Today's hydrogen bombs are fusion reactions with a nuclear fission warhead attached. A maintained fusion reaction is not currently possible on earth. Sidenote: The radiation comes from fission, not fusion. IF this nitrogen fusion happens during an explosion, the fusion would be localized because there isn't enough magnetic strength or gravity to contain the plasma, aka The heat would dissipate too quickly to maintain fusion. (fusion requires a temp of 100 million k, the temperature in which electrons are stripped from protons and plasma is formed)
That doomsday theory isn't plausable. I bet comic book writers made up that speculation