EP0081084A1 - Moulded body for encapsulating radioactive wastes, and process for manufacturing this body - Google Patents

Abstract

Shaped bodies are known for the safe long-term integration of radioactive and toxic waste, which have a matrix of graphite and an inorganic binder, preferably nickel sulfide. Increased leaching resistance is achieved with moldings that have a waste-containing core and a waste-free shell made from the same graphite matrix.

Description

Shaped body for the integration of radioactive waste and process for its production

The invention relates to a shaped body made of graphite and an inorganic binder for the safe long-term incorporation of toxic. and radioactive waste, and a process for its production.

• - Die Wiederaufarbeitung der Brennelemente mit Rückführung des Brennstoffes in die Brennelementfertigung sowie Abtrennung, Konditionierung und Endlagerung der Spaltprodukte.
• - Die direkte Endlagerung der abgebrannten Brennelemente.

Spent fuel elements from nuclear reactors have to be disposed of after a certain interim storage period. Two options are discussed:

• - The reprocessing of the fuel elements with return of the fuel to the fuel element production as well as separation, conditioning and final storage of the fission products.
• - The direct disposal of the spent fuel.

In both cases, a suitable, in particular corrosion and leach-resistant, binding matrix or a corresponding container material must ensure that the highly active waste that is stored remains at the storage location for a thousand years or more and does not return to the biosphere.

The radioactive waste generated during the reprocessing of spent fuel elements must be brought into a form suitable for disposal. A high level of waste is economically necessary. This requires a substantial volume reduction - for example by evaporation - before the solidification step.

Several methods are known for solidifying highly radioactive waste. For example, the waste is first calcined in a fluidized bed between 350 and 900 ° C. A mixture of oxides is obtained which is incorporated as a powder or granulate in a glass-like or ceramic matrix and thereby solidified to form a product that can be disposed of.

Methods are known for embedding medium and low-level waste, according to which the waste materials e.g. heated with bitumen and subjected to an extrusion process. The radioactive waste is incorporated into the bitumen mass, hot filled into barrels and finally stored.

Another method is to fix the radioactive waste in cement or concrete. Here, the waste is usually processed in the form of a salt concentrate or slurry, which extends to about 70 - 30 wt .-% composed len of solid Bestandtei ⁼ - 80 percent of liquid and 20.. The slurry is mixed with cement and allowed to set. If necessary, this step can be carried out directly in the final storage casks.

Furthermore, methods for conditioning radioactive waste are known, in which the waste is preferably disposed of in a polymerizable resin is mixed in at room temperature, and then polymerized into a solid block.

These known methods have a number of disadvantages, in particular for higher activity concentrations. The waste is glazed at high temperatures, usually above 1,000 ° C. At this temperature, some salts are already volatile and have to be processed using complex methods, e.g. exhaust gas purification. This applies in particular to the active compounds of cesium and ruthenium. The thermal conductivity of the glass matrix is relatively low. In order not to exceed an inadmissibly high central temperature of the containers caused by the residual heat, the waste concentration and block diameter are therefore limited to values of approximately 20% by weight or 20 to 30 cm. Furthermore, due to the differences in the thermal expansion coefficients of glass and container material, mechanical stresses occur during cooling, which can lead to undesirable stress corrosion and crack formation in the glass. The required cooling time for glass waste containers can take several days to produce crack-free containers. This additional process step therefore requires expensive hot cell space.

Bituminization is only applicable to relatively low activity concentrations, e.g. for the so-called liquid medium-active waste of approx. 0.1 - 1 Ci β, δ activity. Temperatures of 150 - 200 ° C are necessary, which requires complex safety precautions, e.g. against fire, required. In addition, under radiation, bitumen forms radiolysis gases, e.g. Hydrogen.

