Segregation behavior of Fe and Gd in molten corium during solidification progress

Abstract

For a criticality assessment of the fuel debris generated by the Fukushima–Daiichi Nuclear Power Plant accident, knowing the segregation of neutron absorber materials, i.e., Gd, B, and Fe, in the fuel debris is necessary. Although B may mostly evaporate during melting, Fe and Gd are expected to remain in the molten corium. To understand the redistribution behavior of Gd and Fe during the solidification of the molten corium, solidification experiments with simulated corium (containing UO2, ZrO2, FeO, and Gd2O3 with a small amount of simulated fission products such as MoO3, Nd2O3, SrO, and RuO2) were performed using a cold crucible induction heating method. The simulated corium was slowly cooled from 2500 °C and solidified from the bottom to the top of the melt. An elemental analysis of the solidified material showed that the Fe-concentration in the inner region increased up to approximately 3.4 times that in the bottom region. This suggested that FeO might be concentrated in the residual melt and that, consequently, the concentration of Fe increased in the later solidification region. On the contrary, the Gd concentration in the periphery region was found to be approximately 2.0 times higher than that in the inner region, suggesting the segregation of Gd in the early solidified phase and to remain with the residual, UO2-rich phase even after oxidative losses. No significant segregation was observed for the simulated fission products.