A 'failed' nuclear detonation known as a 'fizzle' can yield relatively small blast but still be very deadly due to radiation emitted. https://en.wikipedia.org/wiki/Fizzle_(nuclear_explosion)
Of course fallout, especially from explosions on the ground to destroy hardened bunkers, silos, and sub pens can generate terrifying levels of fallout. It's just the prompt radiation from the explosion itself that usually isn't a concern.