EP3170187A1 - Method for the decontamination of contaminated graphite - Google Patents

Abstract

The present invention relates to the decontamination of contaminated graphite, in particular of irradiated graphite. According to the invention, this is understood as meaning a method for separating volatile radionuclides from contaminated graphite and transforming the graphite together with non-volatile radionuclides into a form appropriate for ultimate storage. The method according to the invention comprises heating up the contaminated graphite to obtain treated graphite, compacting the treated graphite to obtain a formed piece and optionally embedding the treated graphite in a matrix material to obtain an encapsulated formed piece. Depending on the requirements specific to the country concerned, disposal and storage of the formed piece comprising the treated graphite is possible with less demanding safety requirements. As a result, the volume of such material that requires particularly sophisticated and consequently particularly cost-intensive disposal and storage, particularly storage deep underground, can be reduced significantly. This also leads to significantly reduced costs for the disposal of contaminated graphite, large quantities of which occur on an annual basis.

Description

 Method of decontaminating contaminated graphite

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 simultaneous conversion of the graphite, including the non-volatile radionuclides, into a form suitable for final disposal.

Every year, large amounts of contaminated graphite, in particular irradiated graphite, are produced, above all in the dismantling of reactors (there are approximately 240,000 t of such graphite components worldwide). Worldwide, there are a large number of different graphite-moderated nuclear reactors, such as UNGG in France, Magnox and AGR in England or RMBK in Russia. These reactors are usually gas-cooled and use metal-coated fuel elements, which are packed in so-called graphite-packed sieves through the reactor core. As a core material for this type of reactors usually corresponding graphite blocks are used, which serve both as thermal insulation, as a moderator for receiving free neutrons and as gas guide elements. Many of these plants are to be dismantled, so that it is absolutely necessary to have a cost-effective and simple disposal strategy for contaminated, in particular irradiated graphite. A deep geological introduction of such components is associated with high costs. A simple, near-surface final disposal of the components in containers filled with concrete has not been approved worldwide, since the delivery of the contained radionuclides is not reliably prevented.

Irradiated graphite may typically include various radionuclides such as H-3, C-14, Co-60, CI-36, Cs-137, Sr-90. The content of such radionuclides is particularly due to the neutron activation of nitrogen, which is present as an impurity in the graphite or in the cooling gas, but also due to 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. Because of this distribution of radionuclides, the total volume of irradiated graphite is classified as radioactive waste. The total irradiated graphite is sometimes even classified as medium-active waste, depending on the country-specific classification. The disposal of contaminated, in particular irradiated, graphite is considerably impeded, in particular, by those radionuclides which are volatile and thus also mobile, in particular H-3, C-14 and CI-36. Another complicating factor is volatile radionuclides, which are also long-lived, such as C-14 and CI-36. Volatile radionuclides may be present on the surface, especially on the surfaces of the pore system of the irradiated graphite. They may be chemically bound, adsorbed or absorbed. Due to the content of such radionuclides disposal is difficult. Due to the long half-life and the risk of continuous release of volatile radionuclides from the contaminated graphite, this is to be placed under special safety requirements in deep soil regions and thus end with great effort and expense.

For example, the C-14 content of irradiated graphite from Spain prevents its disposal in the El Cabril near-surface repository. In France, only the radionuclide concentration may be considered in the safety records according to the currently valid regulations for near-surface disposal. Even if a matrix material would guarantee a safe enclosure of the irradiated graphite, this must not be included in the safety considerations. Therefore, if such a graphite is safely integrated, a near-surface, space-saving and cost-effective final disposal is not permitted due to the radionuclide content, with volatile radionuclides being considered particularly critical.

For example, the safe incorporation of contaminated, in particular irradiated, graphite into special matrix materials is conceivable and known. In the WO

2010/052321 A1 describes a matrix material for the disposal of radioactive waste, in which the radioactive waste is introduced. The radioactive waste, which can also be irradiated graphite, is either mixed directly with the matrix material and, if appropriate, cold pre-pressed together with matrix material at room temperature. The waste is then introduced into cavities of a pre-pressed shaped body of matrix material and then finally pressed. Alternatively, the waste can be final pressed directly with the matrix mixture to form a finished shaped article. The final pressing takes place at elevated temperatures and elevated pressure. In particular, as a result of the process control, higher temperatures prevail in areas near the edge of the material compared to the interior of the material. Due to the process, volatile radionuclides of the waste accumulate in the interior of the container. In addition, the waste is embedded in the form in which it is obtained without any previous treatment. The manufactured containers thus contains the radionuclides of the waste including volatile radionuclides and must therefore be stored under correspondingly strict safety requirements, in particular in deep soil regions.

