Note: Descriptions are shown in the official language in which they were submitted.
~117471
The present invention relates to a method for the
efficient separation of isotopes from a mixture of gaseous
isotope compounds by isotope-selective excitation of one of the
isotope compounds contained therein and subsequent chemical or
physical separation thereof by means of pulsed laser beams.
In all the methods for isotope-selective excitation,
separation is achieved by~ exciting only one isotope compound so
that the latter preferentially converts by chemical reaction or
otherwise into a chemical compound, whereby this new compound
10 can be separated relatiyely readily by normal mechanical and
chemical means from the original mixture of sub.stances. This
new compound then contains preferentially the desired isotope
such as uranium 235. The separation of isc:topes is of technical
interest especially for uranium, si.nce the fissionable isotope
uranium 235, which alone i.s usable, is present in the natural
uranium only in the amount of û.7% and ~ust be enriched in the
nuclear fuel for light-water reactors to about 2 to 3%.
Excitation of the one isotope compound, howeyer, can
also be used for ionizing the latter and to make it separable
20 thereby electrically, ox to influence its. dipole behayior in
such a manner that deflection by the electric field of the laser
radiation itself becomes possible. Further details on laser-
induced i.s.otope separation methods by means of physical and
chemical separati.on can be found in German Published Non-
Prosecuted Applications Numbers 2 311 584 and 2 324 797. Further
proposals for th.e isotope separation via selective excitation of
molecular energy levels can be found in German Published Non-
Prosecuted Application Number 2 459 989, where the use of
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wa~el.en~ths in the infrared and ultra-Yiolet ran~e is discussed
in particular.
Almost all uranium isotope separation methods start
out with the compound UF6, which uranium compound exhibits
sufficient vapor pressure for e~fecting i.sotope selective
excitation. It has been found, however, th.at selective
excitation performed at normal temperatures does not permit the
attainment of the desired enrichment values because of the over-
lap of the absorption bands, resonance exchan~e and thermally
acti.vated reactions. It has been proposed as an improvement to
expand the ~aseous isotope mixture adiabatically to temperatures
below 100 K and to i.rradiate it before it is condensed, with a
laser beam o~ s:ultable frequency contai.ned in a resonator; see
in this connection German PubIished Non-Prosecuted Application
Number 2 447 762. It haa also been proposed to accomplish the
cooling-down of the isotope mi-xture by a neutral, heavily
undercooled supplemental gas that is to be admitted, instead of
the adiabatic expans.ion, as described in German Published Non-
Prosecuted Application Number 2 651 306.
This and other i.sotope separation methods are feasible,
with the use of so-called continuous lasers, but such equipment
has low output power and the operation is not very efficient,
however.
Performing the isotope separation processes with pulsed
lasers also h.as its problems. Since the laser pulaes, which
are in the order of one nanosecond, are very short and the pulse
repetition frequency i.s maximally about 100 Hz, only a very small
amount of material can be excited by th.e present methods using
~:~1747i
such pulsed lasers, so that the use of ~ulsed lasers in the
manner described is disadvantageous in thi.s. respect. It should
be recalled in this~ connection that the gas mixture put through
must be recirculated a~ain and again because as pre~iously
stated only a very small amount of material can be excited, so
that enormous pump powers are required res.ultin~ in large
capital inyestment and high operati.ng cost.
An object of the present inyention is to provide a
process for the economical application of pulsed lasers for the
separation of isotopes from a mixture of vaporous isotope
compounds, ensuring that the entire mixture of substances put
through is irradiated in its entirety and the molecules that
can be excited are actually excited.
