CH633499A5 - Nonporous glass composition containing a radioactive material in the form of oxides or of elements which is embedded and immobilised by chemical binding in the glass matrix - Google Patents

Description

The present invention relates to a non-porous glass composition containing a radioactive material in the form of oxides or elements coated and immobilized by chemical fixation in the glass matrix, a process for producing said composition and a use of the new composition.

The disposal of large quantities of highly radioactive waste produced in the reprocessing of spent power reactor fuel is a problem of considerable importance for the use of nuclear energy. It is generally accepted that the most promising technique is to transform this radioactive waste into a dry solid form which will make this waste chemically, thermally and radiolytically stable. This problem of stability of a dry solid form is closely related to the security of human life on earth for a period of more than 20,000 years. For example, radioactive waste contains the isotopes Sr90, Pu 240 and Cs137, the half-lives of which are 29 years, 66,000 years and 30 years, respectively. These isotopes alone pose a significant threat to life and must be put into a dry solid form that will be stable for thousands of years. The solid form of radioactive waste should be able to keep the radioactive isotopes immobilized during this time, preferably even in the presence of a water environment.

A method for fixing radioactive material in a dry solid form having a high resistance to leaching and other forms of chemical attack would be usable not only for the disposal of radioactive nuclear residues, but also for the manufacture of useful radioactive sources in industry, medicine and the laboratory.

There is currently no practical, tamper-proof means for the safe disposal, storage and immobilization of harmful radioactive waste. Current storage containers do not provide sufficient isolation and immobilization of radioactive material, sufficient long-term resistance to chemical attack by the environment and sufficient stability at elevated temperatures.

A current technique is that known as dry solids which involves fixing the waste in glasses by glass melting techniques. This technique provides some improvement in isolation and a decrease in the rate of release of radioactive elements when the outer envelopes or containers are destroyed. In addition, glasses remain relatively more stable at high temperatures than plastics and are generally more chemically durable in saline solutions than are metals. Glasses with high chemical durability and low conductivities of alkali metal ions are melted at very high temperatures, for example 1800 ° C and above. These high temperature melting processes are economically bad and, moreover, are dangerous due to the volatilization of pernicious radioactive materials. Because of the overall difficulty in handling radioactive materials and especially because of the danger of volatilization of radioactive material in the atmosphere, attempts have been made to use glasses having relatively low melting temperatures, i.e. say to use glasses with an SiO2 content as low as 27% by weight. Although the problem of volatilization of radioactive materials has thus been reduced, it has not been completely controlled. In addition, the resulting glass composition exhibits greatly reduced chemical durability and increased ion diffusion rates for the radioactive materials present therein. The greater this diffusion rate, the poorer the ability of the glass to keep the radioactive materials immobilized in its matrix. For the long-term imprisonment of radioactive waste, required by current standards, these glass compositions were insufficient. In addition, the gaseous radioactive materials are not trapped at all or are only trapped in very small amounts by the above methods.

As will be seen below from the various aspects of the holders' contributions to the art, new methods are provided for obtaining new compositions and articles for the imprisonment of pernicious and dangerous radioactive material for extraordinarily long periods of time. . Unlike the glass fusion imprisonment techniques, the methods according to the invention do not need to include any steps which would expose the radioactive material to temperatures above about 900 ° C, thereby eliminating the danger for the environment resulting from the volatilization of radioactive material in the atmosphere. In addition, new methods are planned for the fixation and immobilization of radioactive gaseous waste such as Kr, I, Xe, Ra, etc., in a glass matrix.

The invention, that is to say the composition of non-porous glass containing a radioactive material in the form of oxides or of elements coated and immobilized by chemical fixation in the glass matrix, is characterized in claim 1 above.

From a practical point of view, the upper limit of the amount of radioactive material contained in the new composition of non-porous glass will be determined, to a certain extent, by factors such as: the concentration of SiO2 in the glass, the concentration and the type of other ingredients that may be present in the glass such as B203, A1203, Ti02, zirconia, alkali metal oxides and G02, the concentration and type of the radioactive material embedded in the glass, the volume fraction of the porous structure in the porous glass preform, the various techniques used to coat the active material in the glass and other factors.

Radioactive materials that can be coated and immobilized in the glass matrix include natural radioactive elements (isotopes existing in nature and artificial isotopes and existing in the form of liquids, solids or gases), generally in the form of l oxide, such as carbon, rubidium, strontium, lanthanides, cobalt, cadmium, silver, zirconium, molybdenum, technetium, niobium, ruthenium, rhodium, palladium, tellurium, cesium, barium, francium, yttrium, radium, actinides and krypton gases,

iodine, bromine, xenon and radon. Are especially usable in the implementation of the invention radioactive waste from nuclear reactors or other processes producing waste.

A special aspect of the invention relates to new glass articles having improved confining properties and characterized by an outer shell whose composition comprises at least 90 mol% of silica, preferably more than 95 mol%, and whose core interior contains radioactive material. The high silica content of the envelope gives the articles considerably greater chemical durability. The inner core has a lower silica content of the order indicated below, namely at least 75 mol% of silica, preferably 82 mol% and in particular 89 mol%.

The invention also relates to a new process for preparing the new compositions described above.

Radioactive isotopes, as noted above, exist as gases, liquids, or solids. Liquid isotopes are usually combined to give salts or other compounds which, as in the case of solid isotopes, are dissolved in a solution. In the following, such solutions will be treated separately from gas isotopes. In the case of solutions, two modes of implementing the invention are described.

