EP3170187B1 - Process for decontamination of contaminated graphite - Google Patents

Description

The present invention relates to the decontamination of contaminated graphite, including irradiated graphite. According to the invention, this is understood to mean a process for separating volatile radionuclides from contaminated graphite together with converting the graphite including the non-volatile radionuclides into a form suitable for disposal.

Large amounts of contaminated, in particular irradiated, graphite accumulate annually, especially when dismantling reactors (around 240,000 t of such graphite components exist worldwide).

There are a large number of different graphite-moderated nuclear reactors worldwide, such as UNGG in France, Magnox and AGR in England or RMBK in Russia. These reactors are usually gas-cooled and use metal-clad fuel elements, which are packed in so-called sleeves made of graphite and pushed through the reactor core. Corresponding graphite blocks are generally used as core material for this type of reactor, which serve both as thermal insulation, as a moderator for receiving free neutrons and as gas guiding elements. Many of these plants are to be dismantled, so that a cost-effective and simple disposal strategy for contaminated, especially irradiated, graphite is imperative. A deep geological introduction of such components is associated with high costs. A simple, near-surface final storage of the components in containers filled with concrete has not yet been approved worldwide, since the discharge of the radionuclides contained is not reliably prevented.

Irradiated graphite can usually comprise various radionuclides such as H-3, C-14, Co-60, Cl-36, Cs-137, Sr-90. The content of such radionuclides is due in particular to the neutron activation of nitrogen, which is present as an impurity in the graphite or in the cooling gas, but also to the neutron activation of the naturally occurring C-13 isotope. The radionuclides are distributed more or less homogeneously in the entire volume of the irradiated graphite. Due to this distribution of the radionuclides, the entire volume of the irradiated graphite can also be classified as radioactive waste. Depending on the country-specific classification, some of the irradiated graphite is even classified as medium-active waste.

The final storage of contaminated, in particular irradiated, graphite is made considerably more difficult, in particular, by those radionuclides which are volatile and thus also mobile, in particular H-3, C-14 and Cl-36. Another difficulty is volatile radionuclides, which are also long-lived like C-14 and Cl-36. Volatile radionuclides can be on the surface, in particular on the surfaces of the pore system of the irradiated graphite. They can be chemically bound, adsorbed or absorbed. Due to the content of such radionuclides, final storage is difficult. Due to the long half-life and the risk of continuous release of volatile radionuclides from the contaminated graphite, it has to be disposed of in deep soil regions under special safety requirements and therefore with great effort and costs.

For example, the C-14 content of irradiated graphite from Spain prevents its disposal in the El Cabril repository near the surface. In France, only the radionuclide concentration can be considered in the safety certificates according to the currently applicable regulations for near-surface disposal. Even if a matrix material would guarantee the safe inclusion of the irradiated graphite, this must not be included in the safety considerations. If such a graphite is safely bound, a near-surface and space-saving as well as cost-effective final storage is not permitted due to the radionuclide content, whereby volatile radionuclides are viewed particularly critically.

For example, the safe storage of contaminated, in particular irradiated graphite in special matrix materials is conceivable and known. In the WO 2010/052321 A1 describes a matrix material for the final storage of radioactive waste into which the radioactive waste is placed. The radioactive waste, which can also be irradiated graphite, is either mixed directly with the matrix material and, if necessary, cold-pressed together with the matrix material at room temperature. The waste is then introduced into cavities of a pre-pressed molded body made of matrix material and then finally pressed. Alternatively, the waste can be finally pressed directly with the matrix mixture into a finished molded article. The final pressing takes place at elevated temperatures and elevated pressure. In particular, as a result of the process control, there are higher temperatures in the areas of the material near the edges compared to the interior of the material. As a result of the process control, volatile radionuclides of the waste consequently accumulate inside the container. In addition, the waste is embedded in the form in which it occurs without any prior treatment. The Manufactured containers therefore contain the radionuclides of the waste, including volatile radionuclides, and must therefore be stored under strict safety requirements, especially in deep soil regions.

In the WO 2011/117354 A1 Containers are described comprising an impermeable glass-graphite matrix, IGG for short, in which radioactive waste can be embedded in a metal-coated form. This creates a so-called inverse design. The metal shell around the waste acts as a diffusion barrier and prevents the radionuclides contained in the waste from escaping into the IGG. To produce the waste elements, the waste is optionally filled into a metal shell together with a binder and then extruded in the metal shell to form composite bars. No previous treatment of the waste is planned. This means that any volatile radionuclides are still contained in the waste after embedding. Consequently, as explained above, depending on the country-specific classification, final storage in deep soil regions is not unnecessary.

It is therefore an object of the invention to provide a method which enables simple and cost-effective disposal and final storage of contaminated, in particular irradiated graphite, with lower safety requirements. Depending on the country-specific requirements, a near-surface final storage and / or a final storage on the surface should preferably be permitted in order to relieve underground landfills.

• Aufheizen einer Basismischung umfassend kontaminierten Graphit und wenigstens ein Glas zur Abtrennung flüchtiger Radionuklide von dem kontaminierten Graphit, wobei ein behandelter Graphit erhalten wird;
• Verdichten des behandelten Graphits zum Erhalt eines Formkörpers, der zur Endlagerung geeignet ist;
• optional Einbetten des Formkörpers in ein Matrixmaterial zum Erhalt eines ummantelten Formkörpers.

The object is achieved by the method for decontaminating contaminated graphite described here. The method of the present invention comprises the steps:

• Heating a base mixture comprising contaminated graphite and at least one glass for separating volatile radionuclides from the contaminated graphite, whereby a treated graphite is obtained;
• Compacting the treated graphite to obtain a shaped body which is suitable for final storage;
• optionally embedding the molded body in a matrix material to obtain a coated molded body.

The base mixture for separating the volatile radionuclides is preferably heated in the same device as the compression, so that no further one is used Handling of the graphite is required. As a result, the method according to the invention can be carried out even more cost-effectively and quickly.

The shaped body produced by the method according to the invention is suitable according to the invention for the final storage of the treated graphite, ie preferably for safe storage over geological periods, ideally up to 1 million years or longer.

