Method for forming uranium silicide

Abstract

Methods for producing uranium silicide for use as a nuclear fuel. In a first method, a powder composition (optionally in the form of a pellet) comprising uranium dioxide, silicon carbide, and carbon i

Description

Field of the Invention

[01] The present disclosure relates to a method for forming uranium silicide for use as a nuclear fuel; particularly a fuel for research reactors. In particular, the disclosure relates to a method of preparing uranium silicide via a silicocarbothermic route. Background

[02] Uranium silicide (U3Si2), is a nuclear fuel used in research reactors. Current techniques for synthesis of uranium silicide employ uranium metal, in a process which is highly energy and resource intensive.

[03] As an alternative, the synthesis of uranium silicide (U3Si2) starting from uranium dioxide (UO2) has been successfully developed by Westinghouse (US 2018 / 0370808 A1). The technique first combines UO2with carbon to produce uranium carbide (UC), which is subsequently allowed to react with silanes and silicon halides at high temperature to finally yield UsSi2. A downside, however, is that the process is highly complex and involves hazardous, volatile chemicals (e.g., the silanes and halides).

[04] Therefore, a safer and more cost effective alternative route to achieving uranium silicide is highly desirable. The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein. Summary

[05] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[06] The described techniques allow the preparation of uranium silicide (UsSi2) via a silicocarbothermic route from uranium dioxide (UO2), using highly stable, non-volatile reactants.

[07] Suitably, in one aspect of the invention there is provided a method for forming uranium silicide. The method comprises reacting a powder composition (optionally in the form of a pellet) comprising uranium dioxide, silicon carbide, and carbon. Such a reaction suitably does not require any volatile initial reactants, nor generates any particularly harmful products.

[08] In an example, the composition further comprises silicon dioxide and elemental silicon. Adding silicon dioxide and silicon may compensate for the loss of silicon as volatile Si and SiO, which form as intermediate products in the reaction prior to the formation of the uranium silicide. Here, a molar ratio of initial reactants in the powder composition may be substantially 3 parts uranium dioxide, from about 2 to 3 parts silicon carbide, preferably 2 parts, from about 3.5 to 4.5 parts carbon, preferably 4 parts, and when present, from about 0 to 1.4 parts silicon dioxide and about 0 to 3 parts silicon.

[09] In some examples, the reaction may be carried out at a pressure of less than or equal 0.0001 millibar while at a temperature of greater than or equal to 1100 °C, at a pressure of less than or equal to 0.01 millibar while at a temperature of greater than or equal to 1300 °C, or at a pressure of less than or equal to 0.5 millibar while at a temperature of greater than or equal to 1500 °C. In general, the reaction may be conducted at a temperature T (expressed in °C) and pressure p (expressed in bar) which satisfy the inequality Logio <-37.7 + 0.051 • T -2.61 ■ 10-5 ■ T2 + 5.11 • 10-9 ■ T3. These temperatures and pressures are readily achievable in an efficient manner using currently available furnaces and vacuum pumps, thereby providing a cost effective route to obtaining uranium silicide.

[10] Optionally, the method may comprise obtaining the silicon carbide in situ by reacting silicon dioxide with carbon. Here a molar ratio of initial reactants in the powder composition is substantially 3 parts uranium dioxide, from about 10 to 16 parts carbon, and from about 2 to 5 parts silicon dioxide.

[11] In a related aspect of the invention, there is provided methods of forming uranium silicide starting from uranium carbide. One method comprises reacting a powder composition comprising uranium carbide, silicon carbide, silicon dioxide and optionally uranium dioxide. Another method comprises reacting a powder composition comprising uranium carbide, silicon, and at least one of silicon dioxide and uranium dioxide.

[12] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. The term “about”, or substantially, when used herein means + / - 5% of the stated value. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’. Brief Description of the Drawings

[13] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[14] Fig. 1 shows a graph of example pressure and temperature zones for achieving a suitable reaction for the production of uranium silicide. Detailed Description

[15] At least some of the following example embodiments provide an improved method for obtaining uranium silicide via a silicocarbothermic route from uranium dioxide (UO2).

