Components of uranium enrichment plant
The present invention relates to a component of a uranium enrichment plant. More particularly, the present invention relates to a component of a uranium enrichment plant such as a packing, a lining, a shock-absorbing member and the like, which consists essentially of an ethylene-hexafluoropropylene copolymer. 
     One of the typical conventional processes heretofore employed for the enrichment of uranium from uranium ores comprises a step of reacting the uranium ores with fluorine at a high temperature to provide uranium hexafluoride (UF 6 ) which is a volatile compound to be collected in the form of a vapor. Uranium hexafluoride thus collected generally consists of uranium-238 ( 238  U) which constitutes the major part of natural uranium, and the uranium compound thus collected contains therein, as natural uranium usually does, uranium-235 ( 235  U) which is the most important nuclide as a reactor fuel. Accordingly, if  235  U is wanted, it is necessary to separate it from uranium hexafluoride, which is a gaseous mixture of  238  UF 6  and  235  UF 6 . The &#34;gaseous diffusion&#34; method has been industrially employed heretofore for this purpose. Recently, however, as the nuclear industry grows more and more promising, processes which are industrially more advantageous have been searched for, and as a result the &#34;centrifuge process&#34; has been developed. This process is basically the application of centrifugal force to the separation of mixed isotopes each having different mass. The centrifuge process is generally carried out by using high speed centrifuge which naturally requires proper shock-absorbing components. In addition, since uranium hexafluoride (UF 6 ) is a gaseous compound, the apparatus to be used in such process has to be completely airtight and accordingly a proper packing or sealant is also required. Further, uranium hexafluoride (UF 6 ) has a tendency to react with various organic materials to convert to uranium tetrafluoride (UF 4 ) with the release of hydrofluoric acid and this is accompanied by the deterioration of said organic materials. This is how shock-absorbing members and packings, etc. of a uranium enrichment plant are attacked and the characteristic properties of them are lost. By the term &#34;shock-absorbing member&#34;, we mean a component for absorbing not only shock but also vibration. For these reasons, novel organic materials which are not attacked by uranium hexafluoride have long been desired. 
     The present inventors have long been studying to find organic materials which have satisfactory anti-UF 6  properties and also meet all other requirements. However, the goal was not easy to reach. For example, polymers derived from ethylene tetrafluoride have good resistance to attack by uranium hexafluoride, but they lack elastic properties. Thus, they are not usable as a packing, a shock-absorbing member and the like. Fluorine-containing ethylene copolymers such as ethylene-tetrafluoroethylene copolymer, ethylene-monochlorotrifluoroethylene copolymer, etc. are known as heat-resisting resins, but they also lack elastic properties and are not usable for the purpose mentioned above. The usability of fluorine-containing type elastic materials containing no ethylene component such as, for example, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer and the like has also been examined, and as a result, it has been confirmed that though they are usable as a packing and a shock-absorbing member, they do not have satisfactory resistance to attack by uranium hexafluoride. 
     In view of these drawbacks inherent in prior art resins and elastic materials, the present inventors made a large number of studies and finally found a material which is suitable for use as a component of a uranium enrichment plant. Based on such discovery, the present invention has been accomplished. 
     That is, the main object of the present invention is to provide a novel component of a uranium enrichment plant which comprises a cross-linked product of ethylenepropylene hexafluoride copolymer. The novel components of the present invention are made of the cross-linked ethylene-hexafluoropropylene copolymer and can be characterized in that they exhibit remarkable resistance to attack by uranium hexafluoride even at high temperatures. 
     The copolymers of the present invention include not only a cross-linked ethylene-propylene hexafluoride copolymer but also a cross-linked terpolymer of ethylene, hexafluoropropylene and another ethylenically unsaturated monomer. Such polymers can be prepared by various processes. A typical process is one in which ethylene and hexafluoropropylene or in some cases mixture of said two monomers and another ethylenically unsaturated monomer are polymerized in the presence of a free radical initiator at a pressure in the range of 40-4,000 kg/cm 2  and at a temperature in the range of 40°-300° C. In other typical processes polymerization is carried out in the presence of additives such as ethane propane, an olefin such as propylene, an aliphatic ketone, an aldehyde and the like; other processes are an emulsion polymerization process, which may generally be carried out at relatively lower pressures, a suspension polymerization process, an irradiation-induced polymerization by means of gamma-rays and the like. 
