EP0241364A1 - Process for immobilizing nuclear wastes in a borosilicate glass - Google Patents

Abstract

The invention relates to a process for the immobilization of nuclear waste in a borosilicate glass. In the process, the following are mixed simultaneously: a silica-based gel precursor, a concentrated aqueous solution of a boron compound, and concentrated aqueous solutions of the other constituents of the final glass, i.e. a solution (solutions) of the waste to be treated and a solution of the vitrification adjuvant, with vigorous stirring, mixing taking place at between 20 DEG and 80 DEG C., preferably at 65 DEG -70 DEG C., in proportions corresponding to the desired composition of the glass, the said mixture having an acid pH, preferably a pH of between 2.5 and 3.5, and the said mixture is dried, calcined at between 300 DEG and 500 DEG C. and then melted. The invention is applied to the treatment of solutions of nuclear waste, especially to solutions of fission products.

Description

High-level nuclear waste - such as fission products - or long-lived waste such as actinides is currently immobilized in borosilicate glasses which offer sufficient guarantees of safety for man and the environment.

The French Atomic Energy Commission (CEA) has developed an industrial vitrification process for fission products (PF).

This process (known as AVM) consists in calcining the FP solution and in sending the calcinate obtained, simultaneously with a glass frit, to a melting furnace. In a few hours, at a temperature of the order of 11OO ° C., a glass is obtained which is poured into metal containers.

The glass frit is composed mainly of silica and boric anhydride, plus the other oxides (sodium, aluminum, etc.) necessary for the total formulation of calcinate + frit to give a glass which can be produced by known glass techniques and fulfilling the conditions of safety for storage (conditions on leaching, mechanical strength, etc.).

In the melting furnace, there is digestion of the calcinate which is incorporated into the glass structure. The temperature should be chosen high enough to hasten digestion but without having a detrimental effect on the life of the oven.

To facilitate the formation of a glass structure containing all the necessary elements, including the FPs, the applicant has developed a process in which the constituents of the glass are mixed in an aqueous medium so as to form a gelled solution.

It is also known that obtaining a glass from a gelled solution (also called "gels route") can be done at temperatures lower than those necessary with the oxides ("oxides route").

The main objective is the manufacture by gels of glasses before the same formulation as those currently prepared by the oxide route, as the examples will show, but any borosilicate formulation acceptable for packaging waste can be prepared.

In the following text, the following terms will be used in the sense defined below:
. vitrification adjuvant : it is all the constituents of the final glass other than the constituents originating from nuclear waste and except B and Si. This adjuvant therefore does not contain active nuclear elements. In the AVM process, it is included in a glass frit; in the process which is the subject of the invention, it is an aqueous solution,
. final glass : this is the glass in which nuclear waste is immobilized,
. soil : it is a solution of orthosilicic acid; this unstable evolves by polymerizing. Commercial soils, such as Ludox ® (from Pont de Nemours), are stabilized solutions in which partially hydrated silica particles are found, these colloidal particles are polymers whose evolution has been stopped but can be unlocked by acidification for example,
. gelled solution or gel : it is a homogeneous solution, of variable viscosity which goes from a flowing solution to a fixed mass, according to the progress of the polymerization.

A way is known for preparing the gels in an aqueous medium, known as the sol-gel method, which consists in using a sol in water and in destabilizing it by modifying the pH, thus causing this solution to gel.

The related publications are:
SARZYCKI J. -Jl of Materials Science 17 (1982) p 3371-3379
JABRA R. - Revue de Chimie Minérale, t. 16, 1979, p 245-266
PHALIPPOU J. - Glasses and Refractories, Vol. 35, ^No. 6, November, December 1981.

The literature describes the preparation of a SiO₂-B₂O₃ glass by the sol-gel method:
- addition of a Ludox solution brought to pH: 2 to a solution of aqueous ammonium tetraborate also brought to pH: 2;
- mixture with stirring for 1 h (with possible addition of ammonia to bring the medium to pH: 3.5 very favorable for gelling)
if the solution obtained is free of precipitation or flocculation, it is considered to be a satisfactory gel
- drying 8 h at 100 ° C then 15 h at 175 ° C under vacuum of 0.1 mm Hg
- hot pressing (45O bars - 5OO to 9OO ° C - 15 min to 5 h) to densify and vitrify (fusion is another means).

Until now, only binary or ternary glasses have been prepared by this method because the multiplicity of cations present makes it difficult to control gelation and even to obtain it.

