GB2124015A - Method for improving the properties of solidified radioactive solid wastes - Google Patents

Abstract

In a process for solidifying radioactive solid wastes, the matrix material, which hardens upon the addition of water, is, after hardening, a practically shrinkage-free, heat and acid resistant, X-ray amorphous, hydrate phase free material that contains polymeric sodium aluminium silicates or sodium zinc silicates. The solid wastes are incorporated into a suspension of the unhardened matrix material containing between 10 and 20 volume percent water, and the resulting waste-matrix is then left to harden. Unbound water may be removed after hardening by heating the product and a temperature up to 100 DEG C.

Description

SPECIFICATION Method for improving the properties of solidified radioactive solid wastes The present invention relates to a method for solidifying radioactive solid wastes in a matrix material that hardens upon the addition of water.

Radioactive solid wastes are understood to include, for example, (a) solid chunky wastes, such as, for example, fuel element casings or core components, etc.; (b) slurries of fine grained solid wastes, such as, for example, feed clarification sludges from the reprocessing of irradiated nuclear fuel and/or breeder materials; and (c) alpha-radiator containing ashes and other combustion residues, etc.

In the past, cement-water suspensions have been used in many places for the solidification of such solid, chunky or granulate wastes. The resulting waste cement rock products have the drawback that the comparatively high amount of water therein, due to the influence of the radioactive radiation emanating from the wastes, is radiolytically chemically decomposed, thus releasing large quantities of hydrogen and oxygen. This could result in dtawbacks for the operational safety of the intermediate and final storage of these products. At higher temperatures in a final depository, water is released from the products, resulting in considerable pressure build-up in the product itself and in the container enclosing the product. If the container corrodes, this could possibly lead to the release of radioactivity from the container into the environment.Moreover, it has been found that the cement solidification products produced in the past, when stored over longer periods of time in salt solutions, lose their previously sufficiently good properties, such as pressure resistance, etc.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved process for solidifying radioactive solid wastes, such as, for example, fuel element can sections, other radioactive metallic wastes, alpha-radiator containing ashes and other combustion residues, feed clarification sludges from the reprocessing of irradiated nuclear fuel and/or breeder materials incorporated, in a matrix material which binds with water to thereby provide a solidified block which exhibits improved properties for long term storage.

It is another object of the present invention to provide such a process to produce solidified blocks which exhibit, compared to those produced according to the prior art processes, increased radiation resistance and reduced radiolysis gas formation, increased corrosion resistance, especially with respect to concentrated acids and concentrated acid solutions, for example with respect to a quinary solution (i.e. an aqueous solution of the composition, in weight percent, of: 24.7% MgCI2; 2.3% MgS04; 1.9% NaCI; 3.3% KCI; 67.9% H20) and good thermal shock resistance.

To achieve the foregoing objects and in accordance with its purpose, the present invention provides a process for solidifying radioactive wastes in a matrix material which hardens upon the addition of water, comprising: employing as the matrix material a material which, after hardening, is a practically shrinkage-free, heat and acid resistant, X-ray amorphous, hydrate phase free material containing polymeric sodium aluminium silicates or sodium zinc silicates. A suspension of the unhardened matrix containing between 10 and 20 volume percent water is provided, and the solid wastes are incorporated in the suspension to provide a waste-matrix mixture which is then left to harden.

Preferably, after hardening, the waste matrix is subjected to a heat treatment at a temperature between room temperature and 100"C so as to remove unbound water.

In one embodiment of the invention, the matrix material suspension can be a prefabricated suspension into which the solid wastes are vibrated. In another embodiment of the invention, the solid wastes can be stirred into a suspension of the matrix material with water.

It is to be understood that both the foregoing description and the following detailed description are exemplary and explanatory, but are not restrictive of the invention.

DETAILED DESCRIPTION OF THE INVENTION Matrix material suitable for the process according to the present invention have become known in the field of the manufacture of deep bore holes for oil and natural gas recovery. They are called "hydrothermal cements" and are described by W. A. Mallow and J. K. Dean, in Transactions of the ASME-Journal of Pressure Vessel Technology, Paper No. 77/Pet-27, pages 1-5 (1977). These materials can be obtained commercially from various manufacturers.

Although they are called hydrothermal cements, these materials have nothing in common, with respect to manufacture on the one hand and with respect to composition on the other hand, with cements employed in the construction industry. The hydrothermal cements which can be employed in the present invention essentially comprise SiO2 sand, SiO2 flour, sodium silicates and Al(OH)3 or ZnO. Water adsorption during hardening is at a maximum of 20 volume percent.

The hardened hydrothermal cements comprise amorphous, polymeric silicates which do not contain the water of hydration characteristic of portland cements. The hydrothermal cements are prepared from alkali metal silicates, and polvalent metal oxide (Al(OH)3 or ZnO) and generally are prepared from a mixture of spray-dried sodium silicate hydrate powder and anhydrous sodium silicate powder. The alkali metal silicates are converted to high molecular weight, amorphous silicate moieties during hardening by substitution of the monovalent alkali metals by the polyvalent (Al,Zn) metal. Water provides the vehicle for dissolving and ionizing the silicious and metallic oxides, ensuring their reactivity and subsequent solidification.Generally, the dry hydrothermal cement component is mixed with water to form a suspension containing 10 to 20 volume percent water, and the solid wastes are incorporated into this suspension as by stirring or vibrating and the resulting waste-matrix mixture is then left to harden.