In WO 201 1/1 17354 A1 containers are described comprising an impermeable glass-graphite matrix, IGG short, in which radioactive waste can be embedded in metallumhüllter form. This achieves a so-called inverse design. The metal shell around the waste acts as a diffusion barrier and prevents the leakage of radionuclides contained in the waste into the IGG. To produce the waste elements, the waste is optionally filled together with a binder in metal sheaths and then extruded in the metal shell to composite pressed bars. A prior treatment of the waste is not provided. Thus, any volatile radionuclides are still contained in the waste after embedding. Consequently, as explained above, disposal in deep soil regions will not be dispensable depending on the country-specific classification.

It is therefore an object of the invention to provide a method that makes possible a simple and cost-effective disposal and disposal of contaminated, in particular irradiated graphite, with lower safety requirements. Preferably, a near-surface disposal and / or a final disposal on the surface should be permitted depending on country-specific requirements to relieve underground landfills. The object is achieved by the method for decontaminating contaminated graphite described herein. The method of the present invention includes

Steps:

Heating a base mixture comprising contaminated graphite and at least one glass to separate volatile radionuclides from the contaminated graphite to obtain a treated graphite;

Compacting the treated graphite to obtain a shaped article suitable for disposal; optionally embedding the shaped body in a matrix material to obtain a coated shaped body.

The heating of the base mixture for the separation of the volatile radionuclides is preferably carried out in the same device as the compression, so that no further Handling of graphite is required. As a result, the method according to the invention is even more cost-effective and faster to carry out.

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

The molded article may preferably be disposed of and stored under reduced safety requirements as compared to storage of contaminated graphite which has not undergone decontamination according to the invention. Depending on the safety requirements for final disposal, safe and near-surface final disposal and / or even safe disposal of the shaped body produced according to the invention on the surface are permitted. As a result, the volume of such material, which requires a particularly complex and thus particularly cost-intensive disposal and storage, in particular an 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 occurrence of high amounts of contaminated, in particular irradiated graphite. In addition, the cost of disposal of contaminated graphite can be significantly reduced.

"Contaminated graphite" is a graphite containing proportions of radionuclides Preferably, contaminated graphite is a graphite having an activity of> 10 ³ Bq / g, in particular> 10 ⁴ Bq / g or even> 10 ⁵ Bq / g Thus, according to the invention, the "contaminated graphite" is 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 be due to contamination in the graphite, for example, if the graphite is part of fuel assemblies, converted. However, the content of radionuclides can also be caused by neutron activations during the irradiation of the graphite or impurities in the graphite. The term "contaminated graphite" thus also includes an "irradiated graphite" according to the invention, which has radionuclides as a result of the irradiation. Common radionuclides which may be present in contaminated graphite include H-3, C-14, CI-36, Co-60, Cs-135, Cs-137, 1-131, 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 inventive method 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 according to DIN 1343 (publication date 1990-01) or when the contaminated graphite is heated to at least 350 ° C. and at most 1600 ° C. under a pressure of less than 15 MPa, preferably less than 10 MPa, more preferably less than 5 MPa, in the gaseous state or in the form of gaseous chemical compounds or under the conditions mentioned in the gaseous state or gaseous compounds can be converted. Gaseous compounds of radionuclides are in particular those of radionuclide in elemental form and / or in the form of oxides or halides of the radionuclide. In any case, volatile radionuclides are H-3, C-14, CI-36, 1-131, 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, CI-36, 1-131, Cs-135 and Cs-137. It may contaminate one of the mentioned volatile radionuclides

Graphite present. It is also conceivable that mixtures comprising at least two or more of said volatile radionuclides present in the contaminated graphite.

In particular, radionuclides selected from H-3, C-14 and CI-36 can be separated particularly advantageously from the contaminated graphite with the method according to the invention. The process according to the invention is therefore particularly suitable for

Decontaminating a contaminated graphite comprising at least one volatile radionuclide selected from the group consisting of H-3, C-14, CI-36 and mixtures thereof.

Preferably, the contaminated graphite is 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 especially> 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 B / g The method according to the invention is particularly suitable for contaminated graphite having relatively medium or high total activities of volatile radionuclides allows separation of the volatile radionuclides, thereby a particularly effective and cost-saving disposal of the contaminated graphite is possible. In particular, in embodiments in which this radionuclide is contained in the contaminated graphite, the activity of CI-36 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 aforementioned preferred minimum activities 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 may include other non-volatile radionuclides. 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 lead radioactive isotopes and mixtures thereof. The list is exemplary and not exhaustive. Any other radionuclides may be present in the contaminated graphite in addition to the at least one volatile radionuclide, which are not explicitly mentioned here.