With the foregoing and other objects in view, there is
provided in accordance with the invention a method for the
separation of isotopes from a mixture of vaporous isotope
compounds by the application of pulsed laser beams to effect
selective excitation of one of the is:otope compounds contained
therein and subsequent chemical or physi.cal separation of the
~ excited isotope compound, the improvement comprising
a) flowing a stream of the mixture of vaporous isotope
compounds periodically at constant intervals,
b) continuing the flow of the mixture of Yaporous isotope
compounds. during each period of flow-for a s.hort time in the
range of milliseconds,
c) adiabatically expanding each.stream of the mixture of
vaporous isotope compounds flowing per~odically to cool the
mi.xture to a temperature below 100 K,
il:l7L~7~L
d) flowing the cooled mixture of yaporous isotope compounds
to an irradiation zone at said intervals, and
e) flowing the cooled mixture through the irradiation zone
ànd subjecting the cooled mixture during the time of flow
duration to consecutive pulses of a plurality of consecutively
fired lasers to excite substantially all of the one isotope
compound to be selecti.vely excited during flow of the mixture
of vaporous isotope compounds through the irradiation zone.
In accordance with the invention there is provided
apparatus for controlling the ~low of a mixture of vaporous
isotope compounds and adiabatically expanding the mixture of
vaporous isotope compounds to cool the ~ixture to a temperature
below 100 K, for the separation of isotopes from the mixture
by the application of pulsed laser beams to effect selective
excitation of one of the isotope compounds, contained therein
which comprises
a) a slot-shaped expansion nozzle,
b) a valve whose outlet extends oyer the entire width of
the inlet to th.e expansion nozzle,
c) sai,d valYe havi.ng a rotor, rotatable at a constant
speed, with two di.ametrically opposite lon~i.tudinal slots,
d) a housing with a vapor inlet surroundin~ and sealing
the rotor, said rotor when aligned with one slot facing the vapor
i.nlet and the other s,lot facing the inlet to the expansion
nozzle permits the flow of Yapor through sald valYe~
Other features which are considered as characteristic
for the invention are set forth in the appended claims.
Although the invention is illustrated and described
47~
herein as embodied i.n a method for the separation of isotopes
by isotope-selective excitation, it is: neverth.eless not intended
to be limited to the details shown, s.ince ~arious modifications
may be made therein with.out departing from the spirit of the
i.nvention and within the scope and range of equivalents of the
claims.
The invention, however, toge.ther with addi.tional
objects and advantages thereof will ~e best understood from the
following description when read in connecti.on with the accompany-
ing drawings, in which:
Figure 1 sh.ows a valve with a rotor, an expansion
nozzle and an irradiation space and diagrammatically illustrates
the relationship between the irradiation s.pace, expansion nozzle
and valve in accordance with the invention,
Figure 2 shows a timing diagram i:llustrati.ng the
switching sequences of the valve as well as the firing sequences
of the lasers,
Figure 3 diagrammatically illustrates directing
radiations from di.fferent lasers into the i.rradiation space by
means of mirrors, and
Figure 4 illustrates conducti.ng the radiation through
a cylinder lens into the irradiation space.
In accordance with the invention, the vaporous mixture
of the isotope compounds as well as of supplemental gases known
per se and/or reaction partners are adiabatically expanded at
constant time interYals, always for a flow duration in the
range of milliseconds, via a nozzIe, and is thereby cooled down
to below 100 K. The cooled gas mixture fed at interyals to an
1117471
irradiation space is exposed duri.ng the entire flow duration to
consecutive pulses of consecutiYely fired lasers for substan-
tially, quantitati.vely complete isotope-specific excitation.
This method is thus based on the combination of at
least two characteristi.cs: To send the gas mixture which is to
be fed to the isotope separation facilit~ at continuously
repeated i.nter~als for a sho~i~t flow duration through the
irradiation space, and to cover it in its entirety radiation-
wise by a multiplicit~ of consecuti~ely fired laser equipments
of the same frequency. This is based on the following
considerations:
Our own tests have shown that with dynami.c cooling of
a gas mixture, the selective exci.tati.on and s:eparation of the
desired isotope compound must take place on a travel distance
of about 2 cm. W;th a jet velocity of the materi.al mixture
entering the irradiation space of about 500 m~sec, the flight
time of a molecule through. this distance is about 40 ~sec. If
one now irradi.ates this enti.re zone of 2 cm length with a laser
pulse and makes a further laser pulse follow-after another
4Q ~sec, then the irradiated regions of the materi.al mixture
jet follow each other without gap. To this end, however, a
laser with a pulse repetition frequency o~ 25 kHz would be
requi.red, but these likewise do not yet exi.st with the desired
: wavelengths and powex ranges. The requi.red pulse energies are
about 1 Ws in the W range ~0.3 to 0.4 pm). and about Q.025 ~s
in th.e IR range C16 ~m).
