Note: Descriptions are shown in the official language in which they were submitted.
BACRGROUND OF THE INVENTION.. .
Field of the Invention~
This invention relates to molecular and atomici~otope separation, particularly applicable to separation of
--1-- , 1,
i"
X ' .
.
- . . , , - ,, . ,, " .. , - .
s~, . ~
. :, ' - ' ` ' ' ^
- ' : ,
.
Uranium-235 from other uranium isotopes includ~ng Uranium-
238.
De~crlption of the Prior Art:
Specific isotopes of a given element are used for
many purposes including medical treatment, tracer studies of
chemical and biological processes, and preparation of target
materials and fuel for nuclear reactors. One of the most
common and desirable processes is the separation or enrich-
ment of Uranium-235 from other uranium isotopes, particular-
ly Uranlum-238. The basic process presently in use for such
separation is gaseous diffu~ion, requiring a complex cascad-
ing network and large energy inputs.
Alternative methods being considered include
centrifugal separation and, more recently, processes taking
advantage of the isotope shiits in atomic or molecular
mixtures so as to ~orm ions o~ a desired isotope, and then
separating the ions Examples of the latter processes
include those described in the above-referenced patent and
also U.S. Patent No. 3,443,087 in the name of J. Robieux et
al. Another U.S. Patent No. 3,558,877 in the name of
Jerome Pressman teaches the use of a light beam to deflect
selected isotopes to promote separation. Still another U.S.
Patent, No. 3,944,825 in the name of Richard H. Levy et al
utilizes selectiYe ionization ~nd an expand$ng plasma to
achieve separation. The references, while representing
desirable improvements in the art, are not without defi-
ciencies. Among the de~iciencies is the necessity for
separation of ions from a neutral background requiring
electric or magnetic fields formed from complex energy
consuming separating devices. Another deficiency is the
-2-
8~)
power required in some proposed processes to ionize to a
sufficiently high energy state. Further, the occurrence of
charge exchange reactions can complicate the proposed
methods. It is desirable to alleviate the complexities and
requirements associated with such separating devices and
procedures and to provide further alternatives in the field
of isotope separation.
SUMMARY OF THE INVENTION
This invention provides method and apparatus for
isotope separation from atomic and molecular isotopic mix-
tures, particularly applicable to separation of isotopes of
uranium. The invention eliminates reliance upon electric
and magnetic field separation means and is generally unaf-
fected by charge exchange possibilities. A basis relied
upon for several of the preferred embodiments is that recoil
momenta are imparted to atoms or molecules undergoing selec-
tive irradiation resulting in ionization, dissociation or
dissociative electron attachment reactions. Accordingly, a
molecular or atomic isotopic gaseous beam having velocity in
a given direction can be selectively irradiated such that a
desired isotopic ~pecies, because of isotropically distributed
recoil momenta, acquires a velocity in a direction different
than the direction of the beam. me desired products of
these reactions can therefore be collected down~tream of the
irradiation area by simple mechan~cal means such as a slit
plate upon which the products condense or from which they
are continuously remo~ed. me balance of the beam, contain-
ing a higher abundance of undesired species, passes through
the slit.
In one embodiment a gaseous atomic isotopic beam
lil~8~
containing, for example, a low abundance of a first isotope
and a high abundance of a second isotope is irradiated so as
to ~electively excite the first isotope. The excited first
lsotope is further irradiated at a wavelength which takes
advantage of the isotope shift and selectively ionizes the
isotope and imparts isotropically distributed recoil momenta
to the ions. The ions therefore move in a direction differ-
ent than the direction of the beam and can be collected
down~tream with comparati~ely simple mechanical separation
means. Once the ions acquire momenta and a new direction
f~om the recoil reaction, the occurrence o~ neutrallzation
or charge exchange reactions ls of little consequence since
it is the direction o~ the flow, and not the charge, which
provides the basis for separation.
