To answer that question, you have to wonder first: How does a TL have to look that people will NOT use a nuke after they invented them?
Some possibilities:
- When they invent nukes, all countries are already living happily in peace, so they don't need it.
- They invent it, but aren't at war in that moment, and until a new war emerges they found out so much about the consequences of radiation they they'll say: "No... we can't do that, not even against our worst enemies."
- Two countries who are enemies discover them almost at the same moment and a Cold War emerges before they can use their nukes.