oxygen-enriched air oven is more difficult.

At the end of the incineration step ii, a liquid bath is formed resulting from the melting, at the bottom of the crucible. Bitumen not yet burnt, with a density barely greater than 1, floats on the liquid with density 2.7 to 3, in particular if the mud coated in the bitumen contains a lot of barium sulphate. The liquid bath has a density of 2.7 when there is no barium sulfate in the salts, which is part of the variability of the salts. For a content of 50% of barium sulphate in the salts, maximum value observed, additives are added so as to obtain a content of 30% of barium oxide (resulting from the thermal dissociation of the sulphate) in the molten liquid ( maximum incorporation value for melting at 1600 ° C.), in which case the density of the liquid is 3.6. The density of 3 corresponds to 15% of barium oxide in the liquid.

In an embodiment of the invention in which the ground material is subjected to the incineration and melting stage, this bath comprises at least two phases, the upper phase consisting of oxides and phosphates and optionally of molten sulphates, the lower phase consisting of the product of the melting of the containers, in particular of molten steel.

In a particular embodiment, a charge of the ground material or a charge of the bituminous waste fragments injected into a crucible fills approximately 15% of the crucible, which implies the repetition of step // until reaching the capacity limit in volume of the liquid bath of the crucible.

In the context of the present invention, periodically and / or when the crucible reaches its capacity limit, a step of heating by electric arc or by plasma is carried out, in particular in order to treat any unburnt matter present on the furnace roof, then a heating step by Joule effect or by electric arc or by plasma, so as to obtain a temperature of the order of 1600 ° C., ensuring (i) the end of the thermal dissociation of sulfates, in particular of barium, (ii) obtaining a bath comprising a single, homogeneous upper phase (containing the oxides and phosphates), and (iii) a bath without gas bubbles (which would increase the volume of final product). The lower phase of the bath contains the material, in particular the steel of the molten barrels.

Then the heating is stopped and the electrodes are reassembled, then the crucible full from the sky is separated from the furnace, which is associated with a new crucible, to start a new incineration / fusion cycle (batch).

In another particular embodiment of the invention, any unburnt residues of the ground material or fragments of bituminous waste present at the level of the roof of the furnace in step iv are treated by electric arc or plasma heating. Advantageously, this heating step is carried out at a temperature between 1100 and 1800 ° C. In the case where plasma is used, the plasma torch is designed to efficiently heat the roof of the oven so that the combustion of unburnt materials is carried out. The accumulation of unburnt materials in the furnace roof only becomes troublesome when the plutonium (Pu) mass balance approaches the safe value avoiding criticality phenomena. This step is thus carried out periodically and / or when the crucible reaches its capacity limit, the frequency depending on the quantities of incoming and outgoing plutonium and the uncertainties about these quantities.

In the context of the invention, the expression “liquid bath” is used interchangeably with the expression “molten bath” and means the product resulting from the melting of the residue resulting from the incineration and from the thermal dissociation of the sludge from bituminized waste, including where appropriate from the melting of the container, in particular from the drum and the overflow in which the bituminized waste was contained. Said cast iron is advantageously complete or almost complete at the end of the combustion step and, where appropriate, the heating of the liquid bath produced.

In one embodiment of the invention where the ground material is separated into bituminous waste fragments and pieces of metal containers and where only the bituminous waste fragments are subjected to the incineration and melting step, the liquid bath does not contain than the melting of the residue resulting from the incineration and thermal dissociation of the bituminous waste sludge without the melting of the metal container.

In the context of the present invention, at the end of the treatment of bituminous waste, the molten bath obtained is chemically neutralized for essentially:

(i) produce a solid final matrix after cooling and acceptable in final storage, and in particular avoid the formation of a separate phase of alkali and alkaline earth sulfates, or if this phase is formed, allow it to be absorbed.

(ii) limit as much as possible, taking into account the final storage objective, the volume of this matrix.

In a particular embodiment of the invention, at least one additive promoting the dissociation of sulfates is added in step ii. The additive thus promotes the obtaining of a satisfactory fusion (the liquid bath comprises a single and homogeneous phase, without bubbles) at an acceptable temperature for the oven (<1800 ° C.). Since radioactive bituminous waste is very variable in composition, the quantity of additive also varies. The precise amounts depend on the composition of the salts.

For example, in the presence of barium sulfate, the stoichiometry of the formation of barium silicate from sulfate in the presence of silica added to the silica already present in the form of diatoms is determined.

In a preferred embodiment of the invention, the additive is selected from silica, alumina, a mixture of the two.

