Method for manufacturing a 14c radionuclide

Abstract

The present invention related to a method for manufacturing at least one carbon-14 [14C, herein after] labeled alkaline earth metal carbonate from at least one neutron-irradiated metallic nitride, wherein the method comprises at least the following successive steps: a) contacting at least one neutron-irradiated metallic nitride with at least one aqueous solution to produce a gaseous stream (G) and an aqueous suspension (S); b) contacting said gaseous stream (G) with an acidic solution to produce at least one 14C-labeled enriched gaseous stream; c) subjecting said at least one 14C-labeled enriched gaseous stream to an oxidation process to produce at least one oxidized gaseous stream; d) treating said at least one oxidized gaseous stream with an alkaline solution comprising at least one alkali metal hydroxide base to produce a 14C-labeled aqueous solution comprising at least one 14C-labeled alkali metal carbonate; and e) treating said 14C-labeled aqueous solution comprising at least one 14C-labeled alkali metal carbonate with at least one alkaline earth metal salt solution, thereby obtaining a precipitated 14C-labeled alkaline earth metal carbonate.

Description

[0001]“Method for manufacturing a14C radionuclide” FIELD OF THE INVENTION The present invention relates to a method for manufacturing a14C radionuclide comprising at least one14CO32-salt from a neutron-irradiated metallic nitride. BACKGROUND OF THE INVENTION Radionuclides or radioisotopes find wide-ranging applications in various fields, including industry, research and development, agriculture and medicine. The manufacturing of specific radionuclides in high yield and isotopic purity, radiolabeled lead compounds, radiation sources and other products based on radionuclides constitute important activities of several national nuclear (research) programs. Among said radionuclides carbon-14 (14C) is by far the most frequently used radionuclide by the pharmaceutical industry. Indeed, at one or more stages during the development process of new active pharmaceutical ingredients (APIs), said APIs are radiolabeled with14C to study metabolic pathways and establish mass balances. The14C-radiolabeled APIs are commonly manufactured via the use of a14C precursor, such as Ba14CO3. Since only very low activities of14C can be safely administered to humans, it is important to avoid the risk of14C contamination originating from other sources and high isotopically pure14C precursor materials need to be obtained. As a consequence, these14C precursor materials are made with high specific activity material. Typically,14C activities in the order of 10 GBq14C per gram are used. In general, the manufacturing of high specific activity14C is carried out by thermal neutron irradiation of nitrogen-14 (14N) rich materials, such as metallic nitrides, in nuclear reactors. The irradiation of metallic nitrides, for instance aluminum nitride, with neutron fluxes allows the (n,p) nuclear transmutation reaction of a14N atom into a14C radionuclide. In a following manufacturing step, after neutron irradiation, the generated14C radionuclide needs to be extracted out of his matrix and subsequently converted to a14C precursor. Particularly, during the extraction step of the generated14C radionuclide, it is important to avoid any contamination with other carbon radioisotopes, such as12C, in order to maintain the isotopic purity of the produced14C radionuclide as high as possible. Various methods for extracting14C from neutron-irradiated metallic nitrides are known in the art, thereby including wet oxidation and dry thermal oxidation. Wet oxidation methods include a wet oxidation of the neutron-irradiated metallic nitride in presence of reactive chemicals, resulting in the oxidation towards gaseous14CO2. Alternatively, dry thermal oxidation methods include dry procedures involving the treatment of the neutron-irradiated metallic nitride using high temperatures, such as 1250 °C, often in the presence of an oxygen rich atmosphere to oxidize the carbon-containing compounds in said metallic nitride.14CO2 is then trapped in alkaline solutions, precipitated as14CO32-, followed by one or more filtration steps, washing steps, and drying steps. As per every method involving the use of radioactive matter, handling said radioactive matter, the risk associated with said methods and the waste management are of major concerns. For instance, with reference to The Journal of Chemical Physics 1947, 15, 6, 374 – 375, P.E. Yankwich describes the production of14C by neutron irradiation of beryllium nitride. Neutron-irradiated beryllium nitride powder is dissolved by warming in a 6 N solution of sodium hydroxide. Next, a saturated potassium permanganate solution is added as an oxidant and a stream of oxygen is started to sweep the gases evolved into a combustion tube. The solution is then brought to a boil and excess 6 N sulfuric acid is added to liberate carbon-14 dioxide. Table 1 on page 375 describes that the formed14C species involve CO, CO2, CH4, CN-, and non-gaseous oxidizable14C species such as CH3OH,and HCOOH. It is further described that in an alternative experiment in which neutron- irradiated beryllium nitride powder is dissolved in 6N sulfuric acid instead of a 6N solution of sodium hydroxide, no significant departure from the activity distribution in Table 1 is detected. Similar observations are disclosed by P.E. Yankwich et al. in J. Am. Chem. Soc.1956, 78, 8, 1560 – 1562 upon dissolving neutron-irradiated beryllium nitride crystals in 2 N sulfuric acid and 12 N sulfuric acid, respectively. Starting from neutron-irradiated aluminum nitride, the disclosure of Zlokazov, S. B. et al. concerns a direct wet oxidation process using potassium dichromate oxidizing agent. The gas mixture formed in oxidation of the neutron- irradiated aluminum nitride is subsequently supplied into an absorber with a nearly saturated potassium permanganate solution, i.e. to remove nitrogen oxides (NOx), acidified with sulfuric acid. Then, the gas mixture is bubbled through four tandem absorbers with ultrapure recrystallized sodium hydroxide. In following precipitation and filtration steps,14C is isolated from the alkaline solutions in the form of white powdered barium carbonate (Ba14CO3). CN 102849772 A describes a direct wet oxidation process wherein neutron-irradiated aluminum nitride is reacted with an aqueous inorganic acid solution, such as a mixture of sulfuric acid and phosphoric acid, in the presence of a CrO3 oxidizing agent. Then, the resulting14C-containing gaseous phase is transferred to an oxidation furnace containing metal oxides. In this oxidation furnace, an oxidation reaction is initiated in order to generate14CO2. This14CO2 is then absorbed in an alkaline solution to trap the14CO2 as comprised therein. Finally, via the addition of BaCl2 the targeted Ba14CO3 is produced and precipitated. A similar direct wet oxidation process using CrO3 oxidizing agent is described in US 2,532,490 A. The above-described wet oxidation methods known in the art for the extraction of14C from neutron-irradiated starting materials have the disadvantage of requiring the use of hazardous and toxic reactive and / or oxidizing chemicals; such as potassium permanganate, potassium dichromate, chromium trioxide, phosphoric acid, sulfuric acid, and sodium hydroxide; in aqueous solution, thereby finally producing a problematic radioactive liquid waste material, which further imposes significant challenges in terms of environmental radioactive waste management. In addition, since the above-described wet oxidation methods are based on direct bulk oxidation of the neutron-irradiated starting materials, the isotopic purity of the resulting14C radionuclides may be adversely affected by the presence of12C impurities in the starting materials. Furthermore, such bulk oxidations, which require the use of excess amounts of hazardous and toxic reactive and / or oxidizing chemicals, may lead to significant safety and health issues in handling and reacting large batches of neutron-irradiated starting materials. GB 1189639 A is also concerned with a wet procedure for extracting14C from neutron-irradiated metallic nitride, in particular neutron-irradiated magnesium nitride. Neutron-irradiated pulverized magnesium nitride is first hydrolyzed by dissolving said nitride at 0 °C in a dilute 1 N to 5 N sulfuric acid solution. The hydrolysis reaction is subsequently produced by heating. After hydrolysis of the magnesium nitride, a distribution of chemical forms of the14C radioactive carbon is obtained, notably including14CH4,14CHN,14CO2 and14CNNH2.14CH4 is further converted into14CO2 by means of an electrical discharge in the presence of oxygen. The above-described hydrolysis wet procedure as described in GB 1189639 A for the extraction of14C from neutron-irradiated magnesium nitride has the disadvantage of finally producing significant amounts of a liquid mixture of radioactive waste materials, which further imposes significant challenges in terms of environmental radioactive waste management (e.g. liquid sorption followed by incineration). Furthermore, the acidic hydrolysis protocol as described in GB 1189639 A does not allow to successfully extract nearly the total amount of14C as initially present in the neutron-irradiated starting material. With reference to dry thermal oxidation methods, Hata K. et al. describes a dry procedure for the release of14C from neutron-irradiated aluminum nitride (Journal of Nuclear Science and Technology 1973, 10, 89-94). Particularly, a fine powder of neutron-irradiated aluminum nitride is heated at about 1100 °C and oxidized under oxygen stream, during which14CO2 is liberated as a consequence of oxidation and trapped in a caustic solution. A similar dry procedure is described in CN 102849772 A. Since the above-described dry thermal oxidation methods involve the treatment of the neutron-irradiated starting materials using high temperatures in the presence of an oxygen rich atmosphere for the extraction of14C from said neutron-irradiated starting materials, said dry thermal oxidation methods have the disadvantage of being energetically demanding with respect to the requirements in terms of high temperatures in combination with oxygen rich atmospheres in order to allow the oxidation reaction to proceed. Furthermore, such dry bulk oxidations, which require the use of high temperatures in combination with oxygen rich atmospheres, may lead to significant safety issues in handling and reacting large batches of neutron-irradiated starting materials. In addition, since the above-described dry thermal oxidation methods are based on direct bulk oxidation of the neutron-irradiated starting materials, the isotopic purity of the resulting14C radionuclides may be adversely affected by the presence of12C impurities in the starting materials. Accordingly, there is a further need to provide for an improved environment-friendly and safer method for extracting14C radionuclides from neutron-irradiated metallic nitrides, which method allows to lower the amount of radioactive waste and simplifying the management, treatment, and handling of the secondary radioactive waste eventually produced, while maintaining a high recovery of the14C radionuclides in isotopically pure form. SUMMARY OF THE INVENTION The inventors have now surprisingly found that it is possible to obtain a method for manufacturing a14C radionuclide comprising at least one14CO32-salt that meets the above-mentioned needs. Therefore, there is now provided a method for manufacturing at least one carbon-14 [14C, herein after] labeled alkaline earth metal carbonate from at least one neutron-irradiated metallic nitride, wherein the method comprises at least the following successive steps: a) contacting at least one neutron-irradiated metallic nitride with at least one aqueous solution to produce a gaseous stream (G) and an aqueous suspension (S), wherein said aqueous solution has a conductivity of less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A; wherein said gaseous stream (G) comprises14C-labeled methane and / or 14C-labeled carbon monoxide and at least one nitrogen-based contaminant; wherein said aqueous suspension (S) comprises at least one14C-labeled carbon-based hydrolysis product; b) contacting said gaseous stream (G) with an acidic solution to produce at least one14C-labeled enriched gaseous stream, said at least one14C- labeled enriched gaseous stream being enriched in the molar quantity of 14C-labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said gaseous stream (G); c) subjecting said at least one14C-labeled enriched gaseous stream to an oxidation process to produce at least one oxidized gaseous stream, said oxidized gaseous stream being depleted in the molar quantity of14C- labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said14C-labeled enriched gaseous stream, and said oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said14C- labeled enriched gaseous stream; d) treating said at least one oxidized gaseous stream with an alkaline solution comprising at least one alkali metal hydroxide base to produce a14C- labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; and e) treating said14C-labeled aqueous solution comprising at least one14C- labeled alkali metal carbonate with at least one alkaline earth metal salt solution, thereby obtaining a precipitated14C-labeled alkaline earth metal carbonate. DETAILED DESCRIPTION OF THE INVENTION The term “comprising”, as used in the claims, should not be interpreted as being limited to the means mentioned thereafter; such a term does not exclude other elements or steps. The term should be read as specifying the presence of the stated features, integers, steps, or components, without, however, precluding the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a method comprising steps A and B” should not be limited to the method consisting only of steps A and B. This means that, with respect to the present invention, the only relevant steps of the method are A and B. Accordingly, the terms “comprising” and “including” encompass the more limiting