What was a major concern of scientists who left nazi germany for the united states before world war 2

One major concern of scientists who left Nazi Germany for the United States before World War 2 was: They feared Germany was trying to develop nuclear weapons.

They predicted that if the Nazi was successful in developing the nuclear weapon, they could wipe out almost 1/3 of the worlds and destroyed the lands core nutrient for hundreds of years. They flee because they wanted to prevent this from happening.

I'm pretty sure that's called a volcano