Note: Descriptions are shown in the official language in which they were submitted.
113744Z
1 The present invention relates to a method for the
2 separation of isotopes and more partlcularly, the present
3 invention relates to a method for the separation of isotopes
4 employing infrared radiation.
In accordance with the teachings of this invention, it
. has been discovered that when separating isotopes of an
7 element by selective photodissociation of a mixture of
8 molecules containing isotopes of the element where the
g absorption spectra of the molecules are shifted but overlap-
10 ping, the compounding effect of multiple photon selectivity
11 is related to the number of photons which must be absorbed to
12 produce dissociation by those molecules with the largest
13 amount of thermal energy in the mixture, as well as the
14 number of such molecules having such thermal energy. In
15 accordance with the teachings of this invention, 2 method of
16 separating isotopes of an element is taught which includes
17 the steps of:
18 (1) providing a vapor of a compound of the element
19 having an isotopically shifted but overlapping
infrared absorption spectrum associated with the
21 isotopes of the element which does not change
22 appreciably upon the absorption of photons when the
23 compound is maintained within a predetermined
24 temperature range; and
(2) irradiating the vapor with a predetermined fluence
26 of infrared radiation which is preferentially
27 absorbed by an infrared-active molecular vibration
28 of molecuies of the compound containing a predeter-
29 mined isotope of the element, thereby providing
excited molecules of the compound, enriched in the
31 predetermined isotope; while maintaining the vapor
32 of the compound at a temperature within the
33 predetermined temperature range which provides
34 sufficient molecules at thermal equilibrium that
require more than one photon to promote dissocia-
36 ~ion so that upon dissocia~ion ~he isotope
,' ~
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1 selectivity is at least ten percent above the
2 maximum selectivity which can be achieved with an
3 ensemble of molecules of the compound each of which
4 can dissociate by absorbing a single photon,
thereby enabling separation of the excited
6 molecules.
7 In the preferred embodiment of this invention, isotopes
8 Of uranium are separated using a CO2 laser to provide the
g aforesaid infrared radiation, and molecules of a uranyl
10 compound having a formula UO2AA'.L are employed, where A and J
11 A' are monovalent anions, and L is a neutral ligand. In a
12 most preferred embodiment, the anionic ligand would be
13 1,1,1,5,5,5-hexafluoracetylacetonate (hfacac), and the
14 neutral ligand utilized would be tetrahydrofuran (THF) or
15 another base of similar or greater strength. In another
16 preferred embodiment of the present invention, irradiation of
17 the vapor is conducied with a fluence of infrared radiation
18 such that less than about seventy percent of all of the
19 ensemble of molecules dissociate, preferably less than about
20 fifty percent.
21 As a result of experimental work involving the selective
22 dissociation of UO2(hfacac)2.THF as disclosed in Belgian -
23 patent No. 846,225, it was discovered that the selectivity
24 achieved was less than one would have expected theoretically
25 if on the average more than one photon is absorbed by
26 UO2(hfacac)2.THF when selective photodissociation occurs.
27 In accordance with the present invention, it has now
28 been discovered that if a vapor of a compound containing
29 isotopes of an element has an over]apping infrared absorption
30 spectrum associated with the isotopes of that element, the
31 maximum separation factor which can be achieved with
32 molecules starting at thermal equilibrium is a function of
33 the number of and the selectivity of the molecules which
34 require the least number of photons to dissociate from thermal
35 equilibrium. The minimum number of photons necessary to
36 produce dissociation depends on the energy gap between the
1137442
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1 highest thermally populated state and the lowest
2 dissociative state. If the energy gap is equal to or less
3 than one photons' energy, the selectivity will approach
4 the single photon selectivity ~, where c~ is defined as the
ratio of the small signal absorption cross sections of the
6 isotopic species at a given wavelength. This occurs because
7 the molecules have energy contents defined by a thermal
8 ~istribution. The selectivity for those molecules which
g require n photons to dissociate iso~n. The overall selectiv-
ity is reduced because, under conditions such tnat a
11 significant fraction of molecules requiring n photons has
12 dissociated, nearly all those molecules of both isotopic
13 species which require fewer than n photons have also
14 dissociated. These latter molecules comprise the high
energy tail of the thermal distribution. Since they serve
16 only to reduce the overall selectivity, one way to com-
17 pensate for this loss is to increase the energy gap between
18 the highest thermally populated state and the lowest dis-
19 sociative state. If the energy gap is equal to the energy
of n photons, the selectivity will approachcCn, provided
21 certain conditions are met. For example, the irradiation
22 of the vapor has to be conducted such that less than about
23 70% of all the ensemble of molecules dissociate, preferably
24 less than about 50%.