     The hexafluoropropylene content of the end product of the present invention may be varied depending on the desired performances of the end product to be used as a component of a uranium enrichment plant. Generally, however, those which have a hexafluoropropylene content in the range of 10-50% by mole are preferred copolymers having both high elastic properties and good resistance to attack by uranium hexafluoride. 
     As mentioned above, a cross-linked terpolymer of ethylene, hexafluoropropylene and another ethylenically unsaturated monomer can also be used in the practice of the present invention. Examples for such ethylenically unsaturated monomers are methyl, ethyl, n-propyl and n-butyl acrylates and methacrylates, vinyl polyfluoroacetate, vinylidene fluoride, 1,1,2-trichloroethylene, 1,1,2-trifluoroethylene, tetrafluoroethylene, 1,1-chlorofluoroethylene, 1,2-difluoroethylene, 1,1-dichloro-2-fluoroethylene, trifluorochloroethylene and the like. 
     The proportion of said ethylenically unsaturated monomer in the terpolymer is generally in the range of 0-50% by mole, the proportion of hexafluoropropylene in the same is in the range of 10-50% by mole and the balance is ethylene. 
     The copolymers which can be used as components of a uranium enrichment plant according to the present invention are generally those having a melt index (M.I.) in the range of 1-300 (determined at a load of 2.16 kg). 
     The ethylene-hexafluoropropylene copolymers according to the present invention are generally subjected to cross-linking treatment before they are put to use as a packing, a lining, a shock-absorbing member and the like as component of a uranium enrichment plant. Useful cross-linking processes for the purpose include a process comprising the addition of an organic peroxide, a process by means of electron beam irradiation and the like. The term &#34;organic peroxide&#34; herein used means an organic compound having a bond of --O--O-- in its molecule. Typical examples of such compounds include the compounds having the following generic formulas: ##STR1## wherein X represents an alkyl group, an aralkyl group and a group derived therefrom; Y represents an alkyl group, an aralkyl group, a group derived therefrom, and hydrogen, wherein X and Y can be either the same or different. Representative examples of compounds of these types include ketone peroxide, peroxy ketal and the like. 
     The amount of the organic peroxide to be added is in the range of 0.1-10 parts by weight per 100 parts by weight of a fluorine-containing ethylene copolymer. When the amount added is 0.1 part by weight or less on the same standard as mentioned above, the degree of cross-linking is below the desired level. The addition of more than 10 parts by weight based on the same standard is almost of no use, because in such a case the degree of cross-linking reaches a saturation point and therefore it is disadvantageous from an economic viewpoint. The addition in the range of 1-7 parts by weight based on the same standard as mentioned above is most preferred. 
     The cross-linking treatment of ethylene-hexafluoropropylene copolymers is carried out by using a conventional rubber processing unit. That is, the copolymers are kneaded in a rolling mill or Banbury mixer before their first cross-linking is carried out in a press or the like and the second cross-linking in an oven. The first cross-linking is carried out at a temperature in the range of 140°-180° C. for 10-50 minutes and the second cross-linking at a temperature in the range of 190°-220° C. for 0.5-24 hours. 
     The term &#34;ionizing radiation&#34; herein used means gamma-rays, X-rays, beta-rays, alpha-rays, electron beams and the like. Among those, gamma-rays and electron beams are particularly advantageous in the practice of the present invention. 
     The irradiation dose is generally in the range of 1-50 Mrads, preferably in the range of 5-30 Mrads. The dose rate generally employed is in the range of 10 3  -10 10  rads/hr, more preferably in the range of about 10 4  -10 9  rads/hr. 
     The components of the present invention to be used in a uranium enrichment plant can be prepared by applying heat or ionizing radiation to powdered, or particulate, or molded raw material comprising ethylene-hexafluoropropylene copolymer optionally containing proper fillers. Said molded material may be a film, a sheet, a pipe, a rod, or sometimes an O-ring, a lining and the like. 
     In addition to the organic peroxide materials mentioned above, a number of various compounding additives selected from acid-acceptors such as magnesium oxide, lead oxide, zinc oxide, calcium oxide, etc.; inorganic fillers such as carbon black, plasticizers; stabilizers and the like can also be added to the ethylene-hexafluoropropylene copolymers of the present invention. Polyfunctional monomers can also be added thereto as a modifier. Namely, the present invention can be summarized as follows. 
