Possible Clues:

• Nuclear threat
• SALT subject
• "Ivy Mike" test weapon
• Cold war specter
• Its testing awakened Godzilla ... and what's dropped, facetiously, into five puzzle answers
• Big bang maker
• Weapon in a 1952 test
• Weapon since '52
• Cold War concern ... and what created 20-, 32-, 40- and 55-Across?
• Cold War weapon