A theoretical model is developed to predict the critical heat flux (CHF) based on the description of the hydrodynamic behavior of the vapor-liquid interface of a bubble at the heater surface leading to the initiation of CHF condition under saturated boiling. The CHF model considering heater size and gap size effects is developed to account for surface orientation effect, gap size effect, pressure effect, and contact angle effect for downward facing surfaces and gaps with orientations of 0 degrees (horizontal downward facing position) to 90 degrees (vertical position). The CHF in pool boiling and gap boiling was well predicted by the model for different effects. And the CHF model was extended to predict the CHF of the reactor-scaling hemispherical surface. The results indicated that the CHF model could also well predict the experimental data on large scale hemispherical surface. The present work is instructive for the safety analysis of the lower head of PWR in case of core meltdown during severe accident. (c) 2012 Elsevier Ltd. All rights reserved.