Abstract
In the accident of Fukushima Daiichi Nuclear Power Station, various reactor structures melted and formed fuel debris, which accumulated in the primary containment vessel region. This fuel debris is considered to have a heterogeneous distribution of components at different locations. Understanding the distribution of Gd2O3, which may behave as potential neutron absorber materials in the fuel UO2, is especially important from the viewpoint of criticality safety for fuel retrieval. We conducted solidification tests of reactor materials to investigate component segregation and solidification behavior in fuel debris using a Cold Crucible Induction Heating technique at the Research Centre Řež (CVR) in Czech Republic. In particular, melted samples weighing more than 1 kg were melted using the furnace to reproduce macroscopic component segregation in fuel debris. After melting the sample at about 2500°C, the melt was gradually solidified by pulling the crucible out of the heating section to control the solidification direction. As for the simulated fuel debris, a mixture of core materials (UO2, ZrO2, Gd2O3), structural material (FeO), simulated fission products (MoO3, Nd2O3, SrO, RuO2), and concrete materials (SiO2, Al2O3, CaO), were used under four conditions. After the test, the distribution of each component was evaluated using an SEM-EDS analysis of the sample cross-section.
The sample solidified gradually from the bottom to the top with cooling rate of approx. 13 ℃ / min in two samples and 3-4 ℃ / min in other two. and a shrinkage porosity was observed in the upper part of the predicted final solidification zone. Initial solidified areas show fine microstructure. Gd2O3 was concentrated in the periphery of the sample, suggesting that it may segregate in the early solidification region. On the other hand, Fe was concentrated in the central region, i.e., in the late solidification region. Fe is considered to be concentrated in the late solidification position because, thermodynamically, it is distributed to the liquid phase during the solidification process. No significant segregation was observed in the simulated fission products. EDS analysis might not have detected he simulated fission products due to the small amount added and evaporation during heating. In the sample containing concrete, the concrete component formed a glass phase and separated from the fuel oxide phase, although there were differences in microstructure depending on the location of the sample