terms “consisting mainly of” and “consisting of “. As used herein, the terms "optional" or "optionally" means that a subsequently described event or circumstance can or cannot occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. It is understood that the term “quantity” relates to the property of being measurable in terms of amounts. The term “quantity” may denote the mass or weight or moles or any physical or in any case numerical parameter, unless otherwise specified able to indicate at least, at a quantity level, the presence of species. As said above, according to step a) of the method according to the present invention, at least one neutron-irradiated metallic nitride is contacted with at least one aqueous solution, said aqueous solution having a conductivity of less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125- 14, test method A, in order to produce a gaseous stream (G), said gaseous stream (G) comprising14C-labeled methane and / or14C-labeled carbon monoxide and at least one nitrogen-based contaminant, and an aqueous suspension (S), said aqueous suspension (S) comprising14C-labeled methane and / or14C- labeled carbon monoxide and at least one nitrogen-based contaminant, respectively. Within the context of the present invention, the expression “at least one neutron-irradiated metallic nitride” is intended to denote one or more than one neutron-irradiated metallic nitride. Mixtures of neutron-irradiated metallic nitrides can also be used for the purpose of the invention. Within the context of the present invention, the expression “at least one aqueous solution” is intended to denote one or more than one aqueous solution. Mixtures of aqueous solutions can also be used for the purpose of the invention. In the remainder of the text, the expressions “neutron-irradiated metallic nitride”, and “aqueous solution” are understood, for the purposes of the present invention, both in the plural and the singular form. Within the context of the present invention, the neutron-irradiated metallic nitride may be any neutron-irradiated metallic nitride known to the skilled person in the art. Non-limiting examples of suitable neutron-irradiated metallic nitride for use in step a) of the method according to the present invention, as detailed above, notably include neutron-irradiated aluminum nitride (AlN), neutron- irradiated magnesium nitride, neutron-irradiated beryllium nitride, or combination thereof. In an embodiment of step a) of the method according to the present invention, as detailed above, the at least one neutron-irradiated metallic nitride is selected from the group consisting of neutron-irradiated aluminum nitride (AlN), neutron-irradiated magnesium nitride, and neutron-irradiated beryllium nitride. Preferably, the at least one neutron-irradiated metallic nitride is neutron- irradiated aluminum nitride (AlN), in particular with respect to thermal stability and radiation resistance. In an embodiment of step a) of the method according to the present invention, as detailed above, the at least one neutron-irradiated metallic nitride, as detailed above, is preferably provided in the form of a pulverized material, or in the form of a powder. In general, a person skilled in the art is familiar with the neutron irradiation of metallic nitride, thereby allowing the transmutation of the14N, contained in the nitride of the metallic nitride, to14C via a transmutation reaction of the type14N(n,p)14C. Suitable and conventional preparation methods of neutron-irradiated metallic nitrides are generally known in the art. It is understood that the skilled person in the art will carry out said preparation of the neutron- irradiated metallic nitride according to general practice, such as notably using optimal neutron fluxes, temperature, irradiation time, and nuclear reactor type in order to allow the (n,p) nuclear transmutation reaction of a14N atom into a14C radionuclide to effectively and sufficiently proceed meaning that the nuclear reaction14N(n,p)14C at least partially proceeds. For instance, upon irradiating aluminum nitride with predetermined neutron fluxes in a nuclear reactor, the nuclear reaction14N(n,p)14C takes place and the14C nuclear recoiling with a kinetic energy of about 40 keV is trapped finally at a lattice point or an interstitial site of the neutron-irradiated aluminum nitride crystal. As said, according to step a) of the method according to the present invention, the contacting of the at least one neutron-irradiated metallic nitride, as detailed above, with at least one aqueous solution, said aqueous solution having a conductivity of less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A, results in the production of a gaseous stream (G), said gaseous stream (G) comprising14C-labeled methane and / or14C- labeled carbon monoxide and at least one nitrogen-based contaminant. In general, known reaction vessels can be used for carrying out step a) of the method according to the present invention, such as those reaction vessels used in the nuclear domain and known to the person skilled in the art. A non-limiting example of a reaction vessel is for instance notably described in US 2,532,490 A, and its accompanying Fig. 1 described therein, and GB 1189639 A, and its accompanying Fig. 1 described therein. The reaction vessel may be fitted with heating means, mechanical stirring means, a reflux condenser column, and gas introducing means for introducing a gas flow, as detailed below. The inventors have surprisingly found that by contacting in step a) of the method according to the present invention, with the aqueous solution, said aqueous solution having a conductivity of less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A, the neutron-irradiated metallic nitride, as detailed above, is at least partially hydrolyzed and reacted with the aqueous solution. Moreover, upon at least partial hydrolysis of the neutron-irradiated metallic nitride after contacting with the aqueous solution, said metallic nitride undergoes various degradation and decomposition reactions thereby resulting in the effective release of various14C activities upon contact with water and the subsequent formation or production of a corresponding gaseous stream (G), as detailed below, and a corresponding aqueous suspension (S), as detailed below, respectively. Without being bound to this theory, it is inferred that the hydrolysis of the neutron-irradiated metallic nitride allows the opening of the crystal lattice of said neutron-irradiated metallic nitride, thereby making it easier for water to reach places where14C species are present. This observation brings the advantage that a high recovery of the14C radionuclides in isotopically pure form can be successfully obtained. In addition, in contrast to the known wet oxidation methods known in the art for the extraction of14C from neutron-irradiated metallic nitrides and thereby requiring the use of hazardous and toxic reactive and / or oxidizing chemicals in aqueous solution, as detailed above, a less problematic environmental radioactive waste material is produced upon hydrolysis of the neutron-irradiated metallic nitride after contacting with the aqueous solution having a conductivity of less than 10.0 µS / cm, thereby further simplifying the management, treatment, and handling of said secondary radioactive waste. Furthermore, since, in contrast to the known wet oxidation methods known in the art, the use of hazardous and toxic reactive and / or oxidizing chemicals is no longer necessary for effectively reacting and releasing various14C activities from neutron-irradiated metallic nitrides, significant safety and health improvements are obtained, in particular with regard to the handling and reacting of large batches of neutron-irradiated metallic nitrides for extracting14C radionuclides comprised therein. Even more in addition, since, in contrast to the known wet oxidation methods known in the art, the use of hazardous and toxic oxidizing chemicals is no longer necessary for effectively reacting and releasing various14C activities from neutron-irradiated metallic nitrides, no harmful NOx gases are produced thereby improving the environmental friendliness of the method according to the present invention for extracting14C radionuclides from neutron-irradiated metallic nitrides. In step a) of the method according to the present invention, the neutron-irradiated nitride, as detailed above, may be contacted with the aqueous solution, as detailed above, under optimal temperature and time conditions to maximize the yield and the nature of the gaseous stream. In particular, said step a) may be carried out in such a way to produce a gaseous stream (G), as detailed below. According to one embodiment of step a) of the method according to the present invention, step a) is carried out at a temperature equal to or greater than 15 °C, or equal to or greater than 20 °C, or equal to or greater than 25 °C, or equal to or greater than 30 °C, or equal to or greater than 35 °C. The upper limit of the temperature for carrying out step a) of the method according to the present invention is equal to or less than 95 °C, or equal to or less than 90 °C, or equal to or less than 85 °C, or equal to or less than 80 °C, or equal to or less than 75 °C. According to embodiment of step a) of the method according to the present invention, step a) is carried out at a temperature from 15 to 95 °C, or from 20 to 90 °C, or from 25 to 85 °C, or from 30 to 80 °C, or from 35 to 75 °C. When carrying out step a) of the method according to the present invention at a certain temperature, as detailed above, depending on said temperature, the at least one neutron-irradiated metallic nitride, as detailed above, is contacted with at least one aqueous solution, as detailed above, for a given time, such as for instance a time of 1 to 96 hours, or 10 to 72 hours, or 12 to 48 hours, or 16 to 36 hours. By measuring the metallic content, such as the aluminum content, of samples taken at different time intervals by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES), in the art also referred to as inductively coupled plasma optical emission spectroscopy (ICP-OES), the inventors have found that by increasing the temperature for carrying out step a) from 20 °C to 80 °C, the time needed for completing the hydrolysis reaction of the neutron-irradiated metallic nitride, such as neutron- irradiated aluminum nitride, upon contacting with the aqueous solution, and therefore mutatis mutandis the time needed for the effective release of various14C activities and the subsequent formation or production of a corresponding gaseous stream (G), as detailed below, and a corresponding aqueous suspension (S), as detailed below, respectively, can be successfully reduced. According to one embodiment of step a) of the method according to the present invention, the at least one neutron-irradiated metallic nitride, as detailed above, is present in an amount as determined by a weight ratio of the at least one neutron-irradiated metallic nitride to the total weight of the at least one aqueous solution, as detailed below, ranging from 0.01:1.00 to 0.20:1.00, or from 0.01:1.00 to 0.15:1.00, or from 0.01:1.00 to 0.10:1.00, or from 0.01:1.00 to 0.08:1.00, or from 0.01:1.00 to 0.05:1.00. According to one embodiment of step a) of the method according to the present invention, the aqueous solution, as detailed above, has a conductivity of less than 9.5 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A, preferably less than 9.0 µS / cm, preferably less than 8.0 µS / cm, preferably less than 7.0 µS / cm, more preferably less than 6.0 µS / cm. The aqueous solution, as detailed above, for use in step a) of the method according to the present invention may be commercially available, or may be prepared by using any conventional methods known to the skilled in the art. Non-limiting examples of suitable aqueous solutions for use in step a) notably include purified water which may be obtained; optionally in combination with one or more filtration steps selected from microporous filtration, ultrafiltration, or carbon filtering; through one or more purification steps selected from reverse osmosis, electrodialysis, distillation, or deionization. Preferably, the aqueous solution is selected from distilled water or deionized water, more preferably the aqueous solution is deionized water. As said, according to step a) of the method according to the present invention, the contacting of the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above, results in the production of a gaseous stream (G), said gaseous stream (G) comprising14C-labeled methane and / or14C-labeled carbon monoxide and at least one nitrogen-based contaminant, and the production of an aqueous suspension (S), said aqueous suspension (S) comprising at least one14C-labeled carbon-based hydrolysis product. Within the context of the present invention, the expression “at least one nitrogen-based contaminant” is intended to denote one or more than one nitrogen-based contaminant. Mixtures of nitrogen-based contaminants can also be used for the purpose of the invention. Within the context of the present invention, the expression “at least one14C-labeled carbon-based hydrolysis product” is intended to denote one or more than one14C-labeled carbon-based hydrolysis product. Mixtures of14C- labeled carbon-based hydrolysis products can also be used for the purpose of the invention. In the remainder of the text, the expressions “nitrogen-based contaminant”, and “14C-labeled carbon-based hydrolysis product” are understood, for the purposes of the present invention, both in the plural and the singular form. Within the context of step a) of the method according to the present invention, the at least one nitrogen-based contaminant as comprised in the gaseous stream (G) is intended to denote those carbon-free nitrogen-based contaminants, in particular ammonia, which are produced upon contacting of the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above. Upon said contacting, the at least one neutron-irradiated metallic nitride is at least partially hydrolyzed, thereby resulting in the formation or