The published technical documentation sufficiently describes the preparation of porous glass preforms. The preforms usable in the new processes generally contain SiO2 as a major constituent. In the preparation of various embodiments of the invention, the concentration of SiO2 is advantageously at least 75 mol%, preferably at least 82 mol% and in particular at least 89 mol% of SiO2 (and preferably less than 1 mol% of alkali metal oxide). Such glasses are described in the published technical documentation (see EUA patents Nos. 2106744, 2272342 and 2326059). According to the first embodiment, the dopant consisting of radioactive isotopes in the form of salts, oxides or other compounds is diffused in the pores of the preform; the

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preform is then removed from the dopant solution and is dried. The sintering of porous molecularly packed glasses allows the decomposition of the salts into oxides and their incorporation into the glass matrix. The sintering is carried out at temperatures generally much lower than the melting temperatures of the glass, and the risks of evaporation of radioactive material in the atmosphere are thus reduced to a minimum. The resulting non-porous glass thus produced has a finite concentration of dopant everywhere. According to the second embodiment, the dopant solution containing the radioactive compounds is diffused in the pores of the preform; the preform is removed from the dopant solution and immersed in one or more solvent solutions such as water, methanol, ethanol, acetone and nitric acid; the preform is then removed from the solvent solution (s) after a substantially solvent-free region of sufficient thickness has formed on the surface of the preform, and is dried. Following sintering, as in the method of the first embodiment, to form a non-porous glass with an inner core region containing the radioactive dopant and an outer shell region with a silica content of at least 90 mol% , preferably greater than 95 mol%, and containing substantially no radioactive material.

Two methods of implementing the invention are also described below for trapping radioactive gases in the glass matrix. According to the first embodiment, the porous glass preform is dried and then exposed to radioactive gas (gaseous waste) at a pressure determined by the desired concentration of the gas trapped in the glass; the gas is allowed to dissolve in the glass matrix and the pores are sintered at elevated temperatures in the presence of the radioactive gas. According to the second mode of implementation, the porous glass preform is first immersed in a solution containing one or more doping compounds, advantageously in the form of their salts, before drying. These doping compounds can be chosen so as to increase the surface area, which, in turn, increases the solubility of the radioactive gas in the glass, or so as to lower the glass transition temperature of the glass. The concentration of the doping compounds in the glass can be varied, if desired. After precipitation of these doping compounds, the preform is dried and, if necessary, further heated to decompose the doping compounds generally to their oxide form. The preform is then exposed to the radioactive gas waste at a pressure determined by the desired concentration of the gas trapped in the glass, the gas is allowed to dissolve in the glass matrix and the pores are sintered at high temperatures in the presence of the radioactive gas waste .

The variation of the concentration of the dopant in the glass preform before drying and the incorporation of the radioactive gaseous waste can be advantageous for several reasons. If the dopant is another radioactive waste, it will be desirable to reduce its concentration as much as possible near the surface of the glass preform, so as to further reduce the risk of leakage into the environment. Secondly, if the dopant used increases the solubility of the gas in the porous structure of the preform, it will also be advantageous to reduce its concentration as much as possible near the surface of the preform, since this will lead to a reduced concentration of the dissolved radioactive gas. close to the glass surface and thus reduce the risk of environmental leakage. Thirdly, if the dopant lowers the glass transition temperature of the glass matrix and if its concentration is varied so as to have a maximum inside the preform and a minimum in a layer adjacent to the surface, will obtain the corresponding differences in the concentration of dissolved radioactive gases.

In prior EUA patent No. 3,938,974 relating to fiber optics, the applicants limited the range of dopants used because high optical transmission was necessary and the dopants which caused light attenuation by glass could not not be used. However, in the fixation of radioactive substances envisaged here, there is no need to apply such criteria.

A dopant is a compound that is deposited in porous glass. This compound or, in the case of gas, an element or a molecule, can be incorporated into the glass during the slump as it has been deposited, or after transformation into the appropriate oxide. In the case of metals, it is often introduced in the form of a nitrate in solution, or precipitated in the form of a nitrate crystal. Upon heating, it decomposes to give an oxide crystal which, upon collapse, is incorporated into the silica structure.

Examples of dopants include, of course, the radioactive material contemplated and described, alkali metals, alkaline earth metals, boron, germanium, aluminum, titanium, lead, bismuth, phosphorus and rare earths , in the form of nitrates, carbonates, acetates, phosphates, oxides, borates, arsenates and silicates, in hydrated, non-hydrated forms or their mixtures.

The manufacture of the porous glass matrix can be carried out according to any of the available methods used by a person skilled in the art to form a porous glass of any desired conformation, preferably cylindrical or rectangular, of a composition comprising more than 75% silica. It is preferred to form the porous glass matrices in the form of cylinders, in accordance with the methods described in EUA patent application No. 635727 filed on November 26, 1975. Briefly, a glass containing silica, boron trioxide and two alkali metal oxides (such as NazO and K20) and stretched into long rods. By appropriate heat treatment, these rods are subjected to a phase separation giving two phases: a phase rich in silica also containing B203 and alkali metal oxide, and a phase poor in silica containing a greater amount of B203 and d alkali metal oxide. The heat-treated rods are then immersed in an appropriate leaching solution so as to dissolve and remove the phase containing the lowest concentration of silica. Elimination of this phase and subsequent washing gives a porous glass preform characterized by an SiO 2 content greater than 90 mol% which is ready for use as a matrix for coating the radioactive material. Several procedures are followed here, depending on whether the radioactive materials to be coated are in the form of solid materials and liquids or gases.

