EP0112771B1 - Process for dispensing of wastes constituted by radioactive metallic particles, for instance by dissolution dusts from irradiated fuel elements - Google Patents

Description

The subject of the present invention is a process for conditioning waste constituted by radioactive metallic particles such as the fines obtained during the dissolution of the irradiated fuel elements and the dust obtained during the cutting and / or mechanical stripping operations of the irradiated fuel elements. .

In facilities for reprocessing irradiated nuclear fuel elements, the usual practice is to first subject the fuel elements to a preparatory mechanical treatment carried out, for example, by cutting or shearing in order to facilitate the subsequent dissolution of the fuel in a nitric acid solution. During this operation, it is difficult to avoid the formation of dust and these radioactive metallic dusts which are for the most part insoluble in the solutions used for reprocessing will have to be recovered and subjected to conditioning. Similarly, during the dissolution of the fuel elements, certain metallic particles are not attacked because they are insoluble in the nitric solution and they constitute what is generally called "dissolving fines"; these consist essentially of ruthenium, rhodium, palladium, molybdenum and, to a lesser extent, zirconium, niobium, technetium, uranium and plutonium. By way of example, the nature and composition of dissolving and shearing fines from light water reactors and fast neutron reactors are given in the attached table.

These metallic particles constitute highly radioactive waste which is difficult to recover in the short term despite the presence in large quantities of metals of the platinum family.

Also, it is necessary to treat this waste in order to ensure its long-term storage under good safety conditions.

As the quantity and dimensions of these insoluble radioactive particles increase with the rate of irradiation, the problem of the treatment of this waste becomes more and more important with the development of light water reactors and fast neutron reactors whose fuel elements are subject to high combustion rates.

Thus, it is estimated that the treatment of a tonne of uranium coming from light water reactor fuels gives about 3.5 kg of dissolving "fines" and that a tonne of oxide coming from fuel elements of fast neutrons gives 8 kg to 13 kg of "fines" of dissolution.

Also, if we take as an order of magnitude a reprocessing plant with a capacity of 800 t / year for fuels in the light water sector, we would have to process 2800 kg of these fines per year, and in the case of '' a reprocessing plant at 150 t / year for fuels in the fast neutron sector, we would obtain 1200 kg of fines per year. In addition, we must add to these figures, the dust obtained during shearing or cutting of fuels. In the case of light water reactor fuels, the cladding is made of Zircalloy and the quantities of shearing fines produced are generally of the order of 3 kg / t; in the case of fuel elements of fast neutron reactors for which the cladding is generally made of stainless steel, these shear fines represent approximately 1 kg / t of uranium.

However, the processing of dissolving and shearing fines poses certain problems because of their high thermal power linked to their high radioactivity, and also in certain cases because of their pyrophoric nature due to the presence of fine zirconium particles originating from the shearing of sheaths of light water reactor fuel elements.

In addition, it is preferable to treat these dissolving and shearing fines in the early stages of the fuel reprocessing process in order to prevent clogging of the pipes, because these particles which have high specific masses tend to settle in the areas installation calm. The same is true to avoid local overheating, which causes premature attack on containers, and the degradation of organic solvents by radiolysis.

Also, it is envisaged to separate and recover these fines at the outlet of the dissolution installation and to treat them afterwards for their packaging.

Hitherto, for the packaging of radioactive products, various processes have been used, the main ones of which consist in coating the waste in cement or in vitrifying it. However, these known methods are limited when it comes to conditioning radioactive waste constituted by dissolving or shearing fines. In fact, in the case of cements, the high specific power of the fines is detrimental to the mechanical strength of the coating material. In addition, there are risks of radiolysis of the water constituting the cement.

The incorporation of these particles into a glass is only possible after a sufficient period of cooling of the spent fuel; this in order to avoid constituting in the material, hot spots favorable to a heterogeneous development of crystallization and to primers of rupture. This is why the process, which can be used for the conditioning of fines from PWR fuels, which is generally reprocessed after several years of cooling, is no longer suitable for the conditioning of fines from fast fuel, which, in general, is reprocessed fairly quickly after unloading the reactor.

