A nuclear engineering student at Penn State lowered a GoPro camera into the waters of the Breazeale nuclear reactor to show what it looks like when it is powered on.
The video shows the core at first turned off, then powered to 500kW, then to 1MW.
As the output increases, a bright blue glow emanates from the rods. The glow is the result of Cherenkov radiation, the effect of electrons traveling faster than the speed of light in water.