Method for treating radioactive liquid waste and apparatus for carrying out the method

Abstract

The present invention relates to a method for treating radioactive liquid waste from nuclear plants or hospital waste containing various metal, non-metal and organic compounds to convert this liquid waste into a glass body that safely holds the nuclear elements or isotopes in a silica matrix, and the invention also relates to an apparatus for carrying out this method in an automated manner.

Description

[Technical Field]

[0001] The present invention relates to a method for treating radioactive liquid waste from nuclear plants or hospital waste containing various metal, non-metal and organic compounds to convert this liquid waste into a glass body that safely holds the nuclear elements or isotopes in a silica matrix, and the invention also relates to an apparatus for carrying out this method in an automated manner. [Background technology]

[0002] Radioactive by-products, residues, or waste products of human activities are commonly referred to as "nuclear waste." Because of its danger to people and the environment, nuclear waste of all types and origins must be treated and stored according to special procedures that ensure that radiation and nuclear elements or isotopes are contained for very long periods of time.

[0003] Many types of activities and processes generate waste at various levels of concentration and toxicity. Medical materials used in nuclear medicine, disposable clothing provided when visiting nuclear power plants, and other sources of low-activity waste account for approximately 90% of the radioactive waste generated by weight, but only 1% of the radioactivity. Waste with intermediate levels of radioactivity, such as the sheaths of fuel elements used in nuclear power plants, accounts for approximately 7% of the waste by weight and 4% of the total radioactivity. Finally, waste with high levels of radioactivity (e.g., nuclear sludge, recycled waste, or "decommissioned" reactors from no longer active nuclear reactors) accounts for only 3% of radioactive waste, but accounts for 95% of the radioactivity and is the most dangerous due to the high radiation transferred by accidental exposure and the decay times, on the order of millions of years, of some of the radioisotopes they contain.

[0004] Most of the activities cited generate waste in the form of ash, slurry, mud, sludge, solutions, or dispersions containing metal ions, which must be disposed of in a safe manner. For a description of the problems involved in treating waste from nuclear power plants, see the article "Nuclear Fuel Recycling: More Problems Than Value," by Frank N. von Hippel, Scientific American, April 2008.

[0005] A typical composition of radioactive sludge from a nuclear power plant is reported in the table below. [Table 1]

[0006] Disposal of these wastes generally requires a conditioning step, such as inactivation and conversion of the waste into a form suitable for storage, and storage of the conditioned waste in a suitable site, either natural or industrially created.

[0007] Many techniques have been reported for the conditioning of nuclear waste.

[0008] Some work has focused on the use of iron-containing phosphate glassy systems as hosts for radioisotopes, as described in U.S. Pat. Nos. 5,750,824 and 5,840,638, and GB 2,371,542 A.

[0009] Another type of material being evaluated for immobilizing radioisotopes is silica-based glass, see for example the article "Glass packaging guaranteed for millions of years," E'.Y. Vernaz, Clefs CEA, No. 46 (2002), pp. 81-84.

[0010] One technique that has been proposed for producing silica-based glass bodies is sol-gel.

[0011] The sol-gel technique is widely known in chemistry and involves hydrolyzing a compound or mixture of one or more trivalent or tetravalent metals or metalloids in an aqueous or aqueous-alcoholic solution (sol) to form a compound in which one or more groups linked to the metal or metalloid are hydroxy groups, and then reacting the hydroxide species thus formed by condensation (i.e., elimination of water molecules and formation of oxygen bridges between two metal or metalloid atoms) to form a 3D network of bonds coextensive with the starting sol and containing the solvent (gel). The preferred element for forming gels according to this route is silicon. The wet gel thus obtained can then be dried naturally, possibly in an oven, or supercritically dried to obtain a porous solid. In the art, dried gels obtained by drying in air (heated or unheated) are referred to as "xerogels," while dried gels obtained by supercritical drying are referred to as "aerogels." Natural drying (leading to xerogels) can cause fracture in the dried gel and thus the final glass body, and the tendency for fracture in xerogels increases with size. Supercritical drying of wet gels ensures that the dried gel and the final glass body will not break, but when applied to wet gels obtained from aqueous or aqueous-alcoholic solutions, it requires specialized equipment (autoclaves), temperatures between 240 °C (ethanol, the alcohol typically used in sol-gel processes) and 370 °C (water), and pressures between approximately 60 bar (ethanol) and 217 bar (water). Therefore, employing such drying methods is only justified when there is an absolute need to accurately replicate the shape of the starting wet gel to obtain a shaped final piece, and is generally not suitable for large-scale industrial applications. The dried gel can finally be compacted by heat treatment (a process known in the art as "sintering"), typically at temperatures above 1000 °C. It is possible to add other components to the starting sol that cannot themselves form a gel but can be incorporated into the network of bonds between the gel-forming elements, resulting in the final wet gel and the final porous or dense solid when the wet gel is dried and subsequently subjected to sintering.

[0012] This approach to immobilization of nuclear elements or isotopes has been utilized in several patent documents such as U.S. Pat. Nos. 4,514,329 and 5,494,863, as well as EP 1 667 938 A1 and WO 2010 / 043698 A2.