It has also been envisaged to package radioactive waste consisting of oxide or glass particles obtained from solu tions, using metal matrices as described in patents FR-A 2 387 093 and GB-A 1 446016. However, the products obtained by these processes have a heterogeneous structure, the particles of oxides or radioactive glasses being dispersed in the metallic matrix. In addition, the process described in patent FR-A 2 387 093 involves the preparation of a finely divided powder from a solution of radioactive waste containing a salt of the metal forming the matrix, then carrying out a step of hot compression.

Thus, this method cannot be used for the treatment of dissolving fines because it leads to the formation of a ceramic-metal, having from the thermal point of view the same drawbacks as glass. In addition, the mixture of very energetic fines in an oxide, which is a poor conductor of heat, leads to significant increases in temperature in the mixture, to agglomeration of the mixture and to the impossibility of obtaining a fine powder for sintering.

Likewise, the process of patent GB-A 1 446016 cannot be suitable for the treatment of dissolving fines because, taking into account the very small dimensions of dissolving fines, it will be impossible to obtain a homogeneous dispersion of the fines in the metal matrix. by pouring it into a container containing the dissolving fines. Therefore, the products obtained will not have satisfactory characteristics for long-term storage.

The present invention specifically relates to a process for packaging radioactive waste consisting of dissolving fines and / or parting and / or mechanical stripping fines, which overcomes the drawbacks of the methods currently known.

The process according to the invention for conditioning waste constituted by radioactive metallic particles insoluble in nitric solutions, is characterized in that said particles are suspended in a liquid, in that the suspension is subjected to a heat treatment of evaporation by injection of said suspension on a hot bed of a powder of a metal or an alloy chosen from the group comprising copper, nickel, zinc, copper alloys, alloys nickel, zinc alloys and stainless steel, and in that the dry mixture of powder and metal particles obtained after this heat treatment is subjected to a melting carried out at a temperature sufficient to melt the metal powder or of alloy and form compounds defined between the metal of the powder and at least part of the metallic constituents of the radioactive particles.

According to the invention, the metal or alloy powder is thus used to fix, by chemical bonding, in the form of defined compounds, the metallic constituents of the radioactive particles, which has many advantages.

Indeed, the choice of a metal or an alloy as a material for fixing radioactive waste makes it possible to solve the problems posed by the elimination of heat from radioactive particles because the metals have good thermal conductivity, which does not This is not the case with cement, glass and cermets in which large temperature gradients develop which can cause the appearance of cracks and an increase in the leaching rate because it increases with temperature. Furthermore, thanks to the good thermal conductivity of metals, it is possible to increase the rate of fixed radioactive particles and thereby reduce the volume of the packaging.

However, the level of radioactive particles fixed in the blocks obtained after solidification of the mixture is generally limited to 10% by weight.

In addition, the choice according to the invention, of a powder of copper, nickel, zinc, copper alloy, nickel alloy, zinc alloy or stainless steel to constitute the medium of fixing of radioactive waste makes it possible to obtain products which retain this waste better and which moreover have satisfactory characteristics over time. Indeed, these materials can form defined compounds with most of the radioactive metallic constituents of the waste particles. Thus, when using a copper powder, the rhodium which is the most radioactive of the mixture of fines to be treated forms a compound defined with copper, which dissolves in the matrix giving an alloy consisting of a solid solution Cu- Rh. The same is true for palladium and zirconium. The use of cupronickel makes it possible to obtain a solid solution also with the fission molybdenum.

With regard to resistance over time, it is known that copper, nickel, zinc and their alloys as well as stainless steel have better resistance over time than cement or glasses, which makes it possible to ensure better containment of radioactive waste, limit the exchange surface with the surrounding environment and avoid the risks of fracturing which are significant in the case of cement or glass matrices. In addition, if the metal powder used is appropriately chosen, certain constituents, in particular the platinoids, can be recovered later, after deactivation; thus, in the case of copper, this can be achieved by subjecting the conditioned waste in the copper, after deactivation, to a chemical treatment of selective dissolution of the copper.

Preferably, according to the invention, a powder of copper or a copper alloy, for example bronze, cupronickel or a copper and zirconium alloy, is used.

Generally, for the implementation of the process of the invention, the fines for dissolving irradiated fuels and the shearing fines are conveyed in suspension in a liquid such as water. Indeed, to recover these fines after dissolution of the irradiated fuels, the dissolution solution is subjected to a clarifica tion using either a centrifugal decanter or a pulsed filter.