In accordance with this invention, it has thus been
26 recognized that in order to increase selectivity employins
27 compounds under conditions herein under discussion, it is
28 desirable to maintain the temperature cf _he compound
29 curing irradiation at a temperature ~nich provides ~he
over-~helming majority of the moiecules at thermal equil brium
31 in states such that .hey require more than one photon to
32 promote dissociation therein. It should be understood
33 that a de ~inimis number of molecules can be in a state tnat
34 requires only one pho.cn ~o dissociate, but in crder to
35 practically practice 'his ;nven~ion, the number should be
36 limi~ed to one in .~hich some compounding effect is observable.
1137~4Z
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1 For some chemical compounds a lower temperature
2 limit exists below which the advantages of this invention
3 are lost. Thus, if the temperature is lowered such that the
4 infrared absorption spectrum of these species changes
5 appreciably upon absorption of photons, compounding o. the
6 selectivity may not be observed.
7 In accordance with this invention, when compounds having
8 a formula of ~O2AA'.L are employed, where A and A' are
9 monovalent anions and L is a neutral ligand, it is preferred
10 that the neutral ligand L be a base stronger than T~F, toward
11 the uranyl ion, that is, L should have an equilibrium
12 constant for exchange with tetrahydrofuran of greater than 1.
13 The reason for this is that the base strength of the neutral
14 ligand is one of the determining factors in the bond strength
15 between that ligand and the uranyl moiety, and this in turn
16 is a factor in the thermal stability of the molecule, and,
17 therefore, the energy required for dissociation. Irrespec-
18 tive of a compound's dissociation energy, when the average
19 thermal energy content of the compound approaches its
20 dissociation energy a significant number of molecules
21 require only one photon to dlssociate and the selectivity of
22 an isotope separation process performed under such conditions
23 is limited.
24 In accordance with this invention, it has been recog-
25 nized that, all other things being equal, greater base
26 strength is desirable, and furthermore that it is most
27 preferred to operate the process while maintaining the
28 temperature of the compound towards the lower end of its
29 temperature range of volatility.
In particular, the vaporizable compounds used in this
31 invention, and which have an isotopically shifted but
32 overlapping infrared absorption spectrum as discussed above,
33 w111 preferably have the general formula UO2AA'.L, where A
34 and A' represent monovalent anions and L a neutral ligand as
35 discussed above. The anions A and A' which can be employed
36 in this process will generally have conjugate acids which
' .
1137442
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l have boiling points of less than about 200C. ald PKa valUes
2 of 4.8 or less. These anions ma~ be monodentate or polyden-
3 tate, and preferably both A and A' will be the same anion.
4 Pre~erable anions for use in connection with the
compounds of the present invention include, in addition to
6 the 1,1,1,5,5,~-hexafluoroacetylacetonate anion discussed
7 above, anions such as l,l,l-trifluoroacety' - acetonate
8 (CF30CHCOCH3), 3-trifluoromethyl-1,1,1,5,5,5-hexa-
9 f uoroacetylacetonate ((CF3CO)2CCF3), 1,l,1,3,5,5,5,-hepta-
10 fluoroacetylacetonate, ((CF3CO)2CF)1,1,1,2,2,3,3,7,7,7-deca-
11 fluoro-4,6-heptanedionate (CF3COCHCOC3F7), fluorinated
12 tropolonates, and others.
13 As for the neutral ligand L which is employed in
14 accordance with the above formula, reference is again made to
15 the preferable minimum basicity requirements discussed above.
16 As noted, it is preferred that L have a base strength
17 greater than that of tetrahydrofuran towards the uranyl ion,
18 or more particularly a basicity measured by the equilibrium
19 constant for the following reaction:
K
21 Uo2(hfacac)2.THF + L = UO2~hfacac)2.'~ + THF
22
23 ln which K will be greater than l. .~ in this case is measured
24 in an anhydrous nor.-coordinating solvent such as benzene,
25 methylene chloride or chloroform.
