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.
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Each weekday, SPACE.com looks back at the history of spaceflight through photos (archive).