Patent Application
20140245885
This invention relates to a method and device for implementing the said method, designed to separate, in a gaseous mixture, at least one isotopic compound from another or others contained in said mixture.
More particularly, the invention is aimed at separating stable isotopes of the same body, in order to isolate one of them for its properties and characteristics that make it attractive for the applications and uses envisaged.
There are several methods of separation, most of which are operational, based firstly on the conversion into gaseous form of the body, and secondly on the difference in atomic weights of the isotopes.
The invention relates to methods wherein the gas, possibly after mixing with another gas, namely an inert gas such as helium, argon or nitrogen, flows through a device designed to create a gaseous flow at supersonic speed which is subjected to means of excitation and chemical or physical modification such as lasers or electron beams that have a greater effect on the isotope sought than on the other or others. Then, the isotopes thus modified in gaseous form are, after ionisation, separated by electromagnetic means that make one of the isotopes migrate to the outside of the gaseous flow resulting in the enrichment of the sought isotope.
In general, the gaseous flow is generated by a nozzle, such as a so-called de Laval nozzle, for example, and the excitation and modification means are lasers or electron beams.
For example, the gaseous compound may be SF6, UF6, or WF6, MoF6. In the case of uranium the isotope sought is then U235; in the case of sulphur, the isotope sought is S34 and in the case of molybdenum, the isotope sought is Mo99.
Thus, the aim of these methods is to enrich with the sought isotope the external or internal part of the gaseous flow after separation.
The methods and devices for isotope separation need to reconcile contradictory constraints.
That is because they must simultaneously:
The known methods and devices are still very far from optimising the work required for separating isotopic species. As a result of the low enrichment rates achieved in a single pass of gas in common devices such as centrifuges, the installations must be designed in a serial manner to obtain acceptable enrichment rates. That means that the installations have to be large, with high power consumption in relation to the minimum thermodynamic work required for enrichment. The patent FR 2 370 506 describes an example of such installations.
Other methods or devices, such as that described in the US patent U.S. pat. No. 4,119,509, lead to a flow regime that is not compatible with the efficient separation of isotopes. That is because the device does not inhibit diffusion, resulting in a mixture of isotopologues in the gaseous flows, greatly limiting the efficiency of the method.
The invention discloses a method and device that constitute a significant advance in the relevant field by allowing high-efficiency isotope separation with reasonable quantities of gas and power, using a small installation.
To that end, according to the invention, the method for the isotopic separation of at least two different isotopes of a body in gaseous form comprises the steps stated in claim 1.
The ion recovery step is preferably carried out with ion recombination.
The gas may, for example, be SF6, UF6 or MoF6.
The molecules are excited by vibrations with the help of at least one laser, for example of the CO2 type in the case of UF6 gas.
The gas is mixed with at least one first inert gas, such as helium or argon.
The speed of the gaseous flow is supersonic.
Advantageously, the means to create the laminar or slightly turbulent flow is a nozzle, of the de Laval type, preferably with flat geometry comprising:
The nozzle is supplied with a gaseous flow that firstly comprises a central or inner jet formed of a mixture of gas to process and a first other gas, namely an inert gas, and secondly a peripheral or outer jet formed of a second other gas, namely an inert gas.
The two jets are substantially isolated from each other. In the case of a revolution nozzle, the two jets are concentric.
The first and second inert gases may be the same, and are selected from helium, nitrogen, argon and hydrogen.
The nozzle is supplied by means of a chamber supplied with gas, which has:
The internal deflector is located upstream from the throat of the nozzle or extends downstream up to it.
A mixture of said gas and a first inert gas flows in the inner part of the deflector and a flow of a second inert gas is provided between the deflector and the convergent of the nozzle. These two gases are independent and come from two separate tanks.
After expansion in the divergent of the nozzle, part of the isotopes present are made to migrate from the inner flow to the outer flow.
The nozzle is generally planar in shape, so as to increase the efficiency of the process by minimising the mass flow of the species flowing in the boundary layers and maximising the mass flow of the species flowing in the isentropic core, where selective laser radiation absorption occurs.
The nozzle may for example have the following dimensions:
The exit section of the internal deflector is positioned near the throat of the nozzle or offset by a few millimetres upstream from said throat.
These dimensions correspond to a speed of Mach 4, which itself corresponds to a temperature of 40° K. at the core of the flow. That allows satisfactory selectivity of laser radiation absorption and thus guarantees a high enrichment rate.
The width of the exit section of the conduit ranges between 5 and 20 times the width of the throat, and is preferably close to 7 times the width of the throat.
The length of the divergent conduit is at least 2000 mm and/or ranges between 20 and 30 times the height of the throat.
In the case of a revolution shape, the nozzle has the following dimensions as an example when it is of the revolution type:
The diameter of the exit section of the conduit ranges between 3 and 6 times the diameter of the throat, and is preferably between 3 and 4 times
The length of the divergent conduit is at least 1300 mm and/or ranges between 13 and 30 times the diameter of the throat.
Whether the nozzle is planar or of the revolution type, the diffusion speed Vd is far smaller than the average speed Vq in the section in question of the nozzle Vd=D/e, where D is the diffusion coefficient and e is the distance that separates the walls opposite the external and internal deflectors.