26 Preferable neutral ligands which meet these pre-
2, requisites for use in the process of this invention include
28 trimethylphosPhate (TMP, (CH30)3P-O);triethylphosphineoxide
29 ((C2H5)3P=O); and hexamethylphos- ~horamide ((CH_)"-N)3P=o);
dimethylsulfoxide ((CH3)2S=O; pyridine (C~H_N), etc.
31 Still another advantage which can be realized by
32 utilizing uranyl compounds containing these neutral ligands
33 in this process arises from the fact that these ligands tend
34 to shift the frequency at which these molecules absorb as
35 compared to UO2(hfacac)2.THF, for example. This shift
36 results from an increase in the strength of the bond between
` ~137442
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1 these ligands and the U02 moiety, and it is particularly
2 preferred to use such ligands which shift the position
3 of the absorption band of the uranyl asymmetric stretch
4 into correspondence with C02 laser transitions more favorable
5 with respect to isotopic selectivity and laser efficiency.
6 P~EFE~RED EMBODIMEN~
7 Irradiation of a uranium-containing compound in
8 accordance with the present invention was carried out in a
9 molecular beam such as that disclosed in Belgian Patent ~o.
lO 867,647. The uranium-containing compound which was employed .
11 in that patent was U02(hfacac)2.(THF). It was found that when
12 this compound's photodissociation and IR absorption spectrum
13 were observed in the gaseous state, each exhibited a maximum
14 at 956 cm (nearly resonant with the P(6) transi~ion of 10.6
15 ~.m C02 laser) and a full width at half maximum intensity of
16 about 7.8 cm l.
17 Specifically, the U02(hfacac)2.THF compound was placed
18 in a heated oven having a .005 inch orifice and heated to
19 about 115C. The molten material exhibited a vapor pressure
of about 0.2 torr at this temperature, and a molecular beam
2L was produced at the oven orifice. This molecular beam was
22 crossed by a CW 10.6 ~m C02 laser, and a selectivity for
23 preferential dissociation of the uranium 235 containing
24 species of 1.28 + 0.14 was achieved during operation on the
P(4) transition at a laser energy fluence of 5.l mJ/cm2 with
26 the contact time of the laser with the molecules being about
27 5 lls. A depletion of l.9 percent of the U-235 containing
28 species was observed. The photodissociation products of this
29 process were found to be U02(hfacac)2 and THF.
Further tests demonstrated that significantly increased
31 selectivities can be obtained if the temperature is lowered.
32 In these experiments, the isotope selective dissociation of
33 U02L2.THF was measured for the P(lO), 10.6 ~m C02 laser
34 transition. Irradiation on this transition selectively
dissociates the uranium 2,8 containing molecule. In the
36 results summarized in Table A the molecular beam was crossec
` 1~37442
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1 by a pulsed C02 TEA laser with a pulse width of about 400
2 nanoseconds (FWHM). The results in Table A demonstrate that
3 the isotope selectivity increased dramatically as tne
4 temperature was reduced.
Table A
6 Oven No. of Laser ~luence Isotope Dissociation
7 TemO- Laser Pulses (mJ/cm ) Selectivity Fraction
8 C (D8/D5)* ( 8)
9 12020,480 48 1.17 + 0.15 .7
11020,480 120 1.22 + 0.11 .9
ll 84 24,576 48 1.25 + 0.22 .5
12 75 14,336 52 1.o7 + 0.22 .4
13 65 9,216 48 1.91 + 0.37 .3
14 *D8 is dissociation fraction for U238 bearing species.
D5 is dissociation fraction for u235 bearing species.
1~ Additional experiments have been performed using U02
17 (hfacac)2.trimethylphosphate (TMP). The TMP has a base
18 strength stronger than THF with respect to the U02(hfacac)2
l9 complex. The oven was heated to about 100C to produce a
molecular beam of U02(hfacac)2.TMP which was then irradiated
21 with a pulsed C02 TEA laser operating on the P(4), 10.6 ~ m
22 transition, a transition which preferentially dissociated the
23 uranium 235 containing species. The selectivity under these
24 conditions was measured to be 1.7 + 0.2. Thus, the selec-
tivity achieved with this compound is considerably higher
26 than that achieved with the U02(hfacac)2.THF compound at an
27 equivalent temperature. The value of the single photon
28 selectivity is found to be about 1.4 for U02(hfacac)2.TMP.
29 Thus, the selectivity achieved experimentally is higher
for compound of the class U02(hfacac)2.THF with a more
31 strongly bound base, L.
