EP0195723B1 - Process and device for conditioning low and middle level radioactive waste solutions with hydraulic binders - Google Patents

Description

The present invention relates to a method and a device for conditioning, by hydraulic binders, radioactive effluents of low and medium activity.

The conditioning by hydraulic binders of radioactive effluents of low and active activity has been practiced for a long time in the nuclear industry, because of the commonly accepted simplicity of the corresponding processes, and of the low cost of the coating material.

• - un prétraitement chimique fasiant passer l'effluent radioactif à conditionner sous une forme bétonnable (solide dispersé dans une phase aqueuse);
• - une réduction de volume du produit traité;
• - et un malaxage du produit obtenu avec une quantité adéquate de liant pour obtenir le béton.

It is recalled that such conditioning must make it possible to manufacture, from a given chemical effluent, a concrete that can be stored in a given environment and regulatory context, said manufacturing having to take place in a nuclear context and at the lowest cost. The known methods for producing such packaging generally include the following steps:

• - a fasiant chemical pretreatment pass the radioactive effluent to be conditioned in a concrete form (solid dispersed in an aqueous phase);
• - a reduction in the volume of the product treated;
• - and mixing the product obtained with an adequate quantity of binder to obtain the concrete.

• - un système continu basé sur une réduction de volume par évaporation à pression atmosphérique et l'utilisation de malaxeurs horizontaux; un tel système est particulièrement adapté aux fortes capacités et aux activités importantes;
• - un système discontinu dans lequel la réduction de volume s'effectue par combinaison d'une séparation liquide/solide et une évaporation de la phase liquide, et l'utilisation de malaxeurs industriels à axes verticaux; un tel système est plus particulièrement adapté au traitement des solutions diluées dans des installations fixes.

For the treatment of borated effluents, in particular from PWR power plants, we have already recommended:

• - a continuous system based on a reduction in volume by evaporation at atmospheric pressure and the use of horizontal mixers; such a system is particularly suitable for high capacities and important activities;
• - a discontinuous system in which the volume reduction takes place by combination of a liquid / solid separation and an evaporation of the liquid phase, and the use of industrial mixers with vertical axes; such a system is more particularly suited to the treatment of diluted solutions in fixed installations.

The present invention relates to a particular mode of implementation of batch packaging systems, said mode of implementation being particularly suitable for the use of mobile and transportable stations.

• - ledit borate de calcium tétrahydraté est réalisé en ajoutant, à la solution à traiter, une quantité de chaux calculée de façon à former le borate B₂0₃CaO;
• - l'opération de réduction de volume est réalisée par évaporation, sous pression réduite, des solutions et/ou suspensions, ladite pression réduite étant comprise entre 0,1 et 0,3 bar (1 bar = 10⁵ Pa) environ et appliquée après la phase de préparation des cristaux de borate de calcium tétrahydraté;
• - et que ladite réduction de volume est menée à bien jusqu'à l'obtention, en fin de procédé, d'une suspension bétonnable contenant d'environ 30 à environ 70 % de matière sèche.

The conditioning process according to the invention aims at conditioning, by hydraulic binders, borated radioactive effluents of low and medium activity by chemical pretreatment aiming at the preparation of crystals of calcium borate tetrahydrate, reduction of volume and kneading of the product obtained with at least one hydraulic binder, characterized in that:

• - Said calcium borate tetrahydrate is produced by adding, to the solution to be treated, an amount of lime calculated so as to form borate B ₂ 0 ₃ CaO;
• the volume reduction operation is carried out by evaporation, under reduced pressure, of the solutions and / or suspensions, said reduced pressure being between 0.1 and 0.3 bar (approximately 1 bar = 10 ⁵ Pa) and applied after the phase of preparation of calcium borate tetrahydrate crystals;
• - And that said reduction in volume is carried out until the end of the process, a concrete suspension containing from about 30 to about 70% of dry matter.

• - la première consiste, après l'adjonction de chaux, dans la formation, à une température de l'ordre de 60°C, de germes de sels de calcium; au cours de cette étape le mélange conserve sensiblement une viscosité constante;
• - la seconde qui succède à la première, et qui se déroule à la même température, consiste dans la formation de sels mixtes ce qui se traduit par une augmentation progressive notable de la viscosité du mélange.
• - la troisième, qui succède à la seconde, consiste dans la formation, à une température de l'ordre de 40 - 50°C, de cristaux de borate de calcium à 4H₂0; cette formation de cris taux se traduit par une diminution de la viscosité du mélange (P₂);
• - la quatrième, qui succède à la troisième, consiste dans le mûrissement des cristaux formés; ce mûrissement réalisé également à une température de 40 - 50° C est constitué en fait par un grossissement progressif des cristaux (P₃).

