FR2520151A1 - METHOD AND APPARATUS FOR SEPARATING AND REMOVING US NUCLEAR FUELS - Google Patents

Abstract

METHOD AND APPARATUS FOR SEPARATING AND REMOVING SPENT NUCLEAR FUELS. IT INCLUDES A SEPARATION CONTAINER 10 HOUSING A MELTED METAL 16 SERVING AS A SOLVENT IN A REACTION REGION 14, A REFLUX REGION 18 PLACED ABOVE AND ADJACENT TO REACTION REGION 14, AND A POROUS FILTER ELEMENT 46 DEFINING THE BOTTOM OF THE SEPARATION CONTAINER, SUPPORTING THE METAL SOLVENT. SPENDED FUELS ARE ADDED TO THE METAL SOLVENT. A GAS CONTAINING NON-OXIDIZING NITROGEN IS INTRODUCED INTO THE SEPARATION CONTAINER, FORMING SOLID ACTINIDE NITRIDES IN THE METAL SOLVENT FROM THE ACTINIDES FOR FUEL, LEAVING OTHER FISSION PRODUCTS IN SOLUTION. A PRESSURE OF APPROXIMATELY 1.1 TO 1.2.10PA IS APPLIED TO REGION OF REFLUX 18, MOVING THE MELT METAL SERVING SOLVENT AND SOLUBLE FISSION PRODUCTS FROM THE CONTAINER, LEAVING SOLID ACTINIDE NITRIDES IN THE CONTAINER OF SEPERATION.

Description

The present invention generally relates to a method and a

apparatus for reprocessing

  and the separation of spent nuclear fuel

  Fission products in a molten metal as a solvent It relates more particularly to a method and apparatus for reprocessing and separating

  spent fuel elements in a separation device

  The invention is characterized in that it contains a molten metal material as a solvent, and has a porous filter element for removing the molten solvent and the soluble compounds contained therein from the separation apparatus. A molten metal solvent, usually tin, has been used as a solvent and as a reaction medium in the reprocessing and separation of elements

  nuclear fuel, including fuel

  actinides, for example in the removal of fission products and other impurities from spent uranium-plutonium and thoriumuranium (plutonium) fuels in the form of oxide, metal or carbide. its sheath, if necessary, then placed in a solution of molten tin maintained at a

temperature of about 1600 ° C.

  If the fuel is an oxide, a carbothermic reduction operation is necessary. For this purpose, the separation vessel housing the molten tin is made of a material which is a source of carbon, preferably graphite. The carbon dissolves in the Tin melts and reacts with the oxides of actinide and fission products, transforming them into a metallic solution and forming gaseous CO For uranium-plutonium fuel, the main reaction is represented by: (U Pu) 02 (s) + 2 C (in solution in Sn) - * (U Pu) (in solution in Sn) + 2 CO (g) (1) During the dissolution of the fuel, volatile fission products are released and are

  swept out of the separation vessel by CO, tan-

  say that the most refractory fission products remain in the molten tin used as a solvent.

  volatile fission products have volati-

  different capacities in molten tin, they are all eliminated in one stage. At the same time as the volatile fission products, a large part of the molten tin used as solvent also evaporates and is

removed from the container.

  Then actinides (in solution) are separated from most of the fission products

  remaining in the molten tin by a nitride

  A non-oxidizing atmosphere containing nitrogen is

  introduced into the container, which leads to the forma-

  actinide nitrides in molten tin

  uranium-plutonium fuel, the reaction is

  the following equation: (U Pu) (in solution in Sn) + 1/2 N (2) (g) (U Pu) Ns) (2) During the nitriding process, molten tin can also evaporate and escape from the separation vessel As soon as the nitriding is complete, the solid actinide nitrides are separated from the molten tin and the fission products remain in the tin used

as a solvent.

  U.S. Patent Nos. 3,843,765, issued Oct. 22, 1974 to Anderson et al, and No. 3,843,766, October 22, 1974, issued to Anderson et al. Illustrate such methods and apparatus for separating with molten tin. The aforementioned patents indicate that actinide nitrides are separated from the present fission products

  as impurities in the separation vessel

  troducing mechanical systems to physically separate them from each other But the apparatus necessary to achieve this type of physical separation

  is not convenient to incorporate into a separation system.

  operating at the temperature of the liquid tin.

