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Natural circulation flow capability for removing decay core power has been demonstrated, and several studies have focused on taking advantage of this fact. This work studies the sequence of events that occur during a station blackout accident, in which natural circulation is the dominant flow pattern in the primary system. To this end, the Test A1.1 carried out in ATLAS facility is analyzed and a TRACE5 model is developed paying special attention on the modeling of heat losses. This phenomenon is very influential in the flow capacity and this is demonstrated through the correlation G~(Q-qloss) m between the net power Q-qloss and the mass flow G, that has been established from simulations under steady-state conditions. The Test A1.1 reproduction shows the TRACE5 code adequacy to investigate natural circulation phenomena, which are difficult to control in a facility. In addition, the heat loss modeling technique using constant heat transfer coefficients is substantiated.

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