The pretreatment used in the invention is known in principle since it consists, after possible neutralization of the effluent so as to cause the Na / B molar ratio to be close to 0.25, to add lime (in solid form or in solution) so as to target the subsequent precipitation of B ₂ 0 ₃ CaO.4H ₂ 0. It is known that the precipitation of this calcium borate takes place in four stages:

• - the first consists, after the addition of lime, in the formation, at a temperature of about 60 ° C, of calcium salt germs; during this step the mixture substantially retains a constant viscosity;
• - The second which follows the first, and which takes place at the same temperature, consists in the formation of mixed salts which results in a notable progressive increase in the viscosity of the mixture.
• - The third, which follows the second, consists in the formation, at a temperature of about 40 - 50 ° C, of calcium borate crystals at 4H ₂ 0; this formation of rate cries results in a decrease in the viscosity of the mixture (P ₂ );
• - the fourth, which follows the third, consists in the ripening of the crystals formed; this ripening, also carried out at a temperature of 40 - 50 ° C, consists in fact of a gradual enlargement of the crystals (P ₃ ).

• - d'une part que l'opération de réduction de volume du mélange par évaporation sous pression réduite ne pouvait en aucun cas être réalisée pendant les seconde et troisième étapes de la précipitation du borate de calcium; ladite réduction de volume ne pourra donc intervenir, à ce stade du procédé de conditionnement, que pendant la première et/ou quatrième étape de l'opération de précipitation;
• - et d'autre part, que l'on pouvait, pour gagner du temps, obtenir la formation du borate de calcium tétrahydraté en passant par l'intermédiaire du borate de calcium hexahydraté; pour ce faire la troisième étape de la précipitation est réalisée à une température comprise entre 10 et 30°C et ce n'est qu'ensuite (quatrième étape) que la température du mélange est portée à 40 - 50°C; dans ce cas il y aura simultanément, dans cette quatrième étape, formation du borate de calcium tétrahydraté à partir du borate de calcium hexahydraté, grossissement des cristaux du borate de calcium tétrahydraté et, de préférence, réduction du volume du mélange par évaporation sous pression réduite.

It has been found and this constitutes another aspect of the present invention:

• on the one hand, that the operation of reducing the volume of the mixture by evaporation under reduced pressure could in no case be carried out during the second and third stages of the precipitation of calcium borate; said volume reduction can therefore occur, at this stage of the packaging process, only during the first and / or fourth stage of the precipitation operation;
• - And secondly, that one could, to save time, obtain the formation of calcium borate tetrahydrate by passing through calcium borate hexahydrate; to do this the third stage of precipitation is carried out at a temperature between 10 and 30 ° C and it is only then (fourth step) that the temperature of the mixture is brought to 40 - 50 ° C; in this case there will be simultaneously, in this fourth step, formation of calcium borate tetrahydrate from calcium borate hexahydrate, magnification of the crystals of calcium borate tetrahydrate and, preferably, reduction of the volume of the mixture by evaporation under reduced pressure .

By the foregoing explanations, it is understood that the mixture obtained at the end of crystallization can comprise calcium borates having several stages of hydration. This is why, when in the present application, it is indicated that calcium borate tetrahydrate is prepared, it should be understood that the aim will be to prepare this product which proves to be the most stable during packaging and in the normal conditions of storage of the packaged product, but that other calcium borates (for example 2.6 or 9 molecules of water) may be present. However, in general, we manage to prepare a mixture of calcium borates in which the borate tetrahydrate represents at least 90% of the borates.

We have seen that the volume reduction operation by vacuum evaporation can take place (and preferably takes place to achieve maximum time savings) during the fourth stage of precipitation. It was also indicated that this same operation could take place, if necessary, during the first stage of precipitation, and this is done when the effluents are diluted.

But the flight reduction operation one can also take place before the crystallization operation begins and must continue after the fourth stage of the crystallization operation to obtain the best volume reduction. It should indeed be remembered that there is a concrete mixture (adding cement) between 30 and 70% dry extract.

