EP0724271B1 - Process and device for treating radioactive waste by hydraulic binding agent - Google Patents

Description

The invention relates to a method and an installation for the treatment of residues. radioactive in the form of sludge, ash, concentrates, corundum and various detritus consisting in immobilizing them by means of a hydraulic binder in order to obtain a block solid of defined mechanical resistance. This invention will therefore find its application in the context of the reprocessing of radioactive waste.

Whether in nuclear power plants or in nuclear waste treatment plants, many precautions must be taken as to the quality of the effluents discharged into the environment from these power plants or these factories. Thus, it is necessary to rid these effluents of all particles radioactive in order to store them in safe places where they are not likely to contaminate the environment.

Thus, for the treatment of these effluents, chemical precipitation, flocculation or other type, in order to remove the particles in the form of sludge.

In addition, other types of residue, such as evaporator concentrates, ash and other products used in the treatment, by projection or filtration, for example, corundum, sand, etc. are also collected and stored.

In this regard and with regard more particularly to radioactive sludge, this are stored in metal drums or tanks which are themselves stored in hangars or, simply, in the open, knowing that in no case such barrels may be buried due to the significant risk of contamination posed their content. Indeed, such metal drums have a limited lifespan so that it it is necessary, periodically, to transfer their contents into new barrels. It is thus understood that these sludges are considered to be particularly cumbersome, so much so that in some countries, including either regulations not favoring the environment, that is to say insufficient means of control, this sludge radioactive yet dangerous in every way, are simply dumped into nature.

In this regard, it is known from the document GB-A-2-218,256, a method and an installation for the treatment of radioactive sludge by vitrification. In fact, it is explained that after reduction of the water content by centrifugation, this sludge is sent to the unit vitrification by different conveying processes. One of these known methods consists in ensuring the transport of this sludge via vapors which, when they condense, proceed to a further dilution of the sludge. More specifically, the invention as described in this prior document consists in mixing at sludge, at least part of the glass particles intended for vitrification. This mixture contained in the appropriate container is homogenized and dried under gas flow inert. Note, moreover, that the glass particles can themselves be previously heated before being mixed with the sludge. In the end, this mud thus mixed to the glass and dried particles are transported towards the unit of vitrification in the form of a fluidized bed by means of an inert gas.

It is also known from the document PATENT ABSTRACT OF JAPAN Vol. 16, no.571 page 145; 11, .12, .1992 JP-A-42 021 799 a sludge treatment process radioactive consisting in taking the sludge from a storage tank to come the pour into a treatment container. To this sludge is added an agent intended for solidify the whole, more particularly the cement or the powder of glass, the whole being then homogenized. After dehydration and assuming that the binder corresponds to cement, the whole is left in the state until solidification. In if glass powder is introduced into the mud, the mixture is heated and put under press so that the glass powder forms a uniform solid substance.

Document DE-A-4,023,118 also discloses a method for processing construction waste slightly contaminated with radioactivity. In the context of this process first of all grinding, the waste likely to be in fine particles. Then, in parallel, we have in a container whose material constitutes a barrier to radioactive emissions, larger sized wastes that do not can be ground. After that we proceed to the preparation of the filling material made from water cement and binder. To this mixture are, moreover, added previously finely ground particles of contaminated material.

One of the objects of the invention is a sludge treatment process consisting of immobilize in such a way that it is possible to store them without risk of contamination precisely thanks to the stability of the solid block resulting from this process immobilization of sludge.

To this end, the invention relates to a process for the treatment of radioactive residues under form of sludge, ash, concentrates, corundum and various detritus, consisting of immobilize by means of a hydraulic binder, characterized in that one mixes several types of radioactive residues which are distinguished by different water contents so as to obtain a sludge to be treated comprising an optimal water content corresponding, substantially, to the amount of water necessary for the hydration of components of the hydraulic binder to ensure the hardening of the mixture.

Advantageously, throughout the treatment process, the sludge and then the mixture hydraulic mud-binder are kept under agitation in order to obtain and then maintain a certain level of homogeneity.

