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The S-CO2 cooled pebble bed reactor (SCPBR) is a type of pebble bed reactors in which the supercritical carbon dioxide used as a coolant. In this study, the thermal-fluid phenomena in the SCPBR core, at different porosities (0.38, 0.4, and 0.42) and pebble diameters (3 cm, 8 cm and 12 cm) are numerically studied by developing a three-dimensional CFD model. ANSYS FLUENT 14.5, was used to carry out the numerical simulation. The results show that for a given pebble diameter, when the porosity is increased, the fluid and solid temperatures are almost the same, the pressure drop decreases, and the heat transfer coefficient increases. Also, it was found when the pebble diameter is increased, the fluid temperature is almost the same, the pressure drop and the heat transfer coefficient decrease, but the solid temperature increases. Keywords: CFD, Heat transfer coefficient, Porosity, Reactor, Pebble diameter