The contaminated graphite may contain, in addition to graphite and the at least one volatile radionuclide, other ingredients added to the graphite, depending on its use, or contained as impurities. The contaminated graphite is preferably derived from fuel element 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 of research equipment 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 may comprise further components besides the contaminated graphite and glass Optionally, at least one oxidizing agent may be present The base mixture is preferably obtainable by mixing the constituents contained therein, in particular the contaminated graphite and the glass and the oxidizing agent Preferably, the base mixture is a homogeneous mixture, ie the constituents are uniform in the base mixture The person skilled in the art is familiar with suitable methods of mixing Powder form, wherein the average particle diameter of the constituents contained therein are preferably less than 100 μηι. If in this invention of a mean grain diameter is mentioned, so it always means the Ferret diameter.

The term "treated graphite" refers to the product obtained by heating the base mixture according to the invention.

The "treated graphite" comprises the constituents of the base mixture, but preferably has a markedly reduced content of volatile radionuclides. According to the invention, the treated graphite is further processed by compacting to give a shaped body which is suitable for disposal.

The treated graphite is thus preferably one which has a markedly reduced content of volatile radionuclides. A "significantly reduced content" of volatile radionuclides is present in accordance with the invention if the content of at least one volatile radionuclide of the volatile radionuclides contained in the contaminated graphite in the treated graphite is at least 60%, preferably at least 70%, more preferably at least 80% and even more preferably reduced by at least 90% based on the amount of volatile radionuclide in the contaminated graphite.

Most 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 by methods known to those skilled in the art. The person skilled in the art is able to choose a suitable detection option, depending on the radionuclide. In particular, stand the

Liquid scintillation spectrometry, the total alpha-beta activity measurement,

Mass spectrometry, a Neutronenaktivierungsanalyse and optionally a radiochemical separation as a method for the selective and quantitative determination of volatile radionuclides available.

If the contaminated graphite H-3, the treated graphite is preferably such that at most only 25%, more preferably at most only 15% and more preferably less than 5% and most preferably less than 2% H-3 based on the amount of H-3 in the contaminated graphite. When the contaminated graphite comprises C-14, preferably less than 65%, more preferably less than 55%, and even more preferably less than 50% of C-14 in the treated graphite is present in the contaminated graphite, based on the amount of C-14. If the contaminated graphite comprises CI-36, it is preferred that the treated graphite contain less than 80%, more preferably less than 60% and even more preferably less than 50% of CI-36, based on the amount of CI-36 in the contaminated graphite.

If H-3 is present in the contaminated graphite, the treated graphite is preferably one in which the activity at H-3 is <10 ³ Bq / g, more preferably <10 ² Bq / g, and most preferably H-3 no longer detectable in the treated graphite with conventional detection methods. If C-14 is present in the contaminated graphite, 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 CI-36, the activity on CI-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 may be a material which is no longer radioactive, that is to say a free, or only weakly active one. This also applies to the molding, which is obtained according to the invention by compacting the base mixture. Depending on the country-specific classification, the shaped body according to the invention can therefore be a material which is no longer radioactive, that is to say a free, or only weakly active material. In particular, the shaped body preferably has a markedly reduced content of volatile radionuclides.

According to the invention, the heating of the base mixture to separate the volatile radionuclides from the contaminated graphite, preferably the volatile radionuclides are separated during heating of the base mixture of contaminated graphite. The radionuclides are preferably "separated" from the contaminated graphite when a treated graphite is obtained which has a markedly reduced content of volatile radionuclides This is ensured, in particular, by the composition according to the invention of the base mixture and the process procedure according to the invention The separation of the volatile radionuclides can be carried out by They contribute to the liberation of volatile radionuclides from the contaminated graphite due to their oxidising effect, in particular, such substances may contribute to the opening of closed pores in which they are trapped Volatile radionuclides are and / or trigger the implementation of chemically bonded radionuclides under the process conditions to gaseous compounds.

In preferred embodiments, the use of oxidizing agents is dispensed with, so that no oxidizing agents are added to the base mixture. Surprisingly, the glass in the base mixture already has an optimum oxidative effect, so that the process according to the invention can be made even more cost-effective and simpler. In alternative embodiments, in which oxidizing agents are added to the base mixture, the content of these substances should preferably be at most 8 wt.% And more preferably at most 5 wt.%, More preferably at most 2 wt Do not exceed the total weight of the base mix. If an excessive amount of oxidizing agents is used, the material of the equipment used is attacked, which reduces the life of the equipment. Preferably used oxidizing agents are organic peroxides.

