Patent Number: 047724315
Section: summary

High-level nuclear waste, such as fission products, or nuclear waste with a long half-life, such as actinides, is currently immobilized in borosilicate glasses which offer adequate safety guarantees to man and the environment. The Atomic Energy Commission (AEC) has developed an industrial process for the vitrification of fission products (FP). This process (called AVM) consists in calcining the solution of FP and sending the resulting calcinate, at the same time as a glass frit, into a melting furnace. A glass is obtained in a few hours, at a temperature of the order of 1100.degree. C., and is run into metal containers. The glass frit is composed mainly of silica and boric oxide together with the other oxides (sodium, aluminum etc.) necessary so that the total formulation of calcinate+frit gives a glass which can be produced by the known glassmaking techniques and which satisfies the storage safety conditions (conditions pertaining to leaching, mechanical strength, etc.). In the melting furnace, the calcinate is digested and becomes incorporated into the vitreous structure. The chosen temperature must be sufficiently high to hasten the digestion, but must not have an adverse effect on the life of the furnace. To facilitate the formation of a vitreous structure containing all the necessary components, including the FP, the Applicant Company developed a process in which the constituents of the glass are mixed in an aqueous medium to form a gelled solution. Furthermore, it is known that a glass can be obtained from a gelled solution (or by the so-called "gel method") at temperatures below those required with oxides ("oxide method"). The aim is essentially to manufacture, by the gel method, glasses having the same formulation as those currently prepared by the oxide method, as will be shown in the examples, but any borosilicate formulation acceptable for conditioning waste can be prepared. In the remainder of the text, the following terms will be employed with the meanings defined below: vitrification adjuvant: This comprises all the constituents of the final glass other than the constituents originating from the nuclear waste and except for B and Si. This adjuvant therefore contains no active nuclear components. In the AVM process, it is included in a glass frit; in the process forming the subject of the invention, it is an aqueous solution. final glass: This is the glass in which the nuclear waste is immobilized. sol: This is a solution of orthosilicic acid; the latter, being unstable, changes by polymerizing. Commercial sols, such as Ludox.RTM. (du Pont de Nemours), are stabilized solutions containing partially hydrated particles of silica; these colloidal particles are polymers whose polymerization has been stopped but can be unblocked, for example by acidification. gelled solution, or gel: This is a homogeneous solution of variable viscosity, ranging from a solution which flows to a solidified mass, depending on how far the polymerization has advanced. A method, called the sol-gel method, is known for preparing gels in an aqueous medium; it consists in using a sol in water and destabilizing it by modifying the pH, thus causing this solution to gel. The following publications refer to this method: J. SARZYCKI--J. of Materials Science 17 (1982) p 3371-3379 R. Jabra--Revue de Chimie Minerale, t. 16, 1979, p 245-266 J. Phalippou--Verres et Refractaires, Vol. 35, no. 6, November, December 1981. The preparation of an SiO.sub.2 -B.sub.2 O.sub.3 glass by the sol-gel method is described in the literature: addition of a solution of Ludox, adjusted to pH 2, to a solution of hydrated ammonium tetraborate, also adjusted to pH 2; mixing by stirring for 1 h (aqueous ammonia being added, if necessary, to bring the pH of the medium to 3.5, which is very favorable for gelling); if the resulting solution shows no precipitation or flocculation, it is considered to be a satisfactory gel; drying for 8 h at 100.degree. C. and then for 15 h at 175.degree. C. under a vacuum of 0.1 mm Hg; and hot pressing (450 bar--500 to 900.degree. C.--15 min to 5 h) in order to densify and vitrify the product (an alternative method is melting). Only binary or ternary glasses have so far been prepared by this method because the presence of a multiplicity of cations makes it difficult to control gelling and even to achieve it. Thus, to produce a glass having the same composition as the glass frit used in the present vitrification process, the following would be necessary: B.