• des moyens d'agitation pour homogénéiser les résidus radioactifs contenus dans des fûts destinés à constituer un lot de boues à traiter ;
• un conteneur dans lequel sont déversés, après homogénéisation, les résidus radioactifs contenus dans les fûts ;
• des moyens d'agitation ayant pour fonction d'homogénéiser le lot de boues introduites dans le conteneur ;
• une unité de malaxage ainsi qu'une unité de moulage ;
• et des moyens de pompage permettant, respectivement, de prélever le contenu des fûts en vue de l'introduire dans le conteneur puis de prélever une quantité déterminée de boue homogénéisée contenue dans ce conteneur en vue de la transférer vers l'unité de malaxage.

The invention also relates to an installation for implementing the above method. Thus, this installation includes:

• stirring means for homogenizing the radioactive residues contained in drums intended to constitute a batch of sludge to be treated;
• a container into which the radioactive residues contained in the drums are poured, after homogenization;
• stirring means whose function is to homogenize the batch of sludge introduced into the container;
• a kneading unit as well as a molding unit;
• and pumping means making it possible, respectively, to take the contents of the drums with a view to introducing it into the container and then to take a determined quantity of homogenized sludge contained in this container in order to transfer it to the mixing unit.

The advantages obtained thanks to this invention consist precisely in that transforming radioactive sludge into an immobilized compact block which, in addition, homogeneous is, therefore, chemically and mechanically stable, it is possible without risk for the environment, to bury these blocks arranged in an envelope confining.

Consequently, the present invention, through the method and the installation allowing its implementation, allows to consider a real treatment of this waste radioactive.

• la figure 1 est une vue schématisée et en plan de l'installation permettant la mise en oeuvre du procédé de traitement de résidus radioactifs tels que des boues ;
• la figure 2 est une vue en élévation de l'installation telle que représentée dans la figure 1.

The invention will be better understood on reading the description which follows, accompanied by drawings illustrating an exemplary embodiment.

• Figure 1 is a schematic plan view of the installation allowing the implementation of the process for treating radioactive residues such as sludge;
• FIG. 2 is an elevation view of the installation as shown in FIG. 1.

The present invention consists of a method for treating radioactive residues such as chemical co-precipitation sludge or evaporator concentrate, ash, sand and other spray treatment products that are found, whether in a nuclear power plant or at a waste reprocessing plant nuclear and generally all producers of radioactive waste.

As visible say Figures 1 and 2 of the accompanying drawings, the invention also relates, to an installation allowing the implementation of this treatment process.

In fact, according to this process, the residues which are in the form of fine particles must be distributed homogeneously in a liquid medium, essentially, by water. So, in the case of sludge treatment, this is all first homogenized by stirring because, as will appear further in the description, this homogenization will depend on the stability of the product finally obtained. In the case of dry residues, or with low water content, a water supply can be necessary in proportions, which, again, are further defined in the description. In fact, such dry residues are, according to a feature of the invention, mixed with sludge to be treated in order to obtain a water content in the proportions desired.

It is understood that the method is applied systematically to the treatment of sludge, whether sludge such as recovered through a settling tank or other or obtained by water supply or corresponding to a mixture of sludge and residues dry.

Ultimately, once this radioactive sludge is homogenized as perfectly as possible, we just add, according to the process, a hydraulic binder in proportions which are a function of the mechanical resistance of the solid block that will result after drying.

This radioactive sludge - hydraulic binder mixture is again made homogeneous by agitation, precisely so that we obtain this homogeneity in this block solid, after setting of said hydraulic binder.

In fact, the present invention, through the process, aims to reach an optimum in terms of yield: quantity of sludge treated - final volume of sludge immobilized, and, at the same time, the amount of wasted hydraulic binder.

Thus, starting from mechanical considerations, we know the proportions of binder hydraulic to mix with a given volume of dry residue. We also know the volume of water necessary for the hydration of the components of the hydraulic binder and ultimately allowing the mixture to harden.