• 1. si l'effluent de départ a une concentration en acide borique correspondant à environ 200 à 250 g/I; il est possible de ne pas effectuer de réduction de volume avant la quatrième étape de l'opération de cristallisation. La mise sous pression réduite du mélange aux fins de réduction de volume, sera donc réalisée seulement au cours de ladite quatrième étape et pourra être prolongée au-delà de ladite étape si nécessaire;
• 2. si l'effluent de départ a une concentration en acide borique inférieure à environ 200 g/I, l'expérience a montré qu'il est alors souhaitable d'effectuer également une réduction du volume durant la première phase de l'opération de cristallisation et/ou avant ladite première phase. Cette réduction de volume peut en effet être entreprise sur la solution (ou suspension) d'effluent reçue, avant toute addition de chaux audit effluent, l'avantage d'opérer cette réduction de volume à ce moment du procédé est que l'on peut réduire la pression sur un liquide chauffé à une température supérieure à 60° C.

Given this objective and the various stages to which the operation of volume reduction by evaporation under reduced pressure can be applied, the following rules and possibilities can be stated:

• 1. if the initial effluent has a boric acid concentration corresponding to approximately 200 to 250 g / l; it is possible not to carry out a volume reduction before the fourth stage of the crystallization operation. Reducing the mixture under reduced pressure for the purpose of reducing volume will therefore be carried out only during the said fourth stage and may be extended beyond the said stage if necessary;
• 2. if the initial effluent has a boric acid concentration of less than about 200 g / l, experience has shown that it is then desirable to also carry out a reduction in volume during the first phase of the operation of crystallization and / or before said first phase. This reduction in volume can indeed be undertaken on the solution (or suspension) of effluent received, before any addition of lime to said effluent, the advantage of effecting this reduction in volume at this point in the process is that one can reduce the pressure on a liquid heated to a temperature above 60 ° C.

• - soit de viser la préparation d'une suspension bétonnable à environ 30 % en poids de matière sèche;
• - soit de viser la préparation d'une suspension bétonnable contenant jusau'à environ 70 % en poids de matière sèche, ce qui donnera naissance à un volume de béton inférieur.

In all cases, and whatever the treatment phase where the volume reduction is carried out, it is understood that this reduction is carried out by placing the solution or the suspension at a pressure of between 0.1 and 0.3 bar at the temperature of said solution or suspension in said phase. It is easy to see that by using the invention it is possible:

• - Either to target the preparation of a concrete mix suspension at around 30% by weight of dry matter;
• - Either to target the preparation of a concrete suspension containing up to about 70% by weight of dry matter, which will give rise to a lower volume of concrete.

• - d'un dispositif (double enveloppe par exemple) de maintien de la température du produit à traiter 3 une valeur déterminée à l'avance entre 10 et 100° C environ;
• - d'un agitateur efficace présentant de préférence plusieurs régimes d'agitation et qui puisse être utilisé avec des fluides (ou suspensions) de viscosités très nettement différentes (ces viscosités différentes étant rencontrées dans la période de cristallisation et dans la période de bétonnage);
• - et d'un dispositif permettant de réaliser dans l'appareil une pression réduite allant de 0,1 à 1 bar. Il est clair, en effet, que, si l'opétion de réduction de volume est réalisée en amenant la pression à être comprise entre environ 0,1 et environ 0,3 bar, il est possible et parfois souhaitable, pour des raisons de confinement, de réaliser tout ou partie des autres opérations en opérant sous une légère dépression, par exemple à 0,7 ou 0,9 bar.

An essential advantage of the invention is that all the operations can be carried out in a single device and possibly for questions of conveniences and volumes treated in two devices of the same type on condition of using a mixer-kneader-evaporator which is provided ;

• - a device (double jacket for example) for maintaining the temperature of the product to be treated 3 a value determined in advance between 10 and 100 ° C approximately;
• - an effective agitator preferably having several agitation regimes and which can be used with fluids (or suspensions) of very distinctly different viscosities (these different viscosities being encountered in the crystallization period and in the concreting period);
• - And a device allowing to realize in the device a reduced pressure going from 0.1 to 1 bar. It is clear, in fact, that, if the volume reduction option is carried out by bringing the pressure to be between approximately 0.1 and approximately 0.3 bar, it is possible and sometimes desirable, for reasons of confinement , to carry out all or part of the other operations by operating under a slight depression, for example at 0.7 or 0.9 bar.