Th.is difficult~ is overcome wi.th. the pre~ent inYention
in that the required puls:e repetiti:on frequency is formed by
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the consecutiye firing of a multi,plicity of identical lasers.
Since, however, the number of these lasers is li~ited, not only
for economic reasons but also because of the ph~sical arrange-
ment (they all must be arranged so that their radiation goes
through the irrad;~ati,on space by the same path), provision is
now made that the mater~al mi:xture jet tra~els: through the
irradiation chamber only during the radiati`on time of these
consecutiyely fired lasers. This is accomplished by inserting
ahead of the expansi`,on nozzle a val~e, which transports at
periodic inte~Yals, which correspond to the pos.sible pulse
repetition fre~uenc~ of a laser, a length-limited material
mixture jet through the irradiati.on space.
This procedure will now ~e explained, to illustrate
the invention in greater detail, with, the aid o~ an example,
referring to Figures 1 to 4.
Figure 1 shows diagrammatically the spatial relationship
between the irradiation space 3, the nozzIe 2 and the valve 4.
The procedure is now as follows: The material mixture 16, i.e.
the mixture of vaporous isotope compounds together with
supplemental gas and reaction partneL, flows through the line
45 into the valYe 4. The latter cons~ists of a tubular housing
41, to which are connected on the one $ide the feed line 45 and
on the other side the expansion nozzle 2. Inside this housing
rotates, with constant speed of rotation, a rotor 42 proYided
w1th slots 43 and 44 whi.ch are likewi,se arranged di.ametrically
opposite each other. When these slots. 43 and 44 establish a
connection between the feed li.ne 45 and the nozzle 2, a material
mixture jet o~ always the same length'will traveI through the
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1~17~71
irradiation space 3. The length o~ this i.rradi.ation space is
designated with s. With a jet veloci.ty of 500 m/sec, a molecule
traverses this distance in 40 ~sec. If now the front of the
gas stream leaving the nozzle has arrived in ~ront of the end
of the irradiation space 3, the fi.rst laser is fired, and the
next one at intervals of 40 ~sec. each. Thus, a division of
the material mixture jet into irradiation zones gl to gn is
obtained, which follow each other without a gap. In Figure 1
is shown that situation, in which just the last material jet
region gn is being irradiated by the last (the nth) laser. The
total length of the material mixture jet leaving the nozzle 2
at intervals is then obtained from the number of lasers n times
the length of the irradiation space s.
Figure 2 shows in a timing diagram the switching
sequences of the valve 4 as well as of the n lasers. The valve
4 opens the flow path at intervals of ~.Tl, where ~Tl will be
assumed to be, for instance, 100 milliseconds = 100 msec = 10o.
The open time of the Yalve, on the other hand, is., for instance,
1 msec = 1OOO and i.s designated ~T2. During this time range
~T2, the lasers 1 to n fire consecutiyely~ wh.ere in the numerical
example chosen, the firing interval between two lasers is
40 ~sec = 40 microseconds = 1 ooo 0OO, i.e., that time which it
takes a molecule for traversing the irradiation space 3.
Introduci.ng the radiations coming from di~ferent lasers
into the irradiation space 3 are now shown diagrammatically in
Figures 3 and 4. In this connection, it should also be mentioned
that the expansion nozzle 2 is designed in the form of a slit.
At its narrowest point, it has a gap of, for instance, 0.1 to
'7.L
0.5 mm and e.xtends over a width.of about 1 m. Figures 3 and 4
show this nozzle 2 in a top view, i.e., onto the broad side.
The number of lasers is obtained according to the numerical
example given as ~T2 ~ 1 msec. di.vi.ded by 40 ~sec, or n = 25.
