This is why all the people shrieking about how evil the public was/is for 'irrationally' hating nuclear and how we need more nuclear (here, on twitter, and reddit) are idiots, especially when they talk about it providing "grid stability" because of those naughty, highly variable renewables.
Grid stability comes from fast-reacting power sources like natural gas, hydro, and energy storage systems like pumped hydro and batteries. The most famous example of this is in the UK, where pumped hydro is fast enough to react to "The Great British Kettle Surge" aka TV pickup:
https://en.wikipedia.org/wiki/TV_pickup
Video covering Electric Mountain (9GWHr capacity) by Tom Scott: https://www.youtube.com/watch?v=6Jx_bJgIFhI
Any time someone opens their mouth and says nuclear is the solution to "days with no sun or wind", you can immediately discount everything else that comes out of their mouth as being from a person who doesn't understand the first thing about electrical grids/generation.
Even if it were feasible to change power levels fast enough to react to wind and solar generation changing from a physics standpoint, no power authority in the world would keep nuclear generation (the most expensive form of power generation already) capacity in reserve for a cloudy day.
What we need are more renewable sources, more energy storage, and a wide-scale HVDC distribution network to be able to cost-effectively move power from areas where there is strong generation by renewables to areas that need it, or at least into storage.