The accomplishment of the objectives of achieving a high safety level and reducing the cost through the reliance on Natural Circulation (NC) mechanisms requires a methodical understanding of those mechanisms and more particularly the instability phenomena. Instabilities occur generally due to external and internal disturbances. External disturbances include fluctuations in mass flow rate, fluctuations in inlet enthalpy, fluctuations in heat supply rate, etc. Internal disturbances include generally flow regime transitions. This requires specification of stable operating procedures, which in turn requires deeper understanding of the parametric trends. Therefore, the level of knowledge for the thermal-hydraulic phenomena for the specific geometric conditions and governing heat transfer conditions should be deeper when NC is involved.