     Uranium enrichment plants for producing enriched uranium for use as a reactor fuel generally require various components made of an elastic material which has good resistance to attack by uranium hexafluoride at a high temperature. Examples of such components include a packing, a shock-absorbing member and the like. Heretofore, no proper elastic material has been found which satisfies all such requirements as mentioned above. Thus, the only countermeasure to the problem of UF 6  attack has been to change such component parts as often as necessary, a temporary and unsatisfactory solution. 
     In a series of investigations with respect to high polymer elastic materials having improved chemical and heat resistances, the present inventors have carefully examined the mutual action and reactivity between various elastic materials and uranium hexafluoride. As a result, they have discovered that an elastic material prepared by cross-linking a copolymer consisting of ethylene and hexafluoropropylene has remarkably superior resistance to attack by uranium hexafluoride as well as acceptable resistance to high temperatures and acceptable elasticity as compared with any other elastic material and accordingly is very suitable as a component of a uranium enrichment plant. Based on such discovery, the present inventors have accomplished the present invention. 
     The present invention will be explained in more detail by the following examples. But, it will be understood that these examples are given here only to illustrate and not to limit the invention. 
    
    
     EXAMPLES 1-3 
     Each of three different ethylene-hexafluoropropylene copolymers containing different fractions of hexafluoropropylene was molded into a sheet 1.0 mm thick, to the surface of which a sheet of Mylar film was closely adhered to shut off air. Then, the film was subjected to electron beam irradiation from an electron beam accelerator for a dose of 12 Mrads to cross-link the copolymer. A dumbbell specimen of ASTM-D-1822, Type-L was cut out of each cross-linked copolymer film. Each specimen was placed in an about 1000 ml (internal volume)-stainless steel vessel, which was deaerated by vacuum before gaseous uranium hexafluoride was introduced thereto. Heat was applied to keep the gaseous UF 6  at 100° C. and at a pressure of 500 mmHg for 7 days. Then, the temperature was lowered and the UF 6  gas was removed before the vessel was opened to take the specimen out of it. The weight difference before and after the experiment was examined and a tensile test was carried out to determine the change in the mechanical strength. The results of these experiments are summarized in Table 1.  As is obvious from this table, the novel component of the present invention made of ethylene-hexafluoropropylene copolymer is not substantially attacked by uranium hexafluoride even at 100° C. No substantial decrease in the tensile strength was observed after the experiment. In contrast, the result of comparative experiment carried out under the same conditions except that a conventional fluororubber (vinylidene fluoride-hexafluoropropylene copolymer) available in the market was used instead of the copolymer of the present invention showed that a remarkable weight increase due to the formation of solid uranium tetrafluoride in the test sample of fluororubber was observed and the mechanical strength of the sample remarkably decreased. Thus, the superiority of the material of the present invention was clearly demonstrated. 
     
                       TABLE 1                                                     
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(Results of Examples 1-3)                                                 
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       Conditions for preparing samples                                   
       Composition of elastic raw                                         
                         Dose for                                         
       materials (% by weight)                                            
                         cross-linking                                    
Example No.                                                               
         Hexafluoropropylene                                              
                        Ethylene (Mrads)                                  
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1        52             48       12                                       
2        62             38       12                                       
3        70             30       12                                       
______________________________________                                    
       Experiments for checking resistance to attack by                   
       uranium hexafluoride (by exposing a specimen to                    
       100° C., 500 mmHg for 7 days)                               
            Results of tensile test (Value                                
            after exposure based on 100* (%)                              
         Weight                     Modulus at                            
         increase Tensile   Extension                                     
                                    100%                                  
Example No.                                                               
         (%)      strength  at break                                      
                                    elongation                            
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1        0.3      97        98      100                                   
2        0.2      100       100     100                                   
3        0        100       100     100                                   
Comparative                                                               
example  57.9     51        30      --                                    
(market                                                                   
fluororubber)                                                             
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 *providing the value before exposure is 100)                             
 
    
     EXAMPLES 4-6 
     The same copolymers as those used in Examples 1-3 were used but cross-linking of them was carried out by using an organic peroxide (tertiarybutyl peroxybenzoate) instead of radiation. In particular, 3 parts by weight of tertiarybutyl peroxybenzoate and 2.5 parts by weight of triallyl isocyanurate were mixed with 100 parts by weight of the raw material copolymer to knead them well with one another before the mixture was pressed at 150° C. for 40 minutes to carry out both cross-linking and molding into the form of a sheet, simultaneously. Then, a dumbbell specimen was cut out of the molded sheet. The resistance against gaseous uranium hexafluoride and the mechanical strength of each specimen were examined in the same manner as mentioned in Examples 1-3. The results of these experiments are summarized in Table 2. As is obvious from this table, the material of the present invention prepared by cross-linking by chemical means is of superior quality and is not substantially attacked by uranium hexafluoride even at a temperature as high as 100° C. 
