If President Vladimir V. Putin of Russia follows through on his threats to use a nuclear weapon in Ukraine, he is likely to turn to a specific type. Tactical nuclear weapons have a fraction of the strength of the Hiroshima bomb and of the super bombs and city busters that people worried about during the Cold War. What exactly are these weapons, how did they develop and what would it mean if Mr. Putin resorted to them in the war in Ukraine? Guest: William J. Broad, a science reporter and senior writer for The New York Times.