The Zaporizhzhja nuclear power plant lost its only operating high voltage line
in a missile attack yesterday. Already repaired, but this time the reactors 5 and 6 were in operation
. As IAEA's Rafael Grossi told: it's the sixth time, one day our luck will run out. aljazeera
Without the power line, the alternators have no electric load, they don't brake the turbines, all accelerate. A typical speed safety factor is only 1.2× before turbines break. Such machines are about 40m long, and they leave little time to react.
That accident wouldn't be nuclear... except if the spinning rotor of a turbine or alternator punches through the stator and accelerates on the ground, which did happen with series-connected electric motors. At older French nuclear power plants, some rotors would accelerate towards the next reactor.
As the turbines stop and absorb no more heat power, so must the reactors, and quickly. Problem: the radioactivity still produces some 10% (100MW!) of the fission power, and at the VVER-1000 and most designs it must be evacuated actively with pumps.
Future concepts seek passive removal of the radioactivity heat by natural convection, or a small engine that converts this heat into enough power for the pumps. Others would offer enough capacity to absorb this heat. Very few operating power plants have such features.
That's why a nuclear reactor needs power after shutdown. They aren't safer when unplugged. At Zaporizhzhja the Diesel generators have started every time. At Fukushima they didn't.
Without cooling power, the pressure vessel would vent the vapor or burst. Unlike Chernobyl, the VVER-1000 at Zaporizhzhja have a confinement dome. Alas, it resists only if helped by sprinklers that condensate the vapor if they have power. irsn.fr
That's more or less what failed at Fukushima.