  It is also very expensive. Accordingly, it is an object of the invention to provide an apparatus and method for reprocessing

  and separate spent nuclear fuels, among

  which actinide fuels and fission products, in which the actinide fuels are

  separated fission products at high temperature.

  Another object of the invention is to provide an apparatus and a method for separating and reprocessing spent nuclear fuels, wherein actinide fuels are separated from the fission products without

use mechanical means.

  Another object of the invention is to provide an apparatus and a method for separating and reprocessing spent nuclear fuels, in which a transport system for physically removing actinides from

  fission products is not necessary.

  Other purposes, advantages and novel features of the invention will be partly enumerated

  in the description below, and they will appear for

  on the other hand to specialists in the review of the following

  or may be taught by the practice of the invention.

  The aims and advantages of the invention can be realized

  achieved and achieved by means of instrumentation and

  particularly indicated in the claims

attached.

  To achieve the foregoing and other objects, and in accordance with the purpose of the present invention as embodied and generally described herein, the nuclear fuel reprocessing and separation apparatus employs a molten metal as a solvent A separating vessel is provided which has a reaction region for accommodating the

  metal as a solvent, and a reflux region

  above the reaction region and adjacent thereto The two regions are independent with respect to temperature and are capable of functioning

  at different temperatures An opening is practically

  A porous filter element is disposed in a wall element of the separating vessel, it constitutes the bottom of the separation vessel and supports the metal used in a wall element of the separation vessel.

  as a solvent A system for varying the temperature

  In addition, systems for evacuating the separation vessel and systems for introducing various

atmospheres are provided.

  In another aspect of the invention, the method

  spent nuclear fuel reprocessing and separation units comprising actinide fuels, volatile and non-volatile fission products, uses a molten metal as a solvent There is provided a separation vessel which has a reaction region housing the metallic solvent , and a region of reflux arranged

  above and adjacent to the reaction region

  The separation vessel further includes an opening in a wall member of the vessel adjacent to the reflux region, and a porous filter element disposed in a wall element.

  container defining the bottom of the container, and suppor-

  The spent nuclear fuel is introduced into the separation vessel, after which the nitrogen-containing atmosphere is introduced.

2 ZE 2015 1

are

  non-oxidizing solid actinide nitrides are

  in the metal solvent, while the nonvolatile fission products remain in solution. The refluxing region is then subjected to a sufficient pressure to force the molten metal used as the solvent to pass through the porous filter, leaving the actinide nitrides in

  the reaction region of the separation vessel.

  The presence of the porous filter element allows the separation of the solidified actinide nitrides from the molten metal solvent and the nonvolatile fission products remaining in solution without the need for a transport system to collect the nitrides. actinides solid, then physically transport them out of the molten solution In addition, one

  saves money by eliminating the

  expensive mechanical transport system The separation of

  the present invention is successfully carried out at temperatures of

  necessary to maintain the metallic solvent

in the molten state.

  The attached drawing, which is incorporated in and forms part of this memo, illustrates a pattern of

  realization of the invention With the description, it is

  intended to explain the principles.

  Figure 1 is a diagram of an apparatus for reprocessing and separation of spent nuclear fuel. Used nuclear fuels, including

  actinide fuels in the form of oxide,

  or metal, and volatile and nonvolatile fission products, are reprocessed and separated successfully using a separation vessel housing a liquid metal solvent in a reaction region of the vessel, and a reflux region disposed at above the reaction region An embodiment of a

  Such separation apparatus is illustrated in FIG.

  The separation container 10 is formed of wall elements 12 made of a carbonaceous material.

  12 of the container should preferably: (1) be refractory

  and able to contain the system at

  erations significantly higher than the reaction temperatures; (2) be inert to actinides under the reaction temperature and solvent conditions used; (3) possess low neutron absorption characteristics; and (4) be a carbon source for the carbothermic reduction of actinide oxides. Examples of carbon-containing materials include carbides, hydrocarbons, and the like.

  graphite Graphite is the preferred material.

  The separation vessel 10 defines various distinct regions: a reaction region 14 housing a molten metal as a solvent 16; a reflux region 18 placed all the way against the reaction region 14 and above it; and a condensation region 19 adjacent to and immediately above the refluxing region 18. The condensation region 19 is a dome-shaped structure, which can be separated from the

  remainder of the container 10, defining the top of the container.

  A reservoir 20 is formed in the condensation region 19 at its intersection with the reflux region 18. The reservoir 20 is defined by an insulating wall 22 and a

  wall element 12 of the container.

