Yes and no. Not all gas-cooled reactors can work completely through convection cooling without melting, some do need fans even to just remove decay heat. Also,

Yes and no. Not all gas-cooled reactors can work completely through convection cooling without melting, some do need fans even to just remove decay heat. Also, reactors containing graphite (as many pebble-bed reactors will) do need a oxygen-free atmosphere, otherwise the graphite will ignite. So only convection of the proper uncontaminated protective atmosphere will work.