Transient 3D simulation for heating and melting process of PWR core after SBO

•To simulate the heating and melting progress of a reactor core after SBO accident, a numerical model has been established based on the 2D transient heat conduction of each rod, coupled considering residual heat of core, convective and radiation heat transfer, zirconium water reaction.•A transient 3D simulation code with high precision has been developed.•An accurate and complicated model of radiation heat transfer has been applied.•The detailed temperature contours of core during the melting process have been shown.