[0013] Despite the progress that has been achieved, there is still a felt need in the field for an efficient process for the immobilization of nuclear isotopes in vitreous bodies.

[0014] It is therefore an object of the present invention to provide a method for the treatment of radioactive liquid wastes that allows the immobilization of radioactive elements and isotopes in long-term stable solid objects, and another object of the present invention is to provide an apparatus for carrying out this method in an automated manner. Summary of the Invention

[0015] These objects are achieved, in a first aspect, by the invention, which consists in a sol-gel method for the treatment of radioactive liquid waste, making it possible to immobilize in a solid glass body the radioactive elements and / or isotopes initially present in the radioactive liquid waste, said method comprising the following steps: (a) preparing an aqueous dispersion of silica in the form of finely divided particles having a size of less than 100 μm and having a silica concentration of 10 to 90% by weight, while controlling the pH at a value not exceeding 4 by addition of an acid; (b) adding to the dispersion prepared in step (a) an alkoxysilane, a compound of general formula Si(OR)4, where R is a linear or branched C1-C4 alkyl radical and the molar ratio alkoxysilane / silica is 0.4-0.7; (c) feeding the dispersion of step (b) together with the radioactive waste liquid into a mixer equipped with a stirring system, a pH meter, and an outlet nozzle at the end of the hydrolysis reaction, while controlling the mixing ratio so that the pH value of the resulting mixture exceeds 4.8; (d) discharging the mixture obtained in step (c) through an outlet nozzle into one or more molds or onto a flat surface; (e) placing the mold containing the mixture in an oven at a temperature of 60-70°C for at least 48 hours to dry it, thereby obtaining one or more dried porous gel bodies; (f) treating the dried porous gel body obtained in step (e) at a temperature of 850-1350°C to obtain one or more glass bodies consisting of a silica matrix in which radioactive elements and / or isotopes are embedded.

[0016] In a variation of the above process, the total amount of alkoxysilane required for the reaction may be added partly to the aqueous dispersion of silica in step (a) and partly to the radioactive liquid waste used in step (c).

[0017] In another variation of the method, the aqueous dispersion of silica from step (a) and the radioactive waste liquid are first mixed, and the entire amount of alkoxysilane to be used in the reaction is added to the mixture thus obtained.

[0018] In a second aspect, the present invention relates to an apparatus capable of carrying out the above method by automated means, said apparatus comprising: - a tank for aqueous dispersion of silica, placed on the balance; - reservoir for alkoxysilanes; - tanks equipped with mixing equipment for the mixture of aqueous dispersions of silica with alkoxysilanes; - pipes connecting the tank for the aqueous dispersion of silica and the reservoir for alkoxysilanes to the tank for the mixture of aqueous dispersion of silica and alkoxysilanes; - a volumetric valve on the pipe connecting the reservoir for alkoxysilanes to the tank for the mixture of aqueous dispersion of silica with alkoxysilanes; - a volumetric valve on the pipe connecting the tank for the aqueous dispersion of silica with the tank for the mixture of the aqueous dispersion of silica with alkoxysilane; - a thermometer inserted into the tank for the mixture of aqueous silica dispersion and alkoxysilane; - a reservoir for radioactive waste placed on the balance; - Agitation tanks for radioactive waste; - Pipes connecting the reservoir for radioactive waste to the agitation tank for waste; - Mixing tanks or equipment for mixing aqueous dispersions of silica with alkoxysilanes and radioactive waste liquids; - a pipe connecting the tank containing the mixture of the aqueous dispersion of silica and the alkoxysilane to the above-mentioned mixing tank or device; - Pipes connecting the agitation tank for waste liquid to the above mixing tank or device; - microprocessor or personal computer; - a means controlled by a microprocessor or a personal computer for mixing the aqueous dispersion of silica, the alkoxysilane and the radioactive waste liquid, and controlling the resulting mixture to have a predetermined pH value; - Electrical / data lines connecting the volumetric valve on the pipe connecting the reservoir for the alkoxysilane to the tank for the mixture of the aqueous dispersion of silica and the alkoxysilane to a microprocessor or personal computer; - Electrical / data lines connecting the volumetric measuring device connecting the tank for the aqueous silica dispersion to the tank for the mixture of the aqueous silica dispersion and alkoxysilane to a microprocessor or personal computer; - Electrical / data lines connecting the thermometer to a microprocessor or personal computer; - a conveyor belt for transporting the mixture leaving the mixing tank or mixing device in an oven, in a mould or on a belt surface. [Brief explanation of the drawings]

[0019] [Figure 1-3] 1 to 3 show schematic diagrams of possible embodiments of the device of the present invention. [Figure 4] FIG. 4 shows a possible heat treatment profile according to the present invention for vitrifying a dried gel. DETAILED DESCRIPTION OF THE INVENTION

[0020] In its first aspect, the present invention resides in a sol-gel method for immobilizing radioactive elements and / or isotopes.

