Note: Descriptions are shown in the official language in which they were submitted.
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The present invention concerns a method for separating mixtures
of substances, particularly of isotopes or isotope compounds, based on the
principle of selective excitation, dissociation or ionization of the one
substance, for instance to make possible a cnemical reaction with a
separately fed-in other reaction partner. Such excitation can be brought
about by means of electromagnetic waves, particularly by laser radiation,
the frequency of which is adjusted so that the radiation is absorbed
selectively by the isotope to be separated. Such a method has become known,
for instance, from the German Offenlengungsschrift 1,959,767, laid open
on June 3, 1971.
` Use of such methods has shown that in addition to the chemical
reactions made possible by the selective laser excitation, which permit a
normal separation of the reaction product which contains only the excited
isotope, also other reactions take place which have an adverse effect on the
desired degree of selectivity. Such other are caused by an overlap of the
absorption bands, resonance interchange and thermally activated reactions.
The problem therefore arose, to improve this method so that, on
the one hand, the selectivity is increased and, consequently, the yield is
also improved substantially~
According to the present invention~ this problem is solved by
decompressing the initially vaporous substances of the mixture adiabatically
to temperatures below 100 X a~d irradiating them, still before they are con-
densed, with an electromagnetic wave, preferably by a laser beam of suitable
` frequency located in a resonator. The electromagnetic wave is adjusted here
as to bandwidth and frequency position in such a manner that the Q-branch
of the rotational vibration spectrum of the substance to be excited, is
covered. In this process, the cooling-down has the effect, and is pushed
so far, that the molecular vibrations are largely frozen and the frequency
` distribution of the rotational energies is shaped so that the P- and R-branch
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of the isotope that is not to be excited, does not overlap much with the
Q-branch of the one to be excited. Furthermore, use is made here particularly
of a step-wise excitation.
For a further explanation, this method is illustrated by the
example of the separation of the uranium isotopes 235 and 238; it should be
pointed out here, of course, that the application of this method is in no
way limited thereby.
The action of the method according to the invention is based on
the combination of two measures:
1) Lowering the temperature of the mixture to below 100 K.
Thereby, a very considerable narrowing of the absorption bands of the
isotope mixture, in this case of UF6, is achieved, whereas at room temper-
ature the absorption bands of both isotope compounds U 235 F6 and U 238 F6
overlap in a large range, so that irradiation, using a definite frequency
by means of a laser would lead to an almost equal excitation of both isotope
compounds. The obtainable selectivity is therefor low. However, due to
the narrowing of these abso~ption bands particularly of the Q-branches ~ -
obtained by the heavy cooling according to the invention, overlap between
the Q-branches practically no longer exists or is greatl~ reduced. The
absorption maxima of the two isotope compounds, plotted versus the frequency,
are distinctly separated. This means, however, that if a laser frequency
which corresponds to the absorption maximum of the uranium 235 F6-compound
~` is radiated, the other isotope compound uranium 238 F6 is practically not
~`~ excited, or only much more weakly so.
`~ 2) Stepwise excitation makes it possible to excite with the
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~` relatively low frequencies at which the molecule exhibits strong absorption
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(for infrared-active fundamentals and simple combination vibrations), and
still to achieve the high excitation energy desired for seleetive chemical
reaction. For this reason and by excitation in a resonator, one can get
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along with relatively little laser power. This can be smaller by several
orders of magnitude than for the excitation of the same intensity in a
single-quantum process of the same end energy, although stepwise excitation
is usually possible with low losses, only if the power density is chosen so
high that the activation rates are higher than the interfering deactivation
rates (without stimulated emission). The excitation preferably takes place
here with the fundamental frequency V3~ which is 624 cm for U 235 F6.
Depending on the availability of lasers, other vibrations, e.g., the
combination vibration vl ~ V3 of the U 235 F6, can also be excited, however.
This stepwise excitation by means of the fundamental frequency is possible
because the energy difference between the lower excitation levels are little
different. Preferably, the U 235 F6 molecules at resonance are involved.
In cases where this share is too small, the number of collisions is set,
by suitable choice of the density or less divergent direction of the flow,
to values so high that the other molecules get into the state necessary for
absorption during the stay in the reaction zone. The occupation density of
the excited states is therefore increased.