[16] In one embodiment, the method comprises reacting a powder composition comprising at least uranium dioxide, silicon carbide, and carbon. The reaction may be conducted in a furnace (e.g., a research furnace) at suitable temperature and pressure (i.e., under vacuum), as discussed further below.

[17] As an alternative to powder form, the method may utilise pellets of reactants for ease of handling and to limit contact between the reaction mixture and the underlying furnace crucible (which is a potential source of contamination). In one example, pellets may be obtained by uniaxially compacting the powdered reactants at a compacting pressure of from 100 to 500 MPa (megapascals). Lubricants such as zinc stearate or lithium stearate may be added to a suitable pressing die to improve the mechanical stability of the pellet, but their presence does not significantly affect the synthesis of LhSi? due to the very small amount that is retained on the pellet.

[18] An example reaction is as follows (whereby upwards arrow f indicates a volatile product): 3 UO2 + 2 SiC + 4 C ->• U3Si2 + 6 CO f (1)

[19] As shown, a preferred molar proportion (or ratio) of UO2: SiC : C is substantially 3:2:4. Preferably, the proportion of uranium dioxide is generally always about 3 parts of the composition. The remaining reactants however can be varied. For example, silicon carbide may form from about 2.0 to about 3.0 parts of the starting composition, preferably from about 2.0 to about 2.5 parts, while the carbon may form from about 3.5 to about 4.5 parts of the starting composition.

[20] Suitably, other than U3Si2, the reaction ideally produces only carbon monoxide (CO). CO is gaseous, and as such is automatically separated from the solid (in powder or pellet form) reaction products during synthesis, as it is removed by the vacuum pump and leaves no residues.

[21] While CO is a noxious gas, there is ample experience on its industrial handling and exhaust treatment systems for CO are well established: e.g., a gas flare can combust CO into far less harmful CO2. CO is a far less dangerous and volatile product than silane (SiH4) and silicon halides. As much is reflected in the fact that the (UK) Workplace Exposure Limit in ambient air for a 8-hour shift is 23 mg / m3 for CO, and 0.67 mg / m3 for SiFk

[22] In some implementations, solid uranium oxycarbide (UCp-xjOx, with x ranging between 0 and 0.5) may be formed as an intermediate product and remain mixed with uranium silicide in the produced solids. Uranium oxycarbide is not significantly more hazardous than UO2 or U3Si2, so its presence in the reaction environment is not a safety concern. However, its presence does present a challenge for separating desirable uranium silicide form the uranium oxycarbide. The formation of uranium oxycarbide via reaction (1) is accompanied by the formation of silicon and silicon oxide. Example reactions are as follows: 3 UO2 + 2 SiC + 4 C 3 UC0.50O0.50 + 2 Si | + 4.5 CO f (2) 3 UO2 + 2 SiC + 4 C 3 UCo soOo 17 + 2 SiO f + 3.5 CO J (3)

[23] These reactions occur concurrently and in varying proportion, depending on the reaction conditions, and reactions (2) &(3) represent two ideal cases whereby all initially present silicon atoms are lost as elemental Si and as SiO, respectively. The resulting ideal oxycarbide phases will equilibrate into a single one in reaction (4): x UC0.50O0.50 + y UC0.83O0.17 —> UCzO(i-z; (4) where x, y, and z are real numbers, with (x + y) = 1 and z ranging between 0.83 and 0.5.

[24] Reactions (2) and (3) have the effect of depleting the reaction composition of silicon and oxygen atoms, leaving behind an excess of uranium and carbon.

[25] In one example, these silicon losses may be minimised by reducing the temperature at which the process is carried out (also desirable for energy saving), as this can prevent, or at least limit, the volatilisation of Si and SiO from the reaction environment, allowing them to react with the respective oxycarbides through reactions (5) and (6) (and their linear combinations), thereby converting the oxycarbide intermediate into the desirable uranium silicide. 3 UC0.50O0.50 + 2 Si —> UsSi2 + 1.5 CO f (5) 3 UCo 83O017 + 2 SiO U3Si2 + 2.5 CO f (6)

[26] Suitably, silicon losses may be offset by adding a tailored mixture of silicon dioxide (SiO2), elemental silicon (Si), and SiC to the reaction composition. That is, the initial powder composition may comprise uranium dioxide, silicon carbide, carbon, and at least one of silicon dioxide and silicon.

