6.1.11 Fission

6.1.11 Fission: Describe nuclear fission, explain how a chain reaction arises and is controlled in a reactor, and state the advantages and problems of fission power.

Fission releases energy because the products are more tightly bound than the original large nucleus.

Definition

Nuclear fission is the splitting of a large unstable nucleus, such as uranium-235 or plutonium-239, into two smaller nuclei.

Trigger

The nucleus absorbs a neutron, becomes unstable and splits, releasing further neutrons and a large amount of energy.

Energy release

The energy appears mostly as the kinetic energy of the fragments and neutrons, which heats the surroundings.

Products

The fragments are themselves radioactive, which is the source of nuclear waste.

Energy source

One kilogram of uranium-235 releases millions of times more energy than a kilogram of coal.

Chain Reactions

Chain reaction

Neutrons released by one fission can cause further fissions, so the reaction sustains itself.

Uncontrolled

If each fission triggers more than one other, the number of fissions grows rapidly, as in a nuclear weapon.

Control rods

Rods of boron or cadmium absorb neutrons and are moved in or out to keep the reaction steady.

Moderator

Material such as water or graphite slows neutrons so that they are more likely to cause fission.

Coolant

A fluid carries heat from the core to a heat exchanger, producing steam that drives turbines and generators.

Reactor component	Function
Fuel rods	Contain the fissile uranium
Control rods	Absorb neutrons to control the rate
Moderator	Slows neutrons
Coolant	Removes heat to make steam
Concrete shielding	Absorbs radiation from the core

A power station gives a large and reliable electrical output from a very small mass of fuel.

Fission does not emit carbon dioxide while operating, so it helps to reduce greenhouse gas emissions.

Spent fuel and other waste stay radioactive for thousands of years and must be stored securely.

Accidents, though rare, can release radioactive material over large areas, so safety systems and regulations are strict.

Building and decommissioning a plant is expensive and takes many years, which affects the cost of the electricity.

Question: In a simple model each fission releases two neutrons that each cause one further fission. Starting with one fission in generation 1, how many fissions occur in generation 10 if no neutrons are lost?

Generation 1 has 1 fission; each generation doubles the number of fissions.
Generation n has 2ⁿ⁻¹ fissions.
Generation 10 has 2⁹ fissions.
2⁹ = 512.

Answer: 512 fissions in generation 10, showing how rapidly an uncontrolled chain reaction grows.

6.1.11 – Key Points for the Exam

Nuclear fission: The splitting of a large nucleus into two smaller nuclei with the release of energy.
Chain reaction: A self-sustaining series of fissions caused by released neutrons.
Control rod: A neutron-absorbing rod used to control the rate of fission.
Moderator: A material that slows neutrons in a reactor.
Fissile: Able to undergo fission after absorbing a neutron.

Common Mistakes

Confusing fission with fusion.
Saying the control rods take part in the fission rather than absorbing neutrons.
Stating that nuclear power stations produce carbon dioxide from the fission reaction itself.
Forgetting that fission products are radioactive and need long-term storage.