Effects of Urea and Cerium Addition in ZrO₂ Microspheres Characteristic as UO₂ Surrogate Material for High Temperature Gas- Cooled Reactor Fuel

Zirconia microsphere as a surrogate nuclear fuel material for high temperature reactor was successfully prepared through an external gelation process using ceria to stabilize the crystal on a high tetragonal phase composition. The obtained gel microspheres are prone to cracking phenomena during the early stage of the heat treatment after the aging-washing process. This study was conducted to address this issue by introducing urea to the pre-neutralized nitrate sol in the broth preparation stage to produce crack-less zirconia microspheres. Urea was introduced by varying its composition against the total metal mol. The effect of the stabilizer on the zirconia phase was investigated by various compositions of cerium. A comparative thermal analysis reveals urea's subsequent decomposition allowing crack-less microsphere production by various heating rates at 50 – 250 oC. Zirconia microspheres were then characterized using scanning electron microscopy (SEM) and X-ray diffractometry (XRD) to obtain surface morphology and crystal phase composition by semi-quantitative calculations. Electron microscopy of the kernel surface showed a decrease in the kernel cracking rate as the urea composition increased, significantly by the urea addition of 2.5 times the mol of metal. Based on semi-quantitative analysis of the sintered kernels' X-ray diffraction pattern, the mixture of zirconia crystal phases insignificantly changed by increasing the molar ratio of Ce/Zr, though approximately higher lattice constant and crystallite size of the desired tetragonal phase is obtained, and accordingly lead to lower microstrain. (authors)