An experimental study of natural convection heat transfer in a one-twelfth scaled model of a sealed storage cask of the Integral Monitored Retrievable Storage facility containing 12 canisters is performed. A uniform power dissipation per unit length is supplied to each canister (heater rod), and the cask (outer cylinder) is maintained at a constant temperature. Correlations representing the convective Nusselt number as a function of the Rayleigh number for each rod as well as one for the rod bundle as a whole are reported. Two outer cylinders in a vertical position with exactly the same dimensions, one with internal fins and the other without the fins, are used in these experiments. Comparisons of the heat transfer data obtained for these cylinders show that the conduction regime terminates at a much lower Rayleigh number in the case of the cylinder without internal fins. Moreover, the critical Rayleigh number (onset of convection) obtained for this case is in good agreement with the one predicted by a general correlation for square array rod bundles as reported previously. A significant result of the present study is that, once the rod bundles reach their respective convective flow regime, the heat transfer can be predicted with a single correlation. Using a recommended length scale, the correlation predicts the convective Nusselt number for all the available rod bundle data (present and previously reported results) with an average deviation of 7.2%.