Nuclear power is more economical because of the capacity factor of the plant. This is particularly true when you compare it to solar and wind.
Solar and wind typically have capacity factors between 20 and 30% while a nuclear plant is typically 96 to 98%. Put another way, a nuclear plant gets up and running and stays online for months, even years, steadily producing massive amounts of power 24/7. Wind and solar--particularly solar--produce power intermittently when the wind blows or the sun shines. Even when producing the output is highly variable.
Thus, neither wind or solar can be relied on for base loading of the grid unlike nuclear. That means you have to duplicate capacity and have backup power plants that can be quickly brought on line and taken off to make up for solar and wind's unreliability. That adds lots of costs that otherwise wouldn't be present with a stable, reliable production system.