CA1072048A - Isotope separation process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The instant invention relates to a process for separating a material into two or more parts in each of which the abundances of the isotopes of a given element differ from the abundances of the isotopes of the same element in said material. More particularly, the invention relates to a method for the isotopically selective excitation of gas phase molecules by infrared radiation and the conversion of these molecules either by the same radiation or by an excited state chemical reaction to form a product which may be separated by means known in the art. The in-stant invention is useful for but not limited to the separation of the principal isotopes of uranium.

Description

~ ~ ~ 4 8 The instant Invention relates to a proce~s fQr separating a material into two or more parts in each of which the abundances oE the isotopes of a given element dif~er from the ahundances of the isotopes of the same element in said material. More particularly? the inven~ion relates to a method for the isotopically selective excitation of gas phase molecules by infrared radiation and the conversion of these molecules either by the same radiation or by an excited state chemical reaction to form a product which may be separ-ated by means known in the art. The instant invention is useful for but not limited to the separation-of the-principal isotopes of uranium.
This application relates to U.S. Patent 3,937,956 and to U.S. Patent 4,003,809, both of R.K. Lyon; that is, to isotope separation processes wherein, in a first step IR photon absorption is utilized to selectively excite one isotope of an isotopic mixture, and said excited isotope is converted in a second step to a Eor= which can be recovered from said mixture.
In order that the instant invention may be clearly understood, it is usefu~ to review the prior art relating to photochemical isotope separation.
U.S. Patent 2,713,025 and British Patent 1,237,474 are good examples of processes for the photochemical separation of the isotopes of mercury. The first requirement for a photochemical isotope separation is that one finds

2~ con~itions such that atoms or molecules of one isotope of a given element absorb light more strongly than do atoms or molecules of another isotope of said element. ~ercury is a volatile metal and readily forms a vapor of ~ . .

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1 atoms. Said atoms absorb ultraviolet llght at 2537 A. The 2 absorption line Of ~g202 is displaced by about 0.01 A with ~ :

3 respect to the absorption line of Hg200~ Since the absorp- :

4 tion lines are extremely narrow, one may by use of light in a critically narrow wavelength region excite either Hg200 or 6 Hg202 7 The second requiremen~ for a photochemical iso~ope 8 separation is that those atoms or molecules which are e~ci-9 ted by light undergo some process which the atoms or mole~
lo cules which have not been excited do not undergo~ or atleas~
1I do not undergo as rapidly. A quantum of 2537 A ultraviolet 12 light imparts an excitation of 112s7 Kcal/mole to the mercury -~
13 atom which absorbs it~ The number of mercury atoms which at 14 room tempera~ure are thermally excited to this energy i9 van-ishlngly s~all, hence the atoms excited by ligh~ are not di-16 luted by atoms excited by thermal means. Atoms of this high 17 excita~ion readily undergo reactions with H20 (as taught in 18 the U.S. patent) or with 2~ HCl or butadiene (as taught in 19the British patent), said reactions not occurring at room :
temperature with unexcited mercuryr 21Uranium, however, ls a highly reractory metal~
22 boilL~g only at extremely high temperatures. Thus, use of 23 the above-described process wit~ uranium atoms instead o~
24 mercury involves obvious dificultiest The most volatile form o~ uranium is UF6~ U235F6 and U238F6 both absorb ultr~
2h violet light and do 90 to exactly the same exten~ at all 27 wavelengths in the W ; hence, UV excitation of UF6 does not 23 satisy the first requirement of photochemical isotope sepa- :
29 ration However, UF6 will also absorb infrared l~ght in the 30 region around 626 cm-l (the ~ 3 band) and 189 cm~l (the ~ 4 ,~ .

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1 band). Both the ~3 and ~4 bands of U235F6 are shifted 2 slightly toward higher energy with respect to the ~ 3 and ~4 3 bands of U238F6 respectively, but the size of these shifts 4 is small compared to the width of the bands; in other words, the infrared absorption spectra of U238F6 and ~235F6 do not 6 exactly coincide, bu~ they ov~rlap at all wavelengths so -7 that i one lsotope absorbs light, so, to a substantlal de-8 gree, will the other~ Hence, the i~frared excitation of UF6 9 by absorption of a single IR photon is a p~ocess of limited ~-0 isotopic selectivity.
11 Since the means of staging isotope separation are 12 we~l known7 this limited selectivity may s~ill be useul;
13 however, the second requirement for isotope separation is 14 also a matter of some difficulty for UF6. UF6 molecules which are excited by a single photon o IR light receive a 16 small amount of energy and are only slightly different from 17 unexcited molecules. Thus, if one is to convert ~he excited 18 molecules while leaving the unexcited molecules unconverted, 19 a con~ersion means is required which is highly selective with respect to the energy content of the molecules.
21 There is the further difficulty that since the en-22 ergy provided by photon excitation is small, molecules may be 23 thermally excited to the same energy level Thus, if a group 24 of molecules each absorb an IR photon, these ~olecules which were photoexcited with isotopic selectlvity will be diluted 26 with molecules produced by unselective thermal excitation.
27 The photoexcited molecules will rapidly disappear but the 28 thermally excited molecules are continually replenishedO Thu~
29 as time passes after the excitation the d~lution of selecti~
ly excited molecules with unselectively excited molecules in-:

