Stoichiometric Effects on Performance of High-Temperature Gas-Cooled Reactor Fuels from the U-C-O System

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

“…The controlled crystal growth step involved using hexamethylenetetramine (HMTA)-urea stock solutions that were boiled for 60 min or less. Before this discovery, all the other crystal growth steps (when utilized) could reduce the tap density to only ~1.3 g/cm 3 , which was not sufficiently low for use in ideal pellet pressing. The use of the boiled HMTA-urea solution allowed the tap density to be lowered to 0.93 g/cm 3 , with the ideal density being about 1.0 g/cm 3 .…”

“…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.…”

“…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. In this work it was discovered that the uranium crystallinity controlled the tap density or porosity of the microspheres.…”

“…In this work it was discovered that the uranium crystallinity controlled the tap density or porosity of the microspheres. By optimizing the above-mentioned crystal growth parameters, it was possible to decrease the tap density of microspheres from 2.7 to 1.27 g/cm 3 . However, this density was not low enough for use in ideal pellet pressing.…”

“…It was found that boiling HMTA-urea stock solutions for certain periods of time could lower the tap density of the microspheres to 0.93 g/cm 3 . Furthermore, the boiling process could be controlled so that stock solutions could be prepared and used to obtain any targeted tap density between 1.27 and 0.93 g/cm 3 . Microspheres with tap densities <1.0 g/cm 3 proved to be ideal for the DIPRES process.…”