A method for producing uranium nitrides from gaseous stock

Abstract

The invention relate to a method for producing a powder comprising UN2 and / or U2N3 and / or UN, the method comprising the steps of: a) heating a reaction zone in a vessel containing an NH3 atmosphere to a temperature in the range of 700 – 1200 °C to drive a reaction between UF6 and NH3, the pressure in the vessel being from 1 up to 45 atm; b) continuously injecting gaseous UF6 as a process gas into the reaction zone, optionally co-injecting with a shield gas around the UF6; c) continuously injecting gaseous NH3 as a process gas into the reaction zone, the amount of NH3 controlled to be in stochiometric excess for the reaction UF6 + 2 NH3 -> UN2 + 6 HF; d) reacting the process gases in the reaction zone, thereby forming the powder and gaseous HF; e) continuously, evacuating the powder and process gases including HF and excessive NH3 from the reaction zone; f) separating the powder from the process gases. The invention also relates to a reaction vessel having walls comprising a ceramic and / or a metallic material that preferably have low or zero catalytic activity for ammonia dissociation.

Description

[0001] A METHOD FOR PRODUCING URANIUM NITRIDES FROM GASEOUS STOCK

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for making uranium nitride containing powders from uranium hexafluoride.

[0004] BACKGROUND

[0005] The present invention relates to a process for conversion of gaseous uranium hexafluoride (UFe) to uranium nitride powders, which can be used to produce nuclear fuels.

[0006] UN is a promising candidate for new nuclear fuel. It has high density, high thermal conductivity, high melting point, and irradiation resistance are further properties that make UN fuel pellets an interesting option as a nuclear fuel.

[0007] Use of the isotope15N is preferable because the predominant isotope,14N, has a significant neutron absorption cross section which affects neutron economy and, in particular, it undergoes an (n,p) reaction which produces significant amounts of radioactive14C which would need to be carefully contained and sequestered during reprocessing or permanent storage.

[0008] Known UN manufacturing process are however complex and costly.

[0009] A method for generating UN is carbothermic reduction of uranium oxide (UO2). Another method suggested is hydriding of metallic uranium to form UH3 followed by nitriding to form U2N3 which can be heated to be decomposed to UN.

[0010] US 2022 / 0333254 Aldiscloses an electrochemical method for UN production comprising two steps: 1) exposing UFe gas to a nitrogen bearing salt at or near a cathode to produce UxNy, where x is an integer of 1 or 2 and y is an integer of 1 to 3; and 2) converting UxNyto UN and nitrogen gas.

[0011] US 7 582 232 Bl discloses a method for producing UN by converting uranium oxide to a uranium fluoride salt and heating in ammonia gas to produce uranium nitride.

[0012] Other methods for producing uranium nitride, including exposing uranium tetrafluoride or uranium hexafluoride to ammonia gas, are discussed in the following articles: Ekberg, C., Costa, D., Hedberg, M. et al (2018). Nitride fuel for Gen IV nuclear power systems. Journal of Radioanalytical and Nuclear Chemistry, 318(3): 1713-1725. http: / / dx.doi.org / 10.1007 / sl0967-018-6316-0

[0013] Zagoraios, G. (2022). Synthesis of uranium nitride fuel from UF4 stock. Master Thesis at

[0014] Reactor Physic Division, KTH. https: / / kth.diva- I=diva2%3 Al 705317&dswid=-4248

[0015] In the latter it is suggested to produce UN powder in a batch synthesis, where solid UFe is heated to 800 °C under a stream of NH3 gas, producing UN2 and NH4F as a by-product.

[0016] However, there are several problems that needs to be solved to make this process workable in industrial scale. The present invention relates to improvement of this method.

[0017] RU2241269C2 describes a process for making a primary target comprising uranium nitride (UN). The target is used for the formation of a radioactive isotope molybdenum 99 as source for technetium-99. The document suggests forming uranium nitride from UFe N2, or NH3 and H2 in vapor phase, where uranium nitride is formed on a substrate in a low pressure chamber (100 to 3000 Pa). The uranium nitride is deposited as a growing layer on the surface of the substrate. An example using CVD (chemical vapor deposition) is disclosed is described in more detail. Here a layer of uranium nitride is formed on an inner wall of a metal tube. The tube is simultaneously used as a substrate and a reaction chamber. According to the document it is necessary to heat the substrate to carry out the chemical reaction. Furthermore, the described process uses an internal pressure of 10 to 1 000 Pa. Obviously, the layer of uranium nitride stays in the reaction chamber during the process.

