You have seen the image in movies and TV, but it is not a case of fancy editing. Nuclear reactors really do glow a brilliant blue color.
But why? How come deadly radiation can look so pretty? The answer comes from Russian scientist Pavel Alekseyevich Cherenkov, who named the phenomenon Cherenkov Radiation.
Let’s take the full explanation head on, and then dissect it step by step. Cherenkov Radiation is caused by a charged particle moving through a medium faster than light would in that same medium.
The speed of light in a vacuum is a universal constant, c, or 186,000 miles per second. However, when light travels through a medium that is not empty, the photons interact with the other particles in its way, slowing the photon down. For example, in water, light travels only 0.75c, or 75% the speed of light.
Cherenkov Radiation is caused when a charged particle moves faster than light would in a medium. So, taking a nuclear reactor as the example, which is surrounded by cooling water, charged particles are moving through the water at greater than 0.75c. This commonly happens in nuclear reactors as the particles being emitted from the radioactive material are being shot off (or are decaying) at high energies, therefore moving very quickly.
As the speeding charged particle (most commonly an electron) moves through the water it disrupts the electromagnetic (EM) field in water. That is to say, the charged particle excites (increases their energy state) the water’s electrons as it passes by. In order to return to equilibrium, the water’s excited electrons release energy in the form of photons, which make up visible light. And because of the properties of this radiation, the photons that you see are of higher frequencies (shorter wavelengths). The shorter the wavelength, the more blue the light appears.
This is why nuclear reactors glow. The charged particles that they emit travel through water faster than light normally would. In common circumstances, photons produced this way would destructively interfere with each other and you would not be able to see anything glow. However, when the particle travels faster than light would in a medium, the photons constructively interfere, resulting in a brilliant glow.
Another way to think about this would be to consider a sonic boom. A sonic boom happens because the plane is traveling faster than the speed of sound in the air. The sonic boom then is a shock wave caused by the sound waves bunching up together as they cannot travel faster than the front of the plane. Cherenkov Radiation is similar. The charged particle traveling through the water creates a shock wave of photons, creating the glowing light that we observe.