EP1430487B1 - Method for the detoxification of an object made from ceramic, graphite and/or carbon contaminated with at least one toxic agent in particular a radiotoxic agent - Google Patents

Description

The invention relates to a method for disposing of a contaminated with at least one toxicant, in particular radiotoxic article of ceramic, graphite and / or coal.

The disposal of radiotoxic contaminated articles is a significant economic factor, particularly in connection with the operation, decommissioning and disposal of nuclear facilities. In order to avoid contamination of the environment with toxic materials or to keep it as low as possible, a considerable technical and thus financial expense is necessary. In nuclear installations, graphite, coal and such basic ceramic materials are used in a variety of ways, e.g. as moderators or as structural and insulating materials. Radioactive contamination of these materials occurs through neutron activation of existing chemical contaminants or through adsorption or diffusion of fission products. The well-known disposal of low-level radioactive waste through sinking in the sea is ecologically questionable and has not been practiced for many years according to the relevant London Agreement by Western countries.

Landfilling of low level radioactive waste in repositories or interim storage facilities is also problematic because of the high volumes and flammability of carbonaceous materials on the one hand and the limited storage capacity on the other. So can at a disused carbon moderated Reactor several 100 tons of low-level radioactive waste.

The likewise known burning of the low-level radioactive waste is problematic, in particular in the case of carbon structures, since in this case radionuclides, such as tritium and C ^{14, are released} into the atmosphere.

From the DE 197 37 891 A1 a method of the type mentioned is known in which the object to be disposed of is first heated to remove a portion of the toxic substances by outgassing or thermal decomposition. The removed portion of the toxicant is collected and the partially decontaminated article and the collected toxicant are then separately added to further disposal steps. The high-level trapped toxics are disposed of in a manner already available for more radioactive waste, e.g. B. filters from nuclear facilities, is known. Due to the reduced concentration of radiotoxic agents, the partly decontaminated article may be handled with less stringent requirements and may be stored temporarily or temporarily. It can be treated to protect against fire and leaching the surface of the object, for. B. by reaction with liquid silicon to form SiC or by pyrolytic deposition of carbon to close the pores. In addition, in this way any stored, built by radiation-induced disturbance of the crystal lattice energy (Wigner energy) and the associated risk of self-heating is controlled eliminated.

If the objects to be disposed of are from nuclear installations, these are generally massive blocks with dimensions of approximately 0.5 to 1 m in height and width and up to approximately 2 m Length containing significant amounts of different radiotoxic agents. They fall, for example, in decommissioning of graphite-moderated nuclear reactors or in normal operation of gas-cooled reactors by replacing ceramic components, eg. B. fuel element casings or in the form of moderator columns in material test reactors (MTR) or in the form of ceramic or carbonaceous adsorbent from cleaning equipment of all kinds.

The material to be disposed of is usually flammable and therefore already represents a special problem for storage. In addition, especially reactor graphite and coal are very susceptible to chemical attack and leaching. Reactor graphite is a graphite mold with extremely high porosity, especially used for nuclear installations. Charcoal is also highly porous.

The present invention has for its object to provide a method of the type mentioned above, with which the utilization of given storage capacities can be optimized and at the same time an efficient protection against burning and leaching is given. The object is achieved in a method of the type mentioned, in which the object is comminuted into granules, the object is baked before, during and / or after crushing to reduce its contamination under inert gas atmosphere or vacuum and then the granules either with a liquefied enveloping material is poured around, or mixed with the enveloping compound in powder or granular form and then heated to liquefaction of the enveloping material, the liquefied enveloping material either solidifies itself to a ceramic or chemically reacts with the granules to a ceramic.