[0021] As mentioned above, this method allows for three types, depending on the solution and time in which the addition of alkoxysilane is carried out. The method is described below with reference to the first type, which includes steps (a) to (f) defined above. Those skilled in the art will have no difficulty in modifying the method to carry it out according to the second and third types.

[0022] The first step of step (a) consists of preparing an aqueous dispersion of silica powder having a silica concentration of 10-90% by weight and a particle size of less than 100 μm, wherein the pH of the dispersion is controlled by the addition of an acid so that it always remains at a value of 4 or less.

[0023] Preferably, the dispersion has a silica content of 20-65% by weight, more preferably 30-40% by weight. The silica can be any form of silica powder with a particle size of less than 100 μm, preferably less than 10 μm, even more preferably less than 5 μm. A preferred type of silica is so-called "fumed silica" or "pyrogenic silica," which is a highly dispersed, highly porous form of silica obtained by the combustion of SiCl4 with oxygen. Fumed silica is commercially available from various manufacturers, such as Aerosil® OX50, manufactured and sold by Evonik Resource Efficiency GmbH (Germany). Addition of the silica to the water is preferably carried out under vigorous stirring, using a disperser, such as an IKA® Ultra-Turrax® series (IKA-Werke) or similar equipment.

[0024] The addition of silica to water to form the dispersion is carried out under controlled conditions to ensure that the pH never exceeds 4; preferably, the pH of the dispersion at this step is maintained in the range of 1.5 to 3, more preferably 2 to 2.5. The pH of the dispersion is maintained in the desired range by the addition of an acid, which may be an organic acid such as formic acid or acetic acid, but is preferably an inorganic acid such as hydrochloric acid, nitric acid, phosphoric acid, or sulfuric acid.

[0025] Step (a) of the method is carried out at room temperature, ie without cooling or heating the system, and therefore typically at a temperature in the range of 18-30°C.

[0026] In step (b), an alkoxysilane is added to the silica dispersion obtained as described above to achieve an alkoxysilane / silica molar ratio of 0.4 to 0.7. Alkoxysilanes are compounds of the general formula Si(OR)4, where R is a linear or branched C1-C4 alkyl radical. Preferred alkoxysilanes that can be used in the present invention are tetramethylorthosilane, Si(OCH3)4, commonly referred to as "TMOS," and in particular tetraethylorthosilane, Si(OCH2CH3)4, commonly referred to in the art as "TEOS." The preferred alkoxysilane / silica molar ratio is 0.5 to 0.6. For example, in a preferred formulation according to the present invention, 2 liters of TEOS are added to a dispersion prepared with 1 kg of fumed silica. The alkoxysilane is hydrolyzed with water according to the following reaction:

[0027] Si(OR)4+ 4H2O → Si(OH)4+ 4ROH

[0028] This reaction results in a temperature increase which can rapidly reach 35-45°C, depending on the alkoxysilane / water ratio (which in turn depends on the silica concentration in the dispersion from step (a)). The end of the hydrolysis reaction is indicated by a decrease in temperature.

[0029] Once the hydrolysis of the alkoxysilane in step (b) is complete, step (c) of the method is carried out. In this step, the dispersion from step (b) is fed into a mixer equipped with a stirring system, a pH meter, and an outlet nozzle, while the radioactive waste liquid is simultaneously fed. In an alternative embodiment in which the pH values ​​of the dispersion from step (b) and the radioactive waste liquid are measured separately, these two liquid phases are fed into a mixing device without a pH meter; this possibility is described in more detail below with reference to the operation of the device in Figure 3. The mixing ratio of the two liquid phases must be such that the resulting mixture has a pH value above 4.8. The mixer must be efficient enough to mix the two liquid phases very quickly during co-addition to the mixer tank, thereby ensuring that the resulting mixture always has a uniform (or nearly uniform) composition. This condition is necessary to obtain a homogeneous distribution of the radioactive elements and / or isotopes in the gel network and the final glass body, so that these elements and / or isotopes are efficiently immobilized in the 3D network of bonds in the silica phase. Conversely, if mixing is inefficient in this step of the process, the elements and / or isotopes will be poorly bound, forming a silica phase that embeds "bubbles" of the radioactive waste liquid, allowing it to be easily released from the final glass.

[0030] Since radioactive liquid waste typically has a pH value in the range of 5 to 14, it is usually effective to bring the pH of the mixture to the desired range above 4.8 by mixing with the acidic dispersion of step (b). If the pH of the radioactive liquid waste is low (close to 5) and the mixing ratio of the two liquid phases requires a large amount of radioactive liquid waste (which may therefore impair the gelling properties of the entire system), it is possible, according to the present invention, to add a base, e.g., NaOH or KOH, directly to the radioactive liquid waste or to the mixer tank.

[0031] The present inventors have used the compositions reported in Table 1 above and 1.0 to 1.3 g / cm 3When working with radioactive waste liquids with a density of 1000 MPa, it was observed that the optimal mixing ratio of radioactive waste liquid / silica dispersion was approximately 4:1 to 1:1, and that these ratios made it possible to obtain a final glass body with a volume of approximately 5% to 15% of the starting liquid mixture, thereby optimizing the space occupation in the disposal site.

