Cellular convection of argon gas that takes place in narrow annuli of component penetrations in FBR top shield has been investigated. The 3-D CFD simulations have been validated against in-house experimental data. The circumferential temperature difference in the shells, which is a result of cellular convection is given special attention. Cellular convection is seen to be highly sensitive to geometric and thermal conditions. For Rayleigh numbers (Ra) less than ∼300, cellular convection effect is seen to be very weak. For Ra exceeding ∼3 × 10 8, natural convection is vigorous with insignificant circumferential temperature variations. In the intermediate range of Rayleigh numbers, cellular convection assumes great significance. The maximum circumferential temperature difference in the component shell is seen to occur at Ra ∼ 3 × 10 5, which corresponds to a gap width of 50 mm. The number of convective cells increases with decrease in gap width as well as increase in forced cooling. The circumferential temperature difference is found to be directly proportional to annulus height, suggesting significant incentives in adopting shorter penetration heights. However, the circumferential temperature difference is found to be a weak function of annulus diameter. The number of cells is unfavorably less in an eccentric annulus compared to a concentric annulus. Appropriate correlations have been proposed to determine the circumferential temperature difference and number of convective cells to serve as a guideline for thermal design of top shield of future FBR. © 2012 Elsevier B.V.