|Cherenkov radiation glow.|
From Wiki- Cherenkov radiation (also spelled Cerenkov or Čerenkov) is electromagnetic radiation emitted when a charged particle (such as anelectron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The charged particles polarize the molecules of that medium, which then turn back rapidly to their ground state, emitting radiation in the process. The characteristic blue glow of nuclear reactors is due to Cherenkov radiation. It is named after Russian scientist Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to characterise it rigorously.
IAEA BLOG LINK.