No, you can get to 100%. A key is hydrogen (or perhaps ammonia) to handle rare long lulls in production. The round trip efficiency on that is lousy, but the capital cost is low (which is more important for that segment). A simple cycle gas turbine plant is only 5% of the cost of a nuclear plant of the same rated power.

Try playing with this online modeling tool:

https://model.energy/

With the plausible numbers there, new nuclear is optimized out of the mix.

This will be more expensive than fossil fuels with free CO2 release, but presumably CO2 taxes get ramped up to the point that's no longer true.