Putin can't assume that Ukraine itself, a 50 year nuclear weapons power with the largest reactor in Europe and with many nuclear scientists, hasn't had a little "Manhattan Project" of its own going underground in some remote place since 2014 or earlier. Also, Ukrainian spies can easily move about Russia so Putin has lots of extra thinking to do if he thinks only the West can counter a barbarous nuclear strike. After all, nearly all of Russian Culture and history resides in 2 cities and the rest of world is everywhere else on the planet. Not as strong a nuclear position as it may seem to some.