Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
.
10~ 4~ ::
''`'',
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-
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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
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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.
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