Abstract: PHOENICS 3 2, a three dimension CFD code, is used to simulate the heat transfer and flow in the bypass and the upper plenum of the core of 200 MW nuclear heating reactor (NHR 200). In order to guide the convective flow, several schemes with and without the baffle are calculated, and the impact of the mass flowrate of the core bypass is considered. The effect of natural convection on the calculated region is so small that it can't change the direction of main flow. The important flows in the calculated region except the main flow are a recirculating flow which occurs in the bottom of the bypass of core due to the change of flow area and another big recirculating flow which occurs in the upper plenum due to the change of flow area and natural convection. Laying the baffle on the top of spent fuel zone can suppress the impact of the recirculating flow on the upper plenum and decrease the change of the temperature on the bypass of core.