According to one broad aspect of the invention there is provided
` a method for separating intermixed isotopes forming vaporous mixture which
is irradiated by an electromagnetic wave having a wave length selectively
absorbed largely by one of the isotopes so that one isotope is excited to
be separated after selective chemical reactions with a chemical partner or
selective dissociation or selective ionization~ wherein the improvement
compr~ses cooling said mixture to temperatures below about 100 K and
irradiating the cooled mixture by said wave.
According to another broad aspect of the invention there is
provided an apparatus for separating vaporous intermixed isotopes by mixing
them with a vaporous chemical partner to form a vaporous mixture which is
irradiated by an electromagnetic wave having a wave length absorbed largely
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by one of the isotopes so that that one isotope is excited and selectively
chemically reacts with the chemical partner; said apparatus comprising an
evacuated chamber, irradiating means for radiating a beam of said wave
transversely through said chamber, nozzle means adjacent to said beam for
ejecting a flat jet of said mixture into said chamber transversely through
the beam, and means beyond said beam on its other side from said nozzle,
for peeling of portions of said jet which failed to pass through said beam
while allowing the portions that passed through the beam, to continue onward
into said evacuated chamber.
The accompanying drawings sohematically show examples of suitable
apparatus for practicing this method, the various figures being as follows:
Figure 1 in longitudinal section shows the complete apparatus;
Figure 2 shows an example of the laser used; and
Figure 3 shows a modification of the Figure 2 laser.
As already mentioned, the two uranium isotope compounds U 235 F6
and U 238 F6 are to be separated in this example, and specifically, by means
of a chemical reaction with hydrogen bromide as the chemical partner.
According to the separating apparatus schematically shown in
Figure 1, the~ul~anium compounds are contained in the supply vessel 2 and the
reaction partner is in the supply vessel 3. They are fed to distribution
` chambers 32 and 22 via valves 31 and 21, and get from there to a mixing
chamber 23, which is followed by a slit-shaped discharge nozzle 24. The
latter alread~ forms part of the vacuum chamber 1 which is equipped with
cooling walls 14, 15 and 16. Unspent reaction partner as well as volatile
reaction products can be exhausted from the chamber 1 via connected pumps -
; S and 6. In front of the nozzle 24, the laser beam 4 goes lengthwise through
the jet of vapor
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issuing from the nozzle, which represents a mixture of UF6 and Hbr This
laser beam 4 is generated in the laser device proper, see Figure 2, and is
built up by mirrors 42 and 43 to values so high that the losses are equal to
the energy supplied. The position of the walls of the vacuum tank 1 or its
t~indow is between the mirrors (see Figure 2) as is indicated here by dashed
lines l. In the interior of the vacuum vessel there is the peeler 11, which
forms a slit-shaped nozzle and takes care that the particles coming from the
zone of the laser radiation are separated from those coming from other zones
o~ the jet.
l~ The vapor pressure of the UF6 is adjusted through temperature
control in the supply vessel 2, to a value slightly above the total pressure
in the mixing chamber 23 of 3300 Torr. The temperature of the reaction part-
ner in the supply vessel 3 is assumed to be adjusted so that in the mixing
chamber 23, a mixture temperature adjusts itself which is slightly above the
condensation temperature at the desired UF6 partial pressure. For 300 Torr
~ partial pressure of the UF6, corresponding to a vessel temperature of 314 K, a
- mixture temperature of 320 K is chosen. Then, the HBr gas must be fed-in with
a temperature of 290 K. At that temperature, HBr has a vapor pressure of
15,000 Torr. The desired mixture ratio, the ratio of the molecule concentra-
~0 tion UF6/HBr = l:lO, is set via the valves~31 and ~1. So that as few thermal
` reactions as possible take place in the mixing chamber, the latter, and there-
ore, with a given throughput, the dwelling time, are kept as small as possible.The dwelling time is in the order of lO 3 seconds. In order to prevent
reactions at the walls of the mixing chambers, these chambers are designed
aerodynamically (not shown) in such a manner that the correct mixture ratio
occurs only in the zone of the vapor jet which is to be engaged later by the
laser bea~. In praticular, there should be as little UF6 as possible in the
outer zones. By lining the walls of these mixing parts with plastic, e.g.,
with polytetrafluoroethylene ~Teflon), catalytic action by them to release
chemical reactions is largely prevented. Through these measures, clogging of
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of the narrow discharge slit is prevented and the peeled-off UF6 portion,
which was not engaged by the laser beam, is reduced. The walls of the nozzle
passages may be heated to temperatures higher than that of the substances 10w-
ing through the passages to prevent condensation at the walls of the nozzle.
