Chain Reaction

The Physics of Control

One Neutron, Two Problems

When a U-235 atom splits (fission), it releases 2 or 3 extra neutrons.

The Mathematical Dilemma:

Exponential Feedback

Neutrons fly out of an atom at 14,000 km/s. They are moving too fast to be absorbed by another Uranium atom.

The Moderator (Water or Graphite) "bounces" the neutrons around, slowing them down to "thermal" speeds so they can trigger more fission.

Kinetic Energy Transfer

Like a cue ball hitting a cluster of billiard balls.

Materials like Boron or Cadmium are like "neutron sponges." They eat neutrons without splitting.

Rods Up: More neutrons available → Power increases.

Rods Down: Neutrons absorbed → Power decreases.

k = 1.0: Every fission leads to exactly one more. This is Criticality—the perfect steady state.

Scram: An emergency shutdown where all control rods are dropped into the core instantly.

Water becomes less dense (bubbles/steam).

Neutrons stay fast; fewer fissions occur.

This is a Negative Void Coefficient. The reactor naturally wants to shut itself off if it gets too hot.