Nuke in a Volcano
Some of the erupting material will be lava, but that isn’t the dangerous bit. The deadliness comes from the plumes of ash. For example, the Icelandic volcano Eyjafjallajökull virtually brought global air travel to a standstill with plumes of fine dust. In this case, the dust traveled all the way to Italy, Russia and towards the East Coast of America. In our nuclear bomb example, this dust would also be radioactive. And the power of the chain reaction in the bomb would push more ash into the environment for a longer period of time, causing long-term contamination hazards.