The prevention of the formation of a separate phase of alkali and alkaline earth sulfates, or its absorption, is based on the one hand on the heating power and the temperature in the oven and on the other hand, on the addition of additives, if necessary, facilitating the dissociation / decomposition of the sulfates, which has the effect of reducing the temperature of thermal dissociation of the sulfates.

Barium sulphate alone dissociates around 2000 ° C. In the presence of silica, this temperature decreases to 1300 ° C. According to a particular embodiment of the invention, the oven is designed to operate at a maximum temperature of 1800 ° C. The presence of silica therefore makes it possible to dissociate barium sulphate at a temperature acceptable by the oven, according to one of the reactions:

BaSÛ4 + S1O2 -> BaSiOs + SO3 or BaSÛ4 + S1O2 -> BaSiOs + SO2 + ½ O2

The additives added for carrying out the process are for example silica or alumina which promote the thermal dissociation of the sulfates by dissolving the associated cations (in particular calcium or barium depending on the nature of the salts which have been coated in the bitumen) in a matrix composed of a mixture of oxides, mainly lime, alumina, silica, or barium oxide, depending on the nature of the salt which has been coated in the bitumen. This mixture of oxides is more than ternary. Indeed other metals are present, in particular Na, K, Mg, Fe (and, in less proportion, Cu, Mn, Ti, U). Experience shows that complex mixtures from three components, preferably of different valences, are rather favorable for obtaining many eutectics melting at low temperature (below 1800 ° C, preferably below 1600 ° C).

The addition of silica or alumina to promote the dissolution of the oxides is optionally decided during the course of the process. Indeed, silica is present in salts in the form of diatoms, and alumina also, in the form of hydroxide. If the silica and alumina initially contained in the treated bituminous waste do not prevent the sulphate phase from forming (detected by cooled camera fitted to the oven, if necessary by spectral analysis) and the oven temperature is not sufficient to dissociate the sulfates, additives such as silica and / or alumina will be added.

The amount of additives added is from 0 to 30% relative to the mass of salts (the diatoms being counted as salts), preferably from 0 to 20%.

In another embodiment of the invention, the additive can comprise or can be chosen from fluorine, lithine, soda, boron oxides as fluxes which, added in a mass amount less than or equal to 5% , reduce the melting temperature up to 300 ° C less.

In a particular embodiment of the invention, the crucible is made of ceramic, in particular alumina.

An alumina crucible can be an advantage. Indeed, if the melting and dissociation step of the sulfates requires alumina, the crucible material spontaneously provides the alumina.

In a particular embodiment of the invention, the dimensions of the crucible are determined in order to be able to quantify an overall volume reduction factor. The constraint is to fill the storage container optimally, with one or more cooled crucibles. The storage container for MAVL is known to date as being a parallelepiped of maximum dimensions (external) 2.9 x 2.9 x 2.25 m. The crucible can have a diameter of 1 m on a height of 0.7 m. As far as possible, the invention uses crucibles of large volume, the increase in volume resulting from the principle of the lost crucible being all the less the larger the crucibles.

In the context of the present invention, the product of each incineration / fusion batch remains in its crucible. Thus, after cooling, the crucible and the solid mass resulting from the incineration and fusion of the bituminous waste and, where appropriate, their containers are packaged together, the last in the first, in a final storage container.

In a particular embodiment of the invention, the crucible consists of an internal refractory layer and an external steel casing ensuring the mechanical strength and the coupling with the furnace roof.

In the context of the invention, the method is therefore based on the principle of the "lost crucible" which has the following advantages: It almost completely avoids possible accumulations of plutonium at the level of the treatment installation, this accumulation being capable of generating a risk of criticality; It facilitates handling, which is necessarily remote-controlled for the levels of radioactive activity of the treated waste (even FAVL); It responds to the problem of abrasion of ceramic crucibles by bitumen burning.

The drawbacks which are linked to the cost of supplying crucibles and the impact on the reduction in volume of the final product are relatively insignificant.

Due to the use of lost crucibles, the present invention allows a volume reduction factor of bituminous waste of about 4, in particular for a useful crucible diameter of 1 m on a useful height of 0.7 m.

The crucible is advantageously designed to be relatively inexpensive and above all to add little volume. In a particular embodiment of the invention, the refractory of the crucible is itself inexpensive, and above all not very thick. This is possible provided that the crucible is cooled by circulation of water on its external surfaces, for example by means of a cold mantle, as shown in FIG. 3. The refractory layer of the crucible has, for example, a thickness of 15 at 100 mm, in particular 50 mm.

In a particular embodiment of the invention, the refractory of the crucible consists of alumina or of material based on graphite.