[0032] In step (d) of the method, the dense mixture obtained in step (c) is dispensed into one or more molds through the mixer tank or the outlet nozzle of the mixing device. As described below, this step of the invention foresees natural drying in an oven, thus forming a xerogel. As explained above, these molds tend to fragment if produced at too large a size, so the molds used in this step preferably have a size not exceeding 5 cm in any one of the three spatial directions. The reason for reducing the size of the xerogel in terms of obtaining shaped pieces is that fragmentation of the material obtained at the end of the step increases the overall area per unit weight of the material, thereby increasing the leaching surface of radioactive elements and / or isotopes when the final glass body is buried in a special disposal facility. The molds used in this step can have a cylindrical, cubic, or prismatic shape. The molds are preferably made of plastic or metal with an inner surface covered with a layer of polytetrafluoroethylene (PTFE), ensuring easy release of the final dried gel from its surface. The pH of the dense mixture in step (c) ensures gelation of the silica component in 5 to 30 minutes.

[0033] In an alternative embodiment, which will be described in more detail below with reference to FIG. 2, in step (d), the dense mixture obtained in step (c) is dispensed onto a flat surface through an outlet nozzle of the mixer tank.

[0034] Once the mixture in the mold has gelled, it is transferred to an oven (preferably ventilated) at 60-70°C for at least 48 hours for mixtures dispensed into molds in step (d) or 0.5-1 hour for mixtures dispensed onto a flat surface in step (d) to obtain a dried xerogel. During this step, the gel shrinks in all dimensions due to a phenomenon known as syneresis, which is well known in the sol-gel field. The resulting dried gel has a volume of approximately 50%-75% of the volume of the starting wet gel, which facilitates its release from the mold walls. The resulting dried gel is porous and has a chalky appearance and uniformity.

[0035] Finally, in the last step (f) of the method, the dried porous gel body obtained in the previous step is densified by treatment at temperatures between 850 and 1350 °C, resulting in one or more glass bodies consisting of a silica matrix in which radioactive elements and / or isotopes are embedded. A typical heat treatment profile involves a heating ramp rate between 5 and 10 °C / min, a hold at the maximum temperature for 2 to 15 minutes, typically 4 to 6 minutes, followed by cooling, either naturally or forced by ventilation.

[0036] In a variation of the above method, the total amount of alkoxysilane used in the reaction may be subdivided into two portions, the first portion being added in step (b) to the aqueous dispersion of silica prepared in step (a), and the second portion being added to the radioactive waste liquid before mixing with the aqueous dispersion of silica in step (c).

[0037] In another variation of the method, the aqueous dispersion of silica from step (a) and the radioactive waste liquid are first mixed, and the entire amount of alkoxysilane to be used in the reaction is added to the mixture thus obtained.

[0038] In a second aspect, the present invention relates to an apparatus for carrying out the above-mentioned method in an automated manner. Various embodiments of the apparatus are shown schematically in Figures 1 to 3, in which like reference numerals correspond to like elements.

[0039] The apparatus 100 of Fig. 1 comprises a tank 110 for the aqueous dispersion of silica and a reservoir 112 for the alkoxysilane, both arranged on a balance 111. The tank 110 and the reservoir 112 are connected via piping to a tank 113 equipped with a mixing device 114, where the aqueous dispersion of silica and the alkoxysilane are mixed in the desired ratio. Control of the correct mixing ratio between the aqueous dispersion of silica and the alkoxysilane is achieved by a microprocessor (or personal computer) 120, which controls the opening of valves V1 and V2 located on the piping connecting the reservoir 112 and the tank 110 to the tank 113, respectively, via lines L1 and L2. Lines L1 and L2, as well as all lines L1 and L2 described below, are connected to a tank 113 equipped with a mixing device 114, where the aqueous dispersion of silica and the alkoxysilane are mixed in the desired ratio. Control of the correct mixing ratio between the aqueous dispersion of silica and the alkoxysilane is achieved by a microprocessor (or personal computer) 120, which controls the opening of valves V1 and V2 located on the piping connecting the reservoir 112 and the tank 110 to the tank 113, respectively, via lines L1 and L2. N The lines L7 and L8 constitute both electrical lines for providing power to the devices / actuators to which they are connected, as well as data lines for transmitting information (e.g., temperature, pH, ...) to the microprocessor. The apparatus 100 also comprises a tank 116 containing acid, connected to the tank 110 via a line containing valve V5, and a pH meter 117 connected to the microprocessor 120 via line L8. The system formed by the tank 116, pH meter 117, valve V5, and lines L7 and L8 operates as a feedback loop. The microprocessor 120 continuously receives data regarding the pH in the tank 110 and controls the opening of valve V5 to ensure that the pH in the tank 110 is always below 4. The microprocessor 120 is also connected via line L5 to a thermometer 115 immersed in the mixture generated in the tank 113.

[0040] The device also comprises a reservoir 130 for radioactive liquid waste placed on a balance 131. The reservoir 130 is connected via a pipe to a tank 132 for the liquid waste, which is equipped with a stirrer 133.

