So years ago I worked at a government nuke site. We made the plutonium from uranium. The fuel rods came out of the reactor and then sat in a 50' deep "cooling" tank for 6 months or so until they could be processed. These underwater rods glowed that aquamarine color seen in the movies. It was explained to me it was nuclear particles going from the speed of light in a solid (atom) to speed of light in a liquid. The particle realizes the speed of light is slower in the water and has to give off energy in the form of light to slow down. I think they called it Cherenkoff radiation. Maybe they were messing with me... but the aqua color was way cool!