Thus, to develop a berre of the same composition as the glass frit used in the current vitrification process, it would be necessary:
B₂O₃, SiO₂, Al₂O₃, Na₂O, ZnO, CaO, Li₂O, ZrO₂.

However, it is known that:
the boron makes gelation very difficult (in the HITACHI process described below, the boron is also added after formation of the gel) in particular due to the high insolubility of many boron compounds and promotes recrystallization in mixed gels,
- aluminum promotes precipitation to the detriment of gelling, which is opposed to the desired result,
- Sodium, calcium and zirconium lead to the formation of crystals which later constitute fragile points which can cause local destruction.

The multiplicity of components therefore leads those skilled in the art to question how to introduce them and their order of introduction.

• 1) Westinghouse et le US Department of Energy ont développé un procédé de vitrification de solutions actives avec préparation de gels mais en milieu alcoolique (alcogels) - Brevets US 4 43O 257 et US 4 422 965 - Leur procédé peut se résumer ainsi :
      - mélange et hydrolyse des constituants inactifs du gel en milieu alcool-eau, les constituants étant introduits sous forme X(OR)_n
  par exemple : Si(OR)₄, B(OR)₃...
      - élimination de l'azéotrope eau-alcool et obtention d'un gel sec,
      - addition de la solution de déchets nucléaires (le composé final contenant 3O-4O % au maximum en déchets) ajustée à pH : 4 à 6,
      - séchage,
      - fusion.
      Le gel préparé à partir de composés X(OR)_n en milieu alcoolique peut être obtenu plus facilement car on ne se heurte pas aux problèmes de solubilité et de plus l'effet peptisant de l'eau à haute température est éliminé par emploi de l'alcool.
      Inconvénient majeur d'un tel procédé : le milieu alcooli­que sujet à incendie, explosion... d'où la nécessité d'éliminer l'alcool avant introduction des déchets nucléaires, ce qui oblige à une opération supplémentaire peu pratique à mettre en oeuvre.
• 2) Le procédé HITACHI dans lequel le gel est obtenu à partir de la solution de PF dans une solution de silicate de sodium, le bore (sous forme B₂O₃) n'étant rajouté qu'après gélification, ce qui oblige à effectuer une calcination du gel à au moins 6OO°C pendant le temps nécessaire (3 h, cité comme exemple) à la diffusion du bore dans la matrice silicate pour former la structure borosilicatée, l'homogénéité du produit restant un problème.
• 3) Publication : UETAKE N. - Nuclear Technology, Vol 67, novembre 1984.

The complexity of the components in the vitrification process, with both those
- vitrification adjuvant (Al₂O₃, Na₂O, ZnO, CaO, Li₂O, ZrO₂) plus B₂O₃ and SiO₂,
- FP solution to be vitrified (around twenty different cations!)
has led manufacturers to develop two processes on the basis of gels:

• 1) Westinghouse and the US Department of Energy have developed a process for vitrifying active solutions with preparation of gels but in alcoholic medium (alcogels) - US Patents 4,430,257 and US 4,422,965 - Their process can be summarized as follows:
  - mixing and hydrolysis of the inactive constituents of the gel in an alcohol-water medium, the constituents being introduced in X (OR) _{n form}
  for example: If (OR) ₄, B (OR) ₃ ...
  - elimination of the water-alcohol azeotrope and obtaining a dry gel,
  - addition of the nuclear waste solution (the final compound containing 3O-4O% maximum in waste) adjusted to pH: 4 to 6,
  - drying,
  - merger.
  The gel prepared from compounds X (OR) _n in alcoholic medium can be obtained more easily because there is no problem of solubility and moreover the peptizing effect of water at high temperature is eliminated by using l 'alcohol.
  Major drawback of such a process: the alcoholic environment subject to fire, explosion ... hence the need to eliminate the alcohol before the introduction of nuclear waste, which requires an additional operation that is impractical to implement.
• 2) The HITACHI process in which the gel is obtained from the solution of PF in a solution of sodium silicate, the boron (in B₂O₃ form) being added only after gelation, which requires calcination of the gel at least 6OO ° C for the time necessary (3 h, cited as an example) for the diffusion of boron in the silicate matrix to form the borosilicate structure, the homogeneity of the product remaining a problem.
• 3) Publication: UETAKE N. - Nuclear Technology, Vol 67, November 1984.

It emerges from this analysis that the mixing of solutions (PF + other components) from the start seemed impossible, all the components which are not compatible with each other for the preparation of gels.

