Nuclear does not have the same function than biomass.

A biomass power station is (relatively) cheap to build but the fuel is expensive. So it make sense to have it as a backup and only use it when necessary.

On the other hand nuclear is expensive to build but the fuel is cheap. So building a nuclear power station as a backup does not make sense, it needs to run all the time.

This is the basic ideas, but in practice nuclear is actually beneficial to renewables. The electricity network operator did several scenarios for the French electrical production in 2050. In their scenarios, having around 13% of nuclear in the mix divided by almost two the amount of solar, wind turbines and batteries needed.