The crushing of the object allows adaptation of the material to be disposed of to predetermined shapes, eg. B. the shape of transport containers or the geometry of the deposit, and thus optimizing the use of space. A comminution to granules with grain sizes of at most a few cm allows almost any shape. In the case of a liquefied coating mass, which reacts chemically with the material of the granules, the granules can fully react under appropriate conditions. As a rule, at least the larger grains of the granules will only react in the range of a surface layer of a few μm to about 1 mm thickness to form a ceramic. In the case of mixing the granules with the powdery or granular shell mass, this can also be added before, during or after heating liquefied enveloping material. In any case, the comminution and the coating of the granules with the enveloping mass also causes the contamination remaining after the heating to dissipate substantially in the volume of the container. This is particularly advantageous for objects to be disposed of, which have a high surface contamination and no or only small volume contamination. The conversion to a volume contamination can lead to much lower requirements for the handling of the container due to the then lower surface dose rate and the given self-shielding. Thus, it is not necessary to remove almost the entire amount of the toxicants by baking, whereby the decontamination less expensive, z. B. at relatively low temperatures could be performed. Another advantage of comminution arises when the heating takes place on the granules themselves, as due to the shorter diffusion paths, a more efficient heating can be achieved.

Furthermore, the method according to the invention should be carried out so that the comminution takes place under an inert gas atmosphere. As a result, the risk of ignition and deflagration is reduced and avoided in the case of carbonaceous objects, the reaction with atmospheric oxygen to form CO or CO ₂ , which would normally contain C ¹⁴ and therefore must not escape. Suitable atmospheres are z. Nitrogen and argon.

The parameters to be used in the process according to the invention, such as temperature, residence times, pressure, ratio of the mass of the shell mass to the mass of the article, the particle size, are each dependent on the nature of the original concentrations of the toxics and the desired properties of the container. Should a Toxikum due to the high stability of its binding to the article not or can be expelled only by uneconomically long periods of time by heating the article to the required extent, the corresponding toxicant by chemical reaction with a suitable substance, eg. As a halogen, in a thermally removable from the object chemical compound, for. As a halide transferred.

The heated toxics are - as already known from the prior art - separately collected and disposed of.

The mixture produced from the granules and the coating mass can be filled into a suitable container for producing a container which can be stored temporarily or in a repository. This can be done in the flowable or moldable state of the batch to completely fill the container with the batch. The use of metallic containers is known. However, these have the disadvantage that, due to oxidation of the metal in aqueous solution, hydrogen can form and collect in the repository.

It may therefore be advantageous to carry out the method according to the invention in such a way that the mixture produced from the granulate and the enveloping mass is poured into a container from a non-contaminated ceramic. As a result, the hydrogen formation described above and thus a risk of explosion in the repository is avoided.

Furthermore, the method according to the invention can be carried out in such a way that, after the mixture has been poured, the container is completely filled with a non-contaminated liquid filling compound which solidifies to form a ceramic. In this way the container becomes so closed. that the mixture has no contact with the environment. The filling mass, the z. B. can solidify to the same ceramic, from which the container is made, thus closing the latter.

However, the method according to the invention can also be carried out in such a way that the mixture produced from the granulate and the coating mass is used in the flowable or moldable state for filling the intermediate spaces into other contaminated waste containing containers which can be stored temporarily or in a repository.

In this way, the capacities of the containers used and thus the given storage capacities can be used almost optimally. The remaining contamination in the mixtures is so low that filling the interstitial spaces of other disposable wastes can not exceed limit values that would endanger final disposal.

Furthermore, the method according to the invention can be carried out in such a way that, in order to produce a container which can be stored interim or repository, the mixture produced from the granulate and the coating mass in the flowable or moldable state is completely enveloped by another contaminated waste.

The mixture can enclose the other contaminated objects without a gap and thus form a container adapted to the content itself. If the residual contamination, in particular in the area of the surface of the batch, is sufficiently low, the container can be temporarily stored or disposed of without further encasing. For this purpose, he can use a suitable form, for. B. a cuboid or cylinder abandoned.

