In an interview with The New York Times, Dr. Ulrich Kühn talks about a possible nuclear escalation in the Ukraine war. The danger is extremely low, but not zero. American domestic politics also carries a potential risk of escalation. For example, the case of a chemical weapons use by the Russian military could lead to domestic political pressure on the U.S. government, which could then react unexpectedly strongly to such a scenario.
You can read the interview from 16th of March here.