  The creation of different regions in the container 10 allows the formation of a thermal gradient

In this one.

  The regions 14, 18 and 19 are all three temperature independent. In general, the reaction region 14 is maintained at a higher temperature than the regions 18 and 19. When the reflux region 18 and the condensation 19 are maintained at lower temperatures, the evaporated metal from the solvent 16 enters the reflux region 18, condenses thereon with a series of deflectors 24, preferably made of graphite and returns to the The regions 14 and 18 may be maintained at the same temperature, while the condensation region 19 is maintained at a higher temperature.

  In this case, the evaporated metal solvent

  spends region 18 to reach region 19, where it condenses It is collected in liquid form in-the

tank 20.

  In the present invention, the metal solvent

  that must have a good solvent power for uranium and other actinide metals; have a voltage of

  sufficiently low vapor to allow the condensation

  solvent in the reflux region; do not easily form nitrides; and have a sufficiently low neutron cross section Suitable solvents include lead, zinc, bismuth and tin, as well as combinations thereof. Preferred Solvent

is tin.

  To achieve the thermal gradient in the

  container 10, a first inductive heating element

  26, connected to a suitable power source is disposed outside the container, surrounding the region

  of reaction 14 A second heating element

  duction 28, also connected to a power source

  appropriate, is placed on the outside of the container,

  By rotating the reflux region 18 Each of the induction heating elements is independent of the other, which allows the formation of the thermal gradient in the

  container 10 Other means of heating and cooling

  These means include, for example, but are not limited to radiant heating with resistance heating elements, electronically shocked heating of the vessel and direct electrical resistance heating applied to the heater. Container Several cooling coils 30 externally surround the condensing section 19 A circulating medium such as water, steam, air, helium, etc. flows through the coils 30 to cool the cross section.

condensation of the container 10.

  The separation vessel 10, the induction heating elements 26, 28 and the cooling coils 30 are all housed in a metal vacuum tank 32 The tank 32 may also be an inert gas chamber The tank serves as a holding vessel secondary use for spent nuclear fuels and it is an additional security for the case where liquid would be spilled The top cover 34 of the tank 32 is removable, it is a means of introducing spent fuel into the container 10 A. This effect, removes from the condensing region 19 a flange 36 connecting the container wall elements which define the reflux region 18 and the condensing region 19 for example, the dome is separated from the other sections of the container 10 Used fuel is then mechanically introduced or discontinuously in the container A removable graphite sleeve 38 may, if desired, be included and disposed to Before reprocessing each new batch of spent fuel, a new graphite jacket is placed in the container 10. This reduces the wear on the containers.

  inner walls of the container 10.

  A pipe 40 passes through the lower wall 42

  of the tank 32 and the wall element 12 of the container 10.

  A valve 43 is disposed in the pipe 40 The pipe 40 is in communication with pump and vacuum systems (not shown), various recovery vessels (not shown) for collecting and

  keep CO, volatile fission products, etc. -

  removed from the container 10, and also a holding container

  naked (not shown) housing a gas containing non-oxidizing nitrogen such as N 2 Electrical cables for the induction heating elements 26 and 28, pipelines for the coils 30 and other necessary auxiliaries are also provided through the wall

lower 42.

  A porous filter element 46 carrying the metal solvent 16 is disposed in the wall element of the separation container 12 in a position defining the bottom of the container 10, and more precisely the bottom of the reaction region 14. variable dimensions, and it can extend over the entire width of the bottom of the container 10 The porosity of the filter 46 must be sufficient for it to constitute a support

  for the molten metal 16 serving as a solvent when

  a pressure of about 105 Pa at the reflux region 18 and at the condensation region 19, while permitting the flow of solvent and soluble compositions therein when a pressure of about 1.1 is applied. at 1.2 105 Pa at the reflux region The pore size of the filter material should preferably be

from about 10 to 100 microns.

  If the graphite jacket 38 is included in the separation vessel 10, the bottom of the jacket 38

  is of a porous nature similar to that of filter 46.

  As preferred filtering materials, mention will be made of

  phite, stainless steel and other materials that can

  may be used to form the separating container

  The preferred material is graphite. When applying a pressure of about 1.1 to 1.2 × 10 5 Pa of the nitrogen-containing non-oxidizing gas, or an inert gas such as argon, at the separating vessel 10 above the metal solvent 16, this causes the flow of the solvent from the vessel into a recycle chamber 48 disposed under the vessel 10. The recycle chamber 48 is cooled by circulation of a cooling medium,

  such as water, through its walls The metal solvent

  league 16 as well as the nonvolatile fission products are collected in the recycle chamber 48, cooled and removed therefrom. A drainage tube 49 is removably attached to the separation vessel 10 and

  is also removably attached to the

  The presence of a drainage tube 49 is born

  necessary to separate the recycling chamber from the

  manager of the separation vessel much hotter.