     
                       TABLE 2                                                     
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(Results of Examples 4-6)                                                 
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Conditions under which specimens were prepared                            
Compositions of elastic                                                   
                   Organic                                                
raw materials      peroxide   triallyl                                    
(% by weight)      (tertiarybutyl                                         
                              isocyanurate                                
Example                                                                   
       Hexafluoro-         peroxy   (parts by                             
No.    propylene  Ethylene benzoate)                                      
                                    weight)                               
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4      52         48       3        2                                     
5      62         38       3        2                                     
6      70         30       3        2                                     
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Experiments for checking resistance to attack by uranium                  
hexafluoride (by exposing a specimen to 100° C., 500 mmHg          
for 7 days)                                                               
            Results of tensile test                                       
Weight      Value after exposure based on 100 (%)*                        
Example                                                                   
       increase Tensile  Extension                                        
                                  Modulus at 100%                         
No.    (%)      strength at break elongation                              
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4      0.6      95       96       100                                     
5      0.6      95       97       100                                     
6      0.5      97       97       100                                     
Compar-                                                                   
ative                                                                     
example                                                                   
       57.9     51       30       --                                      
(market                                                                   
fluoro-                                                                   
rubber)                                                                   
______________________________________                                    
 *providing the value before exposing is 100.                             
 
    
     EXAMPLES 7-9 
     The same copolymers as those used in Examples 1-3 were used as raw materials but cross-linking was carried out by using an organic peroxide (tertiary peroxybenzoate). Carbon (MT carbon) was used as a filler as is usually employed in the conventional rubber making process, and lead oxide or magnesium oxide or both of them were used as an acid-acceptor added to the raw materials. In particular, 20 parts by weight of MT carbon, 3.7 parts by weight of tertiary butyl peroxybenzoate, 2.5 parts by weight of triallyl isocyanurate, 15 parts by weight of magnesium oxide and 10 parts by weight of lead oxide (this compound was used only in Example 9) were added to 100 parts by weight of the copolymer as a raw material to knead them well with one another and the resulting mixture was pressed at 150° C. for 40 minutes to carry out both cross-linking and molding at the same time to obtain a molded product in the form of a sheet. Then, a dambbell specimen was cut out of the molded sheet. Tests for checking the resistance against gaseous uranium hexafluoride and the mechanical strength of each specimen were carried out in the same manner as stated in Examples 1-3. The results of these tests are summarized in Table 3. As is obvious from this table, the material obtained was of excellent quality and is not substantially attacked by uranium hexafluoride even at a temperature as high as 100° C. 
     
                                           TABLE 3                                 
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(Results of Examples 7-9)                                                 
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Conditions under which specimens were prepared                            
Compositions of  Organic                                                  
elastic raw      peroxide                                                 
                      Triallyl         Lead                               
materials        (tertiary-                                               
                      isocyanu-  Magnesium                                
                                       oxide                              
(% by weight)    butyl                                                    
                      rate MT carbon                                      
                                 oxide (parts                             
Example                                                                   
     Hexafluoro- peroxy                                                   
                      (parts by                                           
                           (parts by                                      
                                 (parts by                                
                                       by                                 
No.  propylene                                                            
            Ethylene                                                      
                 benzoate)                                                
                      weight)                                             
                           weight)                                        
                                 weight)                                  
                                       weight)                            
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7    53     47   3.7  2.5  20    15    0                                  
8    20     80   3.7  2.5  20    15    0                                  
9    53     47   3.7  2.5  20    15    10                                 
__________________________________________________________________________
Experiments for checking resistance to attack by uranium hexafluoride     
(by exposing a specimen to 100° C., 500 mmHg for 7 days)           
                  Results of tensile test                                 
          Weight  Value after exposure based on 100 (%)*                  
          increase                                                        
                  Tensile Extension                                       
                                   Modulus at                             
Example No.                                                               
          (%)     strength                                                
                          at break 100%                                   
__________________________________________________________________________
7         6.5     95      90       100                                    
8         5.1     100     90       90                                     
9         7.4     95      70       80                                     
Comparative                                                               
example   57.9    51      30       --                                     
(market                                                                   
fluororubber)                                                             
__________________________________________________________________________
 *providing value before exposure is 100.