Process for the isotopic enrichment of chlorine fluorides and conversion into isotopic enriched actinide chloride salts

Abstract

A process for the isotopic separation of a feed stream of gaseous chlorine-fluoride compound comprising a serially connected cascade of separation units wherein the separation in the separation unit is based on mass difference to obtain a 37Cl enriched product stream of the chlorine-fluoride compound and the 37Cl enriched chlorine-fluoride compound. The invention further pertains to the preparation of 37Cl enriched chlorine and actinide and alkaline chloride salts and the application in molten salt reactors.

Description

[0001] P37113PCOO / MOV

[0002] Title: Process for the isotopic enrichment of chlorine fluorides and conversion into isotopic enriched actinide chloride salts

[0003] Field of the invention

[0004] The invention pertains to a process for the isotopic separation of a feed stream of gases based on a serially connected cascade of separation units, wherein the feed stream is a gaseous chlorine-fluoride compound, the separation in the separation unit is based on mass difference and the product stream is37CI - enriched chlorine-fluoride compound and the preparation of a37CI-enriched actinide chloride salt from the37CI - enriched chlorine-fluoride compound.

[0005] Background of the invention

[0006] Molten salt reactors have recently been rediscovered as an interesting source of energy and many developments are currently ongoing to meet the need for alternative nuclear energy sources. Molten salt reactors rely on the use of molten salts as a solvent, blanket and a as source of nuclear fuel. Accordingly, an interest has been developed in providing such salts. In the past many (mixtures of) salts have been tested and suggested. These typically are halogenide salts chlorides, fluorides and bromides).

[0007] Chlorine has two isotopes found in nature: Chlorine-35 (35CI, CI-35) which makes up about 75.78% of natural chlorine and Chlorine-37 (37CI, CI-37) which constitutes about 24.22% of natural chlorine. In Molten Salt Reactors (MSRs), chlorine can be part of the molten salt fuel in the form of actinide chloride or other chloride salts like NaCI. When salts containing35CI are used in a reactor, it has a tendency to capture neutrons, which can produce chlorine 36 (CI-36,36CI).36CI is a long-lived radioactive isotope with a half-life of approximately 301,000 years. In contrast,37CI has a much lower neutron absorption cross-section than35CI. This means that37CI is far less likely to capture neutrons and become radioactive, thereby reducing the production of36CI and minimizing radioactive waste.

[0008] MSRs rely on a careful balance of neutron production and absorption to sustain a controlled chain reaction.35CI's higher neutron absorption cross-section means it absorbs more neutrons than37CI, effectively wasting neutrons that could otherwise contribute to the fission process. This absorption can slow down the chain reaction and reduce the reactor’s efficiency. By using37CI-enriched salts, neutron absorption by chlorine is minimized, allowing more neutrons to participate in the fission process. This improves the reactor’s performance and overall efficiency.

[0009] Hence, there is a need to produce compounds or salts enriched in37CI at an industrial scale. Clusius and Dickel enriched the amount of37CI in HCI gas to 99.5% using thermal diffusion,where temperature gradients separate lighter and heavier isotopes. This method works in both gas and liquid phases, with heavier37CI concentrating on the cooler side, as shown in Rabinovich and Ivakhnik, Chlorine Isotope Separation by Liquid-Phase Thermal Diffusion, Atomnaya Energiya, Vol. 45, No. 3, 197-200, 1978. Liu et al. , Journal of the American Chemical Society, 97(9), 1975 and Brenner et al., Journal of the American Chemical Society, 99(14), 4554-4561, 1977 used laser-induced reactions to separate chlorine isotopes by reaction of ICL (iodine monochloride) with bromobenzene on laboratory scale.

[0010] Summary of the invention

[0011] The present inventors have found a method to produce37CI-enriched chlorine compounds via the use of chlorine-fluoride compounds in combination with a mass-based separation technique.

[0012] In a first aspect, the invention provides a process for preparation of37CI-enriched salt for molten salt reactors by the separation of a feed stream of gases for providing an enriched product stream containing a37CI - enriched chlorine-fluoride compound suitable for conversion to37CI-enriched actinide or alkali chloride salts, comprising a serially connected cascade of separation units, the process comprising:

[0013] (a) introducing the feed stream into a first separation unit within the cascade;

[0014] (b) operating the first separation unit to produce a product stream enriched in a desired component and a tail stream depleted in the desired component;

[0015] (c) feeding the product stream from the first separation unit as a feed stream into a subsequent separation unit downstream in the cascade;

[0016] (d) recycling the tail stream from the first separation unit as a feed stream to a preceding separation unit upstream in the cascade;

[0017] (e) repeating steps (b) through (d) for each separation unit within the cascade, such that the enrichment of the desired component in the product stream progressively increases along the cascade; and

[0018] (f) collecting a final enriched product stream from the last separation unit in the cascade and a final depleted tail stream from the first separation unit,

[0019] wherein the feed stream is or comprises a gaseous chlorine-fluoride compound; wherein the separation in the separation unit is based on mass difference; wherein the enriched product stream contains37CI - enriched chlorine-fluoride, further comprising the steps of

[0020] (g) conversion of the chlorine-fluoride compound enriched in37CI into37Ch, and (h) preparing37CI-enriched actinide chloride salts or37CI-enriched alkali chloride salts by chlorination of, respectively, an actinide compound or an alkali compound with the obtainedIn a second aspect, the invention provides37CI-enriched alkali and actinide salts.

[0021] In a third aspect, the invention provides the use of the37CI-enriched alkali and actinide chloride salts in a molten salt reactor.

