Abstract:
A nuclear fuel electrorefiner for recovering uranium from nuclear material containing uranium. A cylindrical vessel with having a longitudinal axis has a product collector movable axially of the vessel. Circular cathodes extend axially of and radially spaced inwardly of the vessel with a plurality of generally polyhedron-shaped anode baskets having at least one face aligned with a radius of said vessel and circumferentially spaced from adjacent anode baskets and concentric with respect to the cathodes in the vessel. A plurality of axially extending metal rods are insulated from and placed between the anode baskets. Mechanism outside of the vessel rotate the anode baskets and the metal rods with respect to the cathodes, and an electrical power supply in selective electrical communication with said cathode and said anode baskets and said metal rods to cause uranium values to move between the and when current flow is in a first direction uranium values in said anode baskets and the metal rods to the cathodes.

Description:
[0001] The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy (DOE) and The University of Chicago representing Argonne National Laboratory. 
     
    
     
       FIELD OF INVENTION  
       Background of the Invention  
         [0002]    Electrorefining is a metallurgical separation technique that has been used to recover uranium from fission products and other components of spent fuel from the Experimental Breeder Reactor-II (EBR-II), as well as to purify impure plutonium metal. The technique is well suited to this application because the separation is based on changes in oxidation state and is accomplished by the addition or removal of electrons at electrodes rather than the use of chemical oxidizing or reducing agents, which can significantly increase the volume of waste generated.  
           [0003]    For the last six years, personnel at Argonne National Laboratory (ANL) have mounted a significant effort to increase the throughput of this process as it is applied to the treatment of spent EBR-II fuel. In that time ANL personnel have moved from a small prototype device to a working unit that was demonstrated successfully at a throughput of 150 kg uranium/month with actual spent fuel in the Fuel Cycle Facility at ANL-West. The throughput still needs to be substantially increased, however, to handle the large inventory of EBR-II blanket fuel and other spent metal fuels at DOE sites. The basic electrochemistry of the process is now well understood and optimized. Consequently, increasing the throughput has focused primarily on improving the engineering of the process.  
           [0004]    In the high-throughput electrorefiner design, uranium was loaded into anode baskets that rotated in a channel between cathode tubes. The anode assembly and the cathode tubes were submersed in a molten LiCI-KCI eutectic. The salt also contained 5 to 7 wt. % uranium cations. Uranium and the elements in the fuel that are less noble than uranium were oxidized at the anode and formed cationic species that dissolved in the molten salt. Uranium cations were then reduced at the cathode, and the reduced uranium metal deposited on the cathode surface. Scrapers mounted on the rotating anode basket assembly dislodged the electrodeposited uranium, which fell to the bottom of the unit where it was collected.  
           [0005]    The schematic of FIG. 1 presents a cross-sectional rendering of the prior art high-throughput concept. FIG. 1 shows 12 anode baskets with attached scrapers in the unit. Four cathode tubes form three channels within which the anode baskets rotate. After being scraped off the cathode tubes, the uranium product is collected in a basket attached to the bottom of the outer cathode tube. With a 10-in. (25-cm) diameter module, uranium can be removed from spent EBR-II fuel at an average rate of nearly half a kilogram per hour. In the Fuel Cycle Facility at ANL-West, four of these units are operated in a single process vessel. One of the key steps required in the opening profile is a periodic reversal of the current to remove a dense layer of uranium that builds up on the surface of the cathode and is not dislodged by the scrapers.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, it is an object of the present invention to provide next-generation high-throughput electrorefiner that is capable of treating as much as 20 metric tons of uranium per year.  
           [0007]    Another object of the invention is to provide an electrorefiner with a peripherally driven anode assembly, independent removal of the product collector, and electrically isolated stripper rods located between the anode baskets.  
           [0008]    Yet another object of the present invention is to provide a nuclear fuel electrorefiner for recovering uranium from nuclear material containing uranium, comprising a cylindrical vessel having a longitudinal axis containing at the bottom thereof, a product collector movable axially of the cylindrical vessel, at least one cathode generally circular in horizontal cross section extending axially of and radially spaced inwardly of the vessel, a plurality of generally polyhedron-shaped anode baskets extending axially of the vessel, each anode basket having at least one face aligned with a radius of the vessel and circumferentially spaced from adjacent anode baskets and concentric with respect to the cathode, mechanism outside of the vessel rotating the generally polyhedron-shaped anode baskets with respect to the cathode, and an electrical power supply in selective electrical communication with the cathode and the anode baskets for causing uranium values in the anode baskets in the presence of a molten electrolyte containing uranium cations to move between the anode baskets and the cathode where uranium values are reduced to uranium metal and thereafter uranium metal is transported from the cathode to the product collector axially removable from the vessel without removing either the anode baskets or the cathode.  