Consequently, if the sludge to be treated has a low water content, it is necessary add water that can be called clean to the mixture to obtain the result research. Under these conditions, we can see that, compared to the volume of the solid block obtained, the volume of treated sludge has not been optimized. Indeed, by operating in this way, we have unnecessarily treated a given amount of clean water added to the mixture.

In the same way, by treating sludge with a high water content, i.e. greater than the amount of water required for hydration of the optimum volume of the hydraulic binder to obtain the desired mechanical strength, there are no other solutions than put more hydraulic binder capable of capturing this excess water. Through therefore, the quantity of hydraulic binder finally wasted is greater than that really necessary, not to mention that, again, this results in a solid block of volume unnecessarily increased compared to the quantity of residues actually treated.

The solution according to the invention consists in carrying out one or more mixtures of different types of residues which are distinguished, precisely, by their water content to obtain a sludge whose water content is that necessary for the hydration of the quantity of binder hydraulic which must be added to this sludge to immobilize it in the form of a block meeting, from the point of view of mechanical strength, the required requirements.

Concretely, sludges with a high water content could, for example, be mixed sludge with low water content or, simply, "dry" residues of the sand or ash type, be added to it.

According to the method, it is also provided, before spoiling the hydraulic binder and after homogenization of the sludge, to carry out physico-chemical analyzes of it. In particular, a measurement of the pH is carried out as well as a measurement of the content in water. Depending on these measurements, it is thus possible to readjust, if necessary, the pH or further modify the water content of the sludge.

In addition, it is planned to check the compatibility of the sludge with the hydraulic binder. For this purpose, homogenized sludge is taken from one or more samples of an imposed volume, to which is added a prefixed quantity of binder hydraulic. This mixture based on a sample of mud is contained in a mold which, after immobilization of this sludge, allows easy demolding.

This immobilized mud sample (s) is then analyzed. On the basis of the results of the latter, we proceed to the necessary readjustment, whether from a point of view the water content of the sludge or the quantity or nature of the hydraulic binder there is place to implement. Such tests on samples are repeated as many times as necessary.

Preferably, these tests on samples are followed by a full-scale test. Thus, adjusted and homogenized sludge, the quantity of sludge is taken corresponding to the volume of the solid block that one wishes to achieve. At this amount of mud given is added the hydraulic binder in proportions as determined in through the test previously conducted on a sample. Note that at this stage, it can still be readjusted the water content, especially if it is less than the desired water content.

It is only after a finding of a satisfactory result at the level of these tests in size nature that the sludge treatment process enters the continuous operating phase until the total treatment of a given batch of sludge.

It should indeed be observed that the tests, whether in reduced size or in size real, lead to results which are strictly dependent on the characteristics physical and chemical of the batch of mud that has been previously prepared and corresponding to a mixture of different types of mud and other radioactive residues. In final, a batch of mud corresponds to the quantity of mud prepared in a container of a specific volume.

According to a first embodiment from such a batch of mud, it is possible to design a given number of blocks of mud immobilized with a predetermined volume. More precisely, the following operations are repeated a number of times: a defined quantity of mud from the batch prepared for introduction into a mixer in which also pours the hydraulic binder. After homogenization, the mixture is poured into a recyclable container: at this time, we also introduce a hook handling in this hydraulic mud-binder mixture; after checking the socket, the block immobilized is removed from the recyclable container and placed in a container of storage which can contain several of these blocks.

According to a second embodiment, more corresponding to the illustrated installation in the attached figures, the batch of prepared and homogenized sludge is mixed using a hydraulic binder whose characteristics and quantities have been determined through tests on samples, this in order to come fill almost a container specific containment, especially in fiber concrete meeting the characteristics physical and mechanical nuclear storage. Finally, this container constitutes, in in this case, a lost formwork and its internal wall is lined with a sheet of material compressible, for example in polystyrene capable of showing variations dimensions of the block being solidified.

As already specified above, the present invention also relates to an installation allowing the implementation of the process which has just been described.

This installation, more particularly represented in FIGS. 1 and 2, comprises, essentially, stirring means 1 for homogenizing the sludge previously contained in drums 2. This installation is also provided with a container 3 into which the sludge contained is poured, after homogenization in several drums 2 and corresponding to a batch 4 of sludge to be treated.