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3, Na.sub.2 O, ZnO, CaO, Li.sub.2 O, ZrO.sub.2. Now, it is known that: boron makes gelling very difficult (in the HITACHI process described below, boron is actually added after the gel has formed), particularly because of the high insolubility of a large number of boron compounds, and favors recrystallization in mixed gels; aluminum favors precipitation to the detriment of gelling, which opposes the desired result; and sodium, calcium and zirconium lead to the formation of crystals which subsequently constitute fragile points capable of causing local destruction. Due to the multiplicity of components, those skilled in the art are questioning the method of introducing them and the order in which they are introduced. The complexity of the components in the vitrification process, namely: those of the vitrification adjuvant (Al.sub.2 O.sub.3, Na.sub.2 O, ZnO, CaO, Li.sub.2 O, ZrO.sub.2) plus B.sub.2 O.sub.3 and SiO.sub.2, and at the same time those of the solution of FP to be vitrified (around twenty different cations!), led industrialists to develop two processes based on gels: (1) Westinghouse and the U.S. Department of Energy developed a process for the vitrification of active solutions involving the preparation of gels, but in an alcoholic medium (alcogels)--U.S. Pat. No. 4,430,257 and U.S. Pat. No. 4,422,965. Their process can be summarized in the following way: mixing and hydrolysis of the inactive constituents of the gel in an alcohol/water medium, the constituents being introduced in the form X(OR).sub.n, for example Si(OR).sub.4, B(OR).sub.3 etc.; removal of the water/alcohol azeotrope to give a dry gel; addition of the solution of nuclear waste (the final compound containing a maximum of 30-40% of waste), adjusted to pH 4 to 6; drying; and melting. The gel prepared from compounds X(OR).sub.n in an alcoholic medium can be obtained more easily because solubility problems are avoided and, furthermore, the peptizing effect of water at high temperature is eliminated by using alcohol. The major disadvantage of this type of process is that the alcoholic medium is prone to fire, explosion etc., so the alcohol has to be removed before introduction of the nuclear waste; this necessitates an additional operation which is rather impractical to carry out. (2) The HITACHI process, in which the gel is obtained from the solution of FP in a solution of sodium silicate, the boron (in the form of B.sub.2 O.sub.3) not being added until after gelling; this necessitates calcining the gel at 600.degree. C., or above, for the time required for the boron to diffuse into the silicate matrix to form the borosilicate structure (for example 3 h); the homogeneity of the product remains a problem. (3) The publication: N. Uetake--Nuclear Technology, Vol. 67, November 1984 This analysis shows that it seemed impossible to mix the solutions (FP+other components) at the start of the process, not all the components being mutually compatible for the preparation of gels. The homogeneity of the gel was also a problem. It was necessary to stir, but not too much. In fact, specialists considered that the gel should finish forming at rest for several hours (after mixing by stirring), any stirring being capable of causing local destruction at that stage. The Applicant Company has found a process for the immobilization of nuclear waste in a borosilicate glass wherein all the constituents of the glass are introduced at the same time into a mixing zone, mixing taking place with vigorous stirring in an aqueous medium under given conditions of temperature (25.degree.-100.degree. C., preferably 65.degree.-70.degree. C.) and pH (acid, preferably between 2.5 and 3.5) and in given proportions (according to the desired composition of the final glass), and the constituents of the glass being composed of: a silica-based gel precursor, a concentrated aqueous solution of a boron compound, and concentrated aqueous solutions of the other constituents, i.e.: a solution of the nuclear waste to be treated, and PA2 a solution of vitrification adjuvant. The mixture obtained is dried, calcined (300.degree.-500.degree. C.) and finally melted (1000.degree.