Use of boiled hexamethylenetetramine and urea to increase the porosity of cerium dioxide microspheres formed in the internal gelation process

Cerium dioxide (CeO2) is a commonly used simulant for plutonium dioxide and for plutonium (Pu) in uranium (U) and Pu oxide [(U, Pu)O2] mixtures used in nuclear fuel development. This effort developed CeO2 microspheres with a various porosities and diameters. The internal gelation technique has only been used to produce CeO2 microspheres with limited initial porosity. Previous studies have shown that the crystallite size and porosity of mixed U and thorium oxide microspheres and the (U, Pu)O2 microspheres from the internal gelation process increased when an equal molar solution of hexamethylenetetramine (HMTA) and urea is gently boiled for 1 h prior to its use in the gelation process. In this study with cerium, the combination of ammonium cerium nitrate and 1-h boiled HMTA-urea failed to produce a stable feed broth. However, when the 1-h heated HMTA-urea was combined with unheated HMTA-urea in 1-3 vol ratio or the boiling time of the HMTA-urea was reduced to 15-20 min, a stable solution of HMTA, urea, and Ce was formed at 273 K. This new Ce solution produced very porous CeO2 microspheres, which are suitable simulants for uranium microspheres.