In this container 3 are again introduced stirring means 5 which, again have the function of homogenizing the batch of sludge introduced into this container 3.

This installation is further provided with a kneading unit 6 in which is introduced, according to a discontinuous cycle, a given quantity of sludge from the container 3, as well as the volume of hydraulic binder allowing, ultimately, to immobilize this defined quantity of mud. In addition, under this kneading unit 6 is disposed a molding unit 7 at the level of which there is at least one container 24 able to contain said immobilized hydraulic binder-binder mixture.

Advantageously, this installation also includes pumping means 9, 10, allowing, respectively, to take the contents of the barrels 2 with a view to introducing it into container 3, then take a determined quantity of homogenized sludge contained in this container 3 in order to transfer it to the mixing unit 6.

Figures 1 and 2 schematically illustrate the entire installation. So we notes that this installation comprises, first of all, an intermediate airlock 11 in which are arranged the drums 2, defining a batch of sludge to be treated 4 and coming from a storage room 12. Downstream of this intermediate airlock 11 is the airlock opening barrels 13 in which the barrels 2 will be opened successively in order to to homogenize their content which will then be sampled by means of pumping 9 in order to transfer it to the container 3 located at an airlock 14 in downstream.

It should be noted that when opening a keg 2, we proceed to a certain number operations such as visual inspection of the contents of the keg and measurement of the quantity of mud contained in the latter. We also remove, where appropriate, the waste solid. Note, moreover, that on this open barrel 2 is placed a cover 15 specific to even to be crossed by the rod 16 of a turbine 17 connected to a portable mixer 18, the assembly constituting the stirring means 1.

Furthermore, between the barrel opening airlock 13 and the airlock 14 in which the container 3 is a control and test airlock 19. Inside this airlock are carried out the various measurements, radiological and pH for example, as well as the tests on samples to check the compatibility of the sludge with the hydraulic binder.

As foreseen by the process, once the batch of mud is perfectly homogenized in the container 3, this by means of appropriate agitation means 5 which, again, can be in the form of a portable mixer 20, a quantity is taken from it given in order to transfer it to a mixing unit 6. This is, in fact, in an airlock 21 downstream of the airlock 14 and includes a mixer 22 surmounting a support frame 23 under which can be placed a specific storage container 24 in which is poured the homogenized mud-binder mixture for hardening.

One can also imagine, according to another embodiment not shown, of have, under the support frame 23 a container 24 in which one comes to deposit blocks immobilized sludge, after hardening in a recyclable mold. In this regard, this recyclable mold can be placed under the mixer 22, on a support structure mounted at inside container 24.

The movements of the container 24 are facilitated by a raceway 29 on which it rests by means of suitable rolling means.

Once this container 24 is filled, it is closed by means of a cover, while the space preserved between this cover and the said solid block (s) is filled with a suitable filling mortar.

Note that at each of these airlocks 11, 13, 14, 19 and 21, can be fitted means of ventilation. Such ventilation means are, are, of course, specific to the domain concerned. Thus, they necessarily include filtration means avoiding contamination of the outside atmosphere.

To return more particularly to the hydraulic binder used in the context of treatment method according to the invention, it is preferably a mortar without ready-to-use removal according to the characteristics of the waste. To this mortar will be able be added a number of additives necessary for the chemical stability of the sludge immobilized.

As for container 24, it is preferably fiber concrete containers metallic capable of ensuring, at least partially, the function of confinement. Through elsewhere, the conditioning and / or treatment of solid waste taken from drums 2 or further conditioning and decontamination of these once emptied, will carried out according to traditional methods in the field considered. These operations are likely to be carried out in airlocks adjacent to those previously mentioned. Of more, this installation will be completed, preferably, by a specific room containing the radiological control apparatus.

The advantages resulting from the present invention consist, of course, in that one is at even, for the first time, to offer a real treatment of radioactive sludge which, being, until then, stored in barrels, posed a huge risk to the environment. Indeed, these barrels were likely to break through over time, this under the action of corrosive agents.