The molded body can preferably be disposed of and stored under reduced safety requirements compared to the storage of contaminated graphite which has not been subjected to any decontamination according to the invention. Depending on the safety requirements for final storage, safe and near-surface final storage and / or even safe final storage of the molded article produced according to the invention is permissible on the surface. As a result, the volume of such material, which requires particularly complex and therefore particularly costly disposal and storage, in particular underground storage in deep soil regions, can be significantly reduced. The latter is extremely advantageous in view of the very limited storage capacities and the regular accumulation of large amounts of contaminated, in particular irradiated, graphite. It can also significantly reduce the cost of disposing of contaminated graphite.

"Contaminated graphite" is a graphite that contains proportions of radionuclides. A contaminated graphite is preferably a graphite which has an activity of> 10 ³ Bq / g, in particular 10 10 ⁴ Bq / g or even 10 10 ⁵ Bq / g. According to the invention, the “contaminated graphite” is therefore preferably at least a weakly active material with activity values in the middle range of the usual range for “weakly active”, in particular even a medium-active material.

The radionuclides may have entered the graphite as a result of contamination, for example if the graphite is part of fuel assemblies. The radionuclide content can, however, also be caused by neutron activations during the irradiation of the graphite or impurities in the graphite. According to the invention, the term “contaminated graphite” thus also includes an “irradiated graphite” which has radionuclides as a result of the irradiation. Common radionuclides that may be present in contaminated graphite include H-3, C-14, Cl-36, Co-60, Cs-135, Cs-137, I-131, Sr-90, Pu-239, U-235 and other radioactive isotopes of uranium, Th-232 and other radioactive isotopes of thorium, Pb-203 and other radioactive isotopes of lead and mixtures thereof.

The method according to the invention is suitable for such contaminated graphite which comprises at least one volatile radionuclide. A "contaminated graphite" according to the invention thus comprises at least one volatile radionuclide.

According to the invention, volatile radionuclides are radionuclides which, under standard conditions in accordance with DIN 1343 (date of issue 1990-01) or when the contaminated graphite is heated to at least 350 ° C. and at most 1600 ° C. when subjected to pressure below 15 MPa, preferably below 10 MPa, more preferably below 5 MPa, in the gaseous state or in the form of gaseous chemical compounds or can be converted into the gaseous state or gaseous compounds under the conditions mentioned. Gaseous compounds of the radionuclides are in particular those of the radionuclide in elemental form and / or in the form of oxides or halides of the radionuclide. Volatile radionuclides are in any case H-3, C-14, Cl-36, I-131, Cs-135 and Cs-137. The contaminated graphite thus preferably comprises at least one volatile radionuclide selected from the group consisting of H-3, C-14, Cl-36, I-131, Cs-135 and Cs-137. One of the volatile radionuclides mentioned can be present in the contaminated graphite. It is also conceivable that mixtures comprising at least two or more of the volatile radionuclides mentioned are present in the contaminated graphite.

In particular, radionuclides selected from H-3, C-14 and Cl-36 can be separated from the contaminated graphite particularly advantageously with the method according to the invention. The method according to the invention is therefore particularly suitable for the decontamination of a contaminated graphite which comprises at least one volatile radionuclide selected from the group consisting of H-3, C-14, Cl-36 and mixtures thereof.

The contaminated graphite is preferably one in which the total activity of volatile radionuclides is> 10 ^-1 Bq / g, more preferably> 10 ¹ Bq / g, even more preferably> 10 ² Bq / g and in particular> 10 ³ Bq / g. In embodiments, the total activity of volatile radionuclides in the contaminated graphite is> 10 ⁵ Bq / g and in particular> 10 ⁶ Bq / g. The method according to the invention is particularly suitable for contaminated graphite which has relatively medium or high total activities of volatile radionuclides. Since the method according to the invention enables the volatile radionuclides to be separated off, the contaminated graphite can be disposed of in a particularly effective and cost-saving manner.

In particular, the activity of Cl-36 in embodiments in which this radionuclide is contained in the contaminated graphite is preferably> 10 ^-1 Bq / g, in particular> 10 ¹ Bq / g, and preferably> 10 ³ Bq / g. In embodiments in which the contaminated graphite comprises C-14, the activity of C-14 should preferably be at least> 10 ² Bq / g, in particular> 10 ⁴ Bq / g and preferably> 10 ⁶ Bq / g. If the contaminated graphite comprises H-3, the activity of H-3 is preferably> 10 ³ Bq / g, more preferably> 10 ⁵ Bq / g and even more preferably> 10 ⁷ Bq / g in the contaminated graphite. If the preferred minimum activities mentioned are exceeded, the advantages of the method according to the invention are particularly evident.

In addition to the at least one volatile radionuclide, the contaminated graphite can include other radionuclides that are not volatile. Such radionuclides include, in particular, Co-60, Sr-90, Pu-239, U-235 and other radioactive isotopes of uranium, Th-232 and other radioactive isotopes of thorium, Pb-203 and other radioactive isotopes of lead and mixtures thereof. The list is exemplary and not exhaustive. Any other radionuclides can be present in the contaminated graphite in addition to the at least one volatile radionuclide, which are not explicitly mentioned here.

In addition to graphite and the at least one volatile radionuclide, the contaminated graphite can contain further constituents which, depending on its use, have been added to the graphite or are present as impurities. The contaminated graphite preferably originates from fuel balls and / or reflector blocks and / or the reactor core. This list is not exhaustive. In particular, the contaminated graphite can also come from thermal columns from research facilities and sleeves from Magnox and UNGG reactors.

According to the invention, a “base mixture” is a mixture which comprises the contaminated graphite and at least one glass. The base mixture can contain other components besides the contaminated graphite and the glass. Optionally, at least one oxidizing agent can be included. The base mixture particularly preferably consists of the contaminated graphite and the glass and optionally the oxidizing agent. The base mixture is preferably obtainable by mixing the components contained therein, in particular the contaminated graphite and the glass and the oxidizing agent. The base mixture is preferably a homogeneous mixture, ie the constituents are distributed uniformly in the base mixture. Suitable mixing methods are known to those skilled in the art. The base mixture is preferably in Powder form before, the average grain diameter of the components contained therein are preferably less than 100 microns. When this invention speaks of an average grain diameter, it always means the Ferretsche diameter.

