Unless you're an Iranian, I think you're thinking about the wrong nuclear agency. Although "we" have never acknowledged it, "we" destroyed (some of) "their" centrifuges (see "stuxnet"). I'm not aware of any incidents like that involving American nuclear facilities--and I'm pretty sure that'd have made the technical press if not the mainstream media.
http://www.theblaze.com/stories/201...-s-nuclear-agency-three-times-in-three-years/ I can not find the one on the facility, but I will try to find it. Wasn't in the mainstream news for sure, just as the power grid attack wasn't.
Typically, critical networks are "air gapped" meaning there is no physical connection between those critical networks and the internet. To move data between those two networks requires a sneakernet (put on your sneakers and carry your data over to the other computer). Social engineering is the biggest payer in the game right now because promising some moron a million dollars or a new password if they just click that link is far easier than brute force penetration into some network.
Would make a lot more sense that way, that's for sure.