Heat Transfer Mechanism by Natural Circulation for Cooling Material in Nuclear Reactors as a Passive Safety System

: Natural circulation in nuclear reactor cooling systems can be modeled based on computational fluid dynamics (CFD). Modeling is done to study the fluid flow in a closed loop system that occurs due to differences in fluid density. The closed loop system model is fitted with a heater and cooler on the opposite side. Because the density of the fluid depends on the temperature, then by adjusting the temperature difference between the heater and the cooler it can produce a fluid flow that occurs naturally. The initial condition in this study uses water as a working fluid with flow properties that are laminar and incompressible. Variations in temperature differences between heaters and coolers are done to get the temperature distribution and fluid flow velocity. The model is built for time-dependent conditions so that the time needed to transfer heat in a closed loop system can be known. Variations in temperature differences between heaters and coolers are carried out until the maximum conditions of water temperature to remain in the liquid phase. For this condition the maximum temperature is set to 80  C. This research was also developed by using several other types of fluids to determine the effect of density on fluid flow velocity. Other fluids used are gasoline, liquid helium, liquid sodium, and liquid mercury. The height of the closed loop system at the beginning of this study was used by three meters which then varied for heights of up to five and eight meters. Model testing is also carried out for working temperatures with differences between heaters and coolers above 80  C.