The reaction is;
Co-59 + n → Co-60 (T½ = 5,7 years) → β + Ni-60 + 2γ
The radioactive Cobalt-60 can be spread over large areas and contaminate the surrounding things, plants, humans and animals. As little as 1 gram Co-60 can contaminate one squarekilometer, so it will be uninhabetable. The Cobalt will waporize in a nuclear explosion, then condense to solid form, in form of dust.
The cobalt bomb occurs in at least two movies, the most famous is;
"Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb", the other is "On the Beach". It's also named in the James Bond-movie Goldfinger, where the bad guy has plans a little, but dirty bomb in Fort Knox to contaminate the Gold, with "Cobalt and Iodine".
Newly manufactured Co-60 has an activity at 10 Sv at a hypotetical detonation fallout. Lethal dose is in 30 minutes is about 5 Sv. It takes 142 years (27 half-lives) to the Cobalt (wich has decade to Nickel) to be safe.
There is no evidence that a country has manufactured cobalt bombs. In a hypotetical bomb, for examle, the tamper can be made of Co-59.