the production of, amongst others, gaseous ammonia as a nitrogen-based contaminant and solid metallic hydroxide. Within the context of step a) of the method according to the present invention, particularly depending on the temperature at which step a) is carried out, it is further understood that the gaseous stream (G) may further comprise, in addition to14C-labeled methane and / or14C-labeled carbon monoxide and at least one nitrogen-based contaminant, as detailed above, other14C-labeled components resulting from the contacting of the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above. Non-limiting examples of such other14C-labeled components notably include14C-labeled carbon dioxide,14C-labeled methanol,14C-labeled formic acid, and14C-labeled hydrogen cyanide. Within the context of step a) of the method according to the present invention, an aqueous suspension (S) is intended to denote an aqueous liquid phase comprising a solid phase comprising undissolved solid particles dispersed in said aqueous liquid phase. It is further understood that said solid phase as comprised in the aqueous suspension (S) may comprise solid metallic hydroxide particles, such as aluminum hydroxide (Al(OH)3) particles and / or solid aluminum oxyhydroxide (AlO(OH)) particles, which are produced upon contacting of the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above. Within the context of step a) of the method according to the present invention, particularly depending on the temperature at which step a) is carried out, the at least one14C-labeled carbon-based hydrolysis product as comprised in the aqueous suspension (S) is intended to denote those14C-labeled components which are produced upon contacting of the at least one neutron- irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above, and which during step a) of the method according to the present invention remain soluble in the aqueous suspension (S) instead of evolving into the gaseous stream (G). Non-limiting examples of14C-labeled carbon-based hydrolysis products notably include14C-labeled carboxylic acids, such as14C-labeled formic acid, and14C-labeled alcohols, such as14C-labeled methanol. It is furthermore understood that the aqueous suspension (S) may further comprise the at least one nitrogen-based contaminant, as detailed above, in particular ammonia, which is also produced upon contacting of the at least one neutron-irradiated metallic nitride with the at least one aqueous solution. Upon said contacting, the neutron-irradiated metallic nitride is at least partially hydrolyzed, thereby resulting in the formation or the production of, amongst others, gaseous ammonia as a nitrogen-based contaminant and solid metallic hydroxide. While part of the produced gaseous ammonia will evolve in the gaseous stream (G) as the nitrogen-based contaminant, the remaining part of the produced gaseous ammonia may dissolve in the aqueous suspension (S), thereby increasing the alkalinity of said suspension (S). According to one preferred embodiment of step a) of the method according to the present invention, during and / or after the step of contacting the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above, said step a) further comprises a step of sparging a carrier gas flow into the aqueous suspension (S), as detailed above, to sweep one or more of the produced14C activities and / or nitrogen-based contaminants from the aqueous suspension (S) into the gaseous stream (G), as detailed above. Non-limiting examples of suitable carrier gas flows notably include an air flow, an oxygen flow, a nitrogen flow, an argon flow, an helium flow or combination thereof. It is further understood that said step of sparging a carrier gas flow into the aqueous suspension (S), as detailed above, advantageously allows the gaseous stream (G) to be further and more efficiently removed from the reaction vessel used for carrying out step a). In addition, said step of sparging a carrier gas flow into the aqueous suspension (S) advantageously allows dissolved ammonia comprised in the aqueous suspension (S) to be at least partially evacuated into the gaseous stream (G), as detailed above, thereby shifting the gas-phase equilibrium and further driving and favouring the underlying hydrolysis reaction of the at least one neutron-irradiated metallic nitride, as detailed above, upon contacting with the at least one aqueous solution, as detailed above, into the various14C activities. According to one preferred embodiment of step a) of the method according to the present invention, during and / or after the step of contacting the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above, said step a) further comprises a step of sparging an air flow into the aqueous suspension (S), as detailed above, to sweep one or more of the produced14C activities and / or nitrogen-based contaminants from the aqueous suspension (S) into the gaseous stream (G), as detailed above. According to a more preferred embodiment of step a) of the method according to the present invention, during and / or after the step of contacting the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above, said step a) further comprises a step of sparging a purified air flow into the aqueous suspension (S), as detailed above, to sweep one or more of the produced14C activities and / or nitrogen-based contaminants from the aqueous suspension (S) into the gaseous stream (G), as detailed above. Within the context of the present invention, the expression “purified air” is intended to denote air which is substantially free of carbon monoxide and carbon dioxide. For the purpose of the present invention, the expression “substantially free of carbon monoxide and carbon dioxide” means that the air comprises equal to or less than 1.0 ppm, or equal to or less than 0.5 ppm, or equal to or less than 0.1 ppm of the sum of carbon monoxide and carbon dioxide. For the purpose of the present invention, the use of a purified air flow as the carrier gas flow advantageously reduces, or even avoids, any contamination with other carbon radioisotopes, such as12C, originating from the carrier gas flow in order to maintain the isotopic purity of the produced14C radionuclide as high as possible. According to another embodiment of step a) of the method according to the present invention, the metallic nitride, prior to neutron irradiation, is advantageously pre-treated by subjecting said metallic nitride to heat treatment in an oxygen flow at a temperature from 800 to 950 °C for 5 to 30 hours in order to reduce, or even avoid, any contamination with12C radioisotopes, as disclosed in RU 2084979 C1. As said above, according to step b) of the method according to the present invention, the gaseous stream (G), as detailed above, is contacted with an acidic solution, to produce at least one14C-labeled enriched gaseous stream being enriched in the molar quantity of the14C-labeled methane and / or14C- labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said gaseous stream (G), respectively. Within the context of the present invention, the expression “at least one14C-labeled enriched gaseous stream” is intended to denote one or more than one14C-labeled enriched gaseous stream. Mixtures of14C-labeled enriched gaseous streams can also be used for the purpose of the invention. In the remainder of the text, the expression “14C-labeled enriched gaseous stream” is understood, for the purposes of the present invention, both in the plural and the singular form. As said above, according to step a) of the method according to the present invention, the gaseous stream (G), as detailed above, comprises at least one nitrogen-based contaminant, in particular ammonia, which is produced upon contacting of the neutron-irradiated metallic nitride, as detailed above, with the aqueous solution, as detailed above. Upon said contacting, the neutron-irradiated metallic nitride is at least partially hydrolyzed, thereby resulting in the formation or the production of, amongst others, gaseous ammonia as a nitrogen-based contaminant and solid metallic hydroxide. Via the contacting of the gaseous stream (G) in step b) with an acidic solution, the ammonia as comprised in said gaseous stream (G) is removed therefrom and is subsequently trapped into said acidic solution as an ammonium species. Via said removal of ammonia from the gaseous stream (G), the14C-labeled enriched gaseous stream is produced, said14C-labeled enriched gaseous stream being enriched in the molar quantity of the14C-labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said gaseous stream (G), respectively. Within the context of the present invention, it is understood that the acidic solution is chosen in such a way that the14C-labeled enriched gaseous stream fulfils the requirement of being enriched in the molar quantity of14C- labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said gaseous stream (G), as detailed above. In other words, the acidic solution is chosen in such a way that said acidic solution is capable of trapping the nitrogen- based contaminant, as detailed above, as comprised in said gaseous stream (G) to produce the14C-labeled enriched gaseous stream depleted in the molar quantity of the nitrogen-based contaminant. Non-limiting examples of suitable acidic solutions notably include aqueous hydrochloric acid solution, aqueous hydrobromic acid solution, aqueous sulfuric acid solution, aqueous phosphoric acid solution, and aqueous nitric acid solution. According to one embodiment of step b) of the method according to the present invention, the acidic solution for contacting the gaseous stream (G), as detailed above, to produce the at least one14C-labeled enriched gaseous stream, as detailed above, is selected from the group consisting of aqueous hydrochloric acid solution, aqueous hydrobromic acid solution, aqueous sulfuric acid solution, aqueous phosphoric acid solution, and aqueous nitric acid solution; preferably selected from aqueous hydrochloric acid solution or aqueous sulfuric acid solution. More preferably, the acidic solution is an aqueous sulfuric acid solution. As for the concentration of the acidic solution, the person skilled in the art can choose said concentration of the acidic solution in such a way that the14C-labeled enriched gaseous stream fulfils the requirement of being enriched in the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in the gaseous stream (G), as detailed above. In other words, the concentration of the acidic solution is, inter alia, dependent on the amount of the neutron-irradiated metallic nitride which is contacted with the aqueous solution and is chosen in such a way that said acidic solution is capable of trapping the nitrogen-based contaminant, as detailed above, as comprised in said gaseous stream (G) to produce the14C-labeled enriched gaseous stream depleted in the molar quantity of the nitrogen-based contaminant. According to one embodiment of step b) of the method according to the present invention, the acid is present in the acidic solution at a concentration from 0.50 to 1.50 mol / L, or from 0.75 to 1.25 mol / L, or from 0.90 to 1.10 mol / L. In general, according to standard practice, it is further understood that the contacting of the gaseous stream (G), as detailed above, with the acidic solution, as detailed above, can be carried out by any suitable method of contacting a gas phase with a liquid phase known to the person skilled in the art. Non-limiting examples notably include sparging the gaseous stream (G) into the acidic solution through use of orifices, porous headers, or spargers. In general, known gas – liquid mixing vessels, such as bubblers and bubble column reactors, can be used for carrying out step b) of the method according to the present invention, such as those gas – liquid mixing vessels used in the nuclear domain and known to the person skilled in the art. A non- limiting example of a gas – liquid mixing vessel is for instance notably described in US 2,532,490 A, and its accompanying Fig.1 described therein. As said above, according to step c) of the method according to the present invention, the at least one14C-labeled enriched gaseous stream, as detailed above, is subjected to an oxidation process to produce at least one oxidized gaseous stream, said oxidized gaseous stream being depleted in the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said14C-labeled enriched gaseous stream, and said oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said14C- labeled enriched gaseous stream. Within the context of the present invention, the expression “at least one oxidized gaseous stream” is intended to denote one or more than one oxidized gaseous stream. Mixtures of oxidized gaseous streams can also be used for the purpose of the invention. In the remainder of the text, the expression “oxidized gaseous stream” is understood, for the purposes of the present invention, both in the plural and the singular form. Within the context of the present invention, the term “oxidation process” is intended to denote a process which allows the oxidation of14C- labeled components, such as14C-labeled methane and14C-labeled carbon monoxide, in the presence of at least one oxidizing agent and at least one catalyst. It is further understood that by oxidizing14C-labeled components, the oxidation state of the14C isotope is increased. By subjecting the14C-labeled enriched gaseous stream, as detailed above, to an oxidation process, the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide as comprised in said14C-labeled enriched gaseous stream is reduced, and hence the molar quantity of14C-labeled carbon dioxide is increased, thereby forming or producing the oxidized gaseous stream. As said above, particularly depending on the temperature at which step a) is carried out, as detailed above, it is further understood that other14C-labeled components which may be comprised in the14C-labeled enriched gaseous stream; such as14C-labeled carbon dioxide,14C-labeled methanol, and14C- labeled formic acid; may also be subjected to the oxidation process and may therefore, i.e. after their oxidation, also contribute to the molar quantity of14C- labeled