Numerical investigation on the CHF model and prediction for the external cooling channel of lower head

External cooling of the lower head can reduce the possibility of the damage to the pressure vessel in serious accidents. However, when the core melt heats the lower head with a high heat flux subcooled boiling under natural circulation may cause bubble accumulation, which results in the deterioration of heat transfer or even radioactive leakage. This phenomenon is essentially the critical heat flux (CHF) on a curved surface with a downward heating boundary. The estimation of this phenomenon is indispensable for reactor safety design. In this paper, improvement in the nucleation density model exhibits higher resulting calculation performance for CHF with new geometries or working conditions. The CHF values and CHF distribution along the ellipsoidal and spherical structures are obtained, and detailed comparison between these two structures is made. The characteristics of elliptical cooling channel prove completely different from those of spherical structure, which requires more experimental studies.