In addition, there was the problem of the homogeneity of the gel. It was necessary to shake but not too much. Indeed, the specialists considered that the gel must finish constituting at rest (after mixing with stirring) for several hours, any stirring being able to then cause local destruction.

The Applicant has found a process for the immobilization of nuclear waste in a borosilicate glass, characterized in that all the constituents of the glass are introduced at the same time into a mixing zone, the mixing taking place with vigorous stirring in an aqueous medium in temperature conditions (25-1OO ° C, 65-7O ° C preferably), pH (acid and between 2.5 and 3.5 preferably) and determined proportions (depending on the composition desired for the final glass), the constituents glass being composed of:
- a silica-based gel precursor,
- a concentrated aqueous solution of a boron compound,
- concentrated aqueous solutions of the other constituents, that is to say:
. the solution of the nuclear waste to be treated,
. the vitrification aid solution.

The mixture obtained is dried, calcined (3OO-5OO ° C) and finally melted (1OOO-115O ° C) to obtain the final glass.

Strong stirring is defined by the stirring speed: the stirring device rotates at more than 5000 rpm, preferably 2OOO rpm, and the thickness of the stirred layer (distance between the wall of the container and a stirring blade) does not exceed 10% of the diameter of the blade. It can for example be a turbine, a mixer or more simply a mechanical agitator rotating in a narrow section.

In the current state of knowledge, there is every reason to think that the agitation must be all the more intense therefore all the shorter the higher the risk of precipitation. It is in fact necessary to ensure that a homogeneous mixture is produced by stirring in a very short time, with respect to the duration of the precipitation; and that the gel also forms as quickly as possible so as to freeze the various ions and, by preventing any diffusion of these ions, to avoid a possible reaction of said ions with one another.

The solutions used are concentrated solutions, with the aim of rapidly manufacturing a gel and minimizing the amount of water to be evaporated, as will be explained in the description and the examples. It is difficult to give an exact concentration limit for each of the compounds, but the concentration of the solutions can reasonably be situated at least 75% of the saturation concentration.

The method can be applied to various nuclear waste solutions. It is particularly suitable for the vitrification of FP solutions alone or with other active effluents, for example the soda solution for washing tributylphosphate used for the extraction of uranium and plutonium; the sodium washing solution can even be treated alone by this process. FP solutions are nitric solutions resulting from the reprocessing of fuels, they contain a large number of elements in various chemical forms and a certain amount of insolubles. An example of composition is given below.

The soda effluent is based on sodium carbonate and possibly contains organic phosphorus traces entrained by washing (example 3).

In the description of the process, a substance containing silica particles, possibly partially hydrolyzed, which is either in the form of a powder which, when dissolved in acid, can produce a sol, or directly in the form of a gel, will be called a gel precursor. 'a floor.

Gel precursors sold commercially and advantageously used in the process can for example be a sol such as Ludox® or else Aerosil® which is formed by hydrolysis in the gas phase of silicon tetrachloride. In an acidic environment, the Aerosil leads to a soil then to a firm gelled mass.

The Ludox is brought in a solution. Aerosil on the other hand can be taken either in solution or directly in the form of powder, depending on the technology used.

The gel precursor is placed in an acidic aqueous medium, according to the process which is the subject of the invention, so that it becomes a gelled solution by polymerization from Si-OH- bonds.

The boron necessary to form the borosilicate structure is brought by the aqueous solution of a boron compound sufficiently soluble such as ammonium tetraborate (TBA) which has a solubility of approximately 3OO g / l or 15.1% B₂O₃. Preferably, the solution is prepared and used at 65-7O ° C. Boric acid can just as easily be used, its solubility is around 13O g / l at 65 ° C or 6.5% B₂O₃, it is increased in the presence of Na⁺ ions when Na / B ≃ O, 23 .

To prepare the solution of the vitrification aid, it is necessary to use compounds containing the desired elements which are soluble in water, at process temperature, which are compatible with each other, which do not unnecessarily add other ions and whose ions not participating in the structure of the final glass are easily removed by heating. They are, for example, nitrate solutions when nitric FP solutions are treated. The solid compounds are always preferably dissolved in the minimum quantity of water so as to minimize the volumes treated and the quantities of water to be evaporated.

The proportions in which these solutions are prepared (with the exception of waste solutions) and mixed depend on the formulation of the final glass desired. It can be considered that the constituent elements of the glass are practically not volatilized and that the composition of the final glass obtained practically corresponds to that of the mixture produced. In the examples, an acceptable glass formulation is indicated. The qualitative and quantitative composition of the vitrification aid is adapted according to the composition of the final glass and that of the waste solution to be treated.