A. Solids and liquids

In the case of solid or liquid radioactive materials comprising nuclear residues, the materials are generally available in the form of an aqueous mixture of salts or other compounds of the various materials to be coated. The high level residues generally consist mainly of the aqueous rafEnat resulting from the first cycle of the well known Purex solvent extraction process as well as various other waste streams. There may be significant variations in the composition of the waste. A typical composition is shown in Table I. Solvent is added to the waste until all of the constituents are substantially in solution. Some treatment facilities also extract individual components from waste mixtures such as Cs and Sr for a particular use.

(Table at the top of the next column)

Rare earths and lanthanides are used without distinction to designate the series of elements comprising the following: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadoli-nium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

In non-waste applications, the radioactive elements may be present singly or in groups, or combined in salts or other compounds. In any case, they must be dissolved in solution or melted.

The porous glass supports (in the convenient form of sticks) are immersed in the dopant solutions containing the radioactive materials for a period long enough to allow

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Table I

Products

of

Concentration

Replacements in the fission solution

(atomic%)

stuffing

Rb

1.0

Rb (N03)

Sr

5.2

Sr (N03) 2

Land

rare *

17.8

Mixture of rare earth nitrates

Zr

20.3

Zr (N03) 4 • 5H20

Mo

18.1

You

2.1

Fe (N03) 2 • 6H20

Ru

11.4

Rh

1.0

Co (N03) 3 • 6H20

Pd

6.2

Ni (N03) 2 • 6H20

You

2.3

Cs

5.2

Cs (N03)

Ba

5.2

Ba (N03) 2

Actinides

4.2

Mixture of rare earth nitrates for the diffusion of the doping material solution (stuffing solution) in the pores. The length of this period depends on the temperature of the stuffing solution, the porous structure and the dimensions of the porous glass host.

Various methods can be used once the stuffing solution has filled the pores with waste or other dopant. According to a method, the porous glass preform is removed from the stuffing solution; then, the dopant is precipitated in the preform in various ways, such as by evaporating the solvent from the solution, lowering the temperature of the solution, by chemical means, etc. If desired, the preform can be washed briefly to clean its surface area. Optionally, the preform can then be immersed in a washing solution such as a solvent having a low solubility for the dopant, for several hours of soaking. Then, the preform is dried, for example in a desiccator, a vacuum chamber, etc., preferably at relatively low temperatures. This process, when supplemented with the drying and sintering described below, gives non-porous glass compositions having a substantially homogeneous concentration of the waste.

According to a second method, the porous glass preform is removed from the stuffing solution; the precipitation of the dopant is allowed to begin; then the preform is immersed for several hours in a solvent having an intermediate solubility for doping materials. This latter soaking allows the formation of a thick skin or envelope on the outer surface of the preform because the dopant is redissolved in the solvent. The soaking in several solvents of decreasing solubility for dopants allows the elimination of the dopant from this region of skin. Once this is done, the preform is dried as illustrated above. This process, once completed by the drying and sintering described below, gives non-porous glass compositions comprising a skin of a dopant concentration substantially lower than the external surface of the preform. As it contains a lower concentration of dopant, this skin acts to protect the interior of the glass against chemical attack and it also has the action of further delaying the diffusion of radioactive materials from the interior of the glass to the surface. where they can react with the environment.

As examples of drying and sintering techniques, the preform (prepared by either of the two methods illustrated above) can be kept under vacuum at low temperatures for about a day, and then heated to a speed of about 15 ° C / h while it is still under vacuum, at a temperature above the decomposition temperature of the dopant and about 100 ° C below the glass transition temperature of the matrix porous glass free of dopant. At this time, any desired redox conditions for the dopant can be obtained in various ways: by heating, still at the sintering temperature, the vacuum preform for a reduced valence state of the dopant; or by heating, at the sintering temperature, the preform under an atmosphere containing hydrogen for a very reduced valence state of the dopant; or by heating, at the sintering temperature, the preform under an atmosphere containing oxygen at an appropriate pressure to obtain a partially or fully oxidized dopant.

This possibility of modifying the oxidation-reduction states of radioactive materials allows fine adjustment of the glass's resistance to chemical attack, since many radioactive materials, especially actinides, are less soluble in water when they are in a reduced state. The possibility of modifying the oxidation-reduction state of these hazardous materials provides additional security against premature dispersion of these materials in the environment.

After sintering at elevated temperatures, the products are cooled to room temperature so that they form a non-porous glass that coats and completely immobilizes the radioactive material in its vitreous structure.

According to another aspect, the invention relates to the storage and immobilization of aqueous solutions of boric acid contaminated with radioactive agents as a result of their use as heat transfer media for nuclear pressurized water reactors. These contaminated solutions contain various radioactive impurities including Co, Cs, Mn, Cr, Sr and Zn as well as inert impurities like Fe. Currently, we get rid of such contaminated solutions by fixing in cement forms and then burial in the ground, a few tens of centimeters. This technique is very unsatisfactory, since the cement has a low load factor, boric acid at room temperature having a solubility of approximately 5%. In addition, the cement is not specially insoluble and it is leached into the water table. Thus, even if it involves disposing of low-level nuclear fuel recycling waste, the harmful effect on the environment and on the population is obvious.