Means to inhibit or reduce the phenomena of nucleation of molecules of the gaseous flow are provided in the conduit or in the flow outside the nozzle, and preferably, said means comprise at least one laser, for example of the infrared type, where the light direction is transversal to the gas flow.
Cooling means may be used in order to reduce the viscosity of the gas at the walls and thus the flow of the carrier inert gas. These cooling means allow gas flow temperatures between 4° K. and 80° K., preferably between 4° K. and 10° K..
The method is based on the uniformity of the flow at its core, which means that the pressure and temperature conditions remain unchanged in the direction of the flow over lengths of a few metres and transversally over several tens of centimetres. The shapes of the nozzles described make it possible to validate these conditions.
The invention will be better understood in the light of the description below relating to illustrative but non-limitative examples, by reference to the figures, wherein:
The description below refers to a particular application, given on an illustrative but non-limitative basis, of the isotopic separation of two uranium isotopes, namely U235 and U238, from a gaseous compound of uranium, UF6.
It is understood that the invention is not limited to such compounds and may be applied to another gaseous mixture, for example SF6.
The device according to the invention as described below by reference to the figures is aimed at separating the two isotopes by isotope enrichment in order to use the isotope sought, such as U235.
The ideas of ‘upstream’ and ‘downstream’ refer to the direction of flow of the moving gases, containing the isotope sought.
The device creates a flow of the gas at supersonic speed with the help of a device called a de Laval nozzle, reference 1, made up of a vessel with an increasing rectangular transverse section (transversal to the gas flow), the dimensions of which at the exit of the nozzle are for example: height between 500 mm and 1200 mm, preferably about 1000 mm, and width between 500 and 1200 mm, preferably about 1000 mm.
The nozzle 1 comprises an upstream supply section 1A, a central section 1B and a downstream section 1C.
The upstream section 1A comprises a convergent with a rounded section comprising a rounded upper plate 3A and a rounded lower plate 3B.
The central section 1B is made up of a throat formed of a parallel upper plate 4A and a parallel lower plate 4B.
The downstream section 1C of the nozzle comprises an upper planar plate 5A and a lower planar plate 5B forming a divergent.
It is understood that the upstream section 1A, the central section 1B and the downstream section 1C also each comprise vertical walls on the sides, which are not visible in the schematic section of
A device 6 is provided upstream from the nozzle 1 to supply the nozzle with the gas to be processed, particularly in a way as to generate a gas mixture jet with particular characteristics in the nozzle 1.
The supply device comprises a chamber 6, which is a parallelepiped for example, with an exit opening made up of the convergent with a rounded section 3A, 3B of the upstream section 1A of the nozzle 1.
Inside the chamber 6, a deflector 7 is provided, formed of an upper curved plate 7A and a lower curved plate 7B respectively.
Each deflector plate 7A, 7B successively comprises:
The convergent plates 8A, 8B have curvatures that are very close or similar to those of upstream plates 3A and 3B, which form both the entry of the nozzle 1 and the exit of the chamber 6. The distance between the plates 8A, 8B from the plates 3A, 3B is preferably constant. The plates are welded edge-to-edge. Details are given below by reference to
The chamber 6 is supplied with:
These two gaseous flows come from separate source tanks (not represented and known in themselves) and the deflectors 7A and 7B are designed so that said gases do not mix with each other before the throat of the nozzle.
All the gases from the chamber 6 enter the nozzle 1 by the throat 4A, 4B.
The gas jet from the nozzle 1 enters a downstream expansion chamber (not represented in
Then, the gaseous flow goes through a magnetic deviation device (not represented and described by reference to
The chamber 6 and the deflector 7 are aimed at generating a flow at supersonic speed in the nozzle, in the form of a central jet, in a planar or revolutionary shape depending on the geometry of the nozzle, surrounded by a so-called peripheral annular jet, as described below.
The chamber 6 is schematically represented by a cube comprising an upper wall 11, a lower wall 12, side walls 13 and 14, a front wall 15 (downstream) and a rear wall 15A (upstream).
In a preferred embodiment, the divergent walls 5A and 5B of the nozzle are cooled by cryogenics, using means known in themselves such as conduits 16 and 17 connected to fluid circulation circuitry and opening into the walls 5A, 5B, wherein the source of cryogenic fluid is not represented and is known in itself.
Devices are provided at the exit of the nozzle 1 that are designed to excite the particles of the gaseous mixture leaving the nozzle. These means are for example infrared type lasers that are known in themselves and referenced 18.
After the excitation step, the gas flow passes through ionisation means 19 made up of electron beams or excimer lasers, that is to say a laser using a gas mixed with a halogen. Under the effect of electronic stimulation, an excimer is formed, which upon returning to its fundamental state emits laser radiation with a determined wavelength. This type of laser currently makes it possible to reach wavelengths below 200 nm in vacuum ultraviolet.
After that, the gas flow enters an isotope separation assembly 20 based on the forced diffusion properties under the effect of electromagnetic fields of the compounds of the gaseous mixture.
