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What causes the blue glow in a nuclear reactor?

You need water to see the glow

You only get the familiar blue glow if the nuclear fuel rods are immersed in water and they have been operating long enough to have become significantly radioactive.

Try Why Do Astronauts Float by Julian Hamm

It's the radioactivity not the fission that's important

When uranium nuclei fission into smaller nuclei then these fission products are always radioactive.  They emit beta radiation, which consists of very fast moving electrons.

Light moves slower in water so you can go faster than it

Nothing can go faster than the speed of light in a vacuum.  But in water light travels slower.  Some of the beta particles go faster than the speed of light in water.

This is called Cerenkov radiation.

The beta particles flip the water molecules so they point in the same direction

The beta particles have a negative electric charge.  The hydrogen and oxygen atoms that make up a water molecule are bound together by electric fields.

As a beta particle rushes by it distorts these electric fields and tends to make the water molecules all point in the same direction.  This is called polarization.

Blue light is emitted when the electric field snaps back to its original shape

After the beta particle has passed, the electric fields of the water molecules settle down and the molecules return to their random orientation.

As the electric field of each water molecule returns to its original shape, a photon of blue light is emitted.

Constructive and destructive interference

If the beta particles are going slower than the speed of light in water then these blue light photons tend to be out of phase with each other and cancel out.  So overall you don't see a blue glow.

When the beta particles are going faster than the speed of light in water then the photons tend to be in phase and the blue glow is amplified.

The brightness of the glow is proportional to the radioactivity

Every fission of a uranium nucleus happens in a random way.  There are dozens of ways it can split to produce two smaller nuclei.

Some of the fission products are beta emitters with a high enough energy to produce the blue glow and some aren't.  Because there are typically trillions of uranium nuclei fissioning at any one time you always have a good mix of both.

But the more fission products you have, the more beta particles that move faster than the speed of light in water and the brighter the glow.

back to Lesson 13: Nuclear Power