Active cooling systems are vulnerable to component failures or loss of electrical power. Consequently, to increase the reliability of the system as a whole, adequate redundancy for these components as well as electrical back-up power sources must be provided. The Fukushima Daiichi accident however demonstrates that, despite redundancy and alternative power sources, an extended loss of cooling can occur. The present paper presents a conceptual design of a passive cooling system with a power removal capacity from 1 kW up to 1 MW. This system, constituted by different fluid loops, can provide adequate confinement in cases it is necessary. The proposed design is modeled with the FlowMaster simulation tool providing both steady state and dynamic responses. The results show the relationship between the system constraints (heat load and temperature difference between hot and cold source) and the design parameters of the system. A valuable application of passive systems is spent fuel storage cooling where adequate heat removal must be provided to the fuel elements at all time and confinement is of outmost importance. The proposed approach is applied to the design of the hot cell cooling in which the spent fuel elements of the future MYRRHA facility are handled.