Isotope separation apparatus and process

An apparatus and process are disclosed for separation of isotopes by gaseous diffusion. The apparatus comprises at least one assembly of microporous capillary tubes having a hydraulic diameter below 0.5 cm, an average pore radius below 200 .ANG., a thickness between 20 and 500 microns, a length of at least 25 cm, a porosity between 10 and 60% and a permeability to air between 10.10.sup.-7 and 100.10.sup.-7 mole/cm/.sup.2 /min/cm of mercury. The tubes are made from an inorganic material and have a circular, square or rectangualr cross-section.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for isotope separation by 
gaseous diffusion by means of porous barriers. More specifically, it 
relates to an isotope separation process making it possible to improve the 
operating conditions of a gaseous diffusion installation, particularly by 
reducing the energy consumed in such an above installation. 
The presently known apparatus for the separation of isotopes by gaseous 
diffusion are generally constituted by an assembly of porous barriers in 
the form of microporous cylindrical tubes, within which is circulated in 
turbulent manner the gaseous mixture to be separated. 
By passing through said tubes, the gaseous mixture is separated into a 
light isotope-enriched fraction which diffuses through the wall of the 
tubes, and a light isotope-depleted fraction which is discharged at the 
outlets from the tubes. 
In view of the fact that passing through a porous barrier only leads to a 
very limited enrichment of the gaseous mixture, to obtain a significant 
enrichment by the light isotope, this operation must be repeated a 
significant number of times. 
Therefore, a gaseous diffusion isotope separation installation generally 
comprises a large number of elementary stages grouped in cascade form, 
each elementary stage being constituted by an assembly of porous barriers. 
In the most widely used type of cascade, the stages are grouped in such a 
way that the enriched fraction leaving the stage is passed to the 
following stage, whilst the depleted fraction leaving the same stage is 
passed to the preceding stage. Before being introduced into their 
respective stages, the enriched and depleted fractions leaving one stage 
must be compressed in order that their pressure is brought to the selected 
value for the diffusion. 
Therefore, the energy necessary for the operation of such an installation 
and specifically the power which is more particularly consumed in the 
performance of the various compression stages is very high, due to the 
large number of stages. 
BRIEF SUMMARY OF THE INVENTION 
The present invention relates to a process for isotope separation by 
gaseous diffusion, making it possible to significantly reduce the specific 
energy (power consumption per separation work unit in KWh/SWU) of an 
isotope separation installation, through the use of porous barriers having 
special characteristics and through the flow conditions established 
through these barriers. 
Therefore, in the gaseous diffusion isotope process according to the 
invention, for the purpose of the separation microporous capillary tubes 
are used having a hydraulic diameter D defined by the formula D=4S/P in 
which S represents the surface of the cross-section of the inner pipe 
defined by the said tube and P the perimeter of said cross-section which 
is below 0.5. cm. The gaseous mixture to be separated is circulated in 
these tubes, so that there is a laminar or almost laminar flow of the 
gaseous mixture therein. 
Advantageously, the gaseous mixture is circulated in such a way that the 
Reynolds number at the tube inlet is below 4000, preferably below 2000 and 
above 200. 
The invention also relates to an isotope separation apparatus for 
performing this process. This apparatus comprises at least one assembly of 
microporous capillary tubes having a hydraulic diameter D defined by the 
formula D=4.S/P, in which S represents the surface of the cross-section of 
the inner pipe defined by the said tube and P is the perimeter of said 
cross-section, below 0.5 cm. 
Advantageously, the average radius of the pores of the tubes is below 200 
.ANG.. 
Advantageously, the length of the microporous capillary tube exceeds 25 cm. 
Preferably, the capillary tubes have a thickness between 20 and 500 
microns, a porosity between 10 and 60% and a permeability to air between 
10.10.sup.-7 and 100.10.sup.-7 /mole/cm.sup.2 .times.mn.times.cm of 
mercury. 
According to an advantageous feature of the apparatus, the tubes are made 
from an inorganic material selected from the group including alumina, 
magnesia, titanium dioxide, silica, chromium oxide, mixed oxides of 
aluminium and magnesium and nickel, as well as certain metal fluorides 
such as magnesium and nickel fluorides. 
According to another advantageous feature of the apparatus, the tubes are 
made from an organic material, for example polytetrafluoroethylene. 
According to the invention, the microporous tubes used can have a circular 
cross-section or a polygonal cross-section, for example a square or 
rectangular cross-section. 
It is pointed out that in the present text the term tube is understood to 
mean a cylindrical surface produced by the displacement of a straight line 
or generatrix which is compelled on the one hand to remain parallel to a 
fixed direction and on the other to meet a fixed, closed planar curve or 
directrix, whose plane intersects the given direction. 
Thus, a tube can have a circular, elliptical, square, polygonal or any 
other cross-sectional shape in which the directrix is constituted by a 
circle, ellipse, square, polygon or the like. 