[27] In a case where silicon dioxide is added to the starting composition, a residual uranium oxycarbide with formula UC0.75O0.25 may react as follows: 9 UC0.75O0.25 + 3.5 SiO2 + 2.5 SiC 3 U3Si2 + 9.25 CO (7)

[28] Here, the silicon dioxide may be added in a molar proportion from about 0.0 to about 1.4 parts, preferably from about 0.0 to about 0.7 parts.

[29] Silicon dioxide may also be used to prepare the silicon carbide in situ by a reaction of silicon dioxide and carbon, as an alternative to starting with previously obtained silicon carbide in the composition. An example reaction is as follows: SiO2 + 3 C SiC + 2 CO ] (8)

[30] Reaction (8) is more favourable than reaction (1), and will occur before the latter starts. Suitably, in an example implementation, all of the initial content of SiC in the composition may be replaced by adequate amounts of SiO2 and C, in the ideal overall reaction (9), which is essentially the sum of reactions (8) (taken twice) and (1): SiO2 + 3 C SiC + 2 CO T (x2) + (8) 3 UO2 + 2 SiC + 4 C U3Si2 + 6 CO ) = (1) 3 UO2 + 2 SiO2 + 10 C U3Si2 + 10 CO ] (9)

[31] Put another way, the starting composition for synthesising uranium silicide may comprise uranium dioxide, silicon dioxide, and carbon. As can be seen from (9), here a preferred molar ratio (or proportion) of UO2 SiO2: C is about 3 : 2 : 10. As with reaction (1), while it is preferable to keep the proportion of UO2 at about 3 parts, the proportion of SiO2 may be from about 2.0 to about 5.0 parts, preferably from about 2.0 to about 4.5 parts, while the carbon may be from about 10.0 to about 16.0 parts, preferable about 10.0 to about 15.0 parts.

[32] It will be appreciated that, advantageously, the number of different types of reactants that must be sourced forthe initial reaction may be reduced. Moreover, the reactants for (9) are less expensive than the reactants for (1), making the process more commercially viable. A downside of exploiting this route are challenges with mechanical stability when using pellets (the greater the proportion of carbon in the initial reaction mixture, the more fragile the pellets obtained thereof will tend to be), but the issue can be addressed with the use, for example, of carbon-based polymeric binders, which is well established in the industry. The benefits of this route generally outweigh the negative.

[33] In the case where elemental silicon (Si) is added to the starting composition, there exists an ideal bounding case in which all of the silicon required to produce U3Si2 is provided as Si in the initial reaction mixture, corresponding to reaction (10): 3 UO2+2 Si + 6 C U3Si2 + 6 CO ) (10)

[34] Including silicon losses to Si and SiO via the uranium oxycarbide process and their offsetting through the addition of SiC, SiO2, and Si to the initial reaction mixture, the overall reaction process may be described as: 3 UO2 + w Si + x SiC + y C + z SiO2 -> U3Si2 + y CO f + j Si J + k SiO J (11)

[35] Here, w, x, y, z, j, k are real numbers representing the stoichiometric coefficients of the reactants and the products. Preferably w ranges from 0.0 to 3.0, preferably from about 0.0 to 2.5, x ranges from about 0.0 to 3.0, preferably from about 0.0 to 2.5, y ranges from about 3.5 to 16.0, preferably from about 3.5 to 15.0, and z ranges from about 0.0 to 5.0, preferably about 0.0 to 4.5.

[36] The above reactions may be achieved in a parameter space defined by the reaction temperature and by the pressure of CO in the reaction chamber - i.e., the CO pressure in the overhead space of the furnace. The pressure of CO can be lowered by actively removing the gas as it forms (e.g., with a vacuum pump or with a flow of inert gas). In the case where the furnace is operated in conjunction with a vacuum pump, it will be appreciated that the CO pressure is substantially the same as (or at least not significantly different too) the overall vacuum in the chamber provided by the pump.

