Nuclear and solar can each scale to ~40% of the annual supply for most grids without storage, but for different reasons they both need increasing amounts of storage as you ramp them past that point.
Solar because the sun doesn’t shine at night and peak consumption is mornings and evening, but Nuclear because demand varies though the day and season while the costs per kWh increase the more its capacity factor drops. France both had lower capacity factors and exchanged a great deal of power with its largely non nuclear neighbors. Exchanging power with less nuclear countries doesn’t scale to a worldwide increase in nuclear.
However nuclear also costs more per kWh as a baseline and runs into similar problems as the percentage of solar energy increases. Without storage, a 20% solar 30% nuclear grid is less profitable for nuclear than a 10% solar 30% nuclear grid. Given the long lifespans of nuclear power plants nobody wants to invest in nuclear if it’s expected to be unprofitable 20+ years from now.