According to the invention, "treated graphite" is the product obtained by heating the base mixture according to the invention.

The "treated graphite" comprises the components of the base mixture, but preferably has a significantly reduced content of volatile radionuclides. The treated graphite is further processed according to the invention by compression into a shaped body which is suitable for final storage.

The treated graphite is therefore preferably one that has a significantly reduced volatile radionuclide content. According to the invention, there is a "significantly reduced content" of volatile radionuclides if the content of at least one volatile radionuclide of the volatile radionuclides contained in the contaminated graphite in the treated graphite by at least 60%, preferably by at least 70%, more preferably by at least 80% and even more is preferably reduced by at least 90% based on the amount of volatile radionuclide in the contaminated graphite.

Very particularly preferably, only at most 50%, preferably at most 40% and even more preferably less than 30% of volatile radionuclides based on the total amount of volatile radionuclides in the contaminated graphite are present in the treated graphite. Even more preferably, only less than 25%, preferably even less than 15%, of volatile radionuclides based on the total amount of volatile radionuclides in the contaminated graphite are present in the treated graphite. The detection and determination of the amount of volatile radionuclides is carried out according to methods known to the person skilled in the art. The person skilled in the art is able to choose a suitable detection option depending on the radionuclide. In particular, liquid scintillation spectrometry, the total alpha beta activity measurement, mass spectrometry, a neutron activation analysis and optionally a radiochemical separation are available as methods for the selective and quantitative determination of volatile radionuclides.

If the contaminated graphite comprises H-3, the treated graphite is preferably one which is at most only 25%, more preferably at most only 15% and particularly preferably less than 5% and very particularly preferably less than 2% Contains H-3 based on the amount of H-3 in the contaminated graphite. If the contaminated graphite comprises C-14, the treated graphite preferably contains less than 65%, more preferably less than 55% and even more preferably less than 50% of C-14 based on the amount of C-14 in the contaminated graphite. If the contaminated graphite comprises Cl-36, the treated graphite preferably contains less than 80%, more preferably less than 60% and even more preferably less than 50% of Cl-36 based on the amount of Cl-36 in the contaminated graphite.

If the contaminated graphite contains H-3, the treated graphite is preferably one in which the activity of H-3 is <10 ³ Bq / g, more preferably <10 ² Bq / g and H-3 is very particularly preferred no longer detectable in the treated graphite using conventional detection methods. If the contaminated graphite contains C-14, the activity at C-14 in the treated graphite is preferably <10 ² Bq / g, more preferably <10 ¹ Bq / g. If the contaminated graphite comprises Cl-36, the activity at Cl-36 in the treated graphite is preferably only <10 ^-1 Bq / g. Depending on the country-specific classification, the graphite treated according to the invention can be a material that is no longer radioactive, that is to say a freely measured material, or one that is only weakly active. This also applies to the shaped body which is obtained according to the invention by compressing the base mixture. Depending on the country-specific classification, the shaped body according to the invention can therefore be a material that is no longer radioactive, that is to say a freely measured material, or only a weakly active material. In particular, the molded body preferably has a significantly reduced volatile radionuclide content.

According to the invention, the base mixture is heated in order to separate the volatile radionuclides from the contaminated graphite; the volatile radionuclides are preferably separated from the contaminated graphite when the base mixture is heated. The radionuclides are preferably "separated" from the contaminated graphite if a treated graphite is obtained which has a significantly reduced content of volatile radionuclides. This is ensured in particular by the composition of the base mixture according to the invention and the process control according to the invention.

The separation of the volatile radionuclides can be enhanced by the addition of oxidizing agents. Due to their oxidizing effect, they contribute to the release of volatile radionuclides from the contaminated graphite. In particular, such substances can contribute to the opening of closed pores in which trapped ones are volatile radionuclides and / or trigger the conversion of chemically bound radionuclides under the process conditions to gaseous compounds.

In preferred embodiments, the use of oxidizing agents is dispensed with, so no oxidizing agents are added to the base mixture. Surprisingly, the glass in the base mixture already has an optimal oxidative effect, so that the process according to the invention can be made even more cost-effective and simple. In alternative embodiments, in which oxidizing agents are added to the base mixture, the content of these substances should have values of preferably at most 8% by weight, more preferably at most 5% by weight and even more preferably at most 2% by weight, based on the total weight of the Do not exceed the base mixture. If too much oxidizing agent is used, the material of the equipment used is attacked, which reduces the service life of the equipment. Organic peroxides are preferred oxidizing agents.

The contaminated graphite is preferably present in the base mixture as graphite powder; the contaminated graphite preferably has an average grain diameter of less than 100 μm, more preferably at most 50 μm and particularly preferably at most 30 μm. If the contaminated graphite is not already in such grain diameters, the contaminated graphite is crushed before heating. Methods of comminution are well known to those skilled in the art. The smaller the grain diameter of the graphite powder, the higher densities can be achieved in the treated graphite or in the shaped body and the better the volatile radionuclides can be separated from the contaminated graphite. Optionally, the contaminated graphite is comminuted before heating.

The glass in the base mixture has, in addition to a binding effect and a certain oxidative effect, in particular also a structuring function and contributes to the production of a particularly dense and non-porous treated graphite or the shaped body obtainable by compression. Glass has the advantage that no gaseous crack products are formed during the heating of the base mixture, which could lead to pore formation in the treated graphite. This means that the glass hardly goes through any implementation processes. Pore formation is thus also effectively prevented due to the procedure according to the invention. In the softened or melted state, the glass wets the contaminated graphite and possibly the other constituents of the base mixture, so that the cavities between the Particles can be closed by capillary or adhesive forces and a dense and almost non-porous molded body can be obtained after compression of the base mixture, which is sufficiently stable for further processing.