  The metal solvent 16, as well as the region

  14, are maintained at a sufficient temperature

  The temperature of the reaction region is preferably from about 1450.degree. to 1800.degree. C., more preferably from 1550.degree. to 17000.degree. C., and more preferably from about 150.degree. The reflux region 18 is initially maintained at a lower temperature than the reaction region, for example at room temperature.

  The reflux temperature must be sufficient

  low for the reflux of the metallic solvent.

  When tin is used as a solvent, a temperature of about 800 to 1200 ° C is maintained. The temperature range is preferably about 950 to 10500 C.

  The temperature of the condensation region 19 is

  than that of the reflux region 18, but above

  at the melting point of the metallic solvent.

  The temperature range of about 232 to about 10500 C is appropriate. At these lower temperatures, the residual metal solvent or volatile fission products that pass through the reflux region 18 are trapped and collected.

ú 520151

  The speed of the carbothermic reduction of

  actinide oxides depends on the amount of carbon

  available in the chosen metallic solvent Carbon

  is very poorly soluble in the preferred metals, particularly

  in the tin However, with the addition of a

  first metal catalyst to increase solubility

  carbon permeability, the speed of reduction

  In order to be effective, the catalyst must increase the solubility of the

  carbon in the metal and not easily form

  Suitable catalysts are cobalt, nickel, iron and their combinations. The preferred catalyst is cobalt. These elements are also particularly useful because they are relatively nonvolatile and do not evaporate from the molten tin used as the solvent. For example, tin is more than 1000 times more volatile than cobalt The catalyst is present

  in the molten metal used as a solvent in a

  weight percent of about 0.1 to 20, preferably

  about 1 to 20, and more preferably about 5 to 10.

  Before introducing the spent nuclear fuel into the container 10, it may be necessary to remove the fuel from its sheath, either mechanically,

  either chemically The spent fuel is broken down into

  relatively small cages (3 mm or less) then introduced into the container 10 with a mechanical feeder or batch. As soon as spent nuclear fuels have been added to the solvent 16 at the appropriate temperature,

  the carbon coming from the graphite wall 12, the

  tributor 52 and the jacket 38 reduces the oxides of

  tinnes, releasing CO As the

  spent fuels dissolve in the metallic solvent

  that the volatile fission products are released, while the nonvolatile fission products remain in solution At the temperature of the reaction region, a large amount of solvent from the solvent

  melt 16 of the reaction region 14 evaporates and penetrates

  in the reflux region 18 together with the volatile fission products and CO In the region

  18, the evaporated solvent is brought to

  because of the lower temperature, and it is returned to the reaction region 14 A very

  of the evaporated solvent reaches the condensation region.

19.

  Volatile fission products, as defined herein, include the fission products listed in Table 1 which have very high, moderate or low volatilities Volatiles of volatile fission products in the reflux region 18 vary greatly at In the reflux region, the less volatile fission products condense and return to molten tin 16 as solvent. These volatile fission products are referred to as condensable fission products. The most volatile fission products do not occur. do not condense in the reflux region 18 or the condensation region 19 Au

  Instead, they flow out of the container through the cana-

  40 and are collected separately in a

  By definition, the most volatile fission products are referred to here as fission products

non-condensable.

  Table 1 lists the estimated volatilities

  fission products at about 16270 C.

2 to 2 C 151

TABLE 1

VOLATILITIES OF FISSION PRODUCTS

  A Very high (1.013 10 Fa to 1.013 10 Fa) I, Br, Cd, Cs, Rb, and Se B Moderate (0.101 Fa to 10.1 Fa) Sb and Te C Low (1.013 103 Pa to 0.101 Pa) Ba, Eu, Sm, and Sr D Very low (less than 1.013 10-4 Pa)

  Ce, Gd, La, Mo, Nd, Pd, Pm, Pr, Rh, Ru, Ta, Y and Zr.

