Why Is the Water Blue in a Nuclear Reactor? - Cherenkov Radiation

Why Nuclear Reactors Really Do Glow

Cherenkov radiation causes water in a nuclear reactor to glow blue.
Cherenkov radiation causes water in a nuclear reactor to glow blue. Argonne National Laboratory

In science fiction movies, nuclear reactors and nuclear materials always glow. While movies use special effects, the glow is based on scientific fact. For example, the water surrounding nuclear reactors actually does glow bright blue! How does it work? It's due to the phenomenon called Cherenkov Radiation.

Cherenkov Radiation Definition

What is Cherenkov radiation? Essentially, it's like a sonic boom, except with light instead of sound.

Cherenkov radiation is defined as the electromagnetic radiation emitted when a charged particle moves through a dielectric medium faster than the velocity of light in the medium. The effect is also called Vavilov-Cherenkov radiation or Cerenkov radiation. It's named after Soviet physicist Pavel Alekseyevich Cherenkov, who received the 1958 Nobel Prize in Physics, together with Ilya Frank and Igor Tamm, for experimental confirmation of the effect. Cherenkov had first noticed the effect in 1934, when a bottle of water exposed to radiation glowed with blue light. Although not observed until the 20th century and not explained until Einstein proposed his theory of special relativity, Cherenkov radiation had been predicted by English polymath Oliver Heaviside as theoretically possible in 1888.

How Cherenkov Radiation Works

The speed of light in a vacuum in a constant (c), yet the speed at which light travels through a medium is less than c, so it's possible for particles to travel through the medium faster than light, yet still slower than the speed of light.

Usually, the particle in question is an electron. When an energetic electron passes through a dielectric medium, the electromagnetic field is disrupted and electrically polarized. The medium can only react so quickly, though, so there is a disturbance or coherent shockwave left in the wake of the particle.

One interesting feature of Cherenkov radiation is that it's mostly in the ultraviolet spectrum, not bright blue, yet it forms a continuous spectrum (unlike emission spectra, which have spectral peaks).

Why Water in a Nuclear Reactor Is Blue

As Cherenkov radiation passes through the water, the charged particles travel faster than light can through that medium. So, the light you see has a higher frequency (or shorter wavelength) than the usual wavelength. Because there is more light with a short wavelength, the light appears blue. But, why is there any light at all? It's because the fast-moving charged particle excites the electrons of the water molecules. These electrons absorb energy and release it as photons (light) as they return to equilibrium. Ordinarily, some of these photons would cancel each other out (destructive interference), so you wouldn't see a glow. But, when the particle travels faster than light can travel through the water, the shock wave produces constructive interference that you see as a glow.

Use of Cherenkov Radiation

Cherenkov radiation is good for more than just making your water glow blue in a nuclear lab. In a pool-type reactor, the amount of blue glow can be used to gauge the radioactivity of spent fuel rods.

The radiation is used in particle physics experiments to help identify the nature of the particles being examined. It is used in medical imaging and to label and trace biological molecules to better understand chemical pathways. Cherenkov radiation is produced when cosmic rays and charged particles interact with the Earth's atmosphere, so detectors are used to measure these phenomena, to detect neutrinos, and to study gamma-ray-emitting astronomical objects, such as supernova remnants.

Fun Facts About Cherenkov Radiation

  • Cherenkov radiation can occur in a vacuum, not just in a medium like water. In a vacuum, the phase velocity of a wave decreases, yet the charged particle velocity remains closer to (yet less than) the speed of light. This has a practical application, as it is used to produce high power microwaves.
  • If relativistic charged particles strike the vitreous humor of the human eye, flashes of Cherenkov radiation may be seen. This can occur from exposure to cosmic rays or in a nuclear criticality accident.