[0041] In this first embodiment of the invention, the means for mixing the aqueous dispersion of silica, the alkoxysilane, and the radioactive waste liquid while controlling the resulting mixture to have a preset pH value is represented by a volumetric valve located on the piping connecting tanks 113 and 132 to a mixer tank equipped with a pH meter, a stirring system, and an outlet nozzle.

[0042] In particular, tanks 113 and 132 are connected via pipes to a mixer 140 equipped with an agitation system 141, a pH meter 142, and an outlet nozzle 143. The piping connecting tanks 113 and 132 includes volumetric valves V3 and V4, respectively, which are controlled by microprocessor 120 via lines L3 and L4. Microprocessor 120 is also connected to pH meter 142 via line L6.

[0043] During operation, microprocessor 120 monitors in real time the pH of the mixture formed in mixer 140 by mixing the dispersion from tank 113 with the waste liquid from tank 132, and adjusts the opening of valves V3 and V4 via a feedback loop to maintain the pH of the mixture formed in mixer 140 at a desired value (above 4.8).

[0044] The dense mixture thus produced in the mixer 140 is then dispensed into molds 160 through an outlet nozzle 143 (indicated by numeral 150 in FIG. 1 ), moved by a conveyor belt 161. The movement of the conveyor belt is synchronized with an element on the nozzle 143 that can close the nozzle after a preset period of release of the mixture 150, thereby moving a new mold under the nozzle when a given volume of the mixture has been dispensed into the previous mold. Finally, the conveyor belt 161 transports the molds into a ventilated oven 162, where the wet gel formed in the mold is dried at a temperature T between 60°C and 70°C.

[0045] Finally, the dried gel body is vitrified by treatment in an oven (not shown in FIG. 1), which can be any type of known oven (e.g., a kiln) capable of reaching a temperature of at least 850° C.

[0046] A second embodiment of the present invention, apparatus 200, is shown diagrammatically in FIG. 2. For simplicity of illustration, tank 116, pH meter 117, valve V5, and lines L7 and L are not shown in FIG. 2, although they are present in apparatus 200 in the same arrangement as shown in FIG. 1. This second apparatus 200 does not use mold 160; instead, dense mixture 150 is deposited directly onto conveyor belt 161, whose movement produces a strip of wet gel 151, which is then transported by the belt into oven 162 for drying; the gel strip in the oven is represented in the diagram by dotted lines as element 151'. Again, the diagram does not show the final vitrification furnace.

[0047] A third possible embodiment of the apparatus of the present invention, apparatus 300, is shown in Figure 3. For simplicity, tank 116, pH meter 117, valve V5, and lines L7 and L are not shown in Figure 3, but they are present in apparatus 200 in the same arrangement as shown in Figure 1. In this embodiment, mixer 140 from the first and second embodiments is replaced by a specific device that performs the same or equivalent function. In particular, if the pH values ​​of the aqueous silica dispersion and hydrolyzed alkoxysilane on one side and the radioactive waste liquid on the other side are known, mixer 140 may be replaced by an engineered dispenser that integrates the functions of mixer 140 and nozzle 143. In this case, since the mixing ratio of the two liquid phases required to achieve a final mixture with a pH > 4.8 is known, pH meter 142 is unnecessary; it is sufficient to have a mixer that ensures control of the mixing ratio. One possible dispenser of this type is the Model 2RD12-3D-EC, manufactured and sold by ViscoTec Pumpenu. Dosiertechnik GmbH (Germany) achieves accurate and consistent dosing of fluids of different viscosities. Figure 3 illustrates the case where a dense mixture 150 is poured into a mold 160, which can also be deposited directly onto a belt 161 as in Figure 2. Compared to the device of Figure 1, the mixer 140, the separate stirring system 141, in particular the pH meter 142, and the line L6 connecting the pH meter to the microprocessor 120 are eliminated, as are the valves V3 and V4 and the lines L3 and L4, which are replaced by lines L9 and L6, respectively. 10 two pH meters 301 and 302 connected to the microprocessor 120 via line L 11 A mixing device 303 is added, connected to microprocessor 120 via a . In this configuration, microprocessor 120 senses the pH of the liquid phases in tanks 113 and 132, calculates the ratio of the flow rates of the two liquid phases that will produce a dense mixture having a pH > 4.8, and controls device 303 to ensure the production of a dense mixture with the desired pH value. Again, the figure does not show the final vitrification furnace. [Example]

[0048] The present invention is further illustrated below by several embodiments, which are provided as non-limiting examples of the scope of the invention.

[0049] Example 1 This example relates to a method of the invention according to the type in which the alkoxysilane is added only to the aqueous dispersion of silica.

[0050] 300 g of fumed silica (Aerosil OX50) was dispersed in 700 mL of water in a beaker using an Ultra-Turrax® mixer, and the system was homogenized for 1 h. As the use of the mixer increased the temperature to 50° C., the dispersion was allowed to cool to room temperature before adding the other ingredients.

[0051] Once the dispersion reached a temperature of 22°C, 0.57g of 1N aqueous HCl was added under vigorous stirring to lower the pH, which reached 2 at the end of the addition, as checked by placing a pH meter in the reaction beaker.

