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In the pool type Fast Breeder Reactors the roof structure is penetrated by a number of pumps and heat exchangers which are cylindrical in shape. Argon gas in reactor is sandwiched between the free surface of sodium and the roof structure and can flow in the annular space between the components and roof structure forming a thermosyphon. These hermosyphons not only transport heat from sodium to roof structure, but also result in cellular convection in vertical annuli resulting in circumferential temperature asymmetry of the penetrating component. There is need to know the temperature asymmetry as it can cause tilting of the components. Experiments were carried out in an annulus model to predict the circumferential temperature difference. Three-dimensional CFD analysis was carried out using COMSOL 3.4 and PHOENICS 3.2 and the results compared with the experimental data. This paper describes the experimental details and the theoretical analysis carried out in COMSOL and PHOENICS.