The environmental cost of Chernobyl is indeed often overblown. Nature in the exclusion zone is arguably off much better now than before!

The cost to humans living in affected areas was massive and high profile, but it’s very questionable if it was higher than that of an equivalent amount of coal-burning plants. Fortunately not a tradeoff we have to debate anymore, since there are renewables with much fewer downsides and externalities still.

Nuclear bombs (at least those being actually used) by design kill people, so I’m not sure what the externalities even are if the main utility is already to intentionally cause harm.