The new methods described here allow the filling of the porous glass preform with such boric acid solutions contaminated with radioactive materials at a low level. The packing operations can be carried out at temperatures which will give load factors several times higher than those which can be obtained in the technique of fixing in cement. For example, the solubility of boric acid in water at 100 ° C is about 30% by weight, which allows load factors about six times greater than those obtained at room temperature using the technique of fixing in cement.

The new non-porous glass compositions of this latter embodiment of the invention are characterized by the coating and immobilization of waste consisting of boric acid contaminated with radioactive materials at a low level (coming from nuclear reactors with pressurized water), this waste being fixed in the glass matrix in the form of boron oxides, the radioactive impurities comprising cobalt and chromium. Other impurities such as strontium, cesium, manganese and / or zinc may also be present in the form of oxides. These compositions generally contain less than 1% by weight of radioactive impurities (in the form of their oxide). However, in the event of serious leakage problems in the pressurized water nuclear reactor, the boric acid waste can contain much higher proportions of impurities.

In Examples 1 and 2, the doping solution contains only cesium nitrate in order to simply show the behavior of the dopant and porous glass system. Cesium was chosen because of its high concentration in waste and because, because it is an alkali metal, its coefficient of diffusion through glass is higher than that of the other constituents, except rubidium (a other alkali metal), which is, however, present at low concentrations5

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tions. All the compositions are indicated on a molar basis. Cesium poses an additional problem because of its high vapor pressure at the high temperatures normally necessary for the melting of glasses, a complication which is avoided thanks to the new methods envisaged here. Thus, of all the radioactive isotopes in reactor waste, Cs probably poses the most problems when trapped in glass and presents the greatest short-term dangers. As a result, a process that immobilizes cesium in a glass matrix will also solve the storage problems associated with other radioactive isotopes. So a process that will immobilize cesium will also immobilize other radioactive isotopes.

In Examples 3 and 4 below, the molecular stuffing method of Example 1 is used. In Example 3, the stuffing solution contains SrN03 which, like CsN03, is present in harmful waste streams. The solution of Example 4 stimulates possible waste at low levels in boric acid. The solutions of Examples 5 and 6 resemble current non-separated waste, Example 6 simulating the major constituents of typical radioactive waste produced in fuel reprocessing elements from reactors of an electricity company.

Examples 1 and 2:

The porous glass preform is obtained by melting a glass having the composition: 60.2 mol% of Si02.32.8 mol% of B203, 3.6 mol% of Na20 and 3.4 mol% of K20, in a platinum crucible near 1400 ° C; the homogenization of the melt is carried out with a platinum agitator, then the drawing of the melt into sticks using a glass primer, the cutting into cylinders or sticks of 7 mm in diameter x 30.5 cm in length, heat treatment of these sticks at 550 ° C for 1.5 to 2 h, annealing of the glass, washing in an aqueous solution containing 2.5% HF for 60 s, leaching in 3N HCl and an aqueous solution 15-20% NH4C1, then wash with water. The resulting porous glass preform is characterized by a porous structure with open cells and has the following composition: 95 mol% of Si02.5 mol% of B203 and less than 0.1 mol% of alkali metal oxide.

A porous glass preform in the form of a rod thus prepared is then placed in a so-called stuffing solution and containing the waste which, in this case, is an aqueous solution of CsN03 (containing 65% by weight of CsN03) for at least 3 h , while the solution is kept at 105 ° C. In Example 1, the CsN03 stuffing solution is then replaced by a mixture of cold water and methanol for a few minutes, until precipitation occurs. of CsN03 occurs in the porous structure, then the stick is soaked in methanol for 3 h. In Example 2, the stuffing solution is replaced with cold water and methanol for less than 2 min, and then precipitation of CsN03 occurs. Then, the glass stick is soaked in water for 3 h at 2 ° C, then soaked in methanol for at least another 3 h at 0 ° C. These two soaks produce a thin skin or envelope of the order of 1 mm around the cylindrical surface of the stick. The skin is devoid of CsN03.

In the two examples 1 and 2, after having maintained the glass rods for 24 h at 0 ° C. under vacuum (of the order of 0.1 mm Hg), the rods are dried under vacuum at increasing temperatures ranging from 0 ° C to 625 ° C to remove the solvent, water and methanol in order to decompose the cesium nitrate dopant into its oxide. A heating rate of 15 ° C / h is used. Additional heating to 840 ° C, which is the sintering temperature of the porous glass rod, is carried out under an oxygen atmosphere. When the porous structure collapses, consolidation is considered complete and the sintered non-porous stick is removed from the oven and brought to room temperature.

Then, the two non-porous rods obtained in Examples 1 and 2 are subjected to determinations of the cesium content and of the associated refractive index profile. The composition of the central part of the sticks of the two examples 1 and 2 is 92.1 mol% of

Si02.4.9 mol% of B203 and approximately 3 mol% of Cs20 (which corresponds to a refractive index of 1.487) with an amount of the order of trace of alkali metal oxide. In Example 1, this composition remains substantially the same up to a very thin layer (or skin) significantly less than 1 mm at the surface. Analysis of the non-porous rod of Example 2 indicates a refractive index of 1.458 in a surface layer 1 mm thick around the rod. This value of the refractive index indicates that there is no measurable amount of Cs20 dopant in this layer.

The single figure is a graphic representation of the fraction of waste released into the environment after 20,000 years of storage for a sintered glass product of Example 2 (curve a) compared to waste preserved by the melting technique of glass from Battelle Pacific Northwest Laboratory, Richland, Washington, USA (curve b).

