[en] A theoretical and experimental study of a natural circulation loop relevant to PWRs is presented. The analysis is based on a one-dimensional model in which the continuity momentum and energy equations are given and solved. The steady-state and transient flow rates and temperatures are derived. The apparatus consisted of an electrically-heated section and two parallel loops with heat exchangers. Both steady-state and transient experiments were conducted to study the effects of the core flow resistance, the power distribution, and the upper plenum design. Flow oscillations were observed under certain conditions, which were accompanied by instabilities and flow reversals. Reasonable agreement (+-30%) is obtained between the analytical and experimental results and with available plant data