[0022] Detailed description of the invention

[0023] Thus, in a first aspect a process for the preparation of37CI-enriched actinide or alkali chloride salts by the separation of a feed stream of gases for providing an enriched product stream containing a37CI - enriched chlorine-fluoride compound suitable for conversion to37CI-enriched actinide or alkali chloride salts, comprising a serially connected cascade of separation units, the process comprising:

[0024] (a) introducing the feed stream into a first separation unit within the cascade;

[0025] (b) operating the first separation unit to produce a product stream enriched in a desired component and a tail stream depleted in the desired component;

[0026] (c) feeding the product stream from the first separation unit as a feed stream into a subsequent separation unit downstream in the cascade;

[0027] (d) recycling the tail stream from the first separation unit as a feed stream to a preceding separation unit upstream in the cascade;

[0028] (e) repeating steps (b) through (d) for each separation unit within the cascade, such that the enrichment of the desired component in the product stream progressively increases along the cascade; and

[0029] (f) collecting a final enriched product stream from the last separation unit in the cascade and a final depleted tail stream from the first separation unit,

[0030] wherein the feed stream is or comprises a gaseous chlorine-fluoride compound; wherein the separation in the separation unit is based on mass difference; wherein the enriched product stream contains37CI - enriched chlorine-fluoride, further comprising the steps of

[0031] (g) conversion of the chlorine-fluoride compound enriched in37CI into37Ch, and (h) preparing37CI-enriched actinide chloride salts or37CI-enriched alkali chloride salts by chlorination of, respectively, an actinide compound or an alkali compound with the obtained37CI2

[0032] There is currently no efficient and industrially scalable route to37CI-enriched chlorine of sufficiently high isotopic purity. In particular, direct enrichment of chlorine-containing species is non-trivial due to the need to avoid introducing additional isotopic complexity and / or reactiveimpurities into downstream actinide chlorination and / or isotopic dilution in downstream conversion steps.

[0033] The invention provides a process in which a gaseous chlorine-fluoride compound acts as an isotopic carrier for chlorine in a mass-based separation cascade. The resulting37CI-enriched chlorine-fluoride is selectively converted into37Ch, and said37Ch is used as the chlorinating agent to form37CI - enriched actinide or alkali chloride salts. This integration ensures that high37CI purity achieved in the gas-phase separation step is preserved through conversion and chlorination, while the dilution of isotopic enrichment is avoided.

[0034] The method involves multiple stages (or separation units) arranged in a "cascade" configuration to progressively increase the concentration of the desired isotope. In the context of the present invention, the “desired component” is the chlorine-fluoride isotopologue containing37CI (e.g.37CIF,37CIF3,37CIF5), while the corresponding35CI-containing isotopologues form the depleted fraction and are preferably withdrawn as a tails stream such that the product stream of the cascade provides the enriched precursor used in the subsequent conversion step.

[0035] The separation unit is suitable for the separation of isotopes. Suitable means that the separation unit is capable of being operated under such conditions that isotopic separation is possible. The principle of operation of the separation unit lies in the exploitation of slight differences in the physical or chemical properties of isotopes due to their mass differences. In cascade processes, multiple stages amplify these small differences to achieve significant separation. The process can be optimized or the number of stages expanded until the desired level of enrichment is achieved. Typically, a cascade consists of a series of separation stages, each with feed, product, and waste (or tails) streams. Feed or feed stream typically describes the input material containing the mixture of isotopes. The product or product stream is the enriched stream containing a higher proportion of the desired isotope. The tails or tails stream is the depleted stream containing a lower proportion of the desired isotope. The product stream is moved further (upstream) in the cascade, while the tails stream is moved further down (downstream). Within the present disclosure, the cascade is configured such that progressive enrichment is achieved towards a product end, while depleted material is withdrawn at a tails end; the specific routing of product and tails between stages may be implemented in various known cascade topologies, provided that the overall enrichment increases stage-by-stage towards the product end. The number of cascades (or stages) can vary between tens and tens of thousands (between 10 and 100.000). Cascades may also be placed in parallel to increase throughput and production.

[0036] A particular advantage of using chlorine-fluoride compounds as the gaseous carrier is that fluorine is monoisotopic, so that the isotopic mass difference relevant for separationoriginates essentially only from chlorine (37CI versus35CI). As a consequence, a mass-based separation unit (e.g. centrifugation and / or thermal diffusion) can act directly on the chlorine isotopic substitution without the presence of multiple fluorine isotopes. In addition, some chlorine-fluorides are suitable for gas-phase handling and can be processed in separation equipment that operates on molecular mass differences, which thus enables a scalable route to highly enriched37CI in a form that is directly convertible to37Ch without introducing additional chlorine-containing reactants. Preferably, the chlorine-fluoride compound is free of multiisotopic atoms other than chlorine, such as carbon atoms.

[0037] Once a37CI-enriched chlorine-fluoride has been obtained, it can serve as a controlled precursor to generate37Ch. Some chlorine-fluoride compounds are highly reactive fluorinating agents, which means that they readily transfer fluorine to suitable reactants. As such, chlorine-fluoride compounds enable an efficient and high-yield conversion into37Ch provided that fluorine is effectively scavenged by a suitable reactant. Accordingly, the invention provides a conversion step in which the chlorine-fluoride compound enriched in37CI is formed into37Ch, e.g. by contacting the37CI with a fluorine-scavenging material. In this manner, fluorine is bound in a stable reaction product, thereby avoiding dilution of the chlorine isotope enrichment and providing a37Ch stream suitable for direct downstream chlorination of actinide compounds.

[0038] The fluorine scavenging may be achieved using one or more of an oxide fluoride, a metal, an element forming stable fluorides, or hydrogen, or combinations thereof. In certain embodiments, the conversion of the chlorine-fluoride compound enriched in37CI to37Ch is obtained by reaction of the compound enriched in37CI with at least one of an uranium oxide fluoride, calcium, silicium, boron and hydrogen. Additionally or alternatively, the conversion of the chlorine-fluoride compound enriched in37CI to37Ch is obtained by reduction of the compound enriched in 37CI with a metal, preferably aluminum, iron or nickel. As such, the fluorine is captured as one or more corresponding metal fluorides and / or hydrogen fluoride, while chlorine is released as37Ch. This approach is particularly advantageous in the present process chain because it uses the reactivity of the enriched chlorine-fluoride carrier to generate37Ch under controlled conditions without introducing additional halogen species that could compromise isotopic purity or downstream compatibility and thereby maintains the isotopic composition set by the upstream cascade.