-1150.degree. C.) to give the final glass. Vigorous stirring is defined by the stirring speed: the stirrer rotates at more than 500 rpm, preferably 2000 rpm, and the thickness of the stirred layer (distance between the wall of the vessel and a stirrer blade) does not exceed 10% of the diameter of the blade. The stirrer can be for example a turbine, a mixer or, more simply, a mechanical stirrer rotating in a narrow cross-section. In the present state of knowledge, there is every reason to think that the stirring must be the more intense and hence the shorter, the greater the risks of precipitation. What is actually required is to create a homogeneous mixture, by stirring, in a time which is very short compared with the precipitation time, and to ensure that the gel forms as quickly as possible so as to solidify the various ions and, by preventing any diffusion of these ions, prevent a possible reaction between the said ions. The solutions used are concentrated solutions so that a gel is produced quickly and the quantity of water to be evaporated off is minimized, 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 given as at least 75% of the saturation concentration. The process can be applied to a variety of solutions of nuclear waste. It is particularly suitable for the vitrification of solutions of FP by themselves or with other active effluents, for example the soda solution for washing the tributyl phosphate used to extract uranium and plutonium, it even being possible for this soda solution to be treated on its own by this process. The solutions of FP are nitric acid solutions originating from reprocessing of the fuel; they contain a large number of elements in various chemical forms and a certain amount of insoluble material. An example of their composition is given below. The soda effluent is based on sodium carbonate and may contain traces of organic phosphorus entrained by the washing process (Example 3). In the account of the process, the term "gel precursor" will be used to denote a substance containing particles of silica which may be partially hydrolyzed; it is either in the form of a powder, which can produce a sol when dissolved in acid solution, or directly in the form of a sol. Examples of gel precursors which are sold commercially and are advantageously used in the process are a sol such as Ludox.RTM. or alternatively Aerosil.RTM., which is formed by the hydrolysis of silicon tetrachloride in the gas phase. In an acid medium, Aerosil produces a sol and then a firm gelled mass. Ludox is used as it is, in solution. Aerosil, on the other hand, can be used either in solution or directly in the form of a powder, depending on the technology employed. The gel precursor is placed in an acid aqueous medium, in accordance with the process forming the subject of the invention, so that it is converted to a gelled solution by polymerization starting from the Si-OH bonds. The boron required to form the borosilicate structure is introduced as an aqueous solution of a sufficiently soluble boron compound such as ammonium tetraborate (ATB), which has a solubility of about 300 g/l i.e. 15.1% of B.sub.2 O.sub.3. Preferably, the solution is produced and used at 65.degree.-70.degree. C. Boric acid can equally well be employed; its solubility is about 130 g/l at 65.degree. C., i.e. 6.5% of B.sub.2 O.sub.3, and is increased in the presence of Na.sup.+ ions when Na/B.perspectiveto.0.23 The compounds, containing the desired elements, which are used to prepare the solution of the vitrification adjuvant should be soluble in water at the temperature of the process, be mutually compatible and not add other ions unnecessarily, and their ions which do not participate in the structure of the final glass should be easy to eliminate by heating. An example would be solutions of nitrates in cases where nitric acid solutions of FP are being treated. Solid compounds are always preferably dissolved in the minimum amount of water so as to minimize the volumes treated and the amounts of water to be evaporated off. The proportions in which these solutions (except for the solutions of waste) are prepared and mixed depend on the desired formulation of the final glass. It can be considered that the constituent components of the glass are not volatilized in practice and that the resulting composition of the final glass virtually corresponds to that of the mixture produced. An acceptable glass formulation is indicated in the examples. The qualitative and quantitative composition of the vitrification adjuvant is adapted according to the composition of the final glass and that of the solution of waste to be treated. The mixture is prepared at between 20.degree. and 80.degree. C. The solutions to be treated are introduced at their existing temperature; on account of its activity, the solution of FP arrives at the treatment unit at between 20.degree. and 40.degree. C. The concentrated solution of the boron compound is kept at between 50.degree. and 80.degree. C. in order to prevent precipitation. The other solutions are produced at ambient temperature. It is then possible either to mix the solutions at the temperature at which they are produced or arrive, or to heat all the solutions (except for the solutions of waste, which are taken as they are) to a higher temperature before mixing them. The latter case has the following advantage. After mixing has taken place and the gelled solution has started to form, polymerization (gelling) develops over a so-called ageing period. This is favored by raising the temperature. It is therefore very advantageous to produce the mixture at between 50.degree. C. and 80.degree. C. In the process forming the subject of the invention, the ageing of the gelled solution takes place during drying, preferably at 100.degree.