The contaminated graphite is preferably present in the base mixture as graphite powder, preferably, the contaminated graphite has a mean grain diameter of less than 100 μηι, more preferably at most 50 μηι and more preferably at most 30 μηι. If the contaminated graphite is not already present in such grain diameters, the contaminated graphite is comminuted before heating. The person skilled in the art is well aware of methods for comminution. The smaller the grain diameter of the graphite powder, the higher the 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, therefore, comminution of the contaminated graphite takes place before heating up.

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 of the molding obtainable by compaction. Glass has the advantage that during the heating of the base mixture no gaseous cracking products are formed, which could lead to pore formation in the treated graphite. This means that the glass goes through little or no conversion processes. Also, due to the process of the invention pore formation is thus effectively prevented. The glass wets in the softened or molten state, the contaminated graphite and optionally the other constituents of the base mixture, so that the cavities between the Particles can be closed by capillary or adhesion forces and a dense and almost pore-free shaped body can be obtained after compression of the base mixture, which is sufficiently stable for further processing.

The process according to the invention makes it possible to produce a shaped body which is preferably essentially free from pores, namely a density of preferably at least 90%, more preferably at least 95%, even more preferably at least 98%, even more preferably in the range of> 99%. and most 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 penetration into the shaped body is further reduced and any non-volatile radionuclides are particularly effectively included in the contaminated graphite. This can even better prevent leakage of these radionuclides into an optional matrix material into which the shaped body can be embedded. The shaped body further preferably has a 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, Alumophosphatgläsern, lead glasses, phosphate glasses, alkali glasses, alkaline earth glasses and mixtures thereof. More preferably, the glass of the base mixture is selected from borosilicate glasses, aluminophosphate glasses, lead glasses and mixtures thereof. Most preferably, the glass of the base mixture is 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, for example to water and many chemicals is explained by the boron content of the glasses. The temperature resistance and insensitivity of

Borosilicate glass to sudden temperature changes are a result of low thermal expansion coefficient of about 3,3x10 ^{"6 K" 1} of borosilicate. Common borosilicate glasses for the registration are, for example, Jenaer Glas, Duran®, Pyrex®, llmabor®, Simax®, Solidex® and Fiolax®. 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% Si0 ₂

7% to 13% B ₂ 0 ₃ 4% to 8% alkali oxides, such as Na ₂ 0 or K ₂ 0

2% to 7% (Al ₂ 0)

0% to 5% alkaline earth oxides, such as CaQ, qQ.

The advantage of aluminophosphate glasses is the high radiation resistance and resistance to high temperatures and water.

Lead glasses are again 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 in the heating of the base mixture, so that the process can be made overall cost and energy saving.

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

Alkaline earth glasses in turn have an increased acid stability, can be easily processed 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 effect and structure effect can be achieved. The average particle diameter of the glass powder is preferably less than 100 μm, more preferably not more than 50 μm, and particularly preferably not more than 30 μm. The smaller the grain diameter, the easier it is for the glass to close any pores between the other constituents of the base mix. It is advantageous if the base mixture contains at least 5% by weight of glass, more preferably at least 7% by weight, even more preferably at least 10% by weight and particularly preferably at least 12% by weight of glass Total amount of the base mixture contained in the base mixture. 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 a space-saving end storage of the graphite, since less contaminated graphite is effectively processed per area. Thus, while sufficient, but as little as possible of glass in the base mixture should be used to supply as much contaminated graphite to the inventive method.

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

The heating of the base mixture preferably initially comprises heating to at least one intermediate temperature which is below the target temperature, before it is heated to the target temperature. Preferably, therefore, the heating of the base mixture runs on the Target temperature at least two-phase. According to the invention, the term "heating phase" refers to specific heating up to a specific setpoint temperature, which can then be maintained for a predetermined time, preferably at least 5 minutes, more preferably at least 10 minutes The second heating phase comprises the further heating from the intermediate temperature to reach the "target temperature." Such a temperature control has proved to be particularly advantageous and has enabled a particularly effective separation of volatile radionuclides and an overall cost-effective and rapid process design Particularly preferably, the content of volatile radionuclides is already markedly reduced already in the first heating phase, so that graphite treated after the first heating phase can be obtained, and the second heating phase is then used for separation Any remaining volatile radionuclides while 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 can not be sufficiently removed in the first heating phase. The intermediate temperature is more preferably between 400 ° C and 500 ° C, more preferably between 420 ° C and 480 ° C, especially at 450 ° C ± 20 ° C.

