Scheduled maintenance. Refueling, replacing worn-out components, that sort of thing. One nine of uptime is pretty good in the "continuously running power plants" business.

Emphasis on scheduled. Wind operators get no choice at all in when they're generating power, and this is true of solar as well, although the fluctuation is easier to predict, especially in very sunny climates.

A fleet of nuclear reactors can be taken offline on a schedule, with advance notice, so they alternate being out of commission. This makes it easier to supply reliable baseline power.