Also if it's online, the electricity needs to go somewhere: any disruption to the electrical grid and it'd have to go off - probably at a bad time. Better to go through the shutdown in a planned manner while the outside situation is (relatively) stable.
Then again, a 5700MW resistor might require a bit of cooling.
If you disconnect the load, you need to stop putting steam through the turbine and dump it somewhere else (i.e. a condenser). All steam turbine plants (including coal) have something similar.
The power would not be dumped into a resistor: the power would be dumped in form of heat with the turbogenerator disconnected from steam. I expect that you can "just" dump it in the condenser and use the standard cooling of the condenser to dump it into the local body of water. If that's insufficient, one can dump steam from the secondary loop into atmosphere (which necessitates adding water there). You can play with a vver-1000 simulator and see how many of these things work; the simulator whose manual I'm citing above can be easily found on the internet.