Magnetic Resonance as a Structural Probe of a Uranium (Vi) Sol-Gel Process
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MAGNETIC RESONANCE AS A STRUCTURAL PROBE OF A URANIUM (VI) SOL-GEL PROCESS Charles M. King*, R. Bruce King**, A. Ronald Garber***, Major C. Thompson* and Bruce R. Buchanan* *Westinghouse Savannah River Company, Aiken, SC **Department of Chemistry, University of Georgia, Athens, GA ***Department of Chemistry, University of South Carolina, Columbia, SC ABSTRACT Nuclear Magnetic Resonance (NMR) investigations on the Oak Ridge National Laboratory process for sol-gel synthesis of microspherical nuclear fuel (UO 2), has been extremely useful in sorting out the chemical mechanism in the sol-gel steps. 13 C, 15 N, and 1H NMR studies on the HMTA gelation agent (hexamethylenetetramine, C6 H12 N4) has revealed near quantitative stability of this adamantane-like compound in the sol-gel process, contrary to its historical role as an ammonia source for gelation from the worldwide technical literature. 170 NMR of uranyl (UO 2++) hydrolysis fragments produced in colloidal sols has revealed the selective formation of a uranyl trimer, [(U02) 3 (ji 3 -O)(9t2 -OH) 3]+, induced by basic hydrolysis with the HMTA gelation agent. Spectroscopic results will be presented to illustrate that trimer condensation occurs during sol-gel processing leading to layered polyanionic hydrous uranium oxides in which HMTAH+ is occluded as an "intercalation" cation. Subsequent sol-gel processing of microspheres by ammonia washing results in in-situ ion exchange and formation of a layered hydrous ammonium uranate with a proposed structural formula of (NH4) 2 [(U0 2 )8 04 (OH)10]- 81120. This compound is the precursor to sintered U0 2 ceramic fuel. INTRODUCTION Beginning in the late 1950s, sol-gel processes were developed for the preparation of nuclear reactor fuels of U, Th, and Pu in the form of microspheres required for high-temperature, gascooled reactors. These fuels are commonly coated with pyrolytic carbon or other ceramics to serve as "pressure vessels" to contain fission products. The sol-gel processes for nuclear fuel were developed at Oak Ridge National Laboratory (ORNL) [1-3] and were based on the gelation of colloidal sols by "internal" gelation methods previously developed for nuclear fuel synthesis in the Netherlands [4]. In this process, hexamethylene tetramine (HMTA) is mixed at about 0°C, in the presence of urea, with a uranyl nitrate solution causing hydrolysis of the uranyl cation to a hydrous uranium oxide network. HMTA functions as a weak base for hydrolysis and is also assumed to decompose to ammonia and formaldehyde. Ammonia is assumed to be produced in-situ or "internally" during the gelation process and is assumed to be the key component causing gelation of the hydrous uranium oxide. All of these steps are conducted in equipment (i.e., cannulae) to form spherical drops [5] which are converted to gelatinous microspheres by suspension in a hot organic liquid (i.e., trichloroethylene). The hydrous uranium oxide in microspherical form is eventually converted to ceramic U0 2 by sintering. EXPERIMENTAL The rate of hydrolysis of HMTA was
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