The simple technique of cementing also has disadvantages. So you get large volumes of waste with the same amount of waste, e.g. 3 to 5 times the volume compared to bitumen inclusion, a relatively poor leaching behavior of the enclosed radioactive waste due to the porosity of the cement, and a radiolysis of the water bound in the cement, which can lead to relatively large amounts of gas, such as hydrogen.

When incorporated into polymerizable resins, hydrocarbon compounds are generally used. Therefore, the radiation of the radioactive waste can increase the brittleness of the synthetic resin and thus jeopardize the mechanical integrity of the container. Such molded articles also have only a relatively low resistance to radiolysis and the release of hydrogen.

From DE-OS 27 56 700 a method for enclosing radioactive waste in a metal matrix is known, which is formed by isostatic pressing of the waste with metal powder at temperatures between 1000 and 1500 ° C. The high pressing temperatures and the large consumption of corrosion-resistant metal make this process seem at least unsuitable for large bodies and for the inclusion of volatile radioactive substances.

In addition, so-called final storage containers are known which absorb the waste materials and are usually designed as multi-layer containers to achieve sufficient long-term corrosion resistance. Corrosion-resistant metallic and non-metallic materials are used as container materials.

DE-OS 29 17 437 describes a process for incorporating radioactive and toxic waste into a good heat-conducting carbon matrix under mild conditions bind, which consists of a mixture of powdered carbon, preferably graphite, with a binder, wherein a corresponding shaped body is formed by pressing with the admixed waste at temperatures above 100 ° C. Suitable binders are organic and inorganic substances, the use of sulfur is advantageous and in a preferred embodiment a mixture of sulfur and nickel is used which forms the water-insoluble nickel sulfide at a pressing temperature of about 400 ° C. Although this matrix is resistant to corrosion and leaching, the waste materials can be removed from the surface layer in the case of the molded articles produced by adding the waste to the matrix starting materials.

For an economical application of this method, the highest possible concentration of waste is in the. Moldings required. In the case of higher proportions of waste in the molded article, the waste is leached out in the long term from ever deeper layers and ultimately from the entire molded article.

It was therefore an object of the present invention to provide a shaped body made of graphite and an inorganic binder for the safe long-term incorporation of radioactive and toxic waste, which is highly dense, corrosion and leach-resistant, so that the incorporated waste does not dissolve even in very long periods of time can be.

This object was achieved according to the invention in that the molded body consists of a core in which the waste is embedded and a waste-free shell made of the same material.

The graphite matrix for core and shell is produced in a known manner by pressing a mixture of powdered graphite and an inorganic binder or the starting components of an inorganic binder at a temperature above 100 ° C. Either sulfur or a stable metal sulfide is used as the inorganic binder. It is advantageous to use an easily compressible natural graphite powder as graphite.

When using sulfur as a binder, a pressing temperature in the range of the melting temperature of the sulfur of approximately 120 ° C. and a pressing pressure of 10-50 MN / m ² , preferably approximately 20 MN / m ² , are used. A core and shell shaped body produced in this way is suitable for toxic and low-level radioactive waste which only generates a small amount of heat. Greater decay heat occurs in more radioactive waste, particularly in the case of highly active waste, which requires high thermal stability of the graphite matrix.

The addition of suitable metal or alloy powders to form stable metal sulfides results in very temperature-resistant binders in the graphite matrix. The chemical reaction between metal and sulfur takes place in the mixture of graphite, sulfur and metal or alloy powder by raising the temperature when pressing the moldings.

For example, lead, iron, nickel, cobalt, copper, molybdenum, vanadium or tungsten can be used as metals. The use of nickel to form nickel sulfide has proven to be particularly advantageous.

The proportion of waste in the core is advantageously between 1 and 70% by volume, preferably between 10 and 50% by volume, so that the core is in the form of a mechanically stable body which has essentially the same physical properties as the waste-free shell.

When using sulfur as the sole binder, a pressing temperature of 130 ° C and a pressing pressure of about 20 to 50 MN / m ^{2 are} sufficient to produce a high-density, corrosion and leach-resistant molded body.

