Unfortunately, nuclear does not fulfill this role from an economic perspective.

Cost of nuclear energy is mostly build cost and maintenance, fuel costs are pretty low. So if you have one, you want to keep it at nominal power for as much time as possible, if you just use it as backup, each kwh is insanely expensive.

For coal and gas, it's the other way round. Building them is cheap in comparison, so is maintenance. They only cost money if they run... So they are pretty good backup plants.

Also... You mean the brand new plant of the EPR Type in Flamanville? Construction was supposed to take 5 years, were now at 15 and counting and cost has gone up from 3.3 to 19(!) billion €