The mixing is carried out between 20 ° and 80 ° C. The solutions to be treated are taken at the temperature at which they are; the FP solution, due to the fact that its activity arrives at the treatment unit between 20 ° and 40 ° C. The concentrated solution of the boron compound is maintained to avoid precipitation between 50 ° and 80 ° C. The other solutions are developed at room temperature. It is then possible either to mix the solutions at the temperature at which they are developed or brought, either to bring all the solutions (except those of the waste taken as is) to a higher temperature before mixing them.

The latter has the following advantage. After mixing has taken place and the gelled solution has started to form, the polymerization (gelling) takes place during a so-called maturation time. The rise in temperature favors it. It is therefore very advantageous to prepare the mixture between 50 ° C. and 80 ° C. The gelling solution matures in the process which is the subject of the invention, during drying, preferably at 100-150 ° C.

The solutions of the glass constituents have different pH values: the gel precursor in solution is acid (such as Aerosil in nitric solution) or alkaline (Ludox), the solution of acidic vitrification adjuvant, the solution of acid waste ( in the case of FP) or alkaline (non-neutralized washing effluent) solutions, the solution of the boron acid (boric acid) or alkaline (ammonium tetraborate) compound. In the process described here, the pH of the mixture must be less than 7 and preferably between 2.5 and 3.5. An adjustment of the pH can be undertaken if necessary.

According to the process which is the subject of the invention, the mixture - from which the final glass is obtained by heating - is prepared from all the components in aqueous solution introduced simultaneously into the mixing zone.

Are to be mixed:

In a mixing zone, the separate solutions A, B, C, D arrive simultaneously (possibly C and D can be brought together).

By mixing, a so-called solution is obtained gelled, its viscosity and its texture changing over time and going from a fluid solution to a gel. The mixture obtained is dried (at 1OO-1O5 ° C preferably) in an oven for example, drying under vacuum is also possible. During this operation, the gel continues to form. A calcination is then carried out between 3OO and 5OO ° C (35O to 4OO ° C preferably) during which the water finishes evaporating and the nitrates decompose in part, the analysis shows that after 2 h at 4OO ° C 30% of the nitrates are still present under the conditions of the example.

The calcination can be done either in a conventional calciner (of the type used in the AVM vitrification process) or in a melting furnace of the ceramic melter type for example.

We always finish the decomposition of nitrates during melting. At the entrance of the oven, the product quickly goes from its calcination temperature to its melting temperature. This is the so-called introduction zone, then in the so-called refining zone, it is at a temperature slightly higher than that of melting and then it is brought to the casting temperature. An interesting value is between 1035 ° C and 1100 ° C for which the viscosity of the glass between 2OO poises and 8O poises allows the glass to be poured under good conditions.

The drying-calcination-melting steps described correspond to heat treatments in defined temperature zones and in different equipment. It is obvious that similar heat treatments in other devices are suitable, for example drying in an oven followed by introduction into a melting furnace designed in several zones, in general any technique for making glass from a gel can be used.

Thus, when a mixture having the formulation of CEA is prepared in an aqueous medium according to the process which is the subject of the invention, it is observed that the ripening times are shortened: 1.5 h are sufficient whereas 5 h were necessary by the way oxides. It is then possible to increase the oven flow rates.

In addition, the formulation worked out by the CEA and which gives all satisfactions can be easily obtained with various wastes.

Indeed, it is possible with the process object of the invention tion to vitrify various wastes, in particular sodium-rich wastes. Sodium, which improves the fusibility of glass, has the disadvantage of making it more sensitive to leaching. The treatment of such waste requires in the oxides path the modification of the glass frit, but it is limited by the fusibility.

According to the process which is the subject of the invention, the composition of the borosilicate matrix prepared in an aqueous medium is adjusted to the type of waste treated. Thus, for sodium-rich waste a borosilicate matrix low in sodium (or even possibly without sodium) can be developed, as the examples will show.

Another important advantage (not previously obtained by other gelation techniques) is that large amounts of gel can be prepared without difficulty with a turbine. We were able to reach in the tests, without being at the limit, 40 kg / h of gel very easily. Apparatus of this simple and removable type can be adapted to the safety constraints applied to nuclear installations.

