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. A uranium-235 nucleus that absorbs a neutron might split into barium and krypton. The two smaller nuclei are called the daughter nuclei, and which pair you get varies from split to split: only the pattern is fixed, not the products.

The chain reaction

Put the steps of a nuclear fission chain reaction in order.

• A neutron is absorbed by a large uranium nucleus
• The nucleus becomes unstable and splits into two smaller nuclei
• Energy is released, along with two or three more neutrons
• These neutrons are absorbed by other uranium nuclei
• Those nuclei split too, and the reaction spreads: a chain reaction

Label the chain reaction

Drag each label onto the right part of the diagram. Read it left to right: something arrives, the big nucleus splits, and something leaves.

Controlling the chain

Each fission gives out more neutrons than it used, so left alone the reaction grows. Everything about reactor design is about holding that growth at exactly one. - In a nuclear reactor the chain reaction is controlled. Control rods absorb spare neutrons, and pushing them further IN slows the reaction down. - In a nuclear weapon the chain reaction is deliberately uncontrolled, releasing enormous energy in an instant. So a reactor is not kept safe by having too few neutrons. It is kept safe by absorbing the spare ones, so that on average each fission causes exactly one more.

Run the reactor

You are watching the instruments in a reactor control room. Decide what each reading means and what to do about it.

• The power output is climbing steadily and will not settle. What does that tell you?
• What should you do with the control rods?
• The power steadies. On average, how many further fissions is each fission now causing?

Fission against fusion

The two are opposites in almost every respect, which is exactly why they are so easy to swap over in an exam. Read down each column:

Which process is it?

A physicist describes a reaction in which two nuclei, each of mass number 2, combine to form a single nucleus of mass number 4, giving out energy. Which process is being described, and what does it need?

• Fusion, and it needs extremely high temperature and pressure
• Fission, and it needs a neutron to start it
• Fusion, and it needs a neutron to start it
• Fission, and it needs extremely high temperature and pressure

Why so hot?

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

• The nuclei are both positively charged and repel each other, so huge energy is needed to force them close enough
• 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

Balance the split

A uranium-235 nucleus absorbs one neutron and splits into barium-141 and krypton-92, plus some neutrons. Mass number is conserved: the total before the split must equal the total after it. Each neutron has a mass number of 1. How many neutrons are released?

Mark the answer

Here is a student's answer about nuclear energy. Tap the TWO sentences that are WRONG.

• In nuclear fission, a large unstable nucleus absorbs a neutron and splits into two smaller nuclei.
• The neutrons released can be absorbed by other nuclei, which is how a chain reaction spreads.
• Control rods in a reactor release extra neutrons to keep the chain reaction going.
• Nuclear fusion joins two light nuclei together and releases energy.
• Fusion is the process used in nuclear power stations, and fission is what powers the Sun.

Nuclear summary

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