A structure for the storage of core assemblies (12) from a liquid metal-cooled nuclear reactor. The structure comprises an enclosed housing having a substantially flat horizontal top plate (48), a bottom plate (50) and substantially vertical wall members (52) extending therebetween. A plurality of thimble members (54) extend downwardly through the top plate. Each thimble member is closed at its bottom end and has an open end adjacent said top plate. Each thimble member has a length and diameter greater than that of the core assembly (64) to be stored therein. The housing is provided with an inlet duct (56) for the admission of cooling air and an exhaust (58) duct for the discharge of air therefrom, such that when hot core assemblies are placed in the thimbles, the heat generated will by convection cause air to flow from the inlet duct around the thimbles and out the exhaust duct maintaining the core assemblies at a safe temperature without the necessity of auxiliary powered cooling equipment.