How fast can a nuclear power station reach full power from shutdown?

Answer 1

It takes a few days to get a plant online after it has been shut down, cooled down and depressurized. Note that it only takes a few minutes to an hour or so to start up the nuclear reactor, depending on the plant. That reactor doesn't take all that long to be made critical. But it takes many hours to heat the primary plant, and then heat up all the steam lines in the secondary plant before you can begin drawing steam to roll the turbines. The limits on the speed at which the plant can be brought online are in the transfer of heat throughout the primary system and the heating of the secondary one. Note that plants are operated to avoid high differential temperatures across the pressure vessel and primary components. And blowing a bunch of steam down a cold steam line will cause the steam to condense. Water droplets and rapidly spinning turbine blades make for a bad mix. It takes a finite amount of time to get the plant "hot" and start generating the megawatts. This all takes a couple or three days or so to do, depending on the size of the plant. Sometimes the plant will sit at a few percent operating capacity while tests and such are run, particularly if the plant is coming back after an extended shutdown. You can guess a few other variables. Bottom line is it takes little time to get the reactor critical, but quite a bit of time (relatively speaking) to heat everything up. Once things are hot in the primary and bleeding steam has heated the secondary, the order is given to open the steam stops and the turbines will begin to spin.