So, to recap the situation:
  • You want a phylactery that will explode with the effect of a nuclear bomb
  • You want it to be non-defusable
  • You have powerful black magic in your world


If you ask me, you're not doing yourself a favor with the whole nuclear-bomb-approach. As others have mentioned, it's actually quite hard to get a nuclear bomb to detonate, since they are designed that way... nobody wants one of them to explode by accident after all. You have the means to simply create a magic macguffin that will behave in exactly the way you describe and there is no need to actually make it a nuclear bomb! By doing that, you add nothing to the story (as far as I can see) and invite this entire line of argument which your players are likely to come up with as well. All this will do is cause frustration if they come up with ideas that you don't want to work.

Just make it an instable source of black magic (maybe the source of black magic that will contaminate the entire realm of destroyed) that they can't defuse and be done with it. Save yourself (and your players) the trouble of that argument if it isn't really necessary.