In another preferred embodiment the recoll momenta
from a dissociative electron attachment reaction provides
the ba~is for ~eparation of ~ 35F6 from an isotop~c mixture
of UF6 including U238F6. me UF6 molecules are formed as a
gaseous beam and the U235F6 is selectively Pxcited by narrow
band radiation of a predetermined wavelength. me molecular
beam i8 then exposed to a beam of electrons promoting disso-
ciative electron attachment and production of U235F5- and
other products including a free F(fluorine3 fragment and an
energy release. A substantial portion of the energ~Y is
distributed as kinetic recoil momenta, thereby changing
the direction of flow of the U235F5-. Mechanical separation
of molecules containing U235, as ions or neutrals, is then
performed downstream of the electron attachment reaction
region~
In another similar embodiment, an isotopic mixture
8$~
of UF6 is again formed into a gaseous beam and exposed to
multi-photon, preferably 2-photon, irradiation. The first
photon selectively excites U235F6 molecules and the second
photon promotes dissociation to products including U235F5.
me energy released upon breaking the bond freeing the
fluorine is distributed as vibrational motion and recoil
momenta, changing the direction of the molecules containing
U235 and thereby facilitating mechanical separation.
In yet another preferred embodiment, an abundance
of ~ 35 is recovered from an isotopic mixture of UF6 by an
isotopically selecti~e irradiation initiated reaction of Br
(hydrogen bromide) and U235F6. UF6 and HBr molecules are
deposited onto a non-reactive surface in a concentration
providing a substantially greater abundance of Br than
UF6. me UF6 and HBr are formed ~nto a solid matrlx so that
the UF6 molecules are distributed throughout the Br and so
that the rotational structure of the V3 mode of v~bration of
the UF6 is collapsed. In the condensed state, the U235F6
molecules in the matrix are then selecti~ely excited. By
proper ad~ustment o~ the molecular temperature by irradia-
tion of the ~3 mode, a reduction of U235F6 molecules to
products such as ~ 35F5 and ~ 35F4 is enhanced and, because
of the differences in volatility, the other reaction pro-
ducts can be driven off as gases while the ~ 35F5 and
U235F4 remain as solids on the deposit~on surface.
It will be apparent to those skilled in the art
that the various steps summarized above each have inherent
ine~ficiencies and present technologioal limitations, It
therefore is to be understood that the term "separation" and
the like refer to increasing the concentration of a desired
8~
molecule or isotope, such as U235, as compared to the feed
concentration. It will also be understood that each of the
di~closed embodiments is compatible with multiple repetitive
stages, as desired for a chosen end-point concentration.
And, it is also to be understood that while the following
description refers to the separation of a desired species,
such as Uranium-235, in the presence of an undesired species,
such as Uranium-238, the actions and reactions directed
toward the desired species can similarly be directed toward
the undesired species, also accomplishing separation.
BRIEF DESCRIPTION OF THE DRAWINGS
me advantages, nature and additional features of
the invention will become more apparent from the following
description, taken in connection with the accompanying
drawings, in which:
Figure 1 is a schematic representation of the i50-
tropic velocity vector distribution of an ion and electron
formed from an atomic beam;
Figure 2 is a side view, in cross-section~ of
apparatu~ in accordance with the invention;
Figure 2A is a view taken at IIA-IIA of Figure 2;
Figure 3 is a schematic representation of a photo-
dissociation reaction;
Figur~ 4 is a schematic representation of the
isotropic velocity vector distribution of the reaction of
Figure 3;
Figure 5 is a schematic representation of UF6
molecules distributed in an HBr matrix;
Figure 6 is a top view, in cross-section, of
apparatus in accordance with another embodiment of the
--6--
_~,
8~
invention; and
Figure 7 is a schematic representation of photon
$nteraction with a matrix surface on a substrate.
DESCRIPTION OF THE PREFERRED EM~ODIMENTS
Isotopic separation of an atomic or molecular
isotopic mixture can be achieved by selectively imparting
added momentum to the desired atomic or molecular species.
One method o~ obtaining added momentum, taking advantage of
the isotopic shift, is through recoil, typically isotropic,
upon fragmentation of the desired atom or molecule resulting
from selective irradiatlon. Fragmentation, as used herein,
re~ers to loss of an electron from an atom or molecule, or
breaking apart of a molecule into plural parts. The frag-
mentation can be of various forms, forming such products as
ions, neutrals and electrons. The recoil phenomenon is
common to those reactions typically termed photo-ionization,
dissociative electron attachment, and photo-dissociation.