An advantageous feature of the process according to the present invention comprises subjecting the waste-matrix mixture, after hardening, to a heat treatment at a temperature between room temperature and 1 00 C so as to remove unbound water.

The process according to the present invention fully meets the requirements regarding the properties of the waste solidification products, and provides solidified blocks which are manufactured in a simple manner at the least possible expense for such a process. Due to the solidification of different solid wastes a volume ratio is preferred to describe the final product as from 1 to 60 vol.% of waste.

The following examples are given by way of illustration to further explain the principles of the invention. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention in any way.

Example 1: This example provides an examination of the pressure resistance of inactive products made of a hydrothermal cement manufactured by Swindell Rust, USA (The Rust Engineering Company): A typical composition of such a hydrothermal cement, after mixing with water, is, for example, in percent by weight: quartz sand 37.5 quartz powder 18.8 (Silica flour) Na silicate, free of water 7.5 (Anhydrous sodium silicate powder) Na silicate, containing water 7.5 (Sodium silicate hydrated powder) Al(OH)3 15.0 H20 13.7 100.0 a) Products were made at room temperature with tap water by stirring or shaking the dry hydrothermal cement with an amount of tap water sufficient to provide a desired water/cement weight ratio, and then leaving the resulting mixture to harden for 24 hours. The water/cement (solids) ratios employed and the properties of the products were as follows: Pressure Ratio of Porosity Resistance Water/Solids (%) (N/mm2) 0.15 19.3 25.0 0.13 15.1 32.3 0.10 14.7 35.0 b) Products were made with a simulated MAW which is an inactive salt solution whose composition essentially corresponds to that of a medium radioactive waste water. The products were made at room temperature and the mixture of cement and simulated MAW was left to harden for 24 hours. The water to cement ratios employed and the properties of the products were as follows: Pressure Ratio of Porosity Resistance Water/Solids (%) (N/mm2) 0.15 14.2 15.0 0.13 14.2 15.0 0.10 16.1 22.5 0.10* 7.1 32.3 *) compacted by vibrating the not yet hardened- mixture into a container.

The pressure resistance of these products is in the order of magnitude of that of solidified blocks produced according to one of the prior art methods (e.g. products produced using ordinary portland cements).

Example 2: This example provides a comparison of the corrosion resistance of inactive solidifed products produced according to the present invention with inactive solidified products produced according to a prior art process.

a) Hydrothermal cement products, as described in connection with Example 1, (1 a or 1 b) produced with a water/solids ratio in the range from 0.10 to 0.15 and a pressure resistance range of 1 5 to 35 N/mm2, were stored in three different corrosive media under varying conditions so as to test their corrosion resistance.

The storage was effected in: (i) diluted HCI solution (1:1 0) at room temperature; (ii) quinary salt solution at 90"C and 1 bar; and (iii) quinary salt solution at 200"C and 100 bar.

In order to determine the state of corrosion, the weight loss and the dynamic E modulus of the samples were determined.

The results of these tests are reported as follows: With storage in HCI solution, there were no changes even after 1 2 months.

With storage in quinary salt solution at 90"C and 1 bar, there were no changes even after 1 2 months.

With storage in quinary solution at 200"C and 100 bar, there were no changes after 4 months.

The samples remained geometrically stable, no chipping or cracking occurred, the samples did not swell, and the sample weight remained constant.

Exemplary values of weight and dynamic E modulus at varying times are given in Table 1 below: Dynamic E modulus: Weight (N/mm2) starting 64.3 g 19.9 after 228 days 64.1 g 19.9 after 331 days 64.3 g 20.3 As can be seen from Table 1, the dynamic E modulus, which can be used as indicator for the mechanical stability of the samples, remained substantially the same.

b) As a comparison hereto, the results for samples made of portland cement (PZ 35) at a water to cement weight rate of 0.45 according to the prior art and under the same storage conditions as in part a) of this example were as follows: With storage in HCI solution (1:1 0) at room temperature, the samples were completely disintegrated after 1 to 2 days.

With storage in quinary solution at 90"C and 1 bar, the samples exhibited clear signs of corrosion (crack formation, swelling, increase in weight, decrease of dynamic E modulus) after 2 months and were completely disintegrated after 3 months.

With storage in quinary solution at 200on and 100 bar, the samples were completely disintegrated after 7 days.

Example 3: This example provides an examination of the development of radiolysis gas in the products of the present invention.

A hydrothermal cement product, as described in Example 1 a, and produced with a water/ solids ratio of 0.1, was hardened for one week, and then dried at 55"C for 2 days and at 90"C for 1 day.

This reduced the H20 content to 1 weight percent, and the pressure resistance was unchanged at 35 N/mm2.

Thereafter, the product was irradiated with 10 MeV electrons to a beta/gamma dose of 108 rad and the resulting quantity of radiolysis gas (primarily H2) was measured.

