PREPARATION OF DENSE URANIUM DIOXIDE PARTICLES FROM URANIUM HEXAFLUORIDE IN A FLUIDIZED BED

Abstract

A fluid-bed method was developed for the direct preparation from uranium hexafluoride of dense, spheroidal uranium dioxide particles for dispersion and packed fuel element applications. The uranium hexafluoride is reacted with mixtures of steam and hydrogen in a bed of uranium dioxide maintained at 650 to 700 deg C, the solid reaction products depositing as a dense layer on the surface of the bed particles. The operating procedure involves alternating periods of uranium hexafluoride feed wi h periods of fluoride cleanup when only steam and hydrogen are fed. Uranium dioxide particles in the size range -- 16 + 140 mesh and having densities of 9.5 g/cc and 9.75 g/cc (up to 89% of theoretical density) were produced at 650 deg C and 700 deg C, respectively, using a uranium hexafluoride feed rate of 25 g/min STA44 lb uranium/(hr)(sq ft reactor cross section)!. The average residual fluoride content of the uranium dioxide was about 300 ppm. The densest material was obtained when 0.75 to 1.4 times the stoichiometric steam requirement (based on the reaction UF/sub 6/ + 2H/sub 2/O + H/sub 2/ yields UO/sub 2/ + 6HF was used. The effect on product density of other process variables such as uranium hexafluoride feed rate, hydrogen concentration, bed height, and duration of hexafluoride feed period was also determined. Further densification of the material to 10.4 to 10.6 g/cc, up to 96.5% of theoretical density, and reduction of the fluoride content to about 5 ppm was achieved by sintering in hydrogen at about 1700 deg C for 2.5 hours. (auth)