In order to better understand the originality of the process which is the subject of the invention in relation to the state of the art, examples will be given, the first example consisting in trying to develop a gelled solution from the teaching of prior art.

Example 1 A conventional process for the preparation of gels applied to the treatment of a simulated FP solution . The solutions

On a laboratory scale, a FP solution was simulated from a typical composition of a real FP solution as follows:

Group 1 represents the inactive elements of the fission product solution and group 2 the PF and insolubles of the same solution.

ZrO₂ and Mo remain solid, they simulate the insoluble. The total amount of water added is 2,972 g.

The simulated FP solution has a pH: 1.3. The final glass composition to be obtained is:

In the percentage composition present, account must be taken of the presence of sodium and nickel in the active oxides (originating from group 2 of the solution defined above).

Thus, the solutions of the vitrification aid are prepared according to the composition of the glass to be obtained and that of the waste solution to be treated.

La solution TBA : (NH₄)₂O,2B₂O₃,4H₂O, 265,2 g dissous dans 663 g d'eau à 65°C - pH : 9,2.For this example, the separate vitrification aid solutions are thus prepared at room temperature:

The TBA solution: (NH₄) ₂O, 2B₂O₃, 4H₂O, 265.2 g dissolved in 663 g of water at 65 ° C - pH: 9.2.

. Operation mode

59 g of TBA solution are placed in a 1 l beaker fitted with a magnetic stirrer (7 cm bar) rotating at 5OO rpm, which is brought to pH 2 by addition of HNO₃

In another beaker, 56 cm³ of Ludox are acidified to pH 2, to avoid the subsequent precipitation of hydroxides such as Al (OH) ₃ at pH: 5-6 or Zn (OH) ₂ at pH: 4.8.

The Ludox solution is introduced, with stirring, into the ammonium tetraborate, the reaction taking place at 65 ° C.-70 ° C. Stirred (magnetic or mechanical stirrer) 30 min while maintaining the temperature. To accelerate the gelation, a small amount of dilute ammonia (0.15 N) is added to obtain pH: 3. There is gel formation.

Each solution of the adjuvant is added separately slowly (drop by drop) with stirring to the mixture. Stirring is continued for 5 to 10 min. The mixture obtained, called gel, has no visible precipitation or flocculation. 235 g of the simulated FP solution are added slowly (drop by drop) while stirring. Precipitates are formed.

To obtain solidification, the mixture is left to stand at 65 ° C.-70 ° C., it takes at least 20 h to obtain a mass which is dried in the oven (90 h at 110 ° C.) immediately after its obtaining, then melted between 1OOO and 115O ° C.

The analysis shows that the molybdenum which sedimented was not homogeneously included in the glass, moreover traces of molybdate are visible. The glass obtained is not acceptable.

With this process, only small amounts of gels (≃ 1OO to 5OO ml) could be prepared. It was not possible to obtain a gel with 1 l of solution (there is precipitation).

The solutions of the glass constituents must be introduced separately or together if they are compatible with each other, otherwise precipitation is observed making the gel non-homogeneous. The gel and the final glass are not always of good quality.

In addition, with this conventional process, it has never been possible to correctly introduce the simulated waste solution. Precipitation and sedimentation have been observed which have the consequence of requiring a higher melting temperature and / or digestion time or producing an unacceptable final glass.

In the tests, a glass of good quality was defined as being a homogeneous glass, not presenting any unfused and bubbles and moreover not showing on the surface traces of molybdate.

Indeed, the molybdate coming from FP solutions poses a major problem: part of the active Mo tends to separate from the solution and sediment so that this phase is not completely dispersed in the mixture therefore it is not fully included in the gelled solution. In addition, molybdenum when it diffuses badly appears on the surface of the glass in the form of traces of yellow and visible molybdate which are considered to be an index of poorer quality.

Example 2 Treatment of a solution of fission products according to the invented process . . The solutions

These are the same as those of Example 1, except that of the vitrification aid.

chacun des composés est dissous dans le minimum d'eau, soit au total 64O g d'eau à 65°C ; pH : O,6.For this example, the vitrification aid solution is prepared as follows:

each of the compounds is dissolved in the minimum amount of water, ie a total of 640 g of water at 65 ° C .; pH: 0.6.

The proportions of the elements Al, Na, Zn, Ca, Li are the same as in Example 1.

. The device

A conventional turbine is used comprising a mixing zone of small volume in which a propeller with several blades rotates so that a mixture with a high rate of shear is produced. In this example, it rotates at 2OOO rpm.

