In the picture – Cherenkov radiation. This is me in 2009 at the Chernobyl Nuclear Power Plant, standing in the hall with the nuclear reactor. The water in the photo is for slowing down neutrons and cooling spent fuel rods. The glow occurs when electrons are ejected from the fuel at a speed exceeding the phase velocity of light in water. Kind of like a sound barrier, but for light. The intensity of this glow can roughly indicate how “fresh” the fuel is in the pool. The brighter and denser the blue, the more active the decay processes are. Interestingly, Cherenkov radiation is the reason why there is no absolute darkness at great depths of the ocean floor.
Hi, I'm Rauf Aliev. 