In any of these processes, conservation of momen-
tum and energy requires, upon fragmentation, that any excess
energy be distributed among the resulting products, in forms
such as vibrational, rotational, and translational motion.
The translational motion imparted ~s referred to as reco~l.
PHOTO-IONIZATION
In the photo-ionization process, a selected iso-
tope in an atomic mixture is ionized through multi-photon
absorption. Two-photon absorpt~on is typically utilized in,
for example, the selecti~e ionization of Uranium-235 in an
atomic mixture including other uranium isotopes, such as
Uranium-238. Uranium is used herein as an exemplary ele-
ment, although the procedure is applicable to other elements.
In accordance with this invention uranium vaporor gas is formed into an atomic beam 10 illustrated in
Figure 1, The beam i5 preferably collimated and ribbon-
shaped. The Uranium-235 in the beam is then s~lectively
irradiated, in accordance with the isotope shift, by radia-
tion of a predetermined wavelength which raises the elec
tronic state but is insufficient to ionize the Uranium-235.
m e initial photon can have an energy of, for example, 3.5
eV. mis initial selective irradiation of Uranium-235 in
preference to Uranium-238, represented as a photon hVl
Flgure 1, results in a beam mixture in which the population
of excited atoms is enriched in Uranium-235. me second
photon, represented as h~2, selectively ionizes the excited
Uranium-235. This second photon can have any energy suffi-
cient to ionize the excited atoms, up to about 6 eV, the
~onization limit for both Uranium-235 and Uranium-238.
When the sum of the two photon energies i8 in
excess of the ionization energy of Uranium-235, an electron,
represented as e in Figure 1, leaves the Uraniu~-235 atom
with a kinetlc energy about equal to the difference between
the total absorbed energy and the ionization potential.
The excess momentum vector has a random direction, as de-
picted by the circular broken lines of Figure 1. L~ne 12
represents th~ distribution of the velocity vectors of the
ionized U235 , and line 14 respresents a similar distribution
of the ~reed electrons. The electron line 14 is of larger
diameter than the ion line 12 to illustrate that the elec-
tron velocity (ve) is substantially higher than the Uranium-
235 ion velocity (~235) in accordance with Equations (1) and
(Z) wherein m refers to mass and I.P. is 'he ionization
potential.
h~ I h 2 ~ I.P. = 1/2 mU ¦v 2351 + 1/2 me IVe I (1)
As a result of conservation of momentum:
mU235VU235 meVe (2)
It is apparent that the mass of the uranium frag-
ment, or any other atomic or ionization fragment, is sub-
stantially greater than the mass of the electron. Accord-
lngly, a substantial portion of the excess energy is given
to the electron. I~ it ls assumed, for example, that the
excess energy i~ 1 eV, the electron velocity is:
1 eV = 1.602 x 10~12ergs = 1/2 me ¦ve j2
Ive I = 5.9~1 x 107 cm/sec
And, from the conservation Equations (1) and (2):
U2351 m 235 ¦ve ¦ = 137.~ cm/sec-eV
If it ~s further assumed that the atomic beam i8
formed at about 20~0K and a pressure o~ approximately 0.01
Torr, the isotopes in the beam have an average velocity
given by:
- (8RT) 1/2 = 4.1g~ x 104 cm/sec
where R is the universal gas constant; T is the absolute
temperature; and M is the atomic mass.
The change in momentum, the recoil momentum,
imparted to the isotopes by the ionization process is iso-
tropic; hence the momentum vector has a random direction, as
illustrated in Figure 1.
111~8~
As the recoil U235 ions acquire a differential
veloc~ty in random directions of about 137.5 cm/sec-eV,
most of the ionized U235 will move out of the beam volume,
ln a direction oi motion difierent than that of the beam.
