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.