The microporous capillary tubes having the aforementioned characteristics 
can be produced by any known process, e.g. spinning, casting, extrusion, 
followed or not followed by isostatic compression and chemical and/or 
thermal treatment or by deposition on a core followed by isostatic 
compression and thermal treatment, said deposition being in particular 
carried out by spraying, dipping or electrophoresis. 
The gaseous diffusion isotope separation process according to the invention 
using porous barriers in the form of capillary tubes with an internal 
diameter at most equal to 0.5 and a length of at least 35 cm and the 
establishment within said tubes of a laminar circulation of the gaseous 
mixture to be separated makes it possible to improve the aerodynamic 
efficiency of the tubes, which also leads to a significant reduction in 
the pressure drop within the tubes. This improvement in the aerodynamic 
efficiency and this reduction of the pressure drop within the tubes leads 
to a specific energy gain. 
It is pointed out that the specific energy of a stage of an isotope 
separation installation, i.e. the specific energy of a porous barrier 
corresponds to the relationship: 
##EQU1## 
in which W represents the energy consumption and .delta.U the separative 
work. 
The energy consumption W is the sum of energy W' necessary for compressing 
to the intake pressure p.sub.e the diffused flow leaving the stage (n-1) 
at pressure p.sub.av and the energy W" necessary for compressing to intake 
pressure p.sub.e the poor flow leaving the stage (N+1) at pressure 
p.sub.s. In a diffuser at balance, the diffused flow and the poor flow are 
equal to Qe/2, Qe being the intake flow of each stage. 
The energies W' and W" corresponding to an adiabatic compression of 
efficiency n.sub.c at temperature T.sub.c are given by the formulas: 
##EQU2## 
in which for uranium hexafluorides 
##EQU3## 
i.e. 1.065, M=0.238 kg, R=8.314 Joules, .tau..sub.c is the compression 
ratio of the diffused gas equal to p.sub.e /P.sub.av and 96 .sub.r the 
compression ratio of the poor gas equal to P.sub.e /P.sub.s. 
The separative work .delta.U is given from the formula: 
##EQU4## 
in which .theta. is the distribution coefficient between the enriched and 
depleted flows, i.e. 0.5 in a diffuser at balance and .epsilon. is the 
isotope separation coefficient of the stage given by the formula: 
##EQU5## 
in which .epsilon..sub.o =43.10.sup.-4 for UF.sub.6 ; 
S is the separation efficiency of the microporous barrier given by the 
formula: 
##EQU6## 
in which P is a constant given by the equation: 
##EQU7## 
.eta.: gas viscosity at process temperature V.sub.2 : average velocity of 
the heavy isotope 
r.sub.p : average radius of pores 
P.sub.e : upstream pressure 
P.sub.av : downstream pressure 
Z is the aerodynamic efficiency given under laminar flow conditions by the 
formula: 
##EQU8## 
in which R.sub.e is the Reynolds number at the entrance to the microporous 
barrier, R.sub.h the hydraulic radius of the capillary tube, L the 
capillary tube length and a a coefficient dependent on the tube 
cross-section. In the case of capillary tubes with a circular 
cross-section, a is equal to 0.04511. 
In the case of uranium hexafluoride, the specific energy can be expressed 
by the formula: 
##EQU9## 
In this formula, W/.delta.U is in KWh/SWU, .sup..delta. U being calculated 
with Qe expressed in kilograms of uranium per annum. 
Thus, with the process of the invention leading to an increase in the 
aerodynamic efficiency value Z and to a decrease in the value .sup..tau. 
r, a lower specific energy is obtained. 
It is apparent from the equation that it is advantageous to work with the 
highest possible value of S, which in practice leads to the limitation of 
the average pore radius values to below 200 .ANG.. 
According to a first embodiment of the apparatus according to the 
invention, the assembly of the microporous capillary tubes is constituted 
by microporous tubes arranged parallel to one another, each of their ends 
being fixed to a plate, called an assembly plate, said tubes being 
arranged within the assembly in rows parallel to a given direction, called 
the first direction, whilst also forming rows parallel to the second 
direction perpendicular to the first direction. 
Advantageously, in this embodiment, the microporous capillary tubes have a 
circular cross-section and the tubess of each row parallel to the first 
direction are in tangential contact with one another. 
This embodiment of the assembly is particularly advantageous because it 
leads to a very compact assembly having a large number of capillary tubes 
without causing a disturbance in the gas flow which has passed through the 
wall of the tubes. 
Thus, the passage of reception channels between the rows of tubes makes it 
possible to pipe the diffused gas and consequently obviates high pressure 
drops in the circuit of the gas separated by diffusion. 
According to a variant of this first embodiment the microporous capillary 
tubes are regularly spaced from one another in each of the rows parallel 
to the first or second directions. 
