[en] Small and medium sized integral type reactors for the diverse utilization of nuclear energy are getting much attention from the international nuclear community. They diversify the peaceful uses of nuclear energy in the areas of seawater desalination, district heating, industrial heat-generation process and ship propulsion. The SMART (System integrated Modular Advanced ReacTor) is a small modular integral type pressurized water reactor, which was developed for the dual purposes application of seawater desalination and small-scaled power generation in KOREA. The reactor is designed for a forced convection core cooling during start-up and normal operating conditions and for a natural circulation core cooling during accidental conditions. The main safety objective of the SMART is to increase the degree of inherent safety features by advanced designs such as a passive residual heat removal system (PRHRS). The passive residual heat removal system removes the core decay heat and sensible heat by a natural circulation in the case of emergency conditions. This study focuses on the flow behavior in the passive residual heat removal system of the integral reactor. The system necessitates a hydraulic head to achieve the required natural circulation flow rate, which in turn, may cause a larger two-phase pressure drop and flow oscillation. Also, it is of interest to investigate the complex effects of the boiling and condensation in such low frequency thermo-hydraulic oscillations. Thermal hydraulic analysis for the passive residual heat removal system has been carried out by means of the MARS code for a full range of reactor operating conditions. The MARS code has been developed at the Korea Atomic Energy Research Institute by consolidating and restructuring the RELAP5/MOD3.2 and COBRA-TF which has the capabilities of analyzing the one-dimensional or three-dimensional best estimated thermal-hydraulic system and the fuel responses of the light water reactor transients. A selected load to analyze the thermal hydraulic characteristics is 100% of nominal power for a forced convection and 4% for a natural convection conditions. The 4% power natural convection condition is achieved through reducing the primary mass flow, closing the main feedwater and steam isolation valves, and opening the PRHRS isolation valves. Parameters are investigated to find the effect of the mass flow on the passive residual heat removal system under natural convection conditions. The stability regimes are identified by the disturbance amplitude of the transient parameter. If the disturbance amplitude is less than ±3%, the regime is a stable regime, more than ±5% it is an unstable regime, and between ±3% and ±5% it is considered to be a transition regime. The mass flow behavior in the passive residual heat removal system is divided into four types depending on the fluid state in both the heat exchanger and the emergency cooldown tank. The disturbance amplitudes of the mass flow are more stable with a decreasing of the height between the steam generator and the heat exchanger, and increasing the hydraulic resistance. And the effect of the initial pressure and N₂ fraction in the compensating tank, and the valve actuation time is small or negligible.(author)