Those who spent tens of millions of man-hours to design, engineer, build, operate and maintain these plants never thought of this?
They did think of it. And our military has plans for many contingencies.
Our problem is not with having the means to retaliate, but having the will.
Shortly after 9-11-2001 GW Bush properly called out Afghanistan as a nation which harbored the terrorists that attacked us.
Then he used our military to go after them. But very soon, he lost the way. He stopped short of winning.
I am not worried about an attack on a power plant. I am worried we lack the political will to kill the human forces that dared knock out the plant.