If the other wastes are completely enclosed by the mixture, these may also be those which themselves are considerably more contaminated than the mixture, eg. B. high-level waste, z. B. fuel. The shielding by the ceramic cladding can be completely sufficient for final disposal. Otherwise, further shielding measures according to the prior art are to be taken.

Both the filling of the interspaces and the wrapping of waste with the mixture can advantageously be carried out before its complete solidification. However, it is also possible to provide this in a later step after separate heating of the batch.

Furthermore, the inventive method can be carried out so that the mixture produced from the granules and the enveloping mass formed into a container, the container filled with further contaminated waste, sealed and interposed with this content or stored.

The mixture is thus processed into a commodity that may be suitable for wrapping even highly radioactive waste. The container may, for. B. cup-shaped. Covering elements, such as covers, can also be molded. A container of two or more parts may, for. By silicon deposited from the gas phase of a silicon-containing compound, or by application of silicon carbide, e.g. made from non-contaminated raw materials are closed. It is also possible to provide the container parts with a thread.

In the production of ceramic containers can be in its production both using the batch and not contaminated materials Substances of high specific gravity, eg. As barium, be added, which improve the shielding effect.

The process according to the invention can also be carried out in such a way that a carbide former is used as the encapsulant in carbonaceous articles.

For this purpose, in particular liquid silicon, which forms SiC with carbon is suitable. The ceramic SiC forms around the grains of the granules a refractory and leaching resistant and abrasion resistant protective layer. Other suitable carbide formers are z. As boron and zirconium.

The method according to the invention can also be designed in such a way that reinforcing ceramic fibers are added to the enveloping mass or to the mixture produced from the granulate and the enveloping mass. The fibers can z. B. as wound fibers, fiber mats or pieces of fiber from a few mm to a few cm in length can be used. They increase the ductility and thus the resistance of the container against cracking and embrittlement when they are present in the container, in the ceramic envelope or in the container contents. In addition, phenolic resins can also be added. When heated, they are decomposed and the resulting carbon can be mixed with the envelope, e.g. As silicon to SiC react. As an alternative to ceramic fibers, metal structures for reinforcing the ceramic are also conceivable.

The process according to the invention can also be carried out in such a way that casting over with the liquefied coating mass or heating of the mixture of coating mass and granules takes place in a vacuum. In this way, it can be avoided, in particular, that gas inclusions hinder the complete wetting of the granulate surface by the liquefied enveloping mass. Furthermore, the vacuum can already be generated during the comminution of the article in order to reduce the risk of deflagration as well as the CO or CO ₂ formation.

Furthermore, the inventive method can be carried out so that the object and the resulting granules are moistened during comminution with a nonflammable liquid.

This provides further protection against fire or deflagration. In addition, an undesirable distribution of dusts is avoided.

The inventive method can also be carried out so that the object is dipped into a non-combustible liquid for comminution.

Both for the moistening and the comminution in liquid water is particularly suitable. If the material of the object to be disposed of water is not wetted, this can be added to a suitable solvent.

The inventive method can also be carried out so that collected during comminution of the object and floating in the liquid dust is collected and encapsulated with the granules with the envelope mass. The dust can be collected by filtering and / or by evaporating the liquid. The dust is thus effectively supplied for disposal.

Furthermore, the method according to the invention can be carried out in such a way that the liquid is collected and returned to the process. Thus, the liquid is circulated, whereby the environment is relieved.

The process according to the invention can also be carried out in such a way that the surface of the mixture is oxidized. As a rule, the enveloping compound, unless it is itself a liquefied ceramic, in the process according to the invention is not continuously a chemical compound to form a ceramic. By oxidizing the entire surface of the container, even in the pores and cracks, it is made resistant to later attacks by fire, leaching or chemical attack. In the case of silicon as the enveloping mass, a resistant surface of ceramic, abrasion-resistant SiO _{2 is} produced. The oxidation is carried out by heating the container in an oxidizing atmosphere, for. As O ₂ or air. The separate heating of the container can be saved if the oxidizing atmosphere is generated as soon as possible after the encapsulation of the granules. High abrasion resistance is desirable to minimize the uncontrolled loss of packaging material. Mohs hardness greater than or equal to 4 would be advantageous.

