In this historical photo from the U.S. space agency, Harold Geisler takes the Plum Brook reactor critical for the first time on the evening of 14 June 1961. This first self-sustaining chain reaction was conducted at very low power.
It wasn't until the following April that the reactor reached its full potential of sixty megawatts. By July 1963, the reactor had completed its first experimental cycle while critical.
For more information browse the Plum Brook Facility Page.
Each weekday, SPACE.com looks back at the history of spaceflight through photos (archive).