In a particular embodiment of the invention, the cold mantle used for cooling can be in several portions which are coupled once the crucible in place. Alternatively, the crucible is kept integral with its mantle when it is detached from the furnace, and is only separated from its mantle at the stage where it is placed in the final storage container. Cooling of the lower part of the crucible is not always necessary, the heat flow released by this lower face being less. If necessary, a refractory allowance is made on the lower face.

Thus, the refractory of the crucible is protected from excessive corrosion by cooling. In a particular embodiment of the invention, the thickness of the crucible refractory is limited to 50 mm or less of alumina.

In a particular embodiment of the invention, the handling at the level of the furnace and the crucible is carried out remotely, for example by carrying out three linear movements comprising the use in particular of an endless screw, of jacks or conveyor (FIG. 2).

In a particular embodiment of the invention, the quantities of radioelements in the crucible full of the liquid bath are measured after the uncoupling of the crucible from the top of the furnace and before the final containerization.

Once the crucible has been detached from the oven, it is lifted, for example with a bridge, to place it in the final storage container which is optionally shielded, and will have been previously placed next to the conveyor, then the final container is closed by so as to ensure the absence of risk of irradiation, then evacuated.

In a particular embodiment of the invention, the fumes generated by the incineration and the melting of the treated waste are recovered by means of at least one outlet for the fumes present at the level of the oven sky.

In a particular embodiment of the invention, after removal from the oven, the fumes are subjected to a treatment in a post-combustion stage in a post-combustion chamber. In fact, combustion (of bitumen or other fuels) produces a mixture of gases consisting of water vapor and CO2, but also CO and various volatile organic compounds; these (CO and VOC) are toxic and / or polluting and therefore cannot be released to the atmosphere. Post-combustion burns CO and VOCs, producing only water vapor and CO2.

In a particular embodiment, the post-combustion step is carried out at a temperature equal to or greater than 850 ° C. and includes a minimum residence time of the treated fumes of 2 s in the after-combustion chamber, in particular in the absence of molecules. chlorinated (this is the case for bitumen waste). These parameters, 850 ° C and 2 s, are imposed by European Union regulations for the incineration of waste containing less than 1% chlorine. They can be modified to increase the duration and temperature of the afterburner treatment.

In a particular embodiment of the invention, the afterburner chamber includes a device for injecting urea or ammonia to suppress nitrogen oxides, in particular until reaching the regulatory criteria, ie 200 mg / m ³ on daily average.

This device for reducing nitrogen oxides in afterburning replaces the more traditional DENOX device usually placed at the end of smoke treatment and operating by catalysis at a temperature lower than that of afterburning (300 ° C instead of 850 ° VS).

In a particular embodiment of the invention, the post-combustion step is followed by a quenching step in which an abrupt cooling to a temperature between 100 ° C. and 200 ° C. of the combustion gases is carried out. for example to convert volatile radioelements (isotopes of cesium and ruthenium) into a solid phase.

This step makes it possible to carry out a brutal cooling of the combustion gases with also the advantage of limiting the formation of dioxins.

The soaking step is essential on the one hand to protect the bag filters and Very High Efficiency particle filters (THE) which are used afterwards, and on the other hand to return in the solid phase potentially gaseous radioelements at high temperature, in particular ¹³⁷ Cs, one of the main radioelements present in bituminous waste.

Most (95%, even 99%) of the actinides and ⁹⁰ Sr remain in the liquid bath, the few percent between 1 and 5% remaining leaving in the fumes in the form of fly ash.

A more significant part, from 10 to 50%, of the ¹³⁷ Cs leaves in the fumes, partially in gaseous form, and is transformed entirely into fly ash after the quenching step (quench).

In known uses of soaking or quenching, a curtain of water is used which must pass through the fumes. According to the invention, taking into account the low smoke flow rates, and also the very radioactive nature that the water thus used would acquire, the quench preferably consists of a water-cooled exchanger. Such exchangers are known in the field and for example consist of a calender inside which circulates the gas to be cooled, this calender being traversed by tubes in which the cooling water circulates. The opposite configuration, where the water circulates in the radiator grille and the gas in the tubes is also possible, but clogs more easily.

In a particular embodiment of the invention, fly ash present at the end of the soaking step is subjected to a filtration step by means of a bag filter in which the fly ash is recovered.

The bag filter collects almost all of the fly ash (up to 99%, even 99.9%). It is a cleanable filter, that is to say that the fly ash can be extracted from the filter, in order to be able to reuse the filter. Such filters are commonly used and known to those skilled in the art in the treatment of flue gases in a variety of industries, including waste treatment and power generation plants.