Furthermore, the method according to the invention can be designed so that containers that from the granules and the enveloping mass produced blends are formed so that they can be placed together with surface contact. Exemplary shapes are those of rectangular or hexagonal cross section. They allow a seamless juxtaposition and thus an optimized use of space. The surface interaction also prevents that acting in the direction of the side surfaces, adjacent container against each other pressing rock pressure in repositories, not as quickly leads to cracks or Zersprödungen in the container as is the case with cylindrical containers, the side only have a linear contact with each other.

Finally, the method according to the invention can be carried out so that it is carried out at the place of use of the object.

In this way, depending on the Kontaminationsart associated with risks transport of the objects to be disposed of be avoided. It is of course also possible to use only individual process steps, eg. B. alone crushing, perform at the site of the object to be disposed of.

In the following, a preferred embodiment of the method according to the invention is shown.

As an example of an object to be disposed of, it is assumed that a moderator element made of carbon is used in a nuclear power plant. Such moderators have a ceramic structure composed of high-purity and porous graphite (reactor graphite) and carbonaceous material. Usually such a moderator is contaminated with different toxic substances. To remove these toxic substances as much as possible, the moderator is in a high-temperature furnace heated by direct current passage. Alternatively, it can also be heated inductively or by separate heating elements. The heating is done in vacuum or under inert gas to prevent the formation of carbon monoxide and carbon dioxide when heated by reaction with atmospheric oxygen, which contain the radioactive C ¹⁴ usually present in irradiated carbon and therefore must not escape in an uncontrolled manner. This procedure is already in the DE 197 37 891 A1 described in detail.

The article is comminuted after baking or even before or during it. This comminution can be done either outside the reactor pressure vessel or within. In the latter case, the movement of large blocks and opening of the enclosure is avoided. For this purpose, the shredding device would have to be introduced into the reactor vessel. With appropriate attachment to a manipulator, it is possible to disassemble the blocks in situ and crush and then remove only the cargo or granules. If the thermal treatment takes place subsequently, this can also be carried out within the reactor pressure vessel, since the required heaters can be very compact.

Since comminution is to be expected with considerable amounts of dust, among other things, to avoid deflagration, the crushing is carried out under inert gas. In addition, the cargo can be moistened or completely in liquid, eg. B. in water, are immersed. A prior thermal trituration of tritium should be done so as not to displace the liquid. The sludge containing the dust can be dried again fed to the disposal process and the liquid can be recycled after condensation.

In the case of heating the granules this can be done continuously or discontinuously under inert gas. The level of temperature depends on the type of toxic substances and the desired decontamination factors. In this case, the graphite cleaning procedure known in the graphite industry can be used essentially. Highly volatile toxic substances, such. B. tritium or cesium are expelled already at relatively low temperatures. Chemical substances bound to carbon have to be removed from the ceramic structure by pyrolysis at higher temperatures. Are individual toxic substances thereby not to be removed from the moderator material, z. B. hard decomposable carbides, these can be converted by the addition and infiltration of gaseous halogen compounds in volatile halides. The toxic substances removed from the moderator material are deposited on condensation plates or collected by means of traps or filters (eg for tritium), where they are then present in a considerably higher concentration than in moderator material.

The partly decontaminated moderator material now contains at most toxic substances that could not be removed by thermal and / or thermochemical treatment. It follows that outdiffusion or leaching of these toxic substances would not take place even over extremely long periods of time.

The thermally and / or chemically pretreated comminuted moderator material is interspersed with liquid silicon, wherein the surfaces of the carbonaceous granules react to SiC under a correspondingly high temperature and a container of Si and SiC can be poured off or extruded with embedded granules. The shape, z. As cylinder, cuboid or flat Rectangular cylinder segments depend on the transport and storage container geometries for the best possible use of space.

