Fission & Fusion

Fission & Fusion ☢️

The nucleus of an atom holds staggering amounts of energy. There are two ways to release it — and they are **opposites**. **Splitting** a giant nucleus is **fission**; **joining** two tiny nuclei is **fusion**. Students mix these up constantly, so let's pin down the difference for good.

Nuclear fission 💥

**Nuclear fission** is the **splitting** of a **large, unstable nucleus** — such as **uranium-235** or **plutonium**. It usually happens when the big nucleus **absorbs a neutron**, becomes unstable, and splits into **two smaller nuclei**. This releases a lot of **energy**, plus **two or three more neutrons**.

The chain reaction

An interactive activity.

True about fission

Select ALL THREE statements about nuclear fission that are TRUE.

• It splits a large, unstable nucleus
• It is usually triggered by the nucleus absorbing a neutron
• It releases energy and further neutrons
• It joins two small nuclei together
• It is the process that powers the Sun
• It never releases any neutrons

Controlling the chain ⚛️

Those spare neutrons make a **chain reaction** possible — and it can be tamed or unleashed: • In a **nuclear reactor** the chain reaction is **controlled**. **Control rods** absorb spare neutrons so the reaction runs steadily and safely. • In a **nuclear weapon** the chain reaction is deliberately **uncontrolled**, releasing enormous energy in an instant.

Nuclear fusion 🌟

**Nuclear fusion** is the opposite of fission: two **light** nuclei **join together** to form a single **heavier** nucleus, releasing energy. It needs **extremely high temperature and pressure** to force the positively charged nuclei close enough to fuse. Fusion is the process that **powers the Sun and all other stars**.

Fission or fusion?

What is the key difference between nuclear fission and nuclear fusion?

• Fission splits a large nucleus; fusion joins two light nuclei
• Fission joins nuclei; fusion splits them
• Fission powers stars; fusion is used in reactors
• Fission releases energy but fusion absorbs it

Match each term to its meaning

Why so hot?

Why does nuclear fusion only happen at extremely high temperatures and pressures?

• The positively charged nuclei repel each other, so huge energy is needed to force them together
• The nuclei are too heavy to move unless strongly heated
• All the neutrons must be removed before the nuclei can join
• Fusion needs a neutron to start it, just like fission

Nuclear summary

Nuclear _____ splits a large, unstable nucleus after it absorbs a neutron, releasing energy and more neutrons that can cause a chain reaction. Nuclear _____ joins two _____ nuclei into a heavier one and needs extremely high temperature and pressure. Fusion is the process that powers the _____.