In this case, the distance d.sub.1 between the planes defined by the axes 
of tubes of two adjacent rows parallel to the first direction preferably 
exceeds the distance d.sub.2 between the axes of two adjacent tubes of a 
row parallel to the first direction. 
Preferably, when the tubes have a circular cross-section distance d.sub.2 
is such that the value of (d.sub.2 -/d.sub.ext) in which d.sub.ext 
designates the external diameter of the tubes is at the most equal to 2 mm 
and the distance d.sub.1 is such that the value of (d.sub.1 -d.sub.ext) is 
between 0.5 and 3 mm. 
According to a second embodiment, the assembly of the microporous tubes is 
constituted by microporous tubes arranged parallel to one another and 
forming in said assembly a first series of ducts, called first ducts 
defined by the inner wall of the tubes, said assembly having a plurality 
of longitudinal partitions integral with at least certain of the tubes and 
distributed between the tubes so as to define with the outer wall of the 
latter a second series of ducts, called second ducts parallel to the first 
ducts. 
In this second embodiment, the tubes and partitions are advantageously 
reciprocally arranged in such a way that the second ducts all have the 
same cross-section which is preferably such that the ratio of the 
cross-section of the second ducts to the cross-section of the first ducts 
is between 1 and 20. 
According to a feature of this second embodiment the tubes of the assembly 
are mounted on two plates, the partitions extending from one plate to the 
other and have openings for the discharge of the separated gas circulating 
in the second ducts. 
According to a variant, the tubes of the assembly are mounted on two 
plates, the partitions extending from one of the plates to the vicinity of 
the other plate in such a way that in the vicinity of said other plate 
openings are provided for discharging the separating gas circulating in 
the second ducts. 
In this second embodiment, the presence of longitudinal partitions which in 
part define within the assembly reception ducts for the gas separated by 
diffusion through the wall of the tubes makes it possible to improve the 
separation efficiency of the installation by creating countercurrent 
effects, i.e. by making the separated gas flow in second ducts in the 
opposite direction to the gaseous mixture to be separated which flows in 
the first ducts. 
For example, this result can be obtained by providing openings in the 
longitudinal partitions arranged so as to only permit a discharge of the 
separated gas circulating in the second ducts in the immediate vicinity of 
the end of the tubes corresponding to the intake of the gaseous mixture to 
be separated. 
Moreover, the presence of the longitudinal petitions give such an assembly 
a good mechanical strength due to the high moment of transverse inertia of 
the system and to the very close and compact network of partitions. This 
also makes it possible to obtain a low or negligible pressure drop in the 
second ducts in which the gas separated by diffusion flows. 
Preferably, the partitions of the assembly are made from the same material 
as the tubes. In this case, the assembly having a plurality of tubes 
joined by longitudinal partitions can be directly manufactured in its 
final form from an organic or inorganic microporous material paste, for 
example by extruding the paste through a spinneret of shape adapted to 
that of the assembly to be obtained, said extrusion being followed by a 
chemical and/or thermal treatment of the thus obtained assembly. 
The extrusion paste used can be constituted by any conventional extrusion 
material complying with the conditions required for the use of the 
assembly. Advantageously, the extrusion paste contains particles of a 
metal oxide such as alumina, magnesia, titanium dioxide and silica coated, 
for example, with organic and preferably thixotropic binders like a 
terpineal seresine mixture. It is also possible to use other binders, such 
as water binders, more particularly from the gum tragacanth group or even 
thermoplastic binders. 
By choosing a spinneret with a suitable geometry, it is possible to obtain 
in a single operation an assembly of microporous tubes joined by 
longitudinal partitions. 
The openings in the partitions can be made during the extrusion operation 
or after baking the assembly obtained, for example by mechanical machining 
or by cutting using laser radiation. 
Preferably, these openings are produced during the extrusion operation by 
stopping the injection of the paste by means of a comb in the locations 
corresponding to the partitions. Thus, the partitions are interrupted over 
a length representing for example 10% of the total length of the tubes, 
which preferably happens at one of their ends. 
However, such assemblies can also be obtained by producing the rows of 
tubes by extrusion and by then assembling said tube rows by means of 
spacers fixed to certain of the tubes by conventional processes. 
According to a third embodiment of the invention, the assembly is 
constituted by an alveolar or honeycomb module, whose walls made from 
microporous material with an average pore radius below 200 .ANG., define 
rows of parallel channels having a square or rectangular cross-section, 
said rows alternately forming a first series of ducts, called first ducts 
having a hydraulic diameter below 0.5 cm and in which the gaseous mixture 
to be separated is circulated, and a second series of ducts, called second 
ducts in which is collected the gas separated by diffusion through the 
wall of said first ducts. 
Advantageously, in this third embodiment, the second ducts are sealed at 
each of their ends, opening being provided in each row of second ducts so 
as to ensure the discharge of the gas separated by the side walls of the 
module. 
