Patent Number: 046630937
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention and Contract Statement This invention relates to a process for preparing gel spheres of nuclear fuel and to apparatus for conducting such process. 2. Division of Background and Prior Art A sol gel process for the production of (U, Pu)O.sub.2 nuclear fuel microspheres is disclosed in U.S. Pat. No. 4,397,778. Because of the higher gelation temperature (about 85.degree. C.) required in such process, silicone oil is used as a gelation medium instead of trichloroethylene (TCE) which is used in conventional internal gelation processes for the preparation of UO.sub.3 spheres. The use of silicone oil, which has higher viscosity and higher surface tension, has caused some difficulties particularly in a continuous mode of operation of the process. Recently, improvements to the process have been disclosed that overcome such difficulties. One of the improvements is a method by which the resistance of the surface tension and viscosity of the silicone oil are overcome by transporting the liquid droplets of a sol or broth into the silicone oil gelation medium by way of a jet stream of silicone oil. Another improvement uses a draft tube and vibrated screen for disengaging the gelled and aged spheres from silicone oil and then transporting the disengaged spheres into a trichloroethylene wash column where the silicone oil is washed from the outer surfaces of such spheres. The application of such improvements to the process has resulted in a continuous internal gelation process for producing nuclear fuel microspheres of predictable size and quality. However, the process is now more complex and more difficult to control and maintain, especially since remote operation is required. It is desirable to have a more simplified process with fewer steps, fewer equipment pieces to control, and one that is more amenable to scale-up and remote operation and maintenance. U.S. Pat. No. 4,124,535 discloses a method of forming gelled spheres by suspending droplets of a slurry in a suspending liquid, such as, perchloroethylene. The slurry includes slurries made from metal oxides, but radioactive materials are not mentioned. U.S. Pat. No. 3,321,560 discloses a method of making uranium dioxide spheres by suspension of a uranium dioxide slurry in trichloroethylene. There is no disclosure of dropping uranium spheres into a moving stream of hot solvent. U.S. Pat. No. 2,820,984 teaches the production of regenerated cellulose pellets. As the drops or globules of regenerated cellulose are formed, they are first dropped into a coagulating bath wherein they are subjected to a preliminary limited regenerating action during which time a thin skin is caused to form on the drops. The bath is caused to flow without turbulence and eddy currents and serves to move the pellets out of the dropping zone. SUMMARY OF THE INVENTION An object of the invention is to provide a simple, continuous process for preparing gel spheres of nuclear fuel, such as, (UPu)O.sub.x. Another object of the invention is to provide apparatus for conducting such process. Other objects and advantages of the invention are set out herein or are obvious herefrom to one ordinarily skilled in the art. The objects and advantages of this invention are achieved by the process and apparatus of the invention. To achieve the foregoing and other objects in accordance with the purpose of the invention, as embodied and broadly described herein. The invention involves an internal gelation process for the continuous preparation of nuclear fuels. The process includes moving a volume of hot perchloroethylene through a trough. Droplets of a nuclear fuel solution are directed into the moving volume of hot perchloroethylene. The droplets of nuclear fuel solution gel to form gel spheres while the droplets are floating on the surface of the moving volume of perchloroethylene. The resultant gel spheres drop into a vertical column of perchloroethylene, wherein the gel spheres of nuclear fuel age while floating as a bed at the top of vertical column. The aged gel spheres of nuclear fuel are separated from the perchloroethylene. Preferably the hot perchloroethylene solution of the first step has temperature between about 75.degree. and about 90.degree. C., and most preferably it has a temperature of about 85.degree. C. Preferably the nuclear fuel solution used in the second step is a U-Pu solution. The vertical column of step (c) is preferably an ager. Preferably the aged gel spheres from step (d) are transported on a moving screen, the entrained perchloroethylene draining off of the aged gel spheres, the drained gel spheres are washed in an aqueous wash column, the wash water is drained from the gel spheres and the aged gel spheres are dried. Also preferably the wash water contains ammonium hydroxide. The process of the invention can be used for both the production of virgin UO.sub.2 fuels and the refabrication of (UPu)O.sub.2 fuels recycled in fuel reprocessing systems. The invention also includes the aged gel spheres of nuclear fuel prepared by the invention process. The invention includes apparatus for continuously preparing aged gel spheres of nuclear fuel. The apparatus includes the vessel and means adapted to contain a flow of hot perchloroethylene therethrough, and means for injecting hot perchloroethylene into one end of a trough-shaped vessel, thereby causing the flow of hot perchloroethylene through the trough-shaped vessel. There is means for forming droplets of nuclear fuel solution and directing the droplets of nuclear fuel into the flow of hot perchloroethylene in the vessel means. The droplets of nuclear fuel rise to the surface of the flow of hot perchloroethylene and gel to form gel spheres of nuclear fuel floating on top of the flow of hot perchloroethylene. There is also column means containing perchloroethylene and attached to the other end of the vessel means so that the interface opening therebetween coincides with the top of the hot perchloroethylene in the vessel means. The gel spheres of nuclear fuel floating on the top of the hot perchloroethylene at the attachment interface are formed into the column means by the build up of floating gel spheres in the vessel means. The top of the perchloroethylene in the column means is below the attachment interface. The gel spheres entering the column means fall onto the top of the perchloroethylene therein. The gel spheres floating on the perchloroethylene in the column means age as they move to the bottom of the column means. Some perchloroethylene from the vessel means come over the attachment means into the column means. Further, there is means for removing the aged gel spheres from the bottom of the column means and separating such removed, aged gel spheres from entrained perchloroethylene. Preferably the vessel means is a trough-shaped vessel. Preferably the injection means includes a horizontal pipe located in the one end of the vessel means located below the top surface of the hot perchloroethylene in the vessel means, the hot perchloroethylene entering the vessel means through the pipe. Also, preferably another horizontal pipe is located in the one end of the vessel means above the horizontal pipe so that the bottom of the rim of the horizontal pipe positioned just above the top surface of the hot perchloroethylene, thereby, when too much hot perchloroethylene is fed into the vessel means, such excess hot perchloroethylene flows out another horizontal pipe so as to maintain the desired level of hot perchloroethylene in the vessel means. Preferably a first vertical baffle is located in the vessel means, the first vertical baffle extending below the top surface of the hot perchlorethylene and the bottom edge of the first baffle being above the bottom surface of the vessel means, and a second vertical baffle is located on the side of the first vertical baffle away from the entrance pipe, the top edge of the second vertical baffle being positioned a minimal distance below the top surface of the hot perchloroethylene in the vessel means, thereby preventing movement of the nuclear fuel droplets toward the entrance pipe or overflow pipe and helping to provide a nonturbulent flow of perchloroethylene in the downstream portion of the vessel means. The column means preferably contain a slow-moving, vertically-oriented ager.