[0039] In certain embodiments of the process according to the invention, the conversion of the chlorine-fluoride compound enriched in37CI as defined herein elsewhere to37Ch is carried out by reaction with an uranium oxide fluoride, preferably IIO2F2 or II3O5F8. An advantage thereof is that fluorine is efficiently captured as uranium fluoride species which can be recycled within the uranium fuel cycle, while chlorine is released as37Ch without introducing foreign elements into the system.The invention further pertains to the step for the preparation of37CI-enriched actinide chloride salts by chlorination of the obtained37Ch with an actinide compound. These actinide compounds are preferably actinide oxides, such as ThC>2, UO2, NpO2, PuC>2, AmCh, or combinations thereof, (ThC>2, UO2, PuCh) to obtain Th37Cl4, U37Cl3, U37Cl4, Np37Ch, Pu37Ch, or Am37Cl3, or combinations thereof, respectively.

[0040] In an additional or alternative embodiment, the37CI-enriched actinide chloride salts are prepared by reaction with metallic actinides including, for example, Th, II, Np, Pu and / or Am, or combinations thereof, using37Ch obtained in the conversion step.

[0041] In an embodiment of the invention, the step of preparation of the37CI-enriched alkali chloride salts is achieved by reaction of the obtained37Ch with at least one of the alkali metals, preferably chosen from the group consisting of Li, Na, K, Rb, or Cs, or their corresponding alkali hydroxide or alkali carbonate form, or combinations thereof The reaction between37Ch and these alkali compounds results in the formation of37CI-enriched alkali chloride salts, where the chlorine isotope in the salt is enriched with37CI. The use of alkali metal chloride salts in this process is advantageous due to their stability and ease of handling in large-scale industrial operations. Moreover, alkali chloride salts serve as an effective medium for introducing the37CI isotope into a system without introducing additional impurities or isotopic dilution, which would be detrimental to downstream processing steps. The use of alkali chloride salts allows for the process to be scaled efficiently and cost-effectively.

[0042] Optionally and preferably, the obtained37CI-enriched alkali chloride salt is reacted with an actinide compound, preferably at least one actinide oxide chosen from the group consisting of ThO2, UO2, NpO2, PUO2, Am02 to obtain Th37Cl4, U37Cl3, U37Cl4, Np37Ch, Pu37Ch, or Am37Cl3, respectively.

[0043] In certain embodiments, the reaction is achieved via pyroprocessing. This method involves the use of actinide metals in a molten alkali chloride bath, where the actinide metal is subjected to a controlled redox potential. Preferably, actinide metals such as thorium (Th), uranium (II), neptunium (Np), plutonium (Pu), or americium (Am) are used to form the respective37CI-enriched actinide chloride salts Th37CI4, LI37CI3, LI37CI4, Np37CI3, Pu37CI3, or Am37CI3. By applying a specific redox potential, the actinide metal reacts with the37CI-enriched alkali chloride salt to form the corresponding37CI-enriched actinide chloride salts.

[0044] In another aspect, the invention pertains to37CI-enriched actinide chloride salts selected from the group consisting of Th37Cl4, U37Cl3, U37Cl4, Np37Ch, Pu37Ch, and Am37Cl3, wherein the relative amount of37CI in the chlorine of the37CI-enriched actinide chloride salt is more than 95%, and wherein said relative amount is determined with respect to chlorine atoms in the salt.In another aspect, the invention pertains to the use of37CI-enriched actinide chloride salts in molten salt reactors.

[0045] According to the invention, the product stream is37CI - enriched chlorine-fluoride compound and is directed to the conversion step to produce37Ch.

[0046] In certain embodiments, the relative amount of37CI in the chlorine of the37CI - enriched chlorine-fluoride compound is more than 95%, 98, 98.5, 99.0, 99.5, 99.8, or 99.9 %.

[0047] In certain embodiments, the tail stream is a37CI - depleted chlorine-fluoride compound tail stream.

[0048] In certain embodiments, the tail stream is a35CI - enriched chlorine-fluoride compound tail stream.

[0049] In certain embodiments, the product stream is a35CI - depleted chlorine-fluoride compound product stream.

[0050] In certain embodiments, the relative amount of35CI in the chlorine of the37CI - enriched chlorine-fluoride compound in the product stream is less than 5.0, 2.0, 1.5, 1, 0.5, 0.2, 0.1%. In certain embodiments, the amount of35CI in37CI - enriched chlorine-fluoride compound in the product stream is less than 2.0, 1.5, 1, 0.5, 0.1%.

[0051] The invention further pertains to a chlorine-fluoride compound enriched in37CI, preferably obtainable by the process as described herein elsewhere.

[0052] In certain embodiments, the chlorine-fluoride compound enriched in37CI is CIF (chlorine monofluoride), CIF3 (chlorine trifluoride) or CIF5 (chlorine pentafluoride), preferably CIF3 or CIF5.

[0053] In certain embodiments, the relative amount of37CI in the chlorine of the37CI - enriched chlorine-fluoride compound is more than 95%, 98, 98.5, 99.0, 99.5, 99.8, 99.9 %.15.

[0054] In certain embodiments, the relative amount of35CI in the chlorine of the37CI - enriched chlorine-fluoride compound is less than 2.0, 1.5, 1, 0.5, 0.1%.Several technologies based on mass differences between isotopes may use these cascaded configurations.

[0055] In the gas centrifuge cascade, a mixture of isotopes (such as235UF6 ad238UF6) is centrifuged at high speeds. Heavier isotopes migrate to the outer edge of the centrifuge, while lighter isotopes remain near the center. Multiple centrifuges are connected in a cascade to gradually enrich the gas stream. Typically, the number of centrifuges can be thousands.

[0056] In the gaseous diffusion cascade, the mixture of isotopes passes through a porous barrier. Lighter isotopes diffuse faster through the barrier than heavier ones. Numerous stages are arranged in a cascade to incrementally separate the isotopes.

