The security of nuclear power stations themselves is overseen in the US by the federal government - there’s a good description here:

https://www.nei.org/resources/fact-sheets/nuclear-plant-secu...

I worked on automated conversion of a large collection of documents for nuclear power stations some years back, and there was a lot of bureaucracy around who could access the documents, and how. I wasn’t supposed to take any files home, for example, which meant I couldn’t copy them onto a laptop. Management claimed that this was due to federal laws, but I didn’t ever investigate the legal basis.