The Mock-Up Reactor (MUR) core as seen from the Plum Brook Facility control room.
In this historical photo from the U.S. space agency, the Mock-Up Reactor (MUR) core is seen from the control room. Since the MUR generated a very small amount of radioactivity, the "swimming pool" within which it was located provided sufficient shielding. A movable bridge directly above the core allowed MUR operators to easily change fuel or manipulate experiments during shutdowns.
For more information browse the Plum Brook Facility Page.
Each weekday, SPACE.com looks back at the history of spaceflight through photos (archive).