Radiation alone provides security. The plant diagram already has two fences with coresponding checkpoints. One needs to stop potential looters, so basically checkpoits with a guard each and a patrol vehicle with a team of two. Make it armed guards if you're in the US where everyone and their grandmother owns not one but several firearms. That plus the usual access cards and CCTV. This isn't a military nuclear weapons lab, it's a civilian power facility.
