Add uranium

Uranium should be an ore at Y10-Y-50 and holding it will give you radiation, but if you sneak (all I can think of) you will get less damage and less often. If you wear uranium armor you will be immune (or use the Radiation enchantment on any armor). Uranium should also be found in dungeon chests and so can the Radiation enchantment book. The enchantment should also be in the enchanting table.