           [0009]    Still another object of the present invention is to provide a nuclear fuel electrorefiner of the type set forth in which a plurality of axially extending metal rods are spaced circumferentially about the longitudinal axis of the vessel and are electrically insulated from and positioned between at least some of the anode baskets.  
           [0010]    Still another object of the present invention is to provide a nuclear electrorefiner of the type set forth in which there are a plurality of concentric radially spaced apart cathodes and a plurality of anode baskets arranged in the annular space(s) or channel(s) between the cathodes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.  
         [0012]    [0012]FIG. 1 is a cross-sectional view of a representative prior art high-throughput electrorefiner design known as the Mark V operated at ANL-West;  
         [0013]    [0013]FIG. 2 is a peripheral drive center removable (PCR) electrorefiner full assembly;  
         [0014]    [0014]FIG. 3 is a PCR electrorefiner full assembly illustrated in FIG. 2 showing an enlarged view of the top;  
         [0015]    [0015]FIG. 4 is a PCR electrorefiner full assembly illustrated in FIG. 2 with the well and heaters removed;  
         [0016]    [0016]FIG. 5 is a PCR electrorefiner full assembly illustrated in FIG. 2 with the crucible removed;  
         [0017]    [0017]FIG. 6 is a PCR electrorefiner full assembly illustrated in FIG. 2 with the outer cathode removed;  
         [0018]    [0018]FIG. 7 is a PCR electrorefiner full assembly illustrated in FIG. 2, showing the removal/insertion of the anode baskets;  
         [0019]    [0019]FIG. 8 is a PCR electrorefiner full assembly illustrated in FIG. 2 showing the removal/insertion of the scraper assembly;  
         [0020]    [0020]FIG. 9 is a PCR electrorefiner full assembly illustrated in FIG. 2 showing a close-up of the bottom;  
         [0021]    [0021]FIG. 10 is a PCR electrorefiner full assembly illustrated in FIG. 2 showing the diverter, sweeper and product collector;  
         [0022]    [0022]FIG. 11 is a PCR electrorefiner full assembly illustrated in FIG. 2 showing the center removal of the product collector;  
         [0023]    [0023]FIG. 12 is a cross-sectional view of a PCR assembly; and  
         [0024]    [0024]FIG. 13 is a cross-sectional view of the PCR assembly of FIG. 12 taken along a different radial line.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The present invention remedies several design inadequacies of the Mark V electrorefiner. Although the PCR design of the present invention includes concentric cathode tubes as well as cathode scrapers and anode baskets in the gap or annular space or channel between the concentric cathode tubes, the present invention is a peripheral drive device in which the center portion of the refiner is free permitting anodes, cathodes and most importantly product to be removed axially independently of each other. Previous designs used a central drive shaft to rotate a plate from which the anode baskets were suspended. In the present design the baskets although suspended from a rotating plate similar to the Mark V electrorefiner is driven by a drive motor positioned outside the vessel so as to free the interior thereof. This inventive design makes possible the center removal of the electrorefined uranium product, unlike the Mark V electrorefiner which requires both the anode and cathode components to be removed prior to removal of the product.  
         [0026]    Another important feature of the present invention is that although cathode scrapers are provided, their use is reduced and in fact some cases may be obviated entirely. In the present design, the anode baskets because of their configuration produce a higher current density close to the cathode wall and scrape uranium product deposited on the cathode into the product collector at the bottom of the unit. In addition, the inclusion of stripper mandrels rods which are electrically insulated from the anode basket(s) is also an important feature. During operation of the Mark V electrorefiner it was found that a hard uranium dentritic coating formed on the cathodes and the only way to remove it was to reverse the current to the vessel thereby electrolytically removing the uranium from the cathode while simultaneously plating the uranium values on the outside of the anode baskets. While effective in removing the hard or dense uranium dentritic coating on the cathodes, this required the outside coating of uranium on the anode baskets to be removed from the anode baskets prior to removal of uranium from the chopped fuel elements inside the anode baskets after current had been reversed once again. This entire reversal requirement slowed down the electrorefining process to a significant extent. With the use of stripper mandrels as hereinafter described, when the current is reversed, uranium is principally plated onto the stripper mandrels as well as on the anode baskets, thereby reducing the amount of uranium which must be removed from the outside of the anode baskets before the uranium values inside the anode baskets are transported to the cathodes.  
         [0027]    It is believed that use of the hereinafter described invention will be able to achieve a 450 kpm throughput. Tables 1 and 2 includes calculations based on the hereinafter described invention for both one channel and two channel PCRs. Two channel PCRs relate to three cathodes and two annular channels of anode baskets with stripper mandrels positioned between each of the anode baskets.  
