Patent GB-998247-A (GB)
Title: Improvements in or relating to coated articles
IPC: G21C7/04
Filed: 1964-04-07
Abstract: Coated articles comprising a core about which is disposed an exterior coating, e.g. coated particles of either a nuclear fuel material or a nuclear poison, have a low density, spongy, shock absorbing coating capable of absorbing thermal and/or irradiation stresses interposed between the core and the coating.  Materials specified are uranium dicarbide or a mixture of uranium dicarbide and thorium dicarbide for the fuel material, boron carbide or gadolinium carbide for the poison, and pyrolitic carbon, silicon carbide, zirconium carbide or niobium carbide for the exterior coating.  The low density, spongy, shock absorbing coating is preferably of pyrolytic carbon.  The exterior coating may have two layers, the outer being pyrolytic carbon and the inner being one of the above carbides. In an example, a particulate UC2-ThC2 fuel mixture is prepared from a mixture of 10 grams of ThO2 and 1 gram of enriched UO2 containing 91 to 93% by weight of U-235, to which carbon is added in an amount in excess of the stoichiometric amount required for the conversion of the dioxides to dicarbides, and a binder of 2% by weight of ethyl cellulose is added to the mixture of dioxides and carbon. The mixture is intimately mixed while dry and a trichloroethylene solvent for the ethyl cellulose binder is then added to form a slurry which is agitated to obtain agglomerated particles of ThO2, UO2 and C of a size of 295 to 500 microns which are oven dried at 149 DEG  F.  The dried particles are mixed with graphite flour in a particle to graphite weight ratio of 8 : 1 and then reacted in a graphite crucible under vacuum at 2200 DEG  C. to melt and densify the particles of UC2-ThC2 which are prevented from coalescing by the graphite flour.  Upon cooling, dense, nearly spherical particles, of a size of 175 to 300 microns, of a solid solution of UC2 and ThC2 are obtained.  A graphite reaction tube 1 inch in diameter is heated to 1200 DEG  C. and He which is preheated to 400 DEG  C. is passed through the tube at 4000 c.c./min.  50 grams of the UC2-ThC2 fuel particles are dropped into the tube and fluidized in the He stream. When the fuel particles reached 1200 DEG  C., acetylene at a partial pressure of 0.65 atmos., is admixed with the He. The acetylene decomposes and deposits low density spongy carbon upon the fuel particles. The acetylene flow is continued until a carbon coating of a thickness of 40 microns is deposited, when the flow is terminated and the temperature of the reaction tube is raised to 1400 DEG  C. At this temperature methane, at a partial pressure of 0.35 atmos., is admixed with the He, decomposing and depositing a dense, retentive, pyrolytic carbon coating over the spongy carbon coating.  The methane flow is continued until a carbon coating of a thickness of 50 microns is obtained, when the flow is terminated and the fuel particles are cooled in helium and removed from the reaction tube.  The low density, spongy, pyrolytic carbon coating is preferably applied to the fuel particles in a thickness of approximately one to two fission "recoil ranges" ("recoil range" being the distance of travel of the fission products within a given material).  Spongy carbon has a "recoil range" of 12 to 25 microns and is preferably applied in a thickness of two "recoil ranges," i.e. from 24 to 50 microns, when nearly all the fission recoils will be absorbed and prevented from striking and rupturing the dense brittle exterior pyrolytic carbon coating.  Fuel materials and poisons coated as described above can be incorporated within various fuel elements for use in nuclear reactors.  The coated fuel particles can be dispersed in a graphite matrix and compressed into fuel compacts, or can be employed in the form of a packed bed of discrete particles disposed in a bore within a graphite body.  In the former method, the coated fuel particles are mixed with powdered graphite in a blender, pitch dissolved in trichloroethylene added to form a paste and the paste spread in a thin sheet which is diced and dried.  The dried paste is pressed at 750 DEG  C. and 4000 p.s.i. to form a fuel compact which can then be heat treated for dimensional stability.  