Molecular dynamics studies for oxidation reaction of Zr metals in high-temperature and high-pressure steam condition

Machida, Masahiko  ; Nakamura, Hiroki  ; Srinivasan, S. G.*; Van Duin, A. C. T.*

Zircalloy has been widely employed as an excellent material covering the fuel rod. The mechanical and thermal properties have been explored by various experiments. In terms of its use as the fuel cladding, the oxidation reaction is an important topic when exposed to high temperature and high pressure steam in severe accidents. Especially, the hydrogen nucleation accompanied by the oxidation is critical because it leads to the crisis of the hydrogen explosion as observed in the Fukushima Nuclear Power Plant accidents. Therefore, in this study, we simulate the oxidation reaction in atomistic level by using ReaxFF simulating the chemical reaction in addition to usual molecular dynamics and clarify how the surrounding situation as the temperature and steam pressure accelerates the oxidation reaction and resultant hydrogen nucleation. The advantage using ReaxFF is to directly trace the oxygen diffusion inside Zircalloy depending on the temperature and vapour pressure together with the oxidation reaction. The calculation results reveal that the oxygen diffusion from the top surface to internal area occurs and nucleated hydrogen molecules move out when increasing the temperature above a certain temperature and pressure.