It is not. Let's say you are a terrorist, and you and your team have taken hold of a nuclear power plant. Then what? It's a problem for the world, but not different from you taking over a hotel or a city hall. You might threaten or even kill hostages, but there's no way you can create a significant nuclear threat. Let's say you force an operator, at gunpoint, to remove some rods from the reactor, or otherwise you get possession of some nuclear material. So what? How are you going to get away? You will be surrounded by hundreds or thousands of SWAT officers. What is the scenario where it will make a difference if they take over the nuclear power plant vs a different building?