Minecraft wiki entry for Nether_reactor, associated text: The reactor cycle would end after 45 seconds, damaging the reactor's structure and changing the world to night. The reactor was replaced with a 3×3×3 box of obsidian containing the core, which adopted a darker, blue-black texture to indicate that the reaction has ended. If the player built a new reactor around the burnt-out core, the netherrack spire would generate, although the rest of the reaction would not take place.