When using nickel sulfide as a binder for the incorporation of more radioactive waste materials, the connection between the inner zone and the shell is preferably carried out at a temperature above 300 ° C. and a pressure of 30 to 100 MN / m ² . The core is preferably also initially pressed at room temperature or at elevated temperature and the compact is inserted into a pre-pressed hollow cylinder with an attached base. After placing the cover plate, the entire molded body is pressed at a temperature above 100 ° C and compressed to over 80% of the theoretical density. With continued pressure, the temperature is raised to above 400 ° C, preferably to about 440 ° C. The molded body, which is ejected after cooling to below 400 ° C., has a density above 90% of the theoretical value and is highly dense and free of continuous pores.

The shaped body according to the invention is chemically extremely stable, i.e. still very corrosion and leach resistant even in highly corrosive media.

The waste-containing core in the interior of the waste-free shell has largely the same physico-chemical characteristics as the casing, so that mechanical stresses that can cause the container to tear are practically eliminated.

The figure schematically shows a molded body according to the invention in an exemplary embodiment.

The core (1) of the cylindrical shaped body consists of a matrix of graphite with an inorganic binder, in which the granular or lumpy radioactive waste (2) is embedded. The core (1) is surrounded on all sides by the waste-free shell (3), with which it is connected without transition. The places where the shell (3) is assembled from preformed annular sections (4) are shown in dashed lines. In addition, the shell (3) is surrounded by a steel shell (5).

The shaped body according to the invention is to be explained in more detail using the following examples:

Example 1:

• 43,3 Gew.-% Naturgraphitpulver, 20,0 Gew.-% Schwefel und 36,7.Gew.-% Nickelmetallpulver.

The press powder used to manufacture the graphite matrix contains the following components:

• 43.3% by weight of natural graphite powder, 20.0% by weight of sulfur and 36.7% by weight of nickel metal powder.

• Aussendurchmesser 450 mm, Innendurchmesser 300 mm, Höhe 200 mm, werden etwa 58 kg des intensiv gemischten und anschliessend granulierten Presspulvers in eine von aussen beheizbare ringförmige Pressmatrize gefüllt. Im Schmelzbereich des Schwefels - bei 130°C - wird das Granulat mit einem Druck von etwa 100 MN/m² zusammengepresst, die Temperatur bei konstantem Pressdruck anschliessend auf ca. 450°C erhöht und dabei der Schwefel zum Nickelsulfid umgesetzt. Nach dem Abkühlen auf 350°C erfolgt das Ausstossen des hohlzylindrischen Formkörpers.

To produce an annular segment with the dimensions:

• Outside diameter 450 mm, inside diameter 300 mm, height 200 mm, about 58 kg of the intensively mixed and then granulated press powder are filled into an externally heatable press die. In the melting range of the sulfur - at 130 ° C - the granulate is pressed together with a pressure of about 100 MN / m ² , the temperature is then increased to approx. 450 ° C at constant pressure and the sulfur is converted to nickel sulfide. After cooling to 350 ° C., the hollow cylindrical shaped body is ejected.

In the way described, 4 ring-shaped segments are produced, which are then assembled into an approximately 800 mm long hollow cylinder by pressing. This requires a temperature of 500 - 600 ° C and a pressure of 50 MN / m ² , a nickel / sulfur powder mixture (stoichiometric ratio 1: 1) mixed with small amounts of natural graphite powder (stoichiometric ratio 1: 1) serves as the connecting intermediate layer reacted to nickel sulfide. The base plate made of pressed powder of the same composition is added using the same procedure. Accordingly, a cover plate is made from the press powder.

A compact made of the same matrix and containing 50 vol.% Waste is inserted into this container-like cavity. The cover plate is then placed on and connected to the hollow cylinder to form a closed shell under the same conditions as specified for the segments.

