Many advanced nuclear reactors adopt methodologies of passive safety systems based on natural forces such as gravity. In one of such system, the decay heat generated from a reactor is removed by isolation condenser (ICs) submerged in a large water pool called the Gravity Driven Water Pool (GDWP). The objective of the present study was to design an IC for the passive decay heat removal system (PDHRS) for advanced nuclear reactor. First, the effect of inclination of IC tube on three dimensional temperature and flow fields was investigated inside a pilot scale (10 L) GDWP. Further, the knowledge of these fields has been used for the quantification of heat transfer and thermal stratification phenomenon. In a next step, the knowledge gained from the pilot scale GDWP has been extended to design an IC for real size GDWP (~10,000 m3). Single phase CFD simulation using open source CFD code [OpenFOAM-2.2] was performed for different tube inclination angles (α) (w.r.t. to vertical direction) in the range 0° ≤ α ≤ 90°. The results indicate that the heat transfer coefficient increases with increase in tube inclination angle. The heat transfer was found to be maximum for α = 90° and minimum for α = 15°. This behavior is due to the interaction between the primary flow (due to pressure gradient) and secondary flow (due to buoyancy force). The primary flow enhanced the fluid sliding motion at the tube top whereas the secondary flow resulted in enhancement in fluid motion along the circumference of tube. As the angle of inclination (α) of the tube was increased, the secondary flow became dominant and resulted into enhanced heat transfer near the tube bottom. Three different heat transfer regimes were identified in the transient period: conduction (0 < t < 0.4 s), quasi-steady (0.4 s < t < 10 s) and fluctuating period (10 s < t < 100 s). The effect of inclination angle (α) was more pronounced in the fluctuating period. The natural convection and heat transfer in the regime of laminar-turbulent transition was studied in the presence of longitudinal vortices. The heat transfer was enhanced in the transition region due to vortices formation and it was reduced in the turbulent regime due to decay of vortices.