- refueling (regularly done, highly plannable)

- insufficient cooling water/ too warm cooling water (regular, seasonal, not very predictable short-term)

- planned maintenance (often combined with refueling)

- unplanned maintenance (accidents)

Whenever these things happen, the entire reactor is producing no power. This is why most nuclear facilities have multiple reactors, so you can rotate these tasks among them and still have some power. Unfortunately, the cooling issue is becoming more and more of a problem, and many sites that used to operate year-round now have to scale down significantly in mid-summer due to lack of cooling water.