The process according to the invention enables the production of a shaped body which is preferably essentially pore-free, namely a density of preferably at least 90%, more preferably of at least 95%, even more preferably of at least 98%, even more preferably even in the range of> 99% and very particularly preferably in the range of> 99.5% of the theoretical density. It is advantageous if the shaped body has a high density, so that the risk of moisture penetrating into the shaped body is further reduced and any non-volatile radionuclides from the contaminated graphite are enclosed particularly effectively. This also makes it even easier to prevent these radionuclides from escaping into an optional matrix material in which the shaped body can be embedded. The shaped body preferably also has good hardness due to the structural effect of the glass.

According to the invention, the glass of the base mixture is preferably selected from borosilicate glasses, aluminophosphate glasses, lead glasses, phosphate glasses, alkali glasses, alkaline earth glasses and mixtures thereof. The glass of the base mixture is particularly preferably selected from borosilicate glasses, aluminophosphate glasses, lead glasses and mixtures thereof. The glass of the base mixture is very particularly preferably a borosilicate glass.

The advantage of borosilicate glasses is good corrosion stability. Borosilicate glasses are also very chemical and temperature resistant glasses. The good chemical resistance to water and many chemicals is explained by the boron content of the glasses. The temperature resistance and insensitivity of the borosilicate glasses to sudden temperature fluctuations are a consequence of the low thermal expansion coefficient of about 3.3x10 ^-6 K ^-1 of borosilicate. Common borosilicate glasses on the day of registration include Jenaer Glas, Duran®, Pyrex®, Ilmabor®, Simax®, Solidex® and Fiolax®.

• 70 % bis 80 % SiO₂
• 7 % bis 13 % B₂O₃
• 4 % bis 8 % Alkalioxide, wie Na₂O oder K₂O
• 2 % bis 7 % (Al₂O)
• 0 % bis 5 % Erdalkalioxide, wie CaO, MgO.

A typical composition for borosilicate glasses is known to the person skilled in the art and is, for example, in percent by weight:

• 70% to 80% SiO ₂
• 7% to 13% B ₂ O ₃
• 4% to 8% alkali oxides , such as Na ₂ O or K ₂ O
• 2% to 7% (Al ₂ O)
• 0% to 5% alkaline earth oxides, such as CaO , MgO .

The advantage of aluminophosphate glasses is their high radiation resistance as well as resistance to high temperatures and water.

Lead glasses are suitable because of the possible absorption of ionic radiation. Phosphate glasses are characterized by low melting points, so that their use is also advantageous. As a result, lower temperatures can be used when heating the base mixture, so that the process as a whole can be designed to save costs and energy.

Alkaline glasses are characterized by low viscosities. As a result, the ability to wet the contaminated graphite is favored. Thus, pores can be closed easily and preferably a high density of the treated graphite can be achieved.

Alkaline earth glasses in turn have increased acid stability, are easy to process and are inexpensive, so that they can also be used according to the invention.

The glass is preferably used in the form of a powder in the base mixture, so that an optimal binding and structural effect can be achieved. The average grain diameter of the glass powder is preferably less than 100 μm, more preferably at most 50 μm and particularly preferably at most 30 μm. The smaller the grain diameter, the easier the glass can close any pores between the other components of the base mixture.

It is advantageous if the base mixture contains at least 5% by weight of glass, more preferably at least 7% by weight, even more preferred are at least 10% by weight and particularly preferred are at least 12% by weight of glass based on the Total amount of base mix included in the base mix. If too little glass is used, a sufficient binding and structural effect can often not be achieved. The base mixture preferably comprises up to 30% by weight, more preferably up to 20% by weight and particularly preferably up to 18% by weight of glass. If too much glass is used in the base mixture, it is no longer possible to incorporate sufficiently contaminated graphite. The moldings according to the invention are then no longer suitable for space-saving final storage of the graphite, since less contaminated graphite is effectively processed per area. Sufficient, but as little as possible of glass should therefore be used in the base mixture in order to supply as much contaminated graphite as possible to the process according to the invention.

When heating the base mixture, i.e. the heat treatment of the base mixture, the base mixture is preferably heated to a target temperature of at least 650 ° C, more preferably of at least 700 ° C and even more preferably of at least 800 ° C and very particularly preferably of at least 1000 ° C. If the target temperature to which it is heated is too low, the glass is softened too little to penetrate between the pores of the other constituents of the base mixture. The volatile radionuclides can often only be inadequately separated from the contaminated graphite at low temperatures. In particular, it may also be necessary for bonds in the graphite to be broken to release volatile radionuclides. The target temperature of the base mixture should preferably not be more than 1600 ° C., preferably not more than 1500 ° C., more preferably not more than 1400 ° C. and even more preferably not more than 1350 ° C., and very particularly preferably not more than 1200 ° C. If the target temperature is too high, the overall process becomes too expensive and there is a risk of undesirable reactions in the base mixture. Target temperatures between 700 ° C and 1300 ° C, in particular between 750 ° C and 1250 ° C, and even more preferably between 800 ° C and 1200 ° C have proven to be particularly suitable. At these temperatures, the glass showed a particularly clear binding and structural effect and the volatile radionuclides could be separated off particularly well.

The heating of the base mixture preferably first comprises heating to at least one intermediate temperature, which is below the target temperature, before heating to the target temperature. So preferably the heating of the base mixture runs on the Target temperature from at least two phases. In this context, according to the invention, the "heating-up phase" refers to the targeted heating up to a specific target temperature, which can then be maintained for a predetermined time, preferably at least 5 minutes, more preferably at least 10 minutes.

The heating very particularly preferably takes place in two phases, the first heating phase comprising reaching an “intermediate temperature” and the second heating phase comprising further heating starting from the intermediate temperature to reach the “target temperature”. Such a temperature control has proven to be particularly advantageous and enables particularly effective separation of volatile radionuclides as well as an overall inexpensive and rapid process design. The content of volatile radionuclides is particularly preferably reduced significantly in the first heating phase, so that treated graphite can be obtained after the first heating phase. The second heating phase then serves to separate any remaining volatile radionuclides while at the same time optimally softening the glass of the base mixture. The intermediate temperature is preferably at least 350 ° C, more preferably at least 400 ° C, even more preferably at least 420 ° C. If the intermediate temperature of the base mixture is too low, there is a risk that volatile radionuclides cannot be removed sufficiently in the first heating phase. The intermediate temperature is particularly preferably between 400 ° C and 500 ° C, more preferably between 420 ° C and 480 ° C, in particular 450 ° C ± 20 ° C.