In a particular embodiment of the invention, the fly ash recovered is reinjected into the oven to be then treated with the bituminous waste during a subsequent treatment cycle. Periodically, the filter is unclogged after a cycle and the collected ash is injected in the next cycle.

The fly ash is reinjected into the furnace and eventually becomes incorporated into the final product, even if some ashes can be re-emitted and undergo several cycles of melting, emission in the fumes, and capture in the filters.

In a particular embodiment of the invention, fly ash remaining after the filtration step by means of the bag filter is subjected to at least one very high efficiency (THE) particle filtration step.

THE filtration efficiency is at least 99.9%, it can reach 99.9999% or more.

After this filtration stage, the radioactivity has essentially disappeared from the gases, the liquid waste generated downstream can be treated by the treatment station, before discharge, for example into the river.

THE filters must be changed once they are clogged and constitute radioactive waste. However, upstream bag filters protect them, and their replacement remains infrequent, for example, every year.

THE filters are however changed by direct human intervention. In order to avoid too high dosimetry, the dose equivalent rate of THE is monitored, and can trigger a change of THE, even if the latter is not clogged.

In a particular embodiment of the invention, the pressure in the oven and in the installation intended for the treatment of smoke is lower than atmospheric pressure for at least one of the stages, in particular for all the incineration stages , melting, post-combustion, quenching, and filtration using the bag filter, and particle filtration of THE. This lower relative pressure compared to the external pressure (vacuum) ensures a speed of air circulation through the possible leaks in the right direction (from outside to inside) and sufficient to oppose backscatter which is caused by turbulence and thermal diffusion. Given the high temperatures prevailing in the oven and the associated turbulent phenomena, it is preferable that a minimum speed of 5 m / s be applied for circulation, which corresponds to a minimum depression of 15 Pa in the oven.

The smoke treatment is therefore designed to operate under vacuum with respect to atmospheric pressure, at least until after the THE stage, so as to confine the radioactive gases and dust inside the process, even in the event of leaks. This depression extends inside the oven.

In a preferred embodiment of the invention, the vacuum of the oven and the afterburner is maintained at least 15 Pa. From the soaking step, a sufficient vacuum can be 5 Pa. contemplated generating a much higher depression for technological reasons. For example, it is preferable to place the extractor downstream of the filters to avoid contamination. The pressure drop in each filter being between 200 and 1000 Pa depending on the clogging state, the total pressure drop is, for three filters (bag filter and two THE stages) between 600 and 3000 Pa. So to ensure 15 Pa of vacuum in the oven, it may be necessary to generate 615 to 3015 Pa upstream of the extractor.

In a particular embodiment of the invention, all of the premises through which non-confined radioactive material passes is also under vacuum, for example the room where the grinding takes place, the room hosting the oven, the room where s '' carry out the characterization and packaging in storage container. The vacuum of the premises is lower than that of the oven, typically 2 Pa.

After the smoke filtering (s), a step intended to suppress the acid gases produced during the incineration / fusion step, such as SO2 and SO3, is carried out, for example by means of a washing tower. The washing tower produces a liquid effluent, in particular a solution of sodium hydroxide and sodium sulphate, which is sent to the liquid effluent treatment station.

Residues such as dust, carbon monoxide (CO), volatile organic compounds (VOC), nitrogen oxides (NOx), sulfur oxides (SOx) and radioactive aerosols are continuously monitored at the level of chimney before evacuation to the atmosphere.

In an alternative embodiment of the invention, step / 7 is divided into two sub-steps: (i) a specific step of incinerating crushed bituminous waste, in particular bits of bituminous waste, and (ii) a melting step to form a liquid bath in a crucible. The specific incineration stage is carried out thanks to a rotary kiln pouring directly into the furnace where the fusion takes place.

The advantage of the specific incineration stage is to ensure almost complete combustion of the bitumen, especially if the oxidizing gas is oxygen.

The use of two ovens rather than a single oven according to the invention, remedies the risk that pieces of bitumen are buried in the molten bath as it is formed, without being able to access the oxidizing gas necessary for their combustion. This risk is nevertheless low, given the respective densities of the bitumen and the bath.

If bitumen pieces are buried, these buried pieces undergo pyrolysis (thermal decomposition in the absence of oxygen) rather than combustion, leading to the formation of graphite inclusions. However the graphite being slightly less dense than the molten bath (2 against 2.7) they are pushed towards the surface, that is towards the oxidizing gas allowing its combustion if necessary.

The graphite inclusions are not detrimental to the quality of the final matrix, but they increase the volume of final waste.

In a particular embodiment of the invention, all handling of radioactive products is carried out remotely: upstream storage of barrels, grinding, injection into the oven, handling of crucibles containing the finished product, handling of filter dust.