-105.degree. C. The solutions of the constituents of the glass have different pH values: the gel precursor in solution is acid (for example Aerosil in nitric acid solution) or alkaline (Ludox), the solution of vitrification adjuvant is acid, the solution of waste is acid (in the case of the solutions of FP) or alkaline (in the case of unneutralized washing effluent) and the solution of boron compound is acid (boric acid) or alkaline (ammonium tetraborate). In the process described here, the pH of the mixture must be below 7 and preferably between 2.5 and 3.5. The pH can be adjusted if necessary. In the process forming 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. The following components are to be mixed: ______________________________________ % of oxide constituents of the glass Temperature ______________________________________ A Gel precursor a % of SiO.sub.2 25.degree. to 80.degree. C. B Boron solution b % of B.sub.2 O.sub.3 50.degree. to 80.degree. C. C Solution of waste c % of oxides 20.degree. to 40.degree. C. D Vitrification adjuvant d % of oxides 50.degree. to 80.degree. C. ______________________________________ The solutions A, B, C and D arrive separately and simultaneously in a mixing zone (C and D may be introduced together). Mixing produces a solution called a gelled solution, its viscosity and texture changing with time and ranging from those of a fluid solution to those of a gel. The mixture obtained is dried (preferably at 100.degree.-105.degree. C.), for example in an oven; drying in vacuo is a further possibility. The gel continues to form during this operation. Calcination is then carried out at between 300.degree. and 500.degree. C. (preferably at 350.degree. to 400.degree. C.), during which the water finishes evaporating off and the nitrates partially decompose; analysis shows that, after 2 h at 400.degree. C., 30% of the nitrates are still present under the conditions of the example. Calcination can be carried out either in a conventional calciner (of the type used in the AVM vitrification process) or in a melting furnace, for example of the ceramic melter type. The decomposition of the nitrates is always terminated during melting. On entering the furnace, the product rapidly passes from its calcination temperature to its melting point. This is the so-called introduction zone. Then, in the so-called refining zone, it is at a temperature slightly above the melting point; it is then brought to the pouring temperatrre. The value is advantageously between 1035.degree. C. and 110.degree. C., at which the viscosity of the glass, between 200 poises and 80 poises, enables 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. Similar heat treatments in other devices would obviously be suitable, for example drying in an oven followed by introduction into a melting furnace designed with several zones; in general, any technique for producing glass from a gel can be used. Thus, when a mixture having the AEC formulation is prepared in an aqueous medium by the process forming the subject of the invention, the refining times are found to be shortened: 1.5 h are sufficient where 5 h were necessary in the oxide method. The throughputs of the furnace can therefore be increased. Furthermore, the formulation produced by the AEC, which is highly satisfactory, can easily be obtained with diverse types of waste. The process forming the subject of the invention in fact makes it possible to vitrify various types of waste, in particular sodium-rich waste. Sodium improves the fusibility of the glass, but has the disadvantage of rendering it more sensitive to leaching. In the oxide method, the treatment of such waste necessitates modifying the glass frit, but modification is limited by the fusibility. In the process forming 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 low-sodium (or perhaps even sodium-free) borosilicate matrix can be produced, as will be shown in the examples. Another important advantage (not formerly obtained by the other gelling techniques) is that large quantities of gel can be prepared without difficulty using a turbine. In tests, it was possible to reach 40 kg/h of gel very easily, and this does not represent the limit. Equipment of this type, which is simple and capable of being disassembled, can be adapted to the safety constraints applied to nuclear plants.