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1 creases and in order to minimize this undesirable dilution ~
2 is clearly necessary that the time lag between excitation and 3 converslon be small.
4 If the irradiation conditions are such that the molecule is caused to absorb more than one infrared photon 6 then the m~lecule will be excited to an energy level well 7 above that populated by thermal means. In fact, the molecu~
8 may be excited to the dissociation limLt 9 To achieve multiple photon absorption at least two limit~ng factors have to be overcome. First, the excita-11 tion process has to be faster than the various molecular re-l2 laxation processes~ Second, a process has to be devised ;;
13 which helps to overcome the anharmonic character o~ molec~r 14 osclllators, namely anharmonicity 4 The solution to the irst problem is to restrict the e~citation pulse width to a tlme 16 shorter than the relevant relaxation times~ There are a num-17 ber of solutions to the second problem. U.S. Patent ~937,956 ;18 and ~ ~ teac~ a means by which anharmonicity may be 19 overcome. This means is the use of a second gas which pro-motes rotational rela~ation between the absorption o IR
21 photon~. Another way is to use a laser which emits not at a ~ ;
22 slngle exact wavelength but a ~inite range of wavelengths, 23 such that ~nharmonicit~ to n-levels is overcome. Finally, 24 multiphoton absorption can be achieved as the result of ~`
power-broadening, where the power-broadening limit is estab-, 26 lLshed by the threshold for laser induced breakdown in the 27 particular gaseous specles9 or mixtures of gaseous species, ~-28 and by the requirement that the isotope selectivity of the 29 exciting radiation not be impairedO The instant invention teaches yet another way that anharmonicity may be overcome .

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''`'', 1 and multiple photon absorp~ion with additionai lsotope e~-2 richment achieved compared with single photon absorption~
3 The instant invention ls a two-step process, the .
4 first step being a combination of isotopically selective ex-citation and conversion, and the second step being the re-6 covery and separation of ~he converted molecules from the 7 unconverted molecules by means known in the art. This com-8 bination excitation and conversion is accomplished by simul-g taneously irradiating gas phase molecules which contain the lo element whose isotopes are to be separated with infrared ra-1 diation at two different wavelengthsO One of these radia- ;
12 tions may be called the resonant radiation because its wave-3 length must correspond to an absorption band of said mole-l~ cules.which in turn corresponds to a mode of molecular motion 15 in which there is participation by atoms of said èlement~
16 If the said gas phase mole.cules are U~6 then it is preferred 17 to use a resonant radiation in one of the following wave~
l8 length ranges: 1888 to 1852 cm~l, 1300 ~o 1280 cm~l, 1170 l9 to 1143 cm~l~ 636 to 613 cm~land 196 to 186 cm~l. There is no high power requirement for the resonance radiation. The 21 second radiation may be called the of-resonant ra~lation and 22 for it high power is required, specifically the power density 23 o~ o~-resonan~ radiatio~ to which the molecules are sub~ec-24 ted must be greater than 106 watts per cm2, preferably great~
er than 107 watts per cm2 and more preferably greater than ~8 26 watts per cm2. There is no requried relationship between the 27 wavelength of the off-resonant radiation and absorption bands 28 o~ the molecules~ hence any high power laser may be used.
29 However, it is preferable that the frequency of the high power ~:
radiation should be close to a resonance of the molecule ex-.