According to the invention, the isotope separation apparatus preferably has 
several microporous capillary tube assemblies. In this case, the apparatus 
advantageously comprises a cylindrical enclosure in which are successively 
arranged a plurality of assemblies of microporous tubes which are parallel 
to the enclosure axis, said assemblies being in each case mounted between 
two plates, called diffuser plates and separated from one another so as to 
provide between two adjacent assemblies and at each of the ends of the 
enclosure successive chambers which alternately constitute distribution 
chambers for the gaseous mixture to be separated in the microporous tubes 
issuing into the latter and collection chambers for the gas leaving the 
tubes issuing into the latter, means for supplying the distribution 
chamber with the gaseous mixture to be separated, means for extracting 
from the said collection chambers the gas leaving the tubes of said 
assemblies and means for collecting the gas which has passed through the 
wall of the tubes of each of said assemblies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an isotope separation apparatus comprising an enclosure 1 
within which are vertically mounted between two horizontal plates 3 and 5, 
called diffuser plates, assemblies 7 of microporous capillary tubes having 
an internal diameter at the most equal to 0.5 cm, advantageously below 
0.25 cm and preferably below 0.12 cm. The diffuser plates are provided 
with an opening for fitting each assembly and are, for example, made from 
metal such as stainless steel, Monel or a steel coated in order to prevent 
corrosion or alternatively can be made from a plastics material such as 
Teflon. 
At the bottom of enclosure 1, the lower plate 5 defines a chamber for the 
introduction of the gaseous mixture to be separated which is supplied 
under high pressure by pipe 9. 
The gaseous mixture then passes into the tube assembly 7 in which it is 
separated into a depleted fraction which is discharged under high pressure 
in accordance with the path of arrows F.sub.1 by pipe 11 and a fraction 
enriched by diffusion through the wall of the capillary tubes which is 
discharged under low pressure following the path of arrows F.sub.2 by pipe 
13. 
In the embodiment of FIG. 1, plates 3 and 5 are interconnected by a 
perforated vertical partition 15 defining with the inner wall of enclosure 
1 an annular passage for discharging low pressure gases towards pipe 13. 
The upper plate 3 is surmounted by a dome 17 defining with plate 3 a high 
pressure gas collecting chamber. 
As can be seen in FIG. 2, an assembly 7 is constituted by a plurality of 
microporous capillary tubes 21 arranged parallel to one another and fixed 
by each of their ends to a plate, called an assembly plate, whereof only 
the upper plate 23 is shown in the drawing. 
These assembly plates are provided with circular openings with a diameter 
substantially equal to the external diameter of capillary tubes 21 and are 
advantageously made from a metal such as nickel or aluminium. The sealing 
with respect to the passages of tubes 21 in the openings of assembly 
plates 23 is brought about, for example, by a phosphate glass such as P 
106. 
As can be seen in greater detail in FIG. 3, the microporous capillary tubes 
21 of assembly 7 are constituted by elementary capillary tubes such as 21a 
and 21b assembled end to end by means of cylindrical sleeves 25 made, for 
example, from aluminium or nickel and having, for example, a thickness of 
approximately 0.2 mm. The seal between sleeve 25 and the elementary tubes 
21a and 21b is provided by a deposit 26 of an aluminium or alumina powder 
applied, for example, by atomizing or spraying. 
The seal between tubes 21a and 21b can also be provided by uranium 
hexafluoride-resistant phosphate glasses such as P 106 or 
fluorine-containing glasses or by means of polytetrafluoroethylene-based 
emulsions or glues. 
According to the invention, the elementary tubes 121a or 21b have a 
thickness between 20 and 500 microns, a porosity of 10 to 60%, a 
permeability to air between 10.10.sup.-7 and 100.10.sup.-7 mole/cm.sup.2 
/minute/cm of mercury. The length of the capillary tubes 21 preferably 
exceeds 50 cm and they generally comprise a plurality of elementary tubes 
such as 21a assembled end to end. 
As can be seen from FIGS. 2 and 4, the tubes 21 of an assembly 7 are 
radially secured by means of elastic fastening means 27 at sleeves 25. 
Thus, sleeves 25 are in contact with one another and also keep tubes 21 at 
a given spacing corresponding to the thickness of said sleeves 25. These 
elastic fastening means comprise, for example, strips of a metallic 
material or a plastics material such as Teflon. Thus, the tubes are 
assembled in accordance with a hexagonal pattern. 
Tubes 21 can be assembled in a hexagonal group and kept at a desired 
spacing by means of a square-meshed, plastic or metallic gauze with a side 
dimension substantially equal to the external diameter of the tubes by 
positioning the latter in the locations of the metal gauze corresponding 
to every other mesh. 
Thus, an assembly of 300 tubes with an external diameter of 1 mm occupies a 
cross-sectional area of 6.65 cm.sup.2, representing a cylindrical group 
with a diameter of 29 mm. 
