Welcome to the mechanics of nuclear power. You already know that large, unstable nuclei can split apart to become more stable. In this topic, we take control of that process.

What you'll learn:

• How firing a slow "thermal" neutron at a nucleus forces it to split (induced fission).
• How one splitting nucleus can cause a cascade of others (a chain reaction).
• The anatomy of a thermal nuclear reactor: the exact roles of the moderator, control rods, and coolant.
• Why we use a simple "elastic collision" model to explain how a moderator works.

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1. Induced Fission and Thermal Neutrons

Spontaneous fission happens entirely by chance. Induced fission is deliberate: we force a heavy nucleus (like Uranium-235) to split by making it absorb an extra neutron.

When a ${}_{92}^{235}U$ nucleus absorbs a neutron, it briefly becomes ${}_{92}^{236}U$. This new nucleus is incredibly unstable and almost immediately fractures into two smaller "daughter" nuclei (fission fragments), alongside a huge release of energy and a few stray neutrons.

However, we cannot just fire neutrons at any speed. If a neutron is travelling too fast, it will simply bounce off the uranium nucleus rather than being absorbed by the strong nuclear force. We need thermal neutrons.

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2. Chain Reactions and Critical Mass

Look closely at the example above. We spent one thermal neutron to trigger the fission, but the reaction spat out three new neutrons.

If those three new neutrons can be slowed down to thermal speeds, they can trigger three more uranium nuclei to split. Those three fissions would produce nine neutrons, which cause nine fissions, producing twenty-seven neutrons... This is a chain reaction.

For a chain reaction to be sustained, you need a certain minimum amount of fuel. If you have a tiny lump of Uranium-235, most of the newly released neutrons will simply escape out of the surface into the air before they hit another uranium nucleus.

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3. The Thermal Nuclear Reactor

To safely harvest the energy from fission, we build a thermal nuclear reactor. The reactor core contains several vital components that work together to maintain a steady chain reaction and extract the heat.

Let's break down the three main components the AQA specification requires you to know: the moderator, the control rods, and the coolant.

The Moderator

The neutrons released in fission are "fast neutrons" (kinetic energies around $1MeV$ to $2MeV$). They are moving far too quickly to be absorbed by Uranium-235. The moderator surrounds the fuel rods and its job is to slow these fast neutrons down until they become thermal neutrons.

How does it work? (The Elastic Collision Model) The moderator relies on elastic collisions between the fast neutrons and the nuclei of the moderator material. From mechanics, you know that when a moving particle collides elastically with a stationary target, the maximum transfer of kinetic energy happens when the two particles have similar masses.

• If a neutron hits a massive Uranium-238 nucleus, it bounces off with almost all its original kinetic energy (like a ping-pong ball hitting a bowling ball).
• If a neutron hits a nucleus with a mass similar to its own (like a proton in water, or a carbon nucleus in graphite), it transfers a large fraction of its kinetic energy to the target nucleus (like a cue ball hitting an 8-ball in pool).

After about 50 collisions in water, a fast neutron loses enough energy to become a thermal neutron.

Materials used: Water (acting as both moderator and coolant), heavy water, or graphite.

Control Rods

While the moderator ensures the reaction can continue, the control rods ensure it doesn't run out of control. Their function is to absorb neutrons, reducing the number of neutrons available to cause fission.

If the reaction rate is too high (reactor getting too hot), the rods are lowered deeper into the core to absorb more neutrons. If the reaction needs to speed up, the rods are raised. In an emergency, all control rods are dropped fully into the core, absorbing almost all thermal neutrons and shutting down the chain reaction entirely.

Materials used: Boron or Cadmium. These materials are chosen because they have extremely high probabilities of absorbing thermal neutrons without undergoing fission themselves.

Coolant

The immense energy released by fission shows up as the kinetic energy of the fission fragments. As these fragments smash into the surrounding fuel lattice, the core heats up dramatically. The coolant is a fluid pumped through the core. Its function is to absorb this thermal energy and transfer it to a heat exchanger outside the core (where it boils water to turn turbines and generate electricity).

Materials used: Water, Carbon Dioxide gas, or liquid sodium. A good coolant must have a high specific heat capacity (to carry a lot of heat energy), a high boiling point (or be kept under high pressure), and must not readily absorb neutrons.

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