Claims (9)

1. Process for treating radioactive residues in the form of sludges, ashes, concentrates, corundum and various detritus, consisting in immobilising them by means of a hydraulic binder, in order to obtain a solid block of a defined mechanical strength, characterised in that several kinds of radioactive residues that differ from each other by different water contents are mixed together, so as to obtain a sludge to be treated which includes an optimal water content substantially corresponding to the amount of water required to hydrate the components of the hydraulic binder to ensure the hardening of the mixture.
2. Process for treating according to claim 1, characterised in that:

   a batch of slurries to be treated is formed starting from radioactive residues that are in the form of sludges of different qualities contained in different barrels (2) ;

   the content of each of these barrels (2) is successively homogenised, before it is poured into a container (3) capable of containing the total quantity of the batch (4) of sludges to be treated;

   after homogenisation of the mixture of sludges contained in this container (3), a quantity determined according to the volume of the solid block to be formed is taken off, to pour it into a mixer (22) ;

   into the mixer (22) is poured the amount of hydraulic binder to guarantee, after hardening, the immobilisation of the quantity of sludges it contains;

   after homogenisation of the sludge-hydraulic binder mixture, it is poured into a container (24) forming a confinement cover.
3. Process according to claim 2, characterised in that

   before taking off a determined quantity of sludge to pour it into the mixer (22) , an analysis of determined chemical and/or physical characteristics of the batch (4) of sludges contained in the container (3) is carried out;

   some parameters, such as the pH and the water content, are readjusted;

   a binder-sludges to be treated compatibility test is carried out on one or several samples of the latter taken off from said container (3) ;

   determined physical and/or chemical parameters of the sludges are readjusted according to the results achieved on the sample or samples;

   these operations are repeated as necessary before proceeding to a test at actual scale, at the end of which said physical and/or chemical parameters of the sludges are once again likely to be readjusted.
4. Plant for implementing the process according to claims 1 through 3, characterised in that it includes:

   stirring means (1) for homogenising the radioactive residues contained in barrels (2) aimed at forming a batch (4) of sludges to be treated;

   a container (3) into which the radioactive residues contained in the barrels (2) are poured after homogenisation;

   stirring means (5) the function of which is to homogenise the batch (4) of sludges poured into the container (3) ;

   a mixing unit (6) as well as a moulding unit (7) ;

   and pumping means (9, 10) allowing, respectively, to take out the content of the barrels (2) to pour it into the container (3), then to take off a determined amount of homogenised sludge contained in this container (3) to transfer it to the mixing unit (6).
5. Plant according to claim 4, characterised in that it includes at least:

   an intermediate lock chamber (11) in which the barrels (2) aimed at forming a batch (4) of sludges to be treated are arranged;

   a barrel-opening chamber (13) in which the successive opening of the barrels (2) is carried out, in order to homogenise their content, which is afterwards taken out, through pumping means (9), to pour it into the container (3) located in a downstream locking-chamber (14);

   a mixing chamber (21) grouping the mixing unit (6) as well as the moulding unit (7).
6. Plant according to any of claims 4 or 5, characterised in that it includes, in addition, a checking chamber (19), preferably located between the barrel-opening chamber (13) and the locking chamber (14) containing the container (3).
7. Plant according to any of claims 4 through 6, characterised in that the mixing unit (6) includes a mixer (22) topping a bearing frame (23) under which can be placed a specific storage container (24) into which the homogenised sludge-hydraulic binder mixture is casted, with a view to its hardening.
8. Plant according to any of claims 4 through 6, characterised in that the mixing unit (6) includes a mixer (22) topping a bearing frame (23) under which can be placed a container (24) into which the immobilised sludge blocks will be arranged, after hardening in a recyclable mould, the latter being placed, under the mixer (22), on a bearing structure fitted inside said container (24).
9. Plant according to any of claims 7 or 8, characterised in that the container (24) is mounted on a runway (29), which makes easier its movements and its positioning under the mixer (22).

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