The nuclear ones, to some extent. The Nuclear Energy Institute claims nearly 9,000 "well-armed and highly trained" security officers guard them,. There are 54 commercially operating nuclear power plants, which means an average of 166 per plant. However, they there are 168 hours a week, so at 40 hours per person per week it takes 4.2 people just to have 1 person always available and that isn't considering vacations, so in practice they will have less than 40 people available at any time.