The turbine used for the tests is manufactured by the company STERMA, the mixing zone has a volume of 1 cm³, the thickness of the agitated layer is of the order of mm. For use on an industrial scale in a nuclear environment, some technical improvements especially concerning the geometry of the blades and the arrival of solutions will be necessary; their purpose is to facilitate operation in an active closed cell.

. Procedure

The solutions arrive separately and simultaneously at the turbine:

The vitrification and PF adjuvant solutions are pumped at the indicated flow rate, and it is their mixture at the overall flow rate of 25.4 kg / h which is sent to the turbine. Thus, there is a flow rate of 36 kg / h in gel. The pH of the gelled solution leaving the turbine is 3.

In this test, 12 min were sufficient to mix the constituents and to produce 7 kg of gelled solution.

• - essai 1 :
  1O,5 kg de mélange ont été concentrés sous vide dans un appareil fabriqué par la Société GUEDU (T : 9O°C, P 63O mmHg), 6,5 l d'eau ont été extraits. La masse récupérée (3,5 kg) est calcinée 2 h à 4OO°C. On obtient 1,8 kg de produit qui sont soumis à une fusion à 1O5O°C pendant 5 h. Un verre de bonne qualité est obtenu.
• - essai 2 :
  5 kg de mélange sont séchés 3 jours à 1O5°C produisant 1,2 kg de produit sec qui est ensuite porté 2 h à 4OO°C où il perd 27,5 % de son poids. Une fusion de 5 h à 1O25°C donne 82O g d'un verre de bonne qualité avec une bonne coulée. Dans cet essai on a observé qu'un gel en masse était obtenu en moins de 3O min à 1O5°C.
• - essai 3 :
  3 kg de mélange étalés sur une plaque avec 2-3 cm d'épaisseur et placés 8 h dans un four à micro-ondes ont donné 55O g de produit qui après 2 h à 4OO°C (la température étant montée régulièrement de celle de séchage à 4OO°C) se réduisent à 5O2 g de produit calciné. La fusion à 1125°C nécessite seulement 1,5 h (dont 1 h d'affinage) pour produire un verre de très bonne qualité avec une bonne coulée.

On 3 samples, the following heat treatments were carried out:

• - test 1:
  10 kg of mixture were concentrated in vacuo in an apparatus manufactured by the company GUEDU (T: 9O ° C, P 63O mmHg), 6.5 l of water were extracted. The mass recovered (3.5 kg) is calcined for 2 h at 4OO ° C. 1.8 kg of product are obtained which are subjected to fusion at 1050 ° C. for 5 h. A good quality glass is obtained.
• - test 2:
  5 kg of mixture are dried for 3 days at 105 ° C. producing 1.2 kg of dry product which is then brought 2 hours to 400 ° C. where it loses 27.5% of its weight. A fusion of 5 h at 1025 ° C. gives 820 g of a good quality glass with good pouring. In this test, it was observed that a bulk gel was obtained in less than 30 min at 105 ° C.
• - test 3:
  3 kg of mixture spread on a plate with 2-3 cm thickness and placed 8 h in a microwave oven gave 55O g of product which after 2 h at 4OO ° C (the temperature being increased regularly from that of drying at 400 ° C.) are reduced to 502 g of calcined product. Melting at 1125 ° C requires only 1.5 h (including 1 h of refining) to produce a very good quality glass with good pouring.

In conclusion, a gel thus obtained treated for 8 h in a microwave oven, calcined for 2 h at 4OO ° C and then melted for 1.5 h at 1125 ° C (1 h of ripening), leads to a glass of very good acceptable quality for the immobilization of nuclear waste and this with a time saving of the order of 3 to 4 hours compared to the process currently used.

. Replacement of ammonium tetraborate with boric acid

The solution of TBA at 15% B₂O₃ is replaced by a solution H₃BO₃ at 6.5% in B₂O₃ formed by dissolution at 65-7O ° C with stirring of 13O g of solid boric acid in 1 l of water (pH = 2 , 7).

Consequently, the flow rate of boron compound is 10.8 kg / h in H₃BO₃ solution, the other solitions being sent at the same flow rates.

It is then obtained about 42 kg / h of mixture which, treated in the same way as above, leads to similar products.

Example 3 Treatment of a soda wash effluent.

Currently, in the vitrification process (AVM) from oxides, it is not possible to treat this soda effluent.