Because the added momentum is small compared to the iso-
tope's initial momentum in the beam, the flow path of the ~ ~-
V235 lons will fan out, maintaining a substantial forward
directional component in the direction of the beam, as
depicted in Figure 2. With a drlit region downstream of the ~ -
ionizatlon of~ ior ex Q le, 300 cm, the U235 recoil ions
w~ll diverge to a maximum Or 0.987 cm per eV of excess
photon energy. -~
One iorm oi apparatus consistent with carrying out
the inventive method is shown in Figure 2. It includes a
means for forming the atomlc isotopic gaseous beam 10 such
as an oven 16. me oYen ~hown includeq a crucible 18 for
holding a BUpply of the ieed~toc~ 20~ such a~ natural
uranlum, It further includes mean~ for vaporizing the
~eedstock 20, such as induction heating coils 22 and heat
shield 240 To facilitate vaporization the oven is main-
tained at vacuum conditions through conduit 26, connected to
vacuum apparatus not shown. For uranium ieedstock the
vacuum ~5 preferably maintained at about 10 8 Torr and the
feedstock heated to approximately 2000K. me atom nux
obtained can be in the region oi 1022atoms cm 2sec 1.
In order to form the high temperature uranium gas
into a beam of desired configuration, collimat~ng means such
as the elongated collimator 28 are used. Although the beam
10 can be of various geometric cross-sections, including
circular, the preferred geometry is ribbon-shaped, as shown
--10--
X
~ 8~ ~S,~22
in Ngure 2A. The beam lO then enters an elongated photo-
ionization region 30, where it is irradiated with photons
h~l and h~2 of preselected wavelength from one or more
lasers 32. The irradiating photons preferably are oriented ~ ;~
to intersect the atomic beam lO at an angle to the beam
direction. The efficiency of the irradiation process can be
increased by use of mirrors which reflect the photons back ;~
and forth through the atomic beam 10. me efficiency and `
throughput of the system can be similarly increased by
utilizlng mirrors and a wide ribbon beam 10 or a plurality
of beams arranged side by ~ide.
The photo-ionization region 30 is also maintained
at vacuum conditions, about 10 g Torr~ through conduit 34
connected to Yacuum maintaining mean~ not shown. Upon
obtaining recoil momentum, the selectively ionized Uranium-
235 diverges from the beam 10 in a h~gh vacuum field free
area. The ma~nstream of the beam 10 therefore continues
through a sllt 36 ~n mechanical separating means such as the
conden~ing collecting surface 3~. The region 40 about the
collecting surface i~ also maintained at vacuum conditions
through conduit ~2. To increase collection efficiency~ the
beam 10 mainstream can continue through another axially
al~gned photo-ionlzat~on and collection region, It w~
also be noted that since the basis for collection is the
differing d~rection Or the Uranium-235 ions, neutralization
of the ions prior to impact upon the collecting sur~ace does
not affect the separa~ion process. ~he collecting surface
can be easily removed when a desired buildup of uranium,
increased in the Uranium-235 concentration, is ach~e~ed.
me uranium can then be remo~ed from the collecting surface
--11--
8~i~
~5,~22
by chemical, scraping or other well known processes. De-
pendent upon the geometry of the apparatus, a buildup of
uranium may also occur on the walls ~ of the photo-
ioniæation region 30, particularly do~mstream of the point
of ionization. The ~ralls 44 are therefore preferably seg-
mented to facilitate removal and subsequent processing to
recover the uranium product.
DISSOCIATIVE ELECTRON ATTACHMENT
ln addition to photo-ionization, the recoil momen-
tum upon fragmentation can also be utilized in dissociative
electron attachment reactions. The process and apparatus
utilized can be similar to that described above, addition-
ally requiring means for exposing the molecular beam to an
electron beam.
In a dissociative electron attachment reaction an
iso~opic mixture of m~lecu es of, for ex~mple5 UF6 is formed
into a gas~ous collimated beam flowing in a ~reset direc~
tion. The beam is then exposed to irradiation at a prede-
termined wavelength so as to selectively excite the vibra-
tional~J3 or other suitable combination mode of the U235F6
molecules in preference to other molecules such as U23~F6.
The energy of the irradiation should be less than the ioni-
zation potential of the molecule and the excitation can be
performed through single or multi~le photon absorbing steps.