The following properties are determined from the molded body:

Example 2:

To incorporate simulated radioactive waste into an inorganically bound graphite matrix, the pulverulent mixture prepared in the same way as in Example 1 from the starting components of the graphite matrix is mixed with approximately the same amount of feed sludge simulate, which consists of molybdenum, molybdenum (VI) oxide, manganese, manganese ( IV) oxide, zirconium, calcium chloride, antimony (III) oxide, stainless steel and nickel powder. This mixture is transferred to the hollow cylinder made of graphite-nickel-sulfur matrix, which is pre-pressed at room temperature and is located in a die that can be heated from the outside. After placing the cold-pressed cover plate, the entire molded body in the melting range of the sulfur - at 130 ° C - is compressed with a pressure of about 100 MN / m ² , the temperature is increased to 450 ° C at constant pressure and the sulfur is converted to nickel sulfide . After cooling to about 350 ° C., the molded article containing the simulate is ejected. In addition to the physical properties mentioned in Example 1, particularly low Cs leaching rates are observed on this body: 3. 10 ^-4 - 5 x 10 ^-6 _c m / d.

Example 3:

The powdery matrix components graphite, sulfur and nickel are first mixed intensively in accordance with Example 1. About 3 cm long sections of fuel rod sleeves (sleeve diameter outside 10.75 mm; wall thickness 0.68 mm) made of Zirkaloy-4 are mixed into the press powder formed. The proportion by weight of the compacted or uncompacted sleeves is 25% by weight.

The sleeve / pressed powder mixture is pre-pressed at room temperature in a floating steel die (inner diameter 50 mm) with a pressure of about 5 MN / m ² . The "core" formed (diameter 50 mm, height 80 mm) has about 50% of the theoretical density.

The pre-pressed "core" is then inserted in a heatable press mold into the parts of the shell which have also been pre-shaped at room temperature and with a pressure of 5 MN / m ² ; the shell consists of a base plate, a hollow cylinder with an outer diameter of 66 mm and a cover plate. After heating to 130 ° C, the molded body is compressed with a pressure of 50 MN / m ² to about 85% of the theoretical density. The almost finished test specimen is heated to a temperature of around 440 ° C under constant pressure. The nickel / sulfur mixture converts into the chemically, mechanically and thermally much more stable nickel sulfide. At the same time, the density increases to over 90% of the theoretical value.

After a holding time of approximately 10 minutes at the reaction temperature, the finished test specimen is cooled to 350-400 ° C. and ejected (diameter 66 mm, height approximately 75 mm).

Claims (9)

1) Shaped body made of graphite and an inorganic binder for the safe long-term integration of radioactive and toxic waste, characterized in that it consists of a core (1), in which the waste (2) is embedded, and a waste-free shell (3) made of the same material. 2) Shaped body according to claim 1, characterized in that it contains a stable metal sulfide as an inorganic binder. 3) Shaped body according to claim 1 and 2, characterized in that the binder consists of nickel sulfide. 4) Shaped body according to claim 1 to 3, characterized in that the core (1) contains 1 to 70 vol.% Waste (2). 5) Shaped body according to claim 1 to 4, characterized in that the shell (3) consists of several assembled individual parts (4). 6) Shaped body according to claim 1 to 5, characterized in that the shell (3) is additionally surrounded by a steel shell (5). 7) Process for the production of moldings according to claim 1 to 6, by pressing a mixture of waste, graphite powder and inorganic binder or the starting components of the binder at temperatures above 100 ° C, characterized in that this compact with a shell of graphite powder and inorganic Binder is pressed on all sides. 8) Process for the production of moldings according to claim 7, characterized in that the shell of preformed hemispherical, ring-shaped or plate-shaped sections is pressed onto the waste-containing pressed body at temperatures above 100 ° C. 9) Process for the production of moldings according to claim 7 and 8, characterized in that the shell is pressed on above 300 ° C with a pressure of 30 to 100 MN / m ² .

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