WO2006012830A2 - Method for treating ceramic contaminated with radiocarbon, in particular reactor graphite - Google Patents

Abstract

The invention relates to the treatment of ceramic contaminated with radiocarbon, in particular reactor graphite, wherein dosed corrosion of the surface of the ceramic, subsequently the surfaces of the pores of the ceramic, takes place. The radiocarbon is disposed, preferably in regions close to the surface. As a result, effective partial decontamination which enables the partially decontaminated material to be either reused, for example as reactor graphite, or to carry out final storage and/or to enrich the removed radiocarbon in the reaction gas of the corrosion and to the use thereof, can be achieved.

Description

 Description

Process for the treatment of a radiocarbon contaminated ceramic, in particular reactor graphite

The invention relates to a method for treating a radio-carbon-contaminated ceramic, in particular reactor graphite.

Radiocarbon is the radioactive ¹⁴ carbon isotope of carbon. It is formed, except in a natural way, by cosmic radiation in neutron radiation fields of nuclear reactors in small amounts as a cleavage product or by activation of the isotope ¹³ C occurring in 1: 1% in natural carbon with an effective cross section of 0.0009 barn, but vor¬ predominantly by activation of nitrogen ( ¹⁴ N) in an n, p reaction with a cross section of 1.81 barn. In addition, the isotope ¹⁷ O with only 0.038% of oxygen present in the air can be converted into radiocarbon via a n, σ reaction with a lower cross-section of 0.235 barn.

Because of its long physical half-life of 5,730 years and its good mountability in biological systems due to a substitution of ¹² C or ¹³ C radiocarbon in carbon compounds with high biological half-life is radiocarbon a problematic contamination in radioactive waste represents and requires special safety precautions. On the other hand, it is also a valuable indicator of biological research because it is built into biological systems like natural carbon and can easily be tracked for radioactive decay.

Removal of at least a considerable part of the radiocarbon from ceramics, in particular from nuclear waste, can therefore be used both for the decontamination of the waste and for the recovery of valuable material for research. Exemplary ceramics include: reactor graphite, carbon, SiC, Al ₂ O ₃ , MgO and Verbundmate¬ materials containing ceramics, eg carbon fiber reinforced SiC.

Particularly high concentrations of radiocarbon are present in most graphitic structures of graphite-moderated reactors (MAGNOX reactor (CO ₂ -cooled reactor type with

Natural uranium fuel assemblies with Magnox shell), Advanced Gas-cooled Reactor (EGR), HTR (High-temperature reactor), RBMK (water-cooled nuclear reactor type Russian)), but also in the thermal columns of research reactors (eg MERLIN, see ATW, Jahr¬ gang 48, Issue 6, June 2003, pages 404-407) , The main part comes mostly from the activation of nitrogen and oxygen. Oxygen and nitrogen are stored z. B. by air access to thermal columns and adsorption. In the case of the thermal decomposition of nitrogen-containing plastics (for example hexamethylenetetramine as binder) in the production of graphites, incorporation into grain boundaries preferably takes place.

DE 197 37 891 C2 discloses a method for disposing of a radiotoxic contaminated article from reactor graphite or coal rock, in which part of the radiotoxins is removed by heating the article therefrom and fed to a further removal step and at the at least a closed layer at the surface of the article by filling in pores with pyrolytic carbon by means Infiltra¬ tion of hydrocarbons above 1000 ⁰ C and / or by chemical reaction with egg ner silicon compound to form silicon carbide in a sealing and / or diffusionshem- mend formed , It is disclosed that the heating takes place in a vacuum or under protective gas, in order to prevent the formation of carbon monoxide and carbon dioxide on heating by reactions with the atmo¬ spherical oxygen, which contain the existing radiocarbon and therefore may not escape uncontrolled. In the method according to the described prior art, other toxicants than radiocarbon, z. For example, tritium or cesium bound to the ceramic structure of the article may be removed. The formation of the closed layer on the surface of the article should in particular prevent an undesired penetration of substances with which the residues remaining in the object, including radiocarbon, could be driven off chemically or by solution during the intermediate or final storage. In addition, however, effective protection against burning and diffusion of the toxicants to the surface of the article is provided. With the described method, the object to be treated is thus not freed to a considerable extent from the radiocarbon contamination.

In DE 197 37 891 C2, the disclosure of which is expressly incorporated here, further disposal methods are shown, but none of which discloses an effective separation of the radiocarbon from a ceramic contaminated therewith. Thus ver¬ remains according to the prior art, the radiocarbon in the object to be disposed of or passes -. B. due to a combustion of weakly contaminated material - in the environment. It is an object of the present invention to provide a method of the type mentioned above, in which radiocarbon can be removed to a considerable extent from a kon¬ taminierten ceramic and which allows subsequent effective disposal or recycling further treatment of the radio-carbon.

