The first photo of the blue glow of a nuclear reactor. Snapped by Alvin Weinberg's gang at Oak Ridge in October, 1951 and sent to Scientific American (where it made the cover) and LIFE (where it made page 52). At the time, it was captioned as a nameless reactor, but today we know it was LITR, the first water-cooled/water-moderated reactor. It was built as a mechanical mockup in advance of the MTR in Idaho. After finishing its mechanical mockup mission, it was loaded with fuel and brought to power to see if there'd be any neutronic effect of a flowing moderator. There was not. Weinberg sent the photo in without higher authorization from his bosses at the Atomic Energy Commission, which apparently caused a bit of a "brouhaha". I'm glad he did it.

Small article from LIFE: "Self-portrait of a pile"