To allow estimates of the diffusivity of cesium ions in order to allow estimates of the loss of radioactive waste as a function of time, electrical conductivity measurements are carried out on the two non-porous glass products obtained as indicated in Examples 1 and 2 above, and on a glass product (fusion process) from Battelle Pacific Northwest Laboratory. The results obtained are shown in Table II.

Table II

Conductivity at two temperatures

Product

Temp.

Conductivity

Diffusivity1

glass

(° C)

(ohm — m) -1

(cm2 / s)

Example 1

100

8 x 10 ~ 18

1 x 10-24

Example 22

100

9 x 10 ~ 16

1.4 x 10 "22

Battelle3

100

3.5 x 10-13

5.3 x 10 ~ 20

Example 1

400

1 x 10-9

3 xlO-16

Example 22

400

3 x 10 ~ 8

1 xlO-14

Battelle3

400

7 x 10 ~ 7

2 x 10 "13

1 Values calculated using the Stokes-Einstein relation; they are therefore only estimated values.

2 The results reported for Example 2 relate to the surface layer only. In the central region, the values are the same as those indicated for example 4.

3 Battelle Pacific Northwest Laboratory, Richland, Washington. This glass product contains less than half the proportion of cesium coated as a dopant in the glass products of Examples 1 and 2.

In Table II, the diffusivity is estimated from the conductivity using the Stokes-Einstein relationship. These results show that the non-porous glass product of Examples 1 and 2, when compared to the glass product (obtained by the melting process from Battelle Pacific Northwest Laboratory) at room temperature, has diffusivities which are lower than several orders of magnitude, despite the fact that the first products contain a proportion of dopant more than double that of the last product. According to the results, the theoretical weight loss after 20,000 years of exposure to a perfect trap (matter that does not allow the accumulation of molecules or atoms scattered at the border, which is the most pessimistic estimate) is shown in the single figure as a function of the storage temperature for a non-porous stick product obtained according to the process described in Example 2.

Example 3:

A porous glass preform (in the form of a stick) prepared as described in Example 1 is immersed in a solution containing 95 g of Sr (N03) 2 per 100 cm3 of water at 60 ° C. The stick is left to soak in this solution for 4 h, then remove the stick from the solution and place it in isopropyl alcohol s

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cold. A precipitation of SrN03 occurs slowly in the porous structure, requiring 2 to 3 hours to be complete. The stick is then dipped in a solution consisting of 66% ethanol and 34% water at 0 ° C for 5 h to remove the dopant SrN03 from an envelope region on the surface of the stick. Finally, the stick is immersed in a cold mixture of 90% ethanol and 10%

of water, then in a cold isopropanol bath. Then, the stick is dried and collapsed (sintered) as in Examples 1 and 2, starting with a step under vacuum for 24 h at 0 ° C. When removed from the oven after sintering, the stick contains strontium incorporated into the structure of the glass in its central region; the envelope region is substantially free of strontium.

Example 4:

A porous glass preform in the form of a stick prepared in Example 1 is soaked for 3 h in a solution at 100 ° C. containing 25 g of H3B03.0.27 g of Fe (N03) 3'9H20,0, 27 g of Cs (N03), 0.27 g of Sr (N03) 2.0.27 g of Co (N03) 26H20.0.27 g of Cr (N03) 3'9H20, 80 cm3 of water and 1 cm3 of HN03 (molar concentration 13.4). The stick is removed from the solution and then immersed in water at 0 ° C for 30 min and dried in vacuo at 0 ° C for 24 h. The stick is then heated at a speed of 15 ° C / h to 625 ° C under vacuum to remove the remaining solvent and to decompose the nitrates of metal dopants, then it is heated at a speed of 50 ° C / h to 850 ° C vacuum to collapse the pores and consolidate the glass. The final product is a non-porous glass containing Si02, B203 and the oxides of Cs, Sr, Fe, Co and Cr as an integral part of the matrix. The water immersion step after the soaking (stuffing) step decreases the concentration of the dopant near the surface of the stick. The stick can also be soaked in methyl acetate or acetone after removal of part of the stuffing solution, so as to form an envelope region on the surface of the stick with lower concentrations of the doping. Rods prepared in the latter manner, however, have a tendency to split longitudinally during the drying and sagging steps. This longitudinal founding of the rods is not, however, harmful for the fixing and immobilization of the radioactive material in the rods.

Example 5:

A solution is prepared comprising:

12.9gdeSr (N03) 2 18gdeRb (N03)

15.9 gdeBa (N03) 2 104.1 g Zr (N03) 4'5H20 153.6 g Fe (N03) 3'9H20 4.2 g Co (N03) 2 6H20

21.3 g of Ni (N03) 2 6H20 12.3gdeCs (No3)

89.4 g of mixture of rare earth nitrates and enough water to cause the dissolution of metal dopants at around 100 ° C. A porous glass preform (in the form of a stick) prepared as described in Example 1 is soaked in the solution (for stuffing) above for 3 h. The stick is then removed from the solution and placed in a 3N solution of HNO 3 at 0 ° C for 10 min. The stick is then soaked in acetone for 2 h at 0 ° C and then placed in a drying chamber maintained at 0 ° C with suction pumping means for a period of 24 h (to remove the solvent from the rod). Then, the stick is heated under vacuum to 650 ° C at a rate of 15 ° C / h. The stick is kept at 625 ° C for 24 h under vacuum, then slowly heated to 840 ° C, also under vacuum, so that a greatly reduced state is obtained for the waste dopants contained in the porous structure of the preform. The rod is sintered in the vicinity of 840 ° C. and forms a solid non-porous glass product with the waste as an integral part of its structure.