The pressing pressure when heating the base mixture is preferably below 15 MPa, more preferably below 12 MPa and particularly preferably below 10 MPa.

If there is a two-phase heating, which is particularly preferred according to the invention, the pressing pressure during the first heating phase is preferably below 5 MPa, more preferably below 3 MPa, even more preferably below 2 MPa and particularly preferably below 0.5 MPa and even more preferably below 0 , 2 MPa and very particularly preferably at normal pressure, ie about 0.101325 MPa +/- 20%. According to the invention, the heating to the intermediate temperature is preferably carried out without external pressure. The second heating phase is preferably carried out at a pressure below 15 MPa, more preferably below 12 MPa and even more preferably below 10 MPa. The pressing pressure in the second heating phase is very particularly preferably between 5 MPa and 10 MPa, more preferably between 6.5 and 9.5 MPa and particularly preferably between 7.5 and 8.5 MPa. Such a Press pressure has proven to be particularly advantageous for separating volatile radionuclides that are still present, while at the same time optimally softening the glass component.

If too high a pressure is exerted when the base mixture is being heated, i.e. if it is heated and compressed at the same time, there is a risk that, as a result of the heating from the outside and simultaneous pressure, volatile radionuclides will accumulate in the center of the base mixture and the volatile radionuclides will not be contaminated Graphite can be separated. Such heating from the outside with simultaneous increased pressure corresponds to the usual procedure for producing an IGG matrix as in the WO 2011/117354 A1 described. A resulting container cannot be stored under reduced safety requirements, especially not near the surface, due to the clear content of volatile radionuclides. It is self-evident that, according to the invention, compacting does not take place before the base mixture is heated. Compression before heating can also make it considerably more difficult to separate the volatile radionuclides and lead to the accumulation of radionuclides in the interior of the base mixture, which is undesirable.

The heating rate during heating is preferably at least 5 ° C / min, preferably at least 8 ° C / min and more preferably at least 10 ° C / min. Such slow heating makes it easier to separate volatile radionuclides from the contaminated graphite. The heating rate during heating should not be too high, that is, preferably below 300 ° C / min, more preferably below 100 ° C / min. If the heating rates are too high, the overall process becomes too expensive and too complex. Heating rates between 15 ° C./min and 20 ° C./min have proven to be particularly advantageous, particularly in the second heating phase.

The heating, that is to say the heating until a target temperature of preferably at least 650 ° C. and preferably at most 1600 ° C. is reached, preferably lasts for at least 5 minutes, more preferably for at least 10 minutes and particularly preferably for at least 12 minutes, and even more preferably over at least 18 minutes, and more preferably over at least 25 minutes. If the heating takes place too quickly, that is to say in a period of time that is too short, there is a risk that the volatile radionuclides cannot be adequately separated from the contaminated graphite. However, it is preferably heated for a maximum of 60 hours, preferably for a maximum of 50 hours and even more preferably for a maximum of 24 hours, particularly preferably for a maximum of 10 hours. If the heating takes too long, there is a risk of side reactions in the base mixture.

A target temperature of the base mixture of preferably at least 650 ° C. and preferably at most 1600 ° C. is preferably maintained for at least 5 minutes, more preferably for at least 10 minutes and particularly preferably for at least 12 minutes. If such a target temperature is maintained for too short a time, there may be a risk that volatile radionuclides which are still present may not be adequately separated from the contaminated graphite. The target temperature is preferably held for at most 15 hours, more preferably for at most 10 hours. If the heating is carried out in two phases, which is preferred, the intermediate temperature is preferably maintained for at least 5 minutes, more preferably at least 10 minutes and particularly preferably for at least 12 minutes. The intermediate temperature can be maintained for up to 30 hours, preferably up to 26 hours and more preferably up to 24 hours. If the intermediate temperature is maintained for too short a time, there is a risk of inadequate separation of the volatile radionuclides, because according to the invention, a significant reduction in the volatile radionuclides can already be achieved in the first heating phase.

The glass viscosity when heating to the target temperature, preferably in the second heating phase, is preferably ≤ 10 ⁵ dPa × s, more preferably <10 ⁵ dPa × s. If the viscosity of the glass is too high during heating, the glass cannot penetrate sufficiently between the pores of the other constituents of the base mixture, so that a sufficiently dense and hard molded body cannot be obtained on a regular basis.

The release of volatile radionuclides is preferably monitored during heating, preferably by on-line measurement. The heating up time and / or the duration of the intermediate temperature and target temperature are particularly preferably matched such that a treated graphite remains which has a significantly reduced content of volatile radionuclides.

The heating is particularly preferably carried out in a vacuum, the residual gas pressure preferably being <10 ^-3 MPa, more preferably 10 10 ^-4 MPa. The heating can be carried out by supplying heat, exposure to electricity, microwaves or other methods for heating a material.

According to the invention, the heating is preferably carried out in such a way that a temperature gradient between the innermost regions of the base mixture and regions of the base mixture near the edges is achieved. In the innermost regions of the base mixture, there are higher temperatures than in regions of the base mixture near the edge, which is according to the invention is referred to as a "negative temperature gradient" to differentiate it from the usually existing temperature distribution with higher temperatures in areas near the edge. A negative temperature gradient is ensured according to the invention in particular by the selection of a suitable heating rate and the duration of the heating and / or the duration of the target temperature and the preferred intermediate temperature. A negative temperature gradient according to the invention leads to transport processes of the volatile radionuclides in such a way that a separation of the volatile radionuclides becomes even better possible.

