[en] The steady state and stability behaviour of a closed natural circulation loop was studied both experimentally and analytically. The experiments were conducted at low steam qualities. Analysis was carried out to predict the steady state and the threshold of instability for various operating conditions. Stability maps under different conditions have been provided. The steady state experimental results have been found to be in good agreement with the analytical predictions. The effect of various two-phase friction factor multipliers and void fraction correlations on steady state mass flow rate is discussed. The various instabilities, which may occur during start-up of a boiling natural circulation loop, were investigated. Type 1 density wave instability was observed in the loop at low pressures and at low powers. Onset of instability and its transition to Type 1 density wave instability have been investigated. (author)