Fission reactor has a cladding encasing a heat generating source including a fissionable nuclear fuel composition. The heat generating source is offset from the surface of the cladding and molten metal is located within the void space formed by the offset. As a liquid, the molten metal will flow and occupy any contiguous network of void space within the fuel cavity and provides thermal transfer contact between the heat generating source and the cladding. The cladding separates the heat generating source and the molten metal from the primary coolant volume.