Because electricity demand fluctuates. It's uneconomical to build enough nuclear capacity to satisfy peak demand. So even if you were to go to 100% nuclear, you'd have to store the excess power to satisfy the peaks.
As gas fueled heating is phased out, most of Northern and Central Europe will get the same kind of seasonal demand as in Norway/Sweden. That is, much higher demand during winter.
There are very few places in Europe (outside of Norway/Sweden) where such long-term storage (sevel months worth) could be built at a competitive price. Currently, storage is usually counted in minutes to single digit hours, even in the optimistic plans.
So, barring some extreme breakthrough in storage technology, the only way to remove the reliance on fossil fuels, is to have significantly more plannable peak capacity than low season consumption.
In the case of nuclear, the main ways to bring prices down would be to create standardized and predictable regulations with a better thought out balance between the economic and health/safety costs, while also encouraging innovations and economies of scale to drive costs down in most other parts of the economy.
If costs could be reduced to €20-€40/MWh, some overcapacity would be ok.
And if you have enough for peak demand (which France for a while mostly had), you run into the opposite problem of having too much power production when you don't need it. Nuclear reactors are not only slow to switch power output, but they cannot be run below a certain output level (like 40-50%) without shutting down entirely. On top of that, running nuclear reactors on average significantly below maximum output drives the cost up further.
Of course running below maximum output drives up cost. Are you disagreeing with basic math? Consumption of fuel is not the main cost of a nuclear power plant, and even if you shut it down completely, it still needs external power for the cooling.
Electricity demand does fluctuate, but so does supply from renewables. I think it is easier to store excess power than to deal with power shortage in case of bad weather.
It is true that the problem of storing energy exists in both cases, but as far as I understand with renewables you lose reliability (in comparison to nuclear power).
That's not how it works. I don't know the actual numbers, but what if peak load is 50% higher than average load? You need quite a lot of battery storage to make that work, especially if the peak load lasts longer than a few minutes (it usually does).
And that's true even assuming instant capacity adaptation. It's just not efficient to keep nuclear power at a capacity lower than their peak capacity.
> I don't know the actual numbers, but what if peak load is 50% higher than average load
You'd know if you read the link I provided.
Nuclear plants in Germany had no issues scaling up and down between 400-600MW and 1200-1400MW per reactor per day.
Now, with renewables you do have this issue. Because due to their intermittent nature you're required to both overbuild them and provide enough grid-scale storage to last for hours.
> It's just not efficient to keep nuclear power at a capacity lower than their peak capacity.
For some politically-motivated definition of efficient. Additional costs to running nuclear plants in load following mode are immaterial.
How is it immaterial to build twice as many nuclear power plants as necessary instead of using energy storage? You'd have to believe battery storage is way more expensive than an idle nuclear plant. But that's just not the case. The real killer in that comparison is that people really don't want to live near a nuclear plant and in any case, regulation and such politics makes it hard or impossible to scale up nuclear power in that way.
You don't get the fact that nuclear power requires just as much storage, right
France has this problem in a hidden way: They have to import power in the summer, because they DONT have that peak capacity, nuclear or otherwise. To provide that with nuclear, they would either need storage or increase the number of their nuclear plants, and probably like 50%.
