Renewables and nuclear -both- have the problem that you can't control when you get the generation.
Nuclear wants to be constant-- both because ramping isn't super fast and because you want to recover large capital costs. Renewables ... produce varying amounts pseudo-periodically and unpredictably, which is even worse. Neither matches conventionally to demand.
The only way around this is technologies that store (hydroelectric, batteries, power-to-gas-to-power, solar thermal) or burning fossil fuels to supplement. Overprovisioning is also necessary: it takes a lot less storage / peaker plants / etc to meet 99th percentile demand with 150% of required generation capacity than 101%.
Renewables and nuclear do complement each other a little bit in reducing volatility.
If I'm not mistaken nuclear in France is actually pilotable quite fast, it is not instantaneous but it is really fast. You cover the risk of shortest term high variance with other means, but they basically already exist (hydro, gaz) and you don't need much.
This is not necessarily the case for all nuclear power plant, it has to be designed for that.
It depends what you call fast. It's my understanding that PWR's like this ramp from 50% to 90% in about an hour.
But nuclear power plants have a huge capital cost, so you really don't want them to operate for half of the day at 40% or they're even more expensive for the power generated.
And going below a minimum output power means a long time to get power back, which is why in North America power prices sometimes go negative (don't want to shut down).
Nuclear wants to be constant-- both because ramping isn't super fast and because you want to recover large capital costs. Renewables ... produce varying amounts pseudo-periodically and unpredictably, which is even worse. Neither matches conventionally to demand.
The only way around this is technologies that store (hydroelectric, batteries, power-to-gas-to-power, solar thermal) or burning fossil fuels to supplement. Overprovisioning is also necessary: it takes a lot less storage / peaker plants / etc to meet 99th percentile demand with 150% of required generation capacity than 101%.
Renewables and nuclear do complement each other a little bit in reducing volatility.