Phase-field simulation of vapor pressure for micropore formation in pseudo-binary system, B₂O₃–UO₂

Abstract

We performed thermodynamic and phase-field (PF) simulations to investigate micropore formation mechanisms in a pseudo-binary system, B₂O₃–UO₂, during the solidification process. In the B₂O₃–UO₂, a solid UO₂ nucleation occurs in liquids below the melting point (T_m). As the temperature decreases further, a monotectic reaction (Liquid → S + Liquid#2) occurs below the monotectic temperature (T_mono ≈ 1845 K). Here, S and Liquid#2 denote the phases of solid UO₂ and second liquid containing low UO₂ concentration. This study calculated that the behaviors of the Liquid and S phases during the cooling process depended strongly on the cooling rates, leading to high vapor pressure of the liquid phase due to the low UO₂ concentration. The vapor pressure of liquid phase showed comparatively high values at high temperatures. In this PF simulation, the UO₂ concentration of the liquid phase decreased rapidly at 1838 K under a low cooling rate (β = 6 K/min). Therefore, the porosity was considered to increase by vaporization of the liquid. On the other hand, under a high cooling rate (β = 120 K/min), the porosity could be lower than under the low cooling rate condition because the UO₂ concentration decreased at lower temperature. The dependency between porosity and a cooling rate is consistent with Yusufu’s experimental results.