Example 6:

In this example, a solution containing 10 major constituents of a typical composition of radioactive waste often called PW8a is used. The relative concentrations of these constituents are representative of the composition of the waste stream with a reconcentration factor of three. The solution contains the following materials:

148.8 g Fe (N03) 3'9H20 21.3 g U02 (N03) 26H20 33.6 g NaN03

18.6 g of Zr (N03) 4-5H20 19.8 g of Nd (N03) 3-5H20 9.0 g of Ce (N03) 3'6H20 4.2 g of La (N03) 3'6H20 4, 2 g CsN03 2.4 g Sr (N03) 2 3.0 g Ba (N03) 2

264.9 g HN03 150mlH20

Sufficient amounts of water and nitric acid are added as indicated above to maintain the above metal nitrates in solution. The preparation of the solution is carried out in a staggered manner with varying temperature levels; the final temperature being 99 ° C.

Six porous sticks prepared as in Examples 1 and 2 are immersed in the above solution maintained at 99 ° C for 3 h, rinsed in water at 20 ° C for 5 s and then placed in a desiccator kept at 20 ° C for 16 h (hereinafter uncoated sticks). Three uncoated sticks are removed from the desiccator and subjected to the following immersion treatments to produce an envelope region on the surface of the stick having a lower concentration of dopants (waste) than the central inner portion of the stick (hereinafter sticks coated): 30 min in isopropanol containing 8% by weight of U02 (N03) 2 6H20.4% by weight of Ce (N03) 2'6H20 and 2% by weight of La (N03) 3 5H20, and 30 additional min in another solution of 80% chloroform and 20% isopropanol. These three coated rods are then dried for 24 h at 0 ° C under less than 1 mm of Hg; then place them in an oven with the 3 remaining uncoated sticks at room temperature and heat them to 625 ° C at a speed of 15 ° C / h under vacuum to remove the solvents and to decompose the nitrates dopants of metals into metal oxides. The sticks are grouped in three pairs, one coated and one uncoated. Each pair is then heated to 50 ° C / h in different environments, namely a, b or c below:

a) 95% of N2 and 5% of H2 under 1 atm (strongly reducing conditions),

b) under vacuum, that is to say less than 1 mm of Hg (moderately reducing conditions); and c) 1 atm of air (oxidizing conditions).

At about 860 ° C, sintering and sagging of the porous structure of the sticks occurs.

Analysis of the 3 resulting pairs of non-porous glass rods indicates that the doping metal oxides are at varying degrees of oxidation. It is quite obvious that, by implementing the invention, it is possible to obtain new non-porous glass products, chemically inert, containing radioactive materials (at their lowest degree of oxidation), for example actinides, radioactive material coated and immobilized for extraordinarily long periods, for example centuries.

B. Gas

Various embodiments of the invention, as indicated below, relate to new methods for the encapsulation and immobilization of radioactive gases in glass.

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Such a new process is illustrated by Examples 7 and 8 in which the porous glass rod, as a host, is partially dried at 0 ° C under vacuum for approximately 24 h, then further dried under vacuum by progressive heating to a drying temperature higher which is 50 to 150 ° C lower than the glass transition temperature of the undoped consolidated glass used as a host. This drying temperature is approximately 625 ° C. for the porous glass stick used. At 625 ° C, one or more radioactive gases are introduced into the closed chamber containing the stick. The temperature is maintained at 625 ° C for approximately 24 h, then the stick is heated at approximately 50 ° C / h to 835-900 ° C, where sintering occurs under an atmosphere of radioactive gases, thus trapping the gases dissolved in the glass matrix. If desired, the heating rates and holding temperatures can be easily changed. Rapid heating can, in some cases, break the stick. This rupture, however, has no effect on the fixation and immobilization of the radioactive gas in the host glass.

According to a second new method, the porous glass rod is immersed in a solution containing one or more dopants. If desired, these dopants can be chosen to increase the solubility of the radioactive gaseous waste in the rod. After diffusion of the dopants in the pores of the glass rod, the rod is removed from the doping solution and it is immersed in a solvent free of dopant. If desired, the concentration of dopants in the porous structure can be varied in various ways. First of all, in order not to have a variation in the concentration of dopants in the glass rod, the solvent bath will be chosen so as to cause precipitation of the dopants uniformly in the pores of the rod. Then remove the solvent and dry the stick. Secondly (see example 9), if a layer free of dopant forming the surface of the rod is desired, the dopant is precipitated as in the first case, but then the rod is transferred to another solvent or to other solvents, so that the dopant in the surface layer is redissolved and can diffuse out of the stick, before drying. Thirdly, if a staggered variation of the dopant in the structure of the rod is desired, the rod is first dipped in a solvent which does not precipitate the dopant and allows its diffusion out of the porous structure. It is then transferred to another solvent or other solvents which cause precipitation, and then the stick is dried.

If desired, the drying step can start at 0 ° C under vacuum. The rods are then heated slowly, that is to say at a rate of 15 ° C / h, at 625 ° C under vacuum to remove the remaining solvent and allow the decomposition of the dopant. At 625 ° C, one or more radioactive gases are introduced into the heating chamber containing the porous sticks and the temperature is maintained at 625 ° C for approximately 24 h. The rods are then heated at a rate of 50 ° C / h to 835-900 ° C, where sintering occurs, thereby incorporating the dissolved gas in the glass matrix.