The fines thus separated are then washed and they are suspended in a stream of water, then the suspension is stored in suitable containers before being treated by the process of the invention.

Taking into account the acidity of the dissolution solution from which the fines were recovered, the suspension obtained is generally acidic and can have a nitric acidity of about 0.8 N.

For the implementation of the process of the invention, the suspension of radioactive particles is subjected to a thermal evaporation treatment carried out by injecting this suspension on a hot bed of metal or alloy powder which will constitute the fixing medium. . Thus, the liquid in the suspension is evaporated simultaneously and a homogeneous mixture of radioactive particles with the metal or alloy powder of the bed which is preferably in motion during this heat treatment.

Advantageously, this treatment is carried out in a substantially horizontal tube heated and driven in rotation about its axis, which contains the bed of metal or alloy powder. Preferably, this tube further comprises means such as a scraper to prevent sticking of the powder particles on the wall of the tube. This scraper can be constituted by a crazy bar of star section, which is supported on the tube in the bed of metal or alloy powder.

When a rotating tube containing the bed of metal or alloy powder is used, the suspension of radioactive particles and the metal or alloy powder are advantageously introduced at one end of the heated tube which is rotated around of its axis, and the dry mixture obtained is recovered at the other end of the tube, then it is transferred to a melting furnace.

Thus, one can operate continuously by forming in the rotary tube a bed of dry material on which the suspension of radioactive particles and the metal or alloy powder is introduced at controlled flow rates.

In order to obtain, during the evaporation heat treatment, a homogeneous mixture of the radioactive particles with the metal or alloy powder of the bed, a metal or alloy powder is advantageously used having a particle size of 100 to 500 μm and, preferably, a tormented surface to facilitate the mechanical attachment of the radioactive particles to the powder, because given their small dimensions (from 0.3 to 15 µm) the particles would risk being entrained by the gases circulating in the device used for thermal evaporation treatment.

Furthermore, to obtain a homogeneous mixture which fixes the radioactive particles well, the volumes of metal or alloy powder are chosen relative to the volume of radioactive particles to be treated, so as to obtain, after solidification, a block having satisfactory qualities. . Generally, the volume ratio between the metal and alloy powder and the radioactive particles is 10, but thermal conditions (release of heat from the ingot produced, cooling conditions, etc.) can cause this ratio to be modified, by example to double it.

The apparatus used to carry out the thermal evaporation treatment can be constituted in particular by a calciner such as that described in patent FR-A 2 262 854.

Preferably, in particular when the starting aqueous suspension has a certain acidity, and the metal or alloy powder used can be oxidized by this acid solution, the dry mixture of powder and metal particles obtained following the evaporation treatment, a reduction treatment with hydrogen before carrying out the fusion. This reduction treatment can be carried out in the rotary tube containing the bed of metal or alloy powder. For this purpose, the rotary tube comprises at least two zones heated to different temperatures and a reducing gas mixture consisting, for example, of circulating in the rotary tube against the suspension and the bed of metal or alloy powder. argon or nitrogen with added hydrogen. Thus, evaporation is carried out in the first zone of the tube and the evaporation treatment is completed by a reduction treatment in the second zone of the tube in order to reduce the oxides possibly formed during the thermal evaporation treatment.

The dry mixture obtained is then subjected to a fusion. This can be carried out in a vacuum induction furnace or in a controlled atmosphere, for example under an argon atmosphere containing hydrogen.

Advantageously, the dry mixture obtained is transferred directly from the rotary tube into the melting furnace by making it flow by gravity into the crucible of the furnace and the melting is carried out at a temperature ranging from 1100 to 1500 ° C.

In the case where copper powder is used, a liquid bath is generally obtained by heating the mixture to a temperature of 1300 to 1500 ° C. After fusion, the liquid bath is poured into an ingot mold. A metallic ingot is thus obtained in which the different radioactive constituents of the fines are alloyed or dispersed.

In some cases, to improve the surface condition of the ingot obtained, a flux can be added to the liquid bath, for example consisting of glass frit to digest the remaining oxides which come from the surface oxidation of the metal powder or of alloy by water vapor. After separation of the glass, upon cooling, an ingot having a clean surface is obtained.

