Reed Research Reactor

TRIGA Mark I at 240 kW

There is a good explanation of the blue glow (Čerenkov radiation) on Wikipedia.

(click images to enlarge

Cerenkov - Jeffrey Bradford Cerenkov - Michael Pollock
Photo by Jeffrey Bradford, 1987 Photo by J.Michael Pollock, 1993
Core Overexposure #1 - Mark Chen Core Overexposure #2 - Mark Chen
This picture is slightly overexposed. Another overexposure, with the pool lights left on.
Photos by Mark Chen, 1998