This object is achieved in a method of the type mentioned in that in a process temperature range by means of chemical reaction of at least one Korrosi¬ onsmediums at least with the radiocarbon at least one volatile reaction gas is formed det, the amount of at least one corrosion medium, the process temperature and / or the duration of the process can be selected such that the chemical reaction takes place essentially only on the surface of the ceramic, including pore surfaces, and within areas close to the surface, and the at least one reaction gas is collected and fed to a separate treatment.

This procedure is based on the finding that a significant part of the radio-carbon sits on the surfaces of the graphite crystals or in the binder and optionally diffuses into the near-surface crystal regions or grain boundaries at the usual operating temperatures. Previously, the experts assumed that radiocarbon is almost completely installed on regular lattice sites of the ceramics in question and thus largely eludes selective removal. In the meantime, it is also assumed that the organic-chemical binder used in the production of technical graphite is not completely graphitized during production, which is why the radiocarbon formed there may also be more easily corroded under certain conditions than is the case with regular graphite crystals ,

The process according to the invention has a number of considerable advantages:

The process can be carried out at relatively low temperatures, so that a stronger involvement of the radio-carbon, z. B. is prevented by diffusing near-surface Ra¬ diocarbon into the crystal lattice or by a Nachgraphitierung.

On the one hand, the concentration of the radiocarbon contamination can be considerably reduced by the method. Although it is not possible to remove the entire radiocarbon from the treated ceramic by acting on the surfaces and the areas near the surface. However, even a distance of z. 50%, which are processed by the method achievable measurements could double the usability of waste storage because the (end) storage capacities are coupled to the total cumulative ¹⁴ C activity. Teildekontaminiertes graphite material can be supplied at sufficiently low residual contamination and a complete conversion to CO or CO ₂ at higher temperatures of, for example, about 1000 ° C in air.

On the other hand, the concentration of the radiocarbon in the collected reaction gases is significantly enriched in comparison to the natural occurrence, which is a good prerequisite for a further use of the radio-carbon as valuable material.

With the method according to the invention, the treated ceramic can be corroded on its surface metered so that the inner regions remain as stable as possible.

The process according to the invention can also be carried out in such a way that oxygen is used as the corrosion medium. When oxygen is used, it reacts with carbon to form carbon monoxide or carbon dioxide, ie volatile carbon compounds.

In this case, the inventive method can also be carried out so that air is used as an oxygen supplier. Alternatively, it is also possible to use pure oxygen or oxygen in a mixture with other gases in a composition different from air. The reaction schieht ge into the aforementioned volatile carbon compounds at temperatures of about 500 ⁰ C. At these temperatures, the Sau¬ erstoff far into an existing pore system penetrate to the crystals on the inner O- berfläche partially oxidize. At higher temperatures, however, a strong oxidation on the outside of the ceramic to be treated, since the pore diffusion is accelerated less by increasing the temperature than the chemical reaction and thus to a depletion of corrosion oxygen in the interior of the pore system of korrodie¬-generating ceramic Medium is coming. The optimum temperature for the metered corrosion thus depends on the chemical reactivity of the corrosion medium, the diffusion rate in the pore system, which can be increased or controlled for example by reducing the pressure or by adding inert inert gases, and the dimensions of the ceramic material to be corroded.

The process according to the invention can also be carried out so that water vapor is used as an oxygen supplier. It has proved to be advantageous, an inert gas To saturate at room temperature with water and then heat the water-inert gas mixture to process temperature.

The process according to the invention can also be carried out in such a way that hydrogen is used as the corrosion medium. In this case, substantially CH _{4 is} formed as the reaction product with carbon.

Basically come as corrosion media all gases or vapors into consideration, which react with carbon and are able to convert the near-surface bound radiocarbon into gaseous reaction products.

It may also be advantageous to carry out the method according to the invention in such a way that the ceramic is comminuted, in particular ground, before starting the chemical reaction. Small crystal dimensions can bring about an extension of the usable temperature range: on the one hand, lower process temperatures are possible because of the larger surface area. On the other hand, small crystal dimensions reduce the extent of the necessary diffusion of the corrosion medium. At higher process temperatures, therefore, in comparison to larger crystal dimensions, the depletion of the corrosion medium in the inner pores described above occurs only to a lesser extent, ie the upper limit of the usable process temperature range shifts further upwards. Too much comminution, which destroys the crystals, is to be avoided, however, as this may destroy the original surfaces and may lose the benefit of the increased radioactive carbon concentration on the surface. The proportion of natural and stable carbon isotopes ¹² C and ¹³ C in the reaction products would increase. This undesirable effect would occur even if the grinding was not too heavy but too high.