During the evaporation heat treatment carried out in the rotary tube, the vapors and gases which escape from this tube can cause radioactive particles which it is necessary to separate. Advantageously, the dust entrained by the vapors released during the recovery is recovered. heat treatment of evaporation, for example by washing gases and vapors, and this dust is recycled in the suspension of radioactive particles to be treated.

For the implementation of the process of the invention, the heat treatment of evaporation is advantageously carried out by heating the rotary tube to temperatures of 250 to 450 ° C and operating under pressure lower than atmospheric pressure.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of nonlimiting illustration with reference to the appended drawing which represents, in vertical section, a device for implementing the method of the invention.

In this figure, it can be seen that the device for conditioning radioactive waste in the form of particles, comprises an evaporation assembly 1 and a melting furnace 2. The assembly 1 comprises a tube 3 produced for example from an alloy sold under the URANUS brand, which can be rotated about its axis by means of an electric gear motor 5 via an assembly 6 with chain and gears. The rotary tube 3 can be arranged either horizontally or in such a way that its axis is slightly inclined, for example up to around 3%, relative to the horizontal. It is provided at its ends with flanges 7 and 9. A ferrule 11 is fixed on the flange 7 and a sealing device 13 is fitted around the ferrule 11 to seal the tube at one of its ends during of its rotation. A conduit 15 connected to a suspension reservoir (not shown in the drawing) crosses the end piece 13 to open at the end of the tube 3 and it makes it possible to introduce into the tube 3 the suspension of radioactive particles at the desired flow rate. A conduit 17 connected to a hopper 19 filled with metal or alloy powder also passes through the nozzle 13 to open into the tube 3. This conduit 17 is provided with a supply screw 21 and it makes it possible to introduce into the tube 3 the metal powder at the desired flow rate. The end piece 13 is still crossed by a conduit 23 for discharging the gases. This conduit then passes through a dedusting installation (not shown in the drawing), in which the entrained radioactive particles are recovered by washing the gas. The particles thus recovered are then recycled to the suspension tank associated with line 15.

At its other end, the tube 3 is closed by a fixed sealing end piece 25 comprising a sealing connection assembly to the melting furnace 2.

At each of its ends, the tube 3 is supported by rollers 26 to support the latter when the latter is in a fixed position or in rotation. A conduit 27 passes through the nozzle 25 in order to circulate in the tube 3 a gas such as argon containing 5% of hydrogen against the current of the powder bed 29 which circulates in the tube 3. A scraper 31 made up a crazy star-shaped bar prevents sticking of the powder particles on the walls of the tube 3 during the heat treatment. To carry out this heat treatment, the tube is placed inside an oven 33 which comprises three heating zones I, II and III in order to be able to bring the corresponding zones of the tube 3 to different temperatures.

The melting installation 2 comprises an induction furnace 41 inside which is placed a crucible 43 receiving the dry mixture of powder and metallic particles, coming from the tube 3, which is transferred by gravity by the light provided for this purpose in the flange 9. Inside the melting crucible opens a conduit 45 for introducing into the crucible a neutral or reducing gas such as hydrogenated argon in order to protect the bed in the crucible and push the vapors towards the nozzle 13. After fusion, the molten bath flows into an ingot mold 47.

An example of implementation of the method of the invention is described below using the device described above with a rotating tube 30 cm in diameter and 80 cm in length.

A suspension containing 50 g / l of dissolving fines having a particle size of the order of a few microns is kept under stirring in the storage tank associated with line 15, and the suspension is introduced into the rotary tube 3 through line 15 at a flow rate of 5 l / h, which corresponds to the introduction of 250 g / h of fines. Also introduced into the tube 3 by the transfer screw 21, 2.5 kg / h of copper powder having a particle size between 500 and 100 μm, and a neutral gas containing hydrogen is introduced through the tube 27. evaporate under a neutral argon or nitrogen atmosphere. The rotation of the tube 3 is adjusted at a speed of approximately 5 revolutions / min. and zones I and II are heated to a temperature of 425 ° C and zone III to a temperature of approximately 350 ° C.

Under these conditions, a bed of powders 29 having a thickness of approximately 3 cm and weighing approximately 13 kg are formed inside the tube 3, which remains in the tube for a period of approximately 5 hours. The temperature of the bed rises to 80, 195 and 250 ° C. in the zones which correspond respectively to the heating zones I, II and III, and the water vapor is evacuated with the purging gas by the conduit 23 while the dry product flows by gravity into the crucible 43 of the melting installation 2.

