Opening the circuits on a steam turbine without reducing the incoming flow of steam is a bad idea: the 'inductive drag' on the generator is where a large fracti

Opening the circuits on a steam turbine without reducing the incoming flow of steam is a bad idea: the 'inductive drag' on the generator is where a large fraction of energy is leaving the system! If you don't do anything the turbine will destroy itself fairly quickly. The next thing you can do is route the steam past the turbine, which will effectively waste some of the thermal energy through the condensors. However in most nuclear power plants these condensers aren't designed to take the full energy output of the reactor, so if you remove all the load from the system you'll need to reduce the reactor's power output, which is not a fast operation to reverse (fossil fuel based power stations will generally be able to react faster in this case). But long and short is if you interrupt the power going out from a nuclear power station it will not be producing useful power for a while! (it won't damage anything but it'll take some time to get it back up to normal operation).