Abstract: In-vessel retention（IVR） of molten core debris via water cooling at the external surface of the reactor vessel is an important severe accident management feature of advanced passive plants. For this concept, it is important to keep the heat load on the vessel wall surface lower than the critical heat flux at any position of the lower head. Based on the existing bubble crowding model in the literatures, a theoretical CHF model is developed suitable for the inclined heater surface in low pressure. In the new model, the effect of orientation on bubble velocity and bubbly layer thickness is taken into consideration. A new method is introduced to calculate the steam quality of both layers. Comparison with the present experimental data shows that the new model satisfies the prediction accuracy. The maximum deviation between the model prediction and experimental data is less than 10%.