'Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium.'
It's going faster than the speed of light (in water). The blue glow is basically a sonic boom but for light.
http://sploid.gizmodo.com/seeing-a-nuclear-reactor-start-up-is-cooler-than-my-sci-1596286481
