Find the reactor's bootup console.
- Go through the doorway revealed by destroying the Barricade Servitor.
- This will lead you down to the Festering Halls.
- Drop down into the pool below and start fighting the Dregs and Thralls.
- Go over to the stairs leading up on the left.
- Head through the door and fight through the Thrall.
- Turn to the left and drop down the opening in the floor.