According to the invention, there is a negative temperature gradient if the smallest measured temperature difference (ΔT) between a central measuring point and at least 2 external measuring points, preferably at least 3 external measuring points, along a horizontal plane within the basic mixture is preferably such that the temperature at the central measuring point is more than 5 ° C, more preferably by more than 10 ° C and particularly preferably by more than 20 ° C and even more preferably by more than 50 ° C higher than the temperature at the external measuring points. However, this temperature difference should not be too high either, since the process as a whole then becomes too cost-intensive and complex. ΔT should therefore be at most 300 ° C, more preferably at most 200 ° C. The horizontal level within the base mixture is selected so that it horizontally divides the base mixture into two halves of equal size based on the volume of the base mixture. The center measuring point and the outside measuring points lie along this horizontal plane.

The "center measuring point" is located at the point of the horizontal plane at which the horizontal plane is cut by a vertical plane which in turn divides the basic mixture vertically into two equal halves based on the volume of the basic mixture. The external measuring points are on the horizontal plane such that the smallest distance between the central measuring point and each of the external measuring points is at least 60%, preferably at least 70% and even more preferably at least 80% of the length of a straight line from the central measuring point to the edge of the base mixture, where the straight line runs in such a way that it intersects the external measuring point and the central measuring point and runs from edge to edge of the basic mixture. This ensures that the external measuring points are sufficiently far away from the center measuring point and sufficiently close to the edge of the base mixture.

The greatest distance between each external measuring point and the central measuring point is selected such that the distance is at most 95% and preferably at most 90% of the length of the straight line from the central measuring point to the edge of the base mixture. This ensures that the external measuring points are not too close to the edge of the base mixture. The temperature curve in the base mixture can thus be ideally represented.

According to the invention, the heating of the base mixture is followed by a densification of the treated graphite, i.e. exerting increased pressure. According to the invention, this means exerting a pressing pressure of preferably at least 20 MPa. A particularly stable and more densely treated graphite can thus be achieved, which can be easily processed further in the method according to the invention. The compression is preferably carried out at elevated temperature, preferably at the target temperature, that is to say at temperatures between 650 ° C. and 1600 ° C., more preferably at temperatures between 700 ° C. and 1400 ° C. and even more preferably at temperatures between 800 ° C. and 1200 ° C.

The compression pressure during compression is preferably up to 250 MPa, more preferably up to 200 MPa, even more preferably up to 180 MPa and even more preferably up to 150 MPa. The pressure should not be too high, because then the process as a whole becomes too expensive and too complex. However, the compression pressure during compression should be at least 20 MPa, preferably at least 30 MPa and more preferably at least 50 MPa and more preferably at least 60 MPa. If the compression pressure was in this range, the treated graphite showed a particularly advantageous compression. The compression takes place preferably under protective gas. Alternatively, compression takes place under vacuum, the residual gas pressure preferably being <10 ^-3 MPa, more preferably ≤ 10 ^-4 MPa.

The compression is preferably carried out in a hot isostatic press, a vacuum hot press or a spark plasma sintering system (SPS). The base mixture is preferably also already heated in one of the plants mentioned, preferably in the same plant as the compression.

The pressing force in the PLC is preferably between 80 kN and 500 kN, particularly preferably between 90 kN and 300 kN, in order to ensure adequate compression. According to the invention, the residual gas pressure in the PLC is preferably at most 10 ^-3 MPa, particularly preferably the residual gas pressure is below 10 ^-3 MPa. Preferably the treated graphite filled into an axial die. The heating of the base mixture according to the invention preferably takes place beforehand in the press mold. In this case, the treated graphite is already in the axial mold.

The base mixture can be heated in this system by applying a current, in particular a direct current, with currents in the range from 3 kA to 8 kA, preferably from 3.5 kA to 5 kA and even more preferably from 4 kA to 4.5 kA , and voltages of 4 V to 10 V, preferably 4.5 V to 8 V, even more preferably 5 V to 6 V. The power consumption should be 15 kW to 30 kW, in particular 20 kW to 25 kW. The direct current is passed directly through the base mixture to heat the base mixture. For compression, a pressure of 50 MPa to 250 MPa is preferably applied under protective gas or in a vacuum. The process enables the production of a molded body with high density even with short process times.

In a further embodiment, hot isostatic pressing is used for the compression. For this purpose, the treated graphite is filled into a container. The base mixture is preferably also heated in this container. The compression is preferably carried out at a pressure between 20 MPa and 200 MPa, preferably in a vacuum.

The pressing pressure of preferably between 20 MPa and 250 MPa can be maintained for up to 15 hours, preferably up to 12 hours and ideally up to 10 hours. If the pressing pressure is maintained for too long, the process is altogether too expensive and complex. According to the invention, the compression preferably also comprises cooling the molded body obtained. A first cooling of the shaped body is preferably carried out while maintaining the pressing pressure of preferably between 20 MPa and 250 MPa to temperatures below 800 ° C., preferably below 600 ° C., more preferably to 500 ° C. ± 5 ° C. The first cooling is preferably carried out over a period of at least 1 min, more preferably 2 min. The period is a maximum of 120 minutes, more preferably a maximum of 60 minutes. A period of 5 minutes for the first cooling has proven to be particularly suitable. After this first cooling, the glass viscosity should be at least 10 ⁶ dPa × s, preferably ≥ 10 ⁶ dPa × s. This is preferably followed by a second cooling to temperatures below 35 ° C., more preferably below 30 ° C. and even more preferably to 25 ° C. ± 5 ° C. with simultaneous pressure reduction.

It is a particular advantage of the method according to the invention that embedding and / or embedding of the shaped body in other materials or metal containers is not is required for safe disposal. Rather, the molded article produced according to the invention is suitable for final storage, ie preferably for safe storage over geological periods, ideally up to 1 million years or longer. The molded body can also be embedded in a matrix material.