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

If a two-phase heating takes place, which is particularly preferred according to the invention, the pressure during the first heating phase is preferably below 5 MPa, more preferably below 3 MPa, even more preferably below 2 MPa and more preferably below 0.5 MPa and even more preferably below 0 , 2 MPa and most preferably at atmospheric pressure, ie about 0.101325 MPa +/- 20%. The heating to the intermediate temperature is carried out according to the invention preferably 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. Most preferably, the pressure in the second heating phase is 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 Compression has proven to be particularly advantageous to separate any remaining volatile radionuclides while optimally softening the glass component.

If an excessively high pressure is exerted during the heating of the base mixture, ie heating and compacting occur at the same time, there is the risk that the volatile radionuclides will accumulate in the center of the base mixture as a result of external heating and simultaneous pressurization and the volatile radionuclides will not can be separated from the contaminated graphite. Such heating from the outside with simultaneous increased pressure corresponds to the usual procedure for the preparation of an IGG matrix as described in WO 201 1/1 17354 A1. Due to the significant content of volatile radionuclides, a resulting container can not be stored under reduced safety requirements, in particular not close to the surface. It is self-evident that compaction does not occur according to the invention prior to heating of the base mixture. Compressing before heating can also make the separation of the volatile radionuclides considerably more difficult 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 facilitates the separation of volatile radionuclides from the contaminated graphite. The heating rate during heating should not be too high, so preferably below

300 ° C / min, more preferably below 100 ° C / min. At too high heating rates, the process is too expensive and too expensive. Heating rates between 15 ° C / min and 20 ° C / min have proven particularly advantageous, especially in the second heating phase.

Heating, ie heating until reaching a target temperature of preferably at least 650 ° C. and preferably at most 1600 ° C., preferably lasts for at least 5 minutes, more preferably at least 10 minutes and more preferably at least 12 minutes, and even more more preferably at least 18 minutes, and more preferably at least 25 minutes. If heating is too fast, ie in too short a time, there is a risk that the volatile radionuclides can not be separated sufficiently from the contaminated graphite. However, it is preferably heated for a maximum of 60 hours, preferably over a maximum of 50 hours and even more preferably over a maximum of 24 hours, more preferably over a maximum of 10 hours. If the heating takes place for too long a period, there is the danger 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 at least 10 minutes, and most preferably at least 12 minutes. If such a target temperature is maintained for too short a time, there may be a risk that possibly still remaining volatile radionuclides will not be sufficiently separated from the contaminated graphite. The target temperature is preferably maintained for a maximum of 15 hours, more preferably for a maximum of 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 may 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 the danger of an insufficient separation of the volatile radionuclides, since according to the invention a significant reduction of the volatile radionuclides can already be achieved in the first heating phase.

The glass viscosity during heating to the target temperature, preferably in the second heating phase, is preferably <10 ⁵ dPa ^χ s, more preferably <10 ⁵ dPa ^χ s. If the viscosity of the glass during heating is too high, the glass can not penetrate sufficiently between the pores of the further constituents of the base mixture, so that it is generally not possible to obtain a sufficiently dense and hard shaped article.

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

The heating is particularly preferably carried out in vacuo, wherein the residual gas pressure is preferably <10 ^{~ 3} MPa, more preferably <10 ^{~ 4} MPa. The heating may be accomplished by the application of heat, current, microwaves or other methods of heating a material. According to the invention, the heating preferably takes place in such a way that a temperature gradient is achieved between innermost regions of the base mixture and regions of the base mixture close to the edge. In the innermost regions of the base mixture are higher temperatures than in near-edge areas of the base mixture, which according to the invention A negative temperature gradient is ensured according to the invention in particular by the selection of a suitable heating rate and duration of heating and / or the duration of the target temperature and the preferred intermediate temperature. An inventive negative temperature gradient leads to transport processes of the volatile radionuclides such that a separation of the volatile radionuclides is even better possible.

According to the invention, a negative temperature gradient in the base mixture is present when the smallest measured temperature difference (ΔΤ) between a center measurement point and at least 2 outer measurement points, preferably at least 3 outer measurement points, along a horizontal plane within the base mixture is such that the temperature at the center measurement point is greater than 5 ° C, more preferably more than 10 ° C, and more preferably more than 20 ° C and more preferably more than 50 ° C higher than the temperature at the outer measuring points. But this temperature difference should not be too high, since the process is then too costly and expensive. ΔΤ should therefore be at most 300 ° C, more preferably at most 200 ° C. The horizontal plane within the base mixture is chosen so that it divides the base mixture horizontally into two equal halves based on the volume of base mixture. The center measurement point and the outer measurement points lie along this horizontal plane.

