To facilitate the IVR (in-vessel retention) of molten core debris strategy at severe reactor accident

the capacity of ERVC (external reactor vessel cooling) is a key parameter

i.e. the CHF (critical heat flux) of the external reactor vessel should be higher than the related location heat flux. In this paper

an analysis model of CHF on the downward facing curved surface for pool boiling has been proposed

which adopts the Helmholtz instability analysis of vapor-liquid interface of the vapor jets which penetrating in the thin liquid film underneath the elongated bubble adhering to the lower head outer surface. When the heat flux closing to the CHF point

the vapor-liquid interface becomes highly distorted which resulted in obvious vapor blanket

it will block liquid to feed the thin liquid film underneath the vapor blanket from the bulk region. As a result

the thin liquid film will dry out gradually. As a consequence

the CHF occurs. Based on the aforementioned mechanism and the energy balance between the thin liquid film evaporation and water feeding

and taking the subcooling of the bulk water into account

the mathematic model about the downward facing curved surface CHF has been proposed. The CHF of the downward facing curved surface for pool boiling increases along with the downward facing orientation

and subcooling has significant effects on the CHF. Comparing the result of this model with the published experimental results

it shows good agreement under the both of saturated and subcooled boiling conditions.