Possible Clues:

• Period spawned by the Manhattan Project
• It was spawned by the Manhattan Project
• Time period that the Manhattan Project kicked off
• Post-WWII period
• Post-Manhattan Project era
• Trinity announced its arrival in 1945 needing a cat I fence in
• Era that began with a blast?
• Explosive time in history?
• It followed the Manhattan Project
• He's a cat I put behind bars, it seems, 10 across