[0052] A 200 mL aliquot of the dispersion thus prepared was placed in a second beaker, and 120 mL of TEOS was added to the second beaker while the mixture was continuously and vigorously stirred with a magnetic stirrer.

[0053] The hydrolysis reaction of TEOS caused the temperature to rise from 20° C. to 34° C. (measured by a thermometer built into the pH meter) in less than 5 minutes. The temperature remained stable for about 2 minutes and then began to decrease, indicating the end of the reaction.

[0054] The dispersion was stirred for 24 hours in a beaker sealed with polymer film.

[0055] Liquid 1 was prepared by diluting radioactive waste liquid obtained from nuclear power plant waste with water, and has the following composition:

[0056] [Table 2]

[0057] β / γ radioactivity (mainly 60 Co and 137 Cs) is 3.5 × 10 7 Equivalent to Bq / kg.

[0058] 700 mL of Liquid 1 was homogenized using an Ultra-Turrax® mixer for 1 hour, then the beaker was covered and slightly stirred overnight. After 24 hours, the pH of Liquid 1 was checked. The pH meter indicated a pH of 6.8.

[0059] 300 mL of the silica and TEOS dispersion was added with stirring to 700 mL of Liquid 1. The resulting mixture was quickly poured into 20 cylindrical PTFE molds, each with a volume of 50 mL, and appeared as a thick liquid.

[0060] The mixture gelled very quickly, and a wet gel formed in the mold in about 5 minutes. The mold was dried in an oven at 60°C for 72 hours.

[0061] At the end of drying, the xerogel showed a volume reduction of about 50% and was easily removed from the PTFE mold.

[0062] The xerogel was placed on a cristobalite plate (i.e., a crystallized quartz plate) and placed in an oven for vitrification according to the thermal profile shown in Figure 4.

[0063] The set of vitrified bodies thus obtained is called Sample 1.

[0064] Example 2 This example relates to a type of process of the invention in which the alkoxysilane is added partly to the aqueous dispersion of silica and partly to the radioactive liquid waste.

[0065] To 250 mL of the dispersion of fumed silica at pH=2 obtained as described in Example 1, 210 mL of TEOS was added under continuous and vigorous stirring.

[0066] The hydrolysis reaction of TEOS caused the temperature to rise from 23° C. to 37° C. in about 5 minutes. The temperature remained stable for about 2 minutes and then began to decrease, indicating the end of the reaction.

[0067] The dispersion was stirred for 24 hours in a beaker sealed with polymer film.

[0068] Liquid 2 was prepared by diluting radioactive wastewater obtained from nuclear plant waste with water in a beaker, and had the following composition.

[0069] [Table 3]

[0070] 700 mL of liquid 2 was homogenized using an Ultra-Turrax® mixer for 1 hour, the temperature was raised to 55° C. and the system was cooled to room temperature.

[0071] When the temperature reached 23° C., the pH was checked. The pH meter indicated a pH of 10.8. Under stirring and pH control, NaOH was added (a few milligrams) until the pH value reached 11.4.

[0072] While stirring vigorously, 2.75 mL of TEOS was added to Liquid 2, resulting in the hydrolysis of TEOS. Under these conditions (base-catalyzed reaction), the hydrolysis of TEOS was slower than in the acid-catalyzed reaction, and no significant increase in temperature followed by a subsequent decrease was observed. The beaker containing Liquid 2 and TEOS was sealed with polymer film, and the hydrolysis reaction was allowed to proceed for 24 hours with magnetic stirring.

[0073] 300 mL of the silica dispersion and hydrolyzed TEOS was added to 700 mL of Liquid 2 and hydrolyzed TEOS with stirring. The resulting mixture was quickly poured into 20 cylindrical PTFE molds, each with a volume of 50 mL, and appeared as a thick liquid.

[0074] Again, gelation of the mixture was very fast, with a wet gel forming in the mold in approximately 5 minutes.

[0075] The mold was placed in an oven and dried at 60°C for 72 hours.

[0076] At the end of drying, the xerogel showed a volume reduction of about 50% and was easily removed from the PTFE mold.

[0077] The xerogel was placed on a cristobalite plate and placed in an oven for vitrification following the same thermal profile as in Example 1 (FIG. 4).

[0078] The set of vitrified bodies thus obtained is called Sample 2.

[0079] Example 3 This example relates to a type of process according to the invention in which an alkoxysilane is added to an already prepared mixture of an aqueous dispersion of silica and a radioactive waste liquid.

[0080] 425 mL of Liquid 1 was poured into a beaker. Using an Ultra-Turrax® mixer, 75 g of fumed silica (Aerosil® OX50) was added to Liquid 1 and the mixture was homogenized for 1 hour. During stirring, the temperature rose to 58°C.

[0081] The mixture was allowed to cool in a covered beaker with slight stirring.

[0082] When the temperature reached 22°C, a pH value of 4.8 was measured.

[0083] 1N aqueous HCl was added with vigorous stirring until a pH of 2 was reached.

[0084] To the mixture thus prepared, 300 mL of TEOS was added.

