It is necessary to access the pool top area for loading and unloading irradiation test pieces by a required irradiation period during a normal operation of an open-pool-type research reactor installed on the bottom of reactor pool with a depth of about 13m. However, when the reactor pool top radiation level exceeds the limit of radiation level by the rising of reactor coolant contaminated by radioactivity due to a natural convection of the pool water, access to the reactor pool top area is denied. In the case of HANARO, a hot-water layer (HWL, hereinafter) is maintained below a depth of 1.2m from the top of the reactor pool in order to reduce the radiation level of the reactor pool top area by suppressing natural convection of the pool water.

After normal operation of the HWL, the pool top radiation level is safely maintained below the limit of the pool top radiation level. For studying the characteristics of the HWL under downward flow pattern of the reactor coolant (hereafter as DFP), the heat loss of the HWL is calculated based on the model for HANARO HWL. The heat loss characteristics of the HWL were reviewed by reactor power level increment.

This paper suggests a HWL heat loss relation formula by an increment of the reactor power level. It was confirmed that the total heat loss of the HWL under DFP of reactor coolant linearly increased larger than that of upward flow pattern (hereafter, UFP). The reason is that the bottom convection loss of DFP increased 4.5 times than that of UFP by the 10 times increment of the core bypass flow rate under DFP than that under the UFP.

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