To make it cost effective, the U.S. has to stop making enemies. Do you know how much it will cost to secure a few hundred nuclear power plants from people willing to blow themselves up?
As much as 200 more low security prisons? 20'000 federal employees (to be generous) is peanuts to a project of that scale. How many attacks have there been on power plants anyway? You cant make modern designs go boom, just melt.
I certainly hope your right that modern nuke plants are safer. But the general public responds more to security theater than real security.
As to "how many attacks have there been"? Well, it only takes one successful one. How many successful (reached their ultimate goal) airline highjackings have their been?
Just imagine the true "green revolution" that could be brought about with a few hundred more nuclear plants in the US alone.