Patent Number: 040102874
Section: description

The improved process is illustrated in the following representative example to form UC.sub.2 microspheres. EXAMPLE A 70-g batch of dried weak-acid resin (Rohm & Haas IRC-72) containing about 34 g uranium was loaded into a 13/8-in. fluidized-bed coater furnace. Using argon for fluidization, the furnace was heated at about 100.degree. C./min. up to about 600.degree. C. and thereafter the heating rate was increased until a temperature of 1125.degree. C. was attained. This carbonized the resin constituents without any structural damage to the particle while retaining the uranium as UO.sub.2. The temperature was maintained at 1125.degree. C. and the gas was changed to acetylene (C.sub.2 H.sub.2), with a minor argon dilution. Under these conditions a carbon buffer coating of about 20 microns was applied in 3 minutes. Since the final desired product was a UC.sub.2 dispersion in a porous carbon matrix, the flow of acetylene was stopped and fluidization was continued solely with argon as the particles were heated to 1800.degree. C. and held at the temperature for 10 minutes. This had previously been shown to be sufficient time-temperature condition for complete conversion of UO.sub.2 to UC.sub.2. The temperature was thereafter reduced to 1275.degree. C. and a low-temperature isotropic coating of 40 microns applied in 5 minutes using undiluted propylene. Particles coated as described above were examined by radiography and metallography and appeared to have satisfactory geometry and physical properties for HTGR fuel. Although not considered to be detrimental, some particles exhibited a void region between the kernel and the inner surface of the buffer coat. This resulted from some kernel densification (shrinkage) during the conversion from oxide to carbon. The total void volume within the coatings is, however, determined by the kernel pore volume prior to coating and thus no special adjustment of void volume by buffer coating thickness is required. If the void region is considered detrimental, less densification during conversion can be effected if the initial portion of the carbonization (up to 600.degree. C.) is carried out at a heating rate of 50.degree.-75.degree.0 C./min. in contrast to the .gtoreq.100.degree. C./min. While the invention has been described and exemplified in terms of uranium-loaded microspheres, it should be understood that other metals such as boron; cadium; rare earth metals such as yttrium, and those having an atomic number in the range 58-71; actinide earth metals having an atomic number in the range 90-105, particularly thorium and plutonium, as well as other heavy metals may be usefully incorporated in spherules of the kind described as a carbide or oxide-carbide phase dispersed within a carbon matrix.