Since a portion of the silicon may not yet have reacted with carbon, an after-treatment is performed under an oxidizing atmosphere, with silica forming at the oxygen-accessible sites, providing additional oxidation protection that also resists long-term corrosion attacks.

Because of the pretreatment, it can be assumed that the surface dose rate of the container is very low and SiC also allows the production of complex shapes, the material can also be poured into small containers, which in turn can absorb other hazardous substances and protect against corrosion and leaching. Furthermore, the still liquid mixture can be used for pouring spaces or for wrapping whole or disassembled reactor fuel elements. In addition to the excellent corrosion resistance, silicon carbide is characterized by good heat conduction.

A sheath made of SiC is much more resistant than metal. Another disadvantage of the metal compared to the SiC is that the oxidation of metals in aqueous solution produces large amounts of hydrogen, which accumulate in the repository and can form a source of danger.

Claims (16)

1. Method for the detoxification of an object made from ceramic, graphite and/or carbon contaminated with at least one toxic agent in particular radiotoxic agent, in which

   a) the object is crushed to a granulate under an inert gas atmosphere,

   b) the object is heated under an inert gas atmosphere or vacuum during and/or after crushing to reduce its contamination and

   c) the granulate is then either

   aa) recast with a liquefied encasing mass
   or

   bb) blended with the encasing mass in the form of powder or granulate and then heated, until the encasing mass liquefies,
   in which the liquefied encasing mass either itself hardens into a ceramic or reacts chemically with the granulate to form a ceramic.
2. Method according to claim 1, characterised in that the mixture produced from the granulate and the encasing mass is poured in a free flowing or malleable state into a container made from a non-contaminated ceramic.
3. Method according to claim 2, characterised in that, after the mixture has been poured, the container is filled completely with a non-contaminated ceramic.
4. Method according to one of claims 1 to 3, characterised in that the mixture produced from the granulate and the encasing mass is used in a free flowing or malleable state to fill up the gaps in other packing drums containing contaminated waste, which may be put into intermediate or final storage.
5. Method according to claim 1, characterised in that to produce a packing drum, which may be put into intermediate or final storage, other contaminated waste is encased completely with the mixture produced from the granulate and encasing mass.
6. Method according to claim 1 or 4, characterised in that the mixture produced from the granulate and the encasing mass is made into a container, the container is filled with other contaminated waste, closed and put into intermediate or final storage with this content.
7. Method according to one of claims 1 to 6, characterised in that a carbide creator is used as the encasing mass in the case of objects containing carbon.
8. Method according to one of claims 1 to 7, characterised in that reinforcing ceramic fibres are added to the encasing mass or the mixture produced from the granulate and the encasing mass.
9. Method according to one of claims 1 to 8, characterised in that recasting with the liquefied encasing mass or heating the mixture made from the encasing mass and granulate is done in a vacuum.
10. Method according to one of claims 1 to 9, characterised in that the object and the granulate produced are moistened with a non-combustible liquid during crushing.
11. Method according to one of claims 1 to 9, characterised in that the object is immersed in a non-combustible liquid for crushing.
12. Method according to claim 10 or 11, characterised in that dust produced whilst the object is crushed, which is floating in the liquid, is absorbed and recast together with the granulate with the encasing mass.
13. Method according to one of claims 10 to 12, characterised in that the liquid is absorbed and fed to the process again.
14. Method according to one of claims 1 to 13, characterised in that the surface of the mixture is oxidised.
15. Method according to one of claims 1 to 14, characterised in that packing drums, which contain the mixture produced from the granulate and the encasing mass, are made in such a way that they may be put next to each other with extensive contact.
16. Method according to one of claims 1 to 15, characterised in that it may be carried out at the place of use of the object.