Here, the "center measurement point" is at the location of the horizontal plane where the horizontal plane is intersected by a vertical plane dividing the base mixture vertically into two equally sized halves relative to the volume of base mixture on the horizontal plane such that the smallest distance between the center measuring point and each of the outer measuring points is at least 60%, preferably at least 70% and more preferably at least 80% of the length of a straight line from the center measuring point to the edge of the base mixture, the straight line being so in that it intersects the outside measuring point and the center measuring point and runs from edge to edge of the base mixture, thereby ensuring 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 largest distance between each outside measuring point and the center measuring point is chosen such that the distance is at most 95% and preferably at most 90% of the length of the straight line from the center measuring point to the edge of the base mixture. This will ensure that the outside measurement points are not too close to the edge of the base mix. Thus, the temperature profile in the base mixture can be ideally represented.

The heating of the base mixture according to the invention is followed by a densification of the treated graphite, i. exerting increased pressure. This means according to the invention the exertion of a pressing pressure of preferably at least 20 MPa. Thus, a particularly stable and dense treated graphite can be achieved, which can be easily further processed in the process according to the invention. Preferably, the compression is carried out at elevated temperature, preferably at the target temperature, ie 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 compacting pressure during densification 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 is too expensive and too expensive. However, the compacting pressure should be at least 20 MPa, preferably at least 30 MPa, and still more preferably at least 50 MPa, and more preferably at least 60 MPa. If the pressing pressure was in this range, a particularly advantageous compaction of the treated graphite was found. Preferably, the compression takes place under protective gas. Alternatively, the compression is carried out under vacuum, wherein the residual gas pressure is preferably <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 plant (SPS). Preferably, the heating of the base mixture already takes place in one of the said systems, preferably in the same system as the compression. The pressing force in the SPS is preferably between 80 kN and 500 kN, more preferably between 90 kN and 300 kN, to ensure sufficient compaction. The residual gas pressure in the SPS is according to the invention preferably at most 10 ^{~ 3} MPa, more preferably the residual gas pressure is below 10 ^{~ 3} MPa. Preferably, the treated graphite filled in an axial mold. Preferably, the heating of the base mixture according to the invention already takes place in the mold before. In this case, the treated graphite is already in the axial mold.

The heating of the base mixture can be carried out in this plant by applying a current, in particular a direct current, with current strengths in the range of 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 for heating the base mixture. For compacting, a pressure of from 50 MPa to 250 MPa is preferably applied under protective gas or in vacuo. The method enables the production of a molded body with high density even at low process times.

In another embodiment, hot isostatic pressing is used for compaction. For this purpose, the treated graphite is poured into a container. Preferably, the heating of the base mixture takes place 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. Too long a maintenance of the pressing pressure makes the process on the whole too expensive and expensive. The compacting according to the invention preferably also comprises the cooling of the shaped article obtained. Preferably, first a first cooling of the shaped body 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 minute, more preferably 2 minutes. The period is a maximum of 120 minutes, more preferably a maximum of 60 minutes. A period of time for the first cooling of 5 minutes has proven to be particularly suitable. The glass viscosity should be at least 10 ⁶ dPa ^χ s after this first cooling, preferably> 10 ⁶ dPa ^χ s. Preference is given to 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 incorporation of the shaped body in other materials or metal containers, not is required for a safe disposal capability. Rather, the shaped body produced according to the invention is suitable for disposal, ie preferably for safe storage over geological periods ideally up to 1 million years or longer. The shaped body can also be additionally embedded in a matrix material. In embodiments of the method according to the invention, the shaped body is therefore embedded in a matrix material. This makes it possible to improve the final storage capacity of the molding even further and to include the treated graphite even safer. In particular, such embedding of the shaped body imparts additional radiation and corrosion stability. The shaped body can be embedded in the matrix material without further intermediate steps not mentioned here, such as further working or processing. In particular, according to the invention, it is not necessary for the shaped body to be introduced into an additional metal shell, for example as a diffusion barrier, before it is embedded in the matrix material. By contrast, the shaped 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 control. Also, a metal shell provides only temporary sufficient diffusion protection due to possible corrosion and cracking during prolonged storage. With the method according to the invention, diffusion of radionuclides from the contaminated graphite into the matrix material is already sufficiently prevented or reduced by the composition according to the invention of the base mixture and the method according to the invention, in particular the heating of the base mixture for separating volatile radionuclides from the contaminated graphite. Therefore, an additional introduction of the shaped body into a metallic shell prior to embedding in the matrix material according to the invention is not required. According to the invention, "embedding" means that the shaped body is enclosed by the matrix material; in accordance with the invention this is referred to as a "coated shaped body". The shaped body of the matrix material is enclosed 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.

