[troyvit]

It's literally the difference between bombing a nuclear power plant and using nuclear weapons. If Russia uses a nuclear weapon it uses the yield it wants, where it wants, when it wants, and leaves a pretty distinct footprint behind.

If Russia blows up Zaporizhzhia using conventional weapons it creates a totally different set of problems for Europe and comes with some deniability. They could say that Ukraine did it. It also wouldn't necessarily trigger mutual assured destruction. They just have to say, "Oops" and I bet they'd get a way with it aside from a raft of "powerful sanctions."