Russia likely also still remember the aftermath of Chernobyl; a nuclear strike so close to their home would hopefully be seen as something to avoid at all costs. On a similar note, Ukraine also still has a few bandages it could take off in the event of a nuclear strike, like an all-out strike on Russian nuclear power plants. Those are undoubtedly well-defended, but there are a lot of them and they only need to succeed in causing fallback to leak from one.