The inventive method can also be carried out so that the ceramic is in a container, wherein the at least one corrosion medium at a suitable process temperature supplied controlled and the reaction gas is withdrawn.

In this way, a continuous process can be carried out in which the desired reduction of the radio carbon content in the ceramic can be achieved in a single heating cycle. Furthermore, it may be advantageous to carry out the method according to the invention such that the at least one corrosion medium is adsorbed by the surface of the ceramic, including the pore surfaces, and then the ceramic is brought to process temperature. It may be advantageous to heat the ceramic under vacuum or in an inert gas atmosphere. The extent to which the surfaces are loaded with the corrosion medium can be controlled, for example, by the chosen partial pressure and the loading time. The degree of loading should be chosen so that the amount of corrosion medium present is just sufficient to achieve the desired level of corrosion during heating. In this case, the removal of the reaction gases by evacuation or by purging with inert gas after the process at temperatures below the process temperature range or during the process can take place. In this procedure, near-surface carbon is reacted, which reacts chemically with the previously adsorbed corrosion medium. The proportion of radiocarbon in the separated carbon is higher in this process than in the treatment in a permanent atmosphere of the corrosion medium. However, this gain goes hand in hand with a decrease in total sales due to lower partial pressures.

The inventive method can also be carried out so that in a further step, the container is evacuated or the ceramic is purged with inert gas to withdraw the reaction gases, and then the process, starting with the adsorbing of at least one corrosion medium on the surface , at least one more time durch¬ is performed. In this way, a batch process with several cycles is given, which, on the one hand, leads to longer treatment times compared to the continuous process, but on the other hand can entail increased proportions of radiocarbon in the reaction gases.

The method according to the invention can also be carried out such that, before adsorbing the at least one corrosion medium, metered corrosion is carried out by controlled addition of the at least one corrosion medium at process temperature such that at least a portion of closed pores is opened. Although the first step in this process primarily serves to open the closed pores, it also already leads to a removal of a portion of the radio-carbon. The opening of the closed pores can, for example, be carried out under a medium of corrosive medium, wherein this treatment step can be carried out at elevated temperatures in a relatively short time, which is just sufficient to open the pores. The corrosion too However, pore opening can also be carried out with the aid of a previously adsorbed corrosion medium in vacuo or in an inert gas atmosphere.

The process according to the invention can also be carried out in such a way that the remaining residual ceramic is recycled after the final removal of the reaction gases, for example as nuclear graphite.

Furthermore, the method according to the invention can be carried out in such a way that the radiocarbon contained in the collected reaction gases is enriched. Subsequently, the radiocarbon can be recycled as a valuable material. If from the extracted radiocarbon again a solid, e.g. In the form of graphite, which wer¬ should be, especially when using steam as an oxygen supplier, a cycle for the corrosion medium be advantageous. The cyclic process might look like this: During the corrosion process, carbon monoxide and hydrogen are formed under appropriate process conditions with the endothermic reaction of graphite with water vapor as reaction products. In the extracted and collected carbon monoxide, a significantly increased proportion of radiocarbon is now present compared to the original ceramic. This proportion can now be further increased by enrichment processes. In a further step, solid carbon and water (steam) can be generated from the carbon monoxide in reaction with the hydrogen. The water can then again serve as a supplier for the corrosion medium oxygen.

It may be advantageous to carry out the process according to the invention so that carbon monoxide or CH ₄ is preferably produced as the reaction gas. Carbon monoxide and CH ₄ have a lower weight than carbon dioxide. With these reaction gases, therefore, the given weight difference between radiocarbon and the stable isotopes of the carbon can be better utilized for an enrichment of the radiocarbon.

If the extracted radiocarbon-containing material is to be stored safely for a long time, the production of leach-resistant, non-combustible storage containers makes sense. This can e.g. by reaction of the radiocarbon-rich material to carbides, e.g. SiC, or ge¬ stone-like carbonates happen. The corrosion medium should be chosen so that the reaction gases are as directly as possible suitable for the further treatment steps.