In embodiments of the method according to the invention, the molded body is therefore embedded in a matrix material. This makes it possible to further improve the final storage capacity of the shaped body and to enclose the treated graphite even more reliably. In particular, embedding the shaped body in this way gives additional radiation and corrosion stability. The molded body can be embedded in the matrix material without further intermediate steps, such as further machining or processing, which are not listed here. According to the invention, it is in particular not necessary for the molded body to be introduced into an additional metal shell, for example as a diffusion barrier, before being embedded in the matrix material. In contrast, the molded body is preferably embedded in the matrix material without an outer metal sheath. This is advantageous because it enables cost-effective storage and simple process management. A metal shell also only provides temporary protection against diffusion as a result of possible corrosion and cracking during longer storage. With the method according to the invention, a diffusion of radionuclides from the contaminated graphite into the matrix material is already sufficiently prevented or reduced by the composition of the base mixture and the procedure according to the invention, in particular the heating of the base mixture to separate volatile radionuclides from the contaminated graphite. Therefore, an additional introduction of the molded body into a metallic shell before embedding in the matrix material is not necessary according to the invention.

According to the invention, “embedding” means that the shaped body is enclosed by the matrix material; according to the invention, this is referred to as “covered shaped body”. The shaped body is enclosed by the matrix material when more than 95%, preferably more than 98% of the outer surface of the shaped body is covered by the matrix material and the outer surface of the shaped body is very particularly preferably completely covered by the matrix material.

According to the invention, the matrix material comprises, as matrix constituents, graphite, which is not contaminated, and at least one inorganic binder, selected from glasses, Aluminosilicates, silicates, borates and mixtures thereof. Such matrix materials are known from the prior art.

The inorganic binder is preferably selected from glasses, in this case it is a so-called impermeable graphite-glass matrix, IGG for short. Glass, as an inorganic binder, has the advantage that there are no gaseous crack products that lead to the formation of pores in the matrix material. In addition, in the softened or melted state, it wets the remaining matrix components and the cavities between the particles are closed by capillary or adhesive forces. This ensures a high density of the matrix material and excellent corrosion resistance.

According to the invention, the glass in the matrix material is preferably selected from borosilicate glasses, aluminum phosphate glasses, lead glasses, phosphate glasses, alkali glasses, alkaline earth glasses and mixtures thereof. The person skilled in the art will choose a suitable glass according to his specialist knowledge. The glass is particularly preferably selected from borosilicate glasses, aluminophosphate glasses, lead glasses and mixtures thereof. The glass is very particularly preferably a borosilicate glass due to the high corrosion stability and high chemical and temperature resistance.

The proportion of graphite in the matrix material is preferably at least 60% by weight, more preferably at least 65% by weight. The graphite content is preferably at most 90% by weight. The proportion of inorganic binder is preferably at least 10% by weight. A maximum of 40% by weight of inorganic binder is preferably contained in the matrix material.

The graphite in the matrix material is an uncontaminated graphite, radionuclides are therefore preferably not detectable therein and / or the graphite only has a natural activity. The activity of the uncontaminated graphite is therefore preferably ≤ 10 ³ Bq / g. It is preferred that the graphite of the matrix material is natural graphite or synthetic graphite or a mixture of both components. It is particularly preferred that the graphite portion of the matrix mixture consists of 60% by weight to 100% by weight of natural graphite and 0% by weight to 40% by weight of synthetic graphite. The synthetic graphite can also be referred to as graphitized electrographite powder. The advantage of natural graphite is that it is inexpensive, that, unlike synthetic graphite, the graphite grain has no nano cracks and that it can be pressed into moldings with almost theoretical density at moderate pressure.

The matrix components, in particular the inorganic binder and the graphite, are preferably used in the form of a powder, so that an optimal binding effect and density of the matrix material can be achieved. The average grain diameter of the glass powder is preferably less than 100 μm, more preferably at most 50 μm and particularly preferably at most 30 μm. The smaller the grain diameter, the easier it is for the glass to close any pores between the matrix components. The graphite powder of the matrix material preferably also has the average grain diameter mentioned.

The production of the matrix material is also known in principle. The preparation of the matrix material comprises mixing the matrix components in powder form to obtain a pressed powder. The press powder can comprise auxiliaries in amounts of a few percent, based on the total amount. These are, for example, pressing aids, which can include alcohols. Granules are preferably produced from the press powder. For the production of granules, the starting components, in particular the two components graphite and glass powder, are mixed with one another, then compacted, and by subsequent breaking and sieving, granules with a grain size of less than 3.14 mm and greater than 0.31 mm are produced.

• Zusammenfügen wenigstens eines Formkörpers mit dem Matrixmaterial zu einem Pressling, wobei das Matrixmaterial bevorzugt in Form eines so genannten "Grundkörpers" mit Kavitäten vorliegt, und
• finales Pressen des Presslings zum Erhalt eines ummantelten Formkörpers. Das finale Pressen erfolgt vorzugsweise durch dynamisches Pressen oder Heißpressen vorzugsweise im Vakuum. Dabei kann ein Pressdruck von vorzugsweise zwischen 80 MPa und 300 MPa zur Anwendung kommen. Das finale Pressen kann ferner ein Aufheizen auf Temperaturen zwischen 800°C und 1400°C umfassen.

The molding according to the invention is preferably embedded in the matrix material by:

• Assembling at least one shaped body with the matrix material to form a compact, the matrix material preferably being in the form of a so-called “base body” with cavities, and
• final pressing of the compact to obtain a covered shaped body. The final pressing is preferably carried out by dynamic pressing or hot pressing, preferably in a vacuum. A pressure of preferably between 80 MPa and 300 MPa can be used. The final pressing can also include heating to temperatures between 800 ° C and 1400 ° C.

In preferred embodiments, the shaped body according to the invention is embedded in the matrix material by joining one or more shaped bodies with the matrix material, which is in the form of a “base body”. According to the invention, the base body is a pre-pressed geometric shape that can take on various configurations, preferably a hexagonal prism, and the one or has several cavities for receiving the molded body (s). The moldings are preferably filled into the cavities. Before the final pressing, the cavity openings are preferably filled with matrix material or covered with matrix material in the form of a further base body made of matrix material. In alternative embodiments, the shaped bodies are introduced into matrix material, which is in powder form, and the mixture is then pressed into a covered shaped body by final pressing.