carbon dioxide as comprised in the resulting oxidized gaseous stream. In particular, step c) of the method according to the present invention notably allows to oxidize all14C species or14C components comprised in the14C-labeled enriched gaseous stream into14C-labeled carbon dioxide. Within the context of the present invention, it is understood that the skilled person in the art will carry out step c) of the method according to the present invention according to general practice in the art. In particular, the skilled person in the art will carry out said oxidation process with respect to selecting any oxidizing agent and catalyst known in the art suitable for carrying out step c) of the method according to the present invention, i.e. oxidizing the14C-labeled enriched gaseous stream, as detailed above, to produce the oxidized gaseous stream, as detailed above. Non-limiting examples of suitable oxidizing agents for carrying out step c) of the method according to the present invention, said step c) comprising the step of subjecting the14C-labeled enriched gaseous stream, as detailed above, to an oxidation process to produce the oxidized gaseous stream, as detailed above, notably include air, in particular purified air, or oxygen gas, in particular purified oxygen gas. Within the context of the present invention, the expression “purified oxygen gas” is intended to denote oxygen gas which is substantially free of carbon monoxide and carbon dioxide. For the purpose of the present invention, the expression “substantially free of carbon monoxide and carbon dioxide” means that the oxygen gas comprises equal to or less than 1.0 ppm, or equal to or less than 0.5 ppm, or equal to or less than 0.1 ppm of the sum of carbon monoxide and carbon dioxide. For the purpose of the present invention, the use of purified air or purified oxygen gas for carrying out step c) of the method according to the present invention, advantageously reduces, or even avoids, any contamination with other carbon radioisotopes, such as12C, originating from the oxidizing agent in order to maintain the isotopic purity of the produced14C radionuclide as high as possible. Non-limiting examples of suitable catalysts for carrying out step c) of the method according to the present invention, said step c) comprising the step of subjecting the14C-labeled enriched gaseous stream, as detailed above, to an oxidation process to produce the oxidized gaseous stream, as detailed above, notably include those catalysts as described Royer, S. et al. in review article ChemCatChem 2011, 3, 24 – 65. Non-limiting examples of suitable catalysts notable include Pt, Pd, Rh, Ir, Ru, Ni, Co, Cu, one or more oxides thereof, and combinations thereof. According to a preferred embodiment of step c) of the method according to the present invention, the catalyst is selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), nickel (Ni), cobalt (Co), copper (Cu), one or more oxides thereof, and combinations thereof; preferably selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), copper (Cu), one or more oxides thereof, and combinations thereof; more preferably selected from the group consisting of platinum (Pt), palladium (Pd), copper (Cu), one or more oxides thereof, and combinations thereof; most preferably the catalyst is platinum (Pt). It is further understood that the catalyst, as detailed above, may be unsupported or supported. When the catalyst is supported, non-limiting examples of suitable supports notably include silica (SiO2); alumina (Al2O3); zirconia (ZrO2); carbon (C); titania (TiO2); magnesia (MgO); ceria (CeO2); and manganese oxides such as MnO2, Mn3O4, and Mn2O3; and combinations thereof. According to one embodiment of step c) of the method according to the present invention, the catalyst, as detailed above, is supported by a support selected from the group consisting of silica (SiO2); alumina (Al2O3); zirconia (ZrO2); carbon (C); titania (TiO2); magnesia (MgO); ceria (CeO2); and manganese oxides such as MnO2, Mn3O4, and Mn2O3; and combinations thereof. More preferably, the support is selected from the group consisting of silica (SiO2); alumina (Al2O3); carbon (C); and combinations thereof. Most preferably, the support is alumina (Al2O3). According to one preferred embodiment of step c) of the method according to the present invention, the oxidizing agent is purified air or purified oxygen gas, and the catalyst is selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), nickel (Ni), cobalt (Co), copper (Cu), one or more oxides thereof, and combinations thereof, wherein the catalyst is optionally supported by a support selected from the group consisting of silica (SiO2); alumina (Al2O3); zirconia (ZrO2); carbon (C); titania (TiO2); magnesia (MgO); ceria (CeO2); and manganese oxides such as MnO2, Mn3O4, and Mn2O3; and combinations thereof. According to another preferred embodiment of step c) of the method according to the present invention, the oxidizing agent is purified air or purified oxygen gas, and the catalyst is selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), copper (Cu), one or more oxides thereof, and combinations thereof, wherein the catalyst is optionally supported by a support selected from the group consisting of silica (SiO2); alumina (Al2O3); carbon (C); and combinations thereof. According to a more preferred embodiment of step c) of the method according to the present invention, the oxidizing agent is purified air or purified oxygen gas, and the catalyst is platinum (Pt), wherein the platinum (Pt) catalyst is supported by alumina (Al2O3). It is further understood that it is generally known in the art that supported catalysts, such as a platinum (Pt) catalyst supported by alumina (Al2O3), are denoted with the term ‘catalyst / support’, such as Pt / Al2O3. In step c) of the method according to the present invention, the14C- labeled enriched gaseous stream, as detailed above, may be subjected to the oxidation process, as detailed above, under optimal temperature, pressure, and time conditions to maximize conversion into the oxidized gaseous stream, as detailed above, said oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said14C-labeled enriched gaseous stream. According to one embodiment of step c) of the method according to the present invention, step c) is carried out at a temperature equal to or greater than 600 °C, preferably equal to or greater than 700 °C, preferably equal to or greater than 800 °C, preferably equal to or greater than 850 °C, more preferably equal to or greater than 900 °C. Advantageously, the upper limit of the temperature for carrying out step c) of the method according to the present invention is equal to or less than 1500 °C, preferably equal to or less than 1300 °C, preferably equal to or less than 1200 °C, preferably equal to or less than 1100 °C, more preferably equal to or less than 1000 °C. According to a preferred embodiment of step c) of the method according to the present invention, step c) is carried out at a temperature from 600 to 1500 °C, preferably from 700 to 1300 °C, preferably from 800 to 1200 °C, preferably from 850 to 1100 °C, more preferably from 900 to 1000 °C. In general, known combustion reactors can be used for carrying out step C) of the method according to the present invention, such as those combustion reactors used in the nuclear domain and known to the person skilled in the art. A non-limiting example of a combustion reactor is for instance the copper oxide type combustion reactor notably described in US 2,532,490 A, and its accompanying Fig. 1 described therein. The combustion reactor may be a single catalyst bed combustion reactor or a multiple catalyst bed combustion reactor, as known by the skilled person in the art, although other catalyst configurations are possible within the scope of step c). As said above, according to step d) of the method according to the present invention, the at least one oxidized gaseous stream, as detailed above, is treated with an alkaline solution comprising at least one alkali metal hydroxide base to produce a14C-labeled aqueous solution comprising at least one14C- labeled alkali metal carbonate. Within the context of the present invention, the expression “at least one alkali metal hydroxide base” is intended to denote one or more than one alkali metal hydroxide base. Mixtures of alkali metal hydroxide bases can also be used for the purpose of the invention. Within the context of the present invention, the expression “at least one14C-labeled alkali metal carbonate” is intended to denote one or more than one14C-labeled alkali metal carbonate. Mixtures of14C-labeled alkali metal carbonates can also be used for the purpose of the invention. In the remainder of the text, the expressions “alkali metal hydroxide base”, and “14C-labeled alkali metal carbonate” are understood, for the purposes of the present invention, both in the plural and the singular form. For the purpose of the present invention, the treatment of the oxidized stream enriched in the molar quantity of14C-labeled carbon dioxide, as detailed above, with an alkaline solution comprising at least one alkali metal hydroxide base allows to efficiently trap said molar quantity of14C-labeled carbon dioxide, thereby producing a14C-labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate. Non-limiting examples of suitable alkali metal hydroxide bases notably include lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), and caesium hydroxide (CsOH). According to a preferred embodiment of step d) of the method according to the present invention, the at least one alkali metal hydroxide base as comprised in the alkaline solution is selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), and caesium hydroxide (CsOH); preferably selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), and potassium hydroxide (KOH). More preferably, the alkali metal hydroxide base is selected from sodium hydroxide (NaOH) or potassium hydroxide (KOH); most preferably, the alkali metal hydroxide base is sodium hydroxide (NaOH). According to a preferred embodiment of step d) of the method according to the present invention, in order to reduce, or even avoid, any contamination with12C radioisotopes, the alkali metal hydroxide base, as detailed above, is recrystallized according to known procedures in the art, for instance following the procedure described in Karyakin, Yu.V. and Angelov, I.I., Chistye khimicheskie veshchestva (Pure Chemical Compounds), Moscow: Khimiya, 1974, p.254, prior to treating the oxidized gaseous stream, as detailed above, with the alkaline solution comprising said alkali metal hydroxide base. As for the concentration of the alkali metal hydroxide base as comprised in the alkaline solution, as detailed above, the skilled person in the art can choose said concentration in such a way that the14C-labeled aqueous solution comprising the14C-labeled alkali metal carbonate is produced. In other words, the concentration of the alkali metal hydroxide base as comprised in the alkaline solution is chosen in such a way the alkaline solution is capable of trapping the molar quantity of14C-labeled carbon dioxide as comprised in the oxidized gaseous stream, as detailed above, to produce the14C-labeled aqueous solution comprising the corresponding14C-labeled alkali metal carbonate. According to a preferred embodiment of step d) of the method according to the present invention, the at least one alkali metal hydroxide base, as detailed above, is present in the alkaline solution at a concentration from 0.2 to 2.0 mol / L, preferably from 0.5 to 1.5 mol / L, more preferably from 0.8 to 1.2 mol / L. In general, according to standard practice, it is further understood that the treatment, i.e. the contacting, of the oxidized gaseous stream, as detailed above, with the alkaline solution, as detailed above, can be carried out by any suitable method of contacting a gas phase with a liquid phase known to the person skilled in the art. Non-limiting examples notably include sparging the oxidized gaseous stream into the alkaline solution through use of orifices, porous headers, or spargers. In general, known gas – liquid mixing vessels, such as bubblers and bubble column reactors, can be used for carrying out step d) of the method according to the present invention, such as those gas – liquid mixing vessels used in the nuclear domain and known to the person skilled in the art. A non- limiting example of a gas – liquid mixing vessel is for instance notably described in US 2,532,490 A, and its accompanying Fig.1 described therein. It is furthermore understood that step d) of the method according to the present invention may be carried out once or multiple times in sequence, for instance using a set-up of more than one reaction vessel for treating the oxidized gaseous stream, as detailed above, with the alkaline solution, as detailed above. When step d) is carried out in sequence, such as by using three reaction vessels in sequence for carrying out said step d), this set-up advantageously allows to more efficiently and quantitatively trap the molar quantity of14C-labeled carbon dioxide as comprised in the oxidized gaseous stream, thereby producing the14C-labeled aqueous solution comprising the14C- labeled alkali metal carbonate. As said above, according to step e) of the method according to the present invention, the14C-labeled aqueous solution comprising the at least one14C-labeled alkali metal carbonate, as detailed above, is treated with at least one alkaline earth metal salt solution, thereby obtaining a precipitated14C-labeled alkaline earth metal carbonate. Within the context of the present invention, the expression “at least one alkaline earth metal salt solution” is intended to denote one or more than one alkaline earth metal salt solution. Mixtures of alkaline earth metal salt solutions can also be used for the purpose of the invention. In the remainder of the text, the expression “alkaline earth metal salt solution” is understood, for the purposes of the present invention, both in the plural and the singular form. For the purpose of the present invention, the treatment of the14C- labeled aqueous