Decay heat removal is a key safety and design issue for the Generation IV gas (helium)-cooled fast reactor. This paper investigates the natural convection capability of the dedicated DHR loops under depressurized conditions while injecting a heavy gas into the system. Investigated is a loss-of-coolant accident using the TRACE code. The goal of the study is to improve fuel/cladding temperature behavior during LOCA transients with the enhancement of passive safety by operation in natural convection only, while accepting 10 bar back-up pressure in the guard containment. The paper investigates the cooling capabilities of different heavy gases. Furthermore, different injection locations and mass flow rates have been tested, in order to address possible core-overcooling problems resulting from rapid depressurization of the gas reservoir. It has been shown that, among the gases investigated, CO2 is the best choice from the thermal-hydraulics viewpoint, being able to cool the core satisfactorily for a broad range of injection rates. N-2 can be envisaged as an alternative solution in case of chemical problems with CO2. Supplementary studies carried out for the CO2 and N-2 injection cases include that of the sensitivity to the number of available DHR loops and to the LOCA break-size. The effect of the resulting neutron spectrum changes on the shutdown-reactivity margin has also been investigated. (C) 2010 Elsevier B.V. All rights reserved.