A value of 1.5 10-4 ml H2/9 Mrad was obtained, which is lower by approximately a factor of 10 than for the radiation of undried samples.

Example 4: This example provides a comparison examination of thermal shock resistance.

A hydrothmal cement product, as described in Example 1 a, and produced to have a water/solids ratio in a range from 0.1 to 0.15 was, after hardening for 1 week, heated to a temperature of 500"C and then quenched with cold water.

In this treatment, the samples remained intact, no macrocracks developed, mechanical stability was not worsened.

If samples made of portland cement are subjected to such a treatment, the samples exhibit macrocracks and fall apart.

Example 5: This example illustrates the fixing of inactive fuel element casings in hydrothermal cement.

Fuel element casing sections having a diameter of 1 cm and a length of 5cm, were added under slight vibration, into a mixture, produced as in Example 1 a, of "Swindress Bond 100', which is a hydrothermal cement made by Swindell Rust, USA, and water, the mixture having a water/solids ratio in a range from 0.1 to 0.15.

After hardening, a product was obtained which completely filled the cavities and interstices in the casing sections. The weight proportion of the casing sections in the final product was 25 weight percent.

After hardening the products at room temperature for 1 week, and then drying the products for 1 to 2 days at 50 to 90 C, their H20 content can be reduced to 1 weight percent.

A reduction in the H20 content inevitably reduces the quantity of radiolysis gases that can possibly develop in the radioactive waste solidification products compared to the quantities developed in the prior art products. The water required for the solidification with the hydrothermal cements employed in the present invention is needed only to mobilize the reaction components or for the reaction itself - i.e. for the formation of the polymeric Na-aluminum silicates which may also contain multivalent (e.g. Zn2+, Al3+) and does no participate in the development of the structure. In contradistinction thereto, the water used in the solidification of the prior art cements, such as, for example, portland cement, etc., is consumed additionally for the hydration of the compounds developed during the hardening process.The residual H20 content after intensive drying at, for example, 2 to 3 days at 200 to 250"C, is at 0.2 weight percent for hydrothermal cement blocks having a water-cement weight ratio of 0.13 and produced in accordance with the present invention, and for portland cement (PZ 35) blocks having a water to cement weight ratio of 0.4 is at 8.7 weight percent, i.e. more than 40 times as much.

A great advantage of the present invention is that the solidification of radioactive wastes with hydrothermal cements brings about increased adhesion of the hardened mass on metal and concrete surfaces, compared to solidification with cements used customarily in the construction industry, which moreover usually shrink during hardening. The waste products produced according to the present invention, when in their containers (e.g. in drums), are therefore less exposed to leaching medium if a leak develops in the container (e.g. as a result of a hole eaten through it) than the conventional waste products in their containers.

The compositions of the water/matrix material mixtures employed according to the process of the present invention can be varied in such a way that increases in temperature (e.g. as a result of disintegration heat from the waste radionuclides) cannot have a negative influence on the hardening process. This may be done by a suitable use of a matrix material-waste ratio.

Example 6: This example illustrates the fixing of dissolver residues (feed clarification sludge) from the reprocessing of irradiated nuclear fuels.

A large enough amount of simulated dissolver residue, whose composition is set forth in Table 2 below, was stirred into a mixture of "Swindress Bond 100' with H20, the mixture having a water/solids ratio of 0.1 to 0.1 5 and- produced as in Example 1 a, so that the proportion of dissolver residue was 1 to 5 weight percent of the final product. After hardening at room temperature for 1 week, a pressure resistance of about 20 N/mm2 was noted.

By drying at 90"C for 1 day, the H20 content in the final product was reduced to 1 weight percent, and the pressure resistance remained unchanged.

TABLE 2 Composition of Simulated Dissolver Residue Compound g/1000g ruthenium 189 ruthenium dioxide 189 Pd (Pd + Rh) 132 MoO3 242 MnO2 44 Sb203 8 Ce203 8 Zr chips 130 1.4541-steel chips 40 1.4568-steel- chips 6 Inconel 718 chips 12

Claims (5)

1. Process for solidifying radioactive solid wastes in a matrix material which hardens upon the addition of water, comprising: employing as the matrix material, a material which, after hardening, is a practically shrinkage-free, heat and acid resistant, X-ray amorphous, hydrate phase free material which contains polymeric sodium aluminium silicates or sodium zinc silicates, incorporating the solid wastes into a suspension of the unhardened matrix material containing between 10 and 20 volume percent water to form a waste-matrix mixture, and then leaving the waste-matrix mixture to harden.

2. Process as defined in claim 1, wherein after hardening, the waste-matrix mixture is subjected to a heat treatment at a temperature between room temperature and 100"C so as to remove unbound water.

3. Process as defined in claim 1, wherein the suspension is a prefabricated suspension into which the solid wastes are vibrated.

4. Process as defined in claim 1, wherein the solid wastes are stirred into a suspension of the unhardened matrix material with water.

5. Process for solidifying radioactive solid wastes in a matrix material which hardens upon the addition of water substantially as hereinbefore described and exemplified.