FIGS. 5 and 6 show two modes of fitting an assembly in a corresponding 
opening of the upper diffuser plate 3. 
In FIG. 5, assembly 7 is fitted to diffuser plate 3 by means of assembly 
plate 23 bearing on a shoulder 3a of the corresponding opening of said 
plate 3. 
In the fitting mode according to FIG. 6, the assembly plate is connected by 
means of a metal bellows 29 to an annular metal member 31, whose free edge 
is fixed to the edge of the opening of plate 3, for example via a ring 33. 
The diffuser advantageously comprises one thousand assemblies, each having 
300 microporous capillary tubes. These assemblies are arranged according 
to a hexagonal pattern in corresponding openings with a diameter of 
approximately 30 mm in plates 3 and 5. The distance between the centers of 
the assemblies is approximately 37 mm. 
Thus, the 1000 assemblies occupy a cross-section of 4.74 m.sup.2 
corresponding to a circular section with a diameter of 2.46 m. 
In an isotope separation apparatus of this type having 0.06 cm diameter 
capillary tubes, the intake flow rate of one tube is 0.0106 g of UF.sub.6 
/seconds. Thus, the total flow rate of the apparatus with 1000 assemblies 
of 300 tubes will be 3.18 kg of UF.sub.6 /second. 
To prevent pressure drops, the diameters of pipes 9, 11 and 13 are 
definitely fixed in such a way that the gas velocity in each of the pipes 
is 2 m/sec. 
Hereinafter, two exemplified embodiments of an installation for separating 
isotopes from the uranium by uranium hexafluoride diffusion through 
microporous capillary tubes are described. 
EXAMPLE 1 
The alumina tubes have the following characteristics: 
length 100 cm 
internal diameter 0.06 cm 
thickness 330 microns 
porosity 0.2 
average radius of pores 91 .ANG. 
permeability to air at 20.degree. C.: 31.10.sup.-7 mole/cm.sup.2 
/minute/centimeter of Hg. 
By means of such installation, the specific energy is reduced by 
approximately 33% compared with a conventional installation under the 
following operating conditions: 
temperature 70.degree. C. 
pressure at intake of tubes P.sub.e =1 bar 
Reynolds number R.sub.e =1,100 
downstream pressure P.sub.av =P.sub.e /4.5 
pressure drop in tube: 34 millibars 
intake flow rate=3.03.10.sup.-5 mole/sec, i.e. 0.0106 g of UF.sub.6 /sec 
diffused flow=poor flow=0.005325 g of UF.sub.6 /sec. 
EXAMPLE 2 
The tubes have the following characteristics: 
length 100 cm 
internal diameter 0.06 cm 
thickness 360 microns 
porosity 0.2 
permeability to air at 20.degree. C.: 34.10.sup.-7 mole/cm.sup.2 /min/cm of 
Hg 
average radius of pores: 105 .ANG. 
By means of such an installation, a specific energy reduction of 
approximately 28% is obtained compared with a conventional installation 
under the following operating conditions: 
temperature 100.degree. C. 
Reynolds number 1100 
intake pressure P.sub.e =1 bar 
downstream pressure P.sub.e /4.5 
pressure drop in tube: 44 millibars 
intake flow rate: 0.0106 g of UF.sub.6 /sec 
diffused flow=poor flow=0.005325 g of UF.sub.6 /sec. 
FIG. 7 is a perspective view of a microporous capillary tube assembly 7 
corresponding to the first embodiment of the invention. In this assembly, 
the microporous capillary tubes 21 are mounted at the end thereof on a 
plate 23 produced from welded together unitary elements 23a. 
On referring to FIG. 8, it is possible to see that in this first 
embodiment, the microporous tubes 21 are distributed in rows parallel to a 
first direction OX, the plane defined by the axes of the tubes of one row 
being at a distance d.sub.1 exceeding the external diameter d.sub.ext of 
said tube from the plane defined by the axes of the tubes in an adjacent 
row. 
The microporous tubes 21 are also distributed so as to form rows of tubes 
parallel to a second direction OY perpendicular to the first direction OX. 
In a row parallel to direction OX, the tubes are regularly spaced from one 
another by a distance d.sub.2, representing the gap separating the axes of 
two adjacent tubes. 
Preferably, according to the invention, the distance d.sub.1 exceeds 
distance d.sub.2, so as to define between the rows of tubes parallel to 
the first direction OX passages which favor an appropriate flow of the 
gaseous mixture having diffused the wall of tubes 21. In this way, it is 
possible to obtain a very compact assembly by minimizing the pressure 
drops between the tubes for the gaseous mixture which has diffused through 
the walls of the tubes. 
As shown in FIG. 7, tubes 21 are mounted in sealed manner at their ends on 
plates 23, whereof only one is shown in the drawing and made for example 
from Teflon. Plate 23 is formed from unitary elements 23a having a profile 
such that two such elements can completely surround either a row of tubes 
parallel to the first direction OX or a row of tubes parallel to the 
second direction OY. Elements 23a are welded to one another, e.g. by hot 
pressing to form a plate 23 into which are sealingly fitted the ends of 
tubes 21. 