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1 cited by'the low power radiatio~. Beca~se of their high effî-2 ciencies the C02, C0, HF, and DF lasers are preferred. It is ,' 3 alsopreerred that the time during which the moleculesare sub-4 3ect to irradiation be less than 10-5 sæonds, m~re preferably less than 10~5seconds and most preferably less than ~-7 secon~
6 The preferred mode of conversion is unimolecular 7 decomposition, i Oeo the irradiated molecules receive sufff - -,:
'8 cient energy that they decompose. It is, however, within the 9 scope of the instant invention that the ir:radiatqd molecules rçceive an amount,of energy which is insufficient to cause ll decomposLtion but which causes them to react with some other ~ ~-12 gas phase molecule also present in the irradiated volume 13 E~amples of the various chemical conversion processes the ' ,;
14 selectively excited molecules can undergo are described in 15 the Lyon patents mentioned above. '~ ~ "
16 The simplest description of the comb~nation exci=
l7 tation conversion step is in ~erms of sequential process~
8 The resonant radiation causes an isotopic~lly selective ex-19 citat~on of the molecules. The excited molecules have a -greater densi~y o vibrational states ava~lable for urther 21 excitatlon than do the unexcited molecules and thereore more ~ '~
., .
22 readlly undergo ~bsorption o~ the high power of~resonant ra-23 diation leading to their aecomposition. Hlgh power off~
~ 24 resonant radiation means in the context of this application 'I 25 radlatlon which ls not absorbed by the undamental molecular i 26 vihration. This description, however, implies that the high , 27 power of-resonant radiation does not influence the absorp-28 tion of the resonant radiation. It is withln the scope of 29 this invention to operate under conditions such that said implication is valid but it is also within ~he scope of this :

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1 invention to use conditions such that the high power off-2 resonant radiation interacts signi~icantly wlth the absorp-3 tion of the resonant radiationwit~o~tlmpairing isotope selec-4 t~vity. It is further within the scope of this lnvention

5 that the high power laser off-resonant radiation in the con- ;

6 text o~ direct absorpticn of ~he radiation may inelastically

7 scatter according ~o a Raman process such that the equivalent

8 of multiphoton vibra~ional excitation is achieved. In the

9 presence of the large photon yield this may approach a stim-ulated scattering process with a cross sec~ion larger than 11 observed at lower yields. Due to the high density of states 12 of the molecules a resonant Raman æca~tering process may pre~
13 vail.
14 From the above descrlption, the instant invention is readily distinguished from the prior artq Thus, U~S.
16 Patent 3,443,087 teaches the separation o~ U235F6 from 17 V238F6 by selectively exclting one of them with an infrared 1~ laser then ioniæing said excited molecules with ultraviolet 19 light and recovering the ions by means of electric and/or ;
magnetic fields or chemical reactions. In a review entitled 21 "Photochemical Isotope Separation as Applied to Uranium"
22 (Union Carbide Corporation ~uclear Divlsion, Oak Ridge Gas- ;
23 eous Diuslon Plant, March 15, 1972, K-L~3054, Revlgion l, 24 page 29), Farrar and 5mith discuss the above-mentioned paten~
and comment unfavorably on the practicality of the proposed 26 second 9tep o~ photoionization. ~9 an alternative, they 27 suggest photodissoc~ation by single W photons.
28 British Patent 1,284,620, German Patent 1,959,767 29 and German Patent 2,150,232 teach the use o infrared radia~
tion to selectively excite molecul~s wh~ch then undergo a ,~

lQ7~0~8 l chemical reaction which the unexcited molecules undergo more 2 slowly. Only one example of such a reaction is given, the 3 thermal decomposition of U(BH4)40 4 In all the above re~erences the energy given the `
molecules in the photoexcitation step ls explicitly taught 6 to be that of one IR photon, while in the instant invention 7 the molecules are given the energy of several ~R photons.
8 There are also publications involving multiple IR
3~o~
photons~ In U.S. Paten~ 3,93`7,956 and ~,6~P~Ls-thereof, o a process i~ taught in which molecuLes are excited in an iso-ll topically selective manner by resonant in~rared radiation at 12 a required high power density~of at least 104 watts per cm2 13 per torr pressure of the gaseous compound which contains the 14 element whose isotopes are being ~eparated. The isotopically selective decomposition of SF6 by resonant radiation at power 16 densities of 109 watts per cm2 has been observed and report-17 ed in journal articles by Ambartzumlan et al (Soviet Physics 18 JETP 21, 375, 1975) and by Lyman et alO (Applied Physics 19 Letters 27, 87, 1975). All these references share the com-mon requirement that the user mu~t supply a high power laser 21 which operates at a resonant waveleng~h~ i.el the high power 22 laser has to operate at a wavelength dlctnted by the mole^
23 culesD The instant invention is clearly distinguished from 24 and advantageous with respect to these references in that the high power laser may operate at whatever wavelength 26 permits the mos~ e~Elcient laser operation. It is well 27 known that the efflciency and expense o generating high 28 power in~rared radiation is sensitive to the waveleng~h at 29 which the laser must operate. This di~ficulty is greatly reduced in the instant inventlon since the resonant radia-~`