[0057] In the laser isotope separation cascade, lasers selectively excite and ionize specific isotopes. The different isotopes are responsive to distinctive wavelengths based on their nuclear masses. The selectively excited isotopes are separated from the rest of the material. Cascade steps use multiple lasers and collection points to achieve high isotopic purity.

[0058] In the electromagnetic separation cascade, charged particles are separated in a magnetic field based on mass differences and multiple stages are used for progressive separation.

[0059] In the thermal diffusion cascade, the separation principle is based on the relative motion in a gaseous mixture wherein a radial flow of light components flows towards the hotter inside of a column. Consequently, the heavier components move to the outside, typically under conditions of laminar flow in the columns.

[0060] In the aerodynamics cascade, the isotopic compounds in a high-speed gas stream are subject to a turn through a small radius, casing a pressure gradients. The lighter fraction can be extracted towards the inside and the heavier fraction on the outside.

[0061] Thus, in certain embodiments, the separation technique used in at least one of, preferably in each of, the separation unit is selected from one or more from the group consisting of gas centrifugation, ultracentrifugation, thermal diffusion, laser excitation, electromagnetic separation or an aerodynamic process, preferably gas centrifugation, ultracentrifugation and thermal diffusion.

[0062] In some embodiments, combinations of cascade processes are made, such as the combination of centrifugation and thermal diffusion as they are considered to improve efficiency. This means that a respective separation technique is used in a respective separation unit, or that a single separation unit uses multiple separation techniques in said unit.

[0063] In cascade processes, each stage enriches the isotope concentration, making the overall process efficient when scaled. The advantage of cascades is that they can be adjusted to meet different enrichment requirements. Each stage’s efficiency depends on its ability to differentiate between isotopes. Optimizing the number of stages and configuration is preferred for minimizing energy consumption and maximizing separation effectiveness. The cascadeprocess remains foundational in isotope separation due to its precision and adaptability to various methods and materials.

[0064] The chlorine fluoride compound in the process is preferably selected from the group consisting of CIF (chlorine monofluoride), CIF3 (chlorine trifluoride) or CIF5 (chlorine pentafluoride). There is a preference for CIF3. CIF3 is a very toxic, very aggressive composition that for instance is used to remove solid IIO2F2 from walls of processing equipment such as in the cleaning of gaseous diffusion devices due to its ability to remove uranium oxide side products. This also means that the design criteria for processing equipment in terms of resilience and robustness for the use of CIF3 are well within the scope of the skilled person and the described compounds can be used in existing enrichment plants.

[0065] Fluorine has only one isotope which means that the mass difference between isotopic chlorine fluoride compounds depends only on the isotopes of chlorine. Using separation techniques depending on mass difference, such as (ultra) centrifuges and thermal diffusion, with chlorine fluoride compounds, like CIF3, allows to enrich efficiently in37CI. Enriched CI2 can then be produced by a reaction between enriched chlorine fluoride compounds and uranium compounds, very often used in the nuclear industry. Enriched actinide chloride salts and other enriched chloride salts can then be produced using enriched CI2 or other derivates enriched chlorinated compounds. The37CI-enriched chlorine salts can be used in molten salt reactors.

[0066] In embodiments, the relative amount of37CI in the chlorine of the37CI - enriched chlorine-fluoride compound is more than 95%. In embodiments, the relative amount of35CI in the chlorine of the37CI - enriched chlorine-fluoride compound is less than 5%.

[0067] The feasibility of this invention is exemplified below.

[0068] SWU enriched chlorine demand for a gigawatt-scale plant

[0069] The separative work unit (SWU) is a parameter that embodies the amount of physical work needed to enrich a compound to a desired level, given enrichment levels of the feed and tails. It essentially embodies the resources which will be devoted to the enrichment operation, and is thus a very useful unit. It will also be directly proportional to the enrichment cost (Cochran, Robert G., and Nicholas Tsoulfanidis. "The nuclear fuel cycle: analysis and management." American Nuclear Society, (1999).). The feed is natural chlorine, thus having a37CI mole fraction of x / = 0.2424 (24.24%) The tails stream in preferred embodiments has a37CI mole fraction of xw= 0.005 (< 0.5%). A similar calculation would apply to the chlorine fluorides used in this invention. In this example, Pdenotes the annual product amount and xpdenotes the37CImole fraction in the product. From the mass balance, the required feed F, in tons per year, is then given by:

[0070] F = P ■Xp_Xw= 120 tons per year

[0071] Xf xw

[0072] Similarly, the waste rate W is given by

[0073] W = P ■Xp_Xf= 90 tons per year

[0074] Xf xw

[0075] The so-called separation potential associated with each of the streams can be calculated (Benedict, M., Pigford, T.H., Levi, H.W. Nuclear Chemical Engineering. McGraw-Hill (United States), 1981.:

[0076] Xi

[0077] V(Xj) = (2xj - l)ln— —

[0078] ± Xj

[0079] From which the SWU can be computed over a time period T (chosen in this example to be one year):

[0080] SWU = [P • V(xp) + W • V(xw) - F • V(xf)] • T

[0081] = 634 tons SWU

[0082] : The Urenco plant in Eunice, New Mexico, has (in 2024) 4600 tons SWU per year, bringing the described process for37CI enrichment example is well within the means of existing commercial plants. This order of magnitude indicates that the enrichment demand can be compatible with existing commercial enrichment capacity. It is also reasonable to expect that the chlorine inventory considered would suffice for a commercial MSR power plant of gigawatt scale.

[0083] Gas centrifugation is a suitable technology to separate the35CI and37CI isotopes. Indeed, for the separative power of centrifuges to be maximum, the gas density and mass difference have to be as high as possible. The maximum separative capacity per unit length is given by:

[0084] >

[0085]

[0086] Where D is the diffusion coefficient, p is the density, V the peripheral speed of the centrifuge, T the temperature, R the gas constant, and M2-M1 the molar mass difference between the two isotopic compounds to separate (e.g. for Cl, the mass difference between35CIF3 and37CIF3). The second factor in the expression is called the circulation efficiency, and further depends on the radius of the centrifuge, a, and N, the depleted stream flow rate. In the limits where a

[0087]

[0088] 0 and N 00, the second term reduces to 1 and the expression reduces to the first factor only: this limit was shown (Cohen, Karl P., and George Moseley Murphy. The Theory of isotope separation as applied to the large-scale production of U-235. Vol. 1. New York: McGraw-Hill, 1951.) to be the maximum possible separative capacity per unit length for a centrifuge without any axial back diffusion. Given a peripheral speed and temperature, this maximum depends onthe product between the density and the diffusion coefficient of the gas pD, and the molar mass difference.