This type of equipment is marketed for example by the company GUEDU (France), the mixing principle used in the corresponding mixer-dryers having been applied to the pharmaceutical and fine chemical industry (mixing, chemical treatments, vacuum evaporation) as in the field of civil engineering since many years.

The paste obtained at the end of the volume reduction operation by evaporation under reduced pressure is kneaded, after cooling, with a hydraulic binder. Said paste could previously have been fluidized, either by acidification to pH <10, or either by adding a fluidizer, or by adding an untreated effluent.

The composition of the concrete obtained (solid / water / binder) is fixed by the following specifications, for a given hydraulic binder: consistency (function of the quantity of liquid in the paste) and mechanical resistance (function of the binder / water ratio). The dry extract of the paste on concentrate is therefore fixed by these two criteria, and will give the volume of condensate to be obtained.

• - du ciment Portland;
• - du ciment à base de laitier de haut fourneau;
• - du ciment à ajouts semi-liants (pouzzolanes, cendres volantes ...

The type of binder to be used will depend on the specifications on the final product and will commonly be:

• - Portland cement;
• - cement based on blast furnace slag;
• - cement with semi-binding additions (pozzolans, fly ash ...

The typical performances tested both on the laboratory scale and on the pilot scale, inactive or active, are the following:

Volume reduction

Up to 420 g H ₃ BO ₃ per liter of concrete, i.e. a reduction in volume (effluent / concrete) of 3.5 for an initial effluent at 21,000 ppm of boron.

• . résistance à la compression > 200 bars après 28 jours de temps de cure en air humide,
• . résistance à l'eau après 3 mois d'immersion, pas de modification de la résistance mécanique, variation volumique < 1 %
• . lixiviation: Co 60 : < 10-⁶ cm/jour < 10-⁴ m/jour

Quality of the mix (based on 420 g H ₃ B0 _3/1 concrete)

• . compressive strength> 200 bars after 28 days of curing time in humid air,
• . water resistance after 3 months of immersion, no change in mechanical strength, volume change <1%
• . leaching: Co 60: <10- ⁶ cm / day <10- ⁴ m / day

The following nonlimiting examples illustrate the invention.

Example 1

Boric acid neutralized with sodium hydroxide, in an Na / B molar ratio = 0.26, and 240 g / l in boric acid is treated. It is assumed that the production of concentrates for a PUR 900 MWe unit is 2.5 m ³ / year.

A mobile conditioning station is equipped with a GUEDU type 2500 mixer drier, with a useful capacity of approximately 1600 1 and of approximately 10 m ² of total exchange surface.

The evaporative capacity under the conditions of use of the process is of the order of 80 kg / h. 1.6 m ³ of borated effluent are fed into the apparatus, then lime is introduced, in a Ca / B molar ratio = 0.5. The precipitation reaction is carried out overall in 5 h, the speed of rotation of the mixer being controlled by the measurement of a viscometer with vibrating needle. This results in a suspension of insoluble calcium borates mainly in the form CaO, B ₂ 0 ₃ , 6H ₂ 0, the reaction yield being 85%. The dry extract at 60 ° C of the solution obtained is approximately 32%.

The suspension obtained above is then overconcentrated under an absolute pressure of 0.25 bar, at around 60 ° C., to a dry extract of 45%. The tetrahydrate CaO, B ₂ 0 ₃ , 4H ₂ 0 is mainly obtained, which is stable. The operation takes around 7 hours. Then, after cooling, CLK cement is added, such that the free water / cement ratio is 1.45. After 10 min of kneading, 1160 I of concrete with density 1.625 are obtained.

The total duration of the operating cycle is approximately 13 h. Volume reduction 1.38 (effluent volume / concrete volume).

• 20 heures (en équivalent).

Processing of the annual production of a tranche:

• 20 hours (equivalent).

Example 2

Boric acid neutralized with sodium hydroxide is treated in a Na / B molar ratio = 0.25 and at 120 g / I in boric acid.

In GUEDU 2,500, 16,000 liters of initial solution are fed and concentrated by evaporation under vacuum to 240 g / 1, at a pressure of 0.1 bar absolute, gradually feeding the initial solution so as to maintain the constant volume.

The evaporation capacity under these conditions is 120 kg / h. After treatment of 3,200 l of initial solution, 1,600 l of solution containing 240 g / l of boric acid is obtained, the operation lasting approximately 14 hours.