solution comprising the14C-labeled alkali metal carbonate, as detailed above, with the alkaline earth metal salt solution allows to efficiently precipitate the14C-labeled alkaline earth metal carbonate. The inventors have surprisingly found that step e) of the method of the present invention allows to produce a precipitated14C-labeled alkaline earth metal carbonate as a14C radionuclide in a high isotopically pure form. Non-limiting examples of suitable alkaline earth metal salts notably include calcium hydroxide (Ca(OH)2), calcium nitrate (Ca(NO3)2), calcium chloride (CaCl2), strontium hydroxide (Sr(OH)2), strontium nitrate (Sr(NO3)2), strontium chloride (SrCl2), barium hydroxide (Ba(OH)2), barium nitrate (Ba(NO3)2), and barium chloride (BaCl2). According to a preferred embodiment of step e) of the method according to the present invention, the alkaline earth metal salt comprised in the alkaline earth metal salt solution, as detailed above, is selected from the group consisting of calcium hydroxide (Ca(OH)2), calcium nitrate (Ca(NO3)2), calcium chloride (CaCl2), barium chloride (BaCl2), barium nitrate (Ba(NO3)2), and barium hydroxide (Ba(OH)2). More preferably, the alkaline earth metal salt is selected from the group consisting of barium chloride (BaCl2), barium nitrate (Ba(NO3)2), and barium hydroxide (Ba(OH)2). Most preferably, the alkaline earth metal salt is barium chloride (BaCl2). In step e) of the method according to the present invention, the14C- labeled aqueous solution comprising the14C-labeled alkali metal carbonate, as detailed above, may be treated with the alkaline earth metal salt solution, as detailed above, under optimal temperature, concentration, the rate of addition of the alkaline earth metal salt solution to the14C-labeled alkali metal carbonate, time allowed for precipitation, ageing of the precipitated14C-labeled alkaline earth metal carbonate, etc. to maximize the yield of the precipitated14C-labeled alkaline earth metal carbonate. As for the concentration of the alkaline earth metal salt, as detailed above, as comprised in the alkaline earth metal salt solution, as detailed above, the skilled person in the art can choose said concentration in such a way that the14C-labeled alkaline earth metal carbonate is precipitated from the14C-labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate, as detailed above. In other words, the concentration of the alkaline earth metal as comprised in the alkaline earth metal salt solution is chosen in such a way the14C-labeled alkaline earth metal carbonate is precipitated from the14C-labeled aqueous solution comprising the14C-labeled alkali metal carbonate. According to a preferred embodiment of step e) of the method according to the present invention, the alkaline earth metal salt, as detailed above, is present in the alkaline earth metal salt solution at a concentration from 0.10 to 1.00 mol / L, preferably from 0.25 to 0.75 mol / L, more preferably from 0.30 to 0.50 mol / L. According to one embodiment of step e) of the method according to the present invention, prior to treatment with the alkaline earth metal salt solution, as detailed above, the14C-labeled aqueous solution comprising the14C-labeled alkali metal carbonate, as detailed above, is first heated to a temperature from 30 to 80 °C, or from 50 to 70 °C. According to another embodiment of step e) of the method according to the present invention, prior to treatment, both the14C-labeled aqueous solution comprising the14C-labeled alkali metal carbonate, as detailed above, and the alkaline earth metal salt solution, as detailed above, are first heated to a temperature from 30 to 80 °C, or from 50 to 70 °C. According to a further embodiment of the method of the present invention, the precipitated14C-labeled alkaline earth metal carbonate as obtained in step e) may be further separated through one or more (vacuum) filtering steps, followed by one or more washing steps, one or more (vacuum) drying steps, and packaging. According to one further embodiment of the method according to the present invention, said method further comprises at least the following successive steps: f) filtering, in particular vacuum filtering, the aqueous suspension (S), as detailed above and as produced in step a), to produce an aqueous filtrate (F) comprising the at least one14C-labeled carbon-based hydrolysis product, as detailed above, and thereby removing at least one metallic hydroxide comprised in said aqueous suspension (S), said at least one metallic hydroxide which is produced in step a) upon contacting the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above; g) vacuum evaporating, optionally combined with heating, the aqueous filtrate (F), as detailed above, to dryness, to produce a second gaseous stream (G’), said second gaseous stream (G’) comprising the at least one 14C-labeled carbon-based hydrolysis product, as detailed above; h) subjecting the second gaseous stream (G’), as detailed above, to an oxidation process to produce a second oxidized gaseous stream, said second oxidized gaseous stream being enriched in the molar quantity of 14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said second gaseous stream (G’); i) treating the second oxidized gaseous stream, as detailed above, with a second alkaline solution comprising at least one alkali metal hydroxide base to produce a second14C-labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; and j) treating the second14C-labeled aqueous solution comprising the at least one14C-labeled alkali metal carbonate with at least one second alkaline earth metal salt solution, thereby obtaining a second precipitated14C- labeled alkaline earth metal carbonate. For instance within the context of step f) of the method according to the present invention, when the neutron-irradiated metallic nitride is neutron- irradiated aluminum nitride, the aluminum hydroxide comprised in the aqueous suspension (S) may be aluminum trihydroxide (Al(OH)3) and / or aluminum oxyhydroxide (AlO(OH)). Within the context of step f) of the method according to the present invention, an aqueous filtrate (F) is intended to denote the liquid phase of the aqueous suspension (S), as detailed above and as produced in step a), after filtration step f), wherein the aqueous filtrate (F) comprises the at least one14C- labeled carbon-based hydrolysis product. Via step f) of filtering the aqueous suspension (S), any metallic hydroxides which are produced in step a) via the at least partial hydrolysis of the neutron-irradiated metallic nitride are removed from said aqueous suspension (S), thereby forming or producing the aqueous filtrate (F). As said before, particularly depending on the temperature at which step a) is carried out, the aqueous suspension (S) comprises at least one14C-labeled carbon-based hydrolysis product, as detailed above, which during step a) remains in the aqueous suspension (S) instead of evolving into the gaseous stream (G). Non- limiting examples of14C-labeled carbon-based hydrolysis products notably include14C-labeled carboxylic acids, such as14C-labeled formic acid, and14C- labeled alcohols, such as14C-labeled methanol. Therefore, after step f) of filtering the aqueous suspension (S), the14C-labeled carbon-based hydrolysis product is comprised in the aqueous filtrate (F). The inventors have surprisingly found that by contacting in step a), as detailed above, with the aqueous solution, said aqueous solution having a conductivity of less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A, the neutron-irradiated metallic nitride, as detailed above, is at least partially hydrolyzed and reacted with the aqueous solution. Upon at least partial hydrolysis of the neutron-irradiated metallic nitride after contacting with the aqueous solution, said metallic nitride undergoes various degradation and decomposition reactions thereby resulting in the effective release of various14C activities upon contact with water and the subsequent formation or production of at least one metallic hydroxide which is substantially insoluble in the aqueous suspension (S), as detailed above, and which may now be easily removed from said aqueous suspension (S) by filtering step f). When the neutron-irradiated metallic nitride is completely hydrolyzed, i.e. thereby resulting in the effective release of all14C activities, the resulting metallic hydroxide will be practically free of radioactive contamination after filtration and subsequent washing, and can therefore be further handled and managed in a simplified manner as a chemically inert low level radioactive solid waste material. In this way, said chemically inert low level radioactive solid waste is already effectively separated from the tritiated liquid radioactive waste material as obtained in step b), as detailed above, for contacting the gaseous stream (G) with the acidic solution to produce the14C-labeled enriched gaseous stream, and in steps d) – e), as detailed above, for treating the oxidized gaseous stream with the alkaline solution to produce the14C-labeled aqueous solution comprising the14C-labeled alkali metal carbonate followed by precipitation of the latter in presence of the alkaline earth metal salt solution. It was found that an amount of tritium was formed or produced during neutron irradiation of the metallic nitride, said amount of tritium which became subsequently trapped in these liquid phases of steps b) and d) – e) when performing the method steps according to the present invention. For the purpose of the present invention, it is further understood that via step g) of vacuum evaporating the aqueous filtrate (F), as detailed above, to dryness, the at least one14C-labeled carbon-based hydrolysis product which is produced in step a) and as comprised in the aqueous filtrate (F), as detailed above, evolves into the second gaseous stream (G’). Within the context of step h) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step c) of the method according to the present invention, as detailed above, equally apply to step h) of the method according to the present invention. Within the context of step i) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step d) of the method according to the present invention, as detailed above, equally apply to step i) of the method according to the present invention. Within the context of step j) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step e) of the method according to the present invention, as detailed above, equally apply to step j) of the method according to the present invention. According to a further embodiment of the method of the present invention, the second precipitated14C-labeled alkaline earth metal carbonate as obtained in step j) may be further separated through one or more (vacuum) filtering steps, followed by one or more washing steps, one or more (vacuum) drying steps, and packaging. Alternatively, according to another embodiment of the method according to the present invention, said method further comprises at least the following successive steps: f') filtering, in particular vacuum filtering, the aqueous suspension (S), as detailed above and as produced in step a), to produce an aqueous filtrate (F) comprising the at least one14C-labeled carbon-based hydrolysis product, as detailed above, and thereby removing at least one metallic hydroxide comprised in said aqueous suspension (S), said at least one metallic hydroxide which is produced in step a) upon contacting the at least one neutron-irradiated metallic nitride, as detailed above, with the at least one aqueous solution, as detailed above; g') partially evaporating the aqueous filtrate (F), as detailed above, to produce a residual aqueous filtrate (F’) and a third gaseous stream (G’’); h') subjecting the third gaseous stream (G’’) to an oxidation process to produce a third oxidized gaseous stream, said third oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said third gaseous stream (G’’); i') treating the third oxidized gaseous stream with a third alkaline solution comprising at least one alkali metal hydroxide base to produce a third14C- labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; j') treating the third14C-labeled aqueous solution comprising at least one14C- labeled alkali metal carbonate with at least one third alkaline earth metal salt solution, thereby obtaining a third precipitated14C-labeled alkaline earth metal carbonate; k') contacting the residual aqueous filtrate (F’) with at least one non-carbon based, inorganic sorbent material to produce a mixture (M) wherein the residual aqueous filtrate (F’) is adsorbed onto said at least one non-carbon based, inorganic sorbent material; l') subjecting the mixture (M) to an oxidation process to produce a fourth oxidized gaseous stream; m') treating the fourth oxidized gaseous stream with a fourth alkaline solution comprising at least one alkali metal hydroxide base to produce a fourth 14C-labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; and n') treating the fourth14C-labeled aqueous solution comprising at least one 14C-labeled alkali metal carbonate with at least one fourth alkaline earth metal salt solution, thereby obtaining a fourth precipitated14C-labeled alkaline earth metal carbonate. Within the context of step g’) of the method according to the present invention, it is further understood that the at least one14C-labeled carbon-based hydrolysis product, as initially comprised in the aqueous filtrate (F), as detailed above, may be comprised in the residual aqueous filtrate (F’), or may be comprised in the third gaseous stream (G’’), or may be comprised in both the residual aqueous filtrate (F’) and the third gaseous stream (G’’). Within the context of step g’) of the method according to the present invention, it is further understood that for partially evaporating the aqueous filtrate (F), as detailed above, said aqueous filtrate (F) may be heated to a temperature from 75 to 100 °C, preferably from 80 to 100 °C. According to one embodiment of step g’) of the method according to the present invention, said step g’) comprises partially evaporating at least 50.0 percent by volume [vol. %, herein after] of the