A physical and chemical analysis of fast quenched particles of UO2 and ZrO2 mixture

An interaction between molten fuel of a nuclear reactor, which is called corium and mainly consisted of UO2 and ZrO2, and sub-cooled water may result in a steam explosion. It is one of the outstanding reactor safety issues. To investigate the fundamental mechanism behind the recent experimental observation that the composition of the material highly affected the strength of the steam explosion, a physical and chemical analysis for the fast quenched particles of UO2 and ZrO2 mixture at different compositions was performed. Six cases were selected for the study, in which the melt composition was changed, while other initial and boundary conditions of the molten fuel and water interaction tests were maintained the same. It was observed that the cases at eutectic composition resulted in a spontaneous steam explosion, while the cases at non-eutectic composition did not result in a spontaneous steam explosion. Electron probe microanalysis (EPMA) was performed for fast quenched particles along a cross-section. Results demonstrated that the UO2 and ZrO2 mixtures formed a solid solution of U1−xZrxO2. The mechanism for the hydrogen generation during the molten material and water interaction was examined by thermogravity analysis (TGA), X-ray diffraction (XRD) and hydrogen reduction analysis. It was demonstrated that the hydrogen generation was not directly related to the oxidation of UO2. Morphologies observed by scanning electron microscopy (SEM) indicated that the particles from the eutectic mixture had many holes, while the particles at non-eutectic mixture did not. The existence of mush phase for the non-eutectic mixture is suggested to be the reason for the non-explosive nature.