There's individual Nuclear plants of that size and the grid already needs to plan for those to be out unexpectedly (far less predictably than wind in fact) so it doesn't seems to be that big a problem in practice.
At this size (4GWe) it was no big deal, the hydro plants can cover easily. But at 50 or 100% renewables it's much more challenging because entire regions can go down unpredictably. The 1 GWe nukes go down for outages generally in a staggered way (though not always, for instance in a common-cause design concern)