, lQ~20~8 1 tion, i~e. the rad~tion which must be matched to the m~le- ~
2 cules comprising the isotopes which are to be separated, `
3 needs to be generated only at low power.
~ EXAMPLE ~ r Uranlum ore of natural isotopic distribution is 6 conver~ed to UF6 by means well known in the art9 Said UF
7 is simultaneously irradiat~d with infrared low power reson~
8 ant radiation in ~he waveléngth range 636 ~o 613 cm~l and -~-9 with in~rared radiation from a C02 laser at a power density ~ -~
lo greater than 108 watts per cm2~ for a time of less than 10-5 11 seconds, whereby the UF6 is decomposed in aLn isotopically 12 selectlve manner.
13 The low power resonant radiation shou~d suitably 14 be suf~iciently intense to excite the molecule to enable the absorption of the o~f-re~onant high power radiation. The 16 low power radiation intensity should not exceed values which 17 produce such level broadening that isotope selectivity is 18 lost. ~or the ~3 transition of UF6~ 636~613 cm~l, this ~n-19 tensity is Iimited to the range of 100 watts per cm2 to 106 watts per cm2~ ~ ;
21 The high intensity radi~ion should suitably be at 22 a ~requency and have an intensity such that it does not con-23 tr~bute to the level broadenlng to the detriment of Isotope 24 selectivity achieved by the low power radiat~on alone. Tt is preferable that the frequency of the h~gh power radiation 26 9hould be close to a resonance of ~he molecule excited by 27 the low power radiationr 28 The wavelengths~ bandwidth~ energy~ pulse width 29 and pulse te~por~l character of both the low power and high power radiation have to be ad~usted to provide maximum yield

- 10 ~ ' Z1~48 1 at optimal isotope separation.
2 The decomposed molecules are then recovered and 3 separated from the undecomposed molecules thus forming iso-4 topically enrlched and depleted uranium, said separa~ion and S recovery being done by any means known in the art. The tech-6 niques of staging isotope separation are well known and sho~
7 greater depletion of the depleted uranium or greater enrich-8 ment of the enriched uranium be desired, ~he separation proc-9 ess may be repeated according to the well known techniques.
-.:

Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of separating the isotopes of an element which forms a volatile compound having an isotopically shifted but overlapping infrared absorption spectrum, the method including the steps of:
(a) irradiating said volatile compound with a first infrared radiation which is preferentially absorbed by a molecular vibration of molecules of said compound containing a predetermined isotope of said element thereby providing excited molecules of said compound enriched in said molecules of said compound containing said predetermined isotope of said element; and (b) irradiating said volatile compound with a second infrared radiation which is not substantially absorbed by said fundamental molecular vibration of said molecule at an inten-sity sufficient to further excite said excited molecules to undergo a conversion.

2. The method as defined in claim 1 in which said conversion is achieved by decomposition.

3. The method as defined in claim 1 in which said first infrared radiation has an intensity insufficient to produce substantial level broadening.

4. The method as defined in claim 3 in which said conversion is achieved by decomposition.

5. The method as defined in claim 3 in which said second infrared radiation is close to a residence of said molecules of said compound containing said predetermined isotope of said compound.

6. The method as defined in claim 1 in which said volatile compound is a compound of uranium.

7. The method as defined in claim 6 in which said compound of uranium is UF6.

8. The method as defined in claim 1 wherein said volatile compound is irradiated for a time less than 10-5 seconds and the second radiation has a power density greater than 106 watts per cm2.

9. The method as defined in claim 8 in which the irradiation time is less than 10-7 seconds and the power density of the second radiation is greater than 107 watts per cm2.

10. The method as defined in claim 9 in which the irradiation time is less than 10-6 seconds and the power density of the second radiation is greater than 108 watts per cm2.

11. The method as defined in claim 1 in which said second radiation is provided by a CO2, CO, HF or DF laser.

12. The method as defined in claim 11 in which said second radiation is provided by a CO2 laser.

13. The method as defined in claim 1 in which said first radiation occurs at one of the wavelength ranges of 1880 to 1852 cm-1, 1300 to 1280 cm-1, 1170 to 1143 cm-1, 636 to 613 cm-1, and 196 to 186 cm-1.

14. The method as defined in claim 13 in which said volatile compound is UF6 and in which said radiation has a wavelength in the range of 636 to 613 cm-1.

15. The method of claim 1 in which said first radiation has an intensity between 100 watts per cm2 and 106 watts per cm2.