Nanoscale visualization of crack tips inside molten corium-concrete interaction debris using 3D-FIB-SEM with multiphase positional misalignment correction.

Characterizing molten corium-concrete interaction (MCCI) fuel debris in Fukushima reactors is essential to develop efficient methods for its removal. To enhance the accuracy of microscopic observation and focused ion beam microsampling of MCCI fuel debris, we developed a 3D focused ion beam scanning electron microscopy technique with a multiphase positional misalignment correction method. This system automatically aligns voxel positions, corrects contrast and removes artifacts from a series of over 500 scanning electron microscopy images. The multiphase positional misalignment correction method, which focuses on time-modulated contrast, considerably reduces charge-up artifacts in glass phases, enabling 3D morphological observation and analytical transmission electron microscopy of crack tips in two types of MCCI debris at the 3D/nanoscale for the first time. In the Fe-ZrSiO4-based debris, metallic balls composed of Fe, Cr2O3 and ZrO2 with dimples on the surface of about 2-58 µm in diameter were observed at the crack tips. In the (Zr, U)SiO4-based debris, a core-shell structure composed of a (U, Zr)O2 core with a diameter of about 1-5 μm and a (Zr, U)SiO4 shell with a diameter of about 2-9 μm in complex MCCI fuel debris at the crack tips.