The matrix material comprises according to the invention 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 in the art.

Preferably, the inorganic binder is selected from glasses, it is in this case a so-called impermeable graphite glass matrix, IGG short. Glass, as an inorganic binder, has the advantage that no gaseous cracking products are formed which lead to pore formation in the matrix material. In addition, in the softened or molten state, it wets the remaining matrix constituents and the voids 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

Borosilicate glasses, Alumophosphatgläsern, lead glasses, phosphate glasses, alkali glasses, alkaline earth glasses and mixtures thereof. The person skilled in the art will select a suitable glass according to his specialist knowledge. Particularly preferably, the glass is selected from

Borosilicate glasses, aluminophosphate glasses, lead glasses and mixtures thereof. Most preferably, the glass is a borosilicate glass due to the high corrosion resistance and high chemical and temperature resistance.

The graphite content of 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

Wt .-%. Preferably, a maximum of 40% by weight of inorganic binder is contained in the matrix material.

The graphite in the matrix material is an uncontaminated graphite, so radionuclides are therefore preferably undetectable and / or the graphite has only 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 content of the matrix mixture to 60 wt .-% to 100 wt .-% of natural graphite and 0 wt .-% to 40 wt .-% consists of synthetic graphite. The synthetic graphite may also be referred to as graphitized electrographite powder. The natural graphite has the advantage that it is inexpensive, the graphite grain in contrast to synthetic graphite has no nanorises and can be pressed at moderate pressure to give moldings with almost theoretical density. 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 particle diameter of the glass powder is preferably less than 100 μm, more preferably not more than 50 μm, and particularly preferably not more than 30 μm. The smaller the grain diameter, the easier it is for the glass to close any pores between matrix components. The graphite powder of the matrix material preferably also has the mean grain diameters mentioned.

The preparation of the matrix material is also known in principle. The preparation of the matrix material involves mixing the matrix components in powder form to obtain a pressed powder. The press powder may comprise adjuvants in amounts of a few percent, based on the total amount. These are, for example, pressing aids which may comprise alcohols. Preferably, a granulate is produced from the pressed powder. For granule preparation, the starting components, in particular the two components graphite and glass powder, mixed together, then compacted and by subsequent crushing and sieving a granulate with a grain size of less than 3.14 mm and greater than 0.31 mm is made.

The embedding of the shaped body according to the invention in the matrix material is preferably carried out by:

• joining at least one shaped body with the matrix material to a compact, wherein the matrix material is preferably in the form of a so-called "body" with cavities, and

Final pressing of the compact to obtain a coated molding. The final pressing is preferably carried out by dynamic pressing or hot pressing, preferably under vacuum. In this case, a pressing pressure of preferably between 80 MPa and 300 MPa can be used. The final pressing may further include heating to temperatures between 800 ° C and 1400 ° C.

In preferred embodiments, the embedding of the shaped body according to the invention in the matrix material takes place 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 which can assume various configurations, preferably a hexagonal prism , and the one or comprises a plurality of cavities for receiving the / the shaped body (s). The shaped bodies are preferably filled into the cavities. The cavity openings are preferably filled with matrix material before the final pressing or covered with matrix material in the form of a further basic body 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 by final pressing to form a coated shaped body.

In embodiments in which the matrix material is already present as a base body with cavities, a handle-resistant basic body with cavities, ie recesses for receiving the shaped bodies, is first pre-pressed. The pre-pressing takes place, for example, with a four-column press with three hydraulic drives. For the production of recesses serve according to the invention preferably shaped rods, which are composed of two parts: A forming rod member with a larger diameter, which is inserted on a thinner support rod. The matrix material described herein is capable of serving as a corrosion barrier over an ultralong period of time. In particular, the matrix material is essentially free of pores, 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 moisture can not penetrate into the sheathed body. This is ensured on the one hand by the material selection and on the other hand by the manufacturing process. In conjunction with the treated graphite according to the invention, the coated molding can be safely stored for an ultralong period of time.

With the method according to the invention for the decontamination of contaminated graphite, a safe and ultralong near-surface disposal of the graphite or a final disposal on the surface is permitted depending on the country-specific safety requirements. Thus, the present invention enables a volume-saving disposal of high amounts of contaminated graphite.