In embodiments in which the matrix material is already present as a base body with cavities, a handle base with cavities, that is to say recesses for receiving the molded bodies, is first pressed. The pre-pressing is carried out, for example, with a four-column press with three hydraulic drives. According to the invention, shaped rods are preferably used to produce recesses, which are composed of two parts: a shaping rod part with a larger diameter, which is placed on a thinner support rod.

The matrix material described herein is suitable to serve as a corrosion barrier over an ultra-long period of time. In particular, the matrix material is essentially pore-free, namely it has a density which is preferably in the range of more than 90% and particularly preferably> 99% of the theoretical density. It is important that the matrix material has a high density so that no moisture can penetrate into the coated molded body. This is ensured on the one hand by the choice of material and on the other hand by the manufacturing process. In interaction with the treated graphite according to the invention, the coated shaped body can be safely stored for a very long time.

With the method according to the invention for the decontamination of contaminated graphite, a safe and ultra-long final storage of the graphite near the surface or a final storage on the surface is permitted depending on the country-specific safety requirements. The present invention thus enables a volume-saving disposal of large amounts of contaminated graphite.

A particularly preferred embodiment of the method according to the invention is shown in Figure 1.

Examples Example 1: Production of a shaped body for final storage

The tool consisted of two press cylinders and a hollow cylinder jacket. To avoid caking, a graphite foil was placed in the hollow cylinder. The lower stamp was inserted and covered with a graphite foil. A base mixture of 100 g of contaminated graphite comprising the volatile radionuclide H-3 and 20 g of glass 8800 from Schott (borosilicate glass), which had been produced by mixing the components, was introduced into the pressing tool. The filled base mixture was covered with a graphite foil. The upper press ram was then inserted into the tool.

The tool was inserted into a PLC press and pre-pressed to 2 kN using the PLC press ram. Initially, evacuation took place under 1.6 MPa pressure. This step was ended when a vacuum according to the invention was reached. A temperature increase according to the invention followed up to an intermediate temperature of 450 ° C. The pressure was then increased to 8 MPa.

In the second heating phase, the temperature was increased to a target temperature of 1200 ° C. using the method according to the invention, the glass viscosity being <10 ⁵ dPa × s (heating rate 15 ° C./min to 20 ° C./min).

When heating according to the invention, a negative temperature gradient was achieved in the base mixture, ΔT being 10 ° C. During this heating, the H-3 was released from the base mixture and converted to tritiated water in a post-oxidation system. The treated graphite had a significantly reduced content of volatile radionuclides. The H-3 content was reduced by 99% in the treated graphite compared to the amount of H-3 in the contaminated graphite. The treated graphite contained all non-volatile components, e.g. Sr-90 or Co-60.

After the target temperature had been reached, the pressing pressure was increased to ≥ 64 MPa and the base mixture in the Spark Plasma sintering system was compressed to a shaped body with a density of> 98% of the theoretical density. This was followed by a cooling of the treated graphite according to the invention under the increased pressing pressure.

The molding obtained is suitable for safe final storage over very long periods of time and can be stored near the surface or on the surface, in particular depending on country-specific regulations.

Example 2: Embedding the Shaped Body in a Matrix Material to Obtain a Sheathed Shaped Body

The molded body from Example 1 was embedded in a matrix material made of uncontaminated natural graphite and glass. A nuclear-pure natural graphite with a grain diameter of less than 30 µm from Kropfmühl and a borosilicate glass of the same grain size with a melting point of around 1000 ° C from Schott served as the starting components.

The two components were mixed dry in the weight ratio natural graphite to glass 5: 1 and pressed into briquettes with the compactor Bepex L 200/50 P from Hosokawa. The briquette density was approximately 1.9 g / cm ³ . Subsequent breaking and sieving produced granules with a grain size of less than 3.14 mm and greater than 0.31 mm and with a bulk density of about 1 g / cm ³ . A base body was then pre-pressed with cavities for receiving the molded body from Example 1.

The molded body from Example 1 was filled into the cavities and the cavity openings were then filled with matrix material. This was followed by a final pressing at 1000 ° C. The final pressing was carried out as dynamic pressing. The compact was moved alternately with the upper and lower punches in a die under full load. After cooling to 200 ° C., the coated molded body was ejected from the tool.

Claims (11)

1. A method for the decontamination of contaminated graphite comprising the steps of:

   a) heating up a base mixture comprising contaminated graphite and at least one glass for separating the volatile radionuclides from the contaminated graphite, wherein a treated graphite is obtained;

   b) compacting the treated graphite for obtaining a molded body which is suitable for final disposal.
2. The method according to claim 1, wherein the molded body is embedded in the matrix material for obtaining a sheathed molded body, wherein the matrix material comprises non-contaminated graphite and at least one inorganic binder selected from glasses, aluminosilicates, silicates, borates and mixtures thereof.
3. The method according to claim 1 or 2, wherein the base mixture comprises glass in a proportion of 7 % by weight to 30 % by weight.
4. The method according to one of the preceding claims, wherein the glass of the base mixture is a borosilicate glass.
5. The method according to one of the preceding claims, wherein the contaminated graphite and the glass in the base mixture are each present with a mean grain diameter of less than 100 µm.
6. The method according to one of the preceding claims, wherein the base mixture further comprises oxidants.
7. The method according to one of the preceding claims, wherein the contaminated graphite comprises at least one radionuclide selected from H-3, Cl-36, C-14 or mixtures thereof.
8. The method according to one of the preceding claims, wherein in the step of heating up the base mixture target temperatures of at least 800°C and at most 1200°C are used.
9. The method according to one of the preceding claims, wherein during the step of heating up the pressing pressure is less than 10 MPa.
10. The method according to one of the preceding claims, wherein the steps a) and b) are conducted in a hot isostatic press, hot vacuum press or spark plasma sintering plant.
11. The method according to one of the preceding claims, wherein the molded body is embedded in a matrix material and wherein the inorganic binder is contained in the matrix mixture in a proportion of 10 to 40 % by weight and wherein the inorganic binder is a glass.