Abstract This paper discusses the design and analysis of a passive decay heat removal system for a Molten Salt Reactor (MSR) of 450 MWth. Following the disaster at the Fukushima-1 nuclear power station, it is clear that the public will demand improved safety performance if nuclear power is to be accepted as a sustainable source of CO2-free energy. In this scope, thorium-based MSRs have very promising properties in the area of passive safety, resource availability and proliferation resistance. Molten Salt Reactor (MSR) systems can be equipped with an emergency salt drain tank. Under any severe accident, all the fuel salt can be drained by gravity into the drain tank, thus, the primary system can be safely emptied of fissile materials and fission products. The ultimate safety can be assured by the integrity of the fuel salt in the drain tank or in other words, the capability of residual heat removal from the fuel salt in the drain tank. From this point of view, we investigated the feasibility of a passive residual heat removal system for the drain tank of an MSR (FUJI-233Um of 450 MWth). We concluded that a system comprising a large drain tank and 60, large-diameter coolant tubes can withstand the thermal shock due to the hot fuel salt, and therefore we conclude that system is feasible. © 2013 Elsevier Ltd. All rights reserved.