A proposed formation mechanism of the Type-A radiocaesium-bearing microparticles released from Units 2/3 during the Fukushima Daiichi Nuclear Power Plant accident

A large amount of radiocaesium-bearing microparticles (CsMPs) were released into surrounding environment during the Fukushima Daiichi Nuclear Power Plant accident. To clarify the formation mechanism of the Type-A CsMPs, which were released from Units 2/3 and have spherical or ellipsoidal morphologies, oxidation behavior of 304 stainless steel containing 1 wt.% Si at 1200 ºC in steam atmosphere was investigated in this work. Both Fe₂SiO₄ and Ni-Fe-Cr phases were formed and distributed in the porous oxide scale. With the progress of oxidation, Fe₂SiO₄ and Ni-Fe-Cr phases were oxidized, causing severe spallation of the oxide scale somewhere between 90 min and 120 min. Trace amounts of Al and Ti were detected in the silica microparticles transformed from Fe₂SiO₄ oxidation. Furthermore, thermodynamic calculations were performed with the aid of FactSage software, revealing that: (1) when severe spallation of the oxide scale occurs, high levels of Fe oxides can stably exist in silica-based microparticles distributed in the oxide scale; (2) the Type-A CsMPs may be formed in reducing atmospheres. Based on the experimental and thermodynamic results, a completely new formation mechanism of the Type-A CsMPs is proposed. Silicate matrix is inherited from partially oxidized FeO-bearing silica-based microparticles, which are released from the oxide scale due to spallation. Moreover, diffusion of volatile constituents into FeO-bearing silica-based microparticles occurs in the reactor pressure vessel (RPV), not out of the RPV. This new formation mechanism can well explain many characteristics of the Type-A CsMPs.