“Nuclear-weapon states should negotiate and conclude a treaty on no-first-use of nuclear weapons against each other or make a political statement in this regard,” Sun said.
China and India are currently the only two nuclear powers to formally maintain a no first use policy. Russia and the United States have the world’s biggest nuclear arsenals.
I think you overestimate how many nukes it would take to cause the end of the world. Unless you mean “every piece of land is a radioactive wasteland” end of the world.
Radioactive contamination is basically a non concern. Potential massive climatic effects and logistics collapse on the other hand, are.