The magnetic separation device 20 comprises two rectangular plates 21 and 22 with “−” and “+” polarities respectively, parallel to the direction of the gas flow, and two superconducting magnets 23 and 24 with annular shapes that are known in themselves and located on either side of the jet, along two parallel planes transversal to the polarised plates 21 and 22.
At the exit of the magnetic separation device 20, a device 10 is provided for so-called ‘differential pumping’ diffusion, connected downstream to pumps (known in themselves and not represented) by the flow arrow 25.
The device 10 is described by reference to
In the chamber 6, the arrow “f” represents the flow direction of the internal mixture SF6 and the buffer gas (He). The arrows “g” and “g′” represent the external flow of helium.
The shape of the nozzle and its dimensions are calculated on the basis of the following elements and parameters: temperature, pressure, nature and proportion of mixed gases (in the introduction part of the upstream chamber 6 of the nozzle, for example SF6 plus buffer gas).
The upstream convergent deflecting walls 9A and 9B have a curvature radius R1 of 26 mm, whereas the upstream convergent walls 4A and 4B of the nozzle have a curvature radius R2 of 25 mm. The throat 4A, 4B of the nozzle has a height dimension “h” transversal to the flow of about 10 mm.
The distance separating the convergent deflecting plates 9A, 9B from the convergent 3A, 3B of the nozzle 1 is as small as possible, about h/10, all other parameters being equal. Lastly, the distance “I” along the flow direction between the downstream end of the deflecting plates 9A and 9B on the one hand and the start of the throat 3A, 3B on the other hand is for example 18 mm.
As an alternative (not illustrated), the nozzle and the upstream deflector (in the chamber 6) may be made in the form of a revolution body (for example a conical body) where the centre line corresponds to the direction of the gas flow.
The nozzle has a shape and dimensions that are such:
The total flow of the peripheral gas (helium) outside the two deflecting walls 9A and 9B and the gas mixture to process with the buffer gas is distributed, at the downstream exit of the nozzle, that is to say at the ends of the divergent plates 5A, 5B so that:
Cooling by cryogenics of the walls of the divergent 5A, 5B of the nozzle, with liquid helium, for example, makes it possible to reduce and diminish very substantially the mass flows of the processed gas flow.
The properties of the inner flow “I” around the central line of the nozzle are such that there is no diffusion between the outer flow “E” and said inner flow “I” and that the viscosity of the flow is negligible. The Reynolds number is high, that is greater than 2000. It is known that if the diffusion coefficient is of the order of magnitude of the kinematic viscosity, which is the case with many gases, it provides the order of magnitude of the speed of the whole in relation to the diffusion speed. A large Reynolds number thus implies negligible diffusion.
The aim is to ensure that the sought isotopologues are removed to the outer flow, thanks at least in part to forced diffusion, the scale of which depends greatly on the mass, under an electromagnetic field of the particles after ionisation by the ionisation means 19 (
One of the merits and one of the particularities of the invention is to use aggregate ions for separation, because the ions can easily acquire an overall average speed under the effect of an electromagnetic field that is of the same order of magnitude as the average speed of the flow for a specified mass.
The method according to the invention uses the mass distribution of aggregates before ionisation and those aggregates are then separated after ionisation.
The shape and dimensions of the walls of the separator are calculated on the basis of the current lines, so that the separators follow the current lines.
According to one particularity of the method in the invention, natural molecular diffusion inside the jet, between parallel current lines, is considered to be negligible.
The complete ionisation device comprises two types of source:
The IR laser inhibits the nucleation of the excited species to a few %.
Nucleation leads to an increase in the effective ionisation section of the aggregates and a reduction of their volume concentration.
The isotopologue 34SF6 is excited and is thus less aggregated than the other isotopologue (32SF6). The monomers and small aggregates of 34SF6, which are far lighter, are thus more strongly deviated after they are ionised by the electromagnetic means placed downstream as a result of their lower mass.
32SF6 is statistically more aggregated, and thus has aggregates with a greater mass than the isotopologue 34SF6. As a result, the downstream electromagnetic field has a much smaller influence on 32SF6. It would therefore not “leave the jet” as much.
Note that the excitation IR laser is not strictly necessary as nucleation at its outset leads to ‘natural’ enrichment of the least abundant isotopologue but is provided in a preferred form of implementation of the invention. The merit of the IR laser is to increase the mass differentiation between 34SF6 and 32SF6 by inhibiting the aggregation of monomers.
Ionisation may be achieved in two ways:
1. either in the so-called ‘direct’ way, that is to say by a laser that creates cations and electrons, by the reaction: photons+aggregate→aggregate ionised into cations+electrons; this leads to the creation of positively ionised aggregates;
2. or in two successive ionisation stages, by laser or electron beam or electrical discharge, to create negatively ionised aggregates (anions), namely:
The second way above (2) is preferred.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1154820 | Jun 2011 | FR | national |
This application claims priority from International Patent Application Number PCT/FR20121051222 filed on May 21, 2012 which claims priority from French Patent Application No. 1154820 filed on Jun. 1, 2011.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2012/051222 | 5/31/2012 | WO | 00 | 5/23/2014 |