The concentrated solution obtained is then treated as in Example 1. In this second example, the total duration of an operating cycle is of the order of 27 hours.

Example 3

Boric acid is treated at 120 g / l.

An installation using two GUEDU mixers and dryers is used: a type 2,500, with capacity 1,600 and 10 m ² exchange surface, and a type 820, with capacity 500 1, 5 m ² exchange surface.

1.6 m ³ of boric acid solution are fed into the 2,500 type mixer-dryer, and it is neutralized with sodium hydroxide in a Na / B molar ratio = 0.26, lime is added to a Ca / B molar ratio = 0.5.

Then, phase P1 of the pretreatment and the evaporation are carried out simultaneously under an absolute pressure of 0.25 bar. The evaporation capacity under these conditions is approximately 90 kg / h. This phase lasts approximately 2 h 40 min. Evaporation is then stopped during phase P2 of the pretreatment, that is to say approximately 2.5 h, then the concentration of the pretreated solution is resumed, under a pressure of 0.1 bar, until approximately 500 l of suspension are obtained. concentrated, containing the equivalent of 384 g / 1 of boric acid, ie a dry extract of approximately 51%. This last operation lasts approximately 12 hours, or an overall time of this first cycle of 17 hours.

The concentrated solution is then transferred to the GUEDU 820, the evaporating capacity of which will be of the order of 40 l / h. The overconcentration is finished until a dry extract of 56%. The operation lasts approximately 1 hour 30 minutes.

The over-concentrated paste is then acidified to pH 8.

Mix for 10 min with CLK 45 cement, in a free water / cement ratio of 0.8, with the addition of 3% of water-reducing plasticizer (by weight relative to the cement).

We obtain 500 1 of concrete, density 1.78 -. Total duration of a cycle assuming continuous work: 17 h.

Volume reduction: 3.2.

Claims (6)

1. A process for encapsulating low- and medium-level radio-active waste containing borates, by hydraulic cements, by chemical treatment with a view to the preparation of crystals of tetrahydrated calcium borate, reduction in volume and mixing of the product obtained with at least one hydraulic cement, characterized in that:

- said tetrahydrated calcium borate is produced by adding to the solution to be treated a quantity of lime calculated so as to form the borate B₂0₃CaO;

- the operation of volume reduction is carried out by evaporation, under reduced pressure, of the solutions and/or suspensions, said reduced pressure being between about 0.1 and 0.3-10⁵ Pa bar and applied after the phase of preparation of the crystals of tetrahydrated calcium borate;

- and in that said volume reduction is continued until a concretable suspension is obtained at the end of the process, which contains from about 30 to about 70 % of dry matter.

2. A process for encapsulating low- and medium-level radio-active waste containing borates, by hydraulic cements, by chemical treatment with a view to preparing crystals of tetrahydrated calcium borate, reducing volume and mixing the product obtained with at least one hydraulic cement, process in which the phase of preparation of the crystals of tetrahydrated calcium borate comprises a first step of addition of lime and of germination, a second step of formation of mixed salts, a third step of formation of said borate, and a fourth step of maturing of the crystals of said borate, characterized in that a reduction in volume, under pressure reduced from 0.1 to 0.3-10⁵ Pa, is also carried out during at least one of the first and fourth steps, and possibly after the phase of preparation of the crystals of tetrahydrated calcium borate.

3. Process of encapsulation according to one of claims 1 and 2, characterized in that the waste is also subjected to an evaporation under a pressure reduced from 0.1 to 0.3.10⁵ Pa before the phase of formation of the crystals of tetrahydrated calcium borate.

4. Process according to one of claims 1 to 3, characterized in that the whole of the phase of crystallization is carried out under a reduced pressure of between about 0.7 and 0.9.10⁵ Pa.

5. Process according to one of claims 1 to 4, characterized in that the crystals of tetrahydrated calcium borate are prepared by the formation, during the third step, of hexahydrated calcium borate, which is subsequently converted into tetrahydrated calcium borate during said fourth step.

6. Apparatus for carrying out the process according to one of claims 1 to 5, characterized in that said process is carried out discontinuously in one (or two) apparatus constituted by a mixer- malaxator-evaporator and which comprises a device for maintaining the temperature, a stirrer having several stirring speeds and efficient with fluids of variable viscosities, and a device for producing therein a pressure of about 0.1 to 1.10⁵ Pa.