It's never economical to operate an asset at under 100% capacity. Intermittent sources of energy like solar and wind encounter the same problem when they start to saturate demand during peak periods of generation. Install a new solar panel in California, and chances are you won't actually be able to sell any electricity around midday since demand is saturated.

The difference is that nuclear will keep running at night, in the winter, regardless of how strong the wind is blowing. A cheap, but intermittent source of carbon-free energy is not comparable on a dollar-by-dollar basis to a non-intermittent source of carbon-free energy.

The common retort is to use batteries, but let's put this in perspective: France uses 1,219 GWh of electricity daily (note that this is just electricity and doesn't include things like transportation, fuels in smelters, chemical feedstock etc.). 12 hours of storage would be 600 GWh. Seasonal fluctuations in wind and solar are even more extreme, and might need days worth of stored energy. But let's be humble and just see what it'll take to provision 12 hours:

At $150/kWh that'll be 90 billion dollars. These batteries will be good for 2,000 to 5,000 cycles. Let's say 4,000, so it has an 11 year life span. Over the course of 55 years that'd cost $450 billion. Just for the storage, mind you, France has to build the renewable generation on top of the storage.

On the flip side, the Flamanville Nuclear plant has a lifespan of 60 years. You could build 12 Flamanville nuclear plants and satisfy 100% of France's electricity demand. At €19 billion euros, or about $22 billion USD building 12 Flamanville plants would work out to $264 billion. The cost of storage to even out intermittent sources is much more expensive than just building the nuclear plants.