aqueous filtrate (F), as detailed above, to produce the third gaseous stream (G’’), or at least 60.0 vol. %, or at least 70.0 vol. %, or at least 80.0 vol. %, or at least 90.0 vol. %, or at least 95.0 vol. %. In general, known reaction vessels can be used for carrying out step g’) of the method according to the present invention, such as those reaction vessels used in the nuclear domain and known to the person skilled in the art. A non-limiting example of a reaction vessel is for instance notably described in US 2,532,490 A, and its accompanying Fig. 1 described therein, and GB 1189639 A, and its accompanying Fig. 1 described therein. The reaction vessel may be fitted with heating means, mechanical stirring means, a reflux condenser column, and gas introducing means for introducing a gas flow, as detailed below. According to one preferred embodiment of step g’) of the method according to the present invention, said step g’) further comprises a step of sparging a carrier gas flow into the aqueous filtrate (F), as detailed above, to sweep one or more14C activities from the aqueous filtrate (F) into the third gaseous stream (G’’). Within the context of step g’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for the carrier gas flow of step a) of the method according to the present invention, as detailed above, equally apply to step g’) of the method according to the present invention. Within the context of step h’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step c) of the method according to the present invention, as detailed above, equally apply to step h’) of the method according to the present invention. Within the context of step i’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step d) of the method according to the present invention, as detailed above, equally apply to step i’) of the method according to the present invention. Within the context of step j’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step e) of the method according to the present invention, as detailed above, equally apply to step j’) of the method according to the present invention. According to a further embodiment of the method of the present invention, the third precipitated14C-labeled alkaline earth metal carbonate as obtained in step j’) may be further separated through one or more (vacuum) filtering steps, followed by one or more washing steps, one or more (vacuum) drying steps, and packaging. Within the context of the present invention, the expression “at least one non-carbon based, inorganic sorbent material” is intended to denote one or more than one non-carbon based, inorganic sorbent material. Mixtures of non- carbon based, inorganic sorbent materials can also be used for the purpose of the invention. In the remainder of the text, the expression “non-carbon based, inorganic sorbent material” is understood, for the purposes of the present invention, both in the plural and the singular form. Within the context of step k’) of the method according to the present invention, the at least one non-carbon based, inorganic sorbent material is intended to denote those non-carbon based, inorganic sorbent materials which are capable of adsorbing the residual aqueous filtrate (F’) upon contacting, thereby producing the mixture (M). Non-limiting examples of suitable non-carbon based, inorganic sorbent materials for use in step k’) notably include vermiculite, zeolite, kaolinite, montmorillonite, illite, calcite, and lime. According to one embodiment of step k’) of the method according to the present invention, the at least one non-carbon based, inorganic sorbent material is selected from the group consisting of vermiculite, zeolite, kaolinite, montmorillonite, illite, calcite, and lime. Most preferably, the at least one non- carbon based, inorganic sorbent material is vermiculite. As for the amount of the at least one non-carbon based, inorganic sorbent material, as detailed above, for contacting the residual aqueous filtrate (F’) to produce the mixture (M) in step k’) of the method according to the present invention, the skilled person in the art can choose said amount in such a way that the residual aqueous filtrate (F’) is effectively adsorbed onto said at least one non-carbon based, inorganic sorbent material. According to one embodiment of step k’) of the method according to the present invention, upon contacting the residual aqueous filtrate (F’) with the at least one non-carbon based, inorganic sorbent material, as detailed above, to produce the mixture (M) wherein the residual aqueous filtrate (F’) is adsorbed onto said non-carbon based, inorganic sorbent material, the non-carbon based, inorganic sorbent material is present in an amount as determined by a weight ratio of the non-carbon based, inorganic sorbent material to the total weight of the residual aqueous filtrate (F’) ranging from 1:6 to 1:1, or from 1:4 to 1:2. Within the context of step l’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step c) of the method according to the present invention, as detailed above, equally apply to step l’) of the method according to the present invention. Within the context of step m’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step d) of the method according to the present invention, as detailed above, equally apply to step m’) of the method according to the present invention. Within the context of step n’) of the method according to the present invention, it is further understood that each of the definitions, preferences, and preferred embodiments mentioned for step e) of the method according to the present invention, as detailed above, equally apply to step n’) of the method according to the present invention. According to a further embodiment of the method of the present invention, the fourth precipitated14C-labeled alkaline earth metal carbonate as obtained in step n’) may be further separated through one or more (vacuum) filtering steps, followed by one or more washing steps, one or more (vacuum) drying steps, and packaging. Within the context of steps d), i), i’), and m’) of the method according to the present invention, it is further understood that the alkaline solution comprising at least one alkali metal hydroxide base, as detailed above, the second alkaline solution comprising at least one alkali metal hydroxide base, as detailed above, the third alkaline solution comprising at least one alkali metal hydroxide base, as detailed above, and the fourth alkaline solution comprising at least one alkali metal hydroxide base, as detailed above, may be the same or different from each other. Within the context of steps e), j), j’), and n’) of the method according to the present invention, it is further understood that the at least one alkaline earth metal salt solution, as detailed above, the at least one second alkaline earth metal salt solution, as detailed above, the at least one third alkaline earth metal salt solution, as detailed above, and the at least one fourth alkaline earth metal salt solution, as detailed above, may be the same or different from each other. The inventors have now surprisingly found that the above-described method now provides for an improved environment-friendly and safer method for extracting14C radionuclides from neutron-irradiated metallic nitrides, which method allows to lower the amount of radioactive waste and simplifying the management, treatment, and handling of the secondary radioactive waste eventually produced, while maintaining a high recovery of the14C radionuclides in isotopically pure form. EXAMPLES The invention will be now described in more details with reference to the following example, whose purpose is merely illustrative and not intended to limit the scope of invention. Raw materials: − High purity (99.8 %) fresh aluminum nitride (AlN) powder was purchased from MATERION. This fresh AIN powder contained 0.0174 wt. % of residual carbon. For safety reasons, after neutron irradiation, the neutron- irradiated AIN powder (81 MBq14C / g) was down blended with fresh AIN powder to 0.70 MBq14C / g; − Purified air as carrier gas flow (99.999 %): [CO + CO2] < 1.0 ppm-mol; − Low carbonate NaOH (GPR RECTAPUR®; measured value 0.2 % Na2CO3); − Aqueous H2SO4 (analytical grade); and − Platinum (Pt) catalyst supported by alumina (Al2O3), pelletized. − Pyrolyser-6 Trio Furnace System, Raddec International Ltd; and − Liquid scintillation counter, Hidex. General procedure: 20.0 grams of down blended neutron-irradiated AIN powder to 0.70 MBq14C / g was contacted with 400 mL of aqueous solution; said aqueous solution having a conductivity less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A; for a time of 72 hours at 20 °C in a glass Erlenmeyer flask. During this period of 72 hours, the resulting aqueous suspension (S) was constantly stirred while purified air as carrier gas flow (0.3 L / min) was sparged into the aqueous suspension (S) to sweep and remove ammonia as the nitrogen-based contaminant and14C-labeled methane and / or14C-labeled carbon monoxide from the aqueous suspension (S) into the gaseous stream (G). The gaseous stream (G) was subsequently sparged into a glass Erlenmeyer flask containing 400 mL of a 1.0 mol / L aqueous H2SO4 solution in order to remove the ammonia as comprised in the gaseous stream (G) and to trap said ammonia into the aqueous H2SO4 solution as an ammonium species. After such trapping of ammonia in the aqueous H2SO4 solution, the resulting14C- labeled enriched gaseous stream was passed through a small tube furnace (Pyrolyser-6 Trio Furnace System, Raddec International Ltd) and subjected to an oxidation process in presence of purified air as the oxidizing agent. The end section of the small tube furnace was loaded / filled with a platinum (Pt) catalyst supported by alumina (Al2O3), pelletized, and operated at 900 °C. In a following step, the resulting oxidized gaseous stream was treated through a series of three bubblers, the first bubbler containing 400 mL of a 1.0 mol / L aqueous NaOH solution and the second and third bubbler each containing 200 mL of the 1.0 mol / L aqueous NaOH solution, thereby producing a14C-labeled aqueous solution comprising14C-labeled sodium carbonate. After 72 hours of stirring the aqueous suspension (S) at 20 °C in the first glass Erlenmeyer flask, said aqueous suspension (S) was vacuum filtered in order to remove solid aluminum hydroxide particles comprised therein. The resulting filter cake containing said solid aluminum hydroxide particles was washed with a small volume of deionized water. The resulting filtrate of this step of washing the filter cake was subsequently collected and combined with the aqueous filtrate (F). The resulting aqueous filtrate (F) was partially evaporated using an electric heating plate at 100 °C until a 10 mL residual aqueous filtrate (F’) remained in the flask. During this step of partially evaporating the aqueous filtrate (F), purified air as carrier gas flow was sparged into said aqueous filtrate (F) to sweep and remove one or more14C activities from the aqueous filtrate (F) into the third gaseous stream (G’’). The resulting third gaseous stream (G’’) was passed through the small tube furnace (Pyrolyser-6 Trio Furnace System, Raddec International Ltd) and subjected to an oxidation process in presence of purified air as the oxidizing agent. The end section of the small tube furnace was loaded / filled with a platinum (Pt) catalyst supported by alumina (Al2O3), pelletized, and operated at 900 °C. In a following step, the resulting third oxidized gaseous stream was treated through the series of three bubblers, as detailed above, thereby producing a third14C-labeled aqueous solution comprising14C-labeled sodium carbonate. The remaining 10 mL residual aqueous filtrate (F’) was contacted and mixed with vermiculite as the non-carbon based, inorganic sorbent material to produce a mixture (M) wherein the residual aqueous filtrate (F’) is entirely adsorbed onto said vermiculite sorbent material. Then, the mixture (M) was loaded into a quartz glass sample boat and introduced into the small tube furnace (Pyrolyser-6 Trio Furnace System, Raddec International Ltd) and subjected to an oxidation process in presence of purified air as the oxidizing agent (0.3 L / min). The end section of the small tube furnace was loaded / filled with a platinum (Pt) catalyst supported by alumina (Al2O3), pelletized, and operated at 900 °C. In a following step, the resulting fourth oxidized gaseous stream was treated through the series of three bubblers, as detailed above, thereby producing a fourth14C- labeled aqueous solution comprising14C-labeled sodium carbonate. The solution containing the combination of the14C-labeled aqueous solution comprising14C-labeled sodium carbonate, the third14C-labeled aqueous solution comprising14C-labeled sodium carbonate, and the fourth14C-labeled aqueous solution comprising14C-labeled sodium carbonate was left standing for a time of 24 hours at 20 °C. The14C radioactivity of said solution was then measured via liquid scintillation counting as generally known to a person skilled in the art (Liquid scintillation counter, Hidex). In this solution, corrected for the mass losses during e.g. sampling, the initial14C activity (0.70 MBq) was entirely recovered. The solution containing the combination of the14C-labeled aqueous solution, the third14C-labeled aqueous solution, and the fourth14C-labeled aqueous solution was then gently heated to 60 °C, followed by the addition of 25 mL of a 0.3 mol / L aqueous BaCl2 solution, thereby resulting in a precipitated mixture containing the combination of a precipitated14C-labeled barium carbonate, a third precipitated14C-labeled barium carbonate, and a fourth precipitated14C-labeled barium carbonate. The precipitated mixture was vacuum filtered and subsequently washed and rinsed with a few mL of acetone, before being submitting to a drying step for 5 minutes at 105 °C. After cooling to 20 °C, the weight of the precipitate containing the precipitated14C-labeled barium carbonate, the third precipitated14C-labeled barium carbonate, and the fourth precipitated14C-labeled barium carbonate was calculated accordingly (0.6 mg of Ba14CO3).