[0089] To determine the usefulness of a given gas in the enrichment of chlorine, it is instructive to compare this maximum amongst different gases. This is done in Table 1, in which UFe is also included as a benchmark. Following Benedict et al. (cited herein elsewhere), pD = 2.161 ■ 10-4g / (cm-s) recommended by Cohen et al. (cited herein elsewhere), is selected for UFe. In the case of HCI(g) and Ch, the diffusion coefficient are assumed to match that of HCI(g) and Ch respectively diffusing through air (Tang, M. J., R. A. Cox, and M. Kalberer. "Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic compounds." Atmospheric Chemistry and Physics 14.17 (2014): 9233-9247.) : DHCI = 1.55-10'5m2 / s (Tang, M. J., R. A. Cox, and M. Kalberer. "Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic) and DCE = 1.24-10'5m2 / s (D. R. Burgess Jr., “Self-Diffusion and Binary-Diffusion Coefficients in Gases”, NIST TN 2279, 2024). The diffusion coefficient of CIF, CIF3 and CIF5 are not documented. However, these diffusion coefficients can be approximated by taking the diffusion coefficient of molecules of similar molar mass and geometry. For CIF, CIF3 and CIF5, we take the diffusion coefficient of HCI, SO2 and SFe respectively: DCIF = DHCI = 1.55-1 O'5m2 / s, DCIFS = DSO2 = 1.24-10-5m2 / s ( D. R. Burgess Jr., “Self-Diffusion and Binary-Diffusion Coefficients in Gases”, NIST TN 2279, 2024) , DCIFS = DSFS = 2.33-1 O'5m2 / s. Further, AM = M2-M1 in the case of Ch was chosen to be the expectation value of the mass difference: unlike for the other molecules, AM in the latter case cannot be attributed solely to a single chlorine atom, thus E[AM] =2.45 g / mol.

[0090] Table 1 shows the maximum separative capacity per unit length for different gases at varying peripheral speeds.

[0091] Table 1. Maximum separative capacity per unit length for different gases at varying peripheral speeds. Values given in kgSWU / (m year). T = 300 K.

[0092]

[0093] It can be concluded from Table 1 that CIF3 and CIF5 performs much more favorably than HCI as they have a higher maximum separative capacity. CIF3 has a maximum separative capacity comparable to CI2. However, the absolute molar mass of the gas matters as well, and is preferably to be above 80 g / mol for the centrifugation to be efficient (V. D. Borisevich O.E. Morozov, Yu.P. Zaozerskiy, G.M. Shmelev, Yu.D. Shipilov, “On the enrichment of low-abundantisotopes of light chemical elements by gas centrifuges.” Nuclear Instruments and Methods in Physics Research A, 515-521, 2000). From combining this criterion with the separative capacities per unit length calculated in Table 1, it can be concluded that CIF3 and CIFs are superior to Ch. The densities at room temperature (calculated from ideal gas law), mass ratios and molar masses of the different compounds studied are listed in

[0094] Table 2.

[0095] Table 2: Densities at 0.3 atm (~30kPa) and 20°C, mass ratios and molar masses of the different compounds studied.

[0096]

[0097] The centrifuges in most enrichment facilities are designed for uranium enrichment and use gaseous UFe. CIF3 and CIFsare denser than Ch and HCI and have more favorable mass ratios than that of uranium when UFe is enriched. As mentioned, CIFsand CIFs have a molar mass higher than 80 g / mol, and are therefore a very good candidates for centrifugation.

[0098] The best candidate for centrifugation would be CIFs, but is currently much less commercially available than CIF3. For a commercial enrichment of chlorine, CIF3 would therefore be the best candidate. Assuming a centrifuge length of 5 m, the enrichment operation using ultracentrifugation with CIF3 would need between -5800 and -54000 cascades / stages.

[0099] A proof of principle illustrating the increasing efficiency of gas centrifugation with molar mass is the series of experiments made by Borisevich et al. To enrich N-15 using pure N2 (28.0135 g / mol), NH4 (18.03846 g / mol) and NF3(71.002 g / mol). The best results were obtained with NF3 (V. D. Borisevich O.E. Morozov, Yu.P. Zaozerskiy, G.M. Shmelev, Yu.D. Shipilov, “On the enrichment of low-abundant isotopes of light chemical elements by gas centrifuges.” Nuclear Instruments and Methods in Physics Research A, 515-521, 2000.) .

[0100] Thermal diffusion

[0101] Thermal diffusion can also separate compounds comprising chlorine isotopes35CI and37CI. When submitted to a thermal gradient, the differences in molecular mass affect particle mobilityand consequently lighter isotopes tend to migrate toward the warmer region, while heavier isotopes concentrate in the colder region.

[0102] Thermal diffusion separation is commonly used with columns like the one developed by Clausius and Dickel (cited herein elsewhere). In this system, a column is provided, wherein the inner and outer walls of the column induce a temperature gradient, typically a higher temperature (hot) on the inner wall and a lower temperature (colder) on the outer wall to thereby facilitate thermal diffusion and isotope separation.

