AC, simply because they are much more efficient than DC generators.
Neither power is inherently ac or dc. Nuclear fuel is used to heat water that makes steam and the steam turns a generator with mechanical energy. In hydroelectric power the mechanical energy of the water flowing turns a generator. Generators can be either ac or dc, but since ac is much more efficient to provide power at longer distances almost all large scale generators produce AC power.
Those are forms of mechanical power that can be converted into electrical energy through the use of a generator. It is the design of the generator that determines whether or not AC or DC is produced. The vast majority of power produced is AC. Any DC producted would most likely be used for use by the power plant as it cannot be distributed efficiently any great distance.
Alternating Current. AC
The water flow in hydroelectric and the steam flow in nuclear is adjusted so the generator is turned at constant speed and the speed is further adjusted to make the frequency exactly match what is out on the grid the generators are feeding.
Detail would be nice