In a particular embodiment of the invention, the equipment (grinder, hoppers, furnace supply duct, oven, dust duct, filters) are instrumented, for example cameras and neutron counting, so as to detect any accumulation of material likely to generate a criticality risk.

The invention will be better understood on reading the examples and appended figures according to which:

Figure 1: general diagram of the process.

Figure 2: representation of the recovery stage from the complete crucible of an oven at the end of the incineration / fusion of radioactive bituminous waste, by remote handling.

Figure 3: representation of the cooling of the crucible containing the liquid bath by circulation of water on its external surfaces.

Figure 4: representation of separation of bituminous waste fragments and pieces of metal containers by placing a grid under the crusher.

Examples

Grinding test

The grinding test is carried out with bitumen conditioned in steel containers. Bitumen of the AZALT 70/100 type is poured into rectangular boxes, 50 x 50 x 15 cm in size and 2 mm thick, the large upper face of the box being open on the top. The 3 boxes are cooled to -20 ° C before grinding, each containing about 40 kg of bitumen and 10 kg of steel. The test is carried out in a mill type S300 (Supplier SID) with rotary shears opening 1.7 x 1 m, with a power of 200 kW. The findings are as follows:

• Bitumen at -20 ° C behaves mechanically like glass.

• It takes less than 15 s to grind a box with its bitumen.

• The bitumen ground material is in the form of flakes, with a maximum size of around 5 cm, accompanied by a fine powder with a grain size less than 1 mm.

• The steel is shredded rather than ground, and comes in the form of large pieces, typically 20 cm.

• Cryogenic grinding allows the bitumen to be detached or detached from the steel. Indeed, the steel pieces after grinding are almost virgin of bitumen.

For example, a 220 L drum of bituminous waste to be treated according to the invention is thus characterized:

• Bituminous waste constitutes on average 220 kg of asphalt and is packaged in 25 kg of barrel;

• 25 kg of barrel corresponds to 3.2 liters of steel;

• 220 kg of asphalt mix contains an average of 132 kg of bitumen. After incineration, 6.6 kg of ash remains (mainly oxides);

• 220 kg of asphalt mix contains on average 88 kg of radioactive sludge. After fusion, there remain 61.6 kg of matter (mainly oxides);

• Bitumen incineration ash and molten sludge together represent 68.2 kg of material (glass or ceramic, mainly in the form of a mixture of oxides).

If to promote the dissolution of the oxides in the molten bath, it appears necessary to add additives, these are added up to 20% of additives compared to the fused sludge, for a total of 81.8 kg , which compacts into a block of density (minimum) 2.5, therefore a volume of 32.7 L of glass or ceramic;

• The final volume with the steel of the barrel is 35.9 L, or 6.1 times less than the initial volume. Taking into account the crucible, this factor of 6 is reduced to 4.

It is well known to those skilled in the art that the fire behavior of bituminized waste does not depend on the level of activity of the radioactive waste. The feedback obtained from the treatment of FAVL is therefore directly applicable to MAVL while minimizing the risks.

FAVL can be treated in a Classified Installation for the Protection of the Environment (ICPE). For MAVL, a Basic Nuclear Installation (BNI) is required.

Using the assumption of a processing capacity of the FAVL / ICPE or MAVL / INB unit is 20 drums per day, we optimize all costs, capital expenditure (CAPEX) and operating costs (OPEX) with continuous operation, 24/7, the treatment of 5000 drums per year, including downtime for maintenance and resetting of the fissile material inventory (shutdown for cleaning and elimination of residues in the various equipment in order to control the risk criticality).

The process diagram is in accordance with the general description of the invention and the particular parameters given in the Example can be applied, individually or in combination, to the steps defined in the general description.

Receipt of bituminous waste and their crushing

Drums containing conditioned radioactive bituminous waste are transported to the installation by armored conveyor. When they arrive, their dose equivalent flow is measured by rotating the barrel on a turntable, facing a ramp of gamma probes.

The drums are cooled by immersion in liquid nitrogen for 1 h, then stored in cold, at a temperature of -40 ± 5 ° C, awaiting treatment, the storage capacity being 10 drums which corresponds to 12 h of operation. Storage is carried out by a FIFO (First In, First Out) type conveyor system.

As the oven needs it (on average one barrel every 2 h 24 min), a cold barrel is introduced into the crusher. The crusher includes a feed hopper (grinder hopper), sized to receive the largest barrels (conditioned in 380 L stainless steel top), with two axes in reverse rotation at the bottom of the hopper, each axis being fitted with knives.