The discharge nozzle 24 is a slit about 1/100 mm wide and 50 cm
long and opens into the vacuum tank 1. The mixture jet is heavily decompres-
sed adiabatically in the process and expands at the same time considerably in
space~ As wall friction cannot occur here, considerable cooling-down occurs.
With an adiabatic coefficient for HBr of 1.42, a lowering of the pressure by
tho factor 10 is sufficient to lower the temperature of the vapor jet to
about 20 K. It should be pointed out at this point that it is generally
advisable to choose reaction partners with adiabatic coefficients as large
as possible, so that the required low temperature is reached with a lowering
of the pressure of as little as possible. This can be accomplished, of course,
also by means of a substance which does not participate in the reaction. In
this process, the already mentioned heavy concentration of the Q-branch of
the rotational vibration spectrum takes place and as a consequence, high
selectivity. This condition occurs about 2.5 mm behind the discharge opening
24, and at this point runs, parallel to the nozzle 24, the laser beam, which - -
is about 3 mm thick and whose frequency range should cover also the entire Q-
branch including the shifting of the bands with increasing excitation stages.
(With increasing numbers of excitation stages, the excitation frequencies
become somewhat lower). The central part of the gas jet issuing from the noz-
zle 24 goes through the laser beam and is selectively excited here in steps.
As the quanta generated by stimulated emission are returned to the laser beam
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the excitation takes place with relatively high efficiency. The energy
density of the laser beam is chosen here so that with the existing cross ;
section and a sufficiently high number of collisions in the region of the
laser beam (about 50 collisions of each molecule), just the essential part of
the U 235 F6 is reacted. Due to the great selectivity, only a small part of
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the isotope compound U 238 F6 is reacted, as the reactive high excitation
levels of molecules of this isotope are only lightly occupied.
The already mentioned peeler 11 lets through only that part of the
jet which has gone through the laser beam, and the other part is either
condensed (the UF6 at the cooling walls 14) or is pumped off (the HBr via the
pump 5). The mixture of reaction products and starting materials that flies
or ejects through the peeler, can be separated by fractional distillation.
On tl~e collector plate 16, enriched reaction products 7 are precipitated;
these consist, for instance, of enriched UF5 or UF4. Then, mainly U 238 F6 is
condensed at the cooling walls 15. The volatile products such as, for in-
stance, HF and HBr are exhausted via the pump 6. It would also be possible
to freeze-out HBr and to bind the ~E chemically.
~; Essential for the described operation of this apparatus is that
the jet of substance is still in vapor form in the region of the nozzle 24,
i.e., not yet condensed. Substantial condensation takes place only when the
jet of substance hits a cooled wall, as the heat of condensation must be
removed, which is not possible because of the low density of the jet of sub-
stance in the region of the laser radiation, the power density of which is
; about 103 watts/cm2.
In Figure 3 a lasex arrangement similar to Figure 2 is shown, in
which a second additional laser 44 is provided, the radiation 4' of which is
kept in the resonance s~stem formed by the mirrors 46 and 45. The lasers 41,
44 and their rsdiations 4, 4' axe arranged practically on the same axis and
are shown mutuall~ displaced only to clarify the ray pakh~ A second such
laser is advantageous if, starting from a selectively excited state, a still
higher excitation, dissociation or ionization of the isotope molecules is to
be brought about. The generated products can then be separated from each
other in a manner known per se by chemical and/or physical methods. Examples
for this are: Selective chemical reaction, possibly connected with subse-
3 30 quent fractional distillation, fractional distillation of the dissociation
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products, and deflection of the selectively produced ions in an electric
and/or magnetic field.
Depending on the choice of the flow and radiation parameters, very
high degrees of enrichment and very low residual contents can be obtained by
the above methods. With the parameters given, one obtains an enrichment to
about 22%, a residue content of 0.08% as well as an effective UF6 throughput
of about 30 tons per year.
In conclusion, it should be mentioned that this method, which was
described in connection with the separation of isotopes, can also be used for
1~ the preparation of chemical compounds which can otherwise be produced only
~ith difficulty or with low yield.
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