[0018] RU2293060C2 relates to a process for producing uranium mononitride and mixture of mononitrides of uranium and plutonium. According to the document uranium hexafluoride can be introduced to a plasma formed from a mixture of nitrogen and hydrogen or ammonia. The plasma is formed in a chemical reactor, preferably a microwave plasma torch or an arc plasmatron.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Fig. 1 show a schematic drawing of the process. DESCRIPTION OF THE INVENTION

[0021] It is an object of the present invention to provide an improved process for conversion of UFe to a uranium nitride powder comprising of UN2 and / or U2N3 and / or UN that can be decomposed to UN powder.

[0022] According to the present invention, there is therefore provided a method for producing a uranium nitride powder comprising of UN2 and / or U2N3 and / or UN, the method comprising the steps of: a) heating a reaction zone 1 in a reaction vessel 2 containing an NH3 atmosphere to a temperature sufficiently high to drive the reaction between UFe and NH3 to form UN2;

[0023] The reaction zone 1 is heated to a temperature in the range of 700 - 1200 °C, preferably 800 -1000 °C. The pressure in the vessel 2 is from 1 atm up to 45 atm, preferably between 1 and 10 atm. The lower limit can be set to 1.5, 2, 3, 4 or 5 atm. The upper limit may be set to 40, 30, 20, 15, 10, or 5 atm.

[0024] Preferably, the walls of the reaction vessel 2 comprise a refractory material that have low or zero catalytic activity for ammonia dissociation at the temperature and pressure of the reaction zone 1. In addition, the material can also aid in protecting the walls 2 from hydrogen embrittlement. The material can be a metallic material, preferably a tantalum alloy, and / or a ceramic material, preferably comprising or consisting of boron nitride, more preferably hexagonal boron nitride. The walls can be made of the material but are preferably lined or coated with the material.

[0025] The nitrogen isotope in NH3 may be the isotope15N and / or14N or any mix between these isotopes. b) Continuously injecting gaseous UFe as a process gas (reactant) into the reaction zone 1. Preferably, co-injecting the gaseous UFe together with an inert shield gas around the gaseous UFe into the reaction zone. The shield gas around the UFe is preferably N2 and / or Ar and / or He. The shield gas will not react with UFe and can optionally be recovered and reused in step h.

[0026] If the nitrogen isotope in the reactant NH3 is15N and N2 is selected as shield gas, then it is preferred that also the shield gas use the isotope15N to avoid isotopic dilution of recycled waste gases.

[0027] Preferably, the temperature of the UFe injector 3 is kept below 400 °C to avoid that UFe undesirably react with the injector material. This can be done by active cooling of the injector 3. The injector can be cooled down to temperature below, 400, 300, 200, or 100°C. By having active cooling of the injector 3, it is possible to place it closer to the reaction zone 1 and it will also minimise corrosion of the injector 3. c) continuously injecting gaseous NH3 as a process gas (reactant) into the reaction zone 1.

[0028] The amount of NH3 controlled to be in stochiometric excess for the reaction UFe + 2 NH3 -> UN2 + 6 HF. This is to ensure that essentially all of the UFe is reacted.

[0029] The nitrogen isotope may be15N and / or14N or any mix between these isotopes.

[0030] The NH3 can optionally be co-injected with the UFe and the shield gas. The NH3 and UFe should however be kept from reacting with each other before they reach the reaction zone 1.

[0031] Preferably, the temperature of the NH3 injector 4 is kept below 400 °C. This can be done by active cooling of the injector 4. The injector can be cooled down to temperature below, 400, 300, 200, or 100°C. By having active cooling of the injector 4, it is possible to place it closer to the reaction zone 1 and it will also minimise corrosion of the injector 4. d) reacting the process gases UFe and NH3, in the reaction zone 1, thereby forming a uranium nitride particles, i.e. a powder comprising UN2 and / or U2N3 and / or UN, as a first reaction product and hydrogen fluoride (HF) as a second reaction product.