[0103] The net transport of the lighter component toward the top of the column T is given by:

[0104] dx

[0105] T = Hx(l — x) — (Kc+ Kd) —

[0106] dz

[0107] Where x is the mole fraction of the lighter component in the mixture, z is the axial column coordinate, H, Kcand Kdare the transport coefficient given by:

[0108] 2nap2g / Thot- Tcold\2

[0109] H = ■ ravgCr, - r2)3- - - 1 - hm

[0110] u. . \ la /

[0111] 2np3g2 / Thot- Tcold\2Kc =. .^"ravg(ri“r2)I \ - T i -aJ / ’kc

[0112] Kd= 2npD ■ ravg(r! - r2) ■ kd

[0113] Where a is the thermal diffusion constant of the gas. D is the ordinary coefficient of diffusion of the gas. g is the acceleration of gravity, p represents the density of the gas. p depicts the viscosity of the gas. Tais the average temperature, at which all the constant are taken, Thotand Tcold. the measured hot and cold wall temperatures in Kelvin or Celsius. r1 is the radius measured from the center or heart of the column, the central axis, to the surface of the hot wall that is oriented towards the gas in the column. r2 is the radius measured from the center or heart of the column, the central axis, to the surface of the cold wall that is oriented towards the gas in the column. Typically, r2> r1. ravgis the average of r, and r2.

[0114] The heavier components diffuse from the hot to the cold part, while the lighter component diffuse from the cold region to the hotter region. hm, kcand kdare “shape factors”, they are constants for a large number of molecules.

[0115] The separation factor q is then expressed by:

[0116]

[0117] L being the column length, H driving the separation and Kc+Kdbeing a resistive term that indicates how the gas will diffuse and includes viscosity and diffusion coefficients.

[0118] Additionally to the column geometry, the temperature gradient and the pressure, the efficiency of the separation depends on the different physical properties of the gas used, i.e. the thermal diffusion constant, the mass difference, the density, and the viscosity. HCI has been historically used and is still used in thermal diffusion to enrich chlorine compounds in37CI.When for example Thot = 623K and Tcoid = 298K, and n = 6.5 cm and r2= 8 cm (case close to Huber et al.( Z. Huber et al., Chlorine Isotope Separations using Thermal Diffusion, Report PNNL-35607, 2024. ). Assuming ideal gas behavior the viscosity, the density and the diffusion coefficient of the gases can be estimated:

[0119]

[0120] Where m is the gas molecule mass, d the diameter of the gas molecule, kB is the Boltzmann constant, T the average temperature, P the pressure, R the ideal gas constant and M the molar mass of the gas. The molecular diameter can be estimated using the covalent bond length between atoms and the geometry of the molecule. The thermal diffusion coefficient a is calculated using the formula:

[0121] k

[0122] a = - P p

[0123] Where k is the thermal conductivity and cpthe specific heat capacity at constant pressure. All the compounds considered are gaseous and have a similar thermal conductivity k « 0.01 W / m ■ K. The specific heat capacity of UFe, HCI and Ch are documented and are taken as:Cp(UF6) = 380 J / kg- K, cp(HCI) = 820 J / kg ■ K, cp(Cl2) = 480 J / kg ■ K. The specific heat capacity of CIF, CIF3 and CIF5 are estimated. CIF is a diatomic polar molecule and its heat capacity is between that of HCI and Ch: cp(C!F) « 600 J / kg ■ K. CIF5 has a geometry close to UFe CP(C1F5) « 400 J / kg ■ K, and the behaviour of CIF3 will be between that of CIF and CIF5 cp(C1F3) « 500 J / kg ■ K.

[0124] The properties are listed in Table 333 for the average temperature (Ta= 460.5K). The diffusion coefficient for UFe, HCI, Ch, CIF, CIF3 and CIF5 are the same as described in example 2. Greene et al provide the shape factors hm, kcand fefor a wide range of radii and temperature (E. Greene, R. L. Hoglund, E. Von Halle, “Thermal diffusion column shape factors: Part 1. Shape factors based on an inverse power repulsion model”, ORNL Report K-1469, 1966.). In this case Thot / Tcold « 2 and r2 / rl « 1.2. This allows to determine hm(taken as the average of ho, hi / 2 and hi), kcand kd in the table provided by Greene et al. In this example, n = 1, so: hm= 7.34-10’4, kc= 9.39-10’8, kd= 2.33-10’1.

[0125]

[0126]

[0127] Based thereon H, Kd, and Kccan be calculated for the different gases:

[0128] T able 44. Values of the transport coefficients for the gases con s i d e r e d .

[0129]

[0130] Taking a column length of L = 162 cm (based on Huber et al.), the separation factor q for the different gases can be calculated. The higher ln(q), the better the separation:

[0131] T able 55: Separation factor q for the different gases cons i d e re d .

[0132]

[0133] The above calculation demonstrate that thermal diffusion can be used to separate chlorine isotopes. As can be deducted from the higher ln(q) values for chlorine and CIFn (n=1, 3, 5) compounds, the process will be more efficient compared to the same process when used for uranium enrichment using UFe.

[0134] Combination of gas centrifugation and thermal diffusion

[0135] A combination of the thermal diffusion and centrifugation processes is achieved by heating the bottom of the centrifuges (heat diffusion effect in the bottom). This causes an axial countercurrent that allows the lighter gas (containing35CI) to move through convection to the top while the heavier molecules (containing37CI) would concentrate near the bottom. It is possible then to collect the depleted stream and the enriched one at different axial levels, the efficiency of the centrifugation process is improved, providing an improved yield or necessitating less centrifuges.Synthesis of actinide salts using enriched chlorine fluorides.

[0136] Actinide salts are synthesized using enriched chlorine fluorides. The enriched chlorine fluorides are converted to isotopically enriched Ch. This can be done using CIF or CIF3 and uranium compounds, like IIO2F2 or II3O5F8. These oxides are known oxide deposits formed on the walls of gaseous diffusion based installations for uranium enrichment, following the reactions equations 1-5 below ( O.B. Gromov, « Separation of a gas mixture containing uranium hexafluoride, hydrogen fluoride, and chlorine trifluoride, Atomic Energy, Vol. 133, No 5-6, March-April 2023, M. Luce and O. Hartmanshenn, “Etude des reactions CIF3-UO2F2 et CIO2F-UO2F2, Rapport CEA-R 3210, Mars 1967.)