Claims

CLAIMS 1. A method for manufacturing at least one carbon-14 [14C, herein after] labeled alkaline earth metal carbonate from at least one neutron-irradiated metallic nitride, wherein the method comprises at least the following successive steps: a) contacting at least one neutron-irradiated metallic nitride with at least one aqueous solution to produce a gaseous stream (G) and an aqueous suspension (S), wherein said aqueous solution has a conductivity of less than 10.0 µS / cm, as measured at 25 °C in accordance with ASTM D1125-14, test method A; wherein said gaseous stream (G) comprises14C-labeled methane and / or 14C-labeled carbon monoxide and at least one nitrogen-based contaminant; wherein said aqueous suspension (S) comprises at least one14C-labeled carbon-based hydrolysis product; b) contacting said gaseous stream (G) with an acidic solution to produce at least one14C-labeled enriched gaseous stream, said at least one14C- labeled enriched gaseous stream being enriched in the molar quantity of 14C-labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said gaseous stream (G); c) subjecting said at least one14C-labeled enriched gaseous stream to an oxidation process to produce at least one oxidized gaseous stream, said oxidized gaseous stream being depleted in the molar quantity of14C- labeled methane and / or14C-labeled carbon monoxide relative to the molar quantity of14C-labeled methane and / or14C-labeled carbon monoxide in said14C-labeled enriched gaseous stream, and said oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxiderelative to the molar quantity of14C-labeled carbon dioxide in said14C- labeled enriched gaseous stream; d) treating said at least one oxidized gaseous stream with an alkaline solution comprising at least one alkali metal hydroxide base to produce a14C- labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; and e) treating said14C-labeled aqueous solution comprising at least one14C- labeled alkali metal carbonate with at least one alkaline earth metal salt solution, thereby obtaining a precipitated14C-labeled alkaline earth metal carbonate.