The length of the knives as well as the distance between the axes are dimensioned to swallow the barrel (rather than bouncing it endlessly on the knives). The spacing between the knives is dimensioned so that the average particle size of the ground material is 100 mm. The hopper, axes and knives are kept cold at -40 ± 5 ° C. The motor and the control system are at room temperature. The engine power is 200 kW.

Feeding of ground material in an oven

The grinder feeds, by gravity, a hopper placed under the axes in reverse rotation, hopper also kept cold at -40 ± 5 ° C (oven hopper). The oven hopper is equipped with a system weighing its contents.

The grinder operates until the oven hopper contains approximately 21.5 kg (15 to 28 kg) of ground material, or 20 to 40 pieces (depending on the size of each piece and the amount of steel associated). Once the oven hopper is loaded, the grinder is stopped.

The oven hopper has two doors, an upper door allowing the hopper to be isolated from the crusher, and a lower door allowing the hopper to be isolated from the oven. This hopper acts as an oven airlock. When loading the oven hopper, the upper hatch is open, the lower hatch is closed.

The pilot of the oven orders the emptying of the oven hopper into the oven (closing of the upper hatch, then opening of the lower hatch) when the fuel begins to run out. The lower hatch is closed as soon as the load has fallen into the oven.

The oven hopper is placed vertically above the oven (as is the grinder, which is above the oven hopper): the load falls into the oven by gravity. The chimney between the oven hopper and the oven is not cooled.

Once the oven hopper has been emptied of its load, its upper hatch is reopened, and the crusher is restarted to prepare a new load. On average, a charge is injected into the oven every 6 min.

Incineration and melting of the ground material and solidification / cooling

Incineration and fusion are carried out in the same arc furnace. The crucible, of useful diameter 1 m and useful height 0.7 m, having a useful volume of 550 L is used. The heating power, 1.5 MW, is required to maintain the bath temperature around 1600 ° C once the bitumen combustion has ended.

The useful volume of the crucible, 550 L, can accommodate the residues from 15 initial bituminous mix drums on average. An incineration / fusion batch, resulting in the production of a lost crucible containing 550 L of final product, lasts an average of 18 h.

These 6 p.m. are divided into three phases:

• At the beginning: injections of ground material, every 6 min, during most of these 18 h;

• Then finishing (production of a homogeneous bath, with a single upper phase, without bubbles), without further injection of ground material, for 15 to 60 min;

• Then cooling (very partial, so that the bath is solidified), evacuation of the cooled crucible and replacement with a new crucible, for 60 min.

The crucible containing the final product is characterized by rotation on a turntable facing a gamma spectrometer with germanium hyperpur, then by counting of coincidences neutrons. The characterization lasts a dozen hours and is carried out in parallel with the next incineration / fusion batch.

Once characterized, the crucible containing the final product is placed in a storage container (external dimensions 1.3 xl, 3 x 1.2 m), the container is closed, and removed by conveyor.

The maximum inventory of bituminous mix drums (or residues from these drums) present simultaneously on the installation is as follows:

• 10 barrels in upstream storage • 1 barrel in the crusher • 15 barrels being treated in the oven • 15 barrels treated for characterization, ie 36 barrels in total. This number should be compared to the hundred or so medium drums (from a radioactive content point of view) acceptable in an ICPE type installation.

Smoke treatment

The oxidizing gas used is air enriched with oxygen at 95% by standard equipment of the PSA (Pressure Swing Adsortion) type producing 850 kg / h of enriched air.

Incineration / fusion produces 940 kg / h of gas (670 Nm ³ / h) of nominal composition (assuming complete combustion, and neglecting the production of NOx and SOx) as follows:

Component CO ₂ H ₂ O o ₂ n ₂ (*) % mass 38 16 42 4

(*): comes from the 5% nitrogen in the air enriched with oxygen.

The post-combustion chamber has a minimum volume of 1.1 m ³ to ensure a residence time of 2 s. The temperature of 850 ° C is usually obtained by the combustion of the gases emitted; however, a natural gas burner provides heating in transitional periods (starting the oven, beginning of the incineration / melting batch).

Urea or ammonia is injected into the afterburner to reduce NOx. The quench is ensured by a tube exchanger. It cools the gas from 850 ° C to 180 ° C, using chilled water rotating in a closed circuit; the chilled water cooling device dissipates the heat through an air heater. The thermal power to be removed is approximately 210 kW. The dimensions of the exchanger are: height 3 m, section 1 x 1 m.

The gas cooled to 180 ° C. is filtered by a bag filter, 99.9% efficiency, then by 2 successive stages of very high efficiency (THE) filters, 99.99% efficiency each.