  During the removal of CO and products

  non-condensable volatile fission, a gas of

  The inert gas such as argon is suitable for this purpose after removal of the volatile non-condensable fission products from the vessel 10, the temperature of the reflux region is raised. 18 of its reflux temperature, for example 950 to 10500 C, at the temperature of the reaction region But

  the condensation region 19 is maintained, while cooling

  if necessary, at a temperature of 232 to 10500 C A

  the high temperature of the reaction region, the

  Volatile condensable fission products will not generally condense in the reflux region. Condensable volatile fission products, together with some evaporated tin, are essentially distilled, condensed and collected in the condensation region 19, in the reservoir. Although a little tin is lost in the distillation operation, much less is lost.

  only if the elimination of all the volatile fission products

  was carried out in the absence of ebb and flow

22 C 151

  condensation that establishes the temperature gradient.

  In addition, the temperature gradient allows a separation

  selectively volatile fission products on the basis of their differences in volatility. For example, condensable and non-condensable volatile fission

Sands are separated.

  When distillation is complete, the temperature

  erature of the reflux region 18 is lowered to the

  indicated reflux temperature of about 950 to 1050 ° C. A nitrogen-containing non-oxidizing atmosphere is then introduced into the vessel. An example of such an atmosphere is pure nitrogen. The nitrogen atmosphere enters the atmosphere. the metal solvent 16 and is dispersed therein with the distributor 52 In addition, the atmosphere containing

  nitrogen can also be introduced via line 40.

  A nitrogen pressure of approximately 105 Pa is preferred. In the solvent 16, solid actinide nitrides are formed, among which are UN, U 2 N 3, Pu N, Pu 2 N 3, Th N, etc.

  Initial soluble fission products remain in solution.

  In order to separate the solid actinide nitrides from the molten solvent and the soluble fission products, the reflux and condensation regions 18 and 19 are

  subjected to a pressure of a gas such as nitrogen, introduced

  The pressure causes the molten solvent and the soluble fission products to flow from the reaction region 14, through the filter 46, and into the reaction zone 14. the recycling chamber 48 The container 10 is then cooled and disassembled, for example it is separated at the flange 36. The remaining actinide nitrides are removed from the container 10 and treated to give usable fuels. presence

  of oxygen to form actinide oxides.

  To accelerate the nitriding process, a second catalyst is incorporated. The second catalyst chosen must increase the affinity of the nitrogen in the metal solvent, but must not form stable nitrides. Suitable catalysts include

  magnesium, calcium, strontium, barium, aluminum,

  minium, manganese, vanadium, chromium, and their

  The second preferred catalyst is calcium.

  Depending on the second catalyst chosen, it may be initially added to the metal solvent as part of the solvent, or just before the nitriding process, given the different volatilities of the catalysts and the potential loss of the second most volatile catalysts, during the reduction process

  carbothermic weight percentage of the second

  relative to the metal solvent is about 1.0 to 10.0, preferably about 1.8 to 2.5, and better

  still about 1.0 The incorporation of the second catalyst

  it entails a multiplication of the nitrous

ration by a factor of about 3.

  The following non-limiting examples are

  given by way of illustration of the invention.

EXAMPLE 1

  In the separation vessel shown in FIG. 1, 50 g of pure shot tin for analysis, 10 g of UO 2 (depleted) and 1 g of cobalt are introduced. Cobalt is added as a catalyst; without

  it, the carbothermic reduction would require 20-30 hours. The reaction region of the container is then heated

  and tin and keep them at a temperature of

  1600 ° C, which forms a molten tin solvent

  keeps the reflux region of the container at a temperature of

  about 10,000 C is added to the molten tin

  10 g of UO 2 (depleted). Argon was then sparged with molten tin for 2 hours. The non-condensable volatile

  collected in suitable cooling traps, -

  CO is oxidized to CO 2 and trapped in an alkaline matrix

  Such as the commercial product sold under the name Ascarite. The temperature of the refluxing region is then raised to about 1600 ° C. The evaporated tin and the previously condensable volatile fission products are collected in a collection container. 15 minutes, the temperature of the refluxing region is lowered to about 1000 ° C. N 2 (under 105 Pa) is introduced into the container through the porous filter, forming nitrides of actinides After about an hour the flow of N 2 is stopped through the filter while N 2 is introduced into the reflux region at about 1.2 105 Pa. Tin and the non-volatile fission products present in the tin melted flow from the recipe through the porous filter

  and are collected in a recycling chamber,

  nitrides of solid actinides.