Tubes 21 can be given a suitable spacing by spacers 25 positioned at 
different levels of assembly 7. The spacers 25 can also be made from 
elementary components of the same type 25a as those used for forming 
plates 23. However, it is not necessary to tightly fix said components 
together in order to obtain a tight system. 
FIG. 9 shows another embodiment of assembly 7 differing from that of FIG. 8 
because the microporous tubes 21 of the rows parallel to the first 
direction OX are in tangential contact with one another. 
In this way, passages are defined between the rows of tubes parallel to the 
first direction OX in which circulates the gaseous mixture having diffused 
through the wall of the tubes. This makes it possible to further improve 
the compactness of an assembly of this type due to the preferential flow 
of the diffused gas to the thus formed passages. This makes it possible to 
prevent significant pressure drops in the diffused gas circuit. 
Preferably, when the overall geometry of the isotope separation apparatus 
permits, these assemblies of tubes are arranged in the apparatus in such a 
way that the passages formed between the rows of tubes parallel to 
direction OX are located substantially in the axis of the diffused gas 
discharge pipe. 
FIG. 10 is a perspective view of an assembly of microporous tubes 
corresponding to the second embodiment of the invention. It is possible to 
see that assembly 7 comprises microporous tubes 21 which are positioned 
parallel to one another and a plurality of longitudinal partitions 
arranged between tubes 21 and fixed to the latter. 
Thus, it is possible to define in the assembly a first series of ducts 24 
bounded by the inner wall of the microporous tubes 21 and a second series 
of ducts 30 bounded by the outer wall of tubes 21 and by partitions 22. 
Tubes 21 are mounted by their other end on plate 23 and by their lower end 
on plate 23'. The partitions 22 extend longitudinally of plate 23 up to 
the vicinity of plate 23', so that in the immediate vicinity of plate 23' 
openings are provided making it possible to ensure the discharge of the 
enriched gas circulating in the second ducts 30 defined between tubes 21 
by partitions 22 and the outside of the wall of tubes 21. 
Obviously, the openings for discharging the gas circulating in the second 
ducts can be differently positioned within assembly 7. Thus, the 
partitions 22 can extend longitudinally from plate 23 to plate 23' and can 
be provided with openings distributed between the said plates. 
Preferably and as illustrated in FIG. 10, the openings of the second ducts 
30 are positioned in the immediate vicinity of end plate 23' of the 
assembly. This corresponds to the entrance (arrow F.sub.1) and to tubes 21 
of high pressure gas in order to create in the second ducts a counterflow 
effect by circulating the enriched gas (arrow F.sub.2) in the opposite 
direction to the gaseous mixture flowing in microporous tubes 21. 
The fitting of assembly 7 to plates 3 and 5 of enclosure 1 is brought about 
by means of end plates 23, 23' made, for example, from Teflon and fixed to 
tubes 21 and partitions 22. 
To this end, during the manufacture of assembly of microporous tubes 21 
joined by partitions 22, the latter are interrupted in the vicinity of the 
end of the tubes so as to make it subsequently possible to introduce the 
ends of tubes 21 into the corresponding openings of Teflon plates 23, 23' 
which have been machined beforehand. In this way, plates 23, 23' can bear, 
in the manner shown in FIG. 11, on the partitions 22 and sealing can then 
be obtained between tubes 21, partitions 22 and plates 23, 23' by casting 
Teflon on plates 23, 23'. 
Sealing can also be obtained by casting a material which resists the 
corrosion of fluorine-containing products, e.g. by means of a phosphate 
glass such as P 106. 
Obviously, when partitions 22 have to be interrupted in the vicinity of an 
end plate to provide openings in the second duct, the corresponding end 
plate is mounted on tubes 21 without it bearing against partitions 22. A 
seal is subsequently only provided between tubes 21 and the end plate. 
In the same way, assembly 7 can be fitted to plates 3 and 5 via a metal end 
fitting fixed to each end of the assembly and secured in the corresponding 
openings of FIGS. 3 and 5. 
On referring to FIGS. 12 to 15, it is possible to see a number of examples 
of microporous assembly of tubes having a circular or square 
cross-section. 
FIG. 12 is a part cross-sectional view of a tube assembly with a circular 
cross-section in which the tubes are distributed with a rectangular 
spacing. In this assembly, the microporous tubes 21 are distributed in 
parallel rows in two orthogonal directions OX and OY. The tubes 21 of rows 
parallel to direction OY are in tangential contact with one another, 
whilst the tubes 21 of rows parallel to direction OX are regularly spaced 
from one another in each row, the adjacent tubes of one row being joined 
by partitions 22 parallel to direction OXY. 