[37] In general, lower pressure equals lower temperature, and as such lower pressure is generally preferred as there is a correspondingly lower power requirement for achieving a suitable reaction temperature (the savings in heating power generally being greater than increases in power to achieve lower vacuum). In addition, lower pressure / temperature result in lower equipment wear and a slower rate of reaction for unwanted or parasitic processes. Suitably, the process becomes more commercially viable.

[38] Several types of vacuum pumps which are effective at removing CO exists which are able to achieve different degrees of vacuum. In an example, a rotary vane pump may be used which may achieve an ultimate vacuum pressure of about 0.06 mbar (millibar), after taking account of typical small leaks that may develop as the seals of the furnace expand with temperature. In another example, a commercially available furnace fitted with a turbomolecular pump can reach ultimate vacuum pressures as low as 10-6 mbar at room temperature and 10-4 mbar at 1500 °C.

[39] Figure 1 shows an example region for which the synthesis of U3Si2 from the above composition is possible (shaded area). The dashed lines represent the ultimate vacuum pressures of the two vacuum pumps mentioned above. The conditions conducive to the synthesis of U3Si2 with a given vacuum pump connected to the furnace are therefore those which fall in the shaded area and are above the relevant dashed line (e.g., generally above 1100 °C for the turbomolecular pump and above 1400 °C for the rotary pump).

[40] Suitably, in an example the reaction may be carried out at a pressure of less than or equal 0.0001 mbar while at a temperature of greater than or equal to 1100 °C. In another example, the reaction may be conducted at a pressure of less than or equal to 0.01 mbar while at a temperature of greater than or equal to 1300 °C. In another example, the reaction may be conducted at a pressure of less than or equal to 0.5 mbar while at a temperature of greater than or equal to 1500 °C.

[41] It will be appreciated that intervening ranges may also be appropriate. For example, the reaction may be conducted at a pressure greater than 0.0001 mbar and less than or equal to 0.01 mbar while at a temperature of greater than or equal to 1100°C and less than or equal to 1300 °C. In another example the reaction may be conducted at a pressure greater than 0.1 mbar and less than or equal to 0.5 mbar while at a temperature of greater than 1300 °C and less than or equal to 1500 °C.

[42] A mathematical description of the shaded region - i.e. only describing the shape of the region in which the reaction may occur, but not containing any numerical reference to the physical phenomena that shape it - may be given as: £0^1° <-37.7 + 0.051 ■ T - 2.61 ■ 10 5 • T2 + 5.11 ■ 10 9 ■ T3, where the left-hand member is the base-10 logarithm of the pressure of CO (expressed in bar) and the right hand member is an expression of the temperature T (expressed in °C).

[43] It will be appreciated that the staring reactants of the composition - e.g., uranium dioxide, silicon carbide, carbon, and if present silicon dioxide - may be obtained by any suitable approaches known in the art.

[44] For example, UO2 may be specifically obtained as part of the present method via transformation of Uranium hexafluoride (UFe) with hydrogen and water by a suitable ammonium uranyl carbonate process or uranium diuranate (ADU) process.

[45] In addition / alternatively, large quantities of uranium dioxide in the form UO2+X (X from 0 to 0.25, more typically X from 0.02 to 0.15) are manufactured yearly for the nuclear fuel industry. Any value of X is usable with the present technique since in the reaction conditions of the present method (i.e., high temperature, presence of carbon, actively pumped vacuum) UCh+x will react with carbon or spontaneously decompose in two concurrent processes: UO2-X + X C UO2 + X CO 1 (12) UO2+X UO2 + X / 2 O2 $ (13)

[46] Varying x slightly affects the optimal ratio between UO2+X and C in the starting composition.

[47] As an alternative to uranium dioxide, other oxides of uranium such as U3O8 and UO3 may be used in the starting composition, as like UO2+X they will convert into UO2 in the projected conditions both by spontaneous decomposition and by reduction with carbon. For the sake of simplicity of notation, U3O8 can be also described as UO2 67, so that reactions (12) and (13) can apply for X = 0.67 and X = 1 as well.