                                                   TABLE 1                           Throughput calculations - Basis            1-Channel PCR           stripper       (25″ diameter)   cathode   anode   mandrel                    Electrode Area, cm 2     13406   7392   4750       sticking coefficient   0.8   N/A   0.5       deposition current density, A/cm 2     0.135   0.15   0.15       stripping current density, A/cm 2     0.102   0   0.287       2-Channel PCR (36″ diameter)   cathode   anode   stripper                   mandrel       Electrode Area, cm 2     26126   19712   11260       sticking coefficient   0.8   N/A   0.5       deposition current density, A/cm 2     0.178   0.15   0.15       stripping current density, A/cm 2     0.107   0   0.249                  
 
         [0028]    [0028]                                                                                             TABLE 2                           Operational Cycle and Throughput                    A-h   I/A   time/h                            1-Channel PCR                                   Deposition   500   1821.3   0.27           Rods   200   712.5   0.28           Baskets   300   11.08.8   0.27           Strip   400   1362   0.29           Wash           0.10                        peak throughput = 1112 g/hr       avg monthly throughput = 450 kg/mo                2-Channel PCR                                   Deposition   500   4646   0.11           Rods   200   1689   0.12           Baskets   300   2957   0.10           Strip   400   2804   0.14           Wash           0.10                        peak throughput = 2077 g/hr       avg monthly throughput = 841 kg/mo            
         [0029]    Referring now to the drawings, there is disclosed a peripheral drive center removal PCR electrorefiner  10  which has an outer vessel  11  cylindrical in design having a cylindrical wall  12  and a horizontal bottom  13 . The cylindrical wall  12  ends in an outwardly extending flange  14  at the top of the outer vessel  11 . An insulator ring  15  is positioned on top of the flange  14  and receives or carries a plate  17  which is normally Z-shaped cross section as is seen in the left hand portion of FIG. 12. Heater shields  18  are suspended from the plate  17 , and there is also provided an insulator ring  19  as seen in FIG. 12.  
         [0030]    A cylindrical crucible  20  is positioned inside the outer vessel  11  and has a vertically extending cylindrical wall  21  and a bottom  22 . The double wall nature of the crucible  20  and the outer vessel  11  are safety features. An annular fixture  25  sits on top of the insulating ring  19  and includes a top flange  26  and a bottom flange  27  interconnected by a vertically extending bight  28 . An outer ring  29  provides support for a top plate  30  and includes a gear ring  33  on the outer periphery thereof which is engaged by a peripheral drive motor  35  having an output gear  36 . A mounting structure  37  mounts the peripheral drive motor  35  to the fixture  25 .  
         [0031]    The top plate  30  has a horizontal flange  40  connected to an upstanding vertical flange  41 , the horizontal flange  40  being provided with spaced apart circular apertures  42 , spaced apart rectangular apertures  43  and spaced apart rectangular apertures  44 , the apertures  43  being wider than the apertures  44 , both having the longer dimension positioned radially of the vessel  11 .  
         [0032]    A slip ring  45  is mounted interior of the flange  41  and is electrically insulated therefrom and carries a plurality of horizontally extending supports from which extend a plurality of mandrel rods  50 , each of the mandrel rods  50  being electrically insulated from the horizontal flange  40  of the top plate  30 .  
         [0033]    A plurality of axially extending circumferentially spaced apart anode baskets  55  are positioned within the apertures  43 . Each anode basket  55  as seen in FIG. 7 is polyhedron-shaped and may be of any convenient design, such as pentahedron or hexahedron. In the preferred embodiment, the anode basket  55  is a hexahedron with the two faces extending radially of the vessel  11  being longer than the two opposed faces extending perpendicularly to the radial faces. More particularly, as seen in FIG. 7, each anode basket  55  includes opposed faces  55 ( a ) which are smaller in dimension than opposed faces  55 ( b ), the faces  55 ( b ) being positioned along the radius of the cylindrical vessel  11 . As is well known in the art, the anode baskets  55  are perforated and may be provided with screens to retain fines within the anode basket during the electrorefining process.  
         [0034]    Scraper assemblies  57  are positioned preferably intermediate each anode basket  55  and each mandrel rod  50 . The scraper assemblies  57  extend axially of the vessel  11  the length of the cathodes  60 ,  65  and are standard in the art. In the inventive PCR  10 , the scraper assemblies  57  may or may not be required depending on the efficiency of the mandrel rods  50  and the anode baskets  55  scraping deposited uranium from the cathodes, as will be described. There are provided two cathodes, an outer cathode  60  having a cylindrical wall  61  and an inner cathode  65  having a cylindrical wall  66 . Cathodes  60 ,  65  are supported at the bottom of the vessel  11  by means of a cathode support  70 , generally U-shaped in vertical cross section to support the outer and inner cathode  60  and  65  respectively, as best seen in FIG. 13.  
