What is stopping Putin from using nuclear weapons?

There was a lot of concern at the outset of Russia's invasion of Ukraine about the possibility that this conflict would lead to the use of a nuclear weapon. That hasn't happened, but is the danger still there? What would happen if Russia used a nuke, and how would that escalate? We asked Dr. Amy Nelson, David M. Rubenstein Fellow in the Foreign Policy program at The Brookings Institution, a nonprofit public policy organization based in Washington, DC.