The UF6 beam is then exposed to a beam of electrons of
sufficient energy to selectively promote dissociative elec-
tron attachment of the excited UF6 molecules in nreference
to other UF6 molecules. ~Jith an asterick denoting an excited
state, alternati~ely stated as a state of population in-
version, and a double plus denoting excess ener~y, the
-12-
45,~22
reackion can be wr~tten as:
I
U235F6 ~ U235F6* e ~ U235F6 ---3 UF5 ~ + 1/2(F2) I E
While the reaction identifies U235F6-, free fluorine, and
energy (E) as the products, it will be understood that other
product~, such as U235F4- are ~lso possible. The dicsocia- ~
ti~e electron attachment reaction offers some advantages ~-
with respect to the abo~e-described photo-ionization pro- -
cess. First, it will be recogn~zed that since the fragments
include free fluorine atoms as opposed to electrons, the
percentage distr~but~on of recoil momentum to the UF5 lon i8
greater than the percentage distribution to a Uranium-23S
atom. Accordingly, the UF5- will move out of the mole¢ular
UF6 be~m with a greater separation. Also, the temperatures
required for w~rklng with UF6 are much lower and w~uld
generally ~e below 1000C *or other ¢ompounds.
PHOTO-DIS80CIATION
In addltion to photo-ionization and disso¢iati~e
eleotron attachment reaction~, recoil momentum can al80 be
advantageously utilized, similar to the proce~ses discussed `
above, in a photo-dissociati~e procedure in con~unction with
a molecular isotopic beam. Here, moleculeR of, for example, -~
SF6 or UF6 are formed into a gaseous collimated molecular
beam. The molecular beam i8 selectively exclted with a
first photon irradiation o~ the vibrational1/3 or ~u~table
combination mode so as to increa~e the excited population in
U235F6 concentration. Neither the first or second photon
irradiation, again, should be of sufficient energy to,
alone, cause dissociation of the U23~F6 or U235F6 molecules.
The molecular beam ~s ~hen further ~rradiated at a predeter-
-13-
s~a
mlned wavelength at an energy sufficient to dissociate the ;
excited U235F6 molecules. It has been established by Rock-
wood, S.D. and Rabideau, S.W., in the IEEE J. Quantum
Electronics, QE-10, 789 (1974), that two-photon irradiation
of SF6 ls achievable, and can be written as:
~ ' hV ,`:
SF6 ~ SF6* ~ SF5 + 1/2 F2 ~
Since the molecular makeup oi SF6 is similar to that Or ~ ;
UF6, lt can be pre~umed that two-photon dissociation is also
achelvable with UF6. me process is illustrated in Figure
4. me bond energies of UF6 and SF6 are ~5.9 kcal/mole and
45.6 kcal/mole, respectively.
Upon breaking Or the bond, a substantial portion ~;
of the energy released goes into recoil motion of the frag-
ments and, as in the above di~cussions, ~acilitates separa-
tion Or the recoll product~ from the mainstream Or the
molecular beam by relati~ely slmple mechanical ~eparation
structure. Since the recoil products are neutrals, electric
or magnetic ~ields need not be used for separation,
MATRIX ISOLATION
Figure 6 illu~trates apparatus useful in ~epara-
tion of a molecular mixture Or uranium ~sotope~ by matr~x
i~olation also utilizlng selecti~e excitation principle~,
The basis for the separation is the isolation of UF6 mole-
cules in a precisely defined en~ronment characteristically
reacti~e abo~e a preselected threshold temperature. The
matrix can be formed by conden~ing a molecular species, ~uch
as UF6, onto a cold surface at the same time that a ma~rix
molecule or atom, inert or reacti~e, such as HBr, is con-
den~ed. Whlle varying ratios o~ the matrix constituents can
-14-
,8~
45,~22
be used, the procedure as applied in lnfrared spectroscopy
usually involves the matrix molecules or atoms at a number
den~ity in excess of 50 to 100 times the primary molecule or
atom.
In accordance with one embodiment of the inven-
tion, UF6 and HBr, preferably in a gaseous state, are
codeposited on a cold unreactive surface, thereby creating a
thin solid layer of ind~vidual UF6 molecules trapped in an
HBr matrix. As illustrated in Figure 5, the matrix forma-
tion makes each UF6 molecule generally unable to interactwlth other UF6 molecules. Additionally, the freezing of the
UF6 into the matrix substantially lessens the molecular
rotational transitions, and specifically collapses the
rotational ~tructure of the V3 fundamental vibration at
623.5 cm~l into a single ~harp line at a slightly different
frequency (see Bar-Ziv, E., Frieberg, M., and Weis~S.,
Spectrochemicha Acta, 1972, 2~A, 2025-202g).