2. The method according to claim 1, wherein the at least one neutron- irradiated metallic nitride in step a) is selected from the group consisting of neutron-irradiated aluminum nitride (AlN), neutron-irradiated magnesium nitride, and neutron-irradiated beryllium nitride.

3. The method according to any one of claims 1 to 2, wherein the at least one neutron-irradiated metallic nitride in step a) is neutron-irradiated aluminum nitride (AlN).

4. The method according to any one of claims 1 to 3, wherein step a) is carried out at a temperature from 15 to 95 °C.

5. The method according to any one of claims 1 to 4, wherein step a) is carried out at a temperature from 20 to 90 °C.

6. The method according to any one of claims 1 to 5, wherein step a) is carried out at a temperature from 25 to 85 °C.

7. The method according to any one of claims 1 to 6, wherein step a) is carried out at a temperature from 30 to 80 °C.

8. The method according to any one of claims 1 to 7, wherein step a) is carried out at a temperature from 35 to 75 °C.

9. The method according to any one of claims 1 to 8, wherein step a) is carried out for a time of 1 to 96 hours.

10. The method according to any one of claims 1 to 9, wherein step a) is carried out for a time of 10 to 72 hours.

11. The method according to any one of claims 1 to 10, wherein step a) is carried out for a time of 12 to 48 hours.

12. The method according to any one of claims 1 to 11, wherein step a) is carried out for a time of 16 to 36 hours.

13. The method according to any one of claims 1 to 12, wherein the aqueous solution in step a) has a conductivity of less than 9.5 µS / cm.

14. The method according to any one of claims 1 to 13, wherein the aqueous solution in step a) has a conductivity of less than 9.0 µS / cm.

15. The method according to any one of claims 1 to 14, wherein the aqueous solution in step a) has a conductivity of less than 8.0 µS / cm.

16. The method according to any one of claims 1 to 15, wherein the aqueous solution in step a) has a conductivity of less than 7.0 µS / cm.

17. The method according to any one of claims 1 to 16, wherein the aqueous solution in step a) has a conductivity of less than 6.0 µS / cm.

18. The method according to any one of claims 1 to 17, wherein step a), during and / or after the step of contacting the at least one neutron- irradiated metallic nitride with the at least one aqueous solution, comprises a step of sparging a carrier gas flow into the aqueous suspension (S).

19. The method according to claim 18, wherein the carrier gas flow is selected from the group consisting of an air flow, an oxygen flow, a nitrogen flow, an argon flow, a helium flow or combination thereof.

20. The method according to any one of claims 1 to 19, wherein the acidic solution in step b) is selected from the group consisting of aqueous hydrochloric acid solution, aqueous hydrobromic acid solution, aqueous sulfuric acid solution, aqueous phosphoric acid solution, and aqueous nitric acid solution.

21. The method according to any one of claims 1 to 20, wherein the acidic solution in step b) is selected from aqueous hydrochloric acid solution or aqueous sulfuric acid solution.

22. The method according to any one of claims 1 to 21, wherein the acidic solution in step b) is an aqueous sulfuric acid solution.

23. The method according to any one of claims 1 to 22, wherein the oxidation process of step c) comprises an oxidizing agent selected from air or oxygen gas, and a catalyst selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), nickel (Ni), cobalt (Co), copper (Cu), one or more oxides thereof, and combinations thereof.

24. The method according to any one of claims 1 to 23, wherein the oxidation process of step c) comprises an oxidizing agent selected from air or oxygen gas, and a catalyst selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), copper (Cu), one or more oxides thereof, and combinations thereof.

25. The method according to any one of claims 1 to 24, wherein the oxidation process of step c) comprises an oxidizing agent selected from air or oxygen gas, and a catalyst selected from the group consisting of platinum (Pt), palladium (Pd), copper (Cu), one or more oxides thereof.

26. The method according to any one of claims 1 to 25, wherein the oxidation process of step c) comprises an oxidizing agent selected from air or oxygen gas, and a catalyst being platinum (Pt).

27. The method according to any one of claims 23 to 26, wherein the catalyst is supported by a support selected from the group consisting of silica (SiO2); alumina (Al2O3); zirconia (ZrO2); carbon (C); titania (TiO2); magnesia (MgO); ceria (CeO2); and manganese oxides such as MnO2, Mn3O4, and Mn2O3; and combinations thereof.

28. The method according to any one of claims 23 to 27, wherein the catalyst is supported by a support selected from the group consisting of silica (SiO2); alumina (Al2O3); carbon (C); and combinations thereof.

29. The method according to any one of claims 23 to 28, wherein the catalyst is supported by a support being alumina (Al2O3).

30. The method according to any one of claims 1 to 29, wherein the at least one alkali metal hydroxide base as comprised in the alkaline solution in step d) is selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), and caesium hydroxide (CsOH).

31. The method according to any one of claims 1 to 30, wherein the at least one alkali metal hydroxide base as comprised in the alkaline solution in step d) is selected from the group consisting of lithium hydroxide (LiOH), sodium hydroxide (NaOH), and potassium hydroxide (KOH).

32. The method according to any one of claims 1 to 31, wherein the at least one alkali metal hydroxide base as comprised in the alkaline solution in step d) is selected from sodium hydroxide (NaOH) or potassium hydroxide (KOH).

33. The method according to any one of claims 1 to 32, wherein the at least one alkali metal hydroxide base as comprised in the alkaline solution in step d) is sodium hydroxide (NaOH).

34. The method according to any one of claims 1 to 33, wherein the alkaline earth metal salt comprised in the alkaline earth metal salt solution in step e) is selected from the group consisting of calcium hydroxide (Ca(OH)2), calcium nitrate (Ca(NO3)2), calcium chloride (CaCl2), strontium hydroxide (Sr(OH)2), strontium nitrate (Sr(NO3)2), strontium chloride (SrCl2), barium chloride (BaCl2), barium nitrate (Ba(NO3)2), and barium hydroxide (Ba(OH)2).

35. The method according to any one of claims 1 to 34, wherein the alkaline earth metal salt comprised in the alkaline earth metal salt solution in step e) is selected from the group consisting of calcium hydroxide (Ca(OH)2), calcium nitrate (Ca(NO3)2), calcium chloride (CaCl2), barium chloride (BaCl2), barium nitrate (Ba(NO3)2), and barium hydroxide (Ba(OH)2).

36. The method according to any one of claims 1 to 35, wherein the alkaline earth metal salt comprised in the alkaline earth metal salt solution instep e) is selected from the group consisting of barium chloride (BaCl2), barium nitrate (Ba(NO3)2), and barium hydroxide (Ba(OH)2).

37. The method according to any one of claims 1 to 36, wherein the alkaline earth metal salt comprised in the alkaline earth metal salt solution in step e) is barium chloride (BaCl2).

38. The method according to any one of claims 1 to 37, wherein the method comprises at least the following successive steps: f) filtering the aqueous suspension (S) in step a) to produce an aqueous filtrate (F) comprising the at least one14C-labeled carbon-based hydrolysis product and thereby removing at least one metallic hydroxide comprised in said aqueous suspension (S), said at least one metallic hydroxide which is produced in step a) upon contacting the at least one neutron-irradiated metallic nitride with the at least one aqueous solution; g) vacuum evaporating, optionally combined with heating, the aqueous filtrate (F) to dryness, to produce a second gaseous stream (G’), said second gaseous stream (G’) comprising the at least one14C-labeled carbon-based hydrolysis product; h) subjecting the second gaseous stream (G’) to an oxidation process to produce a second oxidized gaseous stream, said second oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said second gaseous stream (G’); i) treating the second oxidized gaseous stream with a second alkaline solution comprising at least one alkali metal hydroxide base to produce a second14C-labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; and j) treating the second14C-labeled aqueous solution comprising the at least one14C-labeled alkali metal carbonate with at least one second alkaline earth metal salt solution, thereby obtaining a second precipitated14C- labeled alkaline earth metal carbonate.

39. The method according to any one of claims 1 to 37, wherein the method comprises at least the following successive steps: f') filtering the aqueous suspension (S) in step a) to produce an aqueous filtrate (F) comprising the at least one14C-labeled carbon-based hydrolysis product and thereby removing at least one metallic hydroxide comprised in said aqueous suspension (S), said at least one metallic hydroxide which is produced in step a) upon contacting the at least one neutron-irradiated metallic nitride with the at least one aqueous solution; g') partially evaporating the aqueous filtrate (F) to produce a residual aqueous filtrate (F’) and a third gaseous stream (G’’); h') subjecting the third gaseous stream (G’’) to an oxidation process to produce a third oxidized gaseous stream, said third oxidized gaseous stream being enriched in the molar quantity of14C-labeled carbon dioxide relative to the molar quantity of14C-labeled carbon dioxide in said third gaseous stream (G’’); i') treating the third oxidized gaseous stream with a third alkaline solution comprising at least one alkali metal hydroxide base to produce a third14C- labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; j') treating the third14C-labeled aqueous solution comprising at least one14C- labeled alkali metal carbonate with at least one third alkaline earth metal salt solution, thereby obtaining a third precipitated14C-labeled alkaline earth metal carbonate; k') contacting the residual aqueous filtrate (F’) with at least one non-carbon based, inorganic sorbent material to produce a mixture (M) wherein the residual aqueous filtrate (F’) is adsorbed onto said at least one non-carbon based, inorganic sorbent material; l') subjecting the mixture (M) to an oxidation process to produce a fourth oxidized gaseous stream;m') treating the fourth oxidized gaseous stream with a fourth alkaline solution comprising at least one alkali metal hydroxide base to produce a fourth 14C-labeled aqueous solution comprising at least one14C-labeled alkali metal carbonate; and n') treating the fourth14C-labeled aqueous solution comprising at least one 14C-labeled alkali metal carbonate with at least one fourth alkaline earth metal salt solution, thereby obtaining a fourth precipitated14C-labeled alkaline earth metal carbonate.

40. The method according to claim 39, wherein the at least one non-carbon based, inorganic sorbent material in step k’) is selected from the group consisting of vermiculite, zeolite, kaolinite, montmorillonite, illite, calcite, and lime.

41. The method according to any one of claims 39 to 40, wherein the at least one non-carbon based, inorganic sorbent material in step k’) is vermiculite.

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