The dimensions of the bag filter are: height 4 m, section 1 x 1 m. Dimensions of the THE (the 2 together, in the same box) are: height 1.2 m, section 0.8 x 0.8 m.

The pressure drop of each filter is monitored. When the pressure drop of the bag filter reaches (typically) 500 Pa, the filter is unclogged at the end of the incineration / fusion batch in progress. The design of the installation includes an installation of the filter at the - almost - vertical point of the furnace, and the dust falls by gravity into the new crucible, through a duct of low obliquity with respect to the vertical. After unclogging, this duct is cleaned by blowing air, causing the dust retained in the duct to fall into the crucible.

When the pressure drop across the upstream THE reaches (typically) 500 Pa, it is removed and conditioned as radioactive waste (of FMA, Low or Medium Activity type). It is then replaced by the downstream THE, and the latter is replaced by a new filter.

The change of THE is made during a maintenance phase, and requires stopping the oven for about 48 hours, in particular for the regulatory efficiency checks with fluorescein.

A gas extractor is placed downstream of the THEs in order to generate a minimum vacuum in the oven of 15 Pa. The vacuum can reach 1500 Pa at the inlet of the extractor due to pressure drops in the filters.

The gas at 180 ° C, purified of dust, passes through a washing tower intended to suppress acid gases, in particular SO2, the dimensions of the washing tower being: height 4 m, diameter 1 m. A 5% sodium hydroxide solution circulates with a flow rate of 1 m ³ / h. The solution is projected in rain from the top of the tower; the gas, circulating from bottom to top, then reacts with the solution. This produces an effluent containing 5 soda and sodium sulfite, in proportion to the sulfate content of the bituminous waste. This effluent can then be entrusted to a radioactive effluent treatment station.

The purified gas undergoes a check before rejection: CO, VOC, NO _X , SO2, radioactive aerosols, the NO _X and SO2 results used to control the smoke treatment.

Claims (24)

