What it looks like when a Nuclear reactor starts up

In science fiction films, nuclear reactors glow. While films use special effects, the glow is based on scientific facts. For example, the water around nuclear reactors really glows bright blue! How does it work? It is due to the phenomenon of Cherenkov Radiation.

Cherenkov radiation definition

What is Cherenkov radiation? Essentially it is like a sonic boom, except with light instead of sound. Cherenkov radiation is defined as an electromagnetic radiation emitted when a charged particle moves through a dielectric medium faster than the speed of light in the medium. The effect is also called Vavilov-Cherenkov radiation or Cerenkov radiation.

Why water in a nuclear reactor is blue

As Cherenkov radiation passes through water, the charged particles travel faster than light through that medium can. So the light you see has a higher frequency (or shorter wavelength). Because there is more light with a shorter wavelength, the light appears blue. But why is there light at all? It is because the fast moving charged particle excites the electrons of the water molecules. These electrons absorb energy and give it off as photons (light) when they return to equilibrium.