I'm not aware of the US or Russia or China or Britain or France having Cobalt in their nuclear weapons...
And no one else has any quantity of nukes.
The US and Russia no longer have any Fusion weapons (Hydrogen bombs) and the rest never made any....
You never heard of .... the Doomsday Machine? The technology required is easily within the means of even the smallest nuclear power -- it requires only the will to do so.