[0137] UO2F2 + 4CIF3= UF6+ 2CIF + Cl2+ O2(1) 3IIO2F2 + 4 CIF3= 3O2+ 2CI2+ 3UF6(2) U3O5F8 + 4CIF3= 3UF6+ CIF + CIO2F + Cl2+ 1.5O2(3)

[0138] UF4+ 2CIF = UF6+ Cl2(4) UO2F2 + 4CIF = O2+ 2CI2+ UF6(5)

[0139] The different gases are separated using techniques like distillation, membrane separation, solvent absorption or material adsorption. As a side product, all reactions produce UFe, which can then be used again for uranium enrichment.

[0140] Alternatively, certain metals and hydrogen can reduce37CIF3by preferentially reacting with fluorine, leaving behind Cl2.

[0141] Aluminum (Al): 3 CIF3+ 2AI -^ 3 Cl2+2 AIF3

[0142] Calcium (Ca): 3 CIF3 + 2 Ca^ 3 CI2 + 2CaF2

[0143] Iron (Fe): 3 CIF3+ 2 Fe^ 3 Cl2+ 2 FeF3

[0144] Hydrogen Gas (H2): 2 CIF3+3 H2Cl2+ 6HF

[0145] The37CI-enriched chlorine37Ch are used in the preparation of a wide a variety of salts.

[0146] Actinide salts, like ThCL, UCI3, UCI4 and PuCh, are obtained by conventional carbochlorination. The synthesis of ThCL, could be done as per the following reactions(A. R. Gibson, J. H. Buddery, J. R. Chalkley, R. P. Marshal, “Thorium metal production by a chlorination process”, Second United Nations International Conference on the Peaceful Uses of Atomic Energy, 1958.):

[0147] ThO2+ C + 2CI2= ThCI4+ CO2(1 ) ThO2+ 2C + 2CI2= ThCI4+ 2CO (2) ThOCh + C + Cl2= ThCI4+ CO (3)

[0148] In an analogous manner UCI4 is synthesized by reacting uranium oxide with graphite and gaseous chlorine(Y.-S. Yang, Y.-H. Kang, H.-K. Lee “Estimation of optimum experimentalparameters in chlorination of UO2 with CI2 gas and carbon for LICI4, Materials Chemistry and Physics 50 (1997) 243-247. ):

[0149] UO2+ 2C + 2CI2= UCI4+ 2CO (1) UO2+ 2CO + 2CI2= UCI4+ 2CO2(2)

[0150] IICI3 is formed indirectly with CI2, by first synthesizing a chloride species MCIX(M=Metal), and by reaction with uranium metal (B. R. Westphal, J. C. Price, R. D. Mariani “Synthesis of Uranium Trichloride for the Pyrometallurgical Processing of Used Nuclear Fuel”, Fray International Symposium, 2011):

[0151] U + 3 / x MClx = UCI3+ 3 / x M

[0152] In particular, UCI3 can also be formed by choosing MCIXto be UCI4.

[0153] Finally, the synthesis of PuCh can be done from PuC>2 using CI2-CCI4 as a chlorinating agent M. D. Ferran, M. H. West “The synthesis of Plutonium Trichloride by Chlorination of Plutonium Dioxide with Phosgene”, Los Alamos report LA-12631-MS, October 1993.The process according to the invention may be described by the following clauses:

[0154] 1. A process for the separation of a feed stream of gases comprising a serially connected cascade of separation units, the process comprising:

[0155] (a) introducing the feed stream into a first separation unit within the cascade;

[0156] (b) operating the first separation unit to produce a product stream enriched in a desired component and a tail stream depleted in the desired component; (c) feeding the product stream from the first separation unit as a feed stream into a subsequent separation unit downstream in the cascade;

[0157] (d) recycling the tail stream from the first separation unit as a feed stream to a preceding separation unit upstream in the cascade;

[0158] (e) repeating steps (b) through (d) for each separation unit within the cascade, such that the enrichment of the desired component in the product stream progressively increases along the cascade; and

[0159] (f) collecting a final enriched product stream from the last separation unit in the cascade and a final depleted tail stream from the first separation unit, wherein the feed stream is a gaseous chlorine-fluoride compound;

[0160] wherein the separation in the separation unit is based on mass difference; wherein the enriched product stream contains37CI - enriched chlorine-fluoride. 2. Process according to clause 1, wherein the chlorine-fluoride compound is CIF (chlorine monofluoride), CIF3 (chlorine trifluoride) or CIF5 (chlorine pentafluoride), preferably CIF3 or CIF5

[0161] 3. Process according to clauses 1-2, wherein the mass difference is an isotopic mass difference, preferably between37CI and35CI isotopes.

[0162] 4. Process according to clauses 1-3, wherein the molar mass of the chlorine-fluoride compound is above 80 grams / mole.

[0163] 5. Process according to clauses 1-4, wherein the product stream contains37CI - enriched chlorine-fluoride compound.

[0164] 6. Process according to clauses 1-5, wherein the separation technique used in the separation unit is selected from one or more from the group consisting of gas centrifugation, ultracentrifugation, thermal diffusion, laser excitation, electromagnetic separation or an aerodynamic process, preferably gas centrifugation, ultracentrifugation and thermal diffusion or a combination thereof.

[0165] 7. Process according to clauses 1-6, wherein the separation technique used in the separation unit is ultracentrifugation combined with thermal diffusion.

[0166] 8. Process according to clauses 1-7, wherein the relative amount of37CI in the chlorine of the37CI - enriched chlorine-fluoride compound in the enriched product stream is more than 95%.9. Chlorine-fluoride compound enriched in37CI, preferably obtainable by the process of clauses 1-8.

[0167] 10. Chlorine-fluoride compound enriched in37CI according to clause 9, wherein the relative amount of37CI in the chlorine of the37CI - enriched chlorine-fluoride compound is more than 95%.

[0168] 11. Chlorine-fluoride compound enriched in37CI according to clauses 9 or 10, which is37CIF (chlorine monofluoride),37CIF3 (chlorine trifluoride) or37CIFs (chlorine pentafluoride), preferably37CIF3 or37CIFs.