The sub~tantial elimination of rotational contri-
bution~ greatly increases the efficient utilization of the
vibrational frequency difference Or about 0.05 cm between
the ~3 vibrational mode5 of U235 and U23g- An efficient
selective excitation of the U235F6 V3 fundamental frequency
can therefore be reali2ed.
Although other isolating species may be used, HBr
is known, through ~York developed at the Union Carbide Corp-
oration, Oak Ridge Gaseous Diffusion Plant reported by ~olf,
A.S. 9 Hobby, W~E., and Rapp, K.E., in Inor~anic Chemistry
1965~ 4, #6, 755-757, to undergo the follo~ng reactions:
2UF6 + 2HBr--~ 2U~5 + 2HF + Br2
8~i~
45,~22
2UF5 ~ 2H8r~ 2U~4 1 2HF I Br2
It was found that ~rhile liquid UF6 which was
allowed to stand overnight in contact wlth liquid HBr at
room temperature produced only a small amount of reaction
products, the reactions proceed vigorously at 65C. The
reactions are therefore strongly temperature dependent.
Although the mechanism of the reaction has not been eluci-
dated, in all probability it proceeds along the s~me axis as
the ~3 fundamental mode:
F F F
10F U - F + H Br~ U - F + H-F I 1/2Br2
~ - F
F F F
Accordingly, with U235F6 isolated and surrounded
by HBr molecules, selecti~ely exc~ting the U235F6 ~3 mode
with sufficient energy to produce an effective vibrational
temperature of 65C wlll result in the molecules' reduction
to UF5, UF4, and so forth. Once the U235F6 molecules have
been selectively reduced in the presence of U23gF6 mole-
cule~, separation of the isotopic species is accomplished
wlth relative ease. For example, because of di~ferences in
volatility, a mtld heating of the matrix under reduced
pressure to about 300 Torr will volatize the U23gF6, HBr,
HF and ~r2, while the solid U235F5 and some U235F~ remain on
the deposition surface. Chemical reactions carr~ed out
under matrix i~olat$on conditions are also discussed in
l'Direct Synthesis And Characterization Of Debenzenechro-
mium(0~ In An Argon M~trix At 1~Kn, John T~r. Boyd, John M.
Iavoie and Dieter M. Gruen, Journal of Chemical Physics,
Vol. 60, No. 10, May, 197~; nMatrix Isolation Infrared Study
-16-
/~-
.8~
~5,~22
Of The Reacticn Between Ger~nanium ~apor And Molecular Oxy-
gen. The Characterization And Mechanism Of Formation Of
Molecular Germanium Dioxide And Ozone", A~ Bos, J. S.
Ogden, and L. Orgee, Journal of Physical Chemistry, Vol.
7~, No. 17, 1974; and "Infrared Spectra Of ~atrix-Isolated
Uranium Oxide Species. I. The Stretching Region~, by S. D.
Gabelnick, G. T. Reedy, and M. G. Chasanov, Journal of
Chemical Physics, Vol. 5~, No. 10, 1973.
It will be apparent that the disclosed invention
offers substantial ad~antages as compared to other laser
excitation methods. The exciting radiation is in the in-
frared region, which, under present technology, is believed
to be the most economical and efficient region for photon
production~ Further enhancing the efficiency of the proce-
dure i8 that the reactions are carried out in a solid state,
thereby enabling a high density of reacting sites. Also,
since no io~ization processes are involved, a relatively
high density of excited molecules can be achieved ~thout
concern for space charge effects. Further, selective i80-
topic occurrences are le~s likely to be scrambled by inter-
molecular collisions as a result of the dilution factor and
general immobility in the solid matrix~ ~nd, since the
final separation can merely be the process of removing
volatiles from a solid product, complex field separation
devices are unnecessary.
