Agreed. We would be into a whole new uncharted territory if either a nuke or a dirty bomb was utilised.No, but the US and UK could well do so if they decided to -- not that I think they would, mind you. I think a dirty bomb would result in a no-fly zone, though, which would either end the invasion or spur Russian escalation, which is why it's not been applied yet. Keep in mind that Biden has already warned of catastrophic consequences of Russia setting off a nuke, and Petraeus has already outlined a possible non-nuclear response as well.
It's hard to say what the Western response would be to a simple dirty bomb