They do it intra-day. You can currently follow that through their monitoring [^1]
> Nuclear has so small margins that if you tell the investors of a new nuclear plant in the US that their reactor will be required to ramp down a couple of times per month, the entire project turns into a huge financial loss just like tha
Nuclear reactors tends to be hugely profitable considering their lifetime (80y in USA) but requires an enormous CAPEX.
Like most infrastructure, any attempt to try to fund them entirely privately with investor seeking a 9% quick ROI is doomed to fail.
Hinkley Point is a very good example of that. It is estimated that the price per MWH would be more than halved if the project would have a funding by public bond instead of the current private investor deathtrap.
[^1]: https://energygraph.info/d/gr_6-QW4z/unit-availability?orgId...
The reason the French can get away with it is because they have to. If you have this much nuclear on your grid, you can't get around it, the reactors need to follow demand. The good news is that because they have so many reactors, they can have them take turns poisoning their fuel and still have enough time to slowly come back up again. So each night a different reactor ramps down and gets poisoned.
Also, French reactors can take the financial hit of decreasing output much better than American reactors. Nuclear has so small margins that if you tell the investors of a new nuclear plant in the US that their reactor will be required to ramp down a couple of times per month, the entire project turns into a huge financial loss just like that. They need to run at 100% all year, or lose billions.