re: Seems you don't understand how dependant we are on others
We live in a material society which sustains billions. Living out the rest of one's life with enough food, shelter, warmth and entertainment would not be a problem unless all these things are destroyed as well.
The question posed merely the death/disappearance of everyone else on Earth, not a nuclear holocaust.
As for why I would be concerned about the reactors, I would be concerned that they would meltdown and irradiate an area if left completely unattended - I was not concerned about securing them from anyone.
Yes, we are dependant upon others, but if most/all humanity were to disappear tomorrow leaving behind, intact, the civilization it has created, a single human should be able to adapt to the new challenges.