[0085] The hydrolysis reaction of TEOS reached a temperature of 36°C in about 10 minutes. The mixture was then cooled overnight with stirring while the beaker was sealed with polymer film. While maintaining stirring, the pH was adjusted to 4.8 by dropwise addition of a 1N aqueous solution of NaOH.

[0086] The mixture thus obtained was poured into a PTFE cylindrical mold and gelation was carried out at this pH value for approximately 1 hour. The mold containing the wet gel was placed in an oven and dried at 60°C for 72 hours.

[0087] At the end of drying, the xerogel showed a volume reduction of about 50% and was easily removed from the PTFE mold.

[0088] The xerogel was placed on a cristobalite plate and placed in an oven for vitrification following the same thermal profile as in Example 1 (FIG. 4).

[0089] The set of vitrified bodies thus obtained is called Sample 3.

[0090] Example 4 This example relates to a test of ion release (leaching test) from a vitrified body obtained by the method of the present invention when it comes into contact with water.

[0091] One sample of each vitrified body of samples 1, 2 and 3 was subjected to a leaching test in accordance with the Swiss Federal Institute for Nuclear Safety Inspection (ENSI) standard ENSI-B05, December 2018 edition.

[0092] With regard to the radioactivity of the derived aqueous solutions connected to the multiple isotopes, the release of multiple metals under the test conditions is summarized in Table 1 for Sample 1, Table 2 for Sample 2, and Table 3 for Sample 3.

[0093] [Table 4]

[0094] [Table 5]

[0095] [Table 6]

[0096] Example 5 This example relates to mechanical tests carried out on samples of the invention. The mechanical resistance of the vitrified bodies is important for their expected use (burial of these objects in a repository) to ensure that the mechanical stresses they may be subjected to several meters underground do not break them down and release fragments, leading to increased mobility due to increased surface area and / or increased leaching of radioisotopes. The tests were carried out at a stress of 15 N / mm 2 The tests were carried out according to the method prescribed by standard ASTM D695-10, which requires a pressure resistance of more than 1000 kJ / cm². The results of the tests are reported in Table 4.

[0097] [Table 7] [Explanation of symbols]

[0098] 100, 200, 300 Equipment for waste liquid treatment 110 Tank for silica water dispersion 112 Reservoir for alkoxysilanes 113 Mixture Tank 114 Mixing equipment 115 Thermometer 116 Tanks for storing acid 117 pH meter 120 Microprocessor or Personal Computer 130 Reservoirs for radioactive waste 132 Mixing Tank 140 Mixer 160 type 161 Conveyor Belt 162 Oven L1~L8 connection lines V1 to V5 valves

Claims

1. A sol-gel method for the treatment of radioactive liquid waste, making it possible to immobilize in a solid glass body the radioactive elements and / or isotopes initially present in the radioactive liquid waste, comprising the following steps: (a) preparing an aqueous dispersion of silica in the form of finely divided particles having a size of less than 100 μm and having a silica concentration of 10 to 90% by weight, while controlling the pH at a value not exceeding 4 by adding an acid; (b) adding an alkoxysilane, of general formula Si(OR) to the dispersion prepared in step (a) 4 where R is a linear or branched C1-C4 alkyl radical and the molar ratio alkoxysilane / silica is between 0.4 and 0.7; (c) feeding the dispersion of step (b) together with the radioactive waste liquid into a mixer equipped with a stirring system, a pH meter, and an outlet nozzle at the end of the hydrolysis reaction, while controlling the mixing ratio so that the pH value of the resulting mixture exceeds 4.8; (d) discharging the mixture obtained in step (c) through an outlet nozzle into one or more molds or onto a flat surface; (e) drying the mixture by placing the mold containing the mixture in an oven at a temperature of 60-70°C for 0.5 to 1 hour if the mixture was dispensed onto a flat surface in step (d), or for at least 48 hours if the mixture was dispensed into a mold in step (d), to obtain one or more dried porous gel bodies; (f) treating the dried porous gel body obtained in step (e) at a temperature of 850-1350°C to obtain one or more glass bodies consisting of a silica matrix in which radioactive elements and / or isotopes are embedded.

2. 10. The method of claim 1, wherein the dispersion prepared in step (a) has a silica content of 20 to 65 wt. %.

3. 3. The method of claim 2, wherein the dispersion has a silica content of 30 to 40% by weight.

4. 4. The method according to any one of claims 1 to 3, wherein the silica used in step (a) has a particle size of less than 10 μm.

5. 5. The method of claim 4, wherein the silica has a particle size of less than 5 μm.

6. The method according to any one of claims 1 to 5, wherein the dispersion prepared in step (a) has a pH value of from 1.5 to 3.

7. 7. The method of claim 6, wherein the pH is from 2 to 2.

5.

8. 8. The method of any one of claims 1 to 7, wherein the acid used in step (a) is selected from among formic acid, acetic acid, hydrochloric acid, nitric acid, phosphoric acid, and sulfuric acid.

9. The method of any one of claims 1 to 8, wherein step (a) is carried out at a temperature of 18 to 30°C.

10. 10. The method according to any one of claims 1 to 9, wherein in step (b) the molar ratio of alkoxysilane to silica is from 0.5 to 0.

