Nuclear Fission and Fusion

(Separate Physics only) The whole of this lesson, section 4.4.4, is assessed in AQA GCSE Physics but not in Combined Science.

Nuclear reactions change the nucleus of an atom and can release enormous amounts of energy. There are two ways to do this, and they are opposites.

• Nuclear fission splits a large nucleus into smaller pieces.
• Nuclear fusion joins small nuclei into a larger one.

Both processes release energy, but they work in very different ways and use very different fuels. Fission powers today's nuclear power stations; fusion powers the Sun and every other star. The rest of this lesson looks at each in turn and then compares them directly.

Fission = splitting apart. Fusion = fusing together. Getting these two words the right way round is the single most common mark lost on this topic.

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

For fission to happen:

1. Spontaneous fission, where a nucleus splits entirely on its own, is very rare.
2. Usually the large nucleus must first absorb a neutron. This makes the already unstable nucleus even less stable.
3. The unstable nucleus then splits into two smaller nuclei of roughly the same size.
4. Two or three neutrons are released, along with gamma rays.
5. Energy is released. All the products (the two nuclei and the neutrons) carry away kinetic energy, and gamma radiation is emitted as well.

The energy released as kinetic energy of the fast-moving fragments is what a power station turns into heat, and then into electricity.

In fission a large unstable nucleus absorbs a neutron, splits into two smaller nuclei of roughly equal size, and gives out two or three neutrons plus gamma rays. Energy is released, carried mainly as kinetic energy of the products.

Fission can be written as a nuclear equation, and the totals of the top numbers (mass numbers) and the bottom numbers (atomic numbers) must balance on each side.

A typical fission of uranium-235:

${}_{92}^{235}\text{U}+{}_0^1\text{n}\to {}_{56}^{141}\text{Ba}+{}_{36}^{92}\text{Kr}+3{}_0^1\text{n}$

Check the mass numbers (top): on the left $235+1=236$; on the right $141+92+(3\times 1)=236$. They balance.

Check the atomic numbers (bottom): on the left $92+0=92$; on the right $56+36+(3\times 0)=92$. They balance.

This example produces three neutrons; other fissions of the same nucleus produce two neutrons and different daughter nuclei, but the total mass and charge always balance.

AQA does not require you to identify the specific daughter elements produced by fission. You do need to know that the products are two smaller nuclei plus two or three neutrons and gamma rays, and that energy is released.

The neutrons released by one fission are the key to a chain reaction. Each neutron can be absorbed by another large nucleus, making that nucleus split and release more neutrons, which go on to cause still more fissions.

                       n
                      /
   n ──► [ U ] ──► fission ──► n ──► [ U ] ──► fission ──► n ──► ...
                      \                          \
                       n                          n

Each fission releases two or three neutrons, so the number of fissions can grow very quickly if every neutron goes on to split another nucleus.

A chain reaction is set up when neutrons released by fission are absorbed by further nuclei, causing them to undergo fission and release yet more neutrons.

Whether this growth is useful or dangerous depends entirely on how the neutrons are managed, which is the difference between a reactor and a weapon.

The same chain reaction can be run in two very different ways.

	Controlled chain reaction	Uncontrolled chain reaction
Where it happens	Nuclear reactor (power station)	Nuclear weapon
Rate of fission	Steady and constant	Grows extremely rapidly
How neutrons are managed	Excess neutrons absorbed to keep the rate steady	Nothing absorbs the extra neutrons
Energy release	Steady, used to generate electricity	Almost all at once, a huge explosion

In a nuclear reactor the chain reaction is controlled. On average, exactly one neutron from each fission goes on to cause the next fission, so the reaction rate stays steady. Extra neutrons are absorbed (control rods do this job) to stop the reaction running away, and the steady energy released heats water to drive turbines and generate electricity.

In a nuclear weapon the chain reaction is uncontrolled. The extra neutrons are not absorbed, so the number of fissions multiplies with each step and an enormous amount of energy is released almost instantly.

Controlled chain reaction, steady energy in a reactor. Uncontrolled chain reaction, explosive release in a weapon. The difference is whether the excess neutrons are absorbed.

Nuclear fusion is the joining of two light nuclei to form a single heavier nucleus. It is the opposite of fission.

When two light nuclei (such as hydrogen nuclei) join together, some of the mass may be converted into the energy of radiation. This is why fusion releases energy.

Fusion is how the Sun and other stars generate their energy: hydrogen nuclei fuse to form helium deep in the star's core, releasing the radiation that eventually reaches us as light and heat.

Fusion is extremely hard to achieve on Earth. The nuclei are both positively charged and repel each other strongly, so they must be pushed together at the very high temperatures and pressures found inside stars before they can join. This is why fusion is not yet used in power stations, even though it uses light, abundant fuel and does not produce the same long-lived radioactive waste as fission.

In fusion two light nuclei join to form a heavier nucleus, and some of the mass is converted into the energy of radiation. Fusion powers the Sun.

Both processes release energy from the nucleus, but almost everything else about them differs.

Feature	Nuclear fission	Nuclear fusion
What happens	A large nucleus splits into two smaller nuclei	Two light nuclei join into a heavier nucleus
Typical fuel	Uranium-235, plutonium (large nuclei)	Light nuclei such as hydrogen
Trigger	Nucleus usually absorbs a neutron	Very high temperature and pressure
Products	Two smaller nuclei, two or three neutrons, gamma rays	A single heavier nucleus
Energy source	Energy released as products move apart	Some mass converted into energy of radiation
Where it is used	Nuclear power stations	Powers stars such as the Sun; not yet used in power stations

Fission splits heavy nuclei; fusion joins light nuclei. Both release energy. The clearest way to remember them is that fission needs big nuclei and fusion needs small ones.

1. Swapping fission and fusion

Fission splits a large nucleus into smaller ones; fusion joins light nuclei into a heavier one. Read the question twice and match the word to the direction of the change.

2. Forgetting the neutron has to be absorbed first

For most fission the large nucleus absorbs a neutron before it splits. Leaving out this step, or saying fission always happens spontaneously, loses marks, because spontaneous fission is rare.

3. Not balancing the fission equation

The mass numbers (top) must add up to the same total on both sides, and so must the atomic numbers (bottom). Remember to count the neutrons released, including the two or three on the right-hand side.

4. Vague descriptions of the chain reaction

State that neutrons released by fission are absorbed by other nuclei, causing them to undergo fission and release more neutrons. Do not just write "it keeps going".

5. Saying fusion releases energy without explaining why

In fusion, some of the mass of the light nuclei is converted into the energy of radiation. That mass-to-energy conversion is the reason energy is released, and examiners look for it.