III-a1a primary hypermatter-annihilation reactor

SJFS III-a1a primary hypermatter-annihilation reactor was a model of reactor produced by Sienar-Jaemus Fleet Systems powerful enough to make an about 3000 meters-long Star Destroyer move. Resurgent-class Star Destroyers were powered by a single, large SJFS III-a1a, housed within a dome-shaped reactor containment vessel on the ships underbelly. The massive reactor was supported by a reinforced reactor superstructure and hull chassis.

Source

 * Star Wars: The Force Awakens: Incredible Cross-Sections