[0169] 12. Process for the conversion of the chlorine-fluoride compound enriched in37CI as defined in the previous clauses to37Ch by reaction with an uranium oxide fluoride, preferably IIO2F2 or II3O5F8, or one of aluminum, calcium, iron or hydrogen.

[0170] 13. Process for the preparation of37CI-enriched actinide chloride salts by carbochlorination of actinide oxides (ThC>2, UO2, NpO2, PuC>2, AmCh) with37Chto obtain Th37CI4, U37CI3, U37CI4, Np37CI3, Pu37CI3, or Am37CI3, respectively.

[0171] 14.37CI-enriched actinide chloride salts selected from the group consisting of Th37CI4, U37CI3, U37CI4, Np37Ch, PU37CI3, and Am37Cl3, wherein the relative amount of37CI in the chlorine of the37CI-enriched actinide chloride salt is more than 95%.

[0172] 15. Use of the37CI-enriched actinide chloride salts selected from the group consisting of Th37CI4, U37CI3, U37CI4, Np37Ch, Pu37Cl3, and Am37Cl3, in a molten salt reactor.

Claims

CLAIMS1. A process for preparation of37CI-enriched salt for molten salt reactors by the separation of a feed stream of gases for providing an enriched product stream containing a37CI - enriched chlorine-fluoride compound suitable for conversion to37CI-enriched actinide or alkali chloride salts, comprising a serially connected cascade of separation units, the process comprising:(a) introducing the feed stream into a first separation unit within the cascade; (b) operating the first separation unit to produce a product stream enriched in a desired component and a tail stream depleted in the desired component;(c) feeding the product stream from the first separation unit as a feed stream into a subsequent separation unit downstream in the cascade;(d) recycling the tail stream from the first separation unit as a feed stream to a preceding separation unit upstream in the cascade;(e) repeating steps (b) through (d) for each separation unit within the cascade, such that the enrichment of the desired component in the product stream progressively increases along the cascade; and(f) collecting a final enriched product stream from the last separation unit in the cascade and a final depleted tail stream from the first separation unit, wherein the feed stream is or comprises a gaseous chlorine-fluoride compound; wherein the separation in the separation unit is based on mass difference; wherein the enriched product stream contains37CI - enriched chlorine-fluoride, further comprising the steps of(g) conversion of the chlorine-fluoride compound enriched in37CI into37Ch, and (h) preparing37CI-enriched actinide chloride salts or37CI-enriched alkali chloride salts by chlorination of, respectively, an actinide compound or an alkali compound with the obtained37Ch2. Process according to claim 1, wherein the conversion of the chlorine-fluoride compound enriched in37CI into37Ch is obtained by reaction of the compound enriched in37CI with at least one of an oxide fluoride, preferably a uranium oxide fluoride such as IIO2F2 or II3O5F8, calcium, silicium, boron and hydrogen, or by reduction of the compound enriched in37CI with a metal, preferably aluminum, iron or nickel.

3. Process according to claim 1 or 2, wherein the step of preparation of the37CI- enriched actinide chloride salts is achieved by reaction of the obtained37Ch withat least one actinide oxide chosen from the group consisting of ThC>2, UO2, NpO2, PuO2, AmC>2 , to obtain Th37Cl4, U37Cl3, U37Cl4, Np37Cl3, Pu37Ch, or Am37Cl3, respectively.

4. Process according to claim 1 or 2, wherein the step of preparation of the37CI- enriched alkali chloride salts is achieved by reaction of the obtained37Ch with at least one of the alkali metals, preferably chosen from the group consisting of Li, Na, K, Rb, or Cs, or their corresponding alkali hydroxide or alkali carbonate form, or combinations thereof.

5. Process according to claim 1 , 2 or 4, further comprising the step of reacting the obtained37CI-enriched alkali chloride salt with an actinide compound, preferably at least one actinide metal chosen from the group consisting of Th, II, Np, Pu, or Am to obtain Th37Cl4, U37Cl3, U37Cl4, Np37Ch, Pu37Cl3, or Am37Cl3, respectively.

6. Process according to claim 5, wherein the reaction is performed in a molten alkali chloride bath under controlled redox conditions.

7. Process according to any one of the claims 1-6, wherein the gaseous chlorinefluoride compound is selected from the group consisting of CIF (chlorine monofluoride), CIF3 (chlorine trifluoride) or CIF5 (chlorine pentafluoride), or mixtures thereof, preferably CIF3 or CIF5.

8. Process according to any one of the claims 1-7, wherein the mass difference is an isotopic mass difference, preferably between37CI and35CI isotopes.

9. Process according to any one of the claims 1-8, wherein the molar mass of the chlorine-fluoride compound is above 80 grams / mole.

10. Process according to any one of the claims 1-9, wherein the separation technique used in the separation unit is selected from one or more from the group consisting of gas centrifugation, ultracentrifugation, thermal diffusion, laser excitation, electromagnetic separation or an aerodynamic process, preferably gas centrifugation, ultracentrifugation and thermal diffusion or a combination thereof.

11. Process according to any of the claims 1-10, wherein the separation technique used in at least one of the separation units is ultracentrifugation combined with thermal diffusion.

12. Process according to any of the claims 1-11, wherein the relative amount of37CI in the chlorine of the37CI - enriched chlorine-fluoride compound in the enriched product stream is more than 95%.13.37CI-enriched actinide chloride salts selected from the group consisting of Th37Cl4, U37CI3, U37CI4, Np37Ch, Pu37Ch, and Am37Cl3 or37CI-enriched alkali chloride salts obtained via the process according to any one of claims 1 - 12, wherein the relative amount of37CI in the chlorine of the37CI-enriched actinide chloride salt is more than 95%.

14. Use of the37CI-enriched actinide chloride salts selected from the group consisting of Th37CI4, U37CI3, U37CI4, Np37CI3, Pu37CI3, and Am37CI3or37CI-enriched alkali chloride salts according to any one of the claims 1 - 12 in a molten salt reactor.

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