In exemplified manner, it is possible to assemble in this way 200 
microporous tubes having an internal diameter of 1 mm and an external 
diameter of 1.5 mm with partitions of length 1.5 mm and thickness 0.3 mm. 
The thus obtained assembly has a cross-section of 2.85 cm with a side 
dimension of 3 cm. It comprises rows of 10 tubes in direction OX and 20 
tubes in direction OY. The ratio of the cross-section of the first ducts 
defined within tubes 9 and the second ducts defined by the partitions and 
the outer wall of tubes 9 is 3.48. 
FIG. 13 is a part sectional view of an assembly of tubes with a circular 
cross-section in which the microporous tubes are distributed with a square 
spacing. 
In this assembly, the microporous tubes 21 are distributed in rows parallel 
to two orthogonal directions OX and OY. The tubes 21 of rows parallel to 
direction OY are regularly spaced from one another and partitions 22 
parallel to direction OY are provided between two adjacent tubes of each 
row. The tubes 21 of rows parallel to direction OX are regularly spaced 
from one another and partitions 22 parallel to direction OX are provided 
in every other row of tubes parallel to direction OX so as to join two 
adjacent tubes of the same row. 
For example, in this way, 169 microporous tubes have been assembled having 
internal diameters of 1 mm, external diameters of 1.50 mm with partitions 
of length 0.75 mm and thickness 0.3 mm. In this assembly, the ratio of the 
cross-section of the second ducts to the cross-section of the first ducts 
is 6.45. 
Such an assembly with 169 tubes has a square cross-section of 2.85 cm side 
dimension, each row of tubes parallel to the direction OX or direction OY 
being formed by 13 tubes. 
FIG. 14 is a part cross-sectional view of an assembly of tubes with a 
circular cross-section in which the tubes are distributed with a 
rectangular spacing. 
In this assembly, the microporous tubes 21 are distributed in rows parallel 
to two orthogonal directions OX and OY. The tubes 21 of rows parallel to 
direction OY are in tangential contact with one another and tubes 21 of 
rows parallel to direction OX are regularly spaced from one another. In 
this assembly, the partitions 22 are distributed obliquely between two 
successive rows of tubes parallel to direction OY so as to join a tube of 
a row parallel to OX to a tube of the adjacent row parallel to OY. 
Moreover, this assembly has partitions 22' between two adjacent tubes of 
two rows of tubes parallel to OX located on the periphery of the assembly. 
As an example, 200 tubes have been assembled in this way having an internal 
diameter of 1 mm and an external diameter of 1.5 mm oblique partitions 22 
of length 2.12 mm and partitions 22' of length 1.5 mm, all the partitions 
having a thickness of 0.3 mm. 
In this assembly, the ratio of the cross-section of the second ducts 30 
defined tubes 21 by partitions 22 to the cross-section of the first ducts 
is 2.86. 
FIG. 15 is a part cross-sectional view of an assembly of tubes with a 
square cross-section distributed with a rectangular spacing. 
In this assembly, the microporous tubes are distributed in rows parallel to 
two orthogonal directions OX and OY. The tubes 21 of rows parallel to 
direction OY are in contact with one another by their edges and the tubes 
21 of rows parallel to direction OX are regularly spaced from one another. 
Two adjacent tubes are joined by a longitudinal partition parallel to 
direction OX. 
As an example, 200 square tubes were assembled in this way having an 
external side of 1.6 mm and an internal side of 0.46 mm. The length of the 
partitions 22 joining two adjacent tubes of a row parallel to direction OX 
is 1.5 mm and its thickness is 0.3 mm. In this assembly, the ratio of the 
cross-section of the second ducts 30 defined by partitions 22 and the 
outer wall of tubes 21 to the cross-section of ducts defined within tubes 
21 is 16. 
Several assemblies like those illustrated by those in FIGS. 12 to 15 can be 
mounted between plates 3 and 5 of the apparatus of FIG. 1 by leaving a gap 
of approximately 5 mm between each assembly to permit the discharge 
towards outlet 13 of diffused gas leaving each assembly. 
FIG. 16 shows another type of assembly corresponding to a third embodiment 
of the invention. In this drawing, it can be seen that assembly 7 
comprises an alveolar or honeycomb module, whose walls made from 
microporous material define two rows of channels designated alternatively 
by the references 101 and 103. 
The channels of rows 101 constitute a first series of ducts 105, called 
first ducts, having a hydraulic diameter below 0.5 cm. The channels of 
rows 103 constitute a second series of ducts 107, called second ducts. 
It should be noted that in each row the channels have a square or 
rectangular cross-section and that two adjacent channels have a common 
wall. The gaseous mixture to be separated is circulated in the first 
series of ducts 105 (arrow F.sub.1), whilst in the second series of ducts 
107 is collected the gas separated by diffusion through the wall of the 
first ducts (arrow F.sub.2). 