[0032] In the reaction zone 1, UN2 and HF are formed by the following reaction.

[0033] UF6+ 2 NH3-> UN2+ 6 HF

[0034] Once UFe and NH3 are in contact, the reaction is very fast.

[0035] Preferably, UN2 is formed but depending on processing conditions, U2N3 and / or UN may also form. Since the end goal for a nuclear fuel is UN, any powder UN2 and / or U2N3 and / or UN is acceptable.

[0036] Thus, the present invention provides UN2 from a chemical reaction between UFe and NH3. Hence, this makes it possible to produce a powder comprising

[0037] UN2 and / or U2N3 and / or UN without using a plasma chemical reactor. e) continuously evacuating the powder comprising UN2 and / or U2N3 and / or UN and the process gases including HF and excessive NH3 from the reaction zone 1 and preferably from the reactor vessel 2.

[0038] The process gases comprise the reaction product HF, excessive NH3 and the shield gas if such is used. They may also include small amount of N2 and H2 from decomposition of NH3.

[0039] The formed powder particles are very fine and will therefore tend to follow the process gases.

[0040] By continuously evacuating the powder from the reaction zone 1, catalytic decomposition of NH3 can be minimized. The reason for this is that the powder comprising uranium nitride particles of UN2 and / or U2N3 and / or UN act as a catalyst that facilitates NH3 decomposition to nitrogen and hydrogen gas at the elevated temperature of the reaction zone. By continuously removing the powder this catalytic decomposition can be minimised, and the production yield of the powder can thus be increased. This gives a higher yield of the powder and a more efficient use NH3. f) separating the powder comprising uranium nitride particles of UN2 and / or U2N3 and / or UN from the process gases.

[0041] The particle separation could for example be made by evacuating the process gases to a cyclone separator or any other known kind of separator for solid particles from gas streams.

[0042] The separation may also include sedimentation of the powder particles below the reaction zone and evacuation by mechanical means. A combination of sedimentation below the reaction zone and recovering of powder particles from the gas stream is of course possible.

[0043] In this step, the temperature is preferably kept high enough to keep HF to form NH4F, the temperature is preferably above 150 °C. For practical reasons the temperature may be kept in the range of 200 - 400 °C. g) Optional absorption step, in which the process gasses after step f) are run through an absorber for adsorbing HF, thereby providing purified process gases.

[0044] After the separation of the powder, the process gasses can be run through a suitable absorber such as CaO to absorb HF. For the same reasons in as in the separating step g), the temperature is preferably kept above solidification temperature of NH4F, 150 °C. For practical reasons the temperature may be in the range of 200 - 400 °C.

[0045] If using CaO as absorber, the absorption of HF results in calcium fluoride. h) Optional recovery step, in which remaining NH3 and / or the optional shield gas is recovered from the purified process gases for reuse.

[0046] After the adsorption step g) remaining NH3 can be recovered and reused. Other gases included the optional shield gas can also be recovered and reused.

[0047] Particularly any gas species containing the isotope15N are economically sensible to recover and reuse. i) Optional decomposing step, in which UN2 and / or U2N3 in the powder is decomposed to UN.

[0048] The UN2 and U2N3 in the powder can be decomposed to uranium mononitride (UN) by the following sequence of reactions:

[0049] 4UN22U2N3+ N2

[0050] 2U2N3 4UN +N2

[0051] This can be done by heating the powder under vacuum and / or inert atmosphere at temperatures between 1000 - 1300 °C, more preferably at temperatures between 1050 - 1200 °C. Complete 100% conversion to the UN occurs within ten hours, usually within 2 - 5 hours. This is preferably done in a separate furnace.

[0052] It is preferred that structural components that are exposed to the process gases comprises a material that have low or zero catalytic activity for ammonia dissociation at the relevant temperature. The material can be applied as a coating or lining. Alternatively, the entire component can be made of the material. A further benefit which such materials, is also to protect the components from hydrogen embrittlement.