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

Examples

Example 1: Preparation of a shaped body for disposal The tool consisted of two press cylinders and a hollow cylinder shell. To prevent caking, a graphite foil was introduced into the hollow cylinder. The lower punch was inserted and covered with a bottom 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 prepared by mixing the components, was introduced into the pressing tool. The filled base mixture was covered with a graphite foil. Subsequently, the upper punch was inserted into the tool.

The tool was inserted into a SPS press and pre-pressed to 2 kN with the SPS press punch. First, under 1, 6 MPa pressure an evacuation took place. This step was terminated upon reaching a vacuum according to the invention. This was followed by an inventive temperature increase up to an intermediate temperature of 450 ° C. Subsequently, the pressing pressure was increased to 8 MPa.

In the second heating phase, the temperature was increased with the method according to the invention ren to a target temperature of 1200 ° C wherein the glass viscosity was <10 ⁵ dPa ^χ 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, wherein ΔΤ was 10 ° C. During this heating, the H-3 was released from the base mixture and reacted in a post-oxidation plant to tritiated water. The treated graphite had a markedly reduced content of volatile radionuclides. The content of H-3 was reduced by 99% in the treated graphite with respect to the amount of H-3 in the contaminated graphite. The treated graphite contained all non-volatiles, e.g. Sr-90 or Co-60.

After reaching the target temperature, the pressing pressure was increased over a period of time to> 64 MPa and the base mixture in the spark plasma sintering plant was compacted into a shaped body with a density of> 98% of the theoretical density. Subsequently, under the increased pressing pressure, cooling of the treated graphite according to the invention took place.

The resulting molded article is suitable for safe disposal over very long periods of time and, in particular, depending on country-specific regulations, can be stored close to the surface or on the surface. Example 2 Embedding of the Shaped Body in a Matrix Material to Obtain a Sheathed Shaped Body

The molded article of Example 1 was embedded in a matrix material of uncontaminated natural graphite and glass. The starting components used were a nuclear grade natural graphite having a grain diameter of less than 30 μηι from Kropfmühl and a borosilicate glass of the same grain size having a melting point of about 1000 ° C. from Schott.

The two components were mixed dry in a weight ratio of natural graphite to glass 5: 1 and pressed into briquettes with the compactor Bepex L 200/50 P from Hosokawa. The briquette density was about 1.9 g / cm ³ . By subsequent crushing and sieving, a granule having a grain size of less than 3.14 mm and greater than 0.31 mm and having a bulk density of about 1 g / cm ^{3 was} prepared. Subsequently, a base body was pre-pressed with cavities for receiving the molding of Example 1.

The molding of Example 1 was filled in the cavities and the

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

Claims

claims 1 . A method of decontaminating contaminated graphite comprising the steps of: a) heating a base mixture comprising contaminated graphite and at least one glass to separate the volatile radionuclides from the contaminated graphite, thereby obtaining a treated graphite; b) compacting the treated graphite to obtain a shaped body which is suitable for disposal. 2. The method of claim 1, wherein the shaped article is embedded in the matrix material to obtain a coated molded article, wherein the matrix material comprises uncontaminated graphite and at least one inorganic binder selected from glasses, aluminosilicates, silicates, borates and mixtures thereof. 3. The method of claim 1 or 2, wherein the base mixture comprises glass in a proportion of 7 wt .-% to 30 wt .-%. 4. The method according to any one of the preceding claims, wherein the glass of the base mixture is a borosilicate glass. 5. The method according to any one of the preceding claims, wherein the contaminated graphite and the glass in the base mixture in each case with a mean grain diameter of less than 100 μηι present. A method according to any one of the preceding claims, wherein the base mixture further comprises oxidizing agent. A method according to any one of the preceding claims, wherein the contaminated graphite comprises at least one radionuclide selected from H-3, CI-36, C-14 or mixtures thereof. 8. The method according to any one of the preceding claims, wherein the heating of the base mixture to target temperatures of at least 800 ° C and at most 1200 ° C takes place. 9. The method according to any one of the preceding claims, wherein the pressing pressure during heating is less than 10 MPa. 10. The method according to any one of the preceding claims, wherein the steps a) and b) are carried out in a hot isostatic press, vacuum hot press or spark plasma sintering plant. 1 1. A method according to any one of the preceding claims, wherein the shaped body is embedded in a matrix material and wherein the inorganic binder is contained in a proportion of 10 to 40% by weight of the matrix mixture and wherein the inorganic binder is a glass.