Fabrication of Uranium Dioxide Microspheres by Classic and Novel Sol-Gel Processes
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Fabrication of Uranium Dioxide Microspheres by Classic and Novel Sol-Gel Processes A. Deptuła1, M. Brykala1, M. Rogowski1, T. Smolinski1, T. Olczak1, W. Łada1, D. Wawszczak1, A. Chmielewski1, and K.C. Goretta2 1 2
Institute of Nuclear Chemistry and Technology, 16 Dorodna Street, 03-195 Warsaw, Poland Air Force Research Laboratory, Arlington, VA 22203-1768, U.S.A.
ABSTRACT Uranium dioxide (UO2) microspheres were fabricated by two sol-gel processes. First used was a classical process variant, as developed at Oak Ridge National Laboratory, consisting of (1) reduction of commercial uranyl to U(IV) nitrate; (2) preparation of a sol by precipitation of uranium hydroxide, its peptization, and solvent extraction of nitrates; and (3) gelation to microspheres by extraction of water through addition of a dewatered 2-ethyl-1-hexanol emulsion. Substantial improvement in microsphere production was achieved by application of a sol-gel process in which ascorbic acid was used as strong complexing agent. In this method, the reduction step was omitted and uranyl (VI) ascorbate sols/hydroxyl sols were formed from a suspension of either a uranium trioxide or a uranyl nitrate solution. Gelation through water extraction yielded perfect microspheres. Other metals can be easy added to these sols. Thermal treatment of the UO2 microspheres by calcination and reduction in hydrogen atmosphere was designed on the basis of differential thermal analysis and thermogravimetric analysis. INTRODUCTION The main application for UO2 microspheres is as fuel in a high-temperature gas-cooled reactor (HTGR), such as a generation IV reactor [1-4]. The HTGRs created in the early 1960s required a special procedure for production of fuels. It came to be known as the Oak Ridge National Laboratory (ORNL) sol-gel process [5]. Considerable progress in production of these nuclear fuels has been reported in international conferences over the years [6-9]. Other sol-gel processes for nuclear technology have also been reported [6-10]. In the sphere-pac process, for example, gel-derived spheres of nuclear materials, preferably of three sizes with diameter ratios of approximately 40:10:1, are loaded directly into rods and packed by low-energy vibration to achieve a smear density of 88% of theoretical. Pelletization of sol-gel-produced microspheres can also produce suitable fuels [6-9]. CLASSIC SOL-GEL PROCESSES In the literature, three main families of sol-gel processes for production of UO2 microspheres are described [5-10]: 1. As developed at ORNL water extraction from a U(IV) sol, and, recently, by internal gelation [10]. 2. External gelation by neutralization of UO22+ with ammonia, as developed at, e.g., SNAM (Italy), KEMA (The Netherlands), and NUKEM (Germany).
3. Internal gelation process by neutralization of UO22+ with ammonia generated by decomposition of hexametylene tetramine at elevated temperature, as developed at KEMA (The Netherlands) and KFA (Germany). Microspheres ≤ 150 µm were prepared by mechanical stirring of sols in a solvent. For higher-diameter so
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