The apparatus shown in Figure 6 can be used to
carry out the inventive method, and includes four basic
regions denoted I throu~h IV. Region I is the matrix f~rma-
tion area and includes a series of nozzles 60 and 62 lJhich
respecti~ely deposit UF6 an~ HBr from the inlets 6~ and 66
-17-
111~8~
i
at a molecular number ratio o~ UF6 to Br in the range of
1:50 to l:100. me UF6 and Br are deposited (as liquids or ~-
possibly gases) onto a moving substrate 68 which iæ cooled
to a temperature less than 100K. me spraying region is
~; preierably maintained at a pressure of about 10 4 Torr or
less, through vacuum maintaining means (not shown) connected
to conduit~ 70 and 71. Cooling means such as a refrigera~
tion coil 69 ln contact with or near the moving substrate
can be utilized to cool the substrate. ~ ;
The substrate is non-reactive with the matrix
materials and can include materials such as gold or platinum
depo~ited on a copper or other band. Copper is preferred
because o~ its good thermal heat transier characterlstics.
me thickneqs o~ the matrix mixture can be ad~usted by
j varylng the amount deposlted and/or the speed of the sub-
strate 68. The actual mass throughout the entlre system
is al~o controlled by the number of spray nozzles, the mass
flow through the nozzles, the irradiation absorptlon efii-
ciency, and the laser power. me typlcal advantageous
denslty diiferentlal between this embodiment and the molecu-
lar beam teachings will be apparent from a comparison o~ the
number density in the beams at 1 Torr and the number density
ln the diluted matrix, For example, ~or a uranium feed
having approx~mately 0.7% U235, at 20C and a dilution of
100:1, the number density of U235 as UF6 in the solid matrix
is approximately 9.3 x 1016/CC. A molecular beam maintained
at 1 Torr, which represents the maximum pressure reasonably
applicable to molecular beams, will have a U235 number
density of about 2.6 x 10l4/CC. me d~f~erential ccn~tl-
tutes approximately a 350 times greater number density in a
-18-
/
.
~ 5,~22
matrix than in a molecular beam, which ad~antageously allows
higher throughput and greater efficiency.
Selecti~e excitation and chemical reaction of the
deposited solid matrix occurs in region II. The matrix in
region II is preferably maintained at t~mperature and pres-
sure conditions similar to region I to allevlate the poten-
tial for additional side reactions. A conduit 72 connected
to appropriate apparatus can be used for this purpose.
Selective irradiation of the matrix is performed by irradia-
tion means such as a laser head 74. In this region, thesystem ef~iciency is effected by the irradiation absorption
efficiency and the power density of the laser 74. m e
substrate speed can be compatibly ad~usted with the laser
power available, being correspondingly faster for a high
power and lower for a low power,
Since the output of many lasers is polarized~ the
laser head i8 preferably aligned, with respect to the sur-
face o~ the ~ubstrate, at Brewster's Angle. As illustrated
ln Figure 7, this arrangement lessens reflective losses in
the laser beam transmtssion. At Brewster's Angle substan-
tially f~ll transmission of the radiation i5 into the matrix.
With a platinized or gold coated copper substrate, the beam
of photons will be reflected back into ~he matrix as at "A"
with only small losses as at "B". ~eviations from the
Brel~ter's Angle compliment at the matrix-substrate inter-
face 76 ~111 cause additional re~lections back into the
matrix as shown at "A", ~ransmission of ~hotons at Brew~
ster's Angle thus creates a high efficiency configuration
for photons subsequent to entering the matrix, as ~ell as
assuring essentially 100% penetration of the photon beam
--19--
8~i~
into the matri~. Further assisting the photon reaction
efficiency, illustrated at "C", is the result that photons
re-emitted by an unreacting relaxation are subjected to the
same internal reflections, enhancing the reactive absorption
probability ~or U235F6 selective excitation.
Upon excitation causing a ~ 35F6 molecular temper-
ature in excess of 650C, the reduction process discussed
above rapidly occurs, producing U235F5 and U235F4.
m e matrix and substrate then move to region III
where separation of the reduction products occurs. Heating
means such as a heating coil 78 warm the substrate and
matrix to a temperature in the range of 600C. Because of
the differing volatilities of the constituents, this mild
heating drives off the unreacted U238F6, excess HBr, HF, and
Br2, leaving behind the U235F5 and some U235F4 as a solid
residue. m e volatilized compounds are pumped from the warm
substrate through apparatus connected to conduit 80.
