Palo Verde Nuclear Power Plant is the largest in the USA, producing over 3 gigawatts, 35% of Arizona's total power capacity. It supplies electricity to Phoenix and Tucson, as well as San Diego and Los Angeles.
The armed guards, gates, fences and barriers were useless on the night of September 29th 2019.
"Officer noticed several drones (5 or 6) flying over the site. The drones are circling the 3 unit site inside and outside the protected area. The drones have flashing red and white lights and are estimated to be 60 to 91 meter above the site. Drones were first noticed at 20:50 MST and are still over the site as of 21:47 MST".
The drones departed at 22:30, 80 minutes after they were first spotted. It were not simply consumer drones, but something larger and more capable, as estimated over 60 cm diameter.
The next night events were repeated. Four drones were flying in, through, and around the owner-controlled area, the security owner-controlled area, and the protected area.
Local police from Maricopa County were dispatched to find the drone operators, but with no success. The site is reportedly due to receive drone detection gear, but not counter-drone jammers or other defensive equipment that might stop such incursions.
It is highly likely that such sites are still vulnerable to drone overflights.
However, drones would probably not go for a brute-force approach, but would use their ability to strike pinpoint targets to hit control systems and failsafes.
Drones might well shut the plant down, taking out 35% of Arizona's electricity at a stroke.