Abstract
In the Fukushima Daiichi Nuclear Power Plant, a meltthrough severe accident happened in 2011. The fuel debris of various compositions, such as the core elements, the control rods, and the cladding tube, was solidified at the pressure vessel. Yusufu et al. made the simulated fuel debris of B2O3-UO2 and investigated the relationship between the microstructure and the mechanical properties of the fractural strength. It revealed that the simulated fuel debris was a porous medium, which depended on the cooling rates. The B2O3-UO2 debris at high cooling rates showed low fracture strength, compared with that at the low cooling rates due to the porosity density.
In this study, we have performed the phase-field and multiphase-field simulations in the pseudo-binary B2O3-UO2 system. The phase-field simulation focused on the formation process of the liquid-solid structure at a wide range of temperatures and concentrations. This model implicitly treated monotectic reaction using the Gibbs free energy. Randomly nucleated UO2 particles grew up and combined with each other, depending on the cooling rate. The cooling rates affected the concentration at the liquid phase and the growth rates of UO2 particles. On the other hand, the multiphase-field simulation focused on the monotectic reaction around the monotectic temperature ( Tmono ~ 1841K). The monotectic reaction is L#2 →L#1 FCC : “L#1” and “L#2”are liquid phases, “FCC” is solid phase of UO2. We investigated the formation of the solid-liquid microstructures (“FCC” and “L#1”) by the monotectic reaction. This model explicitly treated the reaction. After the formation, the liquid phase of “L#1” could vaporized because of high B2O3 concentration (x 0.96).