         [0035]    As illustrated in FIGS.  8 - 13 , there is provided a product diverter  72  which is mounted to or depends from the cathode support  70 , the product diverter  72  having a frustoconical portion  73  and a vertical flange portion  74  which may be mounted, as illustrated in FIGS. 12 and 13 to the outer cathode  60  and/or the cathode support  70 . As is known, the product diverter  72  would be electrically insulated from the cathodes  60  and  65  in order to prevent a product from being deposited thereon during the electro refining process. The frustoconical portion  73  of the product diverter  72  is reenforced by a plurality of circumferentially spaced apart struts  76  which may be welded or otherwise fixed to either or both of the inner vessel bottom  22  and the cylindrical wall  21 . As best seen in FIG. 10, the product diverter  72  may also include a cylindrical pan  77  in addition to or in lieu of the bottom  22  of the crucible or inner vessel  20 .  
         [0036]    Ring structure  78  is internal of the vertical flange  74  and may provide support for a plurality of blade shaped sweepers  79  which may be rotatable independent of the product collector  80  which is a cylindrical container having wall  81  and a bottom  82 . A handle  83  extends vertically upwardly through the product collector  80  and preferably is aligned with the longitudinal axis of the PCR  10 .  
         [0037]    The materials from which the PCR  10  is made are well known in the art and may include any suitable metal for the inner container or crucible  20  and the outer vessel  11 , representative metals being steel, stainless steel, Inconel or other well known metals in the nuclear electrorefining art. Similarly, insulation between the various metal components of the PCR  10  are also well known and are art recognized. Although there is shown in the above figures two cathodes  60  and  65  and a plurality of circumferentially spaced anode baskets  55  between the two cathodes, it should be understood that a larger plurality of concentric cathodes and concentric anode baskets may be employed as illustrated in the prior art shown in FIG. 1, wherein four cathode tubes form three channels within which the anode baskets rotate. A similar configuration may be used in the present invention with rotational driving mechanism being located exterior to the outermost cathode thereby freeing the center of the PCR  10  for removal of the product collector  80  axially of the vessels.  
         [0038]    As is well known in the art, electrical power supplies are available and when a current flows between the anode baskets  55  and the cathodes  60  and  65 , uranium values in the anode baskets are transferred through the molten salt electrolyte which contains uranium cations and plate out on the cathode as uranium metal by reduction at the cathode. During operation of the Mark V electrorefiner, dense uranium dendrites build-up on the cathodes requiring scrapers as illustrated in the present invention to remove the uranium on the cathode; however, the dense inner coating often cannot be scraped and can only be removed by reversing the current flow so that uranium metal plated onto the cathodes is dissolved in the electrolyte and plates on the outside of the anode basket. The present invention significantly reduces this problem by providing a plurality of axially extending metal mandrel rods  50  which may be of any suitable metal from the anodes but electrically connected to said cathodes  60 ,  65  by an independent power supply so that during current reversal onto uranium metal pates onto the rods  50 . Accordingly, uranium metal when dissolved from the cathodes  60 ,  65  will plate onto the mandrels  50  in lieu of the anode baskets  55 . This is important because the uranium values which plate on the rods  50  reduce or eliminate the amount of uranium metal plating on the anode baskets  55  during current reversal. Because in the Mark V electrorefiner, uranium metal plates onto the outside of the anode baskets during current reversal, when the current is again reversed, the uranium metal on the outside of the baskets must be removed first prior to any uranium values inside the baskets being transported via the electrolyte to the cathodes.  
         [0039]    As seen in Tables 1 and 2, calculations show a significantly improved transfer rate throughput for the PCR electrorefiner of the present invention. While the Marl V electrorefiner has demonstrated a 150 kg/mo throughput, the PCR of the present invention is calculated to be able to move 450 kg/mo. for a second channel refiner and 840 1/mo or a two channel electrorefiner and this is a significant advantage over the current state of the art.  
         [0040]    In other respects, operation of the current PCR electrorefiner  10  and the Mark V are similar. Both use the same materials of contraction, the same anode and cathode materials, the same electrolytes and operating cycles. The PCR  10  of the present invention provides faster throughput, easier handling and less down time and maintenance, although requiring an extra power supply to the mandrels or rod  50  connected to the cathodes  60 ,  65 .  
         [0041]    While particular embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications and improvements may be made, for example in the processing of the materials or in the electrode and/or cell design without departing from the true spirit and scope of the invention.