6.

11. The alkoxysilane used in step (b) is tetramethylorthosilane (Si(OCH 3 ) 4 ) and tetraethylorthosilane (Si(OCH 2 CH 3 ) 4 The method according to any one of claims 1 to 10, wherein the compound is selected from the group consisting of:

12. 12. The method according to any one of claims 1 to 11, wherein in step (c), the mixing ratio of the radioactive waste liquid to the silica dispersion is in the range of 4:1 to 1:

1.

13. 13. The method according to any one of claims 1 to 12, wherein in step (d) the mixture obtained in step (c) is distributed into moulds having a size not exceeding 5 cm in any one of the three spatial directions.

14. 14. The method of any one of claims 1 to 13, wherein step (f) is carried out by heating the dried porous gel body with a heat ramp of 5-10°C / min, maintaining the maximum temperature for 2-15 minutes, and allowing the resulting glass body to cool to room temperature.

15. An apparatus (100; 200; 300) for carrying out the method according to any one of claims 1 to 14 by automated means, comprising: a tank (110) for the aqueous dispersion of silica, placed on a balance (111); a reservoir (112) for alkoxysilanes; a tank (113) equipped with a mixing device (114) for the mixture of the aqueous dispersion of silica and the alkoxysilane; - pipes connecting the tank for the aqueous dispersion of silica (110) and the reservoir for alkoxysilanes (112) to the tank for the mixture of aqueous dispersion of silica and alkoxysilanes (113); - a volumetric valve (V) on the pipe connecting the reservoir for alkoxysilanes (112) to the tank for the mixture of the aqueous dispersion of silica and alkoxysilanes (113); 1 ); - a volumetric valve (V) on the pipe connecting the tank for the aqueous dispersion of silica (110) to the tank for the mixture of the aqueous dispersion of silica and alkoxysilane (113); 2 ); a thermometer (115) inserted in the tank (113) for the mixture of the aqueous dispersion of silica and the alkoxysilane; a tank (116) for containing acid, with a valve (V 5 a tank connected to the tank (110) for the aqueous dispersion of silica through a line in which a pH meter (117) inserted in the tank (110) for the aqueous dispersion of silica; a reservoir (130) for radioactive waste placed on a balance (131); - stirred tank for radioactive waste (132); - a pipe connecting the reservoir (130) for radioactive liquid waste to the stirred tank (132) for the liquid waste; a mixing tank (140) or a mixing device (303) for mixing the aqueous dispersion of silica, the alkoxysilane and the radioactive waste liquid; - pipes connecting the tank (113) containing the mixture of the aqueous dispersion of silica and the alkoxysilane to the mixing tank (140) or to the mixing device (303); - pipes connecting the stirring tank for waste liquid (132) to the mixing tank (140) or to the mixing device (303); - a microprocessor or personal computer (120); - means controlled by a microprocessor or a personal computer for mixing the aqueous dispersion of silica, the alkoxysilane and the radioactive waste liquid, and for controlling the resulting mixture to have a predetermined pH value; - Volumetric valve V 1 to a microprocessor or personal computer (120) 1 ); - Volumetric valve V 2 to a microprocessor or personal computer (120) 2 ); - a line (L) connecting said thermometer to a microprocessor or personal computer (120); 5 ); - Valve (V 5 ) to a microprocessor or personal computer (120). 7 ); - a line (L) connecting the pH meter (117) to a microprocessor or personal computer (120) 8 ); A conveyor belt (161) for transporting the mixture leaving the mixing tank or mixing device in an oven (162), in a mould (160) or on a belt surface.

16. 16. The device (100; 200) according to claim 15, wherein the means controlled by a microprocessor or a personal computer include: a mixing tank (140) equipped with an agitation system (141), a pH meter (142) and an outlet nozzle (143); - a volumetric valve (V) on the pipe connecting the tank (113) containing the mixture of the aqueous dispersion of silica and the alkoxysilane to the mixing tank (140) 3 ); - a volumetric valve (V) on the pipe connecting the waste agitation tank (132) to the mixing tank (140) 4 ); - Volumetric valve (V 3 ) to a microprocessor or personal computer (120). 3 ); - Volumetric valve (V 4 ) to a microprocessor or personal computer (120). 4 ); - a line (L) connecting the pH meter (142) to the microprocessor or personal computer (120) 6 ).

17. 16. The apparatus (300) of claim 15, wherein the microprocessor or personal computer controlled means comprises: - a mixing device (303) equipped with an outlet nozzle (143); a pH meter (301) for measuring the pH value in the tank (113) equipped with a mixing device (114) for the mixture of the aqueous dispersion of silica and the alkoxysilane; a pH meter (302) for measuring the pH value in the stirred tank (132) for radioactive liquid waste; - a line (L) connecting the pH meter (301) to the microprocessor or personal computer (120) 9 ); - a line (L) connecting the pH meter (302) to the microprocessor or personal computer (120) 10 ); - a line (L) connecting the mixing device (303) to a microprocessor or personal computer (120) 11 ).

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