In addition to nuclear fuels and poisons, the invention is also applicable to catalysts which have brittle exterior coatings so as to improve their stability at high temperatures.ALSO:Coated articles comprising a core about which is disposed an exterior coating, e.g. coated particles of either a nuclear fuel material or a nuclear poison, have a low density, spongy, shock absorbing coating capable of absorbing thermal and/or irradiation stresses interposed between the core and the coating. Materials specified are uranium dicarbide or a mixture of uranium dicarbide and thorium dicarbide for the fuel material, boron carbide or gadolinium carbide for the poison, and pyrolitic carbon, silicon carbide, zirconium carbide or niobium carbide for the exterior coating.  The low density, spongy, shock absorbing coating is preferably of pyrolytic carbon.  The exterior coating may have two layers, the outer being pyrolytic carbon and the inner being one of the above carbides.  In an example, a particulate UC2-ThC2 fuel mixture is prepared from a mixture of 10 grams of ThO2 and 1 gram of enriched UO2 containing 91-93% by weight of U-235, to which carbon is added in an amount in excess of the stoichiometric amount required for the conversion of the dioxides to dicarbides, and a binder of 2% by weight of ethyl cellulose is added to the mixture of dioxides and carbon. The mixture is intimately mixed while dry and a trichloroethylene solvent for the ethyl cellulose binder is is then added to form a slurry which is agitated to obtain agglomerated particles of ThO2, UO2 and C of a size of 295-500 microns which are oven dried at 140 DEG F.  The dried particles are mixed with graphite flour in a particle to graphite weight ratio of 8:1 and then reacted in a graphite crucible under vacuum at 2200 DEG C. to melt and densify the particles of UC2-ThC2 which are prevented from coalescing by the graphite flour. Upon cooling, dense, nearly spherical particles, of a size of 175-300 microns, of a solid solution of UC2 and ThC2 are obtained.  A graphite reaction tube 1 inch in diameter is heated to 1200 DEG C. and He which is preheated to 400 DEG C. is passed through the tube at 4000 c.c./min. 50 grams of the UC2-ThC2 fuel particles are dropped into the tube and fluidized in the He stream.  When the fuel particles reached 1200 DEG C., acetylene at a partial pressure of 0.65 atmos. is admixed with the He. The acetylene decomposes and deposits low density spongy carbon upon the fuel particles. The acetylene flow is continued until a carbon coating of a thickness of 40 microns is deposited, when the flow is terminated and the temperature of the reaction tube is raised to 1400 DEG C.  At this temperature methane, at a partial pressure of 0.35 atmos., is admixed with the He, decomposing and depositing a dense, retentive, pyrolytic carbon coating over the spongy carbon coating.  The methane flow is continued until a carbon coating of a thickness of 50 microns is obtained, when the flow is terminated and the fuel particles are cooled in helium and removed from the reaction tube. The low density, spongy, pyrolytic carbon coating is preferably applied to the fuel particles in a thickness of approximately one to two fission "recoil ranges" ("recoil range" being the distance of travel of the fission products within a given material).  Spongy carbon as a "recoil range" of 12-25 microns and is preferably applied in a thickness of two "recoil ranges", i.e. from 24-50 microns, when nearly all the fission recoils will be absorbed and prevented from striking and rupturing the dense brittle exterior pyrolytic carbon coating.  Fuel materials and poisons coated as described above can be incorporated within various fuel elements for use in nuclear reactors.  The coated fuel particles can be dispersed in a graphite matrix and compressed into fuel compacts, or can be employed in the form of a packed bed of discrete particles disposed in a bore within a graphite body.  In the former method, the coated fuel particles are mixed with powdered graphite in a blender, pitch dissolved in trichloroethylene added to form a paste and the paste spread in a thin sheet which is diced and dried.  The dried paste is pressed at 750 DEG C. and 4000 p.s.i. to form a fuel compact which can then be heat treated for dimensional stability.  In addition to nuclear fuels and poisons, the invention is also applicable to catalysts which have brittle exterior coatings so as to improve their stability at high temperatures.