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During an open-tank-in-pool type research reactor operation, the primary coolant was designed to remove the
fission reaction heat of the reactor. When the primary coolant flew upward or downward the core by the
driving force of the primary coolant pump to cool the heat generated by the reactor, a part of the primary
coolant which did not pass the core reached the top of the reactor pool by a natural convection. And it
increased the radiation level at the top of the pool. To protect a natural convection of the core bypass flow,
a hot water layer (hereafter as HWL) was installed in the top of the pool. To maintain the HWL, the heat
loss is calculated and evaluated based on the HANARO HWL calculation model as per the reactor power
increment. It was confirmed through this study that it is available to find a relation formula for predicting a
total heat loss for each flow condition as per the reactor power increment.