For the sake of clarity, only 10 lasers are shown in Figure 3
al.though as many as 4a or more lasers may be employed. The
radiation from the lasers is deflected ~ia mirrors 6 onto a
rotating mirror 61, ~h~ch rotates synchronously with the switch-
ing frequency of the ~ndividual lasers and directs the laser
radiation on the same path into the irradiation space 3. In the
latter, the radiation is reflected back and forth at the
mirrored walls, so that the entire amount of material present
in the irradiation space during the lase~ pulse i5 covered by
thi.s radiation of one respective laser.
As already indicated at the outset, two frequencies
can be used for exciting the material mixture. The introduction
of these two different laser radiations is shown in Figure 4.
The latter again shows in a top vie~ the nozzle 2 and the
irradiation space 3, which has a length s. The radiation 91
2a of the lasers L known from Figure 3 ;.s conducted into the
irradiation space 3 via the cylinder lens 8 and is reflected
there back and forth.
This space has, for instance, the dimensions - 100 cm
width, 2 cm length and 0.5 cm thi.ckness. Its side walls
consist of KCl or NaCl ~indows, which are ~apor-deposited with
dielectric multiple layers of 99.7~ reflectivïty at a wavelength
of 0.4 ~m and whi.ch pass the light of 16 ~m unimpeded. There
are two windows: 71 and 72 with window 72 corresponding to a
47 ~
segment of a cylinder mirror with.100 cm radius of curyature.
By these measures the 16 - ~m light as well as the 0.4 - ~m
light is almost completely adsorbed, as the ratio of the
abso:rptivities of UF6 to 16 ~m to that at 0.4 ~m corresponds
exactly to the ~nverse ratio of the light paths. Contrary to
the light of a . 4 pm wavelength, whi`ch is focuss-ed in order to
let it be reflected back and forth sufficiently often in the
irradiation space, the 16 - ~m li~ght i~ opened out to 2 cm x 0.5
cm i.n order to ~llum~nate the entire irradiation space uniformly.
A facili.ty of th~s form is also su~table, of course, for a
process wh~ch requi:res~onl~ one ki.nd of radi.ation or also only
one UV quantum for e~citation and separation.
If the valve 4 were always open, a facility with the
dimensions and data described, if cooled to 50 K, would have
a material mi.xture throughput of 104 m3~h. The power required
to pump the throu~hput therefor would be extremely large. With
the method described, however, the required poser is reduced to
pump lOQ m3/h., i.e., a power uhich can be obtained uithout
~reat e~fort.
To further illustrate this meth.od, some further
statements on th.e dimensi~ns of the fafit-closing YalYe 4 uill
be gi.ven. In accordance with. a nozzIe wi.dth of 1 m, thi.s valve
4 also has a wi.dth of 1 m. In this manner it is possible to
achi.eve a uniform s-upply~of materi.al mi~ture to the nozzle 2
oyer its enti.re width ~i.a the slots 43 and 44 of the rotor 42.
~ith a s-peed of 600 r.p.m. and a rotor circumference of 20 cm,
one obtains for th.e open time ~T2 ~ 1 msec. wi.th. a width of
th.e inlet and outlet slots 43 and 44 of 2 ~m. The dis.tance
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471
between the housing wall and the rotor can be kept relatiYely
small because of the low speed, s.o that a good seal during the
clos:ing times of the valve 4 is obtained.
The numerical data mentioned should, of ccurse, be
cons:idered only as an example; they do not limit the present
method, but should be adapted suitably, according to other
method parameters such as, for instance, pulse repitition
frequencies, numbers of lasers, etc. Nothing was said about
the further processi.ng or the separation proper of the desired
excited isotope compound or the enriched is:otope compound,
because this can be accompli~hed in accordance with known
methods, which ha~e partly been described at the outset. Pulsed
lasers suitable for use iP accordance with.the in~ention are
known in the art. An example of such laser is proposed for IR
by R. ~akobs et al : Applied Physics LetterS Vol. 29(11),
page 710 (.1976). A commerciall~ avai.lable laser (.UY) is made
by Lambda Physik, Cottin~en or Molectron Corp., 177 N. ~olfe
Road, Sunnyvale, CA 94 086.
The frequencies for excitin~ UF6 are 16 ~m in the IR
range and Q.3 - a . 4 ~m in W range.
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