Two exemplary embodiments of the method according to the invention are presented below. 1 shows a diagram of the result of a treatment of one of the AVR reactor (first high-temperature experimental reactor in Forschungszentrum Jülich, see VD / report 729, AVR 20 years of operation, ISBN 3-18-090720-0, VDI -Verlag, Dusseldorf, 1989)) originating graphite powder. As a corrosion medium, oxygen was used as a constituent of water. For this purpose, first argon was saturated with water vapor at room temperature. Subsequently, the graphite powder was heated in the steam-argon atmosphere to 1057 ⁰ C and held at this temperature for 16 hours. FIG. 1 shows the release of the carbon during the treatment period. Curve 1 shows the total mass loss of the graphite powder. This mass loss is expressed in percent by the left scale. Curve 2 represents the percent release of radiocarbon with respect to the original radiocarbon content. Thus, after 16 hours, about 46% of the radiocarbon originally present in the graphite powder is released. Curve 3 shows the ratio between the percent release of carbon monoxide and the release of stable carbon isotopes in percent. As an example, the ratio after about 16 hours of treatment is shown below. As mentioned earlier, about 46% of the radiocarbon is removed. At the same time, the treatment released a little more than 16% of the stable carbon isotopes. The ratio of the percentages 46/16 gives a ratio of about 2.85. The relationship is shown on the right-hand scale of the diagram in FIG. The diagram makes it clear that at the beginning of the

The ratio of the yield of radiocarbon is higher than at later time points. This confirms the predominant deposition of the radiocarbon on the surface, including the pore surfaces.

In an alternative procedure, graphite powder from the experimental reactor MERLIN was first stored in air so that the surfaces of the powder, including the pore surfaces, could adsorb oxygen. This material was then placed in a pure argon atmosphere, heated to 1057 ⁰ C and held at this temperature for about 13.5 hours.

FIG. 2 shows the result of this treatment in a diagram corresponding to FIG. It can be seen that, on the whole, a substantially lower absolute release rate was achieved with respect both to the radiocarbon (curve 5) and also to the stable carbon isotopes (curve 4). However, according to curve 6, the ratio of the percentage release of the radiocarbon to the percentage release of the stable carbon isotopes is higher by more than a factor of 10. Accordingly, the percentage is also the radiocarbon-containing molecules in reaction gases significantly higher than in the treatment under the water-argon atmosphere. The corrosion solely by means of the adsorbed oxygen is particularly suitable when high proportions of radiocarbon are to be achieved in the reaction gases, for example for the recycling of the radiocarbon.

Claims

Patent claims 1. A process for the treatment of a radio-carbon contaminated ceramic, in particular reactor graphite, in which a) in a process temperature range by means of chemical reaction of at least one corrosion medium at least with the radiocarbon at least one volatile Reaction gas is formed, wherein b) the amount of at least one corrosion medium, the process temperature and / or the process duration are selected such that the chemical reaction takes place substantially only on the surface of the ceramic, including pore renflächen, and within near-surface areas, and c) the at least one reaction gas is collected and fed to a separate treatment. 2. The method according to claim 1, characterized in that oxygen is used as the corrosion medium. 3. The method according to claim 2, characterized in that air is used as an oxygen supplier. 4. The method according to claim 2, characterized in that steam is used as Sauer¬ material supplier. 5. The method according to any one of claims 1 to 4, characterized in that hydrogen is used as Kor¬ rosionsmedium. 6. The method according to any one of claims 1 to 5, characterized in that the Ke¬ ramik before starting the chemical reaction comminuted, in particular ground, is. 7. The method according to any one of claims 1 to 6, characterized in that the Ceramic is located in a container, wherein the at least one corrosion medium is supplied controlled at a suitable process temperature and the reaction gas is withdrawn. 8. The method according to any one of claims 1 to 6, characterized in that the at least one corrosion medium is adsorbed by the surface of the ceramic, including the pore surfaces, and then the ceramic is brought to Prozesstempera¬ temperature. 9. The method according to claim 8, characterized in that the ceramic is brought to process temperature under vacuum or in an inert gas atmosphere. 10. The method according to claim 9, characterized in that in a further step, the container is evacuated or the ceramic is purged with inert gas to withdraw the Reaktionsga¬ se, and then the process, starting with the adsorption of the at least one corrosion medium on the surface, at least one more time. 11. The method according to claim 9 or 10, characterized in that prior to Adsorbie¬ Ren of the at least one corrosion medium metered corrosion by kontrol¬ lierter addition of the at least one corrosion medium is carried out at such process temperature that at least a portion of closed pores is opened. 12. The method according to any one of claims 1 to 11, characterized in that after the last withdrawal of the reaction gases, the remaining residual ceramic is re-used, for example as a nuclear graphite. 13. The method according to any one of claims 1 to 12, characterized in that the radio-carbon contained in the collected reaction gases is enriched. 14. Process according to claim 13, characterized in that carbon monoxide or CH ₄ is preferably produced as the reaction gas.