The substrate then continues to region IV, where
the products having an increased U235 concentration are
removed and the substrate is further cleaned and dried prior
to returning to region I. m e removal o~ de~ired products
can merely involve washing, dissolving, or scraping of the
products from the ~ubstrate.
The ~ 35F6 molecules left on the substrate in
region III should coalesce into stranded particles during
vaporizat1on of the other materials. However, ~t can also
be carried away as small particulates with the U238F6 and
HBr gases. In this event, a particulate filtration system,
including a filter 82, can be utilized to trap the solid
U235F5 and U235F4.
-20-
8'~ ~5,~22
'.
The apparatus also preferably includes baffles ~4
between the respective regions which assist in maintaining
the de~ired pressures and separation of the four regions.
The baffles ~2 can be maintained at low temperature thr~ugh
co~ling by liquid nitrogen or liquid carbon dioxide.
EXAMPLE
An exemplary throughput utilizing matrix isolation
can be shown analytically assumi~g, for example, that the ;~
UF6 deposition nozzles 60 have an effective orifice area of
one square centimeter with a UF6 pressure of 1 Torr at
t 273K. The nozzles can be distributed over approxlmately 20
cm. Further assuming a substrate velocity of 0.5 cm/sec., a
deposition of approximately 2.09 grams per hour of trapped
U235F6 is available for photo-excitation, corresponding to ~-
approximately 1.41 grams per hour of pure U235~ as sh3wn by
the follow~ngs
To determine the molecular density (n) of U235F6
where ~ molecules/cm3, at, for example, 273K and 1 Torr
(1/760 atmosphere), it i8 known that
Pv = n RT
6.023 x 1023
Where v = 1 cm3, T = 273K, P = 1 Torr and R = ~2.057 cm3
atm mole~l K 1, and accordingly
n =( 1/760 x 1 ) 6.023 x 1023 = 3.65 x 1016 molecule~/cm3,
2.~57 x 273
corresponding to n = 2.59 x 1014 U235F ~ cm3.
The average velocity (v) of the U235F6 can be
~ ( ~Rr )1/2 wher8 R = ~.317 x 107, T = 273 and
~rM
M = 3~9, corresponding to v = 1.55 x 10~ cm/sec.
-21-
8 ~
45,g22
The flux (F) at these exemplary conditions is
defined by F = n~ v, or 145S x lO~ x _~ 9 x 101~ = 1.006 x
101~ molecules/cm2 sec. Accordingly, to determine the
number of moles and grams deposited during one hour, assum- -
ing one cm2 of orifice at l Torr and 273K, the deposition
1 oo6 x 10l~ 3 235
This corresponds to 6 x 10-3 x 3~9 = 2.09 g U235F~hr. In
terms of pure U235 th~ deposition rate is 6 x 10-3 x 235 =
1-41 g U235 hr.
The exemplary process can be carried out under
reasonable energy requirements. To exemplify the power
output required for the laser to perform the selective
excitation in a single photon excitation process assuming 1
cm2 of orifice and the other exemplary conditions aboYe, ~t
was shown that the deposition rate of U235~6 is 1.006 x
101~ molecules/sec. Accordingly, the minimum number of
~elective photons (h~ ) w~uld be 1.006 x lOlg/sec. The
energy (Joule/h~3) of each photon is 1.9g6 x 10-23 x 625 =
1.2~ x 10-2 J/h~3. Accordingly, the minimum desirable
energy on a time basis is 1.006 x lO-l~ x 1~2~ x 10-2 =
1.25 x 10-2 J/sec, or a laser power of only l.Z5 x 10-Z
watts, or 12.5 milliwatts. Assum~ng a one ~ercent quantum
yield and one percent wall plug efficiency for the laser,
the po~ler re~uired is 125 ~atts, or 319 kw/g of separated
U235. The electrical power expended ~y the laser for separ-
ation of lg U235F6 is approximately 216 k~ and) for a yield
of 1 kilo~ram of three percent enriched UF6, the po~rer
required is 3.32 ~/kg.
-22-
45,~22
There have therefore been described a number of
systems and methods useful in isotopic separation of atomic
and molecular mixtures. It will be apparent that many ;
modifications and additions are possible in view of the
above teachings. It therefore is to be understood that
within the scope of the appended claims, the invention may
be practiced other than as specifically described.
:
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