Defective Reactor is one of the main quests that you get from Thorald during the third chapter of Elex. You must visit an abandoned factory and find a coil that is necessary for fixing Jax's flying machine. During this quest you will encounter various monsters.
How to unlock this quest: Speak with Thorald in Abandoned Cliffs.
The part that you are looking for is located in Company Premises: West Edan in the west part of the map. During your way to the base and inside of it you must eliminate numerous monsters (including Night Shade, Mutant Rotboar, Terror Bird, Slime Drakon, Rottmolch). Avoid the most powerful enemies and never attack too many monsters at the same time.
You must reach the underground part of the facility. Start with locating a lift shaft in one of the bigger buildings (picture 1). Jump down into the lift shaft and use your jet pack while you are falling down. There, you must find the control room from the picture 2. Using the terminal reveals twisting stairs that lead to a reactor.
Use the stairs and reach the lowest floor. Explore the nearby rooms. X48 Reactor Coil is located in a poorly lit room with glowing Elex (the picture above) (2000XP).
Pick up this part and take it to Camp in the Center where a mechanic, called Harley, is repairing Jax's aircraft. Give him the coil (2000XP).