Patent Number: 050376063
Section: description

The following example illustrates a preferred method for making nuclear fuel particles, and nuclear fuel compacts utilizing such particles, of the types generally herein described. However, this example should be understood to in no way limit the scope of the invention which is defined by the appended claims. EXAMPLE Spheroids of uranium oxide and uranium carbide are prepared having a major portion of uranium oxide material. Stoichiometrically, the spheroids can be viewed as having the composition UC.sub.0.3 O.sub.1.7. The particle size of the spheroids is about 350 microns, and the spheroids are considered as being substantially fully dense. The spheroids are heated in a suitable coating apparatus in a fluidized bed to a temperature about 1100.degree. C. using a levitating flow of argon. A buffer coating of spongy pyrocarbon is deposited from acetylene gas at a partial pressure of about 0.5 (total pressure of 1 atmosphere). The acetylene decomposes and deposits low density spongy carbon upon the cores, and flow is continued for sufficient time to deposit a layer about 40 to 50 microns thick having a density about 1.1 gram per cc. The flow of acetylene is then terminated, and the temperature is raised to about 1200.degree. C. A partial pressure of propylene of about 0.07 is injected into the stream, and coating is carried out for about 5 minutes. This produces a thin, anisotropic coating of generally laminar carbon having a density of about 1.9 grams per cm.sup.3. The buffer-coated cores are then heated to about 1400.degree. C., and a mixture containing about 10% propylene, about 10% acetylene, about 40% hydrogen and about 40% argon, all percents are by volume, is employed to deposit a layer of isotropic pyrocarbon about 50 microns thick having a density of about 1.95 g/cm.sup.3 and a BAF of about 1.1. The temperature is then raised to about 1500.degree. C., and hydrogen is used as the fluidizing gas with about 10% by volume of the hydrogen stream being bubbled through a bath of methyltrichlorosilane. After about 1 hour at these conditions, a layer of silicon carbide about 20 microns thick has been uniformly deposited upon the carbon-coated spheroids. Subsequent measurement and examination shows that the silicon carbide is beta-phase SiC having a density of about 3.18 g/cm.sup.3, which is about 99% of theoretical density of silicon carbide. The silicon carbide-coated cores are maintained in this fluidized condition substituting a mixture of equal parts of argon and hydrogen as the fluidizing gas, and the temperature is lowered to about 1400.degree. C. At this temperature, a mixture of 20% of equal parts of propylene and acetylene is injected to again deposit isotropic pyrocarbon having a density of about 1.95 g/cm.sup.3 and a BAF of about 1.1. The temperature of the fluidized particles is then lowered to about 1200.degree. C., and a partial pressure of acetylene of about 0.4 atm. (total pressure 1 atm.) is injected for about 5 minutes. At the end of this time, the particles are cooled to near room temperature in the fluidizing gas stream and then removed and examined. The exterior coating of pyrocarbon shows a density of about 1.1 g/cm.sup.3 and a thickness of between about 30 and 45 microns, with the mean thickness being about 40 microns for the overcoated material. These overcoated particles are employed to fabricate fuel compacts of generally cylindrical shape having a diameter of about 0.5 inch and a height of about 1.96 inches. A charge of approximately 10 grams of these particles is metered into a mold which is being vibrated to assure adequate filling. A metering system such as that shown in U.S. Pat. No. 4,111,335 to Arya, et al. is used; the completed compacts have a fissile core total volume of about 2.24 cm.sup.3 in a compact of about 6 cm.sup.3. Pre-compaction pressure at about 200 psig is employed to initially reduce the size of the mold to essentially that of the desired height of the fuel compact. After pre-compaction is complete, a mixture of petroleum pitch and graphite flour is injected, which mixture contains about 40 weight per cent graphite flour (having a maximum particle size of about 40 microns), based upon total weight of the mixture. Injection into the mold is via a passageway arrangement that extends around one of the end pistons. Injection takes place at a pressure of about 1000 psig, and the temperature of the pitch mixture and the mold are maintained at about 150.degree. C. Once injection is complete, the temperature of the mold is cooled to solidify the binder, and the compacts are ejected from the mold at a temperature of about 30.degree. C. They are then transferred to a circulating nitrogen furnace where they are heated for about 5-10 minutes at a temperature of about 1000.degree. C. in order to carbonize the binder material. The individual coated nuclear fuel particles made in Example I are tested along with particles removed from the coater prior to the application of the final protective overcoating. The earlier-removed particles exhibit a crush strength of about 6 pounds, whereas the particles after application of the protective overcoatings show a crush strength nearly double that value. The completed fuel compacts are then examined for heavy metal contamination which is indicative of substantial fracture of the fission-product-retentive shells during the compact fabrication process. These compacts are tested by what is termed the HCl leach method wherein exposure to gaseous HCl leaches heavy metal, i.e., uranium, from the compacts. The carbonized fuel compacts are loaded into graphite crucibles in a furnace that is then heated to a temperature of about 1400.degree. C. wherein hydrogen chloride gas is circulated for about 8 hours. The gaseous chlorides of uranium (and/or thorium if present) are formed, and by monitoring the amounts of uranium chlorides, the heavy metal contamination can be calculated. The compacts show less than 1.times.10.sup.-5 grams of uranium per total grams of uranium in the compact, thus showing that the desired level of quality is obtained. Compacts made using such coated nuclear fuel particles without the protective overcoatings are similarly examined for heavy metal contamination and are found to exhibit contamination of about 3.times.10.sup.-5 grams of uranium per total number of grams of uranium, thus putting into perspective the reduction which is achieved by the use of the protective overcoatings. The HCl leach step also effects a cleaning of heavy metal contamination from such compacts before they are treated in the final heat treatment furnace. Heating in this furnace for a temperature of about 1700.degree. C. for about one-half hour results in decomposition of any remaining hydrocarbons in the binder and a slight degree of binder graphitization. Following a final inspection for correct dimensions and visual appearance, the fuel compacts are ready for loading into nuclear fuel blocks to form fuel elements. Further testing by burning of one of each of the groups of compacts shows that the protection afforded by the overcoating during the fabrication process results in a decrease in the defective fraction of fission-product-retentive coatings to a level of about 60% of those found in compacts without the protective overcoatings. Although the invention has been described with regard to the best mode presently understood by the inventors, changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is to be found in the claims appended hereto. Particular features of the invention are emphasized in the claims that follow.