According to a third new process, it is possible to produce a radial variation in the concentration of radioactive gases dissolved in the structure of the glass. Here, the porous glass rod is doped with a material that will lower its glass transition temperature like potassium in the form of potassium nitrate. After complete diffusion of the dopant in the pores of the rod, the dopant is precipitated in the pores either thermally by cooling the rod, or by introducing a solvent having a very low solubility for the dopant. The rod is then immersed in a solvent having a solubility of 5 to 10% for the dopant, so as to remove the dopant from a layer adjacent to the surface of the rod. After the solvent has dried, the rod will have a high concentration of dopant inside and a low concentration in its surface layer. As a result, the stick will have a higher sintering temperature for its surface layer and a lower sintering temperature for its interior. The sample is dried under vacuum at 0 ° C for approximately 24 h. It is then heated to 600 ° C under vacuum at 15 ° C / h. At 600 ° C, one or more radioactive gases are introduced into the chamber containing the stick. The temperature is then slowly raised until the inner region of the porous glass rod is sintered, while remaining below the temperature at which the outer region of the rod sintered. The radioactive gases are then removed from the environment of the rod and the temperature is raised to sinter the surface layer. By this method, the interior region of the rod will contain dissolved radioactive gases, while the surface layer will be substantially free of dissolved radioactive gases.

Example 7:

A porous glass preform prepared as described in Example 1 is immersed in methanol for 3 h at room temperature. It is then transferred to a drying chamber maintained at 0 ° C for 24 h with a suction pumping means operating so as to dry the stick. The stick is heated to 650 ° C at a rate of 50 ° C / h to complete drying. At 650 ° C., gaseous Kr is introduced into the chamber containing a stick at 1 atm. The stick is then kept at 650 ° C for 18 h in a Kr environment and heated to 880 ° C so that a sintering in the Kr environment is obtained, thus trapping the gaseous Kr dissolved in the glass matrix.

Example 8:

The procedure of Example 7 is repeated, except that instead of krypton, iodine gas is introduced into the chamber at 650 ° C. by boiling iodine pellets so as to obtain an atmosphere of 'iodine at a pressure of 0.1 bar. The stick is kept for 18 h at 650 ° C. in this iodine environment and it is heated to 880 ° C. to obtain a sintering of the pores of the stick, thus trapping the iodine gas dissolved in the glass matrix, as shown by a pink color of the product. resulting non-porous glass stick.

Example 9:

This example illustrates the incorporation of both a dissolved solid dopant and a gaseous dopant in the glass matrix.

A porous glass preform prepared as described in Example 1 is immersed in a 67% by weight solution of CsN03 at 105 ° C for 4 h. The rod is then transferred and immersed in pure water at 0 ° C to cause precipitation of cesium nitrate in the pores of the rod. Total precipitation takes 30 s. The stick is left in water at 0 ° C for 3 h to cause a change in the concentration of cesium nitrate in the pores, giving a cylindrical layer free of cesium nitrate adjacent to the surface. The stick is then transferred and immersed in methanol at 0 ° C and is kept there for 3 h to further reduce the concentration of cesium nitrate on the surface. Drying is started immediately afterwards by placing the stick in a vacuum chamber at 0 ° C for 24 h. Then, the stick is heated to 625 ° C at a rate of 15 ° C / h to complete drying and to cause the decomposition of cesium nitrate to cesium oxide. The stick is kept under vacuum at 625 ° C for 24 h. Gaseous krypton is then introduced into the chamber until a pressure of 1 atm is reached. The rod is kept at 625 ° C. for an additional 24 h in this environment of Kr, then heating to 50 ° C./h at the sintering temperature of the rod so as to encapsulate and thus immobilize the dissolved Kr at the same time as Cs in the glass matrix.

As a result of the improved coating of radioactive materials, especially radioactive nuclear waste according to the invention, these radioactive materials can find an advantageous benefit for humanity instead of constituting an irrepressible threat to life on earth. Some examples of useful applications will be discussed below.

After sintering the porous glass, the radioactive elements are completely embedded in the glass matrix. These radioactive elements are chemically bound and immobilized in the new glass item and in such a form represent a safe confinement,

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of an extraordinarily long duration, pernicious and dangerous streams of radioactive nuclear waste. New glassware can be transported with little risk of the radioactive isotopes leaking into the environment. They can be placed in underground shelters for long-term storage. These shelters 5 should preferably be more than 300 m below the ground surface.

Certain radioactive isotopes, including isotopes which do not come from radioactive waste, can be used for advantageous applications as soon as for storage. These applications include sterilizing food items and drugs by destroying bacteria, viruses and other microorganisms; use in medical and industrial instruments, in teaching devices and in research. For sterilization, glassware containing selected isotopes including radioactive cesium, strontium and cobalt can be used for food preservation and for the recovery of nutrients from sewage sludge for inclusion in the food cycle. These applications are not widely used commercially today because suitable containers must be found to prevent contamination of food items with radioactive isotopes and to prevent the associated high probability of radiation poisoning that would result from their ingestion. The present invention will allow the use of radioactive materials which are produced in large quantities in nuclear reactors by safely coating them, as described herein, to sterilize food products.

