How is Russia bombing a nuclear power plant any different from Russia using nuclear weapons. The issue is not the nuclear power plant, it's the mad man willing to cause the end of the world to win his war in Ukraine.
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."