Advantageously, in such an assembly, the second ducts 107 are sealed at 
each of their ends and in the upper part of the module openings 109 are 
formed in each row of channels 107 to ensure the discharge of the gas 
separated at the side walls of the module. 
Such openings can be formed in the following way. After manufacturing the 
module, slots such as 111 are formed in the rows of channels 103, e.g. by 
machining with a suitable tool. The upper parts of the module 
corresponding to rows 103 are then sealed by covering them with a tight 
material 113. As a result, it is possible to discharge the gas diffused by 
the side walls of the module and to circulate the gaseous mixture to be 
separated in ducts 105 by introducing it into the upper part of the 
module. 
FIG. 17 is a perspective view of an isotope separation apparatus with a 
plurality of assemblies of microporous tubes. 
This apparatus comprises a vertical cylindrical enclosure 1 in which are 
successively vertically arranged four assemblies of microporous capillary 
tubes 7a, 7b, 7c, 7d. Each of these assemblies is mounted between two 
plates, called diffuser plates, such as 3a and 5a . . . 3d and 5d. These 
assemblies are separated from one another so as to provide successive 
chambers C.sub.1, C.sub.2, C.sub.3, C.sub.4 and C.sub.5 between two 
adjacent assemblies such as 7a and 7b and at each of the ends of the 
enclosure. 
It should be noted that chambers C.sub.2, C.sub.3 and C.sub.4 are defined 
by the diffuser plates of two adjacent assemblies, e.g. chambers C.sub.2 
by plates 5a and 3b of adjacent assemblies 7a and 7b and that they are 
connected to the tubes of said two assemblies. 
Conversely, the end chambers C.sub.1 and C.sub.5 are only linked with the 
tubes of a single assembly such as 7a or 7d. 
The successive chambers C.sub.1 to C.sub.5 alternately constitute 
distribution chambers for the gaseous mixture to be separated and 
collection chambers for the gaseous mixture leaving the tubes of the 
assemblies. 
Thus, chambers C.sub.2 and C.sub.4 form distribution chambers respectively 
making it possible to distribute the gaseous mixture to be separated in 
the microporous tubes of two adjacent assemblies 7a, 7b, 7c, 7d. In the 
same way, chambers C.sub.1, C.sub.3 and C.sub.5 form collection chambers, 
chamber C.sub.3 serving to collect the gaseous mixture from the tubes of 
two adjacent chambers 7b and 7c, whilst chambers C.sub.1 and C.sub.5 only 
collect the gas from assemblies 7a and 7d respectively. 
The gaseous mixture to be separated by diffusing through the microporous 
tubes of different assemblies of the apparatus is introduced into chambers 
C.sub.2 and C.sub.4 by pipes 9.sub.2, 9.sub.4. Following the arrows 
F.sub.1 this gas then passes into the microporous tubes of each of the 
assemblies of the apparatus and is collected in collection chambers 
C1.sub.1, C.sub.3, C.sub.5 (arrow F.sub.3). It is then discharged from the 
apparatus by the extraction pipes 11.sub.1, 11.sub.3, 11.sub.5. 
By passing through the capillary tubes of assemblies 7a to 7d by diffusion 
in the direction of arrows F.sub.2 through the wall of the tubes, the 
gaseous mixture is separated into an enriched fraction which is collected 
following arrows F.sub.2 in an annular collection space 40 located at the 
periphery of the enclosure. It is then discharged therefrom by a pipe 13 
located at the base of the enclosure. 
It is pointed out that in such an apparatus the assemblies such as 7a to 7d 
may comprise one or more groups in which the microporous capillary tubes 
are directly sealingly marked on diffuser plates 3a and 5a, 3b and 5b, 
etc or in which the microporous tubes are fastened at each of their ends 
to group plates by means of which they are mounted on diffuser plates in 
the manner described hereinbefore. (FIGS. 5 and 6). 
As an example, an apparatus of this type has been produced in which is 
installed four assemblies each having 750,000 microporous capillary tubes 
with an internal diameter of 1 mm, an external diameter of 1.5 mm, a 
length of 1.50 mm, a porosity of 20%, a permeability to air of 
20.10.sup.-7 mole/cm.sup.2 /min/cm of Hg and an average pore radius of 100 
.ANG.. These assemblies are arranged in a cylindrical enclosure having an 
external diameter of 2.7 m and a total length of 9.50 m. The height of the 
successive chambers is 70 cm. The total gas flow which can be treated in 
an installation of this type is 99 kg of uranium hexafluoride per second. 
The intake flow rate into each assembly is 24.75 kg of uranium 
hexafluoride per second. This uranium hexafluoride is separated in each 
assembly into a depleted fraction with a flow rate of 12.375 kg/ sec per 
assembly and an enriched fraction with a flow rate of 12.375 kg/sec per 
assembly. 
An installation of this type is able to ensure the isotope separation at a 
high uranium hexafluoride flow rate.