Indeed, this AVM process uses the vitrification adjuvant in the form of solid glass frit, a known composition is:
SiO₂ 55-6O% by weight
B₂O₃ 16-18% by weight
Al₂O₃ 6-7% by weight
Na₂O 6-7% by weight
CaO 4.5-6% by weight
ZnO 2.5-3.5% by weight
Li₂O 2-3% by weight

This composition limits the amount of admissible sodium in the effluent to be vitrified, since the sodium content cannot be increased too much which lowers the resistance to leaching.

One could think of reducing the sodium content of the glass frit and even bringing it to zero so that the final glass (frit + calcine of soda effluent) has an acceptable sodium content (9 to 11% by weight). But then, we are faced with the difficulty posed by the development and fusion of a glass low in sodium (and consequently richer in silica).

The present invention makes it possible to manufacture a borosilicate glass with the soda effluent having a composition close to that giving any satisfaction in the AVM process. In addition, the ripening temperature can be significantly lowered or the ripening times shortened.

For the tests, we therefore simulated a soda solution with 12O g Na₂CO₃ in 1 l of water (pH = 9). The gel precursor chosen is Ludox AS4O. The TBA solution: 312 g / l TBA, 4H₂O.

To obtain a glass having the composition of that obtained by the AVM process, the following vitrification aid solution is prepared for one liter of aqueous solution:
Al (NO₃) ₃, 9H₂O 2O9, O g
Ca (NO₃) ₂, 3H₂O 98.5 g
LiNO₃ 53.7 g
Zn (NO₃) ₂, 6H₂O 49.7 g
Fe (NO₃) ₃, 6H₂O 73.5 g
Mn (NO₃) ₃, 6H₂O 18.2 g
Ba (NO₃) ₂ 5.5 g
Co (NO₃) ₂, 6H₂O 11.3 g
Sr (NO₃) ₂ 4.1 g
CsNO₃ 8, O g
Y (NO₃) ₃, 4H₂O 71, O g
Na₂MoO₄, 2H₂O 16.6 g
Monoammonium phosphate 2.8 g

In this solution, the components Fe, Mn ... phosphate were introduced so as to obtain a final glass with a composition close to that given in Example 2.

Each of the solutions is kept in a thermostatically controlled bath (temperature 65 ° C). There are 4 diaphragm pumps that have been adjusted beforehand to obtain the desired flow rates.

These solutions are sent simultaneously by the pumps to a high speed mixer (capacity 1.5 l).

The debits set are:
. TBA solution ............ O, 12 kg / h
. Adjuvant solution ....... O, 25 kg / h
. Ludox solution .......... O, 15 kg / h
. Na₂CO₃ solution ........ O, 21 kg / h

The operation is carried out for 1.5 h still in the form of agitation. The contents of the mixer bowl are poured into a beaker and left to stand for 2 hours. A homogeneous, practically solid mass of opalescent color is formed. This mass is distributed on a plate to form a layer of approximately 20 to 30 mm thick and the plate is placed in an oven heated to 105 ° C. for 24 h.

Dry particles of the order of cm³ are thus obtained. It is placed in an oven to calcine, the temperature is regularly raised over 3 hours to 400 ° C. and it is maintained for 3 hours. The calcinate obtained is crushed into particles of 1 to 3 mm.

A Joule effect electric oven with sufficient capacity is set at 115O ° C. We fill a platinum crucible with a third of the prepared powder and place it in the oven. After 30 min, the crucible is recharged with a second third of powder, likewise a third time. The crucible is then left to remain for 2 h in the hot oven and then poured onto a refractory plate. It is annealed at 50 ° C. for 8 h in order to obtain a sufficient surface sample and the temperature is slowly lowered, which gives a glass plate of intense black coloration and of perfect visual homogeneity.

A chemical analysis gives the following average composition:
SiO₂ 45.6%
B₂O₃ 14%
Al₂O₃ 4.9%
Na₂O 1O%
CaO 4%
Li₂O 2%
Fe₂O₃ 2.9%
MnO₂ O, 95%
BaO O, 55%
CoO O, 5%
Cs₂O 1%
SrO O, 35%
Y₂O₃ 4%
MoO₃ 2%
P₂O₅ O, 3%

This example shows how the composition of the vitrification aid can be adjusted.

Example 4 Treatment of soda effluent with Aerosil .

The vitrification adjuvant, TBA and waste solution solutions are the same.

On the other hand, instead of using Ludox AS4O as a gel precursor, we will take the Aerosil ® sold by the company DEGUSSA. The gel precursor is formed by gradually pouring, with stirring, the Aerosil into water acidified with HNO₃3N (pH: 2.5) so as to obtain a solution of 150 g of silica per liter.

