Nukes Destroy Magic
"I don't understand, master," To'Ril complained, "Why is the knowledge of True Alchemy forbidden? Now that I understand this 'atomic theory', it seems so simple. Split off one proton and two neutrons from each mercury atom, and one has a gold atom."
"I will show you why," Krin sighed at her apprentice, "他の場所にいる... Teleport."
To'Ril blinked as he always did after teleportation. Then he blinked again. Before him spread out an empty wasteland. Only scrub grass and small twisted trees. It was silent, perverse.
"This was once the Forest of Eldin, the great home of the elves," Krin explained, "Until the cursed dwarven alchemist Mahaten decided he would end the final elf-dwarf war by splitting a sphere of lead in the heart of the city. You see, split atoms produces a great deal of heat and energy. It was enough to reduce the forest to this. That alone should be enough for you to understand why this knowledge is forbidden, but it is far worse. 雷電は敵を倒す... Lightning!"
A familiar bolt of energy shot out of Krin's hand. It was a common attack spell, one of To'Ril's favorite. Only the bolt broke apart and vanished after only ten feet.
"The child atoms created by splitting the lead are poison," she told him, "Any living creature that spends too much time in the cursed zone will sicken and die. But so to, will the curse eat away at magic. The reason I teleported us here, is that any closer would be impossible. The wasteland is also a zone of anti-magic. No spell will work in there. No blessing will protect you. We cannot cleanse it. From what the best of us can tell, it will be at least four thousand years until the poison has weakened enough for even rudimentary spells to work. That is the price of True Alchemy. Would you pay it, my apprentice?"
The radioactive fallout produced by a nuclear reaction has a secondary effect. It interferes with the binding/weaving of mana into a spell. As a result the area around the nuke effectively becomes an anti-magic zone. Or at least becomes one until enough of the fallout (cursed/poison atoms) have decayed.
Mana Propagates Fission
Normally, if one applies enough pressure to induce fission in Uranium 235, the Uranium is the only (or at least the main) element split. Unfortunately, mana, the Nth fundamental force and the energy of magic has different ideas. It causes the fission of not just the initial target, but all atoms within a certain range. Splitting a single Iron atom will near instantly reduce everything in about 200 meters into superheated Hydrogen, and a lot of nasty by products. As well as the commiserate air burst and thermal radiation release from splitting that many atoms. Of course, if there is any oxygen present, then a secondary Hydrogen burn explosion will happen.
True, such a reaction can be prevented by removing all mana from the area first. But a spell like that is so draining and complex, the energy would be better spent on attack spells. Splitting the atom is better left forgotten.