I'll be honest. It's highly unlikely. But as long as nuclear weapons exist, there is always a possibility of it. I'm from Ireland and I remember losing a week of sleep when i seen a news article recently saying Putin was threatening to sink my country into the ocean with an unstoppable underwater nuclear missile called Satan 2 which sounded absolutely terrifying (you can read the article here) and it made my anxiety go through the roof! I've lately realized we shouldn't stay worrying about stuff which is out of our control tho. Moreover, if Putin is off his rocker and did decided to kill us all, i can't imagine the people who actually launch the nuclear warheads going through with it as they will die too.