Complex sentence: Concerns over scenarios involving nuclear accidents or incidents on American soil are not recent; as early as the 1960s, officials were concerned that a nuclear weapon might be smuggled into the country, or that a nuclear-weapon equipped airplane might crash and contaminate surrounding areas.
Split into simpler sentences: Concerns over scenarios involving nuclear accidents or incidents on American soil are not recent. As early as the 1960s, officials were concerned that a nuclear weapon might be smuggled into the country, or that an airplane carrying a nuclear weapon might crash and contaminate surrounding areas.