Crystal habit and phase attribution of U(VI) oxides in a gelation process

Production of Depleted UO₂Kernels for the Advanced Gas-Cooled Reactor Program for Use in TRISO Coating Development

“…1 Since the late 1970s, researchers at ORNL have studied and fully developed the internal gelation process for making UO 2 , (U,Pu)O 2 , ThO 2 , and (UO 2 + UC 2 ) microspherical fuels. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The internal gelation process used is one of the sol-gel processes developed for the preparation of microspheres of nuclear fuel in which chilled clear broth droplets containing acid-deficient uranyl nitrate (ADUN), hexamethylenetetramine (HMTA), and urea are heated causing homogenous gelation and solidification of the droplets that, after washing treatments, can be dried calcined, and sintered to ceramic kernels of the required density. In the 1990s, ORNL researchers extended the boundaries of the technology by developing the process to make hydrous metal oxide spheres of Ti, Zr, Fe, and other cations for such uses as sorbents, waste forms, catalysts, getters, and dielectrics.…”

“…In a study by M. H. Lloyd et al, 2 transmission electron microscopy and X-ray diffraction analyses showed that the variations in the gel-forming properties yielded gel spheres with a wide range of crystallite structures and compositions. The uranium phases consisted of thin platelets of diameters varying from ~100 to 5000 Å.…”

Control of Urania Crystallite Size by HMTA-Urea Reactions in the Internal Gelation Process for Preparing (U, Pu)O₂Fuel Kernels

“…1 During the late 1970s and throughout the 1980s, researchers at ORNL studied and developed the internal gelation process for making UO 2 , (U,Pu)O 2 , ThO 2 , and (UO 2 + UC 2 ) microspherical fuels. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] More recently, as a task of the Advanced Gas-Cooled Reactor (AGR) Program at ORNL, about 2 kg of UO 2 kernels with diameters of 500 ± 20 µm and 3.5 kg of UO 2 kernels with diameters of 350 ± 10 µm were prepared for use in the triisotropic (TRISO) coating development. 17 The internal gelation process used is one of the sol-gel processes developed for the preparation of microspheres of nuclear fuel in which chilled clear broth droplets containing acid-deficient metal nitrate (ADMN), hexamethylenetetramine (HMTA), and urea are heated, causing homogenous gelation and solidification of the droplets.…”

“…In the development of the internal gelation process, a number of process parameters have been determined that serve as useful tools in controlling the quality of the kernels produced. 2,8,[10][11][12]17 Understanding the crystal morphology of uranium is essential for the preparation of UO 2 , (U,Pu)O 2 , and (UO 2 + UC 2 ) kernels. The crystal morphology of uranium and the concomitant microsphere properties are significantly affected by (1) the HMTA/U mole ratio, (2) the NO 3 − /U mole ratio, (3) the concentration of uranium in the broth, and (4) the gel-forming temperature.…”

“…All of these process parameters were extensively studied, especially in the development of (U,Pu)O 2 kernels for the DIPRES (DIrect PREss Spheroidized) process. 2,8,[10][11][12]17 The primary goal of the DIPRES process was to prepare highly porous, low-density calcined microspheres that could be used as direct-press feed for Fast Breeder Reactor pellet fabrication. Ideal pellets were ones with near-theoretical density (11.08 g/cm 3 ) whose matrices showed little or no evidence of sphere remnants upon examination.…”