EXAMPLE 2

  g of Th O 2 are separated from other fuels

  used under the same conditions as in the example

ple 1.

EXAMPLE 3

  g of (U, Pu) O 2, that is to say of a mixture of

  U 02 and Pu O 2 are separated from other nuclear fuels.

  worn areas under the same conditions as in Example 1.

15 1

Claims (2)

  Apparatus for the reprocessing and separation of spent nuclear fuels comprising actinides as fuels and fission products in molten metal as a solvent, characterized in that it comprises: a vessel (10) having a reaction region ( 14) housing the molten metal (16) serving   of solvent, a reflux region (18) disposed   said reaction region (14), said reflux region (18) being operable at a temperature different from said reaction region, said vessel having an opening in a wall member of said container in a position adjacent to said reaction region (18); this reflux region; means (26) for heating said container; means for evacuating the container; and   means (40) for introducing a   mosphere containing non-oxidizing nitrogen in this vessel, to form actinide nitrides in the   within this molten metal serving as a solvent.   2 Apparatus according to claim 1, characterized in that said container (10) further comprises a condensation region (19) disposed above and adjacent to said reflux region (18), said condensation region (19) being able to operate at a temperature different from that of these reflux regions (18) and the reaction (14).   Apparatus according to claim 1,   characterized in that it further comprises a   put (38) disposed in this container. ú_ 20151   Apparatus according to claim 1,   characterized in that said container (10) is substantially actually formed of graphite.   Apparatus according to claim 1, characterized in that a porous filter element (46) is arranged in a wall element of the separating container defining the bottom of this container   separator, supporting this metallic solvent.   Apparatus according to claim 1, characterized in that said filter element (46) has   a pore size of about 10 to 100 microns.   7 Reprocessing and separation process   spent nuclear fuel containing   actinide fuels, volatile fission products   non-volatile, characterized in that a separation vessel (10) made of a carbon-containing material is provided and a   molten metal (16) serving as a solvent placed in this   separating vessel, said separating vessel having an opening in a wall member of the vessel; a first catalyst is added to this metal solvent which increases the solubility of the carbon in this solvent; used nuclear fuel is introduced into this separation vessel; CO is formed in this molten metal   used as a solvent, from active fuels which are oxides.   Process according to Claim 7, characterized in that it further consists in eliminating   these volatile fats and the CO of this recipe pice to reaction. ú 520151   Process according to claim 8, characterized in that it further comprises: introducing an atmosphere containing   non-oxidizing nitrogen in this separation vessel.   after removing these volatile fission products and this Co, to form solid nitrides from these actinide fuels in this molten metal solvent; and separating these solid actinide nitrides from this molten metal solvent and   of these non-volatile fission products.   Reprocessing and separation process   use of spent nuclear fuel containing actinides as fuels, volatile and non-volatile fission products in molten tin as a solvent, characterized in that a separating container (10) formed of a material is provided containing said carbon, said separation vessel defining a reaction region (14) housing said molten tin (16) as a solvent, and a reflux region (18) arranged   above this reaction region (14) and adjoining   of the latter, this separating container com-   both an opening in a wall member of the container in a position adjacent to this reflux region; a first catalyst which increases the solubility of the carbon in this solvent is added to this solvent tin;   this reaction region (14) is maintained.   at a temperature sufficient to bring these activities   nides serving as fuel in a solubilized state; said reflux region (18) is maintained at a temperature sufficient to reflux said solvent tin, said reflux temperature being lower than that temperature of the reaction region; spent nuclear fuel is introduced into this tin serving as a solvent;   CO is formed in this molten tin   solvent from actinides as fuels, which are oxides; the CO and the volatile fission products in this molten tin solvent are allowed to evaporate with a little molten tin and flow into this reflux region; CO and volatile fission products which are not condensable at this reflux temperature are removed from this reflux region through this opening of the vessel, leaving less volatile tin and fission products evaporated in this reflux region ( 18); and the evaporated less volatile tin and fission products placed in this reflux region (18), which are condensable at this reflux temperature, are allowed to cool sufficiently and return to this molten tin solvent.   in this reaction region (14).   Process according to Claim 7, characterized in that the temperature of the reaction vessel is varied by evacuating the separating vessel and introducing various atmospheres into this separation vessel and in that this content of the separation vessel is filtered through a porous filter (46) disposed in a wall element of the container of separation.   2 z 2 C 151 12 Process according to claim 11, characterized in that this filter has a pore size from about 10 to 100 microns.