[48] In another embodiment of the invention, the method may be initiated with uranium carbide as one of the starting reactants, replacing suitable amounts of UO2 and C in reaction (11) according to reaction (14), not necessarily with the totality of UO2 being replaced in this way: UO2 + 3 C -> UC + 2 CO f (14)

[49] In this embodiment, the method comprises reacting a powder (or pellet) composition comprising uranium carbide, silicon carbide, silicon dioxide, and optionally uranium dioxide, or reacting uranium carbide and silicon with at least one of silicon dioxide and uranium dioxide. As above, the reaction may be conducted in a furnace (e.g., a research furnace) at suitable temperature and pressure (i.e., under vacuum) as per the previous embodiment.

[50] Example reactions are given by: 3 UC + 1 / 3 SiC + 5 / 3 SiO2 -► U3Si2 + 10 / 3 CO f (15) 3 UC + 0.5 Si + 1.5 SiO2 U3Si2 + 3 CO J (16) 2 UC + UO2 + 2 Si U3Si2 + 2 CO] (17) 2 UC + UO2 + 4 / 3 SiC + 2 / 3 SiO2 ->• U3Si2 + 10 / 3 CO 1 (18)

[51] Uranium carbide may be obtained from reacting uranium dioxide with carbon as in reaction (14), and is a likely intermediate product in reaction (1). As such, while it is possible to separately synthesise UC, such a process is presently not preferred as the added synthesis step increases potential for contamination and uncertainty. UC2 may also be used, but it would not be as straightforward and would require a UC2-UO2«-SiO2-SiC mixture to work.

[52] In summary, exemplary embodiments of an improved method for synthesising uranium silicide have been described. The method may be utilised industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[53] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

[54] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[55] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[56] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[57] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 31 05 24

Claims

1. A method for forming uranium silicide, comprising reacting a powder composition comprising uranium dioxide, silicon carbide, and carbon, wherein the reaction is conducted at a temperature and pressure which satisfy the inequality Log10 <-37.7 + 0.051 ■ T - 2.61 ■ 10^-^ + 5.11-10-9^3.

2. The method of claim 1, wherein reacting the composition of uranium dioxide, silicon carbide, and carbon comprises forming uranium oxycarbide as an intermediate product prior to the formation of the uranium silicide.

3. The method of claim 1 or 2, wherein the composition further comprises silicon dioxide.

4. The method of any preceding claim, wherein the composition further comprises silicon.

5. The method of any preceding claim, wherein a molar ratio of initial reactants in the powdercomposition is about 3 parts uranium dioxide, from about 0 to about 3 parts silicon carbide, from about 3.5 to about 6 parts carbon, and when present from about 0 to about 1.4 parts silicon dioxide and from about 0 to about 3 parts silicon.

6. The method of any preceding claim, wherein the silicon carbide is obtained in situ by reacting silicon dioxide with carbon.

7. The method of claim 6, wherein a molar ratio of initial reactants in the powder composition is substantially 3 parts uranium dioxide, from about 10 to about 16 parts carbon, and from about 2 to about 5 parts silicon dioxide.31 05 248. The method of any of claims 1 to 7, wherein the reaction is carried out at a pressure of less than or equal 0.0001 millibar while at a temperature of greater than or equal to 1100 degrees Celsius.

9. The method of any of claims 1 to 7, wherein the reaction is conducted at a pressure of less than or equal to 0.01 millibar while at a temperature of greater than or equal to 1300 degrees Celsius.

10. The method of any of claims 1 to 7, wherein the reaction is conducted at a pressure of less than or equal to 0.5 millibar while at a temperature of greater than or equal to 1500 degrees Celsius.

11. The method of any preceding claim, wherein the uranium dioxide is in the form UO2+X, where X is from about 0 to about 0.25.

12. The method of claim 11, wherein X is in the range from about 0.2 to about 0.15, inclusive.

13. The method of any preceding claim, wherein the powder composition comprises pellets of reactants.

14. The method of claim 13, further comprising obtaining the pellets of reactants by compacting powdered reactants under pressure between 100 and 500 megapascals.

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