[0053] Above 800 °C a refractory material is suitable. This can be a ceramic and / or metallic material, such as but not limited to tantalum and / or boron nitride. Preferably, the ceramic material comprises or consists of boron nitride, more preferably hexagonal boron nitride. Preferably, the metallic material comprises or consists of a tantalum alloy. The ceramic and / or a metallic material are preferably provided in the form of coating or lining.

[0054] One example of structural components that can be exposed to temperatures above 800 °C are the walls of the reaction vessel 2.

[0055] Further examples of components that can be exposed to temperatures above 800 °C are the injectors 3, 4. However, with cooling of the injector it may be possible to use further materials.

[0056] At lower temperatures than 800 °C the range of suitable materials gradually increases, i.e. in addition to refractory materials other materials can be used. For instance, copper can be used below 800 °C. Thermoplastic coating may also be employed such as but not limited to PTFE, PEEK, PF A, ETFE, E-CTFE. Such materials can e.g. be suitable for components used in step f), g) and h) where the temperatures are lower.

[0057] The process can produce uranium nitride powder of isotope15N or14N, or any mix between these isotopes. This is done by controlling the isotope of the nitrogen bearing reactant NH3.

[0058] Use of the isotope15N is preferable because a uranium nitride fuel of this isotope allow a more efficient and compact reactor design. However, the isotope14N is much more abundant and can therefore be chosen for cost and availability issues. The relative volume fraction of15NH3to14NH3can e.g. be 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 95, 100 %.

Claims

CLAIMS1. A method for producing a powder comprising UN2 and / or U2N3 and / or UN, the method comprising the steps of: a) heating a reaction zone in a vessel containing an NH3 atmosphere to a temperature in the range of 700 - 1200 °C to drive a reaction between UFe and NH3, the pressure in the vessel being from 1 up to 45 atm; b) continuously injecting gaseous Fe as a process gas into the reaction zone; c) continuously injecting gaseous NH3 as a process gas into the reaction zone, the amount of NH3 controlled to be in stochiometric excess for the reaction Fe + 2 NH3-> UN2+ 6 HF; d) reacting the process gasses in the reaction zone, thereby forming the powder comprising UN2 and / or U2N3 and / or UN, and gaseous HF; e) continuously evacuating the powder and process gases including HF and excessive NH3 from the reaction zone; and f) separating the powder from the process gases.

2. The method according to claim 1, wherein in step b) co-injecting UFe with a shield gas around the UFe.

3. The method according to claim 1 or 2, further comprising the step of: g) absorbing HF from the process gases, thereby providing purified process gases.

4. The method according to any one of claims 3, wherein CaO is used as an absorber in step g).

5. The method according to claim 3 or 4, further comprising the step of: h) recovery of NH3 and / or the optional shield gas from the purified process gases.

6. The method according to any one of the preceding claims, further comprising the step of: i) decomposing UN2 and / or U2N3 in the powder to UN.

7. The method according to any one of the preceding claims, wherein the walls of the vessel comprise a ceramic and / or a metallic material that preferably have low or zero catalytic activity for ammonia dissociation.

8. The method according to claim 7, wherein the ceramic material comprises boron nitride, preferably hexagonal boron nitride.

9. The method according to claim 7 or 8, wherein the metallic material is a tantalum alloy.

10. The method according to any one of claims 7 - 9, wherein the ceramic and / or a metallic material is in the form of coating or lining.

11. The method according to any one of the preceding claims, wherein the temperature of the injector for UFe is kept below 400 °C.

12. A reaction vessel comprising an injector for gaseous UFe connected to a source of UFe, an injector for gaseous NHa connected to a source of NH3, and a discharge for evacuating powder and process gases, wherein the walls of the reaction vessel comprise a ceramic and / or a metallic material that preferably have low or zero catalytic activity for ammonia dissociation.

13. The reaction vessel according to claim 12, wherein the ceramic material comprises boron nitride, preferably hexagonal boron nitride.

14. The reaction vessel according to claim 12 or 13, wherein the metallic material is a tantalum alloy.

15. The reaction vessel according to any one of claims 12-14, wherein the ceramic and / or a metallic material is in the form of coating or lining.