An investigation on the physical and chemical behaviors of fuel debris during severe accident progression

Present paper investigates the potential configuration of post-accident fuel debris for a severe accident in a nuclear power plant. The physical and chemical behaviors of molten core material are investigated by experimental efforts. A series of melting and solidification experiments utilizing a cold crucible technique was performed using corium of 4.5-15 kg at various compositions. Considering that U/Zr atom ratio and Zr oxidation index could vary substantially during the accident progression, corium melts at compositions with U/Zr in the range of 0.94-1.14 and Zr oxidation index in the range of 36.3-100% were investigated. It was found that the solidified corium separated into a metal rich and an oxide rich layer in cases of partially oxidized corium. Depending on the compositions, metal rich layer was placed either on top or at the bottom of the oxide rich layer. Inductively Coupled Plasma-Atomic Emission Spectroscopy was used to estimate the chemical compositions of the layers and X-Ray Diffraction was used to find the crystal structure of the layers. In the second series of experiments, a molten corium of 13 kg in the form of liquid jet is poured into a pool of water. Melt water interaction resulted in fragmented and quenched particles with a range of size distributions. Scanning Electron Microscope and Electron Probe Micro-Analysis were used to analyze the physical and chemical behaviors of the particles. Partially oxidized corium melt resulted in a quenched particles with irregular shape. The composition of the particles varied among different particles and also within a particle. Fully oxidized melt resulted in quenched particles with homogenous composition among different particles and homogenous distribution of composition within a particle. The shape of these particles was spheroid in most of cases.