The safe coating of radioactive waste in glass articles according to the present invention also allows their use in medical, industrial, teaching and research instruments where radiation sources are necessary for ionization, sterilization, irradiation of patients, control. of quality and various materials for the detection of defects in materials by gamma irradiation.

1 drawing sheet

Claims (3)

633,499 1. Composition of non-porous glass containing a radioactive material in the form of oxides or of elements coated and immobilized by chemical fixation in the glass matrix, this composition being characterized by a) at least 75 mol% of SiO2, b) a radiation activity greater than 1 mCi / cm3 of the composition, c) high chemical durability in aqueous solution, and d) chemical fixation of said materials in the form of oxides or elements in the siliceous structure, thus making the composition extremely stable to chemical attack. 2. Composition according to claim 1, characterized in that the SiO2 content is at least 82 mol%, preferably at least 89mol%. 3. Composition according to claim 1, characterized in that a proportion of at least 0.1 mol% of radioactive material is coated and immobilized in the glass matrix, said composition having an SiO2 content of at least 82 mol %. 4. Composition according to claim 3, characterized in that at least 5 radioactive compounds are coated and immobilized in the glass matrix. 5. Composition according to claim 4, characterized in that the radioactive compound is an oxide of the following group of elements: carbon, rubidium, strontium, lanthanides, cobalt, cadmium, silver, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium , palladium, tellurium, cesium, barium, francium, yttrium, radium and actinides; or a member of the group consisting of krypton, iodine, bromine, xenon and radon. 6. Composition according to claim 1, characterized in that the radioactive material consists of radioactive nuclear waste, also comprising radioactive gases. 7. Composition according to claim 1, characterized in that said oxides are the oxides of boron, cobalt and chromium fixed and immobilized in the glass matrix, these oxides being derived from waste of weakly radioactive contaminated boric acid. 8. Composition according to claim 7, characterized in that it further comprises the oxides of cesium, manganese and strontium. 9. Composition according to claim 1 in the form of a glass article, further characterized by e) an outer envelope or skin of glass comprising at least 90 mol% of Si02 and substantially free of radioactive material, an inner core being formed by the composition according to claim 1. 10. Composition according to claim 9, characterized in that the envelope or skin composition comprises at least 95 mol% of SiO2, and in that the inner core composition comprises at least 82 mol% of SiO2. 11. Glass composition according to claim 9, characterized in that the radioactive material in the core comprises a cesium oxide. 12. Method for producing the composition according to claim 1, characterized in that a) impregnates the pores of a porous glass preform with a predetermined quantity of radioactive material, this preform being characterized by a porous structure with open cells , b) heats the preform to a sintering temperature to cause its porous structure to collapse, and c) obtains a non-porous glass product which contains an amount of coated and immobilized radioactive material sufficient to provide radiation activity greater than minus 1 mCi / cm3 of glass product. 13. Method according to claim 12, characterized in that the preform contains at least 89 mol% of SiO2. 14. Method according to claim 13, characterized in that the radioactive material comprises at least five radioactive isotopes. 15. The method of claim 13, characterized in that the radioactive material comprises radioactive nuclear waste. 16. Method according to claim 13, characterized in that the preform, after the impregnation step, is treated to remove the radioactive material from its surface so as to produce an outer surface layer which is substantially free of radioactive material . 17. Method according to claim 13, characterized in that a dried porous glass preform is impregnated with a radioactive gas isotope. 18. The method of claim 13, characterized in that the heating step is carried out in a reducing atmosphere of 15 so as to transform the radioactive material into a material of a lower valence state. 19. The method of claim 12, characterized in that the radioactive material is chosen from normally radioactive gases and volatilizable materials which give radioactive gases, in 20 which a) the pores are impregnated with a porous glass preform with a dopant so as to increase its surface area, this preform being characterized by a porous structure with 25 open cells and an SiO2 content of at least 75 mol%, b) the dopant is removed from the surface of the preform so as to produce an outer skin or envelope which is substantially free of the dopant and an inner core region containing significant amounts of dopant, C) the preform is dried, d) the pores of the dried preform are impregnated with the radioactive material mentioned above, e) the preform is heated to a sintering temperature so as to cause the porous structure of the 35 preform and to coat and thus immobilize the radioactive material in the glass matrix. 20. The method of claim 19, characterized in that the dopant is introduced into the pores of the glass in the form of a non-radioactive solution. 21. Method according to claim 19, characterized in that the temperature of the heating step is adjusted so as to first cause the porous structure of the inner core region to collapse while remaining at- below the temperature at which the skin, or the outer envelope, of the preform sintered, the 45 radioactive gases from the environment are removed from the preform, after which the temperature is raised to cause sintering and collapse of the porous structure of the skin or outer shell. 22. The method of claim 12, according to which a) The pores are impregnated with a porous glass preform with 50 a liquid stream of radioactive nuclear waste, this preform being characterized by a porous structure with open cells and an SiO 2 content d '' at least 76 mol%; b) the precipitation of solids including radioactive material is caused from the liquid stream in the pores 55 of the preform, c) the preform is dried, d) the preform is heated to a sintering temperature so as to cause its porous structure to collapse, and e) the non-porous product which contains the coated and immobilized radioactive nuclear waste 60 is cooled. 23. The method of claim 22, characterized in that the product is stored below the surface of the earth. 24. Use of the composition obtained by the process according to claim 65 as a source of radioactivity for the sterilization of sewage sludge by destroying microorganisms therein. 2 3 633499