Passive safety is a primary motive behind the development of small and medium sized reactors of various coolants. After the Fukushima accident, there is an increased interest in a nuclear reactor's reliance on passive safety systems. Most of the existing passive systems, regardless of the reactor type, utilize buoyancy force to drive the cooling flow. Hence, it is essential to evaluate if the naturally developed cooling flow is sufficient to maintain the heated surface temperature of the fuel elements below the design limit. Evaluating passively driven flows can be quite a challenging task in both two phase natural circulation systems and also in single phase natural circulation systems. Previous research works have found that single phase heat transfer can be deteriorated and becomes uncertain when the driving force of a system is shifted from external force (forced convection) to self generated buoyancy force or a combination of both (natural or mixed convection). In this paper, single phase gas, water, and liquid metal reactors with passive systems are reviewed briefly. A simple theoretical analysis of each reactor type is performed to find the tendency of the shift in the operating heat transfer regime into the deteriorated region. The analysis results show that single phase water system can maintain operation within the forced convection regime but the operating regime gets closer to the deteriorating heat transfer regime as the system's physical size reduces from a large nuclear power plant to the small and medium reactor scale. The gas cooled system has a high tendency to operate in the deteriorated heat transfer regime when the driving force changes from forced to natural. Meanwhile the liquid metal system demonstrates more margins to operate outside from the deteriorated heat transfer region compared to the two other fluid systems. However further studies are needed to clearly identify the boundaries of the deteriorated heat transfer regime for each coolant since the deterioration greatly depends on the thermophysical properties variation of the coolant and the near-wall flow behavior of the coolant with respect to temperature change. (C) 2013 Elsevier B.V. All rights reserved.