The flow rates are adjusted to the values indicated:
TBA: O, 37 kg / h
Adjuvant: 0.75 kg / l
Aerosil: 1.3 kg / h
Na₂CO₃ solution: 0.63 kg / h

The procedure is identical to the previous example 1 in all respects, except for the drying which is carried out in a vacuum oven which allows the duration to be reduced to 4 h. The result is the same. You can't tell the difference between the two glasses. In particular, we find the same chemical analysis (except for precision).

The two examples 3 and 4 illustrate the invention for the treatment of soda effluent with different gel precursors, but they do not constitute a limitation thereof. We can, in particular, combine them to vary the operating mode without departing from the scope of the invention, for example by bringing the silica both in the form of Ludox and Aerosil simultaneously.

In Examples 3 and 4, the neutralized soda effluent was treated alone. It is obviously advantageous to simultaneously treat the non-neutralized soda effluent (therefore as it leaves the extraction units) and the FP solutions which are nitric, so as not to consume nitric acid and increase the volumes of waste. For this, the washing water of the nitrous vapors, loaded with nitric acid, is added to the soda effluent to neutralize it, the resulting liquid being mixed with the FP solution in predetermined proportions. The vitrification adjuvant solution will then be suitable for this treatment.

All the solutions have been prepared in a minimum of water - they are close to saturation - so as not to increase the drying times, the volumes of liquid to be handled and also the active gas emissions, since the water to be evaporated is contaminated by radioelements, which obliges the operator to process said discharges. It may be necessary, without damage to the process, to further dilute these solutions for pumping or flow issues.

Furthermore, in the examples, the boron compound used is ammonium tetraborate tetrahydrate, this to allow comparison with the prior art easier. However, in existing vitrification plants, the use of TBA poses problems in the treatment of gaseous effluents rich in ammonia and nitrous vapors which are capable of recombining to produce dangerous ammonium nitrate under certain conditions.

For these reasons, boric acid is preferred under the conditions of the process which is the subject of the invention.

The description thus clearly shows that the process developed by the applicant is different from the HITACHI process described in the prior art in that all the components of the final glass are introduced simultaneously to form a gelled solution. Boron, unlike the HITACHI process, is introduced before the gel formation and not after. It is therefore part, from the start, of the structure whereas in the HITACHI process it is dispersed in the silicate structure previously prepared.

The Applicant believes that the gelled solution produced according to the invented process forms more quickly than the compounds react with one another to precipitate. The gelled solution obtained has the structure of the desired final glass and in this solution the ions can no longer migrate.

Indeed, it is estimated that, during mixing, under the conditions indicated, the phenomenon of thixotropy takes place, so that a homogeneous dispersion of the ions occurs. After this mixing phase, the solution sees its viscosity increase, so that the ions are trapped in the medium. They can no longer react (precipitation, sedimentation ...), the environment is blocked.

This effect is, according to the applicant, due to the choice of solutions used and to the method of agitation used to produce their mixture.

Claims (8)

1. Method for immobilizing nuclear waste in a borosilicate glass, characterized in that:
- are mixed simultaneously:
. a silica-based gel precursor,
. a concentrated aqueous solution of a boron compound,
. concentrated aqueous solutions of the other constituents of the final glass comprising the solution (s) of the waste to be treated and the solution of the vitrification aid
with vigorous stirring, the mixture taking place between 20 ° and 80 ° C., preferably 65 ° -70 ° C., in the proportions corresponding to the composition of the glass desired, said mixture having an acidic pH and preferably between 2.5 and 3.5;
- Said mixture is dried, calcined between 3OO and 5OO ° C and then melted. 2. Method according to claim 1, characterized in that the mixing is carried out with a stirring device rotating at more than 5OO rpm and preferably 2OOO rpm and in which the thickness of the stirred layer does not exceed not 10% of the diameter of the stirring device. 3. Method according to claim 2, characterized in that the mixing is carried out in an apparatus chosen from a turbine and a mixer. 4. Method according to claim 1, characterized in that the gel precursor is a sol. 5. Method according to claim 1, characterized in that the gel precursor is a Ludox®. 6. Method according to claim 1, characterized in that the gel precursor is Aerosil ®. 7. Method according to claim 1, characterized in that the boron compound is ammonium tetraborate. 8. Method according to claim 1, characterized in that the boron compound is boric acid.