[en] The phenomena of steam venting and critical heat flux for natural convection boiling on the outer surface of a heated hemispherical vessel surrounded by a thermal insulation structure were investigated experimentally. The objectives were to observe the behavior of the boiling-induced two-phase motion in the annular gap and to determine the flow effect on the critical heat flux. High-speed photographic records revealed the presence of violent cyclic ejection of the vapor masses generated by boiling on the vessel outer surface which resulted in a buoyancy-driven, upward, co-current two-phase flow through the channel. When boiling was taking place at high heat flux levels, the flow through the minimum gap was found to be highly unsteady and chaotic. Measurements of the local boiling heat fluxes and the local wall superheats were made under steady-state boiling conditions covering the entire range of nucleate boiling up to the local critical heat flux. (authors)