When the crucible contains 20 to 40 kg of product, the supply of tube 3 is interrupted to go to the melting phase. This can be done in about 1 hour 30 minutes when operating under 23 KW. After fusion, the liquid bath is poured into the ingot mold 47.

Ingots of 20 to 40 kg are thus obtained, which have satisfactory properties.

In other tests, a stainless steel powder having a particle size of 150 to 300 μm was used, and the introduction of this powder into the tube 3 was carried out at a flow rate of 2 kg / h. With the same operating conditions, a satisfactory mixture of fines is obtained with stainless steel powder and by melting at around 1500 ° C., ingots are obtained which have satisfactory properties.

To optimize the solubilization of the molybdenum from the fines, a cupronickel powder was used containing for certain tests: 80% copper and 20% nickel, for other tests: 60% copper and 40% nickel. After fusion, ingots were obtained, the analysis of which confirms the formation of a solid solution with Mo.

Although this figure shows a melting installation operating discontinuously, one could also use a continuous melting installation, the metal flow being ensured by a continuous casting nozzle.

Tests carried out with other waste consisting of radioactive metallic particles, have shown that satisfactory results could be obtained starting from copper powders or stainless steel powders having particle sizes of 40 μm to 1.25 mm, the most suitable range being 100 to 500 μm, by circulating in tube 3 argon containing 5% hydrogen at a flow rate of 800 I / h and by heating zones I, II and III of tube 3 to temperatures from 300 to 500 ° C with a rotation speed of the tube 3 from 5 to 15 rpm. and a residence time of the products in the tube 3 of approximately 5 hours.

When copper powder is used as the metal powder, it is necessary to avoid as much as possible the oxidation of the copper and to operate in the presence of hydrogen to reduce in the last zone of the tube 3 the oxides possibly formed.

Claims (12)

1. Process for conditioning waste consisting of radioactive metal particles insoluble in nitric acid solutions, characterized in that the said particles are suspended in a liquid, in that the suspension is subjected to an evaporative heat treatment by injecting the said suspension onto a hot bed of a powdered metal or alloy chosen from the group comprising copper, nickel, zinc, copper alloys, nickel alloys, zinc alloys and stainless steel, and in that the dry mixture of powder and metal particles obtained after this heat treatment is subjected to a melting performed at a temperature sufficient to melt the powdered metal or alloy and form defined compounds between the metal of the powder and at least part of the metallic constituents of the radioactive particles.

2. Process according to Claim 1, characterized in that the dry mixture of powder and metal particles obtained after the evaporative heat treatment is subjected to a reductive treatment with hydrogen before the melting is performed.

3. Process according to either of Claims 1 and 2, characterized in that the said bed of powdered metal or alloy is in motion during the evaporative heat treatment.

4. Process according to any of Claims 1 to 3, characterized in that the evaporative heat treatment of the suspension is performed in a heated, substantially horizontal tube driven in rotation about its axis, containing the said bed of powdered metal or alloy.

5. Process according to Claim 4, characterized in that the suspension of radioactive metal particles and the powdered metal or alloy are introduced at one end of the said tube, in that the dry mixture obtained is recovered at the other end of the said tube and in that the mixture is transferred into a melting furnace.

6. Process according to any one of Claims 2 to 5, characterized in that the reductive treatment is carried out in the rotating tube by establishing, in this tube, at least two zones heated to different temperatures and by circulating a reductive gaseous mixture in countercurrent to the suspension and to the bed.

7. Process according to Claim 6, characterized in that the said gasesous mixture is argon or nitrogen to which hydrogen has been added.

8. Process according to any one of Claims 1 to 7, characterized in that the dusts carried away by vapours released during the evaporative heat treatment are recovered and in that the said dusts are recycled into the suspension of radioactive particles to be treated.

9. Process according to any one of Claims 1 to 8, characterized in that the powdered metal or alloy has a particle size of 40 µm to 1.25 mm.

10. Process according to Claim 9, characterized in that the powdered metal or alloy has a particle size of 100 to 500 j.Lm.

11. Process according to any one of Claims 1 to 10, characterized in that the said powder is powdered copper or copper alloy.

12. Process according to any one of Claims 1 to 10, characterized in that the said powder is a powdered stainless steel.

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