claims 1. Process for the treatment of radioactive bituminous waste, said waste being packaged in metal containers and containing radioactive sludge coated in bitumen, comprising the steps of: at. Cooling of conditioned waste at a temperature between -200 ° C and 10 ° C, in particular at -20 ° C; b. Cryogenic grinding of conditioned waste at a temperature between -200 ° C and -10 ° C, in particular in the range from -30 ° C to -10 ° C, more particularly in the range from -22 ° C to -18 ° C, so as to crystallize the bitumen and make it brittle by forming a shred consisting of bituminous waste fragments and pieces of metal containers, said bituminous waste fragments being detached from the pieces of metal containers; vs. In an electric furnace incineration and fusion of the ground material from step b or incineration and fusion of the bituminous waste fragments, the incineration and fusion being carried out in a crucible until a liquid bath is obtained, the pressure inside the oven being kept below atmospheric pressure; d. Uncoupling of the crucible containing the liquid overhead bath from the furnace after incineration and melting according to step c and recovery of the crucible comprising the liquid bath; e. Cooling of the liquid bath until a non-dispersible and chemically neutralized solid mass comprising the residues of treated bituminous waste is obtained; and f. Packaging of the solid mass at the end of step e and of the crucible containing it, in a storage container. 2. Process for the treatment of radioactive bituminous waste according to claim 1, characterized in that the bituminous waste to be treated comprises bitumen, oleates, diatoms, water, salts, and tributyl phosphate (TBP), salts resulting from the treatment of effluents contaminated with radioactive materials, in particular the salts being formed in radioactive bituminous waste by coprecipitation with calcium carbonate, calcium phosphate, or barium sulfate. 3. A method of treatment of radioactive bituminous waste according to claim 1 or 2 further comprising after the cryogenic grinding step in step b and before the incineration and fusion step in step c, a separation step on the one hand bits of bituminous waste and on the other hand pieces of metal containers. 4. Process for the treatment of radioactive bituminous waste according to claim 3, characterized in that the step of separating the fragments of bituminous waste and pieces of metal containers is carried out by placing under the crusher a grid made up of spaced steel bars of the order of the maximum estimated size of the bituminous waste fragments, in particular of the order of 50 mm, and arranged obliquely, so that the bituminous waste fragments fall through and the pieces of metal containers slide laterally. 5. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 4, characterized in that step c comprises steps of: i. Injection of a charge of the ground material or of a charge of the bituminous waste fragments into the furnace provided with an empty crucible or a crucible partially filled at the end of a previous treatment cycle for radioactive bituminous waste; ii. By means of the oven's electrodes, initiation of the incineration of the ground material or bituminous waste fragments by heating, by electric arc or by plasma, in the presence of oxidizing gas and further combustion of the bitumen by adding the ground material or fragments of bituminous waste to be treated in controlled quantities by adjusting the oxidizing gas so as to maintain the temperature in the range of 1100 to 1800 ° C to form a liquid bath resulting from the fusion and comprising the salts contained in the radioactive sludges released by the combustion of the bitumen and molten and / or thermally dissociated as well as where appropriate the product of the fusion of the containers, optionally by providing additional heating by electric arc or by plasma, iii. If necessary, repeat step // until the volume capacity limit of the crucible liquid bath is reached; iv. Optionally electric arc or plasma heating to treat any residue from the ground material or bituminous waste fragments contained in the oven and, v. Optionally heating the liquid bath, by Joule effect or by electric arc or by plasma, to a temperature in the range of 1300 to 1800 ° C, in particular to 1600 ° C, so as to carry out the thermal dissociation of alkali and alkaline sulfates - earths contained in radioactive sludge. 6. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 5, in which the temperature of the liquid bath causes thermal dissociation of at least part of the salts contained in the radioactive bituminous waste, in particular sulphates , nitrates, carbonates and hydroxides, to produce oxides. 7. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 6, characterized in that the bituminized waste contains at least 30% by mass of bitumen. 8. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 7, characterized in that the waste to be treated is packaged in metal drums, for example in steel, if necessary with a stainless steel overlay, for example in metal drums from 150 L to 250 L, in particular 217 or 225 L. 9. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 8, characterized in that, at the end of step b, the particle size of the bituminous waste fragments is equal to or less than 50 mm, preferably between 20 mm and 50 mm, and the particle size of the pieces of metal containers is greater than 50 mm, preferably between 60 mm and 300 mm, in particular approximately 200 mm. 10. A method of treatment of radioactive bituminous waste according to any one of claims 5 to 9, characterized in that the oxidizing gas is air. 11. Process for the treatment of radioactive bituminous waste according to any one of claims 5 to 10, characterized in that the oxidizing gas is air enriched in oxygen, for example air comprising 20 to 99.9%, in particular 95% oxygen. 12. Process for the treatment of radioactive bituminous waste according to any one of claims 5 to 11, characterized in that at least one additive promoting the dissolution of the oxides in the liquid bath, in particular barium and calcium oxides, is added in step / 7. 13. Process for the treatment of radioactive bituminous waste according to claim 12, characterized in that the additive is selected from silica, alumina, a mixture of the two. 14. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 13, characterized in that the crucible of the furnace is made of alumina. 15. Process for the treatment of radioactive bituminous waste according to any one of claims 5 to 14, characterized in that the step of heating the liquid bath by electric arc to treat any unburnt residues of the ground material or fragments of bituminous waste present at the top of the oven in step iv is carried out at a temperature between 1100 and 1800 ° C. 16. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 15, characterized in that the fumes generated after the incineration and the fusion of the treated waste are recovered by means of at least a smoke outlet present at the sky above the oven. 17. Process for the treatment of radioactive bituminous waste according to claim 16, characterized in that after leaving the oven, the fumes are subjected to a treatment in a post-combustion stage in a post-combustion chamber. 18. Process for the treatment of radioactive bituminous waste according to claim 17, characterized in that the afterburner chamber includes a device for reducing nitrogen oxides. 19. Process for the treatment of radioactive bituminous waste according to claim 17 or 18, characterized in that the post-combustion step is carried out at a temperature equal to or greater than 850 ° C and comprises a minimum residence time of the treated fumes of 2 s in the afterburner. 20. Process for the treatment of radioactive bituminous waste according to any one of claims 17 to 19, characterized in that the post-combustion step is followed by a quenching step (Quench) in which an abrupt cooling at temperature between 100 ° C and 200 ° C of the combustion gases is carried out, for example to convert the gaseous radioelements into a solid phase. 21. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 20, characterized in that fly ash present after the soaking step is subjected to a filtration step by means of a bag filter up to 'recovery of fly ash. 22. Process for the treatment of radioactive bituminous waste according to claim 21, characterized in that the fly ash recovered by unclogging the bag filter is reinjected into the furnace to be treated with the radioactive bituminous waste. 23. Process for the treatment of radioactive bituminous waste according to claim 21 or 22, characterized in that fly ash remaining after the filtration step by means of the bag filter is subjected to a filtration step by means of a Very High Efficiency particles (THE) and recovered on said THE filter. 24. Process for the treatment of radioactive bituminous waste according to any one of claims 1 to 23, characterized in that the pressure is lower than atmospheric pressure for at least one of the stages, in particular for all the incineration stages , melting, post-combustion, quenching, and filtration using the bag filter, and particle filtration of THE.